真空渗碳处理对Cr35Ni45Nb合金高温持久寿命的影响

Effects of vacuum carburizing treatment on high-temperature creep rupture life of Cr35Ni45Nb alloy

采用真空渗碳方式对服役2.5年的乙烯裂解炉管钢Cr35Ni45Nb进行了加速渗碳处理,并进行了一系列的高温(1080℃)持久试验;分析了合金在接近实际服役温度下的渗碳特性和持久试验中合金内氧化与蠕变交互作用所引起的材料组织转变、空洞萌生及裂纹形核扩展等规律,重点讨论了渗碳对于合金高温持久寿命的影响机理。结果表明:在高温应力及渗碳因素的叠加条件下,Cr35Ni45Nb炉管近表面碳化物-基体相界面发生内氧化,造成低应力下较易形成空洞和裂纹,而且,裂纹更容易在柱状晶区枝晶间萌生扩展。渗碳时间越久,渗碳层深度越大,Cr35Ni45Nb钢炉管的持久寿命越长。主要原因在于,渗碳造成枝晶间或晶界碳化物增多,降低了合金的蠕变速率,延长了持久寿命;同时,渗碳过程形成的相当于合金内部摩擦力的渗碳应力也部分抵消了外部应力的作用。此外,尽管渗碳强化Cr35Ni45Nb耐热钢,但降低了其高温塑性,同时也会与服役环境下温差热应力及热疲劳损伤等发生交互作用,导致实际服役炉管的早期失效。

Cr35Ni45Nb alloy serviced for 2. 5 years was carburized through vacuum carburizing treatment,followed by a series of hightemperature creep rupture tests at 1080 ℃ under different conditions. Systematically analyses were carried out on the carburization characteristics at the service temperature and the microstructure transformation,void initiation and crack propagation induced by the interaction between internal oxidation and high-temperature creep. Then effects of carburization on creep rupture life were discussed emphatically. The results show that the interdendritic or intergranular oxidation results in inclined formation of voids under low stress and accelerates crack initiation and propagation inward,while the propagating cracks improve the internal oxidation in return. In addition,cracks are inclined to initiate and propagate along the interdendritic boundaries. It is also shown that the longer the carburizing time or the deeper the carburizing depth,the longer the creep rupture life. Carburization can cause an increase of carbide content at both the intergranular and intragranular sites thus reducing the creep rate,increasing the anti-creep properties and finally prolonging the creep rupture life.Comprehensively,the internal reason for life increase due to carburization is that the carburizing stress equivalent to internal friction offset the effects of external stress. Although the carburization can strengthen the alloy,it can reduce the high-temperature plasticity and interact with thermal stress and thermal fatigue damage in service environment,which always can lead to premature failure of the tubes.