The Bainite in Low Carbon Low Alloy High Strength S teels*
By Yasuya OH M ORI,** Hiroo OHTANI,** and Tatsuro KUNITAKE**
Syn op sis m orphology and the formati on m echanism of the T he mor/lhology qf the bainite in some low carbon low 0110)' high bainite in such low carbon low a lloy high streng th strength sleels has been investigated by means oj dilalomell)' and both olltica l steels. and electron microscollies. The results indica Ie lila t Ihe bainite can be devided into three distinct Iylles k)' the lIlor/lilolog)' of Ihe cemenlite /Irecillita II. Exp erimental Procedure lion. The /Jainile f is Jormed al Ihe lem/lCra tures abol'e SUO C alld is the 1. .I[aleria/s carbide-Jree ha initic ferrile. The bainite f f is formed in Ihe intermediale The chemi cal compos ition of the steel s used are !' lem/lerature range or b), coolin/!, at intermediale cooling ra te. A lthongh shown in T abl e I . The steel 7 is a typical of an some amount oj cementile Ilarticles areJonned wilhin Ihe grains, the bainite 2 !I consisted mainly oj Jerrile laths with cementile la)'ers between them, 80 kg /mm quenched and tempered type high streng th being a i)'/Iical oj u/I/)er bainite. The bainite 111 is Jormed at the temliCr steel containi ng 0. 12% carbon, a nd was melted in a atures close 10 the Jl1s or during cooling at as Jasl as the u/'/ler crilical I t high frequency induction furnace, being roll ed into cooling ra te, Ihe morl)hology oj cementile Jormation is similar in a/l/Jeaf{/Ilce the pla tes 25 mm thi ck. In order to simplify the to Ihe high carbon lower hainite. !-Iowever, these bainites had the lalh-like carbide obse rvati on, the a mount of carbon in the steels Jerrite lIlor/lllOlog_y exhibiting Ihe same ( III ). [ II O}. habit. This is ;) a nd 6 was increased to 0.22%. They were melted in di.f!erentJrom the high carbon lower bainite wilh plale-likeJerrile. There .. a 100 kg high fr equency inducti on furnace simulta ne Jore, it seems reasonable 10 classify Ihe bainites observed in low carbon ously a nd divid ed into two 50 kg ingots, sma ll amount low alloy high strength sleels as the u/)/)er bainile. of vanadium being added to one of them . T hey were forged into the shap e of bars 12 mm square in dia 1 I. Introduction m eter. Sincc Davenport and Ba in2 ) found the baini te, the bainite reacti ons in high a nd m edium carbon low a ll oy 2. e.e. T. Diagrams steels have been the subjects of numerous investiga The d ecompos ition or the austenite during con tions, and it has been es tab lished tha t bainite can be tinuo us cooling was examined by means of dilatometry divided into two distinct typ es by the formation tem using a Formaster dila tometer. The specimens for perature. They are call ed upper and lower ba inites these measurements were machined into the shape or and the temperature se parating both structures is be 3 mm in diameter a nd 10 mm in leng th. They were li eved to be a bout 350 C .1 ),2) The upper ba inite com a ustenitized at I 200°C for 5 min by the indueti on prises the bunc hes of the carbide-free ferrite la ths w ith heating a nd the cooling process was con troll ed either the cementite layers between them .2 ) - 4 ) Whe reas the using gases or electri cally. The tempera ture/time a nd .. lower bainite has the pl ate-like m orphology a nd in tem pera ture/dila tatio n curves were recorded simul volves fin e cem entite platelets on a specifi c ferrite ta neously and the C. C. T . curves \\'ere d etermi ned rrom pla ne.2 ) - 6) these resul ts. The cooling ra te d esrirhed in the foli o\-\" H owever, in low carbon low a ll oy hi gh stl-ength ing cha pters refers to the avarage value between 800° steels whose tv1 s temperatures a re above 350°C, the and 500°C. existence of lower bainite was reported. i) ,8) a nd the temperature separa ting both bainites was prop osed to 3. T. T. T. D iagrams rise to much higher than 350°C. 7) This indicates tha t For the exami nation of the iso thr f' mal d ecomposi there exist som e differences between plain carbon tion of a ustenite, the specimens of I X 5 X 10 mm3 were bainite and the low carbon low a ll oy bainite. heated in a n argo n a tmosphere for :5 min a t I 200°C The aim of the present investigation is to revea l the a nd q uenched into the lead ba th kept a t various tem-
Table I. T he chemical compositions of the steels used (\Vt 9 ~)
Steel C Si M n P S C u Ni Cr Mo V Sol. AI
7 0.1 2 0 .30 0 . 83 0 .004 0 . 005 0.30 1. 11 0 .53 0 .49 0.03 0.03 1 ~ 5 0 .22 0.24 0.83 0.007 0. 011 0 .30 1.06 0.54 0.51 0 .029 • 6 0.22 0 . 24 0 .85 0.008 0.012 0.30 1.05 0.54 0.51 0. 02 0.024
* Presented at the 80th ISI.I M eeting, O ctober, 1970. in A magasaki. l\[a nllscript rece ived D ecembe r 2, 1970. ** Centra l R('s('arch Laboratories, S ll m itomo l\[etal Jnci ll stri es, Ltd., Nishin agaslI-hondori, AlIl agasaki 660.
