Carbide reactions (M3C-M7C3-M23C6-M6C)

Carbide Reactions (M3C-->M703->M2306->M6 C) During Tempering of Rapidly Solidified High Carbon Cr-W and Cr-Mo Steels A. INOUE AND T. MASUMOTO Carbide t rans format ions of MaC ~ MTCa ~ M23C6 ~ MaC and crysta l lographic re la - t ionships among these carbides were examined by t ransmiss ion e lect ron microscopy. Two kinds of high carbon-chromium stee ls containing tungsten or molybdenum were quenched rapidly f rom the melts and tempered at temperatures up to 700~ By tem- per ing at 600~ MTCa carbides nucleated mostly on cement i te / fe r r i te inter faces and grew inward the cementi te by in-s itu t ransformat ion . In-s i tu t rans format ions f rom M7C3 to MeaC6 and f rom M2aC6 to M6C were also found in these al loy s tee ls during tem- per ing at higher temperatures . Mutual re lat ionships of c rysta l or ientat ions among MaC, M7C3, M23C6 and M6C were decided as fol lows: (0-fl)MaC//(0001)MTC 3, (012)MaC//(1T00)MvC3, (I-00)MaC//(ll~0)MTCa, (0001)MvC/ / (I-2I')M2aC6, ( l l -00)MvCJ / ( l l l )MeaCe, (llg0)MvCg//(10I)M23C6, and (fl0)M2aCe/// 0-10)M6C, ( l - l l )M23CJ/ ( l l~)M6C, ( l l2 )M2aCJ / ( l l2 )M~C. 1. INTRODUCTION IT has been well known ~-14 that four kinds of carb ides prec ip i tate during temper ing of chromium steels in the order of e -carb ide , cementi te (MAC), MTCa and M2aC~, and that the t rans format ion of these carb ides takes place e i ther through a separate nucleat ion or an in-situ manner . However, evidence for the in- situ t rans format ion of MaC - - MvCa ~ M23C6 shown in the prev ious studies t-%%n,la is based on the s imi - lar i ty in the morphology and/or the prec ip i tat ion site of these carb ides . D i rec t evidence for the in-s itu t rans format ion has not been obtained, s ince observa- tions in the prev ious studies were made by means of some indirect methods such as X - ray di f f ract ion and extract ion rep l ica e lect ron microscopy. Moreover , prec ip i tates in tempered steels are usually so smal l that the detai led process of t rans format ion could not be observed even by t ransmiss ion e lectron microscopy. Recent ly the present authors 1~'~6 have made t rans - miss ion e lectron microscop ic observat ions on the carbide t rans format ions during temper ing in 16.5 pct Cr-3.65 pct C* s tee l quenched f rom the melt, *All compositions are in weight percent. and obtained evidence for the in-s itu t rans format ion of MaC to M~C3 and establ ished the crysta l lographic re lat ionship between MaC and M7C3. The purpose of the present invest igat ion is to c lar i fy the t rans forma- t ion process of MaC --* M7C3 ~ M2aC6 - - MGC by means of t ransmiss ion e lect ron microscopy. a l loys. The melts were taken up into quartz tubes of about 3 mm ID by suction and sol idi f ied in the tubes. The chemica l composit ion of the mixed al loys is shown in Table I. F rom these al loys, long r ibbons of 3.0 to 3.5 mm width and 0.04 to 0.05 mm thickness were prepared as the test samples by d i rect ing a s t ream of molten al loy onto the outer sur face of rapidly revolv ing ro l l which was made of steel . The amount of the al loys melted in a run was about 2.5 g, and the rotat ion speed of the ro l l (200 mm in diam) was about 3000 rpm. Specimens of about 30 mm in length were cut f rom the as-quenched r ibbons, sealed in evacuated quartz capsules and tempered for d i f fe r - ent t imes at var ious temperatures up to 700~ These r ibbons were e lect ropol ished in an e lect ro ly te con- taining 90 ml of ethyl alcohol and 10 ml of perch lo r i c acid to obtain thin foi ls . The e lect ron microscope used was of JEM-200B type with a t i lt ing device of 45 deg operat ing at 200 kV. Examinat ions by X - ray di f f ract ion and thermomagnet ic measurements were also car r ied out. In