Effect of fine structure on mechanical properties of hot-forged powder steels

Abstract

Comparative analysis of the substructural parameters and mechanical properties of hot-forged steels reveals regular changes in the mechanical characteristics depending on the degree of imperfection. It is shown that higher imperfection at lower forging temperatures of porous billets can increase the strain hardening of particles. The maximum strain hardening of carbon steels is revealed at the maximal structural imperfection and a density of 7.77 g/cm 3 after forging a billet heated to 1100ºC. The high strain hardening of chromium steel results from high dispersion of coherent scattering regions.Applications of powder metallurgy are expanded depending on the quality and stability of material properties, which are determined by the structure formed.The friction of a powder against walls of the die mold and the friction among particles lead to uneven distribution of density across the briquette. In hot forging of porous billets, the different compaction over their volume and planes diversely oriented toward the applied force predetermines the different level and extent of particulate deformation and the number of interparticle bonds in different cross-sections. The structure-formation processes contribute to the hardening of the material sintered, along with decreasing porosity [1]. The papers [2-4] employ an xray structural analysis to examine changes in the intragrain structure of ferrite at the periphery and internal areas of hotforged samples of powder iron, carbon and chromium steels, depending on the heating temperature before forging and material structure.The objective of this paper is to examine the relationship between nonuniform mechanical properties and fine structural parameters at the periphery and internal areas of hot-forged materials depending on their composition and heating temperature before forging.In hot forging (HF), the time it takes to bring a heated billet from the furnace to the die should be monitored. When a billet leaves the furnace, its temperature decreases and, therefore, deformation occurs at lower temperatures than that of the billet itself, especially on the surface. Depending on the deformation temperature, crystal imperfections may be either healed or may remain and affect structure formation. The billets are hot compacted in a relatively cold matrix, where they are cooled down rapidly.The heat transfer at the periphery of the samples in contact with the die walls is so great that it leads to a substantial temperature gradient across the billet, which is due to the low conductivity of a porous material [5]. In our experiments, the samples were heated to 1100°C in a furnace with a protective environment (argon) before forging. The period during which the billet was held outside the heating area varied from 4 to 20 sec.The paper [2] shows that there is much lower structural imperfection in surface areas, near the punch-compact interface, as compared with the internal volume depending on the heating temperature before forging. Nonmonotonic dependence of the structural imperfection...

Effect of the Deformation Scheme on the Structure and Properties of Hot-Forged Aluminum-Matrix Composites

“…Produced by hot forging, materials possess not only extreme density, but also other specific characteristics, which distinguish them from materials produced by liquid-phase sintering. One of these characteristics is the hereditary fine granulation of material [10][11][12].…”

“…Appearance of billets for HFPB At the same time, the application of schemes with a developed area of shear deformations allows obtaining materials with more dispersed structure, increased strength and ductility. This is due to (i) the growth in the number of areas of fracture of the oxide films at the inter-particle surfaces and (ii) intensification of inter-grain diffusion, accompanied with increase in shear deformations [12][13][14][15].…”

Flow and Densification Behaviors of Sintered P/F-10C50 Steel under Hot Compression

“…As one of the important near net forming technology used to manufacture high strength powder metallurgy parts substituting the conventional casting-and-forging ones, powder forging has been wildly applied [1,2]. It comprises the preparation of preform through conventional pressing and sintering processes and a subsequent closed-die forging in order to obtain the desired final shape and closed full density [3]. In this process, the complex flow and densification processes of sintered material are both significantly influenced by many factors such as strain, strain rate, and forming temperature and should be clarified and precisely controlled [4][5][6].…”