Bearing Steel 52100

Carbide Refining Heat Treatments for 52100 Bearing Steel

C. A. STICKELS

The life of through-hardened 52100 an t i - f r ic t ion bear ing components is improved if the excess carb ides , undissolved during austenit ization, a re smal l and uniformly d i spe rsed . One kind of ca rb ide- re f in ing heat t r ea tment consis ts of 1) dissolving al l carbides , 2) i so the r - mal ly t ransforming the austenite to pea r l i t e or bainite, and 3) austenit izing, quenching and temper ing in the usual manner . Each step in this sequence of t r ea tments was investigated, and the behavior of pea r l i t i c and bainit ic mic ros t ruc tu re s during subsequent austenit izat ion was contras ted with the behavior of f e r r i t e / s p h e r o i d i z e d - c a r b i d e mic ros t ruc tu re s . It was shown that: 1) The usual hardening t rea tments given sphero id ize-annealed bear ing components resu l t in an inhomogeneous mic ros t ruc tu re , poss ibly due to the fas te r dissolution of carb ides near austenite grain boundar ies . 2) Austenit izat ion of pear l i t e or baini te p roduces ve ry uniform d ispers ions of u l t ra - f ine carb ides on the o rde r of 0.1 gm d iamete r or l ess . 3) Specimens with u l t ra - f ine carb ides tend to have more re ta ined austenite . 4) The ra te of coarsening of u l t ra- f ine ca rb ides at austeni t izing t empera tu res of 840~ and below, is slow enough so that conventional furnace heat t rea tments a re sa t i s fac to ry for producing this mic ros t ruc tu re .

IT has been recognized for some t ime that the ro l l ing contact fatigue life of bear ing s tee ls such as 52100 is be t te r , the sma l l e r the s ize of the excess carbide p a r - t i c l e s p resen t in the mic ros t ruc tu re . However, be - cause soft s tee l is needed for fabr icabi l i ty , the mate- r i a l cus tomar i ly supplied to bear ing manufacturers is spheroidized to produce ra the r coa r se carb ides in a soft f e r r i t e mat r ix . The carbide s ize and d ispers ion p resen t af ter hardening heat t r ea tment re f lec t s to a marked degree the s ize and d ispers ion of carb ides in the spheroidized mater ia l . Thus, near ly al l c o m m e r - c ia l ly produced 52100 bear ing components have c a r - b ides which a re too la rge for the bes t fatigue life.

The intent of the p resen t work is to develop heat t r ea tments which could be applied to spheroidized mic ros t ruc tu r e s to produce hardened bear ing components with uniform d ispers ions of fine carb ides .

LITERATURE REVIEW

Litt le has been published of a sys temat ic nature on the effect of carbide s ize on bear ing fatigue life. There a r e sca t t e red r e fe rences to improved per formance with finer carb ides : Tal l ien 1 c la ims a factor of three difference in fatigue life as a function of carbide s ize. Faunce and Justusson 2 c la im an improvement of 2.8 t imes with carbide ref inement in the life at which 10 pct of a sample of bear ings fai ls (B10 life). Invest i - ga tors at T. L Desford Tubes Ltd. found an improvement in B10 life by near ly a fac tor of 2 with f iner ca rb ides , a In none of these r epo r t s is it mentioned if components were compared with the same hardening heat t rea tments , or compared at the same ha rd - ness ; it is not known if differences in amounts of r e - ta ined austenite existed.

A more thorough examination of the effect of carbide

C. A. STICKELS is a Member of the Process Research Department, Scientific Research Staff, Ford Motor Co, Dearborn, Mich. 48121.

Manuscript submitted September 17, 1973.

s ize has been repor ted by Monma, e t a l . 4 These authors p repa red two sets of specimens, one with a mean e a r - hide d iameter of 1.4/zm (sizes ranged up to 2.7/zm) and another with a mean s ize of 0.56 ~tm (s izes ranged up to 1.5 ~zm). Samples were heat t r ea ted to give the same hardness , using a lower austeni t iz ing t e m p e r a - ture for the fine carbide specimens, hut the same t e m - per ing t rea tment for both. Thus, the compar ison was made at about the same mat r ix carbon content. The average fatigue life of the fine carbide ma te r i a l was 2.5 t imes that of the c o a r s e r carbide mate r i a l . These authors also show that excess carb ides do not make a posi t ive contribution to fatigue strength, but up to 3 to 4 vol pet excess carb ide improves wear res i s t ance .

The heat t rea tment used to ref ine carb ides was not specif ied by Tallien;* Monma, e t a l 4 produced thei r two carbide s izes by modified spheroidizing heat t r e a t - ments. (Total t rea tment t ime was about 13 h and 80 h for the fine and coarse carbides , respect ively . ) Differ- ent spheroidize anneals were also used in the T. I. Desford exper iments , a Faunce and ffustusson a p ro - dueed fine carbides by heating to a t empera tu re suf- f icient to dissolve al l carb ides , a i r cooling to room tempera tu re (forming pear l i te) , then heating for two hours at about 760~ and a i r cooling. The la t te r workers z'a'4 used conventional austenit izing, quenching and temper ing t r ea tments following thei r p r e t r e a t - ments .

