六方氮化硼(hBN)表面镀镍对Ni-Cr/hBN固体自润滑材料性能的影响

为了改善六方氮化硼(hBN)固体润滑剂和Ni-Cr合金基体之间的润湿性,采用硝酸镍分解-氢还原法制备Ni包覆hBN粉末(即Ni/hBN粉末).分别用Ni/hBN粉末和未包覆的hBN粉末作为固体润滑剂制备(Ni-Cr)/hBN自润滑复合材料,研究hBN粉末表面包覆Ni对该复合材料组织和性能的影响.结果表明:与hBN相比,...     

Processing and properties of sinter hardened Fe–Cr–Mo steels with admixed extra-fine nickel

Abstract

The processing and properties of chromium–molybdenum, powder metallurgy steels with admixed extra-fine nickel (XF Ni) were investigated. Prealloyed Fe–1·5Cr–0·2Mo powder was blended with different quantities of XF Ni, while a hybrid steel with lower Cr content was prepared by blending Fe–1·5Cr–0·2Mo and Fe–0·5Mo prealloyed powders, with additions of XF Ni and copper powders. These steels were compacted into different part shapes in order to evaluate the effect of part thickness on sinterhardening behaviour. These parts were also subjected to different cooling rates after sintering. This study showed that additions of XF Ni improve the compressibility, densification behaviour and mechanical properties of Cr–Mo steels. Furthermore, the properties of the hybrid steel were shown to be either equal to or greater than those of the reference material. Hardenability of all steels was sufficiently high such that part thickness was seen to have negligible impact. Higher cooling rates generally resulted in improved mechanical properties.     

Structure and mechanical properties of sintered Ni free structural parts

Abstract

Ni, Cu and in some cases Mo are the alloying elements which have traditionally been used in sintered steels. High performance of powder metallurgy (PM) structural parts from Fe powders is reached mainly by alloying of Ni. The use of Mn in Fe base PM structural parts has been avoided because of its high affinity to oxygen. It is difficult to sinter Mn steel, without oxidation, in industrial atmospheres. However, the PM industry follows also possibilities in order to develop Ni free sintered steels which render as high mechanical properties as diffusion alloyed Ni containing sintered steels and further fulfil the requirements of health protection. In recent years Mn have been introduced as alloying element in Fe based structural parts, on laboratory scale and also for pilot scale production. In this paper the factors that contribute to the structure and mechanical properties of sintered Mn steels are summarised.     

THE EFFECT OF PHOSPHORUS ADDITIONS ON THE TENSILE,FATIGUE, AND IMPACT STRENGTH OF SINTERED STEELS BASED ON SPONGE IRON POWDER AND HIGH-PURITY ATOMIZED IRON POWDER

Abstract

The mechanisms operating during the sintering of iron-phosphorus PM alloys are discussed, as well as the factors contributing to the unique combination of strength, ductility, and toughness that is characteristic of these materials. Alloying methods are reviewed with special reference to powder compressibility, tool wear during compaction, and homogenization during sintering. The preferred production method is to add phosphorus in the form of a fine Fe₃P powder to iron powder. The mechanical properties of a number of sintered steels made with and without Fe₃P additions to sponge iron or to high-purity atomized iron powders are reported. Use of atomized powder makes it possible to reach extremely high density by single pressing and the resulting phosphorus-containing sintered steels have very high ductility and impact strength. The fatigue strength is related linearly to the tensile strength, with a correlation coefficient of 0·91. It is concluded that structural factors other than those that control ductility and toughness are responsible for the fatigue resistance of sintered steels.     

Effect of Microstructure and Surficial Oxidation on the Wear Behavior of an UNS S41003 Stainless Steel

The UNS S41003 ferritic stainless steel is a low-carbon alloy that has great corrosion and oxidation performances in wet and aqueous environments, as well as it has high mechanical strength and ductility when compared to the most ordinary low-carbon steels. These great characteristics, allied to its relatively low manufacturing cost, have made it a potential option to replace structural steels in many applications. Although it is a current trend, there are still few published studies that relate this steel manufacturing process with microstructural evolution and mechanical behavior, mainly regarding its wear behavior, which is a substantial and sometimes limiting characteristic for its applications. In this context, this article presents a pioneering study about the use of biphasic microstructures (ferrite–martensite) and controlled surficial oxidation to enrich the wear behavior of a UNS S41003 steel type. This study concludes that, if well planned, both the increase of martensite fraction and the controlled growth of an adherent and compact oxide layer on the steel surface significantly improve its wear performance, decreasing its wear rate up to 93%.     

