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The characteristics of the formation of austenite in eutectoid steel Robinson, Michael Dennis Edward 1947-12-31

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L  G" *> 8 1  I 1H  ] ft-)  C<rf .  1  THE CHARACTERISTICS OF THE FORMATION OF AUSTENITE IN  EUTECTOID STEEL  BY Michael Dennis Edward Robinson  A Thesis Submitted l n P a r t i a l Fulfilment of the Requirements f o r the Degree of Master of Applied Science i n the Department of Metallurgical Engineering  The University of B r i t i s h Columbia September, 19^7  ABSTRACT This paper Is a report of the investigations carried out to determine the effect of mlcrostructure on the rate and nature of the formation of austenite i n some .commercial steels. Previous work i s reviewed and compared with the results of current experiments. b r i e f l y discussed.  Experimental procedure Is  Steels used f o r these experiments were  similar to the following S.A.E. s p e c i f i c a t i o n s ; 1020, 10^5, 1080,  blkQ, and  52100.  Studies of the 1080 type form the  greater part of the work. Results are presented i n graphs of hardness against time-at-austenitlzatlon-temperature and i n a series of photographs showing various stages of transformation f o r different p r i o r structures. Conclusions drawn from r e s u l t s are discussed. The processes of nucleation and growth of austenite i n p e a r l i t e , spheroidite, balnite, martensite, and sorbite are examined. The effects of lamellar spacing, size and distribution, of spheroids, and proeutectold constituents are noted, together with general considerations such as s t a b i l i t y of microstructures.  The effect of chromium i s b r i e f l y discussed.  P o s s i b i l i t i e s for future work on a u s t e n i t i z a t l o n are presented, with emphasis on induction heating and flame hardening.  TABLE OF CONTENTS  Object  -  -  -  -  -  -  -  -  Summary  -  -  -  -  -  -  -  -  page 1 2  Review of Previous Work k Formation of Austenite from P e a r l i t e 6 Formation of Austenite from Spheroidite 8 Formation of Austenite from Other Structures 9 General Considerations 10 Experimental Procedure Specimens Used Rate of Heating of Specimens Formation of P r i o r Structures Austenitizing Specimens Metallurgical Examination of Specimens  -  15 15 16 16 19 19  Graphical Presentation of Results  21  Study of Microstruetures Formation of Austenite from P e a r l i t e Formation of Austenite from Spheroidite Formation of Austenite from Other Structures General Considerations  30 31 34 38 39  Discussion - . Conclusions Pearlite Spheroidite Other Structures General Considerations  *3  <*3 k6  Suggestions for Further Work Suggestions A r i s i n g from Current Work Flame Hardening and Induction Heating  -  50 5° 51  Acknowledgment  -  53  Bibliography  -  -  -  -  -  5^  L  I  S  T  OF TABLES AND ILLUSTRATIONS  page Table 1:  Analyses of Steels Used  -  -  -  15  Table 2:  Origin of P r i o r Structures  -  -  -  18  Plate 'I:  Curve Showing Rate-of-Heatlng of Specimens -  -  -  17  Plates I I to VII: Curves Showing Hardness vs. Tlme-at-Austenitizatlon - Temperature of Specimens  Figure i ;  2k to 29  Schematic Curves Showing Relation between Transformation and Hardness. -  Figures 1 to 82: Photomicrographs Showing Progression of Austenitizatlon i n Specimens -  -  21  facing pages 30 to - •-  kj,  (1)  OBJECT  The object of the following investigations was to study the mechanism of the formation  of austenite from  various p r i o r metallurgical structures.  These investigations  were resolved Into a search f o r the answers to the following questions; 1.  What l s the nature of the nucleation of austeni t e i n p e a r l i t e ? i n spheroidite? i n other structures?  2.  What i s the nature of the growth of austenite in these structures?  3.  How do the size and d i s t r i b u t i o n of lamellae in p e a r l i t e and of spheroids i n spheroidite affect the nucleation and growth of austenite?  4.  What i s the effect of structure on the time required f o r a given degree of austenitization?  5*  What i s the effect of carbon content on the time required f o r a given degree of a u s t e n i t i - zation?  6.  What effect do a l l o y additions have on austenitization? The procedure followed i n carrying out the object  was arranged as follows: (a)  a review of l i t e r a t u r e on the subject of austenitization;  (b)  a series of experiments with suitable specimens;  (c)  metallurgical study of these specimens; and  (d)  a comparison of experimental r e s u l t s with the conclusions of previous workers. This arrangement of procedure i s discussed at  further length i n the following summary.  (2)  SUMMARY  v  The questions presented  i n the statement of the  object form the basis for t h i s t h e s i s .  They are discussed  i n the sections e n t i t l e d "Review of Previous Work", "Graphical Presentation of Results", "Study of Microstructures", and "Discussion", and to some degree i n "Experimental Procedure". The order of discussion i s as follows:  and  (a)  The formation of austenite from p e a r l i t e ;  (b)  The formation of austenite from spheroidite;  (c)  The formation of austenite from other structures;  (d) General considerations. This order has been observed, but not followed"^  s t r i c t l y , i n "Review of Previous Work".  In the review, only  those sources r e l a t i n g to the questions of the object are included.  Some sources have been omitted because they are  repetitious; others presented t h e o r e t i c a l considerations that would have required too much space to discuss i n the review. This study of the investigations of previous workers i n dicated the nature of the experimental  work required f o r the  current investigations'. "Experimental Procedure" outlines the work conducted by the writer.  I t deals :with the size of specimens  used, the derivation of the rate-of-heating curve f o r the specimens, and the methods used to obtain the d i f f e r e n t p r i o r structures.  (3)  The results of the examination of the specimens are given l n the sections e n t i t l e d "Graphical Presentation of Results" and "Study of Microstructures".  The f i r s t section  discusses a group of curves showing hardness versus  time-at-  austenltizing-temperature; the second presents a series of photomicrographs showing d i f f e r e n t p r i o r structures at d i f f e r ent times of a u s t e n i t i z a t i o n .  Conclusions are drawn i n both  sections. The conclusions are summarized i n "Discussion" and compared with those drawn by other workers, mentioned i n the review.  In these conclusions w i l l be found the answers, i n  part or i n whole, to the questions asked i n the object. Other questions a r i s i n g from t h i s research are presented i n "Suggestions for Further Work". section supplement the review.  Parts of t h i s  CO  REVIEW OF PREVIOUS WORK  Austenlte Is the high-temperature phase of carbon s t e e l ; i t has a face-centered cubic l a t t i c e , with the carbon atoms s t a t i s t i c a l l y d i s t r i b u t e d among the i n t e r s t i c e s of the lattice.  Below the c r i t i c a l temperature (Ae^), approximately  1330°F f o r p l a i n carbon steels, the l a t t i c e transforms from face-centered cubic to body-centered cubic or tetragonal; i n either case, the s o l u b i l i t y  f o r carbon decreases and the  excess carbon combines with some of the i r o n to form i r o n carbide.  Depending on the rate of cooling from the austenitic  phase, the carbide i s combined i n d i f f e r e n t ways with the remaining iron, or f e r r l t e . ( 1 , 2 ) . I f the s t e e l i s again heated to above the c r i t i c a l temperature, these aggregates of iron-carbide and f e r r i t e are  re-transformed to austenlte; In other words, when carbon  s t e e l i s heated to above the c r i t i c a l temperature, the phenomenon of a u s t e n i t i z a t l o n takes place. Grossman (3) outlines a u s t e n i t i z a t l o n as follows: "When • — steel reaches the temperature at which i t begins to transform, the very f i r s t minute austenlte c r y s t a l s form within a p e a r l i t e i s l a n d or i n the boundaries between f e r r i t e grains. These small austenlte grains grow across the p e a r l i t e islands and across the, f e r r i t e grains u n t i l they meet other austenlte grains grown from other n u c l e i . When a l l the austenlte grains have thus met, a certain set of grain sizes w i l l have been established. 0  (5)  Bain  (4) accompanying his discussion with several  photomicrographs "by V i l l e l a , says that above the  critical  temperature, « the reaction between f e r r i t e and carbide to form austenlte i s inaugurated. The reaction necessarily begins at the c a r b i d e - f e r r i t e i n t e r face. — • The carbon o r i g i n a l l y In the carbide p a r t i c l e s near the interface has moved out some 0.0003 inch i n as short a time as 5 seconds and i n 60 seconds s u f f i c i e n t carbon to form a complete austenitic matrix has d i f f u s e d outward from the p a r t i c l e s to a distance at least equal to h a l f the spacing between carbide p a r t i c l e s . — This steep concentration gradient i s propitious f o r rapid carbon d i f f u s i o n . " Davenport and Bain (5),  i n discussing the laws  applying to austenitic grain establishment  i n higher carbon  steels, state that the f i r s t nuclei form at c a r b i d e - f e r r i t e interfaces (a) where manganese, or a similar element, i s at higher concentrations,  (b) where carbide, i f lamellar, i s  i n the thinnest plates, or (c) where carbide, I f spheroidal, i s i n the f i n e s t p a r t i c l e s .  In the i n i t i a l stages these  nuclei are very numerous but as. time progresses, favorably located, meanwhile absorbing f e r r i t e and carbide. f o r t h by Grossman (6),  those more  the larger areas of  These premises had already been set who  discussed them with p a r t i c u l a r  reference to low carbon s t e e l s .  