Grange 5 developed ve ry uniform d ispers ions of ve ry fine carb ides in 52100 s teel by heating to a t e m p e r a - ture sufficient to dissolve al l carbides , oil quenching in warm oil, and temper ing to e l iminate re ta ined austenite and form fine temper ca rb ides . When such p ieces were rapid ly induction heated and austeni t ized for about 1 rain, quenched and tempered, carb ides on the o rde r of 0.1 pm d iameter were produced (like Fig. 11(a)). He also repor ted 8 that a s i m i l a r m i c r o s t r u c - ture can be produced by heating to dissolve a l l c a r - bides , then quenching to a t empera tu re at which hain- ite fo rms and holding until the austenite is completely

METALLURGICAL TRANSACTIONS VOLUME 5, APRIL 1974-865

t ransformed, then hardening as before using shor t - t ime austeni t izat ion t rea tments . Grange did not de- t e rmine the effect on fatigue life of carbide ref inement .

EXPERIMENTAL PROCEDURE

The samples used for heat t rea tment were taken f rom a length of spheroid ize-annealed hot extruded 52100 tubing such as is used to make bear ing r aces . The composition of the s teel is given in Table I. Sam- ples were cut from the in ter ior of the tubing wall, avoiding the sur faces . A standard sample s ize of 5• 8• 13 mm was used.

Samples heat t rea ted at elevated t empera tu res for t imes of 30 min and longer were enclosed in evacuated vycor capsules . Heat t rea tments of shor t e r duration were done in lead pots. If p ieces were i so thermal ly t r ans formed af ter austenit izing, the capsules were broken and the specimens immersed in molten salt , held for a fixed per iod of t ime, then water quenched. Capsules were also broken for oil quenching af ter austenit izing; an oil t empera tu re of 54~ (130~ was used and samples were in the oil for seve ra l minutes p r i o r to cooling to room tempera tu re .

Allowing specimens to remain in the quench oil r e - sults in some austenite s tabi l izat ion. It was found, however, that measurements of hardness and re ta ined austenite made af te r temper ing were more consistent if specimens were left 2 to 5 min in the quenchant. Specimens were s l ight ly harder (~0.SRc at the level of R c 63) and had less re ta ined austenite if left in the quench oil only 15 s, but r esu l t s were e r r a t i c .

Conventional metal lographic techniques were em- ployed. In spite of considerable effort, no re l iab le method of etching to reveal austenite grain s ize could be found, unless al l the carbon was in solution before quenching.

Chromium radiat ion was used for measur ing the amount of re ta ined austenite with the spec.imen enclosed in a he l ium-f i l led chamber to reduce intensity losses due to absorpt ion in a i r . By using only the (200) m a r - tensi te and (220) austeni te l ines, in ter ference from carbide l ines was avoided. An independent check (u l t r a - sonic measurements of Young's Modulus on samples cut pa ra l l e l to and perpendicular to the tube axis) showed no p r e f e r r e d orientat ion in the tubing.

RESULTS

The methods used here for carbide ref inement a re modifications of the methods repor ted by Faunce and Justusson e and Grange. B Specimens were heated to dissolve al l carb ides , quenched in sa l t for i so thermal

Table I. Composition of 52100 Tubing

C 1.04 V 0.005 Mn 0.32 W 0.003 S 0.009 Nb < 0.002 P 0.004 Co 0.005 Si 0.19 A1 0.03 Cr 1.35 Ti 0.002 Ni 0.12 Ca < 0.0002 Cu 0.12 Mo 0.01

t r ans format ion of the austenite, then cooled to room tempera tu re . It was hoped that rap id cooling by sal t quenching would minimize the format ion of carb ide f i lms in austenite gra in boundar ies . Complete t r a n s - format ion of the austenite to pea r l i t e or baini te avoids the cracking problems assoc ia ted with high carbon mar tens i t e s . P r e l i m i n a r y exper iments showed that af ter this p re t rea tment , conventional hardening t r e a t - ments produced uniform d i spe r s ions of ve ry fine c a r - b ides .

The sect ions to follow desc r ibe in detai l the exper i - ments done to es tabl ish the feas ib i l i ty of each step of this p rocess ing sequence.

Carbide Dissolution

The mic ros t ruc tu re of the a s - r e c e i v e d 52100 s teel is shown in Fig. 1. The volume f rac t ion of carbide is about 16 pct; an X- ray powder pat tern on carb ides ex- t r ac ted using a b romine-methanol solution shows only (Fe, Cr)zC. A chemical ana lys i s of the ext rac ted c a r - bides indicates the carb ides a r e 9 pct chromium by weight, confirming the r e su l t s of Glowacki e t a l . 7 A

s t ra ight forward calculat ion using the composit ion in Table I shows that e ssen t ia l ly a l l the chromium in the al loy is combined as carbide in the spheroidized condition.

Heating 30 min at 1040~ (1900~ d isso lves a l l the ca rb ides . The austeni te gra in s ize is ASTM 5-6. This t rea tment has been sa t i s fac to ry for carb ide dissolution in seve ra l heats of c om m e r c i a l l y produced 52100 tubing and wire with carbon contents as high as 1.10 pct.

