Strain induced martensite formation in PM nickel steels

Abstract

Powder metallurgy (PM) nickel steels are often selected because they have high strength, high impact resistance and good abrasion resistance. The microstructure of slowly cooled PM nickel steels typically contains pearlite, bainite, martensite and a fair volume fraction of retained austenite. Since volume diffusion is really low at conventional sintering temperatures [1120–1150°C (≈2050–2100°F)], nickel rich areas are usually found where prior admixed nickel particles were located, i.e. at the surface of iron particles and in sintering necks. Therefore, there is a discrepancy between the rather low mechanical properties of austenite and the high strength of PM nickel steel. Hence, the hypotheses that stress induced or strain induced martensite formation takes place during mechanical testing have been investigated. Results show that martensite forms during mechanical testing, and its final volume fraction is proportional to nickel content.     

Dynamic properties of high-density low-alloy PM steels

PM steels with density higher than 7.5?g?cm^?3 were prepared from binder-treated powder, and the effects of alloying elements and microstructure on dynamic performance were studied. Although the addition of 1.5?wt-% Cu improves the tensile strength, it impairs toughness of the alloy by the formation of low-toughness pearlite. The 0.5?wt-% Cr addition promotes the formation of Ni-rich martensite and bainite at the expense of pearlite, which is beneficial to strengthen and homogenise the microstructure. The 0.5Cr alloy exhibits improved tensile strength and fatigue strength without sacrificing impact toughness, which exhibits high potential to be used for dynamic and cyclic loading applications. The tensile strength and fatigue endurance strength under 10⁷ cycles of 0.5Cr alloy achieve 1350 and 562?MPa respectively, and impact energy is as high as 21.1?J. The enhanced fatigue life is attributed to the high density, smaller pore size, rounder pores and composite-like microstructure.     

烧结硬化粉末冶金钢在高性能零件中的应用

粉末冶金烧结硬化是一种经济有效的生产高强度零件工艺过程。但是,其后续机加工相对困难和昂贵,因此尺寸公差控制也是烧结硬化过程中关注的重要目标。本文基于MPIF标准制备了多种烧结硬化材料,并对比了它们在同等压制压力下的测试试棒以及同等密度下工业粉末冶金零件上的强度、延伸率及尺寸稳定性表现。结果表明在工业生产条件下,所有材料都得到了有效烧结硬化并表现出了优秀的尺寸稳定性。在工业零件生产中,Cr M作为一种预合金无铜添加的经济有效的烧结硬化材料,由于它的高淬透性,在0.5%C(质量分数)添加时,表现出高的尺寸稳定性及极佳的性能。     

Tribological and mechanical properties of PM journal bearings

Abstract

Powder materials are widely used in journal bearings since they provide a good tribological performance with journal bearings. These bearings are self-lubricated and can be used in places where no lubricating is possible. In this study, tribological and mechanical properties of copper based CuSn10, ferrous based Fe–graphite and copper+ferrous based CuSnFe–graphite bearings manufactured by powder metallurgy method have been determined and compared. Wear tests were carried out at 20 N load and 1500 rev min^?1 every 30 min for 2·5 h using radial journal bearing wear test rig. Hardness, tensile, compressive, bending and radial fracture mechanical tests of these bearing materials were carried out. As a result, tribological and mechanical properties improved in CuSnFe–C bearings.     

Effect of heating mode and electrochemical response on austenitic and ferritic stainless steels

Abstract

Powder metallurgical (P/M) processing has the main advantage of making near net shape products. Nowadays, in automobile industries, stainless steels have become the most promising material owing to their good corrosion resistance. In the current study, 316L and 434L stainless steel powders were sintered using microwave and conventional methods through powder metallurgy route. The effects of sintering modes on the microstructure, mechanical properties and corrosion responses of 316L and 434L stainless steel composites are investigated in detail. The results showed that the sample prepared through microwave sintering route exhibited significantly superior densification, higher hardness and better corrosion resistance as compared to the conventionally processed counterpart. On the whole, 316L composites showed better corrosion resistance than 434L stainless steels.     

