1. H. Khorsand, S. M. Habibi, M. Arjomandi, H. Kafash
INVESTIGATION OF MANUFACTURING CONDITIONS OF POWDER METALLURGY parts WITH CONTROL OF MICROSTRUCTURE AND MECHANICAL PROPERTIES
H. Khorsand, S. M. Habibi, M. Arjomandi, H. Kafash
Assistant professor, K. N. Toosi. University of Technology, Mechanical Department
M.Sc. of Department of Materials science and Eng., Shiraz University
MSc student, K. N. Toosi. University of Technology, and SAPCo
- MSc student, K. N. Toosi. University of Technology
Department of Mechanical Engi Science & Technology, Narmak, , Tehran, Iran
Abstract
In the present work, two kinds of steel powder were (Distaloy HP-1 & Ultrapac-LA) selected and subjected to powder metallurgy processing. For Ultrapac-LA a heterogeneous microstructure consisting of tempered martensite, nickel-rich ferrite, divorced pearlite and nickel-rich regions surrounding pores is observed. For Distaloy HP-1, in slow cooling rates (0.4 °C.s-1) the amounts of martensite and bainite varies between 50-60% and 30-40% , however, for fast cooling rates (1.2 °C.s-1) the variation is between 70-80% and 10-20% . The mechanical properties of the prepared samples were studied with controlled production conditions such as cooling rate and heat treatment. The results showed that in this steel, increasing porosity plays an important role in the tensile stress and fatigue strength. The ultimate tensile strength and fatigue strength were more than 1000 MPa and 400 MPa in this research (for Distaloy HP-1, 7.2 g.cm-3, 0.5% carbon content and fast cooling rates). Macroscopic examination of the fracture surfaces for all specimens revealed that fatigue crack growth and final fracture regions were brittle and without noticeable permanent deformation.
Conclusion
In these steels, by increasing of graphite addition (from 0.2% to 0.5%) and increasing of density (from 6.6 g/cm3 to 7.2 g/cm3), apparent hardness, ultimate tensile stress and yield stress improve and elongation decreases. By increasing of cooling rate after sintering process, and changing of microstructure (increasing of martensite percent) the increase of apparent hardness and tensile were seen. The mechanical properties of the P materials are superior to The U materials evaluated.
Effect of heat treatment on the mechanical properties which have higher density (about 7.2 g/cm3) is more and with heat treatment of samples that have 0.5%C and 7.2 g/cm3 densities, we acquire 731 MPa for ultimate tensile stress, 246 MPa for fatigue strength and 324 HV5 for apparent hardness.
References
Introduction
Powder metallurgy has a strong growing market. Since most of the parts made by P/M are aimed for the automotive industry, cost is a major driving force. In development work for new grades, not only cost but also performance has to be taken into account. Powder metallurgy process of sintered steels was studied frequently. The mechanical properties of P/M steels are related directly to their microstructure and the level of porosity [1-4]. In addition, the alloying mode has a significant effect on the microstructure and pore morphology in sintered steels. There are four major and distinct alloying techniques used for formulating P/M steels and these establish a classification for the steels: admixed, diffusion alloyed, prealloyed and hybrid [5].
Material and Method
Chemical composition and physical properties of the investigated materials are shown in table 1. These powders were mixed with 0.8 zinc stearate and various graphite contents 0.2 and 0.5%. Fatigue and tensile samples were compacted at ambient temperature at various pressures according to ISO 3928(1977) and 2740(1986) standard. Specimens were sintered in a 75 H2 -25 N2 atmosphere for 40 minutes at 1120°C. A higher density of 7.2 g/cm3 was obtained by double pressing and double sintering. The specimens were cooled at either normal (0.4ºC.s-1) or fast (1.1ºC.s-1) cooling rate.
Results and Discussion
During sintering a variety of phases are formed due to localized variations in composition. The microstructures in the as-sintered condition of the U and the P materials with 0.2 and 0.5% graphite are shown in Figures 1 and 2. Figures 1 shows, a heterogeneous microstructure consisting of tempered martensite, nickel-rich ferrite, divorced pearlite ("divorced" since it is not lamellar, as in conventional pearlite), and nickel-rich regions surrounding pores is observed.
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[3] M.Habibi, H.Khorsand, M. Ashtari & K. Jonghorban, “Fatigue Behavior of PM Steel ULTRAPAC-LA”, PM02 Conf., India, New Delhi, January (2002)
[4] H.Khorsand, M.Habibi, M. Ashtari & H.Yoozbashizadeh, “The Role of Heat Treatment on Wear_Behavior of Powder Metallurgy Low Alloy Steels”, PM02 Conf., India, New Delhi, January (2002)