( 250 ) Research Article Transactions ISIJ, Vol. 11, 1971 ( 251 )
peratures between 3800 and 800°C. The partially means of dilatometry. transformed specimens were then quenched into water In the steel 5 containing 0.22% carbon but no to suppress further isothermal decomposition by the vanadium, the primary ferrite nucleation was not be martensite transformation of the untransformed aus observed at the cooling rate as small as 15°C /min. tenite. The etched specimens were observed optically The starting temperature for the bainite reaction d e and the TTT diagrams were determined by means creased with increasing the cooling rate, but the tem of the lineal analysis for the fraction transformed. peratures where the bainite formed were below 600°C as in the ease of the steel 7. The transition fmm baini te 4. Optical Microscopy to martensite could not be separated a nd the M s tem The optical microscopic observation for these speci perature was about 400°C (Fig. 2) . .. mens was carried out after etching with 2% nitral. In On the other hand, in the steel 6 containing both order to confirm the r eal shape of the acicular struc 0.22% carbon and 0.02 % vanadium, the primary tures, some of the specimens were observed on two ad ferrite nucleation was observed el early during cooling jacent surfaces inclined at about 90° to one another. at 15°C /min (Fig. 3) and the starting temperature of
" the bainite formation did not vary with the cooling 5. Electron Microscopy rate, being a lmost constant at about 600°C. There '-I Both cellulose acetate/carbon two stage rcplicas and fore, it ean be seen that the small vanadium addition thin foils were used for electron micmseopy. For the accelerated the ferrite formation. The critical cooling thin foil preparation, the bulk specimens of I mm thick rate for the martensite formation in this case was was polished down to 0.1 mm chemically. The final slightly smaller than that in the steel 7 but the M tem electropolishing was carried out in the chromic acid / perature was exactl y same as that in the steel 5. phosphoric acid electrolyte. The replicas and the Summarizing these res ul ts, it can be seen that the foils were examined in the JEM-5Y operating at increase of carbon content from 0.12 to 0.22 % re 100 kV and the HU-200 operating at 200 kV respec duced both the Ms temperature from 440° to 400°C ). . tively. and the critical cooling ra te for the martensite trans formation, but the small vanadium addition accelerat- 6. Hardness '\[easuremenls The hardncss of the mi crostructures was measured by a Vickers microhardness tester using a suitable load 900 Auste nitized at 1200"C between 50 and I 000 g . ...
III. Exp erim ental R esults :; 600 ~ 1. C. C. T. D iagrams ~ 15 "C / mi n Figures I , 2, and 3 show the C.C.T. diagrams for the steels 7, 5, a nd 6 respectively. J n the steel 7 con J::: - -~~ ~-\~~- taining 0.12 % carbon, the primary fe rri te formed d ur r ~ l a rt ens it e 300 (2' 9 1'",.,,, ing cooling at 15°C /min but the bainite reaction oc "-.: hardnt'ss curred at the temperatures below about 600°C during 200 A, 'Iu.'nrhf'd faster cooling (Fig. I). The Ms temperature in this case is about 440°C. At the lower temperatures the 4 6 810' Coolin g tim e fr om A C3 sec reaction might change from bainite to martensite transformation, but this could not be d etected by Fig. 2. T h e C.C,T, diagram for thc Cu- Ni- Cr'- M o s tee l containing 0.22 °1) carbon
900 900 Austenitized at 1200"C AC3 826 "C Austeniti zed at 1200 "C l 800 --
; ,.
