Effect of characteristics of the carbon-bearing components of the charge on the structure and properties of hot-worked powder steels. II. Effect of processing factors on the structure and properties of hot-worked powder steels

Abstract

The ultimate goal of hot-forging is to impart the necessary structure and properties to the finished material. Thus, the relative importance of the characteristics of the semifinished product that change during the different stages in tlie production of a hot-worked powder material (HWPM) (stages such as sintering) is determined by the degree to which they affect the course of consolidation, deformation, and particle growth during forging. Studies were conducted on prismatic specimens with the dimensions 55 x 10 x 10 mm. The specimens were obtained by hot-forging of porous compacts. Some of the compacts had been sintered (1100°C, 1 h), while others had not. They were heated in a medium of dried, dissociated ammonia. We studied the effect of the porosity of the compact, the characteristics of the components of the charge, and hot-forging temperature on the structure, mechanical properties, and friction properties of the HWPM. The effect of the porosityof the compact, varied within the range 10-60%, was studied with a forging temperature of 1100°C and compaction work equal to 250 MJ/m 3. The relative density of the specimens was 98-99.5 %. Figure 1 shows the dependence of the average (for 3-10 specimens) mechanical properties after forging on the initial porosity of the compacts II. It was found that a decrease in initial porosity was accompanied by an increase in all of the characteristics examined. A porosity of 15-20% is optimum, since the small improvement in properties that could be gained by a further reduction in porosity would require excessive pressing pressure and accelerate tool wear. The pattern of change in HWPM properties described above is related to the fact that the dissolution of carbon in the iron matrix, closing of the pores, the degree of uniformity of the deformation of the material, and the uniformity of its structure all depend on the porosity of the compact. When II = 30-60%, carbon dissolves unevenly, and coarse-grained austenite is formed in the regions still containing undissolved particles of graphite. During compaction, forging, and subsequent cooling, nonuniform stresses develop and coarse-grained pearlite is formed, which lowers the properties of the material. The high solubility of special low-ash synthetic graphite (SLSG) and pyrolytic carbon (PC) compared to pencil-grade graphite (PG), reported in [ 1 ], explains the superior properties of the hot-worked powder steels obtained with these materials and corroborates the experimental data given above. The substantial variations in the characteristics of the iron powders used to produce HWPMs makes it necessary to conduct a systematic evaluation of the effect of the main components of the charge. We used reduced (PZhV 2.160.26; PZhV 4.450.26) and atomized (PZhR 2.200.28) iron powders produced by the Sulinsk Metallurgical Plant (GOST 9849--86). The higher mechanical properties of the hot-worked materials obtained with the use of powder PZhV 2.160.26 can be attributed to its higher purity compared to PZhV 4.450.26, as well as to the more developed and less oxidized (compared to the atomized powder) particle surface (Table 1). omission The ultimate tensile strength a B of the specimens containing SLSG (such as materials based on PZhV 2.160.26) was 1.54-1.59 times higher than that of the specimens containing PG, while hardness was 1.12-1.28 times greater. The most likely reason for the difference is the lower level of contamination of SLSG and its higher chemical activity. This difference is less