| Warm compaction is a new powder metallurgy (PM) technology which can make parts with high density and high properties at low cost and short procedure. In this paper, partially-diffuse alloyed Fe-2Cu-2Ni-1Mo-1C powders were used as based materials. High density compacts were obtained by die wall lubricated warm compaction technology, then traditional sintering, calcite sintering and sinter-hardening processes were utilized subsequently. Effects of sintering process on the mechanical properties, microstructures and the fracture morphology of the sintered materials were discussed. The symmetric bending ultrasonic fatigue behaviors of Fe-2Cu-2Ni-1Mo-1C sintered materials were studied. Damage mechanism of the sintered materials during ultrasonic fatigue test was investigated. In addition, the sintered specimens which were treated by heat treatment are tested by the symmetric tension-compression fatigue system, and study the effect of heat treatment on fatigue behavior.Experimental results show that: sintering process can affect the microstructure and mechanical properties of the PM materials. In traditional sintering atmosphere,sinter density is 7.30g/cm3,the strength is 690MPa, the elongation is 7.9%,the hardness is 285HB;In molybdenum wire calcination sintering process, sinter density is 7.32g/cm3, the strength is 760MPa, the elongation is 2.5%,the hardness is 311HB; In hydrogen circumatances, powder sintering properties is best: sinter density is 7.35g/cm3 ,the strength is 935MPa,the elongation is 3.5%,the hardness is 302HB. The final microstructure of the sintering materials all consisted of martensite, bainite, pearlite and retained austenite. The amount of martensite in the final microstructure was significantly affected by the sintering process. The microstructure mainly contained martensite and troostite (fine pearlite) at calcination sintering and sinter-hardened process. In the traditional sintering process, martensite and troostite decreased significantly besides pearlite and retained austenite presented more.Fatigue behavior of the Fe-2Cu-2Ni-1Mo-1C material in cyclic range of 104~109 cycles regime was tested. Results showed that the S-N curves of the three sintering specimens displays the characteristic of"continually decreasing type"up to 109 cycles and there were no traditional horizontal plateau beyond 106 cycles. With the increasing number of cycles,the stressσαdecreased continually. Over 107 cycles, fatigue failure occurred yet. According to the Basquin equation, the fatigue limits of traditional sintering specimens were calculated to be 358MPa,280MPa and 219MPa at 106, 107 and 108 cycles, respectively. The fatigue limits of sinter-hardened specimen were 293MPa,239MPa and 194MPa at 106, 107 and 108 cycles, respectively.The observation of the fracture surface showed that fatigue cracks initiate from surface of specimen for short lives (<107 cycles) at high stress levels, fatigue cracks initiate from subsurface nonmetallic inclusions of specimens in ULCF regime. The dark areas are observed by scanning electron microscopy around the inclusions. This indicates that crack initiates mechanism of Fe-2Cu-2Ni-1Mo-1C P/M material are of two kinds. Cracks propagated mainly through trans-crystalline and modes. Cleavage fracture and typical fatigue striations were observed in the crack propagation region. Evidence of dimple ductile fracture was found in the rupture region of the samples.The results of heat-treating of Fe-2Cu-2Ni-1Mo-1C sintering materials in 104~109 cycles regime show that heat treatment have different influence on fatigue strength of Fe-2Cu-2Ni-1Mo-1C sintering materials in different regime, heat treatment enhanced fatigue behavior in LCF and HCF, but heat treatment have a little influence in UHCF regime compared with LCF and HCF. The reason is that heat treatments have different effects on different fatigue regime result from different crack initiation mechanism. In LCF and HCF regime, heat treatment can change material microscopic structure and enhance mechanical behavior, further enhance fatigue behavior that is controlled by surface crack initiation mechanism. But in UHCF regime, heat treatment cannot change the character of intrinsic inclusions and the diffusion rate of point defects, thereby cannot enhance the fatigue behavior that is controlled by interior crack initiation mechanism. Therefore the heat treatments that improve conventional fatigue behavior probably have no effect on UHCF fatigue behavior.
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