3DAP研究高碳高合金钢深冷处理过程的C偏聚行为
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摘要
将高碳高合金钢SDC99分别在1030℃奥氏体化30 min后油淬、在-196℃液氮中深冷处理8 h及210℃回火2 h.采用三维原子探针(3DAP)技术分析了淬火态、深冷态、回火态C原子的空间分布;用XRD研究了3种热处理状态下马氏体轴比、马氏体中含C量的变化;用SEM原位观察深冷前后碳化物的形貌.结果表明,C原子在深冷处理过程中偏聚于马氏体孪晶界,形成厚度为5~10 nm的偏聚区.210℃低温回火过程中,C进一步偏聚形成富C相或与合金原子形成M23C6型碳化物.
Deep cryogenic treatment(DCT) is a supplement to conventional heat treatment, which usually involves cooling the material to liquid nitrogen temperature around-196 ℃ for a given soaking time and then heating back to the room temperature. As claimed in many pioneering researchers, DCT can evidently improve the hardness and wear resistance of high-carbon high-alloy steel and has been widely used to die steels, cutting tools,carburizing steels and barrels. The improvement of mechanical properties by DCT can be attributed to the transformation from retained austenite to martensite, the fine dispersion of nanoscale carbide precipitate and the removal of residual stresses. However, the nanoscale carbide precipitate is still lack of evidence and the interpretation of carbon segregation behavior during DCT is still unconvincing. In this work, the high-carbon high-alloy steel SDC99 is first austenized at 1030 ℃ for 30 min and then immersed in liquid nitrogen for 8 h and finally tempered at 210 ℃for 2 h. The spatial distributions of carbon atom and alloy element concentration in quenched, DCT treated and tempered samples are analyzed by three dimensional atom probe(3DAP), respectively. In addition, the axial ratio and carbon content of martensite are studied using XRD and the carbide morphology before and after DCT are also observed in situ by SEM. The results indicate that after quenching from 1030 ℃ to room temperature, the volume fraction of retained austenite in SDC99 is about 21.1%. The retained austenite is soft and unstable which can easily transfer to martensite at lower temperatures. Carbon atoms will segregate slightly due to self-tempering. However, other alloy atoms do not segregated with carbon atoms. After quenching from 1030 ℃ to room temperature and then cooling in nitrogen for 8 h, the volume fraction of retained austenite in SDC99 will decrease to 7.4%. Carbon atoms will segregate along the twin boundary of martensite and form a segregation area with a thickness about 5~10 nm.There is no carbide precipitate after DCT. Furthermore, carbon atoms segregate again during heating up back to room temperature from-196 ℃. After tempering at 210 ℃ for 2 h, the volume fraction of retained austenite is almost 5.4%. Both carbon and alloy atoms will segregate during tempering at 210 ℃. With the increase of tempering time, the carbon segregation will aggravate and result in a C-rich phase or form the M23C6 carbide combined with other alloy element. This is one of the main reasons increasing the wear resistance of tool steels.
Deep cryogenic treatment(DCT) is a supplement to conventional heat treatment, which usually involves cooling the material to liquid nitrogen temperature around-196 ℃ for a given soaking time and then heating back to the room temperature. As claimed in many pioneering researchers, DCT can evidently improve the hardness and wear resistance of high-carbon high-alloy steel and has been widely used to die steels, cutting tools,carburizing steels and barrels. The improvement of mechanical properties by DCT can be attributed to the transformation from retained austenite to martensite, the fine dispersion of nanoscale carbide precipitate and the removal of residual stresses. However, the nanoscale carbide precipitate is still lack of evidence and the interpretation of carbon segregation behavior during DCT is still unconvincing. In this work, the high-carbon high-alloy steel SDC99 is first austenized at 1030 ℃ for 30 min and then immersed in liquid nitrogen for 8 h and finally tempered at 210 ℃for 2 h. The spatial distributions of carbon atom and alloy element concentration in quenched, DCT treated and tempered samples are analyzed by three dimensional atom probe(3DAP), respectively. In addition, the axial ratio and carbon content of martensite are studied using XRD and the carbide morphology before and after DCT are also observed in situ by SEM. The results indicate that after quenching from 1030 ℃ to room temperature, the volume fraction of retained austenite in SDC99 is about 21.1%. The retained austenite is soft and unstable which can easily transfer to martensite at lower temperatures. Carbon atoms will segregate slightly due to self-tempering. However, other alloy atoms do not segregated with carbon atoms. After quenching from 1030 ℃ to room temperature and then cooling in nitrogen for 8 h, the volume fraction of retained austenite in SDC99 will decrease to 7.4%. Carbon atoms will segregate along the twin boundary of martensite and form a segregation area with a thickness about 5~10 nm.There is no carbide precipitate after DCT. Furthermore, carbon atoms segregate again during heating up back to room temperature from-196 ℃. After tempering at 210 ℃ for 2 h, the volume fraction of retained austenite is almost 5.4%. Both carbon and alloy atoms will segregate during tempering at 210 ℃. With the increase of tempering time, the carbon segregation will aggravate and result in a C-rich phase or form the M23C6 carbide combined with other alloy element. This is one of the main reasons increasing the wear resistance of tool steels.