Davenport and Bain  (5)  also  indicate that the laws of grain growth are c l o s e l y related to and dependent upon the phenomena of the formation austenlte.  of  (6)  The foregoing ideas have also been discussed i n part or i n whole by Carpenter and Robertson (7), Hultgren others.  (8), and  Roberts and Mehl (9) present a very thorough b i b l i -  ography of work done p r i o r to 194-2. These fundamentals of austenitizatlon have been developed more thoroughly by Roberts • and Mehl (9, 10), Digges and Rosenberg (11,12,13,14), and Baeyertz  (15). The Formation of Austenlte from P e a r l i t e Baeyertz  (15) states, i n substance, that:  on heating aggregates of f e r r i t e and p e a r l i t e , the p e a r l l t l c areas become austenitic f i r s t , followed by free f e r r i t e . Similarly, i n aggregates of cementite and p e a r l i t e , the transformation of the p e a r l i t e i s followed by that of the which Is complete only when the A forming at the A  C 1  c m  i s exceeded.  cemetite,  Austenlte  must be of eutectoid content and for this  reason would naturally be expected to nucleate at phase boundaries between f e r r i t e and cementite.  At  first,  "numerous small austenlte grains replace small portions of the cementite lamellae and the f e r r i t e between two cementite lamellae." As the extremely small grains meet each other, the larger probably grow at the expense of the others, t h i s e f f e c t increasing as the grains become established.  On increasing  the temperature, these grains apparently grow i n two ways, the f i r s t by transformation of contiguous  ferrite  and  (7)  cementlte and the second by atomic rearrangement of small p a r t i c l e s of austenlte enveloped by the larger grains, Baeyertz  (15) also mentions two i n h i b i t i n g e f f e c t s ,  (a) undissolved p a r t i c l e s of oxides, carbides, etc., which produce considerable discrepancy i n the size of the austenlte grains and, i n general, tend to reduce the grain size;  and,  (b) , the fact that austenlte does not readily grow across f e r r i t e grain boundaries between p e a r l i t e colonies.  These  effects insure that the resulting austenlte has a grain size equal to or less than the size of the p e a l i t e colonies from which i t was  formed, provided the austenlte so formed i s not  then subjected to a coarsening cycle. Davenport and Bain (5) state that the f i r s t nuclei form at the c a r b i d e - f e r r i t e interfaces where the lamellae are the thinnest.  Baeyertz  (15)  says that the n u c l e i expand  more rapidly along the p e a r l i t e lamellae than across them, thus producing grains roughly rectangular i n section, with the advancing boundaries being commonly perpendicular to the lamellae. Digges and Rosenberg (Ik), i n t h e i r study of a highp u r i t y iron-carbon a l l o y  (0»50  per cent carbon), determined  that, i n a mixture of fine p e a r l i t e and f e r r i t e , austenlte nucleates p r e f e r e n t i a l l y at the f e r r i t e - c a r b l d e interfaces near the p e a r l i t e - f e r r i t e boundaries and at p e a r l i t e  colony  boundaries and secondarily between the p e a r l i t e lamellae.  (8)  The predominant growth occurs In the d i r e c t i o n s of the lamellae* Roberts and Mehl (9)  concluded that nuclei formed at  p e a r l i t e colony boundaries were r e l a t i v e l y few compared to those formed at ferrite-cementite interfaces and that a minor , source was  at f e r r i t e grain boundaries at contact with carbides.  In discussing rates of nucleation and growth of p e a r l i t e , Mehl (10)  showed that the rate of a u s t e n l t i z a t i o n i s the  faster the f i n e r the spacing of the p e a r l i t e . Mehl (9)  Roberts and  stated that as the spacing of the p e a r l i t e decreases,  the rates of nucleation and growth increase and that the overa l l increase i s roughly proportional to the c a r b i d e - f e r r i t e i n t e r f a c i a l area.  Fine p e a r l i t e produces a s l i g h t l y f i n e r  grain than coarse p e a r l i t e on austenitizatlon, since the rate of nucleation increases as the i n t e r l a m e l l a r spacing decreases. The Formation of Austenlte from Spheroldlte Davenport and Bain (5)  state that the f i r s t  nucleation  i n spheroldlte occurs where the carbide i s i n the f i n e s t particles.  Baeyertz (15)  states that the transformation  i n the grain boundary regions of the p r i o r austenlte.  begins The  n u c l e i form at ferrite-cementite phase boundaries around the cementite p a r t i c l e s which l i e i n the f e r r i t e grain boundaries.  (9)  Dlgges and Rosenberg (14) determined that  austenite  nucleates at the f e r r i t e - c a r b i d e interfaces, primarily at the carbide network and occasionally within the network.  At f i r s t  growth i s p r e f e r e n t i a l l y along the network, Roberts and Mehl (9) state, i n substance, that: the austenitizatlon of spheroldlte i s e s s e n t i a l l y s i m i l a r to that of p e a r l i t e and approaches general nucleation.  In low carbon  steels austenlte forms around each p a r t i c l e and grows into f e r r i t e ; i n high carbon steels austenite, forms around some p a r t i c l e s and growth includes the r e s t . The Formation of Austenite  from Other Structures  Baeyertz (15) states that the transformation of martensite i s e s s e n t i a l l y the same as that of spheroldlte, commencing i n the grain boundary regions of the p r i o r austenite.  The nuclei form around cementite p a r t i c l e s which l i e  i n the f e r r i t e grain boundaries.  Martensite  may be considered  as extremely fine spheroldlte, since i t i s necessarily tempered before the transformation  begins.  Roberts and Mehl (9) claim that the carbides i n the martensite p r e c i p i t a t e on heating and that a u s t e n i t i z a t l o n proceeds s i m i l a r l y to that i n spheroldlte.  Differences i n  rate are functions of the degree of spheroidizatlon. No l i t e r a t u r e was available regarding the austeniti z a t l o n of structures other than those already mentioned.  (10)  General  Considerations  In discussing a u s t e n i t i c grain establishment carbon steels, Davenport and Bain  (5)  atate that the  i n higher first  nuclei form at c a r b i d e - f e r r i t e interfaces where manganese, or a s i m i l a r element, i s at higher concentration.  Dlgges and  Rosenberg, (14) studying a 5°-carbon steel, concluded that the i n i t i a l austenite i s fine-grained, regardless of the rate of heating through the Ac-_.  Rapid heating through the Ac -Ac^  range produces fine-grained austenlte.  1  They further con-  cluded that the rate of growth, rather than the rate of nucleation, was  the important factor i n establishing the f i n a l  austenitic grain size of t h i s s t e e l , Baeyertz (15)  concluded from her studies that p r i o r  structure determines only grain size at the time of  formation,  except where undissolved p a r t i c l e s i n the austenite i n h i b i t the action.  Mehl (10)  shows that the rate of a u s t e n i t i z a t l o n  i s the greater the coarser the grain size of the s t e e l , Roberts and Mehl (9) have made what appears to be the most academic approach to the problem.  They assume the pro-  cess to be established as one of nucleation and growth, the nucleation being, a structure-sensitive property.  Above the  equilibrium temperature, atomic mobility i s increased and  the  energy required f o r the formation of austenite decreases as the temperature i s raised.  (11)  The actual l o c a t i o n of the nuclei i s a matter of s t a t i s t i c a l probability, as the number of possible positions i s a function of the i n t e r f a c i a l area of the ferrite-cementite interfaces. Even though the energy required f o r formation i s less at grain boundaries, the effect of f e r r i t e grain boundaries i s small,  ,  as nucleation would have"to begin at the point where a f e r r i t e grain boundary meets a cementite p a r t i c l e .  Nuclei formed at  p e a r l i t e colony boundaries are r e l a t i v e l y few compared to those formed at ferrite-cementite interfaces. According to Roberts and Mehl (9),.the  austenite  nuclei form around cementite p a r t i c l e s l y i n g i n f e r r i t e grain boundaries and often form f i r s t at p r i o r austenite grain boundaries.  In the case of spheroidite and martenslte, a  greater number of cementite p a r t i c l e s l i e i n f e r r i t e grain boundaries than i n the case of p e a r l i t i c structures.  Roberta  and Mehl l o g i c a l l y assume from these and similar factors that the nucleation of austenite Is structure-sensitive, dependent mainly on the size and shape of the carbide p a r t i c l e s and to a much smaller degree upon the presence of f e r r i t e grain boundaries. In discussing the growth of austenite nodules, Roberts and Mehl reason as follows: cementite and f e r r i t e react at the advancing interface to form austenite, the rate of interaction depending on the rate of solution of carbide and also on the rate of migration of carbon atoms i n the austenite.  (12)  Therefore the rate of growth increases with an increase of temperature, as the d i f f u s i o n factor and the effect of carbon concentration gradients increase with temperature.  Hence,  the rate of growth i s also a structure-sensitive property  and  i s presumably dependent on the rate of d i f f u s i o n and perhaps on the rate of the reaction at the interfaces.  