【作者】 周玲；

【导师】 肖志瑜；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 温压技术是以低成本制造高密度、高性能粉末冶金零件的一种新方法。本文以部分扩散预合金Fe-2Cu-2Ni-1Mo-1C粉末为原料,利用模壁润滑温压工艺获得高密度压坯(7.35~7.40 g/cm3),采用网带烧结、钼丝煅烧炉烧结和烧结硬化三种工艺烧结,并对烧结试样进行了合理的热处理工艺。研究了不同烧结工艺下材料力学性能、显微组织及断口形貌的变化;对Fe-2Cu-2Ni-1Mo-1C烧结材料和热处理工艺条件下的材料进行弯曲超声疲劳试验,并用SEM和EDS对疲劳断口进行显微分析,研究了烧结材料的疲劳断裂行为和疲劳断裂机理以及热处理工艺对其疲劳性能的影响。研究结果表明:不同烧结工艺下Fe-2Cu-2Ni-1Mo-1C材料的显微组织及力学性能不同。网带烧结工艺,烧结密度为7.30 g/cm3,抗拉强度690 MPa,延伸率7.9%,表观硬度285HB;钼丝煅烧炉烧结工艺,烧结密度为7.32 g/cm3,抗拉强度760MPa,延伸率2.5%,表观硬度311HB;烧结硬化工艺,烧结密度为7.35 g/cm3,抗拉强度935MPa,延伸率3.5%,表观硬度302HB。烧结材料的最终显微组织均为马氏体、珠光体、贝氏体和残余奥氏体等多种类型组织混合存在。不同烧结工艺影响各相组织含量,钼丝煅烧炉烧结和烧结硬化工艺下马氏体含量相对增加,网带烧结工艺下主要为珠光体组织。Fe-2Cu-2Ni-1Mo-1C烧结材料的弯曲超声疲劳试验结果显示:三组不同烧结工艺处理的试样在104~109循环周次下的S-N曲线均为一条连续下降的曲线,随循环次数的增加,应力幅σα连续降低,不存在传统疲劳概念上的“疲劳极限”和无限寿命,只存在条件疲劳极限。根据Basquin方程求得不同工艺下试样的条件疲劳极限为:网带烧结态试样在106,107,108循环周次下的条件疲劳极限分别为358MPa,280MPa和219MPa。烧结硬化态试样在106,107,108循环周次下的条件疲劳极限分别为293MPa,239MPa和194MPa。Fe-2Cu-2Ni-1Mo-1C烧结试样疲劳断口显微分析显示:在107周次以下的疲劳断裂,疲劳裂纹在试样表面大体积孔洞或表面缺陷处萌生;而在107以上超高周周次的疲劳断裂,疲劳裂纹主要在次表面缺陷处或内部夹杂处萌生,表明粉末冶金Fe-2Cu-2Ni-1Mo-1C烧结材料的疲劳断裂存在两种形式。疲劳裂纹扩展以穿晶断裂为主,可明显观察到裂纹扩展的两个阶段,存在典型的解理和疲劳辉纹形貌,断裂区出现塑形韧窝。Fe-2Cu-2Ni-1Mo-1C烧结材料经热处理后在104～109周次范围的超声疲劳试验结果表明:热处理工艺可明显提高烧结材料在低、高周循环周次下的疲劳性能,但对超高周循环周次下的疲劳性能影响较小,且随着疲劳寿命的增加,影响越来越弱。因此,传统提高Fe-2Cu-2ni-1Mo-1C材料在低、高周阶段疲劳性能的热处理工艺,对超高周疲劳性能的提高将不会起明显作用。原因可能跟疲劳裂纹萌生有关:通过热处理工艺可以改变材料的组织,却不会改变材料本身固有夹杂的性质,比如孔隙率和夹杂物的成分。因此,热处理工艺并不能改变超高周疲劳裂纹的萌生,从而对材料超高周疲劳性能的改善并没有太大作用。更多还原

【Abstract】 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.更多还原