Study On The Preparation And Property Of High-performance Chromium Containing Iron Based Powder Metallurgy Sintered Material

Powder metallurgy iron-based alloys are leading products of the powder metallurgy industry, widely used in automotive and machinery parts and components industries.Chromium has good hardenability, and can enhance the strength of the material and doesn’t decrease the compressibility of powder evidently. Chromium element costs lower and has more resource than scarce resources of Nickel and Molybdenum elements. Degveloping high-performance and low cost chromium containing iron-based powder metallurgy materials is always the objective of powder metallurgy workers and one of the development direction of powder metallurgy. In this paper, different Chromium containing alloy powders were adding to partial diffusion prealloy powder Fe-1.75Ni-1.5Cu-0.5Mo-0.6C, inder to optimize the adding method and Chromium content, and to study effects of sintering process on the properties of materials. The ultrasonic fatigue property and behavior of Fe-1.75Ni-1.5Cu-1Cr-0.5Mo-0.6C were studied. This study provides technical bace for the development and application of Chromium containing sintered steel, which has academic significance and practiced value. The main research results show that:The method of adding chromium affects mechanical properties of iron based sintered materials significantly. Using six chromium containing alloys to add chromium element, the mechanical property of Fe-1.75Ni-1.5Cu-1Cr-0.5Mo-0.6C sintered material added 430 L stainless steel powder is the best, and the performance of the one added ball-milling high-carbon Cr Fe fine powder is second, and the property of sintered material added unball-milling pure Cr is the worst.The content of chromium affects the structure and property of Fe-1.75Ni-1.5Cu-x Cr-0.5Mo-0.6C(x=0, 0.5, 1.0, 1.5) significantly. When 430 L stainless steel powder is used as the adding method, the tensile strength increases first and then decreases, with the increase of chromium content, and when the chromium content is 1.0%, the tensile strength reaches maximum of 924 MPa. The hardness increases with the increase of chromium content, and when the chromium content is 1.5%, the hardness reaches maximum value of 32 HRC.Elongation decreases with the increase of chromium content, and without chromium,elongation reaches maximum value of 2.5%. When unball-milling high carbon Cr Fe powderis used as the adding method, the tensile strength increases first and then rapidly decreases with the increase of chromium content, and when the chromium content is 0.5%, the tensile strength reaches maximum value of 817 MPa. The hardness first increases and then decreases with the increase of chromium content, and when the chromium content is 1.0%, the hardness reaches maximum value of 31 HRC. Elongation decreases with the increase of chromium content and when the chromium content is 1.5%, elongation is only 1.3%. The main organization of different chromium content of the materials are pearlite, bainite, martensite mixed structure. The fracture mode of the material is ductile-brittle mixed fracture, with the increase of chromium content dimples gradually reduces and cleavage gradually increases.When using 430 L stainless steel powder to prepared Fe-1.75Ni-1.5Cu-1Cr-0.5Mo-0.6C,sintering temperature has important influences on the structure and property of the material.The study founded that the performance of the material is worst at the sintering temperature of 1120℃ and the tensile strength is only 505 MPa, and the main structure of the material is pearlite. when sintering temperature reaches 1200℃and 1250℃, the tensile strength is872 MPa and 924 MPa, respectively, and the main organization is pearlite, bainite and martensite mixed structure. Sintered at 1250℃, we also studied effects of cooling speed on the organization and property of the sintered material. The results show that the tensile strength of slow cooling, middle cooling, fast cooling is corresponding to 884 MPa, 924 MPa,958MPa. Elongation decreases gradually and the hardness gradually rise with the cooling speeded. The organization of slow cooling is mainly fine pearlite and bainite. The organization of middle cooling is mainly pearlite, bainite and martensite. The organization of rapid cooling and tempering includes pearlite, bainite and tempered martensite, and a lot of retained austenite also appears and the martensite content increases significantly.The ultrasonic symmetric bending S-N fatigue life curves of Fe-1.75Ni-1.5Cu-1Cr-0.6Mo-0.6C sintered materials are continuous decline curves. Fatigue limits of rapid cooling sinter hardening specimens are 691 MPa, 490 MPa, 347 MPa and 246 MPa, when in 105,106,107, 108 cycles respectively. Fatigue limits of slow cooling normal sintering samples are576 MPa, 453 MPa, 356 MPa and 280 MPa, when in 105, 106, 107, 108 cycles respectively. When under high cyclic stress, the fatigue strength of sinter hardening samples is higher, and while under low cyclic stress the fatigue strength of normal sintering samples is slightly higher. Themagnitude of cyclic stress has a certain influence on the crack source of the material. When under high cyclic stress, fatigue sources mainly locate in edges and corners of surface where stress concentrated or sub surface where the inclusions appears. while under low cyclic stress,fatigue sources mainly distribute in the sub surface where voids gathers or the interior area where the inclusions appears. Clearly fatigue striation can be observed in the fatigue stable growth stage, while under high cyclic stress fatigue striation distance is more wideneder, and with the decrease of stress fatigue striation distance narrows. Obvious morphology of cleavage fracture and dimple can be observed in transient fault zone, and the area of dimple more larger with high stress.Observation of the crack expansion trend along the specimen surface is found that, in rapid cooling sinter hardening and slow cooling normal sintering Fe-1.75Ni-1.5Cu-1Cr-0.6Mo-0.6C two different sintered materials, the way of expanse are mainly transgranular cracking, supplemented by intergranular cracking. Rapid cooling sinter hardening material,under different cyclic stress, cracks mainly along the tempered martensite and bainite. For slow cooling normal sintering material, under different cyclic stress, there is no obvious selection phase of crack propagation, and the crack can be saw along different organization.Cracks in the pearlite are relatively flat, and cracks in the bainite have a certain bend.