650 ℃时效SP2215奥氏体耐热钢管的微观组织和力学性能

Microstructure and Mechanical Properties of SP2215 Austenitic Heat-resistant Steel Tube after Aging at 650 ℃

摘要:

通过OM、SEM、TEM和显微硬度、室温冲击以及高温拉伸试验，研究了650 ℃时效不同时间后SP2215奥氏体耐热钢管的微观组织、力学性能及断裂机理。结果表明，固溶态SP2215钢微观组织由奥氏体，少量孪晶及未溶NbN和Z相组成；650 ℃时效时Cr23C6优先在奥氏体晶界析出，且随时效时间延长逐渐增多、粗化并形成连续网状；球形富Cu相在奥氏体晶内析出且尺寸稳定，时效2012 h约为15 nm；SP2215钢晶内室温显微硬度在时效50 h时基本达到最大值，之后趋于稳定，这与稳定富Cu相的析出强化作用有关；SP2215钢具有明显的高温时效脆化倾向，时效2012 h室温冲击吸收功较固溶态降低约78.5%，室温冲击断裂随时效时间延长由韧性断裂向沿晶脆性断裂转变，这是由Cr23C6在奥氏体晶界析出并逐渐聚集、粗化导致的。SP2215钢在650 ℃，2.5×10-4 s-1应变速率下拉伸时出现锯齿流变（Portevin-Le Chatelier, PLC）现象，锯齿类型为Type(A+B)，随时效时间延长PLC逐渐“减弱”，但锯齿类型不变；随时效时间延长SP2215钢高温拉伸屈服强度基本保持稳定，抗拉强度和断面收缩率逐渐降低，高温拉伸断裂由韧性断裂向准解理脆性断裂转变。

Abstract:

The microstructure, mechanical properties and fracture mechanism of SP2215 austenitic heat-resistant steel tube after aging at 650 ℃ for different times were studied by OM, SEM, TEM, microhardness, room temperature impact and high temperature tensile tests. The results show that the microstructure of solid solution treated SP2215 steel is composed of austenite, a small amount of twins and undissolved NbN and Z phases. Cr23C6 preferentially precipitates at austenite grain boundaries during aging at 650 ℃, and gradually increases, coarsens and forms a continuous network with the extension of aging time. The spherical Cu-rich phase is precipitated in the austenite grains, when the aging time reaching 2012 h, the size of Cu-rich phase is about 15 nm. The room temperature microhardness in grains of SP2215 steel reaches the maximum value after aging for 50 h, and then tends to be stable, which is related to the precipitation strengthening effect of stable Cu-rich phase. SP2215 steel has obvious tendency of high temperature aging embrittlement. The room temperature impact absorption energy of sample aging for 2012 h is about 78.5 % lower than that of solid solution treated sample. The impact fracture mechanism at room temperature changes from ductile fracture to intergranular brittle fracture with the extension of aging time, which is caused by the precipitation, aggregation and coarsening of Cr23C6 at austenite grain boundary. Portevin-Le Chatelier (PLC) appears when SP2215 steel was stretched with the strain rate of 2.5×10-4 s-1 at 650 ℃, and the serrated type is Type ( A + B ). With the increase of aging time, the PLC gradually “weakens”, but the serrated type remains unchanged. With the extension of aging time, the high temperature tensile yield strength of SP2215 steel remains basically stable, the tensile strength and reduction of area gradually decrease, and the high temperature tensile fracture mechanism changes from ductile fracture to quasi cleavage brittle fracture.