以新设计的4种不同W含量(0~4.87%,质量分数)的Nb稳定化奥氏体耐热铸钢为研究对象,通过在1000 ℃、50 MPa条件下的蠕变性能测试和蠕变前后的组织分析,研究了W对奥氏体耐热铸钢1000 ℃蠕变行为的影响机理。结果表明：合金的最小蠕变速率随W含量的增加先降低后增高。合金的主要析出相为Nb(C, N)和富Cr相。W含量的增加对Nb(C, N)的影响很小,但明显促进富Cr碳化物析出,并促使(Cr, Fe, W)₇C₃转变为(Cr, Fe, W)₂₃C₆。而且,过量的W添加会促进金属间化合物χ相析出。在蠕变过程中,合金的基体还会二次析出纳米级的Nb(C, N),阻碍位错运动,从而进一步提高合金的蠕变强度。但是,富Cr相胞团,尤其是胞团内的χ相,会促进蠕变裂纹形核长大,从而提高合金的蠕变速率,降低蠕变寿命。

In order to comply with more stringent emissions and fuel economy regulations worldwide, the operation temperature of exhaust components for automotive gasoline engines is now reaching to as high as 1000 ℃, about 200 ℃ higher than the conventional standard. As a result, the incumbent materials for exhaust manifolds and turbine housings are being pushed beyond their high-temperature strength and oxidation limitations. Therefore, there is an urgent demand from automotive industries to develop novel and cost-effective alloys those durable against these increased temperatures. In this work, the effect of W additions on the creep behavior of a series of Nb-bearing austenitic heat-resistant cast steels is investigated at 1000 ℃ and 50 MPa. Microstructures before and after creep rupture tests are carefully characterized to investigate the microstructural evolution during creep deformation. The minimum creep rate of these alloys shows a trend from decline to rise as the W addition is increased. Microstructural analyses reveal that the W addition does not affect the formation of primary Nb(C, N), whereas significantly improves the precipitation of Cr-rich carbides, as well as accelerating the phase transformation from (Cr, Fe, W)₇C₃ to (Cr, Fe, W)₂₃C₆. Moreover, the excessive addition of W leads to the formation of the interme tallic χ-phase. During creep deformation, the secondary precipitation of nano-scale Nb(C, N) also aids in the strengthening of the creep resistance through pinning the dislocations. However, the cellular Cr-rich phase that contains χ-phase significantly accelerates the nucleation and propagation of creep cracks, thereby increasing the creep rate and decreasing the creep life.