高温奥氏体化对40Cr钢疲劳裂纹扩展行为的影响

The Effect of High Temperature Austenitizing on Behavior of Fatigue Crack Propagation in 40Cr Steel

研究了40Cr钢经不同温度淬火、回火工艺处理后的显微组织和疲劳裂纹扩展行为。结果表明,1200℃高温淬火和1200℃高温奥氏体化后,在850℃冷待15min(1200→800℃)淬火,可获得完全的板条马氏体和较多的马氏体扳条间的残留奥氏体,从而可显著降低200℃回火后的da/dN值,提高Np;但400℃回火后,经1200℃淬火的,其da/dN却明显升高,Np显著降低;而经1200→860℃淬火的,仍具有较低的da/dN和较高的Np。在低温和中温回火态,40Cr钢的疲劳断裂机制均是沿晶与穿晶的混合,在200℃回火态,奥氏体化温度对裂纹扩展初期的断裂机制几乎不产生影响,在400℃回火态,沿晶断口的百分率随奥氏体化温度升高略有增加。在低温和中温回火态,影响疲劳裂纹扩展行为的主要组织因素是马氏体亚结构、残留奥氏体和Fe_3C_2奥氏体晶粒尺寸的影响不明显。

The microstructure and behavior of fatigue crack propagation of40Cr steel under different process of hardening and tempering are investigated.The results show that,after quenching from 1200℃ directly or austenitizing at1200℃ followed by cooling to 860℃ and holding for 15 minutes,then quench-ing,the lath martensite can be obtained completely and the retained austenitebetween the laths of martensite can be increased,therefore da/dN can be decr-eased and Np increased obviously after 200℃ tempering.But after 400℃ temp-ering,the da/dN would be increased and Np decreased obviously when quench-ing from 1200℃,and the da/dN is still very low with very high Np whenquenching from 1200→860℃.After 200℃ or 400℃ tempering,the fatigue fra-cture mechanism of 40Cr steel is a mixture of intergranular and transgranular.After 200℃ tempering,the austenitizing temperature doesn′t effect on thefracture machanism at the beginning of crack propagation.After 400℃ temp-ering,the percentage of intergranular fracture is slightly increased with theaustenitizing temperature being raised.In the different tempering condition,the substructure of martensite,the retained austenite and Fe_3C are the mainstructural factors which effect on the behavior of fatigue crack propagation,and the effect of austenite grain size is not obvious.