The effects of adding Cr, Ni, Mo, V, Si and Mn on the low-alloy heat-resisting Mo-V steel of the base component 0·15% C, 0·8% Mo and 0·3% V were systematically studied.
The results obtained are as follows.
(1) S-curve is affected by Mo more remarkably than by V. When Mo and V are added together, then the noses of A3 and A1 transformation curves appear at about 770 and 700°C respectively and the bainite transformation at below about 575°C. By increasing Si, Cr and Mn, the beginning of A3 transformation is retarded down to below about 700°C, and also that of bainite transformation down to below about 500°C, according to the decrease of temperature.
(2) By the X-ray analysis of the precipitated carbides extracted electrolytically from the isothermal transformed steels, M23C6, Fe3C and VC are found in Mo-V steel, but only Fe3C is found in plain Mo or V steel. No differences are found in the kinds of carbides of Mo-V steel even by changing Cr, Si, Mn etc.
(3) By adding Mo and V together, the hardness as quenched is far higher than by adding them independently of each other, and the hardness increases further by addin Cr, Mn, Si etc. respectively. These results are well understood according to the effects of the added elements on S-curves. The maximum Charpy impact value is obtained by tempering at temperatures between 730-750°C after quenching from 970°C.
(4) The tensile strengths of the Mo-V steels at room temperature are far higher than those of plain Mo or V steel and the ratios of the yield points to the tensile strengths are also high. As far as the variations of Cr, Si, Mn and Ni of the Mo-V steel are less than those described above, the required mechanical properties are obtainable by tempering at the adequate telmperatures between 730-750°C.
(5) By the results of creep-rupture tests of Mo-V steels at 500°C, the followings are confirmed; the effect of Si between 0·03-0·42% is not distinct; by adding Cr up to about 0·93%, the strengths are apt to decrease after long time; by those coataining about 0·45%Cr, the strengths are higher when Mn is lower; by those containing Cr and Ni up to the upper limit of specification the strengths are higher than those containing no Cr and no Ni.