~ ~ 500 15"C / min "c. ~400 ----- r 300 - @V iders hardness
'>- > h 2H 210 196 119 I 00 1~~--:--~~-=CO-----"4 '"""68J02 --"----'4 "'6 -;:8';';\ 0"3---;:-- 4:---C6;:---;;-!8 10' Cooling tim e from AC 3 sec
rig. I. The C.C.T. d iagram 1,) 1' the Cu- Ni- Cr- Mo V Fig. 3. The C,C.T, diagram for th e C lI -Ni -Cr- ~ r o -v steel containing 0. 12°" carbon s teel containing 0,22 ~" carbon
Research Article [ 252 ) Transactions ISIJ, Vol. 11, 1971 ed the primary ferrite nucleation and kept the starting 2. T. T. T. D iagrams temperature of the bainite reaction constant at about The T.T.T. diagrams which were d etermined by 600°C. means of optical microscop y are shown in Figs. 5, 6, Fig ure 4 shows an example of the dilatation curves of and 7. In these cases, the primary ferrite nucleated the steel 6 which vari ed with the cooling rate. On in the tcmperature range between 700° and 600°C, cooling at 15°C /min, the change of the dilatation can and the bainite formation occurred in the range be be separated into two distinct reacti ons, i.e., the pri tween 600°C and M s. I t should, however, be pointed mary Ferri te a nd the baini te formations as d escribed out that the bainite transformation did not complete previously, whereas cooling at 150°C /min revealed at the temperatures above about 500°C. Comparing the two stage dilatational change of the bainite rea c these three figures, it can be seen that reducing the tion, i. e., the gradua l expansion at the beginning of carbon content decreased the time required to com transformation and the subsequent catastrophic forma plete the bainite reaction, but there is no significant tion of the bainite at about 490°C. The former was difference other than this. confirmed to be the carbide-free bainitic ferrite forma Figure 8 shows the r elationship between the frac tion by quenching the teel 7 into water from 500°C tion transformed and the time at various temperatures. ,,-. during the continuous cooling. The increase of cool The incubation time for the bainite transformation did ing rate to 3 300°C /min produced nearly perfect mar not cha nge so largcly with tempera ture, whereas the tensite. final a mount of the bainite which transformed iso thermall y d epended on temperature, i.e., 40% at 500°C, 60 % a t 525°C and 100% at 450°C, as has been bclieved 111 the medium or hi gh carbon low all oy bainite.91 .... 3. Optical Microscopy / / / / Photographs I (a), (b), (c), and (d) shoyl' the micro Aus tenitized at 1200' C / structures of the steel 7 transform ed at various cooling rates. As can be seen in Photo. I (a), when the speci men was quenched into water from I 200°C, a typical low carbon lath martensite was obtained . During cooling at 2 OOO °C /min, the austenite transformed into 3300' C / min the structure whieh exhibited a somewhat difTerent
/ 150 ' C / min 900 Au steniti zed 01 1200'C 800
- 0 . O.IF ~ 700 ' 0 0.7F· Austenite - 0 - O.IF Ferrit e ~ 600 . 0 . 0 0 ~ ?.Q.. Bdln ll e ______/ ______/ .'3 O· / 60 rc ~ 500 . 00 Q. O' E ~ l s f-" 400 357' C l\t'lrtensi te '~ 300
200 300 400 500 600 700 800 T e mp e r ature ('C ) _ -.J 4 6 810 468i02 4 68T03 4 6 810' Time sec Fi g. 4. The variation of the dilatation curve for the Cu Ni- C r- wlo- \ · steel containing 0.22"" carbon with Fi g . 5. The T.T.T. diagram for the Cu- Ni- C r- M o- \ ' cooling rate steel conta ining 0.12°" carbon