the magnetic analys is a magnet ic balance of high sensi t iv i ty was used below 900~ in the magnetic f ield of 1.2 • 106 A / re . The weight of a sample for the magnetic measurements was 25 my. 3. RESULTS F igure 1 shows a mic ros t ruc ture of the as -quenched 18.6 pct Cr -3 .40 pct W-3.63 pct C stee l having a very fine lamel la r s t ructure consist ing of austenite matr ix 2. EXPERIMENTAL PROCEDURES Mixtures of pure metals (Fe, Cr, W and Mo), white cast i ron and graphite were melted under an argon atmosphere in a Tammann furnace to prepare the test A. INOUE and T. MASUMOTO are Research Assistant and Profes- sor, respectively, The Research Institute for Iron, Steel, and Other Metals, Tohoku University, Senai 980, Japan. Manuscript submitted August 20, 1979. Table I. Chemical Composition of Alloy Steels, Wt Pet Equilibrium Number C Cr W Mo Fe Phase at 700~ 1 3.63 18.6 3.40 Balance 1~ + M23C6 2 3.58 17.8 - 3.56 Balance ct + M23C6 3 3.60 18.4 8.31 Balance c~ + MzaC6 + M6C 4 3.61 17.9 - 8.44 Balance ~ + MzaC6 + M6C ISSN 0360-2133/80/0512-0739500.75/0 �9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME METALLURGICAL TRANSACTIONSA VOLUME I1A, MAY 1980-739 Fig. l-Microstructure of austenite and cementite in as-quenched 18.6 pct Cr-3.40 pct W-3.63 pct C steel. (a) Bright field image, (b) selected area diffraction pattern, (c) its schematic key diagram. and a carb ide . The average in ter lamel la r spacing is about 35 nm. An e lect ron d i f f ract ion pat tern taken f rom this foi l and its key d iagram shown in F ig. l(b) and (c) indicate that this carb ide is nei ther hexagonal (or tr igonal) M7C3 nor complex fcc M23C6 but cemen- t ire in a nonequi l ibr ium state. 1~-~~ These resu l ts sug- gest that the cooling rate (105~ during quench- ing of melt could be high enough to suppress both the prec ip i tat ion of M23C6 and M7C3 carb ides and also the t rans format ion f rom austenite to fe r r i te , as in the previous exper iments , ls,16 F igure 2 i l l us t ra tes a thermomagnet ic curve of 18.6 pct Cr -3 .40 pct W-3.63 pct C s tee l obtained dur - ing heating and cooling at a rate of 0.1~ Three t rans i t ion points are seen at about 80, 485 and 700~C as marked on the curve by A, Band C, respect ive ly . Accord ing to t ransmiss ion e lec t ron microscopy and X- ray ana lyses , the large increase in magnet izat ion at 485~ occurs due to the decomposi t ion of austeni te to fe r r i te and cement i te . The inf lect ion point seen at about 80~ is undoubtedly the Curie point of cemen- rite containing large amounts of chromium and tungsten, 2a and the rap id decrease and increase in magnet izat ion around 700~ cor responds to the fe r r i te -austen i te t rans format ion . A typ ica l mic ros t ructure of the 18.6 pct Cr -3 .40 pct W-3.63 pct C s tee l tempered at 500~ for 1 h is shown in F ig. 3(a). F igure 3(b) and (c) a re the se - 120 . . . . . - - , , Hrn =1.2xlO6A/m lOO I o ~ 60 '-~ 4C 2( A 0 ~00 2()0 300 460 500 600 700 800 900 Temperature (~ Fig. 2-Temperature dependence of magnetization for rapidly quenched 18.6 pct Cr-3.40 pct W-3.63 pct C steel. leered area d i f f ract ion pat tern and i ts key d iagram. F rom these f igures it is in fe r red that the decompos i - t ion of austeni te to fe r r i te has been a l ready com- pleted in company with a s l ight growth of cemenf i te . Af ter temper ing at 600~ for 1 h, this cement i te t rans forms to M7C3 carb ide as shown in F ig . 