Structural features of powder materials operating under high temperatures, stresses, and wear

Abstract

It is known that parts of hot zones of metallurgical plants as well as heavily loaded pressing and forging tools operate under a high temperature, elevated mechanical stresses, the action of a steam ambient, slag, surface wear due to contact with the hot metal, etc. The possibility of providing the requisite properties by using high-temperature alloys has already been exhausted because the complexity of their composition intensifies the segregation inhomogeneity of the ingots which cannot be eliminated in the process of hot deformation and homogenizing annealing. As a rule, fracture starts under the conditions of cyclic loads on local structural defects such as accumulations of coarse particles of the eutectic "/'-phase, borides, carbides, nonmetallic inclusions. The chemical inhomogeneity and the structural defects caused by it can be removed by using methods of powder metallurgy that can provide materials with unique properties unattainable in the conventional metallurgy. The present paper is devoted to the special features of the structure of powder nickel alloys.The institute of powder metallurgy has developed powder materials based on nickel alloys and containing up to 50% Co, Cr, Mo, W, Ti, Nb, AI and up to 10% refractory thermally stable compounds (oxides, carbides, nitrides, carbonitrides). They possess high strength, corrosion and wear resistances at 800-I100~ at a satisfactory plasticity and machinability, and diminished capacity for crack formation. The materials are to be used under considerable temperature and stress drops, abrasive wear, and the action of aggressive media (water, slag, sand).The aim of the present research was to study the structure of powder materials in different stages of the manufacturing process (preparation of the powder, plasma facing, extrusion compaction, heat treatment), and after operation. The powders have been fabricated by three methods (Fig. 1). The powder obtained by calcium-hydride reduction was characterized by a mean panicle size of 80 pm and a bulk density of 1.89 g/cm 3 and contained at most 0.3% O~ and 0.2% Ca. An x-ray dillfaction analysis of the powder showed that it contamed a y-solid solution (a-0.252 nm) and a 7'-phase (a = 0.358 nm) and very weak lines of CaO Id 0.252 nm) and ot-Fe (a = 0.2885 nm). The pani-