Austenite Trans format ion

The ra te of i so thermal t ransformat ion of austenite is summar ized in Fig. 2. Dots on the d iagram mark the heat t rea tments used. Lines have been drawn marking 0, 50 and 100 pct t r ans fo rmat ion of austenite . At t ransformat ion t empera tu re of 480~ (900~ and above, proeutectoid carbide f i lms were observed to form in p r i o r austeni te gra in boundaries before t r a n s - formation of the austenite . However, no line is shown for the init iat ion of the proeutectoid carb ide fo rma-

Fig. 1 - - M i c r o s t r u c t u r e of a s - r e c e i v e d 52100 steel . Sphe ro id - ized ca rb ide s in a f e r r i t e ma t r i x . Magnif icat ion 1250 t imes . P i c r a l etch.

866-VOLUME 5, APRIL 1974 METALLURGICAL TRANSACTIONS

tion, because anneals of a few seconds could not be assumed to be " i so thermal" with the specimen size used.

Pearlite is the transformation product formed at temperatures of 620 to 700~ (1150 to 1300~ and bainite is formed from 370 to 590~ (700 to ll00~ Because it is difficult to distinguish pearlite and bainite in the light microscope when the carbide lamellae of pearlite are fine, products having relatively smooth interfaces with the austenite are called pearlite, Fig. 3, and those with jagged or acicular interfaces are called bainite, Fig. 4. This definition is consistent with the known growth habits of these products. 8'9

At temperatures of 480~ and above, the t ransformation product forms initially at pr ior austenite grain boundaries. Since films of proeutectoid carbide exist in these boundaries prior to transformation at these temperatures , the product is actually forming at a carbide/austenite interface. At 675 to 700~ there is a tendency for the initial pearlite colonies to be lo- cated at grain edges (triple points on a section), Fig.

5, and to grow rapidly, with only a few pearlite colonies in each austenite grain. A partially t ransformed specimen will consist of some austenite grains completely t ransformed to pearlite and others t ransformed not at all, Fig. 6.

At somewhat lower temperatures, pearlite or bainite nucleation is more rapid relative to its growth rate, and transformation product is formed along the austen- ire grain boundaries before any individual grain is con- sumed, Fig. 6(c).

J ' ' ' " ' " i ' ' ' " ' " I ' ' ' " ' " I ' ' , oo

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600 \ . . . . . . . . ,,oo "~

~- 550 'r I 0 0 0

�9 5 0 0 :E w 9 0 0 uJ I-- | . . . . . . . 1-

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0.1 I I0 I 0 0

T I M E , M I N U T E S

Fig. 2- - Isothermal Transformat ion diagram for 52100 steel , austenit ized 30 rain at 1040~ ASTM grain s ize 5-6. Points shown the t ransformat ion t rea tments used. Lines are for 0, 50 and 100 pct t ransformat ion of austenite.

(a)

Fig. 3--After carbide dissolution at 1040~ salt quenched to 650~ held 30 s, then quenched to room tempera ture . P e a r l - ite colonies consuming an austenite grain. Note carbides in grain boundaries. Magnification 1250 t imes . Nital etch.

(b)

Fig. 4--After carbide dissolution at 1040~ (a) salt quenched to 595~ held 30 s, then quenched to room tempera ture , (b) salt quenched to 540~ held 5 min, then quenched to room tempera ture . Bainite consuming austenite grains. Magnifica- tion 1000 t imes. Nital etch.


Fig. 5 - -Af te r ca rb ide d i s so lu t ion at 1040"C, sa l t quenched to 705"C, held one min, then quenched to r o o m t e m p e r a t u r e . Note ca rb ide f i lms in g r a in b o u n d a r i e s and fo rma t ion of pea r l i t e at t r i p l e po in ts and along g ra in boundar i e s . Mag- ni f icat ion 1250 t imes . Nital e tch.

Finally, at still lower temperatures (370 to 480~ transformation to bainite occurs in bands, following the pattern of alloy segregation in the material, Fig. 6(d). Banded structures are apparently due to differences in nucleation rate rather than growth rate, because the banding is evident as soon as any t ransformation is visible. Microprobe analyses show that transformation occurs f i rs t in bands low in alloy content. It is not clear whether or not transformation begins at austenite grain boundaries within the bands.

An unusual feature in Fig. 2 is the sluggish t ransformation occurr ing at 480~ (900~ Bainite forms both above and below this temperature, so the sluggish transformation is unrelated to the transition from pearlite to bainite. There is a correlation, however, with the initial distribution of the bainite; above 480~ transformation begins generally at all austenite grain boundaries, Fig. 6(c), while at 480~ and below, banded microst ructures occur, Fig. 6(d).

Networks of carbide in prior austenite grain bounda- r ies are undesirable. Films of proeutectoid grain boundary carbide do not form at transformation temperatures of 425~ and below. However, if a carbide boundary film is present, it may dissolve and/or spheroidize during subsequent austenitization. It will be shown in the next section that no difficulty was experienced in eliminating carbide networks during austenitization in this material. However, one heat of commercial ly produced 52100 steel* has been encoun-

*This heat had 1.10 pct carbon and was high in residual impurities such as Ni, Cu and Mo.

tered in which the carbide films could not be broken up during subsequent heat treatments of the kind cus- tomari ly used to harden 52100 steel. In this material, the only way to avoid a carbide network in the final product was to t ransform at 425~ or below.