4. In this f igure, it is observed that the t rans format ion of cement i te to MvCs carb ide takes p lace by an in-situ mechan ism and the M7C3 carb ide contains numerous interna l faults . The e lec t ron d i f f ract ion pat tern shows re f lect ion spots f rom MvC3 carb ide together 740-VOLUME llA, MAY 1980 METALLURGICAL TRANSACTIONS A Fig. 3-Microstructure of ferrite and cementite in 18.6 pct Cr-3.40 pct W-3.63 pcI C steel tempered for 1 h at 5000(?. after rapid quenching. (a) Bright field image, (b) selected area diffraction pattern, (c) its schematic key diagram. with those f rom cementite and ferr i te . Around these spots, streaks along ill-00] and [1010] d irect ions are seen. This finding indicates that interna l faults lie on the (1]~00) and (i010) planes of M7C3 carbide as pointed out by Beech and Warr ington. 14 This streak has been interpreted 24 to be induced from a st ructure with the faulted vector (a/2 <1010>) which is half of the unit cell repeat distance of the hexagonal crysta l s t ructure . The following or ientat ion re lat ionships between M3C mad MvCa were obtained f rom the Fig. 4(b), allowing for a maximum scatter ing of about ten deg : (0001)MvC a//(0I-1)M 3C, (li00)MTC J/(012)MAC, (1120)MvCJ/(100)M3C. F igures 4(c) and (d) are dark field images taken from a ref lect ion spot of cementite and of MvC3, respect ive ly . The boundary between cementite and MvC3 carbide is straight and the race l ies along the (010) plane of cementite, suggesting that there exists a strong spe- cific crystal lographic relat ionship between the latt ices of the two carbides. These observat ions agree well with the previous resu l t s . 1%16 A t ransmiss ion e lectron micrograph of 18.6 pet Cr-3.40 pct W-3.63 pct C steel tempered at 700~ for 1 h is shown inF ig . 5(a), wherein cementite has d is - appeared completely and most of the carbides are MvC3 except for a l ittle amount of M23C6 carbide. Thus, the in-s i lu t rans format ion from cementite to MvC3 starts at about 600~ and cementite is com- pletely replaced by MvC3 after temper ing at 700~ for 1 h. A mic ros t ructure of carbides obtained after tem- pering at 700~ for 10 h is shown in Fig. 6(a). In this f igure, it is c lear ly shown that a MvC3 carbide t rans - forms to M2aC6 carbide by an in-s i tu mechanism. Reflection spots f rom MvCa and M23Ce carbides are seen in the electron diffraction pattern (b). F ig- ure 6(c) and (d) are dark field images taken from a ref lect ion spot of M23C6 and MvCa, respect ive ly . The boundary between the two carbides is curved and does not lie on any crysta l lographic plane, suggesting that there is no specif ic crystal lographic re lat ion- ship between the latt ices of the two carbides. A fur - ther progress in the in-s i tu t rans format ion from M7C3 to M23C6 was seen in the same steel tempered at 700~ for 24 h. A typical example is shown in Fig. 7(a). The proport ion of M23C6 carbide increased considerably compared with that seen in Fig. 6. METALLURGICAL TRANSACTIONS A VOLUME 11A, MAY 1980 741 Fig. 4-Microstructure showing the in-situ transformation of cementite to MTC 3 in 18.6 pet Cr-3.40 pet W-3.63 pet C steel tempered for i h at 600~ (a) Bright field image, (b) selected area diffraction pattern and its schematic key diagram, (c) and (d) dark field images taken respectively from the 0],~ reflection spot of cementite and the 6331 reflection spot of MTC 3. Fig. 5-Microstructure of MTC 3 and M23C 6 in 18.6 pct Cr-3.40 pct W-3.63 pct C steel tempered for 1 h at 700~ (a) Bright field image, (b) dark field image taken from the reflection spot of MTC 3. F igure 7(c) and (d) are dark field images taken f rom a ref lect ion spot of M23C6 and M~C3, respect ive ly . The boundary between these carbides is curved s imi - lar to that found in Fig. 6. Such an is land- l ike region of M7C3 carbide in M23C6 carbide was frequently ob- served in the Cr -Mo steels subjected to s imi la r heat t reatments . Fur thermore , in the M23C6 carb ides a number of planar faults are seen along the {111} and {100} planes. Deta i ls on these planar faults have been reported e lsewhere . 