800 .. , Au stenitized at 1200 ' C . 0 700 . 0 . 0.2F • IF Ferrite . 0.2F . IF ~ Austenite . 0.2F . IF 600 0.110 0.1 10 0.2F . O.SF OJ... Z ... 500 OJ'" 0- E Ms OJ 400 f-< Ma rtens ite Fi g. 6 . 2 4 6 8 10 2 4 6 8 102 2 4 6 8103 2 4 6 8104 The T.T.T. diagram lor th e C u- Ni- Cr- l\[o Time ( sec ) steel containing 0.22"" carbon
Researc h Article Transactions ISIJ, Vol. 11, 1971 [ 253 J
800 Aus tenitized at 12WC 00 00 700 00 ~ 2F Ferrite ~ Aus tenite · 0 - O.2F e lF 600 o 0 ° O. IF . O.2Fo l r~ 0.5F '"... 02 058 . . 0 108 040B . 40B L .3 : ~00. 2B 048' 20B .." 8 ..~t~ - -."------.-" - - - - -."'- /'"-8513- .... 500 018 058 0 108 0__ - - - 0 658 070 B 0 808 0858 -:i:: '" 058 0 108,- 0508 0808 0 858 0858 0 '"0. 00 018 058 / 80B858o 858 090 8 1008 E Mso 00 0.5B 038/ 0508 0 100 B 400 r'" 28 708 0 80B 0 1008 M M Martens ite Fig. 7. ... 2 4 6 810 2 4 6 8102 2 4 6 8103 2 4 6 8104 The T.T.T. diagram for the Cu- N i- Cr Time ( sec ) M o- V steel containing 0 .22°" carbon
"
Austeniti zed at 1200' C 10 0 - c~ . 80 ~ -0" ? 60 Q. g 40 .~
E 20 ~ , . " ,:'" 0 0~~~~~ 6 810 2 4 6 810' 4 6 8 J03 4 6 810' Time sec )
Fig . 8. The fracti on transformed in the Cu- Ni- Cr- M o steel conta ining 0.2:2"" carbon during iso thCTmal r. transform at ion
etching sensitivity to 2 % nital from martensite, (Photo. I (b». On cooling at 280°C /min, the size of the fer ri te laths increased a nd the carbide reaction was ob served at the lath interfaces, being a typical of upper bainite,2) - 4) (Photo. I (c» . Further reducing the ... cooling rate to 85°C /min produces the large carbide fr ee bainitic fen'ites with the low temperature d ecom position products between them (Photo. I (d». The microstructures of the isothermally tra nsform ed steel containing 0.22% carbon without vanadium are shown in Photos. 2 (a), (b), (c), and (d). Photograph 2 (a) shows the primary ferrite w hich exhibited an irregula r shape nucleated at the a ustenite grain 20,u L-J boundaries at 6 15°C, and the veining structures within the grain can be seen. At the temperatures below Photo. I. Optical microstructures o f the Cu- Ni- Cr- M o- V 600°C, the bainite r eaction was confirmed to occur. steel containing 0.12°" carbon cooled with Ca) The bainitic ferrite which form ed at comparatively 12000°Cfmin, (b) 2 000 Cfmin . (c) 280 C/min, , .. high temperatures above 500°C generall y exhibited Cd) 85 Cfmin the acicul a r morphology. On the surface nearly p erpendicul ar to the growth direction, however, it seems to be in the shape of parallelogram, suggesting the etching sensitivity for nital from martensite and it that these bainitic ferrite had also lath-like m orphology was difficu lt to d etermine w hether thi s being the up w ith a specific growth direction and a habit plane, p er or the lower bainite by mesns of optical micro (Photo. 2 (b». Photograph 2 (c) shows a typical up scopy. An example of such stl'u ctures in the steel 5 is '? per bainite comprising the bunches of ferrite laths shown in Photo. 2 (d) which was obtained by trans with the cementite pa rticles between them. This forming at 425°C . The direct evidence that showed structure was obtained by the transformation at 475°C. these bainites were in the shape of lath was obtained The bainite which formed in the temperature range by the observation of bainitc on the two adjacent sur between 450°C a nd M s exhibited a slight difference in faces inclined at about 90° to on c another, (Photo. 3).