引文
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[30]Li S H.Ph D Dissertation,Shanghai University,2011(李绍宏.上海大学博士学位论文,2011)
[2]Huang J Y,Zhu Y T,Liao X Z,Beyerlein I J,Bourke M A,Mitchell T E.Mater Sci Eng,2003;A339:241
[3]Das D,Dutta A K,Toppo V,Ray K K.Mater Manuf Processes,2007;22:47
[4]Meng F,Tagashira K,Azuma R,Sohma H.ISIJ Int,1994;34:205
[5]Li S H,Deng L H,Wu X C,Wang H B,Min Y A.Mater Sci Eng,2010;A527:6899
[6]Li S H,Deng L H,Wu X C,Wang H B,Min Y A,Min N.Mater Sci Eng,2010;A527:7950
[7]Li S H,Min N,Deng L H,Wu X C,Min Y A,Wang H B.Mater Sci Eng,2011;A528:1247
[8]Li S H,Min N,Li J W,Wu X C,Li C H,Tang L L.Mater Sci Eng,2013;A575:51
[9]Tyshchenko A I,Theisen W,Oppenkowski A,Siebert S,Razumov O N,Skoblik A P.Mater Sci Eng,2010;A527:7027
[10]Zhu C,Cerezo A,Smith G D W.Ultramicroscopy,2009;109:545
[11]Wilde J,Cerezo A,Smith G D W.Scr Mater,2000;43:39
[12]Liu Q D,Liu W Q,Wang Z M,Zhou B X.Acta Metall Sin,2009;45:1281(刘庆冬,刘文庆,王泽民,周邦新.金属学报,2009;45:1281)
[13]Liu Q D,Peng J C,Liu W Q,Zhou B X.Acta Metall Sin,2009;45:1288(刘庆冬,彭剑超,刘文庆,周邦新.金属学报,2009;45:1288)
[14]Liu Q D,Chu Y L,Peng J C,Liu W Q,Zhou B X.Acta Metall Sin,2009;45:1297(刘庆冬,褚于良,彭剑超,刘文庆,周邦新.金属学报,2009;45:1297)
[15]Matteo V,Karen P,Marcel A J S.Acta Mater,2014;65:383
[16]Liu Q D,Liu W Q,Wang Z M,Zhou B X.Acta Metall Sin,2008;44:786(刘庆冬,刘文庆,王泽民,周邦新.金属学报,2008;44:786)
[17]Xie Z J,Ren Y Q,Zhou W H,Yang J R,Shang C J,Misra R D K.Mater Sci Eng,2014;A603:69
[18]Clarke A J,Speer J G,Miller M K,Hackenberg R E,Edmonds D V,Matlock D K,Rizzo F C,Clarke K D,Moor E De.Acta Mater,2008;56:16
[19]Gao G H,Zhang H,Gui X L,Luo P,Tan Z L,Bai B Z.Acta Mater,2014;76:425
[20]Li S H,Xie Y Z,Wu X C.Cryogenics,2010;50:89
[21]Duan C Z,Wang M J.J Iron Steel Res,2008;20(8):38(段春争,王敏杰.钢铁研究学报,2008;20(8):38)
[22]Kurdjumov G V.Metall Trans,1976;7A:999
[23]Gavriljuk V G,Theisen W,Sirosh V V,Polshin E V,Kortmann A K,Mogilny G S,Petrov Y N,Tarusin Y V.Acta Mater,2013;61:1705
[24]Tyshchenko A I,Theisen W,Oppenkowski A,Siebert S,Razumov O N,Skoblik A P,Sirosh V A,Petrov Y N,Gavriljuk V G.Mater Sci Eng,2010;A527:7027
[25]Li J W,Tang L L,Li S H,Wu X C.Mater Des,2013;47:653
[26]Liu Q D,Chu Y L,Wang Z M,Liu W Q,Zhou B X.Acta Metall Sin,2008;44:1281(刘庆冬,储于良,王泽民,刘文庆,周邦新.金属学报,2008;44:1281)
[27]Li S H,Deng L H,Wu X C,Min Y A,Wang H B.Cryogenics,2010;50:754
[28]Li S Y,Liu T Z,Li G.Mater Rev,2003;17(8):80(李士燕,刘天佐,李钢.材料导报,2003;17(8):80)
[29]Li J W,Feng Y,Tang L L,Wu X C.Mater Lett,2013;100:274
[30]Li S H.Ph D Dissertation,Shanghai University,2011(李绍宏.上海大学博士学位论文,2011)