Microstructure and mechanical properties of PM Ni56Fe19Al25 alloy

Abstract

Scanning electron microscopy and X-ray diffraction analysis were used to study microstructure and mechanical properties of PM Ni₅₆Fe₁₉Al₂₅ alloy. The results indicate that as sintered specimen is (β+γ) dual phase structure, and its density is 6·54 g cm^?3 (the relative density is 94·0%), tensile strength is 771 MPa and the total strain is 4·3%. As quenched specimen presents a large superelasticity with the maximum recovery strain of 4·5%, and its tensile strength is 850 MPa and the total strain is 9·2%. The fracture modes of Ni₅₆Fe₁₉Al₂₅ alloy is transgranular, intergranular and tough mixed type.     

Improving pore morphology of PM 316L stainless steels by prealloyed powder prepassivation in 20% nitric acid

Abstract

The effect of prepassivation of prealloyed powder of 316L stainless steels on pore morphology and powder particle shape was investigated. Image analysis technique was used to study the effect of prepassivation, compaction and sintering temperature on the pore morphology of powder metallurgy 316L stainless steels. Porosity, dimension and morphology of the pores were characterised by means of four basic parameters: fraction of surface porosity, equivalent circle diameter, shape factor and elongation factor. In addition, SEM macrographs of powder particles were also investigated by applying the image analysis technique. The Feret’s average diameter and elongation factor were employed to describe the size and roundness of powder particles respectively. Annealing treatment reduced the equivalent circle diameter of pores and simultaneously improved f_shape and f_elong towards higher values. It was proposed that the prepassivation treatment reduced irregularity of powder particles through elimination of sharp corners of powder particles by exposure to acid environment.     

粉末冶金表面滚压致密化链轮工艺的开发

粉末冶金零件以其近终成形、成本低、效率高的优势在汽车行业得到了广泛的应用。但是由于粉末冶金零件表面存在大量的空隙,严重影响了其强度、硬度和耐磨性等性能,使其在某些重载工况中达不到要求,从而限制了其在汽车行业的应用。本文论述了一种粉末冶滚压金表面致密化链轮工艺,通过将粉末冶金技术和滚压致密化技术结合在一起,减小粉末冶金零件表面的空隙,在零件表面形成一层均匀的致密层,而提高其强度、硬度和耐磨性等性能,从而扩大其应用范围。     

Effect of boron on microstructure and mechanical properties of PM sintered and nitrided steels

Abstract

PM Distaloy and Astaloy alloys have many applications in the automotive industry and are used in structural elements with different wear resistance values. Their main features are adequate density, hardness, tensile strength, and good ductility. For the purpose of the experiment presented here, Distaloy SA and Astaloy Mo powders, alloyed with various amounts of elemental boron powder, were used. The Distaloy and Astaloy alloys were produced through mixing, compacting, and sintering at t=1393 and 1473 K, and, after the completed sintering process, they were plasma nitrided at 793 K. Experimental results showed that if boron was added, while sintering, the shrinkage phenomena increased ( 1473 K) and some parameters of those alloys (density, hardness, and tensile strength) were improved. Upon the ion nitriding treatment of the surface of base Astaloy Mo samples, a surface layer was created composed of the ε solution and γ' nitride, whereas the surface layer on the Distaloy SA base was mainly composed of a γ' compound. Boron activates the sintering process of Distaloy SA and Astaloy Mo samples but it has no significant impact on the surface layer's thickness of Distaloy SA alloys as opposed to Astaloy Mo alloys in which boron promotes a greater thickness of surface layers along with a reduced depth of nitrogen diffusion.     

微细Cr-Fe合金粉对粉末冶金钢组织与性能的影响

在Fe-0.5Mo预合金粉末中加入平均粒径为4.65μm的Fe Cr55C600高碳Cr-Fe合金粉末,于1 200℃,采用烧结硬化工艺制备Fe-Cr-Mo粉末冶金合金钢。结果表明:随着Cr-Fe合金粉末量的增加,合金烧结试样的硬度增加,强度增加,合金的显微组织以贝氏体为主。当添加微细颗粒的Cr-Fe合金粉末,使Cr质量分数达到1.7%时,合金烧结试样的综合性能达到最优,抗拉强度为1 210 MPa,硬度为30 HRC,显微组织主要为细小的下贝氏体和马氏体,此时合金中Cr/C质量比约为1.54。     