Austenittzation for Hardening

In order to compare the effect of initial micros t ructure on hardening response, specimens were prepared with the pretreatments shown in Table II. A tempering

treatment of one hour at 175~ (350~ was used in the experiments described in this section.

Fig. 7 shows that for specimens in condition A (spheroidized carbides) increasing austenitizing time at 840~ (1550~ results in increasing hardness and fraction of retained austenite and decreasing volume fraction of excess carbides. From the volume fraction of carbide remaining after 8 h, one calculates that the carbon content of the austenite is 0.55 pct.

Carbides were extracted from samples austenitized from 8 min to 32 h at 840~ and analyzed for chromium, Table III. Since no change in carbide composition is observed for t imes through 30 min, the principal reaction is simply dissolution of existing carbides. The slight decrease in chromium content at longer times may be due to growth of lower chromium M3C carbides at the expense of higher chromium MaC carbides.

The microstructural appearance of the specimens austenitized at 840~ indicates that the process of ca r - bide dissolution is more complex than the regular behavior shown in Fig. 7 would suggest. Specimens austenitized from 8 min to two h have a mottled appearance, most noticeable at magnification of about 200 diam, Fig. 8(a). An examination at higher magnification shows that the undissolved carbides are no longer uniformly distributed. The light-etching material is in a pattern which suggests that it is adjacent to prior austenite grain boundaries, although no grain bounda- r ies can be revealed by etching.

If carbides are dissolving f irs t near grain boundaries, then the austenite in these regions should be enriched in chromium and carbon. This enriched austenite should have a lower martensite start temperature (Ms) than the austenite immediately adjacent to the remaining carbides. To test this, specimens* were austenitized

*For this experiment, the specimen size was 8 • 5 • 3 mm.

for 30 rain at 870~ quenched in salt at temperatures from 163 to 218~ held 15 s, tempered two rain at

Table II. Pretreatments for Microstructural Control

Condition Treatment Microstructure

A As received Spheroidized Carbides in Ferrite Matrix

B Peadite with a thin carbide fdm in Prior Austenite Grain Boundaries

C Bainite. No continuous grain boundary carbide

30min 1040~ salt quench to 650~ hold 30 min, then air cool 30 min 1040~ salt quench to 425~ hold one h, then air cool

Table III. Chromium Content of Extracted Carbides

Austenitization Wt Pct Cr in Initial Condition* Treatment Extracted Carbides

A None 9 A 840~ 9 A 840~ 9 A 840~ h 8 A 840~ h 8 A 840~ h 7 A 900~ 7 B 840~ h 7 B 840~ 5.5

*See Table II.


(a) (b)

(c) (d)

Fig. 6-(a) After 30 min at 1040"C, quenehed to 735~C to allow carbide precipitation on austenite grain boundaries, then quenched to room temperature. (b) Same heat treatment as Fig. 5. (c) Transformed two min at 540"C. (d) Transformed four min at 480"C. Magnification 200 times. The austenite grain size is the same in all four treatments.

218~ then quenched to room t e m p e r a t u r e . Fig. 9 shows that m a r t e n s i t e does indeed fo rm f i r s t in the aus ten i t e which is ad jacent to the undisso lved c l u s - t e r e d ca rb ides .

F ig . 10 shows the effect of aus ten i t i z ing t ime at 840~ on the ha rdnes s and f rac t ion of r e t a ined a u s t e n - ire of spec imens in condi t ions B and C. Peak h a r d - ne s s in these spec imens is achieved af ter f i f teen s e c - onds aus ten i t i z ing t ime . There is a v e r y sl ight de- c r e a s e in ha rdnes s at longer t imes . The f rac t ion of r e t a ined aus ten i te a lso achieves i ts max imum value in a few seconds , and r e m a i n s at e s s en t i a l l y the same value for aus ten i t i z ing t imes up to 32 h.

Up to two min austenitizing time at 840~ B and C specimens have fine, uniformly dispersed carbides, Fig. 11(a). With increasing annealing time, carbide growth occurs, Figs. 11(b) through 11(d). The micro- structures of B and C specimens are similar, except for a faint banding in the C specimens related to banding of the bainite prior to austenitization. No grain boundary carbide films, or orderly sheets of spheroidized carbides left over from such films can be seen in B specimens even for austenitizing times as low as 15 s at 840~

The ch romium content of ca rb ides ex t rac ted f rom B spec imens held 8 and 32 h at 840~ is given in Table


III. Most of the ca rb ides ex t r ac t ed a r e probably the c o a r s e r ca rb ides shown in F igs . l l ( c ) and l l ( d ) . X- r ay powder pa t t e rn s indicate that only MsC c a r b i d e s

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5 9

5~ I I I I I I I I I I J l l l l l l I I I I I I I I I f I I I I I I I J I I l l l l

0.1 1.0 IO I O 0 IOO0 I 0 , 0 0 0

AUSTENITIZING TIME AT 8 4 0 = r ,MINUTES

Fig. 7--Hardness (H) pot retained austenite (RA) and vol pet undissolved carbide (C) as a function of time at 840~ for specimens in condition A (spheroidal carbides). Temper: one h at 175~

"d ~ t a

07.

