12%Cr鋼の低温靱性に及ぼすNiの影響

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Recently a new type of gas turbine-generator has been developed to utilize directly the expansion pressure of evaporating LNG. The material for the turbine blades should satisfy the below requirements.Tensile strength at R.T.>60kgf/mm2 FATT<-75°CAuthors tried to optimize Ni content in a candidate material, 0.1%C-12%Cr-1%Mo steel containing up to 5%Ni.Austenitizing temperatures were fixed at 1000°C for 0 to 2%Ni steels and at 950°C for 3 to 5%Ni steels respectively, which are 100 to 150°C higher than Ac3 for each. Tempering was carried out at 550 to 850°C.The results are as follows.(1) Tensile strength at R.T. is always more than 60kgf/mm2 for all steels with and without Ni.(2) Charpy V-notch impact values at R.T. show maxima when steels are tempered at temperatures 30°C higher than Ac1, which is lowered by Ni addition.(3) Ni lowers FATT significantly and steels containing 3% or more Ni possess FATT below-75°C.(4) The above effect of Ni is to be attributed to the microstructural change of steels with Ni contents. Firstly higher Ni steels show finer structures, which are segmented by reverse transformed austenite along inherent martensite lath boundaries during tempering at temperatures mentioned in(2). Secondly steels containing 4% or more Ni contain much amount of retained austenite, which retards the propagation of crack.
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