OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 6 2 4 Substituting expression (7) into formula (6), we find: ( ) ( ) 3 1 1 3 s c s D d p h − = σ + ⋅ µ ⋅ σ ⋅ − Θ ⋅ (9) Solving dependencies (8) and (5) together, we determined the work of active forces: ( ) ( ) 3 2 2 1 ( ) ( ) 1 4 3 a s c s D d A D d h h π − = − σ + µ σ − Θ ∆ . (10) Yield strength σs of sintered corrosion-resistant steel depends on many factors: chemical composition, structure, porosity, concentration and configuration of foreign inclusions, etc. Various formulas are used to evaluate the effect of porosity on the yield strength of sintered structural materials. In particular, the following expression was proposed in [18]: ( )2 2 1 4 3 s so − Θ σ = σ − Θ , (11) where σsо is the yield strength of compact material. For compact chromium-nickel steels of the austenitic class offset yield strength is (σ0.2) is 250–450 MPa. Therefore, σs of sintered steels, the composition of which is indicated in Table 1, were determined experimentally according to GOST 1497–84, using prismatic tensile test specimens. Table 2 shows some mechanical properties and porosity of prismatic specimens after sintering at 1,150 ºC, 1.5 hours. The ultimate strength of specimens sintered in dissociated ammonia is very low, because even in the case of sintering in the backfill chromium is intensively oxidized, especially at the boundaries of powder particles, due to the interaction not only with oxygen contained in the protective medium, but also with oxygen slammed in the pores of the blank. Ta b l e 2 Physical and mechanical properties of chromium-nickel sintered corrosion-resistant steels Sintering medium Dissociated ammonia Vacuum Backfill SiO2 Backfill SiO2+Al2O3 Powder grade 0.12C-18Cr10Ni-Ti 304L–AW–100 18Cr-4Ni-3Cu 0.12C-18Cr- 10Ni-Ti 304L–AW–100 18Cr-4Ni-3Cu σu, MPa 29.7 45.59 45.10 243.59 237.84 144.15 δ, % 0.13 0.11 0.60 7.84 8.89 0.69 Ψ, % 0.00 0.00 0.00 7.85 12.96 0.57 П, % 32.05 27.61 30.12 25.24 19.25 27.32 ρ, g/сm3 5.58 6.21 6.09 6.32 6.64 6.22 HRB 70.4 90.4 67 74.2 59.1 74.1 In the specimens obtained from a mixture of ferrochrome, iron powders with copper and nickel additives, even after sintering in vacuum, the relative elongation δ and relative contraction Ψ do not exceed 1 % (Table 2). Therefore, in calculations, the yield strength of all steels under study, in which δ and Ψ are less than 1–2 %, was assumed to be equal to the strength limit. Since the plastic properties of steels are influenced not only by chemical composition and structure, but also by the stress-strain state, radial settling of sintered ring specimens with residual porosity of 14–25 % was carried out to evaluate σs. It was found that in the compression zones the relative density during upsetting increases to 0.95–0.97 and cracks appear only in the tensile zones. Taking this into account, σs = 200 MPa was taken in the calculations.
RkJQdWJsaXNoZXIy MTk0ODM1