The effect of silicon and nickel additions on the sulfide spacing and fracture toughness of a 0.4 carbon low alloy steel

This paper discusses the effects of silicon and nickel additions on the mechanical properties of a 0.4 carbon low alloy steel. The four experimental steels used in the study were obtained by making additions of 1.5 wt pct nickel and 2 wt pct silicon, both separately and in combination, to a 0.4C/1.5Ni/1.0Cr/0.5Mn base steel. The base + Ni + Si steel, resulting from combined nickel and silicon additions, has a yield strength of 1682 MN/m² and impact and fracture toughnesses of 65 J and 115 MN/m^3/2, respectively. The other three steels have comparable strength levels but more typical impact and fracture toughnesses of about 30 J and 75 MN/m^3/2, respectively. The microstructures of the four steels are almost identical. The only significant observed difference among the four steels is that the sulfides in the base + Ni + Si steel are almost three times as large as those in the other three steels. As the four steels have similar sulfide volume fractions, there is a similar difference in sulfide spacings. The improved toughness of the base + Ni + Si steel is attributed to the increased sulfide spacing due to the influence of combined nickel and silicon additions on the average sulfide size.     

Revealing the Role of Cerium on Precipitation Behavior of In Situ TiC Particles and Wear Resistance of High-Titanium Wear-Resistant Steels

Compared to conventional martensitic wear-resistant steels of the same hardness, high-titanium wear-resistant steels with in situ TiC particles can significantly improve wear resistance. However, micron-sized TiC particles will decrease the toughness of high-titanium wear-resistant steels. Here, in order to improve wear resistance without reducing impact toughness, we incorporate 0.0025% cerium elements into high-titanium wear-resistant steels. Compared with no cerium steel, the steel containing cerium is demonstrating comparable mechanical properties, with the yield strength of 1283 MPa and impact toughness of 35.6 J, and the wear performance of the steel containing cerium is 1.78 times that of the steel with no cerium. The results show that with the addition of cerium the effective grain size of the steel decreases, and yield strength and toughness increase. The addition of cerium can form intermetallic compounds of Ce₂O₂S, which are used as heterogeneous nuclear particles in TiC to form rare earth composite particles calculated by the 2D mismatch theoretical model of Bramfitt. As the average spacing of the reinforcing phase particles in the steel decreases, the effective grain size of the steel decreases, and the number of reinforcing phase particles increases, the wear resistance of the steel with the addition of cerium is optimized.     

Injection moulding of 316L stainless steels reinforced with nanosize alumina particles

Abstract

This study aims to compare the effect of Al₂O₃ nanoparticle additions on the densification and mechanical properties of the injection moulded 316L stainless steels. The 316L stainless steel and Al₂O₃ nanoparticles were dry mixed and moulded using a wax based binder. The critical powder loading for injection moulding were 60 vol.-% for all samples. Debinding process was performed in solvent using thermal method. After the debinding process, the samples were sintered at 1405°C for 60 and 120 min under vacuum. Metallographic examination was conducted to determine the extend of densification and the corresponding microstructural changes. The sintered samples were characterised by measuring tensile strength, hardness and wear behaviour. Wear loss was determined for all the samples after wear testing. All the powders, fracture surfaces of moulded and sintered samples were examined using scanning electron microscope. The sintered density of straight as well as Al₂O₃ nanoparticles reinforced injection moulded 316L stainless steels increases with the increase in sintering time. The additions of Al₂O₃ nanoparticles improve the hardness and wear resistance with the increase of sintering time.     

Wear Mechanism and Cutting Performance of Conventional and High-Strength PM High-Speed Steels

Abstract

One of the reasons for employing PM techniques in the manufacture of high-speed steels is to avoid the carbide clusters that limit the strength of ordinary high-speed steels. The combination of proper PM techniques, upgrading the steel to an optimum chemical composition, and correct heat treatment results in superior strength and toughness characteristics. The effects of a superior cutting edge strength on the tool wear and the tool performance of some broaches and milling tools are discussed. The life of broaches may be greatly improved while, in the case of milling tools, the machining rate may also be raised. Case studies from actual production are used as illustrations. For high-quality industrial tools, the best way of saving scarce and expensive alloy metals may not be to lower the alloy content of the tool, but to make sure that the tool is capable of machining more components. The crucial factor is the alloy expenditure per machined component. PM/0189     

Dry sliding wear of ferrous PM materials

Abstract

Ashby's map for dry sliding wear of wrought steels has been used as a guide to interpret the dry sliding wear behaviour of PM materials. It has been shown that this map is useful in understanding the acting wear mechanisms and also the experimental wear rates. For given tribological conditions, in terms of normalised pressure and sliding velocity, the sliding wear resistance of PM materials is similar to that of wrought steels, although a closer look at the experimental results highlights the peculiar role of porosity and of a heterogeneous microstructure. In particular, materials with a porosity content of about 10% and with an homogeneous microstructure display the best performances. Since mild wear in metals can be obtained through the formation of protective oxide glazes, steam treatment turned out to be a natural way of increasing the sliding wear resistance of PM ferrous materials. The ‘surface durability’ of steam treated materials was thus shown to depend on the quality of the layer, the applied load and the nature of the counterface. The role of the counterface and the opportunity to adopt other surface treatments to increase the sliding wear resistance of PM ferrous materials are also discussed.     

