蒸気タービン軸用Cr-Mo-V鋼の熱処理とクリープ破断強度について

Abstract

Cr-Mo-V steel is widely used for h-p and i-p steam turbine shafts among many high temperature low alloy steels. And it has been recognized by our creep rupture test results of a number of steam turbine shafts that the creep rupture properties of large-sized Cr-Mo-V steam turbine shafts cover a considerably wide range. It is considered that this variance of the creep rupture properties is caused by the variance of the steel making procedures in a limited sense, that is the imperceptible differences of the melting practice, chemical composition, and heat treatment. To make clear the roles of these variables, a test program has been putting forword. The effects of tempering temperature on the creep rupture strength of a Cr-Mo-V steel is presented here.A Cr-Mo-V steel was austenitized at 950°C for 2h and cooled in a furnace at a rate of 100°C/h, followed by the tempering at 650°C, 675°C and 700°C for 3h. The micro structures were tempered bainite containing a little fraction of ferrite precipitates instead of the fully bainitic structure of actual shafts. The experiments were consisted of the creep rupture test, tensile test and V-notch Charpy impact test for the estimation of the fracture transition temperature. The creep rupture test was conducted with plain and notched bar specimen and the test results were arranged with Larson-Miller's parameter. The tensile test was conducted at room temperature and 538°C.The experimental results may be summarized as follows:(1) As the tempering temperature raised, the creep rupture strength of the plain bar specimen decreased, and the difference between 675°C and 700°C tempering was larger as was expected from the tensile test results and the microstructures of the steels.(2) The creep rupture strength of the notched bar specimens were little changed with tempering temperature.(3) In this range of tempering temperature and rupture time, notch rupture strength ratios at 538°C were more than unity, but the more the tempering temperature decrease, the more the notch rupture strength ratio tends to decrease. This means that there is every indication which shows the marked notch sensitivity of the Cr-Mo-V steel when the tempering was conducted at the comparatively low temperature.(4) The ratio of the creep rupture strength of plain bar specimen to the room temperature ultimate tensile strength was nearly constant irrespective of the tempering temperature. The ratio to the 538°C ultimate tensile strength slightly increased as the tempering temperature lowered, and this may be attribute to the stability of structure during the comparatively shorter rupture time. The ratios of the notched bar strength to the ultimate tensile strength at room temperature and 538°C were both gradually decreased with the lowering of the tempering temperature, according to its tendency to notch sensitivity.