STRESS ACCELERATION FACTOR, Ω AND STRENGTHENING MECHANISMS CONTROLLING MECHANICAL PROPERTIES OF HIGH NITROGEN HEAT RESISTANT STEELS AT ELEVATED TEMPERATURES

强行引入氮元素可以改善耐热钢的力学性能，而对蠕变断裂应变没有任何不利影响，因此被认为是改善耐热钢的最后一种元素。然而，当氮与少量铌和/或钒一起添加时，可以看到氮的最有益效果。为了阐明氮和铌对蠕变特性的联合影响，本研究选择了aSUS310S和含有0.2质量% N和0.45质量% Nb的25Cr-20NiNbN钢。蠕变试验表明，25Cr-20NiNbN的蠕变寿命是SUS310S的100倍，尽管化学成分基本相同，只是添加了少量的这些元素。为了描述假想应变率对应力和温度的依赖关系，推导了这些钢的本构方程。虽然这些钢中的主要取代元素几乎不能与位错相互作用，但一些证据表明，在25cr - 20ninbn钢中，在移动位错周围形成了Cr-N团簇气氛。在25Cr-20NiNbN钢中，除聚类引起的固溶硬化外，析出硬化也起着重要作用。应力松弛和应力变化试验分析表明，有效应力占比约为16%，阈值应力约为190 MPa。利用透射电镜测得的颗粒半径分布数据，根据空洞强化理论计算得到了与实验相同量级的阈值应力。

Forcibly introduced nitrogen has been regarded as the last element for the improvement of heat resistant steels because it improves the mechanical properties without any adverse effect on creep rupture strain. However, the most beneficial effect of nitrogen can be seen when nitrogen is added with a small amount of niobium and/or vanadium. In order to elucidate the combined effect of nitrogen and niobium on creep characteristics, aSUS310S and a 25Cr-20NiNbN steel containing 0.2 mass % of N and 0.45 mass % of Nb were selected for the present study.Creep tests showed that the creep life of the 25Cr-20NiNbN was one hundred times longerthan that of the SUS310S notwithstanding that the chemical compositions were about the same except for a small amount of these additions. In order to describe the stress and temperature dependence of the imaginary strain rate, the constitutive equations for these steels were derived.Although the main substitutional elements in these steels can hardly interact with dislocations, some pieces of evidence were obtained that an atmosphere of Cr-N clusters was formed around moving dislocations in the25Cr-20NiNbN steel. Besides the solid solution hardening due to clustering, it was made clear that the precipitation hardening played an important role in the 25Cr-20NiNbN steel. The analysis of stress relaxation and stress change tests revealed that the fraction of effective stress was about 16% and the threshold stress was about 190 MPa. Using the data on the distribution of particle radius determined by TEM, the threshold stress of about the same magnitude of experimental one was obtained from calculation based on the void strengthening theory.