Influence of Heat Treatment on the Performance Characteristics of a Plastic Mold Steel

Processing of polymers depends on corrosion‐ and wear resistant tool materials being in direct contact with the feedstock material. Plastic mold steels, a special group of tool steels, are successfully applied here, offering both a good wear and corrosion resistance. The lifetime of tools used for plastic processing not only depends on wear and corrosion resistance but also on the mechanical properties of the tool material for example, its flexural strength. In this work, mechanical tests of a commercial plastic mold steel were performed and related to the heat treatment. A close correlation of the amount of retained austenite and carbides and the mechanical properties was encountered. The paper discusses strategies to optimize the mechanical properties by an adequate heat treatment with regard to the resulting corrosion and wear resistance.     

Reciprocating Sliding Wear Behavior of Newly Developed Bainitic Steels

The present work discusses the mechanical properties and wear behavior of newly developed bainitic rail steels with nominal composition of 0.71 pct C, 0.35 pct Si, 1.15 pct Mn, 0.59 pct Cr, 0.40 pct Cu, and 0.20 pct Ni (all in wt pct)). Isothermal transformation has been carried out at different time and temperatures for obtaining different bainitic morphologies. Linearly reciprocating sliding wear behavior of the steels has been studied and compared with that of the conventional pearlitic rail steel. Considerable improvement in mechanical properties of the bainitic steels has been noticed. The hardness of the bainitic steels increases with decrease in isothermal transformation temperature. It leads to enhancement of wear resistance of the bainitic steels compared to the conventional rail steel. Finally, it clearly draws correlation between mechanical properties, wear resistance, and microstructural variation of a series of bainitic rail steels.     

Microstructure and Wear Resistance in Medium and High C Steels Treated by Quenching and Partitioning

The quenching and partitioning (Q&P) steels have shown to be promising candidates to be applied in fields where wear resistance is required. In this study, a medium and a high C steel are heat treated by Q&P and the resulting microstructure, hardness, and wear resistance are characterized. The mechanical stability of the austenite phase under wear test conditions is investigated. It is found that the stability of austenite is very high in the high C steel and decreases in the medium C steel. Additionally, the hardness and wear behavior of the Q&P-treated steels are compared with the results obtained for quenching and tempering (Q&T) treated samples, showing that, although the hardness of Q&P steels is quite lower, the obtained wear rates are similar. It means that in the studied Q&P steels, although the austenite transformation into martensite does not occur considerably, the presence of austenite might play a key role in the wear resistance.     

硫酸介质中铸造奥氏体不锈钢的腐蚀磨损行为

研究了几种铸造奥氏体不锈钢在含有Ｃｌ－的硫酸介质中的点蚀规律,测定了腐蚀磨损率、摩擦因数和表面钝化膜破坏后的修复时间。实验结果表明,高铬、镍、铜不锈钢有较高的耐蚀性,但其腐蚀磨损失重却高于一般的１８－８型不锈钢     

METHODS OF EXTENDING THE APPLICABILITY OF SINTERED STEELS

Abstract

It is a common opinion among users of structural parts that applications for sintered steels are limited to those where requirements for strength are low to moderate. Furthermore, sintered steels of moderate strength are thought to be very brittle. It is the object of this paper to draw attention to significant improvements which have been achieved in the last few years. These are basically a result of powder developments which are based partly on traditional alloying additions, such as Cu, Ni, Mo, and C, and partly on unique combinations of iron powders and phosphorus or on combinations of iron powders, phosphorus, carbon, and/or copper. Unusually favourable combinations of strength and ductility can be achieved with diffusion-alloys based on iron and phosphorus. Components of high-duty sintered steels capable of replacing components of conventional wrought steels can be produced from partially prealloyed combinations of iron, copper, nickel, molybdenum, and carbon. For many applications these materials can also be an alternative to powder-forged steels. All the above powder combinations show consistent and low dimensional changes during sintering so that close tolerances of intricately shaped components can be maintained. Material and processing costs are such that the improved properties can be achieved economically.     

