Effect of Chromium and Molybdenum on the Temper Brittleness of Ni-Cr-Mo-V Steel for Large Turbine Rotor Shaft Forgings

Abstract

The effects of chromium and molybdenum contents on the temper brittleness of Ni-Cr-Mo-V steel, used for turbine rotor shaft forgings, have been investigated. The steels alloyed with 0.52-1.73%Cr and 0.3-0.6%Mo were subjected to embrittling treatment at 500°C for 100hr. The degree of temper brittleness was estimated and compared by the 50% fracture appearance transition temperature (50% FATT) obtained by Charpy impact test (2mm V motch Charpy), and the fracture appearance of each test piece was observed by scanning electron microscope.
The results are as follows:
(1) The degree of temper brittleness increases with the content of chromium and molybdenum.
(2) The embrittled steels have a tendency increasing the ratio of intergranular fracture, however, there is no clear quantitative relationship between the degree of embrittlement and the ratio of intergranular fracture. This tendency is greatly influenced by the austenite grain size, and fine grain steel shows considerable low values in the ratio of the intergranular fracture, even in embrittled speciments.