Research Article [ 254 ) Transactions ISIJ, Vol. 11, 1971
4. Electron M icroscopy Photograph 4 shows the replica image of th e ferrite / a ustenite interface form ed a t 660°C for 15 min, the untra nsformed austenite d ecomposing into martensite during subseq uent quench. The primary ferrite nucle ated at the austenite g rain boundaries with straight ferrite /austenite interfaces, and the sub-structures were not revealed by nita l etching. H owever, by reducing the transformation temperature to 6 15°C, the shape of fcrri te grain becam c much irregular with the veining structure. it is interesting to note that this internal structure is parallel to the ferrite /austenite interface, showing some close relationship with the mechanism of ferrite formation (Photo. 5). On the other hand, in the bainitic range, the car bide-frce bainitic ferrite nucleated at the temperatures above 500°C in the cases of both continuous cooling a nd isothermal transformation and this is keeping w ith the opti cal mi croscopic observation. Photograph 6 shows an example of such a structure of the steel 7 quench ed into water from 500°C during co ntinuous cooling at 280°C /min. It can be scen that this con sisted of the bunch es of ferrite laths with the untrans formed a ustenite between them (whi ch has transformcd •
1
Phot() . :2. Optical Illicrostruclures (, (" the C u Ni- Cr i\lo st(" c l con ta ining O.:l:l "" carbon transf() l"In eci a t (a) 6 15 C for 60 min, ( b ) 570 C fo r 15 sec. (c) 47.'"> C ("or 40 sec. (d) +00 C fo r 25 sec
Bainitic ferrite Photo. 4. The primary ferrite In the Cu- Ni- Cr- M o steel containing 0.2200 carbon transformed a t 660°C
20 J.l
,6
;
Photo. 3. Composite optica l micrograph of th e bainitic ferrit" fo rmed a t 54·0 'C two adjacent surfaces in Photo. 5. The prima ry fe rrite III the C u- Ni- C r- i\Io steel clined a t about 90 C to one anoth e l' conta ining 0.22 ~n ca rbon transformed a t 6 15 C
Research Article Transactions lSI], Vol. 11, 1971 ( 255 )
into martensite during th e subsequent quench). In though the bainitie ferrite conta ined so me internally the temperature range between 500° and 450°C , precipitated cementite, the cementite layers were formed mainly at the ferrite lath interfaces b y the d e composition of the carbon-enriched austenite, pro ducing a typical upper bainite. Photographs 7 (a) • and (b) show such a structure obtained in the steel 5 during the isothermal tra nsformation at 460°C. H ere (a) and (b) are the brigh t fi eld image and the dark " fi eld image using a cementite reRecti on respectivel y. The trace analysis for such bainitic ferrite revealed that it had the lath-like morphology wi th a . { IIOL habit. In the temperature range between SOO °C a nd M s, es pecially below 450°C, the cem entite platelets precipitated within the ferrite grain, and as far - TI as the cementite precipitation is concerned , there is little morphologi cal difference bet\\'een this bainite a nd I ~ a high carbon lower bainite. The bright fi eld image -. and the dark field image using a cementite reRection for such bainite w hi ch formed in the steel 5 at 425°C are shown in Photos. 8 (a) a nd (b) res pectively. It ean J. be seen thal the fine cementite platelets in the sam e ~ orientation precipitated on a sp ecifi c ferrite plane as in ~ the case of lower bainite. For the continuous cooling,
~ . the sam e sort of the structure was obtaincd by cooling as fast as nearly critical cooling rate, (Photos. 9 (a) Photo. 6. Bainitic lC rri te in th e Cu- N i- Cr- l\1o- V steel con and (b)). Photograph 10 shows thc replica image of
taining 0.12 ° 0 ca rbon cooled from a ustenite w ith the bainite /austenite (martensite) interface for this sort 280 C/min a nd qucnched into water from 550 C
... to .~ Photo. 7. A typi cal upper bainite formed a t 460 C in the Photo. 8. The bainitt' formed a l 425 C in the C u- Ni- Cr Cu- N i- Cr- Mo steel co ntaining 0.22"" carbo n i\1o steel CO lltainillg 0.22°" carbon (a) Brig ht field image (a) Bright fi e ld image (b) Da rk field image using a ('t'men lile rcAectioll ( b) D ark field image using a cemenlite re fl ection