Tempering-Induced Microstructural Changes in the Weld Heat-Affected Zone of 9 to 12 Pct Cr Steels and Their Influence on Sliding Wear

Increasing amount of tribological applications is working under alternating high/low temperature conditions where the material is subjected to temperature fatigue mechanisms such as creep, softening due to annealing, and at the same time must withstand mechanical wear due to sliding contact with pairing bodies. Steam turbine valves, gate valves, valve heads, stems, seats and bushings, and contacting surfaces of the carrier elements are some examples of such applications. The purpose of the present study is to evaluate the potential of X20 and P91 steels as materials for applications operating under combined effect of mechanical wear and alternating high/low temperature conditions. It was focused on how the microstructural changes occurring in the weld zone affect the wear properties of the selected materials. Generally, with longer tempering time and higher tempering temperature, the number of carbide precipitates decreased, while their relative spacing increased. Before tempering, the morphology of the steel matrix (grain size, microstructure homogeneity) governed the wear resistance of both steels, while after tempering wear response was determined by the combination of the number and the size of carbide particles. After tempering, in X20 steel larger number of stable M₂₃C₆ carbides was observed as compared with P91 steel, resulting in lower wear rates. It was observed that for both steels, a similar combination of number density and size distribution of carbide particles provided the highest wear resistance.     

Investigation on the Effect of ECAP Routes on the Wear Behavior of AA2014

ECAP is an effective process to improve the mechanical strength and wear resistance along with mechanical and microstructural properties. AA2014 solutionized at 495 °C and aged at 195 °C was subjected to Equal Channel Angular Pressed (ECAP) through route A and B_c at room temperature. It was well proved that the mechanical strength increased due to ECAP in AA2014. In order to investigate their wear behavior after ECAP, dry sliding wear tests were conducted using vacuum tribometer at nominal loads of 10N and 30N with constant speed of 2 m/s for sliding distance of 2000 m. The co-efficient of friction and loss in volume were decreased after ECAP both in route A and B_c. The dominant wear mechanisms observed were adhesive, delamination and stick slip process. In addition to these wear mechanisms, abrasive wear also appeared along with transfer of iron particles from the counter surface to the AA2014 pin. Presence of black powder and oxide formation were observed using EDAX analysis on wear debris. Routes A and B_c showed similar wear mechanisms and characteristics which were better than in unECAPed specimens.     

Understanding contribution of microstructure to fracture behaviour of sintered steels

Abstract

Microstructural features of sintered steels, which comprise both phases and porosity, strongly condition the mechanical behaviour of the material under service conditions. Many research activities have dealt with this relationship since better understanding of the microstructure–property correlation is the key of improvement of current powder metallurgy (PM) steels. Up to now, fractographic investigation after testing has been successfully applied for this purpose and, more recently, the in situ analysis of crack evolution through the microstructure as well as some advanced computer assisted tools. However, there is still a lack of information about local mechanical behaviour and strain distributions at the microscale in relation to the local microstructure of these steels, i.e. which phases in heterogeneous PM microstructures contribute to localisation of plastic deformation or which phases can impede crack propagation during loading. In the present work, these questions are addressed through the combination of three techniques: (i) in situ tensile testing (performed in the SEM) to monitor crack initiation and propagation; (ii) digital image correlation technique to trace the progress of local strain distributions during loading; (iii) fractographic examination of the loaded samples. Three PM steels, all obtained from commercially available powders but presenting different microstructures, are examined: a ferritic–pearlitic Fe–C steel, a bainitic prealloyed Fe–Mo–C steel and a diffusion alloyed Fe–Ni–Cu–Mo–C steel, with more heterogeneous microstructure (ferrite, pearlite, upper and lower bainite, martensite and Ni rich austenite).     

Influences of Manufacturing-Related Microstructural Variations on Fatigue in Carbide-Rich Tool Steels

In cold-work applications, tool steels with high carbide contents are used as cutting and stamping tools. The tool service life is limited by wear resistance and fatigue strength. The relationship between manufacturing-related microstructural influences and fatigue strengths of tool steels has not yet been adequately investigated. To investigate these influences on high-cycle fatigue (HCF) strength (N_G = 10⁷), rotating bending tests are performed on AISI D2 and AISI M2/M3. Raw materials are produced by conventional ingot casting and subsequent hot working (HW) as well as in a powder metallurgy (PM) process with hot isostatic pressing (HIP) and forging. Herein, a statistically validated correlation of process-related defect size and the resulting fatigue strength is presented. Both PM steels show significantly higher HCF strength than the HW steels. Critical defects in PM appear to be exclusively small oxide inclusions. In contrast, fatigue cracks in HW are typically initiated by the fracture of large, blocky eutectic carbides. The main factor influencing HCF strength is defect size. Other critical features of the microstructure include matrix hardness, circularity, and defect type. Improvements in fatigue strength can be obtained by reducing the size of fracture mechanical defects, inclusions for PM, and eutectic carbides for HW microstructures.     

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.