深冷处理对冷作模具钢SDC99组织及性能的影响

Effects of deep cryogenic treatments on microstructure and mechanical properties of SDC99 cold working die steel

对经不同深冷处理工艺的Cr8型冷作模具钢SDC99进行了硬度、冲击韧性、摩擦磨损性能测试,并用SEM、XRD、TEM分析了其相组成及显微组织。结果表明,深冷处理使SDC99钢硬度提高,冲击韧性降低,耐磨性提高,1030℃淬火＋210℃×2 h回火＋（-196℃×24 h）深冷处理＋210℃×2 h回火可使SDC99钢获得最优的综合使用性能,其磨损体积较常规热处理工艺减少23.6%。SDC99钢性能提升的主要原因是深冷处理使亚稳态的块状残留奥氏体转变为马氏体及薄膜状残留奥氏体,深冷处理后残留奥氏体体积分数由常规热处理的17.2%下降至5.5%～10.5%,但并未完全转变。薄膜状残留奥氏体分布于孪晶马氏体及二次碳化物的边界处,松弛应力集中,防止微裂纹的产生,因而改善了材料的综合性能。

Effect of deep cryogenic treatment （DCT） on properties of Cr8-type cold working die steel SDC99 was studied by means of hardness, impact toughness and wear resistance tests. The microstruetual evolution was characterized by means of scanning electron microscope （SEM）, X-ray diffraction （XRD） and transmission electron microscope （TEM）. It is concluded that the hardness and wear resistance of the SDC99 steel increase while the impact toughness decrease. After quenching at 1030 ℃ , first tempering at 210 ℃ for 2 h and then immersed in liquid nitrogen for 24 h and finally tempering at 210 ℃ for 2 h, the quenched steel can obtain the optimal comprehensive performance. The wear volume of DCT sample is decreased by 23.6% compared to that of the common heat treatment one. The results show that the significant improvement in the wear resistance of SDC99 is attributed to the transformation of metastahle blocky retained austenite to martensite and film-like retained austenite after DCT . The volume fraction of retained austenite decreases from 17.2% （common heat treatment） to 5.5%-10. 5% after DCT. But the retained austenite is not completely eliminated. The film-like retained austenite distributes around the twin martensite and secondary carbides, which prevent the concentration of stress and generation of micro cracks, thus improves the comprehensive performance of SDC99.