Interference  with normal growth i s due primarily to impingement, seconda r i l y to grain coalescence (which would speed the  reaction)  and to pro-eutectoid constituents i n hypfo- and hypereutectold s t e e l s . On the effect of banding and dendritic patterns, Roberts and Mehl (9) state that areas with high manganese and  (or) low phosphorus and s i l i c o n w i l l nucleate  first,  l o n g i t u d i n a l l y along the rows. Johnson and Mehl (16),  i n discussing the determin-  ation of austenitic grain size, give the following mathemati c a l relations? Where N N  t  0 and N  v  i s the t o t a l number of nodules per unit volume of t o t a l l y reacted sample, i s the t o t a l number of grains per unit area of polished surface, i s the rate of growth i n mm. per s e c , i s the rate of nucleation,  then N_ = 0.896(N )^  (1) (2)  (13)  Roberts and Mehl ( 9 ) , assuming that N  Y  i s constant  and that the reaction proceeds by general nucleation, show that N  Y  can be calculated from the rate of growth and the time  of h a l f reaction, t ^ , as follows: Q  k /N G V  3  . t  5  0  -  o.9  . t  2  Q  s  0.66  (3)  and y^G  3  -  -  -  (4)  Apparently equation ( 4 ) , using the time for 20 per cent reaction, gives better agreement with actual data than does equation ( 3 ) .  They found that N  V  Increases with  (a) decrease i n the spacing of p e a r l i t e , (b) increase i n temperature, (c) decrease i n the grain size of the s t e e l , and (d) time at a continuous rate, up to 15 or 20 per cent r e action. Roberts and Mehl also outlined the three cases covering nucleation and growth of austenite: (1)  K increases at a faster rate than G with increase i n reaction temperature and the grain size w i l l therefore decrease with increasing heating rate.  (2)  G increases at a faster rate than N with Increase  i n reaction temperature and the grain size w i l l therefore increase with increasing heating rate. (3)  Both N and G increase at the same rate and there w i l l be no e f f e c t of heating rate on the grain s i z e .  general  In conclusion, Roberts and Mehl (9) summarize t h e i r findings as follows: (1)  Austenite forms by nucleation and growth from ferrite-cementite aggregates; both phenomena are structure-sensitive.  (2)  Austenlte formed i n eutectoid steels i s not homogeneous when the f e r r i t e disappears as a structural constituent, f o r undissolved carbide remains, the solution rate of which i s dependent on both time and temperature. Carbon concentration gradients exist a f t e r the carbide i s no longer v i s i b l e .  (3)  'The rate of formation of austenite increases continually with temperature above the Ae^ > in terms of nucleation and growth.  (4)  As p e a r l i t e spacing increases, the rates of nucleation and growth increase. The overa l l increase i s roughly proportional to the c a r b i d e - f e r r i t e i n t e r f a c i a l area.  (5)  The rate of growth i s less f o r aluminum k i l l e d steels while the rate of nucleation remains about the same, accounting f o r the f i n e r grain size i n the former. Where the rate of growth i s about the same, the rate of nucleation i s higher, thus preserving a high N /G r a t i o . y  (6)  Fine p e a r l i t e produces a s l i g h t l y f i n e r grain than coarse p e a r l i t e , since the rate of nucleation increases with f i n e r spacing. Some l i t e r a t u r e on induction heating has been r e -  viewed i n "Suggestions f o r Further Work".  Although some  phases of a u s t e n i t i z a t l o n are discussed i n these references, they were not necessary to the undertaking of the current experimental work, outlined i n "Experimental  Procedure".  (15)  EXPERIMENTAL PROCEDURE  Specimens Used Five Steels were used f o r the experimental work: Steel 1 Steel 2 Steel 3 Steel 4 Steel (5  S.A.E. 1020 S.A.E. 1045 S.A.E. 4l40 Atlas "KK", similar to S.A.E. 52100 Atlas "Maple Leaf Special", similar to S.A.E. 1080.  ,/  ' /  The larger part of the work was done with Steel  , 7; Steels 1 and 2 were used to i l l u s t r a t e transformation l n low carbon steels; Steels 3 and 5 were used to examine the effect of additions of chromium.  The analyses f o r these  samples i s given i n Table I. Steel  1 2 3 5 ^  C  Mn  Cr  0.16 0.72 — 0.43 0.69 ~ 0.48 0.89 0.53 1.03 0.40 1.25 0.82 0.12  —  Mo  Si  —  0.025 — 0,21 0.23 0,28 .05 0.2? —  0,17  Table I : Analyses of Steels Used Bars of these steels were turned to 7/16"  diameter  (tolerance 0.005") and discs 1/8" thick (tolerance 0.002") were cut from the round bars.  A small hole was d r i l l e d  through each specimen close to the circumference, thus providing a means of suspending the specimens while heat treating them.  Several specimens were d r i l l e d r a d i a l l y to  the center and the hot Junctions of chromgl-alumel  thermo-  couples made with 22 gauge wire were wedged into the holes.  1  (16)  Apparatus consisted of two e l e c t r i c salt-bath furnaces (a one-pound and a five-pound capacity),  temperature  control instruments, chromel-alumel thermocouples, a potentiometer, a stop-watch, a quench tank, and a supply of fine chromel wire f o r suspending the specimens i n the s a l t bath. Rate of Heating of Specimens In order to determine the actual time of the specimens at austenitizing temperature, a rate-of-heating curve had to be constructed. This was done by attaching the thermocouples wedged i n the specimens to the potentiometer and then plunging the specimens into the salt bath.  The time required  for the specimen to r i s e to a temperature indicated by a predetermined setting on the potentiometer was measured with a stop-watch.  By repeating t h i s procedure f o r several .  d i f f e r e n t settings of the potentiometer, s u f f i c i e n t data was obtained to construct the curve shown.in Plate I, page 17. Repetition of the whole procedure several times proved the curve to be accurate enough f o r the purpose. Formation of P r i o r Structures Each set of specimens was divided Into groups of ten; each group of ten specimens was heat treated to form a pre-determined p r i o r structure.  F u l l use of '^-Curves'* and  other heat-treating data was made at t h i s point (17,18). l i s t of these structures i s given i n Table I I , page 18.  A  1  i  tSa/rBath  Temp ei-ature,  I4SS°F  ts°  f340°F  1300  \ £1100  i  I  N  SOO  i  j IS  ZS 3S "Time,, Seconds  45  SS  PLATE I Graph showing Rate o f H e a t i n g o f Specimen i n S a l t B a t h at 1455°E 10°. The specimen, 1/8" t h i c k "by 7/16" d i a m e t e r , was suspended i n t h e s a l t h a t h by a chromel-alumel thermocouple, wedged i n a r a d i a l l y d r i l l e d h o l e so t h a t t h e hot j u n c t i o n was a t t h e c e n t e r of the specimen. The l a g i n t h e curve at "A" i n d i c a t e s d e c a l e s c e n c e on p a s s i n g t h r o u g h t h e c r i t i c a l temperature and i s a s s o c i a t e d w i t h the abso r p t i o n o f heat as f e r r i t e t r a n s f o r m s t o a u s t e n ite« The time n e c e s s a r y t o r e a c h a u s t e n i t i z i n g t e m p e r a t u r e , 20 sejconds, h a s been s u b t r a c t e d from from t h e t i b e s of immersion] of specimens i n t h e s a l t b a t h ; hence, P l a t e s I I t o V I I and F i g u r e s 1 t o 82 a r e based on t h e t r u e t i m e s of a u s t e n i t ~ i z a t i o n f o r t h e specimens. (24)  (18)  Steel Used  Preliminary Austenitizatlon  1080  i  n  hr., 1450 n  tt  it  n  ii  tt  tt  u  0  JJ  a  it  H  tt  »  ri  H  it  n  1020 n  tt  a  1045' u  it  52100  tt it  it  H  1045 4140 52100  2 hr.,  tr  3h hr., 1300 hr., 1275 { hr., 1250 { hr., 1225 t nr., 750 2 hr., 550 Brine quenched Quenched drawn at 800 % hr. 8^ hr., 12901340 10 hr., 12 hr., " 14 hr., « 1 hr., 1250 20 hr.,12801330 1 hr., 1250 20 hr.,12801330 1 hr.,1250 20 nr.,12801330 1 hr.,1250 20 nr.,12801330 2 hr.,1250 2 hr.,1250 2 hr.,1250  C Pearlite M " H " R Upper Bainite Lower " Martensite  7b 7c 7d 7e 7m 7n 7o  Sorbite  7P  Spheroldlte  7q 7r 7s 7t  0  11  0  a  tt tt  Resulting Structure  1  M  n n  Succeeding Treatment  1550  tt  n St  M Pearlite PSC U  Specimen Ident'n  7t  " "  7g lf  Spheroldlte M Pearlite  Ig 2f  C Pearlite . Spheroldlte  5t  it  M Pearlite u 11  2g fg  2a 3a 5a  ( i ) Temperatures indicated are i n degrees Fahrenheit, plus or minus 10 degrees. ( i i ) "Succeeding Treatment" followed "Preliminary A u s t e n i t i zatlon without delay. ( i l l ) Spheroidlzlng cycles resembled a sine curve; each complete cycle within the temperature ranges shown required approximately 15 minutes. (iv) A l l heat treating cycles used were determined by r e f e r i n g to "S-Curves". (v) C - Coarse, M - Medium, F - Fine, L- Lower, PS - P a r t l y Spheroidized, U - Unresolved at X730. Lamellar spacing varied from 2 microns for Coarse P e a r l i t e to less than •5 microns to less than 1 micron i n spheroldlte. (vi) O r i g i n a l structures of steels: 1080, annealed; 1020, 1045, and 4l40, normalized; 52100, spheroidized. :  Table I I :  Origin of P r i o r Structures Used i n F i n a l Austenitizatlon Experiments.  (19)  Austenitization of Specimens After the p r i o r structures were formed, each specimen was subjected to the f i n a l heat treatment.  One specimen was  reserved from each group of ten for examination i n the "as l s " condition.  Each of the remaining nine was immersed i n a s a l t  bath, temperature 1455°F plus or minus 10 degrees, f o r a measured time, 10 seconds to 10 minutes, and quenched i n brine. A nine per cent salt solution was used after the recommendations of G i l l  (19).  This treatment produced a group of ten  specimens f o r each p r i o r structure; each specimen i n each group has been austenitized for a d i f f e r e n t period of time. Commercial heat t r e a t i n g salts were used throughout with one exception.  A "home made" salt (20) was used extensively  and proved satisfactory f o r intermittent use at temperatures between 1200 and 1500°F; the composition of t h i s s a l t was as follows - barium chloride 4 8 . 1 $ , potassium chloride 3 » 7 # Q  sodium chloride 21.2$. Metallurgical Examination of Specimens Each specimen was s p l i t across the diameter; one h a l f was mounted f o r metallographic examination (Figures 1 to 82, "Study of Microstructures"): the other h a l f was- tested f o r hardness with a Rockwell hardness tester.  Results of the hard-  ness tests are given i n Plates I I to VII, "Graphical Presentation of Results".  (20)  The hardness readings were converted to the B r i n e l l scale (3000 kg. load, 10 mm. b a l l ) , the only suitable scale covering the complete range of hardnesses.  Rockwell "A",  "B", and "C" scales were used i n the actual determinations. A four per cent solution of p i c r i c acid i n ethyl alcohol proved to be the best general etch f o r metaliographic examination (21). I t s use brought out excellent d e t a i l i n most structures encountered, especially i n p e a r l i t e and spheroldlte; i t was p a r t i c u l a r l y valuable f o r distinguishing martensite (quenched austenlte) from untransformed structures by coloration. True austenitizatlon times (time i n s a l t bath minus 20 seconds, from Plate I) are used i n "Graphical Presentation of Results  u  and "Study of Microstructures". A l -  though the term "austenite" i s used i n discussing these results, the product actually studied was martensite, formed by quenching the specimen to "freeze" the structure at the desired time of a u s t e n i t i z a t l o n .  1  (21)  GRAPHICAL PRESENTATION OF RESULTS The graphs shown i n Plates I I to VII inclusive are derived from the hardnesses found f o r each specimen.  This  i s a r e l i a b l e method f o r determining r e l a t i v e rates of austenitization.  This i s not to say that percentage trans-  formation can be predicted from hardness or vice versa, although a certain degree of f a m i l i a r i t y with the curves permits prediction within rough l i m i t s . According to Roberts and Mehl (9),  increase i n  hardness lags behind per cent transformation up to 10 or 15 per cent transformation.  The curves  then converge and cross and the hardness approaches a maximum at Jj u 85 to 90 per cent transformation, i . e . , the per cent transformation lags behind the Increase i n hardness i n the l a t e r  Time of Austenitizing Temperature Figure i : Schematic Curves Showing Relation Between Transformation and Hardness.  stages of the reaction. The l a t t e r l a g i s apparent i n the current studies.  The  studies do not, however, show the I n i t i a l lag i n increase of hardness.  (22)  Examination of the graphs i n Plates II to VII, pages 2,4 to ZS indicates the following y  conclusions?  Plate I I ; Steels 7b,7c,7d,7e, cut from the same bar of Atlas "Maple Leaf Special"(S.A.E. 1080) are successively f i n e r grades of p e a r l i t e formed at 1300, 1275, 1250, and 1225 F respectively. The curves show that the f i n e r the p e a r l i t e the sooner the hardness increases. Approximately f u l l hardening i s reached at 80 seconds. (2)  Plate I I I : Steels 7q, 7r, 7s 7t, cut from the bar above and spheroidized 8-f, 10, 12, 14 hours of spheroids (range of average sized p a r t i c l e s , O.75 to 1.5 microns) i s not p a r t i c u l a r l y pronounced, the curves d e f i n i t e l y show that the f i n e r the spheroldlte the sooner does the Increase i n hardness occur. Complete hardening i s not attained at 280 seconds, as t h i s time i s not s u f f i c i e n t to austenitize the spheroids completely; maximum hardnesses are approached at 40 seconds a u s t e n i t i z a t l o n of these specimens.  (3)  Plate IV; Steels 7m, 7n, 7o, 7p, cut from the bar above and having p r i o r structures of upper bainite, lower bainite, martensite, and sorbite (tempered martensite) respectively. The graphs show that these structures transform f a r more rapidly than p e a r l i t e or spheroldlte. F u l l hardening Is reached i n 15 seconds f o r these f i n e r structures; there i s no s i g n i f i c a n t d i f ference i n the time necessary to completely harden bainite, martensite, or sorbite.  (4)  Plate V: Steels 7f, 7g, 5f, 5g, the f i r s t two cut from the bar above, the second from the same bar of Atlas "KK" (52100). Specimens (f) were annealed 1 hour at 1250°F and specimens (g) heat treated 20 hours at 1280-1330°F a f t e r austenltlzing \ hour at 1450 F. The curves i n dicate that structures taking longer periods to develop, i . e . , the more stable structures, take longer periods to a u s t e n i t i z e .  (5)  Plate VI: Steels 3f, 3g, 2f 2g, the f i r s t two cut from the same bar of 4l4o, the others from the same bar of 1045• Specimens (f) and (g) treated as i n (4). In addition to confirming the conclusion i n (4), these curves also  (23)  Indicate that the addition of chromium retarded the transformation to austenite^ This i s to he expected, since chromium i s a strong carbide former and i s known to retard the transformation either to or from austenite(22,23). Comparison between steels 2 and 7 (see Plates V I , V) indicates that decreasing carbon below the eutectoid point increases the r e action time* (6)  Plate V I I ; Steels 2a, 3a, 5a, cut from the same bars as 2, 3, 5 (Plates V, V I ) , respectively. A l l specimens austenitized 2 hours at 1550°F followed by annealing 2 hours at 1250°F, p r i o r to f i n a l austenitizing treatments at 1 4 5 5 ° » F  No conclusions are drawn from information i n Plate V I I . These specimens were austenitized one at a time and probable fluctuations of temperature between specimens prohibit accurate comparisons. Specimens 7b, c, d, e were austenitized simultaneously,  however, as were specimens 7<1, *\ s, t and  7m, n, o, p; a l l - specimens (f) and (g) were also austenitized together.  Curves f o r specimens l f and l g were not drawn, as  the range of hardnesses for this s t e e l was so small that such curves were useless f o r comparison.  These l a t t e r speci-  mens, however, furnish some of the best photomicrographs depicting the formation of austenite i n low carbon steel (see Figures 1 to 4, "Study of Micro structures"). The following "Study of Microstructures" i s intended as a companion piece to the graphs discussed i n the present each study.  section; the same specimens have been used f o r  S  ^  S  BrineII Hardness  &  s  &  (3000 Hg., 10 mm.)  |LATE I I Graphs showing Hardness v s . Time at A u s t e n i t i z i n g Temperature. S a l t Bath Temperature, 1455°E± 10°. SAE 1080 - 7b - Coarse P e a r l i t e 7c - Medium P e a r l i t e 7d - Medium P e a r l i t e 7e - F i n e P e a r l i t e N  25  1  S 3  ~_T??—-  I  £  Brine// Hardness  S  S  (3000 Kg.,  s  10 mm.)  PLATE I I I  Gra.phs showing Hardness v s . Time at A u s t e n i t i z i n g Temperature . S a l t Bath Temperature, 1 4 5 5 F ± 1 0 O . SAE 1080 - 7q - F i n e s t S p h e r o i d i t e 7r 7s 7t - Coarsest Spher'ite'' 6  PLATE I V Graphs shewing Hardness v s . Time at A u s t e n i t i z i n g Temperature . S a l t B a t h Temperature, 1455°E±10°. SAE 1080 - 7m - Upper B a i n i t e 7n - Lower B a i n i t e  7o - Mart en s i t e 7p = S o r b i t e "  /  t  PLATE V . \ Graphs showing Hardaess v s . Time at A u s t e n i t i z i n g Temperature . S a l t Bath Temperature, 1455°E± 10° . SAE 1080 - 7f - Coarse P e a r l i t e 7g - S p h e r o i d i t e SAE 52100- 5f - S p h e r o i d i t e / 5g - " (More homogeneoj^ ^  1  Graphs showing Hardness v s . Time at i A u s t e n i t i z i n g Temperature . S a l t Bath \ Temperature, 1455°E±10o. ;! SAE 4140 - 3f - P e a r l i t e j 3g - Coarse P e a r l i t e SAE 1045 - 2 f - P e a r l i t e 2g - Coarse P e a r l i t e  PLATE V I I Graphs showing Hardness v s . Time at A u s t e n i t i z i n g Temperature. S a l t B a t h Temperature, 1455°P±10°P. SAE 1045 - 2a - P e a r l i t e 'SAE 4140 - 3a - P e a r l i t e SAE 52100- 5a - P e a r l i t e  ?ig. l:lfk X730 Annealed 1020  Fig.2: lfC X7 30 Austenitized 5  A u s t e n i t i z e d 15  \ u s t e n i t i z e d 280  sec.  sec  sec.  (30) 5TUDY OF MICROSTRUCTURES Most of the photomicrographs are arranged so that each set represents the progression particular steel.  of transformation In a  Sets have been grouped, where possible, to  show the -differance i n reactions for steels having a r e l a t i v e l y s l i g h t difference i n p r i o r structure. A l l specimens were austenitized i n a s a l t bath at 1455°F, plus or minus 10 degrees. 1?  Reference to Plate I, page  shows that a u s t e n i t i z a t l o n temperature, approximately  1340°F, was  reached at 20 seconds In the salt bath; specimens  required a further 35 seconds to r i s e to the temperature of the bath.  Times mentioned f o r the Figures shown do not i n -  clude the i n i t i a l 20 seconds required to raise the temperature of the specimens to the point where a u s t e n i t i z a t l o n commenced* Figures 1 to k, facing page, are presented as an introduction and show progressive carbon s t e e l (S.A.E. 1020). 1)  austenitizatlon i n low  The o r i g i n a l structure,  (Figure  consists of grains of very fine p e a r l i t e (unresolved at  X 730)  d i s t r i b u t e d i n bands i n a f e r r i t e matrix*  As shown i n  Figure 2, austenitizatlon. of the p e a r l i t e i s effected within 5 seconds; the austenite has already begun to grow i n the surrounding f e r r i t e . austenite  Figures 3 and k show the advance of  (darker areas), p r e f e r e n t i a l l y along the  ferrite  grain boundaries. L i t t l e transformation has occurred between 15 280 seconds.  According to the iron, iron-carbide  and  equilibrium  F i g . 6 : 7"b^ X 715 A u s t e n i t i z e d 15 sec.  F i g . 7 : 7bF X 715 A u s t e n i t i z e d 20 sec.  F i g . 8 : 7"bG X 715 A u s t e n i t i z e d 30 sec.  (3D  diagram (20), the a u s t e n l t e : f e r r i t e r a t i o i n a  completely-  reacted, 0*20 per cent carbon a l l o y i s about 8:7 at 1450°F. Hence, transformation would not be expected to continue a great deal beyond that shown i n Figures 3 and 4.  Comparison  of these figures indicates that, a f t e r the p e a r l i t e has transformed, the austenlte advances along the f e r r i t e grain boundaries and expands l a t e r a l l y u n t i l the l i m i t i n g carbon concentration i s reached* Formation of Austenlte from P e a r l i t e Figures 5 to 8, facing page, and Figures 9 and 10 are studies of transformation occurring i n coarse p e a r l i t e . Figure 5 r P e  r e s e r r t a  ^  e  o r i g i n a l structure, formed by cooling  from 1450 to 1300°F and holding at the l a t t e r temperature f o r 3^- hours. The  Average spacing i s about 1.5 microns per lamella.  