M_s以下温度等温淬火对中碳贝氏体钢相变和性能影响

Effects of below-M_s austempering temperature on baintie transformation and properties of a medium-carbon bainitic steel

设计了马氏体起始相变温度(M_s)以上和以下2个不同温度等温淬火试验,结合热膨胀仪、扫描电镜显微组织、X光衍射和拉伸试验等试验手段,研究了对比于M_s以上温度等温淬火试验,M_s以下等温淬火对中碳贝氏体钢相变、组织和性能的影响。结果表明,贝氏体相变可以发生在M_s温度以下,且其相变动力学被明显促进。相比于M_s以上温度等温淬火,M_s温度以下等温淬火虽然可以加速相变动力学,但导致强度和伸长率下降,因此降低了最终的力学性能。这主要是因为M_s温度以下等温淬火试样组织内部出现了大量的回火无热马氏体(AM)和少量的贝氏体和残余奥氏体(RA)。因此,M_s温度以下等温淬火热处理后的组织性能未必优于M_s温度以上等温处理后组织性能,这主要取决于具体的成分和工艺。

Two austempering experiments with different temperatures above or below martensite starting temperature (Ms) were designed to explore the effects of austempering temperature on bainitie transformation, microstructure and property of a medium-carbon bainitic steel by thermal-dilatometer, scanning electron microscopy, X-ray diffraction and tensile tests. As the results show, bainite transformation was observed at the temperature below Ms as well, which accelerated the transformation kinetic obviously. However, compared to that above Ms, the austempering treatment below Ms deteriorated the strength and elongation, resulting in the decrease of the final mechanical properties. It is mainly attributed to the appearance of tempering athermal martensite (AM), a little amount of bainite and retained austenite (RA) below Ms. Therefore, whether the austempering treatment below Ms could improve the microstructure and property of bainitic steel depended on the composition and heat treatment.