EBS蜡静电荷电和模壁润滑特性研究

采用粉末冶金静电模壁润滑和冷压／温压技术，通过对316L不锈钢粉进行压制和烧结，从而研究出压制过程中润滑剂EBS蜡粉的静电和润滑特性。结果表明：经较长（L）的输粉软管摩擦后，EBS蜡粉表现出较好的荷电性和低压吸附能力；在温压中（110℃）EBS蜡粉比室温下表现出更好的润滑性；高密度生坯中EBS蜡粉作为内润滑剂少量存在对烧结样品的密度和形状并没有出现负面的影响。     

烧结钼钢的动力学性能

许多粉末冶金应用，要求烧结零件能耐高的静力学和动力学工作应力。提高烧结密度是提高动力学性能，获得适当显微组织的关键参数之一。在市场上的低合金钢粉中，低钼合金钢粉令人感兴趣，因为这种合金元素仅对粉末压制性有轻微影响，却很有效地提高淬透性和力学性能。此外，像Ni和Cu关键性元素混合到基础钢粉中或在退火处理过程中扩散粘结到钢颗粒上，也会通过改善显微组织使力学性能提高。本文研究了含有不同量的预合金的低Mo合金钢粉的抗拉和动力学性能。也讨论了将Ni和Cu预混合或扩散粘结到这些钼钢中的作用，特别是对轴向疲劳性能和强度的影响。还与其它类型疲劳试样和／或试验方法所得结果进行了对比。     

Structure and properties of powder austenitic die steels

Possibilities are considered for using powder metallurgy in order to prepare effective austenitic die steels exhibiting high hardness and heat resistance (at temperatures not less than 850 °C). It is shown that in strength properties the material developed are as good as the best austenitic steels used as die tools, and in the level of heat resistance and hardness at room temperature they a considerably better. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 26–30, July–August, 2006.     

Mn含量对粉末冶金铁铜碳低合金钢组织与力学性能的影响

采用铜粉、石墨粉和铁粉为原料,以Fe-74.8Mn-6.9C中间合金粉的形式加入Mn元素,制备粉末冶金Fe-x Mn-(2-x)Cu-0.3C(x=0,0.2,0.4,0.6,0.8,1。质量分数,%)低合金钢,研究Mn含量对该合金组织与力学性能的影响。结果表明,合金组织由铁素体和珠光体构成。加入含Mn中间合金粉对混合原料粉末的压制性能没有明显影响。随Mn含量增加,合金中孔隙的数量增多,尺寸变大;合金密度先升高后降低,Mn含量为0.4%时合金密度最大,达到7.24 g/cm~3;合金硬度先升高后降低,Mn含量为0.6%时硬度最大;合金抗弯强度下降,冲击韧性升高,Mn含量超过0.4%时二者变化均较小。因此Fe-0.6Mn-1.4Cu-0.3C合金具有较好的综合性能,硬度(HRB)和冲击韧性分别达到57.4和8.80 J/cm~2,比Fe-2Cu-0.3C合金分别提高5.3和0.82 J/cm~2,材料呈部分韧性断裂特征。     

Sintering and aging behaviours of Al4CuXMg PM alloy

Sintering and aging behaviours of Al–Cu–Mg powder metallurgy (PM) alloy produced from elemental powders were examined. After evaluating results from thermal analysis, tests were carried out on Al–4Cu alloys with magnesium contents of 0.5, 1 and 2?wt-% and it was found that additions of 1?wt-% Mg was most effective for enhancing the transverse rupture strength (TRS) of the Al–Cu PM alloys for both as sintered and after a heat-treatment conditions. Grain size reduction in the range of 14–45% was achieved by adding magnesium into Al–Cu system. Analyses showed that produced alloys were composed of Al, Al₂Cu, Al₂CuMg and Al₇Cu₂Fe phases. Differential scanning calorimeter and dilatometer analyses revealed that alloys show swelling behaviour after the eutectic melting reaction at 548°C and swelling rates increasing as a function of magnesium content. Both high hardness value (120 HB) and TRS (650?MPa) were achieved via aging of Al4Cu1Mg alloy for 24 hours.     