f i r s t noticeable transformation to austenite has occurred  at 15 seconds, and i s manifested by "fusion" of some of the lamellae (see Figure 6). Figure 7 indicates that nucleation has commenced p r e f e r e n t i a l l y at the boundaries of the p e a r l i t e colonies and that growth has begun to include these colonies before further s i g n i f i c a n t nucleation has occurred. Figure 8 (30 seconds austenitizatlon) shows points of random nucleation and growth a f t e r the primary reaction i s well consolidated. Figures 9 and 10, taken at higher magnification, are perhaps the most interesting of the series.  Interpretation  F i g * 9 ••  7bV X 1120 A u s t e n i t i z e d 20 seconds.  (32)  of the evidence  shown at lower right and top center of Figure  9 and at upper l e f t of Figure 10 indicates the following conclusions regarding the a u s t e n i t i z a t l o n of p e a r l i t e , 1.  Nucleation starts p r e f e r e n t i a l l y at the ferrite-cementite interface near a p e a r l i t e colony boundary.  2.  I n i t i a l growth tends to "hug" the carbide lamella on which nucleation has occurred and proceeds along the side of the lamella for some distance before dissolving the carbide and before crossing the intervening expanse of f e r r i t e to the next lamella.  3.  The preferred growth of austenlte i s along and between the lamellae at the f e r r l t e carbide interface. Growth i s retarded by p e a r l i t e grain boundaries, carbide lamellae, and areas of f e r r i t e (lamellar or not) i n that order when they exist at right angles to the d i r e c t i o n of growth.  k.  Small traces of the carbide lamellae are " l e f t behind" by the advancing austenlte; these are dissolved l a t e r as the transformation continues. Figures 11 to 15 (facing page 33) picture the trans  formation i n p e a r l i t e formed by quenching S.A.E. 1080 from 1450 to 1275°F and holding one-half hour at the l a t t e r temper ature.  The f i r s t signs of a u s t e n i t i z a t l o n occur at 15  seconds (see Figure 12) and are indicated by a s l i g h t thicken ing and "welding" of the carbide lamellae.  No s i g n i f i c a n t  difference exists between the character of transformations i n t h i s and the preceding structure, 7b. An instance of the fusion of the lamellae at the grain boundary, i n the d i r e c t i o n of the boundary, before  ?ig.ll.  7cC  X  715  ? i g . 13: 7cF X 715 A u s t e n i t i z e d 20 s e c .  ?ig.!4: 7cG X 715 A u s t e n i t i z e d 30 s e c .  •'ig.15: 7cH X 715 A u s t e n i t i z e d 40 s e c .  F i g . 16: 7dF X 715 A u s t e n i t i z e d 20 s e c .  F i g . 1 8 : 7dH X 715 A u s t e n i t i z e d 40 sec,  Fig.20;7e"B X715, A u s t . 15 s e c .  1  ' i g . 17: 7dG X 715 A u s t e n t i i z e d 30 s e c .  I f f . 1 9 ; 7eC X 715 A u s t e r i i t i z e d 5 sec.  Plg.21:7eG X 715 A u s t . 30 sec.  Fig.?.2:7eH X 715 tost. 40 s e c .  (33)  transformation proceeds along the lamellae i s shown i n the upper right corner of Figure 14 (austenitized 30 seconds). From t h i s and similar instances, i t may he inferred that the growth of austenite tends to p a r a l l e l p e a r l i t e grain boundaries.  This, however, i s a secondary preference and the  growing areas of austenite are roughly rectangular, with the length p a r a l l e l to the lamellae and the breadth p a r a l l e l to the grain boundaries,  or across the lamellae.  Figure 15 shows the transformation to be about 75 per cent complete at 40 seconds within the a u s t e n i t i z i n g range, or above the Ac^. Figures 16, 17 and 18 show transformation i n p e a r l i t e formed at 1250°F; Figures 19 to 22 show transformation i n p e a r l i t e formed at 1225°F.  These specimens exhibit 7b and 7c,  except that the transformation i s the faster, the f i n e r i s the p e a r l i t e .  Average lamellar spacing i s approximately  1.5  microns per lamella for s t e e l 7b, 1 micron for s t e e l 7c, 0.8 for 7d, and 0.7 f o r 7e. Two more conclusions are added to those already formed regarding transformation i n p e a r l i t e : 5«  6.  The secondary preference f o r the growth of austenite, nucleating as i n 1., l s perpendicular to the p e a r l i t e lamellae next to the p e a r l i t e grain boundaries. This and the preference indicated i n 3» result l n roughly rectangular austenite grain. The f i n e r the spacing of the p e a r l i t e lamellae, the more rapid i s the transformation of p e a r l i t e to austenite, other variables being equal.  7qA X 840 S p h e r o i d i z e d 8-g- h r . SAE 1080  P i g . 2 4 : 7qS X A u s t e n i t i z e d 15  840 sec.  (34)  Formation of Austenlte from Spheroldlte  Figures 23 to 42 show a group of spheroidized structures of S.A.E. 1080 (Atlas "Maple Leaf Special") at various stages of transformation.  Steel 7q, austenitized one-half hour at  1450°F and cooled to and held at 1290-1340°F f o r 8£ hours, i s shown In Figures 23 to 26; s t e e l 7r, 10 hours; s t e e l 7s, 12 hours; and s t e e l 7t, 14 hours, are shown i n Figures 27 to 32, 33 to 36, and 37 to 42, respectively. The maximum diameters of the spheroids i n steels 7q, 7r, and 7t are approximately  1,5, 2, 3, and 4 microns, respectively  average diameters are roughly ,75, 1, 1, and 1,5 microns, respectively.  The structures are progressively more stable  from 7q to 7t, that i s , more of the spheroids approach maximum size i n the longer-treated steels and the spheroids are more evenly distributed. There are two h i n t s of impending transformation at 10 seconds at a u s t e n i t i z i n g temperature for these specimens. On comparing specimens 7rD, 7sD, and 7tD (Figures 28, 34, 38) with specimens 'A f o r the same steels (Figures 27, 33, 37), 1  a "thinning out" of the f i n e r spheroids can be seen i n specimens 'D', indicating that some of them have commenced transformation within 10 seconds.  A s l i g h t difference i n  r e l i e f i s also noticeable;, i n specimens 'A , the r e l i e f Is 1  a l l within the boundaries of the spheroids but i n specimens  g i f t . 2 7 ; 7rA X 840 Spher'zd 10 h r . SAE 1080  F i g . 2 8 : 7rD' X 840 Austenitized 10 seconds  F i g . 2 9 : 7 r E X 840 Austenitized 15 seconds  F i g . 3 0 : 7 r F X 840 Austenitized 20 seconds  F i g . 3 1 : 7 r G X 840 Austenitized 30 seconds  F i g . 3 2 : 7 r K X 840 Austenitized 280 seconds  (35)  f  D, f  t h e r e i s not o n l y r e l i e f w i t h i n the p e r i m e t e r s o f the  s p h e r o i d s but a l s o a f a i n t o u t l i n e around the o u t s i d e o f them. T h i s l a t t e r phenomenon of r e l i e f i n m l c r o s t r u c t u r e i s the o n l y evidence  In any of the photomicrographs o f t h e  a c t u a l nucleation of austenite i n s p h e r o i d i t e .  It indicates  t h a t the s p h e r o i d i s the n u c l e u s and t h a t t h e t r a n s f o r m a t i o n b e g i n s on t h e whole s u r f a c e o f the s p h e r o i d r a t h e r than a t a point.  I t i s perhaps more l o g i c a l t o assume t h a t a  nucleus  forms a t a p o i n t on the s u r f a c e o f t h e s p h e r o i d and t h a t t h i s p o i n t expands r a p i d l y t o form an e n v e l o p i n g f i l m o f a u s t e n i t e about the The  spheroid.  d i f f e r e n c e between specimens D' ,  q u i t e pronounced.  The  and  'E* i s  t r a n s f o r m a t i o n has p r o g r e s s e d  from  l e s s than one p e r c e n t t o about 80 p e r cent i n specimen 7qe, over 5° P ** cent l n 7rE, e  25 p e r cent i n 7 t E .  about 40 p e r cent l n 7sE,  andabout  These i n c r e a s e s have been e f f e c t e d by  h o l d i n g the specimens f o r an a d d i t i o n a l 5 seconds above the Ac^  (see F i g u r e s 24, 29, 35,  39)»  I n these specimens, the  t r a n s f o r m a t i o n has a p p a r e n t l y o r i g i n a t e d - i n the network a t the g r a i n b o u n d a r i e s  spheroid  of the f e r r i t e matrix  and  subsequent growth has t a k e n p l a c e a l o n g the b o u n d a r i e s . f e r r i t e g r a i n boundaries F i g u r e s shown.  are not p a r t i c u l a r l y  These  e v i d e n t i n the  Specimens had t o be etched d e e p l y t o b r i n g  out the b o u n d a r i e s  and the n e c e s s a r i l y l o n g exposures o f  the p l a t e s e l i m i n a t e d the d e t a i l t o such an e x t e n t t h a t p h o t o m i c r o g r a p h s so t a k e n p r o v e d u s e l e s s .  E i g . 3 3 : 7sA X 840 S p h e r o i d i z e d 12 h r . SAE 1080  P i g . 3 4 ; 7sD X 840 A u s t e n i t i z e d 10 sec.  ?ig.35;7sE X 840 A u s t e n i t i z e d 15 sec.  F i g . 3 6 : 7sF Austenitized  X 840 20 sec.  gig.37; 7tA X 840 S p h e r o i d i z e d 14 h r .  gig.38: 7tD X 840 Austenitized 10 s e c .  gig.39: 7tE X 840 A u s t e n i t i z e d 15 s e c .  f i g . 4 0 ; 7tF X 840 A u s t e n i t i z e d 20 s e c . "flb •  v * r. * V!, « -  V  .• e I'-  P  . ov OO  -,  ?ig.41; 7 f r X 840 A u s t e n i t i z e d 30 s e c .  (36)  Figures 26, 32, and kZ show the transformation at 280 seconds.  There i s l i t t l e difference between these speci-  mens and those austenitized f o r 3° seconds (specimens 'G-' ), the transformations being v i r t u a l l y complete i n both cases. Fewer fine spheroids remain at 280 seconds; the coarser spheroids are slower to dissolve.  This i s no doubt a matter  of s t a b i l i t y and concentration gradients.  In the f i r s t ,  instance, the spheroid i s the most stable of the constituents i n normal steels at temperatures below the Ae^;  i t would  naturally be expected to be the most sluggish to react to austenitization.  In the second place, as the concentration  of carbon i n the austenite increases, the d i f f u s i o n of carbon from the combined form to the s o l i d solution decreases. The same considerations may be applied to explain the differences i n rates of formation of austenite i n p e a r l ite.  