研究了C和Nb含量对TP347HFG钢在650 ℃析出相参量(成分、体积分数和尺寸)和持久寿命的影响. 对ASME成分范围内的2种不同C和Nb含量的TP347HFG钢在650 ℃, 230和 150 MPa条件下进行持久实验, 持久寿命分别为199, 420 h和2426, 8837 h, 其中C含量较低Nb含量较高的样品持久寿命较长. 对持久管样的EPMA-EDS+MPSM 和TEM-EDS分析表明, 较低的C含量和较高的Nb含量对应较少的M23C6和较多的 MX, 并阻碍了M23C6的聚集粗化, 同时基体中可保留较多的Cr, 有利于延长持久寿命. 此外, 运用热力学软件Thermo-Calc分析了在500--1300 ℃范围内C和Nb组合含量的变化对各相成分和体积分数的影响, 与实验结果相吻合.
TP347HFG is an austenitic stainless steel which is considered to be among the grades with the highest potential for use in super critical boilers. It has been reported that micro-addition of Nb and a relatively low level of carbon content can obviously enhance creep resistance in this class of materials. Thus, C and Nb content optimization is of significant interest within the composition range from ASME standard. In this study, the effect of carbon and niobium contents on phase parameters (phase composition, volume fraction and size) and creep rupture time and the related mechanism were investigated for TP347HFG steel. Creep rupture tests were carried out under applied stresses of 230 and 150 MPa, respectively, at 650 ℃. The rupture times of two samples with different C and Nb contents were 199 and 420 h at 230 MPa, 2426 and 8837 h at 150 MPa, respectively. The sample with lower C content and higher Nb content corresponded to longer rupture time at each of the stress level. The EPMA-EDS+MPSM (multiphase separation method) and TEM-EDS results show that the creep rupture times were remarkably increased for the sample with relatively lower C and higher Nb contents, which correspond to increasing the volume fraction of nano-scale MX and decreasing that of M23C6 as well as retarding the coarsening of M23C6. On the other hand, more Cr maintained in matrix could also benefit the creep rupture lives. In addition, Thermo-Calc software was used to calculate the mole fraction and the concentration of precipitated phases with various combinations of C and Nb contents over the range of 500-1300℃ and the results were in good agreement with the experimental ones.
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