~> ~ 0

I 0 ~ m uJ~ z z

~ . J

0

(a)

(b)

Fig. 8--Condition A, austenitized 30 min at 840~ quenched, tempered at 175~ Nital etch. Magnification (a) 190 times, (b) 1250 times.

a r e p r e s e n t . While the r ep roduc ib i l i t y of ca rb ide ana lyses f r o m B s p e c i m e n s was not as s a t i s f a c t o r y as the ana lyses of A spec imens , s t i l l i t appea r s that the c o a r s e c a r b i d e s which grow at the expense of the u l t r a - f i ne ca rb ides a r e lower in c h r o m i u m than e i the r the u l t r a - f i ne ca rb ides , o r the sphe ro ida l c a rb ide s in A s p e c i m e n s .

The r e su l t s may be c o m p a r e d with those of Glowacki e t a l . 7 These au thors concluded that c a r b i d e s in 52100 which f o r m in a f e r r i t i c m a t r i x conta in m o r e c h r o m i - um. than those which f o r m in an aus ten i t i c m a t r i x . They found that 1) sphero ida l c a r b i d e s contain 9 pct Cr , 2) the ca rb ides in p e a r l i t e contain 7 pct Cr (a r e - sult obtained indi rec t ly) , and 3) c a r b i d e s which f o r m in aus ten i te at 840~ f r o m a s u p e r s a t u r a t e d sol id so lu - t ion contain 2 pct Cr . Such l a r g e d i f f e r e n c e s in ca rb ide c h r o m i u m content w e r e not o b s e r v e d in the c u r r e n t work, but t he r e is qua l i t a t ive a g r e e m e n t that the MsC ca rb ide in equ i l ib r ium with aus ten i t e at 840~ has a l ower c h r o m i u m content than the MsC ca rb ides p r o - duced by a sphe ro id ize anneal .

Fig. 9--Specimen with spheroidal carbides (condition A) austenitized for 30 min at 870~ quenched in salt at 190~ and held 15 s, then tempered in salt for 2 rain at 218~ and quenched to room temperature. Nita] etch. Magnification 1250 times.

' ' ...... [ . . . . . . . . I . . . . . . . . I . . . . . . . . 1 . . . . . . . .

6 5 S

.u 64

63 2 0 ~

RA | �9

62 e �9 �9 15 Z

Lt~

~ 61 O ~ LU Z

I I I I ~ 59 ] I II1111 I [ I l l l l l I I i i I i i i I I I I I I I I I I I l l l 0 O. I I I0 I 0 0 I 0 0 0 IO,O00

A U S T E N I T I Z l N G T IME AT S 4 0 ~ M I N U T E S

Fig. 10--Hardness (H) and pct retained austenite (RA) as a function of austenitizing time at 840~ for specimens in conditions B and C (pearlite or bainite). There is no systematic difference in the behavior of B and C specimens. Temper: One h at 175~


(a) (b)

(c) (d) Fig. ll--Specimens with a pearli t ie microstrueture (eonditionB) austenitized for various times at 840~ oil quenched and tempered at 205~ Pieral etch. Magnification 1500 times (a) 2 rain, (b) 8 min, (c) 30 min, (d) 2 h.

The effect of aus t en i t i z ing t e m p e r a t u r e on h a r d n e s s , f r a c t i o n of r e t a i n e d aus ten i t e , and vo lume f r ac t i on of c a r b i d e is shown in Fig . 12 fo r s p e c i m e n s in condi t ion A a u s t e n i t t z e d fo r 30 min. The peak h a r d n e s s is a ch i eved at aus t en i t i z ing t e m p e r a t u r e s of 870 to 900~ The amount of r e t a i n e d aus t en t t e i n c r e a s e s with in- c r e a s i n g t e m p e r a t u r e . The r e l a t i o n be tween t e m p e r a - t u r e and amount of r e t a i n e d aus t en i t e is a p p r o x i m a t e l y l i n e a r f r o m 840~ to 980~ M i c r o s t r u c t u r a l e x a m i n a - t ion shows p r o g r e s s i v e d i s so lu t ion of c a r b i d e s with in - c r e a s i n g t e m p e r a t u r e . Only a few s c a t t e r e d und i s so lved c a r b i d e s a r e seen a f t e r 30 min at 980~ C a r b i d e s e x - t r a c t e d a f t e r 30 min at 900~ were found to conta in 7 pc t c h r o m i u m . Appa ren t l y some nuc lea t ion and growth

of low Cr c a r b i d e s a c c o m p a n i e s the g e n e r a l p r o c e s s of c a r b i d e d i s so lu t ion a t h ighe r aus t en i t i z ing t e m p e r a - t u r e s .

