Investigation of mechanical and microstructural properties of austempered steel powder metal

It is known that tensile strength and hardness of powder metallurgy steels are enhanced by adding alloying elements like chromium, molybdenum, copper, nickel. The impact toughness of powder metal steels is commonly low when the strength is increased by adding alloying elements. Besides, internal porosities in the structure of powder metallurgy steels detriment the properties. Austempering known as bainitic quenching heat treatment provides optimum strength and toughness. In practice, this heat treatment cycles producing less distortion and quenching-cracking can alternatively be used instead of quenching plus tempering heat treatment. Also, total heat treatment period can be shorter in the austempering treatment in comparison to quenching plus tempering. Fort his reason, austempering heat treatment process on microstructure and mechanical properties were studied in some alloyed powder metallurgy steels.In this study, with austempering heat treatment, it was intended to produce a bainitic structure in powder metallurgy steels. The effects of various alloying elements on microstructure were also examined. Four different specimen groups of powder metallurgy steels with different chemical composition prepared by conventional powder metallurgy production route. Fort he first group, atomized iron powder (Ancorsteel 1000) was mixed with 0.8 wt% graphite powder, for the second one atomized iron powder was mixed with 0.8 wt% graphite and 3 wt% nickel powders, for the third group iron powder with 0.8 wt% graphite and 3 wt% copper powder were prepared and finally fort he last group of specimens 0,2 graphite+1.75Ni+1.5Cu+0.5Mo (in wt%) pre-alloyed iron based powders were prepared. The mixed powders were cold pressed at 700 MPa with single action and sintered at 1150 °C for 20 min under pure argon gas atmosphere. Pre-alloyed sintered specimens were annealed at 825 °C for 6 minutes and quenched in salt bath at 450 °C for up to 120 minutes. The other sintered specimens were annealed at 850 °C for 6 minutes and quenched in salt bath at 350 °C for up to 120 minutes.The experimental results showed that mainly bainitic and partially martensitic structures formed at short austempering time. However, largely ferritic areas besides bainite and martensite achieved in the pre-alloyed specimens due to low austenization temperature. Locally copper rich and nickel rich areas occurred in the specimen with copper and nickel alloys. As the green densities of specimens were nearly 7.1 g.cm-3, these values were 7.2 g.cm-3 after sintering and heat treatment. The macrohardness values of specimens increased with increasing austempering time; however, microhardness values decreased inversely. The tensile strength also increased with austempering time. As the highest hardness and strength were obtained in the specimen with copper, the lowest hardness and strength were obtained in the pre-alloyed specimen with copper, nickel, molybdenum due to partially presence of soft ferritic areas. It was seen that the fracture elongation of specimens wasn?t related to hardness, strength, chemical composition and microstructure. And impact toughness increased with austempering time owing to decrease in microhardness and increase of bainitic structure ratio. As a result, it was concluded that there are further and detail studies.Key Words : Austempering, Bainite, Powder metallurgy, Mechanical properties