Research Article ( 256 ) Transactions ISIJ, Vol. 11, 1971
of bainite forming in the sleel 5 at 450°C. It can be hibited the same lath-like m orphology with a 'f' 010 Grow th direct ion 100 Fig. 9. The habit plane a nd the growth direction of the bainite laths formed in the temperature range be tween '1·50 C and M s tempera ture .. . Photo. 9. The bainite formed in the Cu- N i- C r- M o- V steel conta ining 0. 12°0 carbon during the continuous cooli ng with I 200 C /min (a) Bri ght image (b ) Dark field image using a cementite reflection Photo. II. Autotempered martensite in the Cu- Ni-Cr- M o V steel containing 0.12 °0 carbon Photo. 10. The interface between the bainite formed at (a) Bright fi eld image 450"C and the un transformed austen i te in the (b) D ark field image using a cementite refl ec Cu- Ti- Cr- l\lo steel containing 0.22°0 carbon ti on Rese<,\rc;:h Article Transactions lSIJ, Vol. 11, 1971 ( 257 ) 450,------100 C u- Ni- Cr- l'v\o • Cu- Ni - Cr- l'v\ o- V 400 • 100 L---~~--~~~~~~7-~~~ 350 400 450 500 55 0 600 650 100 Trans forma ti on t emperature ('C ) - Fi g . 10 . The ha bil plane and the g rowth direction of til(" Fig. 12. The res ults of the hardness measurements for the . low carbon martensite Cu- N i- C r- 1\10 and Cu- Ni- C r 1\1o- V steels con taining 0.22 °" carbon 5. H ardness 2.00 The ha rdness of the continuously cooled steels in . . creased with increasing cooling rate . :3 1.00 o On the other hand, in the case of iso thermal trans '" 0.80 i 0.60 formation, the bainite reaction did not complete at the ~ 0. 40 . temperatures above 500°C and the untransformed -= a ustenite d ecomposed into ma rtensite during the sub ~ 0.20 o seCJ uent water quench. As this resulted in the mixed ! ." 'E structure comprising bainitic ferrite and martensite, ~ 0. 10 the ha rdness increased largely. Figure 12 sho'vVs the 0.08 0.06 variation of the hardness of the steels 5 and 6 with the 0.04 transformation temperature. The measurements were carri ed out either when the tra nsformation was com 600 550 500 450 400 350 Trans formation te mperature (G C ) pleted or after keeping two hours iso thermally for the specimens transformed at the temperatures above rig. I I . The effect of the transformation temperature on the 500°C. At the temperatures below 475°C, softening thickness o r the bainite in the C u- Ni c.:r- 1\10 steel occurred b y raising the transformation temperature, conta ining 0. 22° () carbon but the remarkable increase of the hardness was ob se rved at higher temperatures because of the marten a typical of low carbon lath-like marlensi te and there site transformation of the austenite layers between the is no significant difference between this and the bainites ferrite laths. At the temperatures where the nodula r d escribed above except the morphology of cem entite ferrite formed at the austenite grain boundaries, the formation. Namely, in the bainite formed in the tem measurements of hardness became much easier, and perature range between 500° and 450°C cem entite the d ecrease of the hardness was observed by increasing precipitation was observed at the ferrite lath interfaces the temperature. It can be seen that the retardation as well as within the grain, and in the temperature of this soft ening occurred b y the small vanadium addi range between 450°C and M s the cem entite platelets tion. formed on a specific ferrite plane. Whereas in the autotempered martensite fin e acicular cementite rods IV. Discussions were observed to form a long four equivalent 0 The bainite forming at the temperatures below directions, suggesting that the cementite precipitation 600°C was in the shape of a lath with a < I I I >. growth occurred after austeni te/martensite transformation. direction and a {IIO}. habit plane, a nd this morpho The trace analysis for thi s martensite also indicated logy is similar to that of a low carbon martensite21 the . {I 10}. habit as has been beli eved (Fig . 