F ig . 13 shows the ef fec t of aus t en i t i z ing t e m p e r a t u r e on h a r d n e s s and f r a c t i o n of r e t a i n e d aus t en i t e for B and C s p e c i m e n s . Note that the h a r d n e s s is l e s s d e - pendent on aus t en i t i z ing t e m p e r a t u r e than for A s p e c i - mens . The f r a c t i o n of r e t a i n e d aus t en i t e v a r i e s l i n - e a r l y with the aus t en l t i z i ng t e m p e r a t u r e to about 950~ The s lope of the l a t t e r cu rve i s the s a m e a s the l i n e a r po r t i on of the ana logous c u r v e in F ig . 12. M i c r o s t r u c - ru ra l examina t ion shows m a i n l y u l t r a - f i n e c a r b i d e s at t e m p e r a t u r e s of 790 to 840~ m i x t u r e s of c o a r s e and fine c a r b i d e s at 870 to 900~


66

6 6 -

6 4 -

6 2 -

6 0 -

58 I 750

TEMPERATURE, ~

1400 1500 1600 1700 1800 I I I I I

0

I I I 1 ~

4O

30 z

20 z

- 1 0

I o

h J r~

>, o

.J

t J

800 850 900 950 I000 TEMPERATURE,~

Fig. 12--Hardness (H), pct re ta ined austenite (RA) and vol pct undissolved carbide (C) as a function of austenitizing t e m p e r - ature for specimens in condition A. Austenitizing t ime is 30 min; one h at 175~ temper ,

A U

. J _1 tO

64 U o

m 62 i.al z r ~ r r

58

TE M PE RATURE, ~

1400 1500 1600 1700 1800 I I I 1 I

r

. J , J

v U o n,-

(/3 o~ t~J z 0

- r

I I l I I I

4O

3o

20 ~ Z

IO ~

0 750 800 850 900 950 1000

TE MPERATURE,~

Fig. 13--Hardness (H) and pct re ta ined aasteni te (RA) as a function of austenit izing t empera tu re for spec imens in condition B and C. 30 rain austenit izing t ime; 175~ t emper for one h.

Tempering

The tempering response of samples in the A, B, and C condition, austenitized 30 min at 840~ quenched in oil and tempered for one h, is shown in Fig. 14. Also shown is data for A specimens (designated A1) austenitized for 8 h at 840~

The data for the B and C specimens is essentially the same. Although A1 specimens have the same hardness at low tempering temperatures as B and C specimens, they are significantly softer for tempers above 205~ (400~ For any pretreatment and austenitizing treatment, the amount of retained austenite is about the same with tempers of 150~ and 175~ decreases progress ively with tempers of 200 and 230~ and is close to zero at tempers of 260~ and higher.

Fig. 13 shows that for B and C with a 175~ temper, the hardness is nearly constant for austenitizing temperatures f rom 815 to 900~ although the amount of retained austenite increases with the austenitizing

200

651-

D E G R E E S F A H R E N H E I T 4 0 0 6 0 0 8 0 0 I 0 0 0

i I i I I I I

60

a. J

,., 55 -=2o z

< 5

~ - ~

< 0 Q

Z

45- , . ,

0

4 0 ~ I00 200 3 0 0 4 0 0 500 600

T E M P E R I N G T E M P E R A T U R E ,=C

Fig. 14--Hardness and pet re ta ined austeni te as a function of t emper ing tempera ture for one h t emper ing t ime. A, B and C r e f e r to initial conditions descr ibed in Table II; these spec i - mens were austenit ized for 30 rain at 840~ A1 is mater ia l in condition A austenit ized 8 h at 840~

temperature. The same behavior holds for all tempers in the range 150 to 230~ Thus, for B and C specimens, within the limits of austenitizing temperature cited above, the hardness is for all practical purposes a function only of the tempering treatment. This is not true for A specimens; in these specimens both the austenitizing treatment and the tempering t rea t - ment influence the final hardness, Fig. 15.

DISCUSSION

The kinetics of austenite t ransformation in 52100 steel, as summarized by Fig. 2 and similar diagrams in the li terature, 1~ is well known without being well un- derstood. When the austenitizing temperature is high enough to dissolve all carbides, the initial reaction for temperatures of 480~ and above, is formation of proeutectoid carbide in austenite grain boundaries. At transformation temperatures of 540~ and above, the carbide films are continuous in grain boundaries; at 425~ and below, no proeutectoid carbides can be seen.

Pearli te and bainite c lear ly form initially at carbide/ austenite interfaces at 540~ and above. Pearli te is found at 620~ and above, and bainite forms at lower temperatures . At transformation temperatures of 480~ and below, the kinetics of t ransformation is dominated by the influence of alloy content on nucleation, and banded micros t ructures appear, due to alloy segregation. The size of the bainite laths within the bands seems to be limited by the austenite grain boundaries, but the details of the initial reaction are not


TEMPERING TEMPERATURE ,*F

300 350 400 450 I I I I

67 i

66

u 65 , . . I , , - I "' 64

u 63 B,C o 815-900"C ~: 62 900"C cn ~ 870"C

c~ 840"C o: 60 A <t 815"C "" 59

58 790"C

57 1 I I I 1 150 175 200 225 250

TEMPERING TEMPERATURE , *C

Fig . 1 5 - - H a r d n e s s a s a func t ion of t e m p e r i n g t e m p e r a t u r e f o r v a r i o u s a u s t e n i t i z i n g t e m p e r a t u r e s and s p e c i m e n s in in i t i a l c o n d i t i o n s A, B and C (Tab l e II). 30 m i n a u s t e n i t i z i n g t i m e ; one h at t h e t e m p e r i n g t e m p e r a t u r e .

clear . It is not known ff the bainite reaction begins at an austenite grain boundary.