2~ For h igh-carbon Cr -W and Cr -Mo steels contain- ing la rger amounts of tungsten or molybdenum above about 6 pct, the carbide react ion f rom M23C6 to M6C occur red following the react ions of M3C ~ M~C3 Mz3C~ by prolonged temper ing . A typicaly example of the resu l ts is shown in Fig. 8 for 18.4 pct Cr - 8.31 pct W-3.60 pct C s tee l tempered at 700~ for 120 h. In this f igure, it is observed that the t rans for - mation of Me3C6 to MsC occurs by an i n - s i tu mechan ism. The e lect ron di f f ract ion pattern (b) shows ref lect ion spots f rom M6C carbide together with those f rom M23C6. F igure 8(c) and (d) are dark f ield images taken f rom each ref lect ion spot of 202 of M23C6 and 333 of M~C, respect ive ly . The boundary between the two carb ides is curved s imi la r to that between M~C3 and M23C6. Addit ional ly, this f igure suggests that there may exist a close re lat ionsh ip between the pre - cipitation site of M6C carbide and the planar faults 742 VOLUME llA, MAY 1980 METALLURGICAL TRANSACTIONS A Fig. 6-Microstructure showing the in situ transformation of M7C 3 to M23C 6 in 18.6 pct Cr-3.40 pct W-3.63 pct C steel tempered for 10 h at 700~ (a) Bright field image, (b) selected area diffraction pattern and its schematic key diagram, (c) and (d) dark field images taken resepctively from the 220 reflection spot of M23C 6 and the 74~.6i reflection spot of MTC 3. Fig. 7-Microstructure showing the in situ transformation of M7C 3 to M23C 6 in 18.6 pct Cr-3,40 pct W-3.63 pct C steel tempered for 24 h at 700~ (a) Bright field image, (b) selected area diffraction pattern and its schematic key diagram, (c) and (d) dark field images taken respectively from the 511 reflection spot of M23C 6 and the 4151 reflection spot of M7C3. METALLURGICAL TRANSACTIONS A VOLUME l lA , MAY 1980-743 Fig. 8 Microstructure showing the in situ transformation of M23C6 to M6C in 18.4 pet Cr-8.31 pct W-3.60 pct C steel tempered for 120 h at 700~ (a) Bright field image, (b) selected area diffraction pattern and its key diagram, (c) and (d) dark field images taken respectively from the 202 reflec- tion spot of M23C 6 and the 333 reflection spot of M6C. in M2aC6 carbide. F igure 9 shows the e lectron micro - graph and the selected area diffraction pattern show- hag the i n - s i tu t rans format ion of M23C~ to M~C for 17.9 pct Cr-8.44 pct Mo-3.61 pct C steel tempered at 700~C for 120 h. A number of M~C carbide par - t ic les appear at the M2aC6 ferr i te interface and inside the M2aC6 carbide. The part ic le size and the area of M6C carbide increased with increasing the temper ing t ime. Based on the resul ts descr ibed so far, the temper - ing processes of the Fe -Cr -W-C and Fe -Cr -Mo-C alloys are summar ized in Fig. 10 together with those of the Fe -Cr -C alloy examined previously, t~ The i n - s i tu t ransformat ion of cementite to MvCa starts at about 600~ in all the alloy steels and is completed at about 700~C. The i n - s i tu t ransformat ions of MTCa to Me3C6 and of Me3C6 to M6C take place in the Cr-W and Cr-Mo steels during temper ing for longer t imes than about 1 h at 700~ On the other hand, the de- composit ion of ~ ferr i te and cementite in the Cr-W and Cr-Mo steels takes place at about 500 and 600~ respect ively. These decomposit ion temperatures are much higher than that (about 270~ for chromium steel, indicating that the atomic move- ment for decomposit ion of austenite becomes diffi- cult by the addition of tungsten or molybdenum. DISCUSSION 1) Or ientat ion Relat ionships Among MvC~, M23C6 and M6C �9 For the i n - s i lu t ransformat ion, there should be a definite re lat ionship of crysta l or ientat ion between MvC3 and Me3C6 or M23C~ and M6C s imi la r to that be- tween cementite and MvC3 reported in the previous studies. 