High-Nickel Maraging Steel by Powder Metallurgy (Part III)
On the Sintered Steel with Reduced Concentration of Alloying Elements
1967 Volume 14 Issue 5 Pages 213-220
Details
1967 Volume 14 Issue 5 Pages 213-220
The sintering and age-hardening-characteristics of 18 Ni-7Co-5Mo sintered steel were reported before. In the present paper, a composition of 78Fe-12Ni-5Co-5Mo was chosen for examining the structural and mechanical properties of the sintered steel which was reduced effectively in the amount of alloying elements. Specimens were prepared by mixing reduced iron powder with powders of alloying element, compacted mainly at 6t/cm2 and sintered in hydrogen for lhr at 1300°C. Subsequently, the sintered compacts were aged for 5hrs at 500°C. The results are summarized as follows ;
1) The variations of concentration of alloying elements in the sintered steel were determined by electron-probe' microanalyses. The concentrations were 8-12% for nickel, 4-6% for cabalt and for molybdenum. From these results, therefore, the structural characteristics of the sintered compact may be estimated as follows ; Ms-temperature ; 465-260°C, age-hardenability ; 3-14 HRc and tensile strength after aging; 80-150kg/mm2.
2) The mechanical properties of sintered steel are mostly determined by chemical composition and also by relative porosity. An empirical relationship is established between the porosity and yield strength of sintered steel, given by; Y. Ss/Y. Sm=1-1.58ε2/3, Where, Y. Ss and Y. Sm are the yield strengths of the sintered and forged steel respectively, and & is relative porosity. This relationship shows good agreement among the results obtained with 12Ni-5Co-5Mo and 18Ni-7Co-5Mo steels. The transverse-rupture strength, however, is markedly affected by the stress concentration generated by pores. Consequently, the ratio of the strength of the sintered steel to that of the forged steel is lower than the ratio of tensile property.
3) Addition of copper to sintered steel resulted in higher tensile strength, but in lower transverse-rupture strength, which might be due to the improved age-hardenability in the martensitic matrix. Attempts to obtain compacts with higher density by adding phosphor and boron has been unsuccessful, probably due to the extremely brittle nature of the sintered compacts.