If the temperature and time of austenitization are not sufficient to dissolve all carbides, then the s tructures produced by isothermal transformation of austenite will be different. Specimens austenltized at 840~ for 16 h (a time sufficient to remove local carbon gradients) and transformed at 700~ have no apparent proeutectoid reaction. Austenite grain boundaries are not decorated by any precipitate and pearlite forms in a manner similar to Fig. 6(b). Specimens austenitized only 30 min at 840~ on the other hand, t ransform to a very complex structure which includes pearlite and proeutectoid ferr i te .

Bearing components can be produced with dispersions of ultra-fine carbides beginning with either pearlite or bainite. Austenite t ransforms most rapidly to pearlite, but t ransformation is preceded by the formation of continuous films of proeutectoid carbide in austenite grain boundaries. In this heat, these films redissolved during subsequent austenitization and posed no problem. In other commercial ly produced heats, this is not the case; if the carbon content is at the high side of the specification and the level of residual carbide- forming elements such as Mo is high, grain boundary carbide films can persis t through subsequent austenitization.

A bainitic microst ructure formed at about 425~ will not have carbide films in pr ior austenite grain bounda- r ies . A disadvantage to using bainite is its sluggish rate of formation. A transformation treatment of one h at 425~ was commonly used in these experiments to form bainite. A few small regions were encountered (in over 100 samples given this treatment) in which the austenite had not completely t ransformed and martensite formed on cooling from 425~ Several of these untransformed regions cracked. This could not be tol- erated in a commercial application. More prolonged heating at 425~ or a temper after cooling to room temperature would be necessary to avoid any danger of cracking.

The behavior of pearlitic or bainitic microst ructures during reaustenitization is quite different from the behavior of spheroidized microst ructures . In the latter, at temperatures near 840~ the spheroidal carbides, rich in chromium, gradually dissolve. They do not dissolve uniformly. It appears that those carbides near austenite grain boundaries tend to dissolve first,* en-

*The observed approach to equilibrium at 840~ for spheroidal carbide specimens is too slow to be accounted for by carbon diffusion-limited dissolution of individual carbides, and too fast to be accounted for .by chromium-diffusion limited dissolution. (Diffusion constants were used from Ref. 12). Since the carbides are rich in Chromium, partial dissolution must cause a steep Chromium grad- ient to be established near each carbide, acting to inhibit carbon transport and further dissolution. Austenite grain boundaries, by providing more rapid chromium and carbon transport, may help destroy gradients at adjacent particles. Since the boundaries themselves are mobile, they may play an important role in the dissolution of all carbides.

riching the regions about grain boundaries in carbon and chromium, and locally depressing the Ms t emper- ature. After prolonged heating at constant temperature the gradients of dissolved carbon in the microstructure diminish and the etched microst ructure appears uniform. The mottled appearing microstructure, which is the result of local variations in composition (and Ms) occurs commonly in commercial ly produced bearing components.

When specimens with pearlitic or bainitic micro- s tructures are austenitized, the sequence of events is different. No inhomogeneities associated with austenite grain boundaries are evident. Judging from measurements of hardness and fraction of retained austenite, the austenite becomes saturated in carbon in less than 15 s at 840~ Beginning at about 8 min, carbide growth becomes evident. The original ultra-flne ca r - bides derived from the pearlite or bainite do not coarsen uniformly. New carbides, lower in chromium, may nucleate and grow at the expense of the small c a r - bides. Thus, the shorter the austenitizing time, the more homogeneous is the microst ructure .

Hardened specimens with ultra-fine carbides tend to be harder and to have more retained austenite than hardened specimens with coarse carbides (compare Figs. 7 and 10). Most of the difference can be at tr ib- uted to more rapid carbon saturation of the austenite with fine carbides. However, the amount of retained austenite in B and C specimens austenitized for 8 to 30 min at 840~ was greater than that found in A specimens after 32 h at the same temperature. It is known that the Ms temperature is depressed (thus the amount of retained austenite is increased) as the strength of the austenite increases. 11 It may be that the ultra-fine carbides strengthen the austenite sufficiently to increase further the volume fraction of retained austenite. It is clear from the decrease in hardness with long austenitizing times in Fig. 10, that the fine ca r - bides do contribute to the strength, even at hardness levels of Rc 63 and above.

At tempering temperatures above 200~ the samples with fine carbides are significantly harder than samples with coarse carbides. This is undoubtedly due to en- hanced dispersion strengthening of the tempered mar - tensite by the ultra-fine carbides. Thus, a bearing component with a hardness of more than 61 Rc and no retained austenite can be produced by tempering an ultra-flne carbide microst ructure at 260~

Finally, it has been shown that when pearlitic or


bainitic micros t ructures are austenitized, quenched and tempered, the resulting hardness is quite insensi- tive to austenitizing treatment. While the amount of retained austenite varies with the austenitizing temperature (but not austenitizing time) and tempering temperature, it can be controlled within limits independent of the hardness. For example, referr ing to Fig. 13, samples can be prepared with a hardness of 63.5 to 64 Rc and retained austenite contents of 10 or 25 pct.

