抄録
Although silicon content has been limited to 1% in 17-4 PH steels, the present paper concluded that the addition of silicon up to 3% has good effects on improvement in various properties provided in combination with appropriate heat treatment. Phase changes during and after solidification of these steels were also made clear, the result being summarized as follows:
(1) Silicon tends to form the δ-phase. After the solidification, the γ-phase is increased with lowering temperatures down to about 1050°C, that is the appropriate solution-quenching temperature.
(2) As the silicon content is raised, progressive lowering of MS is found; the MS of the γ in δ plus γ duplex structure in the steel containing 3% Si, exists at 10°C, and the γ in the 3% Si steel tends to transform into a structure which consists only of bainite and martensite by air-cooling from 1050°C, and if, water-quenched, little martensitization occurs in the supercooled γ.
(3) When ordinary water-quenching and tempering are applied, 17-4 PH steel is toughened by a 2% Si-addition with out scarcely affecting the strength.
(4) To improve the mechanical properties of 17-4 PH steel containing 3% Si, it is effective to subzero-cool prior to tempering; and the lower the subzero-temperature and a longer the holding time at this temperature, is more effective for obtaining a steel superior to silicon-free steel in toughness as well as in strength. From this point of view, the steel is found to show a thermal behavior similar to that of 17-7 PH steel.
(5) Addition of silicon improves not only the corrosion resistance to both HCl and H2SO4, but also increases the resistance to stress-corrosion cracking.
(6) The fluidity of molten steels is increased by silicon addition, while the casting shrinkage becomes somewhat larger due to less martensitization at about room temperature.