1%~6 Seven e lectron dif fract ion patterns con- taining ref lect ions f rom MvC3 and Me,C6 or M23C6 and M6C were used to determine the or ientat ion re - lat ionships. As a result , the following re lat ionships were obtained, allowing for a maximum scatter ing of five degrees: (0001)MvCJ/(-i-2~)M2aC ~, (I-10)Me3CJ/(II-0)M~C (I]-00)MvC a//(111 )Me aC6 (i-11 )M23Ce//(1 II)M6C (1120)MvC e/ / (101-)Me aCe (112)Me aCe//(112 )M6C" Based on the information obtained so far on M7C3, M23C~ and M6C, crysta l lographic character i s t i cs of the phase t rans format ion of MvC3 to M23C6 and of Me,C6 to MGC are i l lustrated schematical ly in F igs . 11 and 12. As also demonstrated ear l ie r , 15'1~ the boundary between cementite and MvC3 carbide is 744-VOLUME I1A, MAY 1980 METALLURGICAL TRANSACTIONS A Fig. 9-Microstructure showing the in situ transformation of M2aC 6 to M6C in 17.9 pct Cr-8.44 pct Mo-3.61 pct C steel tempered for 120 h at 700~ (a) Bright field image, (b) selected area dif- fraction pattern and its key dia- gram. Alloy sys- Tempering temperoture(*C) tem(wt.pct] ,?00 300 400 500 600 700,1h 10h 20 3P 50 10C Fe-3.7C- - - - "====::=:~Z ~. r_ MaC ~ I I I l I I I I I I Fig. 10-Structural changes in the two kinds of alloy steels by tem- pering for varying times at different temperatures after rapid quench- ing from the melt. 16.5Cr Fe-3,6C- 18.6Cr-3./~W Fe-3.6C- 17.8Cr- 3.6Mo Fe-3.6C- 18Cr- 8W,or-SMc straight and the t race l ies along the (010) plane of cementite, whereas the boundaries between MTCa and M~sC~ or M23C6 and M6C are curved and the t races do not l ie along any crysta l lographic plane of MTCa, M23C6 or M6C. It is thought f rom these resu l ts that the crysta l lographic re lat ionships of the latt ices be- tween MTCa and M2aC6 or M23C6 and M6C are much weaker than that between cementi te and MTCa carbide. Based on the mutual re lat ionships of the crysta l or ientat ion among cementite, M7C3, M23C6 and M6C determined in the present and previous invest iga- tions, 15'16 the amounts of latt ice misf i t among these carbides were calculated. The resu l ts are sum- mar ized in Table II. The misf i t of the latt ices be - tween cement i te and MTCa carbides is the highest (about 11 pct), that between M7C3 and M23C6 carb ides is about 7 pct and that between M2aC6 and M6C car - bides is about 5 pct. These smal l degrees of misf i t indicate that the or ientat ion re lat ionships among the four carb ides are reasonably maintained in the in - s itu t rans format ion . 2) The Processes of I n -S i tu Trans format ions F rom Cement i te to M7C3, F rom MTCz to M2aC6 and F rom M2aC6 to M6C When a metastable compound prec ip i tates f rom a supersaturated solid solution and subsequently changes into a stable compound, the probable processes may be c lass i f ied into: 1) The metastable compound d isso lves into the matr ix, while the stable compound prec ip i tates sep- arate ly . 2) The crysta l l ine s t ructure of the metastable com- pound changes i n - s i tu to that of the stable compound without composi t ional change. 3) The crysta l l ine s t ructure of the metastable com- pound changes i n - s i tu to that of a stable compound accompanied by composi t ional changes. Atoms diffuse f rom the metastable compound to the matr ix phase and v ice versa . Among these processes , p rocess 1) involves separate METALLURGICAL TRANSACTIONS A VOLUME I1A, MAY 1980 745 (n~O)M,c~//(tN)M,~ / J / Eooo,3 :, . . . . . ) OiO0~r.= (~O0)M~/t(u Ill,c, [O0011M,C~ Fig. 11 Schematic diagram illustrating the orientation relationships between M~Ca and M~3C6, and the internal faults in MTC 3. [IIO]M~C.6 I I LTII~Msc /k Growth 1 [101]lv=~/I [011"~ i n t e r ~ / ~--~ Ir'~'~ i nol MC ~f~ (11~ [11211~3cell [llZIMeC [112]M=3Ce 01T)M,.,C, 0 ff~,cJ" (IT1)M,C Fig. 1 2-Schemat ic diagram illustrating the orientation relationships between M23C 6 and M6C, and the internal faults in M23C6. Table II. Sum