The f i r s t austenite to form, Just above the A c ^  be of eutectoid composition;  must  i f the carbide lamellae are  thin, a small growth of austenite includes the carbide lamellae and the intervening layers of f e r r i t e ; i f the lamellae are r e l a t i v e l y thick, the same growth might not t o t a l l y consume the lamellae or cross the intervening f e r r i t e . In the l a t t e r case, the carbon concentration gradient has been increased by interposing an intermediate  concentration  (austenite) between the carbide and the f e r r i t e . reasonable  I t i s also  to assume that thicker plates of carbide are more  (37)  stable than thinner ones and, hence, retard the transformation to austenite.  Austenitized 2 8 0 seconds. These, then, are the conclusions regarding the austenitization of spheroidite: 1.  Nucleation takes place on the surface of each spheroid as a whole rather than at a point on a c a r b i d e - f e r r l t e Interface; l f the very f i r s t o r i g i n of a n u c l e i i_s a point, then that point expands almost instantaneously to form an "envelope of nucleation" covering the spheroid.  2.  Nucleation occurs p r e f e r e n t i a l l y at those spheroids l y i n g i n the f e r r i t e grain-boundary network.  3.  Austenite grows p r e f e r e n t i a l l y along the f e r r i t e grain boundaries while the remainder of the growth i s i n the form of a l a t e r a l expansion to include the f e r r i t e grains.  4.  The more the more position the time  homogeneous the spheroidite, i . e . , evenly distributed i n size and are the spheroids, the longer i s required f o r a u s t e n i t i z a t i o n .  Ei£ii6:7nA X 715 Lower B a i n i t e SAE 1080  ?ig.47;7nD X 715 Austenitized 10 seconds.  Fig.48:7nS X 715 Austenitized 15 seconds.  Martensite. SAE 1080  F i g . 5 2 : 7 s A X 840 Sorbite(Martensite drawn at 800°F) SAE 1080  Fig.50:7oC X 840 A u s t e n i t i zed 5 seconds.  Fig.53:7nE X 840 Austenitized 15 seconds.  F i g . 54:7pK X 840 Austenit i z e d 280 seconds.  (38)  5»  The l a r g e r the spheroids, the longer i s the time required f o r austenitization*  Formation of Austenite form Other Structures A survey of the structures containing the carbon i n f i n e r dispersion than i n spheroidite or p e a r l i t e i s given i n Figures 43 to  Figures 43 to 45 show the transformation  of upper b a i n i t e ; 46 to 48, lower b a i n i t e ; 49 to $1, martens i t e j and 52 to 5^, sorbite formed by tempering martensite one-half hour at 800°F,  L i t t l e can be said regarding the  nucleation of austenite i n any of these structures, except that I t appears to be general, or occurring at random© P r i o r to nucleation, an agglomeration  of carbides  takes place l n the martensite and sorbite, reducing them to what l s , In.effect, extremely fine spheroidite.  Transform-  ation has apparently started before 10 seconds (see Figure 47) and i s almost complete i n 15 seconds (see Figures 45, 48, 51, 53) at a u e t e n i t l z l n g temperature*  Specimens *E* also  show the two balnites to be more completely transformed at t h i s stage than either the martensite or the sorbite* I f we consider the balnites as extremely  fine  p e a r l i t e , and the martensite and sorbite as extremely  fine  spheroidite, the reason becomes apparent; nucleation and transformation are effected more e a s i l y i n the lamellar structures.  I t l s l i k e l y , though, that the b a l n i t e s undergo  some form of tempering, or agglomeration  of carbides, before  (38)  5*  The l a r g e r the spheroids, the longer i s the time required f o r a u s t e n i t i z a t i o n *  Formation of Austenite form Other Structures A survey of the s t r u c t u r e s containing the carbon i n f i n e r d i s p e r s i o n than l n spheroidite or p e a r l i t e i s given i n Figures 43 to 5^«  Figures 43 t o 45 show the transformation  of upper b a i n i t e ; 46 t o 48, lower b a i n i t e ; 49 to 51,  marten-  s i t e ; and 52 to 5^, s o r b i t e formed by tempering martensite one-half hour a t 800°F.  L i t t l e can be s a i d regarding the  n u c l e a t i o n o f a u s t e n i t e i n any of these s t r u c t u r e s , except that i t appears to be general, or o c c u r r i n g at random* P r i o r to nucleation,. an agglomeration of carbides takes place i n the martensite and s o r b i t e , reducing them to what i s , i n . e f f e c t , extremely f i n e s p h e r o i d i t e .  Transform-  a t i o n has apparently s t a r t e d before 10 seconds (see Figure 47) and i s almost complete i n 15 seconds (see Figures 45, 48, 51, 53) at a u s t e n i t i z i n g temperature*  Specimens 'E* also  show the two b a l n i t e s to be more completely transformed a t i t h i s stage than e i t h e r the martensite or the s o r b i t e . I f we consider the b a l n i t e s as extremely f i n e p e a r l i t e , and the martensite and s o r b i t e as extremely f i n e s p h e r o i d i t e , the reason becomes apparent; n u c l e a t i o n and transformation are e f f e c t e d more e a s i l y i n the l a m e l l a r structures.  I t i s l i k e l y , though, that the b a l n i t e s undergo  some form of tempering, or agglomeration of carbides, before  i  F i g . 55; 7f A X 840 a r l i t e . S A E 1080  F i g . 5 7 ; 7 f D X 840 Austenitized 10 seconds.  F i g . 5 6 ; 7fC X °40 .Austenitized 5 s e c .  F i g . 5 8 : 7 f 0 X 840 ,ustenitized •50 seconds.  F i g . 5 9 ; 7 f K X 840 \ustenitized 280 seconds.  F.ifl.eo: 7gA X 730 P a r t l y spheroidized P e a r l i t e . SAE 1080  F i g . 61: 7gC X 730 A u s t e n i t i z e d 5 seconds.  F i g . 6 4 : 7gK X 730 Austenitized 280 seconds.  (39)  transformation occurs; the graphs i n Plate IV, "Graphical Presentation of Results", indicates t h i s to be so. The following conclusions regarding these finer structures appear reasonable: 1.  Austenite forms i n bainite by random nucleation and growth; the formation resembles that i n p e a r l i t e but proceeds more rapidly.  2.  Austenite forms from martensite and tempered martensite by random nucleation and growth; the structures react as though they were extremely fine, homogeneous spheroidite.  3.  Bainite, martensite, and tempered martensite can be austenitized more rapidly than normal p e a r l i t e or spheroidized structures. General  Considerations  Figures 55 to 64 present a comparison of rates of austenitization i n similar structures having different degrees of s t a b i l i t y : Figures 55 to 59 show the transformation i n a structure formed by annealing steel 7 f o r one hour at 1250°F d i r e c t l y a f t e r austenitizing one-half hour at 1450°F; Figures 60 to 64 show the transformation i n a structure formed by cooling to and holding at the range 1280-1330°F, d i r e c t l y a f t e r the same a u s t e n i t i z a t i o n cycle.  The second p r i o r  structure appears similar to the f i r s t , being only s l i g h t l y coarser. In both steels, the transformation has commenced at five seconds of subsequent a u s t e n i t i z a t i o n (specimens 'C, Figures 56 and 6 l ) .  These specimens are approximately  five  ?ig.67:5fG X 730 Austenitized 30 seconds.  Fig.68:5fH X 730 Austenitized 40 seconds.  Fig.69:r3fK X 7 3 0 Austenitized 2 8 0 seconds.  (40)  per cent transformed.  Specimens 'D* show a s l i g h t difference  i n that more transformation has occurred i n the f i n e r structure; specimen 7fB i s over 90 per cent transformed,  7gD i s  about 85 per cent transformed, at 10 seconds at austenitizing temperature (Figures 57, 6 2 ) .  At 280 seconds, specimen ?fK  has transformed more completely than has specimen 7gK (Figures  59, 6 2 ) . The following conclusion i s indicated: 1.  The more stable the structure, that i s , the more f u l l y i t has developed under similar conditions, the greater Is the time required f o r the austenitizatlon of that structure. A l l the conclusions so f a r presented i n t h i s section,  with the exception of those obtained from the study of Figures 1 to 4, are founded on the examination cut from the same bar of s t e e l .  of specimens  This s t e e l i s Atlas "Maple  Leaf Special", a commercial grade of eutectoid t o o l steel similar to S.A.E. 1080. The remaining photomicrographs, Figures 65 to 82, .show general studies of transformations occurring i n a few other steels.  Figures 65 to 69 show the progress of trans-  formation i n Atlas "KK? similar,to S.A.E. 52100.  The  structure i s well spheroidized and appears to transform in. the normal manner, with a notable exception; some f e r r i t e remains at the end of 40 seconds at austenitizing temperature (Figure 68, specimen 5fH)»  The explanation i s that chromium  i s retarding the transformation; t h i s Indicates the following conclusion:  P i g , 7 0 ; 3fA Pearlitic  ' X 730 4140  f i g . 72: 3fG X 730 A u s t e n i t i z e d 30 seconds.  F i g . 7 1 : 3fD X 730 A u s t e n i t i z e d 10 seconds.  P i g . 7 3 t 3fK X 730 A u s t e n i t i z e d 280 seconds.  (41)  lo  The presence of chromium retards the formation of austenite* Figures 70 to 73, of a S.A.E. 4140 s t e e l ,  t h i s conclusion.  support  Unlike the transformation i n ordinary p e a r l -  i t e , the formation of austenlte i n t h i s case begins i n the pearlite. bordering on the free f e r r i t e and some of the f e r r i t e i s dissolved before the transformation consumes the larger part of the p e a r l i t e (see Figures 71, 7 2 ) .  Figure 73, specimen  3fK, shows undissolved p e a r l i t e remaining even after 280 seconds of a u s t e n i t i a t i o n * z  I t i s probable that most of the  •pearl 31a i s concentrated i n the i n t e r i o r of the p e a r l i t e and that the i n i t i a l transformation occurs i n the comparatively chromium-poor p e a r l i t e and f e r r i t e . Figures 74 and 75 show normal transformation i n a S.A.E. 1045 p e a r l i t e .  The transformation has dissolved the  p e a r l i t e before growing Into the free f e r r i t e .  Inspection of  the inclusions In Figure 75 .does not reveal that they have i n any. way interfered with the reaction.  