The results of these experiments show that there is sufficient flexibility in the process to make carbide r e - fining heat treatments practical. A pearlitic microstructure formed isothermally at about 650~ is the prefer red intermediate structure. To avoid problems with heavy intergranular carbide films forming prior to pearlite, the material used should be from a sup- plier capable of maintaining low levels of residual al- loying elements in his steel.

Since there is no benefit to prolonged austenitization for hardening, induction heating may be used. On the other hand, the coarsening rate of ultra-fine carbides is slow enough so that conventional austenitizing furnaces can also be used. In furnaces, the austenitizing time can be shortened to allow just enough time for the parts to reach the maximum desired temperature.

It is evident from the present work that the microstructure which is developed by conventional heat t rea t - ments of commercial ly produced steel is complex, quite variable (depending strongly on the result of the spheroidizing anneal), and is usually quite inhomogeneous. In contrast, if carbide refining heat treatments are em- ployed pr ior to hardening, the final microst ructure is independent of the spheroidized structure, and is very homogeneous.

The more homogeneous microstructure is expected to have better fatigue propert ies; the magnitude of the improvement has yet to be determined. The prime ad- vantage to carbide refinement, however, may lie in the much superior microst ructural control it is possible to achieve in heat treatment, and the greater resulting uniformity of the bearing components produced.

SUMMARY

1) A study has been made of the heat treatment needed to produce hardened bearing components of 52100 steel with uniform dispersions of ultra-fine carbides. Such microst ructures are produced by first converting a microstructure of spheroidal carbides in ferr i te to pearlite or bainite, then austenitizing, quenching and tempering in a conventional hardening heat treatment.

2) After dissolution of all carbides, pearlite forms rapidly by isothermal transformation at 625 to 650~ while thin proeutectoid carbide films are present in austenite grain boundaries, these redissolve during subsequent austenitization.

3) If the 52100 steel used has comparatively high levels of residual carbide-forming elements, difficulty may be experienced in redissolving carbide films. Then it is preferable to begin with bainite because isothermal transformation to bainite at 425~ suppresses formation of carbide films. The t ransformation is more sluggish, however, than the pearlite transformation.

4) Austenitizing pearlite or bainite produces carbon saturation of the austenite in a few seconds. Thus, the amount of austenite retained on quenching is more con- trollable, since it is not, for practical purposes, a function of austenitizing time.

5) As the austenitizing time is extended, ultra-fine carbides coarsen. While there is no benefit to austenitizing times longer than 15 s at 840~ times up to 30 min produce little carbide coarsening so little degrada- tion in propert ies is expected.

6) Ultra-fine carbides increase the hardness for most combinations of austenitizing and tempering temperature.

7) For a given hardening treat treatment, the level of retained austenite is higher in specimens with ul tra- fine carbides. Most of this effect is due to rapid ca r - bon saturation of austenite.

8) When a microst ructure of spheroidal carbides in ferr i te is austenitized, carbides do not dissolve uniformly. Heat treatments typical of commercia l p rac- tice result in mottled micros t ructures , with clusters of undissolved carbides in a relatively low carbon ma- tr ix surrounded by material with a higher carbon content and fewer undissolved carbides.

ACKNOWLEDGMENTS

Retained austenite measurements were made by Carol J. Kelley, carbide extractions and analyses were done by J. L. Bell, and metallography and heat treatment were done by E. Quick and R. L. Martin, respec- tively, all of the Ford Scientific Research Staff.

RE FERENCES

1. T. E. Tallian: J. Lubrfc Technol. (Trans. ASME), 1967, vol. 89, pp. 73-74. 2. R. L. Faunce and W. M. Justusson: U.S. Patent No. 3,595,711, July 27, 1971. 3. J. H. N. Wheeler: Private Communication, T. I. Steel Tubes (U.S.A.) Inc.,

Larchmont, N. Y. 4. K. Moama, R. Maruta, T. Yamamoto, and Y. Wakikado: Jap. Inst. Metals J.,

1968, vol. 32, pp. 1198-204. 5. R. A. Grange: Met. Trans., 1971,vol. 2, pp. 65-78. 6. R. A. Grange: U.S. Patent No. 3,337,376, August 22, 1967. 7. Z. Glowacki and A. Barbacki: J. Iron SteelInst., 1972, vol. 210, p. 724. 8. R. F. Hehemann: Phase Transformations, pp. 397-432, American Society for

Metals, Metals Park, Ohio, 1970. 9. D. Tumbull and K. N. Tu: Phase 7~ans[ormations, pp. 487-95, American

Society for Metals, Metals Park, Ohio, 1970. 10. Atlas o f Isothermal Transformation Diagrams, pp. 65,133, U.S. Steel Corp.,

1951. 1 I. E. M. Breinan and G. S. Ansell: Met. Trans., 1970, vol. 1, pp. 1513-20. 12. J. F. Elliott, M. Gleiser, and V. Ramakrishna: Thermoehemistry ,for Steel-

making, vol. II, pp. 689-96, Addison.Wesley Publ. Co., Inc. Reading, Mass., 1963.

8 7 4 - V O L U M E 5, APRIL 1974 METALLURGICAL TRANSACTIONS

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Bearing Steel 52100