Ultrasound-assisted drilling of copper and pre-alloyed powder metallurgy steels

Powder metallurgy (PM) steel components are generally considered ‘difficult’ to machine when compared with their wrought counterparts. Thus, efficient and cost effective machining strategies have to be devised. This paper summarises the results obtained by utilising ultrasounds to improve the machining behaviour of copper and sinter-hardenable PM steels. The effect of ultrasonic assisted drilling on tool life with and without admixed MnS is discussed as well. These results are compared to conventional machining methods and show that, for FC-0208 PM copper steels, tool life is increased by 180% and can be several thousand per cents higher if used in combination with admixed MnS. At the same time, the use of ultrasounds permitted to increase productivity by 86%, owing to the possibility of using higher feed rates. Similarly, for FLNC-4408 pre-alloyed PM steels, tool life was increased by 130% and productivity was 60% higher than when no ultrasounds were used.     

少量Mo添加对Fe-0.5Mn-0.5C烧结钢组织和力学性能的影响

采用粉末冶金方法,经1 150℃、H2气氛烧结1 h制得Fe-0.5Mn-xMo-0.5C(x=0,0.25,0.4,0.5)合金。通过金相组织观察、扫描电镜分析及力学性能测试,研究Mo添加对Fe-0.5Mn-0.5C烧结钢显微组织和力学性能的影响规律。结果表明:添加少量的Mo(≤0.5%,质量分数)可以同时提高Fe-0.5Mn-0.5C的烧结密度、抗拉强度、硬度和伸长率。当Mo含量为0.5%时,烧结体的烧结密度、抗拉强度、硬度和伸长率分别达到最大值7.10 g/cm3、426 MPa、92.2 HRB和7.58%。Mo可促进Fe-0.5Mn-0.5C坯体的致密化过程,随Mo含量增加,烧结体的显微组织中珠光体数量增加,片层间距减小;断口形貌图中解理面面积减小,韧窝数量增加。     

淬火温度对60Si2Mn和GCr15钢疲劳裂纹扩展速率的影响

弹钢60Si2Mn和轴承钢GCr15正常淬火（870℃和840℃）及高温淬火（1200℃）的疲劳裂纹扩展速率试验结果表明,高温淬火使弹簧钢疲劳裂纹扩展速率下降,而使轴承钢疲劳裂纹扩展速率升高。     

Effect of nano-SiC content on mechanical properties of SiC/2014Al composites fabricated by powder metallurgy combined with hot extrusion

Nano-SiC particulates (n-SiC_p) reinforced 2014Al matrix composites with different reinforcement volume fractions (0, 0.25, 0.5 and 1?vol.-%) were fabricated by powder metallurgy combined with hot extrusion. The effect of volume fraction of n-SiC_p on mechanical properties of composites was studied at both ambient and elevated temperatures. The increase of n-SiC_p content led to an increase in yield strength (YS) and ultimate tensile strength (UTS) and a slight decrease in elongation which is much better than the composites reinforced with micro-SiC_p. The 0.5?vol.-% n-SiC_p/2014Al composite observed the highest YS and UTS of ～378 and ～573?MPa at room temperature and of ～303 and ～409?MPa at 473?K. The enhancement of the properties is suggested to be induced by uniformly dispersed and well-bonded n-SiC_p reinforcements as well as the age-hardening effect of the more and finer precipitates.     

Effect of copper infiltration on fracture mode in sintered steels

Abstract

The fracture mode of PM steels depends on features, such as pores, densification, diffusion of any added alloying elements, contact area between particles, microstructural homogeneity, and applied load conditions. Consequently, when a sintered steel is infiltrated, several factors that determine the fracture behaviour are affected. The density is raised (in absolute terms, because the infiltrant is usually of higher density, and in relative terms, because activated sintering is promoted by permanent liquid phase sintering). The liquid phase promotes a rounding of the pores which enhances stress transmission from the necks to the particles. Sintered steel with 0˙6%C was infiltrated with three amounts of copper to study the influences of the amount of infiltration and the resultant final density on the fracture mode. Specimens were first uniaxially pressed to 7˙0 g cm^–3 green density, then sintered and infiltrated simultaneously at 1120°C in a 90N₂–10H₂ atmosphere. The mechanical results and microstructures were analysed to evaluate the fracture behaviour and fracture surfaces. The mechanical behaviour was characterised by hardness, tensile and three point bending tests.