Structure formation processes in sintering of stainless-steel-base heterophase materials 1. Sintering of austenitic stainless-steel-base materials with additions of Cr3C2

Abstract

Earlier [1-3], a series of basic and applied investigations of the processes of pressing, sintering, and hot compacting of chromium and chromium-nickel stainless steel powders were presented. These works are devoted primarily to powdermetallurgy steels with a single-phase homogeneous structure basically not differing from the structure of the stainless steels produced by traditional methods. Of interest is development of stainless-steel-base materials with a heterophase structure. Together with good corrosion resistance such materials may possess such specific properties as increased wear resistance. This is obtained by addition to the metallic matrix of various additions such as carbides, intermetallides, nitrides, borides, sulfides, etc. However, certain additions reduce the corrosion and certain mechanical properties of stainless steels, particularly plasticity. In order to provide the optimum combination of properties of such materials it is necessary to optimize their composition and structure. In this work the structure and properties of austenitic chromium-nickel stainless-steel-base materials with additions of chromium carbide Cr3C 2 in a quantity of up to 10% were studied. The powders of types Khl8N12, Khl8N15, Khl8N12, Khl8N12M2, and K.h23N18 stainless steels were prepared by atomization, calcium hydride reduction, and diffusion impregnation [4]. The specimens for tensile, bend, and impact mechanical tests were prepared by a single pressing and sintering in hydrogen and vacuum at 1200~ After sintering the microstructure was investigated and the density and hardness of the specimens and the microhardness of the structural constituents were determined. In addition, selective micro-x-ray spectral and x-ray structural analyses were made for the purpose of studying the composition of the matrix and the carbide inclusions and also of establishing the crystalline lattice parameters of the base. In order to study the influence of the degree of dispersion of the chromium carbide additions on the character of their interaction with the matrix a preliminary investigation including use of types Khl8N15, Kh18N12, and Khl8N12M5 stainless steel powders obtained by the above methods was made. Monodispersed chromium carbide powder with average particle sizes of 30, 80 130, and 200 #m (sifted through -0056, -0100 + 0063, -0160 + 0100, and -0250 + 0160 screens) was used. In all cases the quantity of Cr3C 2 was 10 wt. %. The specimens were sintered in vacuum at 1200~ It was established that the hardness and microhardness of the base of the specimens containing carbides are higher and their density somewhat lower than of the standard specimens. At the same time the microhardness of the carbides themselves also drops significantly as the result of sintering (Fig. 1). With a decrease in carbide inclusion size, all of these rules are strengthened, which is an indication of the more active interaction of the more dispersed carbides with the base. The nature of the powders of the base has significantly less influence on the character of its interaction with the carbides than their degree of dispersion. The more active interaction with the metallic matrix of specimens of atomized type Khl8N12M2 powder is confirmed by the higher absolute and relative increase in microhardness of the base of the these specimens in compared with those prepared from Khl8N12 and Kh18N15 powders prepared by methods of diffusion impregnation and calcium hydride reduction (Figs. 1, 2). The increase by several percent in the chromium content in the metallic matrix and the increase in its crystallin lattice parameters in comparison with the standard specimens simultaneously established by methods of micro-x-ray spectral and xray structural analyses (Table I) are apparently caused by diffusion of chromium and carbon from the carbides into the base. The greatest change in the crystalline lattice parameters of the base is observed with use of dispersed additions with an average