Reference to Figures  1 to 5 establishes the same situation; inclusions i n the S.A.E. 1020 apparently do not interfere with the growth of the austenite. 1.  The following conclusion i s presented: V i s i b l e inclusions d i d not interfere with the nucleation or growth of austenite i n any of the specimens examined during the experiments r e l a t i n g to 'this report. Figures 76 to 80 also show transformation i n S.A.E.  1045; the p e a r l i t e i s a l i t t l e finer and more evenly  distri-  buted than i n the specimens shown i n Figures 74 and 75*  P e a r l i t i c 1045, Aust .lOsec.  F i g . 7 8 : 2 a ? X 1800 Austenitized 20 seconds.  A u s t e n i t i z e d 15 seconds.  Pig.79;2aF X 600 Austenitized 20 seconds.  Fig.80:2aG X 600 Austenitized 30 seconds.  (42)  Figure 76 (specimen 2aD)  shows l i t t l e or no transformation to  have occurred at 10 sdconds austenitization.  Figure 77, at  15 seconds, pictures a "weld" i n the lamellae at the p e a r l i t e grain boundary; the growth i s evidently about to expand i n ward along the lamellae.  Other areas of nucleation are shown  i n the same Figure as random thickening of the lamellae. Figure 78 shows what has happened to a p e a r l i t e grain during 20 seconds at a u s t e n l t i z i n g temperature.  The  untrans-  formed p e a r l i t e i s outlined by the "ghost" structure i n the rest of the grain, now austenitized p e a r l i t e .  Fig.81: 5aF X 1800 52100, austenitized 20 seconds.  Fig.82: 5aH X 1800 52100, austenitized 30 seconds.  Figures 81 and 82 show stages i n the transformation of Atlas "KK",  similar to S.A.E. 52100.  The "ghosts" of  p e a r l i t e are t y p i c a l of chromium steels and are further evidence that chromium retards transformation. The conclusions presented i n this study, together with those from "Graphical Presentation of Results" are summarized and discussed l n the following section of t h i s report.  (43)  DISCUSSION  It i s the plan of t h i s discussion to re-state the conclusions as indicated i n the sections on "Graphical Presentation of Results" and "Study of Mlcrostructures", and to compare these conclusions with those of the workers c i t e d i n "Review of Previous Work".  Most of the following  conclusions are not o r i g i n a l but have been checked by the writer. Summary of Conclusions  Pearlite 1.  Austenite nucleates p r e f e r e n t i a l l y at the carbide-ferrite interfaces of the lamellae near the p e a r l i t e grain boundaries; secondary and minor points of nucleation are at the interfaces of the lamellae within the p e a r l i t e and at positions of random d i s t r i bution. C3,i4).  2.  The growth of austenite i s primarily along the lamellae at the c a r b i d e - f e r r i t e i n t e r face i n f i n g e r - l i k e growths; secondarily, across the i n t e r - l a m e l l a r f e r r i t e , expanding at the base of the "fingers" to include the neighboring lamellae; t h i r d l y , across the lamellae p a r a l l e l and close to the inner "wall" of the p e a r l i t e colony boundaries. 0 5 ) .  3.  Austenitic growth i s retarded by p e a r l i t e colony boundaries, carbide lamellae, and areas of f e r r i t e , i n that order, when i n the path of the advancing austenite. (\5).  (44)  4.  In eutectoid s t e e l , the growth preferences result i n the I n i t i a l austenite grains having a roughly rectangular section. Transformation i s generally complete before a l l traces of the lamellae are dissolved, 0 >9)5  5.  In low carbon steel, the transformation of the p e a r l i t e Is complete, or nearly so, before growth advances i n a f i n g e r - l i k e manner, expanding l a t e r a l l y at the base, (»5).  6.  The transformation i s the f a s t e r , the f i n e r the interlamellar spacing of the p e a r l i t e , other variables being equal. C9). A l l observations contained i n the conclusions  regarding the a u s t e n i t i z a t i o n of p e a r l i t e have been mentioned, at least i n substance, by previous workers. supports conclusions 2 to 5,  Baeyertz  (15)  i n c l u s i v e , on the a u s t e n i t i z a t i o n  of p e a r l i t e ; Roberts and Mehl (9) mention observations i d e n t i c a l , i n substance, with those of conclusions 1, 4, and 6; Digges and Rosenberg (14) are i n accord with conclusion 1, Spheroidite 1.  Nucleation may occur at a point on the surface of a spheroid. However, the point, i f such i t i s , expands so rapidly to cover the surface of the spheroid that nucleation may be considered to commence as an "envelope" covering the surface of the spheroid. (4).  2.  Nucleation occurs p r e f e r e n t i a l l y about those spheroids i n or near the f e r r i t e grain boundaries and where the spheroids are i n the f i n e s t p a r t i c l e s . (i5,5).  3.  Austenite grows p r e f e r e n t i a l l y along the f e r r i t e grain boundary network, expanding l a t e r a l l y to include the f e r r i t e grains. In the early stages the growth i s roughly spherical, with f i n g e r - l i k e projections proceeding along the grain boundaries. (i4) .  (45)  4.  Growth does not readily cross f e r r i t e grain boundaries but tends to f i r s t include the f e r r i t e grain i n which the transformation commenced. The more homogeneous i s the spheroldlte, the more random i s the nucleation and the longer time i s required f o r transformation,  6.  The larger the spheroids, the longer i s the time required for complete transformation. Conclusions 1, 2, and 3, r e l a t i n g to the a u s t e n i t i -  zatlon of spheroldlte, have been previously Indicated.  The  photomicrographs of V l l l e l a , reproduced by Bain (4), support conclusion 1; i t i s unfortunate that Bain gives no references to V i l l e l a ' s work and no analyses of the steels used. clusion 2 i s supported by Baeyertz Bain (5).  Con-  (15) and Davenport and  Conclusion 3 i s confirmed i n the work of Dlgges  and Rosenberg (14). Other Structures 1.  Austenite forms from bainite by random nucleation and growth; rate and character of transformation are what would be expected i n extremely fine p e a r l i t e ,  2.  Austenlte forms from martensite and sorbite by random nucleation and growth; the necessary tempering of these structures reduces them to what resembles extremely f i n e , homogeneous spheroldlte,15*,9).  3.  Bainite, martensite, and tempered martensite transform more rapidly than do normal p e a r l l t i c or spheroidized structures. o  (46)  The essence of conclusion 2, regarding  martensite  and sorbite has been given by Baeyertz (15) and by Roberts and Mehl ( 9 ) . General  Considerations 1.  The more stable the structure, that i s , the more f u l l y I t has developed under similar conditions, the more time i s required f o r the a u s t e n i t i z a t i o n of that structure.  2.  The presence of pro-eutectold constituents retards the transformation to austenite; i n hypoeutectoid steels there are less t o t a l points of nucleation; i n hypereutect o i d steels, the massive carbides require additional time to transform a f t e r areas of eutectoid composition have transformed. In either case, areas of eutectoid comp o s i t i o n transform f i r s t . (i5\3).  3.  The presence of chromium retards the formation of austenite and tends to produce "ghosts" i n the newly transformed structures. C22,23)  4.  Complete transformation i s not necessary to the attainment of maximum hardness i n p e a r l i t e , b a i n i t e , martensite, or tempered martensite. CS)-  5»  Maximum hardness may be obtained i n p e a r l i t e eutectoid s t e e l by austeniting for 80 seconds; i n b a i n i t e , martensite, and sorbite at 15 seconds: maximum hardness may be approached i n spheroidite by a u s t e n l t i z i n g for 40 seconds and are attained i n the coarser spheroidites only beyond 280 seconds. These statements apply only to s t e e l 7 under the conditions of the current experiments.  6.  V i s i b l e inclusions d i d not i n t e r f e r e with the nucleation or growth of austenite i n any of the specimens examined during the current experimenta.  (47)  Conclusion (15)  2 has been previously stated by Baeyertz  and by Roberts and Mehl (9); conclusion 3 i s generally  mentioned i n texts on physical metallurgy, including Heyer (22) and G i l l and co-workers (23). Indicated by Roberts and Mehl (9)  Conclusion  4 has been  (see "Graphical  Presentation  of Results"). 0  Conclusions 4, 5, and 6, under "Spheroldlte" and concerning growth retarding influences and r e l a t i v e rates of growth of austenlte i n spheroldlte, are not s p e c i f i c a l l y mentioned In any of the references  included i n the review.  They have, however, been Inferred i n such discussions and i n studies r e l a t i n g to g r a i n growth and other phenomena. claim can be made as to the o r i g i n a l i t y of these  No  conclusions;  they have perhaps been stated previously i n sources not i n vestigated by the writer. None,of the l i t e r a t u r e investigated contained any s i g n i f i c a n t remarks pertaining to the a u s t e n i t i z a t l o n of bainite; nothing was found that confirmed conclusion 1, "Other Structures".  under  Except f o r the b a i n i t e , the substance of  conclusion 3, under the same section, and regarding rate of transformation,  relative  has been i n f e r r e d previously by many  sources. Conclusion 1,  under "General Considerations",  regard-  ing the effect of s t a b i l i t y of structures, applies to the austenitizatlon of most structures.  