The Bainite in Low Carbon Low Alloy High Strength Steels*

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.

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