研究了一种新型25Cr-20Ni奥氏体耐热不锈钢在750 ℃不同拉应力持久实验中析出相演变及其对性能的影响。结果表明,当持久应力为180 MPa时,持久寿命为32.6 h,析出相包括M₂₃C₆和(Nb, V)(C, N)相。其中M₂₃C₆主要分布在晶界位置,(Nb, V)(C, N)相在晶内弥散析出。当持久应力为150和120 MPa时,随着持久时间延长至98.1 h以上,晶界位置的M₂₃C₆发生长大和Ostwald熟化,但(Nb, V)(C, N)相具有较好的尺寸稳定性。同时,在持久应力为120 MPa条件下,组织中出现σ相。σ相首先在晶界位置析出,当持久应力为100 MPa时,随持久时间延长至752.3 h,σ相也会在晶内析出。研究还发现,大量σ相依附于(Nb, V)(C, N)相析出,这是由于(Nb, V)(C, N)相的析出导致附近奥氏体基体中局部位置C和N元素含量减少,从而促进了σ相的形核。不同应力条件下试样断裂方式均为沿晶断裂,当持久时间较短时,裂纹在晶界M₂₃C₆处产生,引起沿晶开裂。随着持久时间延长,σ相在晶界析出后,裂纹更容易在晶界σ相处产生,导致持久延伸率随持久寿命延长而减小。

25Cr-20Ni austenitic heat resistant stainless steels are widely used as structural materials in nuclear industries and power plants for their excellent corrosion resistance and creep properties at elevated temperature. It is generally accepted that the precipitation during creep is a key factor influencing the creep properties. However, the evolution of precipitates is complicated due to the interaction of the alloy elements. To investigate the precipitation behaviors, a modified 25Cr-20Ni austenitic heat resistant stainless steel has been crept at 750 ℃ under different stresses varying from 100 MPa to 180 MPa. The microstructure observation indicates that M₂₃C₆ and (Nb, V)(C, N) precipitates are formed during 32.6 h creeping deformation under 180 MPa. M₂₃C₆ precipitates are mainly generated at grain boundaries and (Nb, V)(C, N) particles are dispersively distributed in austenitic matrix. The grain boundary M₂₃C₆ carbides are significantly coarsened and Ostwald ripening process happens during 98.1 h creeping deformation under the stress of 150 MPa and 353.0 h creeping deformation under stress of 120 MPa, while (Nb, V)(C, N) carbonitrides show high dimensional stability. With the creep rupture time further prolonging to 353.0 h and 752.3 h under the creep stress of 120 and 100 MPa, respectively, σ-phases are generated first at grain boundaries and then at inner grains. Meanwhile, large amounts of σ-phases are formed around (Nb, V)(C, N) particles, indicating the σ-phase precipitation is accelerated by (Nb, V)(C, N) carbonitrides. Composition analysis and thermodynamic calculation are subsequently performed to elucidate the precipitation mechanism of σ-phase. Carbon and nitrogen depleted zone is detected at the interface between (Nb, V)(C, N) precipitates and austenitic matrix. A correlation between σ-phase and C/N contents has been calculated by Thermo-Calc, which shows that the mass fraction of σ-phase increases with the decreasing C/N contents. According to the thermodynamic calculations and experimental results, it is reasonably inferred that the formation of σ-phase is induced by the carbon and nitrogen depletion in austenitic matrix. Additionally, the fracture surfaces of creep specimens show intergranular fracture under all creep stresses. When the creep time is comparatively short, cracks are inclined to propagate along grain boundaries owing to the low cohesion between grain boundary M₂₃C₆ precipitates and austenitic matrix, resulting in intergranular creep fracture. With the precipitation of σ-phase at grain boundaries after long time creep, the cracks are primarily generated from σ-phase, further deteriorating the creep elongation.