I t can be expanded to  explain why martensite, p e a r l i t e , and spheroldlte are  (48)  progressively slower to react to a u s t e n i t i z a t i o n and why f i n e r structures usually austenitize more rapidly than others* Conclusion 5, under the same section and presenting some quantitative results regarding rates of hardening, i s indicated by the current experiments; t h i s conclusion may he applied with caution to transformations i n steels similar to those used i n these experiments* Conclusion 6, stating that v i s i b l e Inclusions d i d not interfere with austenitization, i s the only one actually at variance with the work of others; Baeyertz  (15) states, i n  substance, that undissolved p a r t i c l e s of oxides, carbides, etc*, produce considerable discrepancy i n the size of the austenite grains and, i n general, tend to reduce the grain size*  While admittedly the inclusions observed i n the S.A.E*  1020 and 1045 during the current experiments were not i d e n t i f i e d with respect to composition, they were not found to Interfere with the austenitization of these steels i n any manner (see Figures 1 to 4, 74, and 75, "Study of Microstructures" )• Some attempts were made to check the equations of Roberts and Mehl (9) (Equations 3 and 4, "Review of Previous Work").  Not enough data was a v a i l a b l e to give other than  rough estimates of the variables Involved and calculated r e s u l t s were unsatisfactory, although the order of the, constants checked (calculated constants were between 0*2 and 0*5, as compared to 0*66 f o r the time of o n e - f i f t h reaction as given by Roberts and Mehl).  (49)  Considerably  more research i s necessary i f the mechanism  of the nucleation and growth of austenite i s to be thoroughly examined.  Some of the possible directions of such i n v e s t i g -  ations are indicated i n the following section, "Suggestions for Further Work",  (50)  SUGGESTIONS FOR FURTHER WORK  Austenitizatlon i s the necessary beginning of p r a c t i c a l l y a l l the time-temperature treating of s t e e l .  cycles used f o r the heat  I t follows that a thorough knowledge of  the mechanism of austenitization should be a prerequisite to such heat t r e a t i n g .  There i s , however, much research to be  performed before the complete f i e l d of austenitizatlon can be thoroughly examined.  Some of ,the possible avenues of research  on the formation of austenite are presented i n t h i s section. Suggestions A r i s i n g from Current Work  Among the phases of a u s t e n i t i z a t i o n not dealt with i n t h i s thesis are the effects of p r i o r austenitic grain size, a l l o y content (nickel, chromium, e t c ) , and rate of heating on the nucleation and formation of austenlte. more quantitative work could be performed,  Considerably  dealing with rates  of austenitization versus p e a r l i t e spacing, spheroid d i s t r i bution and other variables. More work of the type advanced by Roberts and Mehl (9,  10) and by Johnson and Mehl (16) on the prediction of  quantitative rates of nucleation and growth would be valuable* It should be possible to compile tables or graphs showing the times required at various rates of heating f o r (a) the attainment of completely transformed, homogeneous austenite  (5D  and (b) the degree of austenitization necessary f o r maximum , hardening versus composition and microstructure f o r many commonly used steels; the form of such graphs i s indicated i n Plates I I to VII, "Graphical Presentation of Results". Flame Hardening and Induction on Heating  Actual rates of a u s t e n i t i z a t i o n are of v i t a l interest to heat-treaters hardening steels by flame hardening or i n duction heating.  Since both processes are necessarily rapid  for surface hardening, i t i s important to know how long a particular, s t e e l must be austenitized before i t can be hardened.  Austenitization by flame hardening would presumably  require research such as conducted i n the current experiments; Investigations Into the effect of large masses of cold metal adjoining the heated part, that i s , the effect of heat flow, on austenitization are indicated. Induction heating presents another phase of the problem, namely, the effect of the heating c h a r a c t e r i s t i c s of the process on austenitization  A review of the l i t e r a t u r e on i n -  duction heating proved s l i g h t l y confusing l n that some c o n f l i c t i n g opinions were presented.  Tran and Osborne (25,26)  claim extremely fast rates of a u s t e n i t i z a t i o n f o r the process and also thorough austenitization, including solution of free f e r r i t e , of a low carbon s t e e l below the Ae^. Also claimed Is a "superhardenlng" of steels by induction heating. Martin  (52)  and Wiley (27) and E l l i s of induction heating.  (28) discuss these and similar phases  Martin and Van Note (29) review l i t e r -  ature r e l a t i n g to induction heating and a u s t e n i t i z a t i o n with a discussion of the e f f e c t of the pre-austenitlzing microstructure.  Poynter (30) d i f f e r s the most sharply of any with  the claims of Tran and Osborne (25,26).  He sayss  "No evidence i s found to indicate that induction heating results i n more rapid solution and transformation rates - - -. While the evidence obtained regarding the so-called "superhardness ' i s not conclusive, i t seems l i k e l y that the i n creased hardness i s related to the presence of Internal stresses and not to any difference i n the metallurgy" 1  The p o s s i b i l i t i e s of research on the subject of austenit i z a t i o n are f a r more numerous than have been mentioned here. Time or space do not permit further discussion of the subject* In any case, these p o s s i b i l i t i e s w i l l be evident to those f a m i l i a r with the subject of austenitizatlon*  (53)  ACKNOWLEDGMENT  The writer i s indebted to several members of the staff of the Department of Mining and Metallurgy at the University of B r i t i s h Columbia*  Their assistance i n the work  connected with the preparation of t h i s thesis has been of considerable value* The guidance of Professor F. A. Forward, Head of the Department, during the experimental work and his suggestions concerning the presentation of the thesis material were exjtremely h e l p f u l *  W* M* Armstrong, Professor of Physical  Metallurgy and under whose d i r e c t i o n most of the work was conducted, donated much of his time and e f f o r t to unraveling many of the problems encountered; h i s assistance during the photomicrography was p a r t i c u l a r l y valuable* Dr. C* S* Samis, Professor of Metallurgy, made several timely suggestions.  The assistance of Messrs. R.C*  Hammersley, D. A. Scott, and G. B e l l i n determining the analyses of the specimens used i s g r a t e f u l l y acknowledged*  BIBLIOGRAPHY  (1)  Sachs, G., and Van Horn, K.R«, P r a c t i c a l Metallurgy. 1940, Am*Soc.Met.  (2)  Grosvenor, A.W., Seitz, F., and others Metal - Inside Out. 1941, Am.Soc.Met.  (3)  Grossman, M.A», "Grain Size i n Metals, with Special Reference to Grain Growth i n Austenite", Grain Size Symposium. 1934, Am.Soc.Met.  (4)  Bain, E.C., Functions of the A l l o y i n g Elements i n Steel 1959, Am.Soc.Met.  (5)  Davenport, E.S., and Bain, E.C., "General Relations between Grain Size and Hardenabillty and the Normality of Steels", Grain Size Symposium 1934, Am.Soc.Met.  (6)  Grossman, M.A», "On Grain Size and Grain Growth" Trans. A.S.S.T.. Vol. XXI, No.12, 1933.  (7)  Carpenter, H.C.H., and Robertson, J.M., "Structural Changes i n Hypereutectoid Steels on Heating", Jour. I.S.I.. V o l . 127, No. 1, 1933.  (8)  Hultgren, A., "The Ac-j_ Range of Carbon Steel and Related Phenomena", Trans. A.S.S.T.. V o l . 16, 1929.  (9)  Roberts, G.A», and Mehl, R.F., "The Mechanism and the Rate of Formation of Austenlte from Ferrite-Cementite Aggregates", Trans. A.S.M.. V o l . 31, 1943.  (10)  Mehl, R.F., "The Structure and Rate of Formation of P e a r l i t e " Trans. A.S.M.. V o l . 29, 1941.  (11) Rosenberg, S.J., and Digges,' T.G., "Effect of Rate of Heating through the Transformation Range on Austenitic Grain Size", Trans. A.S.M., V o l . 2 9 , 1941.  -  f  (12) .  Dlgges, T.G., and Rosenberg, S.J., "Influence of I n i t i a l Structure and Rate of Heating on the Austenltlc Grain Size of 0,5 per cent Carbon Steels and Iron-Carbon Alloys", J o u r R e s . . Nat.Bur.Stds., Vol. 29, 1942.  (13)  Dlgges, T.G., "Influence of Austenltlc Grain Size on the C r i t i c a l Cooling Rate of High P u r i t y Iron Carbon Alloys", Trans. A.S.M.. V o l . 31, 1943.  (14)  Digges, T.G., and Rosenberg, S.J., "A Metallograph!c Study of the Formation of Austenite from Aggregates of F e r r i t e and Cementite i n an Iron Carbon Alloy of 0.5 per cent Carbon", Trans. A.S.M.. V o l . 31, 1943.  (15)  Baeyertz, M., "Effects of I n i t i a l Structure on Austenite Grain Formation and Coarsening", Trans. A.S.M.. V o l . 30, 1942.  (16)  Johnson, W.A., and Mehl, R.F., Trans. A.I.M.M.E.. Iron and Steel Division,  —  —  — v o l .  135,  P . 4l6,  1939.  (17)  Atlas of Isothermal Transformation Diagrams. 1943, United States S t e e l Corporation, Pittsburgh.  d )  gutting the Heat Treatment to toe Job, 1946 United States Steel Corporation, Pittsburgh.  (19)  G i l l , J.P.,  8  Tool Steels. 1944, Am. Soc. Met. (20)  Metals.Handbook, 1939, Am. Soc. Met.  (21)  V i l l e l a , J.R., Metallographlc Technique for Steel. 1938, Am. Soc. Met. G i l l , J.P., "Alloy Steels", Modern. Steels. 1939, Am. Soc. Met. Heyer, R.H., Engineering Physical Metallurgy. 1941, Van Nostrand. G i l l , J.P., Rose, R.S., Roberts,. G.A., Johnston, H.G., and George, R.B.. Tool Steels, 1944, Am. Soc. Met.  (22) (23) (24)  (25)  Trail, MoA*, ana Osborne, H.B., J r . , "Inherent Characteristics of Induction Hardening", Surface Treatment of Metals. 1 9 4 1 , Am. Soc. Met.  (26)  Osborne, H.B., J r . , "Surface Hardening by Induction", Trans. Electrochem. Soc.. V o l . 7 9 , 1 9 4 1 .  (27)  Martin, D.L., and Wiley, P.E., "Induction Hardening of P l a i n Carbon Steels" Trans. A.S.M.. V o l . 3 ^ , 19^5.  (28)  E l l i s , O.W., "Pseudomorphs of P e a r l i t e l n Quenched Steel" Trans,. A.S.M.. V o l . 32, 1944. . .  (29)  Martin, D.L., and Van Note, W.G., "Induction Hardening and Austenltizing Characteristics of Several Medium Carbon Steels", Trans. A.S.M.. V o l . 3 4 , 1946. .  (30)  Poynter, J.W., "Metallurgical Characteristics of Induction Hardened Steel", Trans. A.S.M.. V o l . 3.4, 19^6.  

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