10). which formed at the temperatures above 350°C or of The thickness of the bainite laths varied with the an Fe- Ni massive martensite.10I The mechanism of tra nsformation temperature, and was measured in the the martensite formation w ith such habit in a Cr- Ni steel 5 using electron micrographs (Fig. I I). It can stainless steel was first explained by Venableslll assum be seen that lowering the temperature reduced the ing that the austenite d ecomposed in to Research Article [ 258 J Transactions ISIJ, Vol. 11, 1971 900 l\us tcn itiz eci 01 1200·C 800 700 ~ F e rrit e g r a in Aust enit e ~,boundary 6001 ~ ( a ) ~ Bainite I ~ 500 ------,------,------ ~ 400 _----'-"M-"-s_----' ~l~ i~i te 111\\ Boinite /I \ ~ ---- ... ', ~=-J 300 ... .. !\ Iartensite , ///<'------2001-__~", I.!...f _____ /' 100lL--:---'-- 6:--::8-:'10'-- 2 4 6 8 J01 4 6 8103 2 4 6 810' ( b) Cooling time from AC 3 sec Fi g. 14. The schematic representation or the tempe ra ture ra nges where 3 lypes of bainite formed in the Cu N i Cr- M o- V slee l during continuous cooling 9001 Soo l l\us tenilizeci al 1200 ·C Fi g. 13. The sc hemalic representation of the bainite forma 7001 u Au st enit e F e n"ite ti on in the temperature range between :... 600 ~ (a) 600 and 500 C. (b) :'00 a nd 450 C. ~ Bainite I ------(c) .~ OO C and M s E500 1 Bai ni te /I " I i\l s -----Bainit e 11'-- ! 400 ---"=---'------~ theory of martensi te with the 12 1 I ], ( I I I ), inhomo ivlart e ll s itc geneous shear. Chilton el a1.l3) also shovved that both 3001 the austenite/ferrite ori entation relationship, which was 200l d etermined b y the close examination of the twin re 100 la ted martensite laths, a nd the habit of the low carbon 1 4 6 8 10 4 -6:--:S:-:'I :: Researc h Article Transactions ISIJ, Vol. 11, 1971 ( 259 J lower bainite. To simplify the foll owing discussion, thermal transformation, it is formed in the tempera these three types of bainite wi ll be call ed the bainite I ture range betwccn 5000 e and M s, but in the continu (a carbide-free bainitic ferrite, BI), the bainite II (a o us cooli ng, it is formed when it is cooled as fast as the typical upper bainite consisted of ferrite laths with critical cooling rate. cementite layers between them, BI1 ), and the bainite (5) The cooling rate separa ting BII and BIll on ... 111 (the bainite with a ( III ) " { I 10). habit with the the continuous cooling produces the large differen ce cementite platelets on a specific ferrite plane similar in in appearance by optical mi croscopy. a ppearance to lower bainite, B1II). The temperature (6) As allthesc baini te have the sam e ( I I I ) " { I I O}.. regions for these three bainite formations are shown in la th-like morphology, it seems reasonable to classify 't Figs. 14 and 15 schematicall y. In the case of con them as upper bainite, as far as the separa tion b e tinuous cooli ng, as the starting temperature of the tween upper and lower bainites is d efined by the fcn-ite bainite reaction in the steel containing vanadium was morphology. a lmost constant at 600o e, BI form ed over the wide (7) Though the cementite in the bainite !!l pre range. However, as the amount of BI was extremely cipitates in a similar fashion to that in a high carbon limited at the large cooling rate, the final structure as lower bainite, the morphology and the habit of ferrite shown in Photo. 7 was obtained . At the intermediate is completely different from those of the lower bainite. cooling rate, though Bl formed initially, the formation (8) The difference among BI, BI I, and Bill can of BII which consisted of the bunches of parallel ferrite b e explained in terms of the mutual relationship be - t laths with the cementite layers between them occurred tween the growth of ferrite laths a nd the carbon dif during the following cooling. This can a lso be seen fusion in austenite. in the dilatometri c measurem ents where the abrupt Acknowledgements transformation controlled by the cementite precipita The authors would like to thank Dr. M . Sumitomo, tion took place aft er the grad ua l formation of BI 'vvhich the managing director o[ th e Sumitomo NIetal In being controll ed by the diffusion of carbon in austenite dustri es, Ltd., [or his permiss ion to publi sh this paper. as shown Fig. 4. 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