Influence of Microstructures on the Creep Rupture Strength of 2 1/4Cr-1 Mo Steel at 566°C (1050°F)

Abstract

The effect of heat treatment on the creep rupture strength of 21/4Cr-1 Mo steel is well known. In order to realize the extrapolation of long-term creep-rupture strength, the microstructure in variously heat-treated specimens before and after testing and their creep-rupture strengths were investigated. The heat treatments are of 2 series, that is‘as quenched’and‘as quench-tempered’with the following cooling rate, water quenching, air cooling, 600°C/hr, 100°C/hr and 15°C/hr. The tempering was made at 700°C (1292°F) for 5hr. Testing temperature was limited to 566°C (1050°F). The kinds, shapes, composition and phase change of the precipitated carbides were investigated by the electron diffraction method of the extracted replica, and by chemical analysis, of the electrolytically separated residue. The relation between these results and the rupture time was considered.(a) The precipitates obtained during heat treating were Fe3C, Mo2C, Cr7C3 and M23C6, among which acicular carbide of Mo2C were formed in every specimen, particularly in ferrite region. M23C6 was found in the bainite structure chiefly. Sometimes Fe3C and Cr7C3 coexisted.(b) Among the‘as-cooled’test pieces, the air-cooled of bainite structure showed the highest creep-rupture strength, decreasing with the decrease of cooling rate.In the test-pieces rapidly cooled such as water-quenching, air-cooling, and 100°C/hr, which showed large decreasing slopes in the stress-time rupture curve, Fe3C and Mo2C precipitated abruptly and grew up.(c) In the tempered pieces, the stress-time rupture curve showed almost the same slope, and the strength decreased with the decreasing of cooling rate. The estimated values for 104hr were 9.4kg/mm2 for the air-cooled and 7.6kg/mm2 for the slow-cooled at 15°C/hr.(d) Both the stress-time rupture curves for some of the‘as-cooled’and the‘cooled-and tempered’crossed at the long duration of almost 104hr. The rupture strengths over this cross point were of much interest from the extrapolation point of view.(e) The elongation of the ruptured test pieces, both the water-quenched and the air cooled, decreased rapidly with the increase in testing duration. This is presumed to be correlated with the precipitation at the grain boundary and the phase change in the steel. This is the example of strong creep resistance showing short rupture time by grain boundary precipitation.Though some of main factors have been made clear from this experiment, possible other factors should be investigated for complete understanding of high temperature properties.