Effects of impurity segregation to grain boundary on intergranular cracking in 2.25Cr-1W steel
Intergranular cracking behaviors in water quenched 2.25 Cr-1.5 W heat-resistant steels are understood in the light of the formation of grain boundary carbides and the impurities segregation to grain boundaries or carbide interface at the grain boundaries. Before stress rupture tests, all materials were water-quenched after holding at 1050 for 1 h. The microstructure showed a typical lath martensite. Heating rate was 1200 /h to a testing temperature and stress rupture test was performed without
... performed without any soaking. At a fixed temperature, time to failure increased as the applied stress decreased. Irrespective of the bulk contents of P, the fracture mode was intergranular except for a condition corresponding to a longer time to failure. From AES analyses, the crater interface where grain boundary carbides had existed showed a segregation concentration of P much higher than that at the carbide-free grain boundaries, resulting from the much higher interface energy. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. These results suggest that the intergranular cracking follows the sequence from the crater interface to the carbide-free grain boundaries.