Development of robust processing routes for powder metallurgy high speed steels

Abstract

This paper reviews progress made in understanding the factors which control the supersolidus liquid phase sintering of high speed steel powders to full density. The correlation between alloy composition and sintering behaviour is discussed for a number of alloy systems. Realising that for complete densification it is necessary for sintering to take place in the liquid +γ+M₆C+MC (or MX) phase region, two approaches have been developed to extend this critical phase field. This enables a scientific development of alloys that are more robust to process variations than currently sintered high speed steels of standard (for wrought materials) compositions. The new alloy systems possess wider process or sintering windows and have lower optimum sintering temperatures. The first approach relies on computer aided alloy design: vacuum sintering windows extending to 30–40 K at temperatures of 1170–1200°C have been achieved for novel Fe–C–4Cr–14Mo(–8Co)systems. The second approach involves sintering vanadium enriched high speed steels (HSSs) in nitrogen rich atmospheres. Such processing promotes the formation of MX carbonitrides in place of the more massive MC carbides. The solidus is lowered and sintering windows of ～30 K at temperatures of 1140–1150°C have been achieved. Compared with wrought HSSs, directly sintered materials have uniform, coarser microstructures. The low levels of residual porosity achieved enable attainment of metal and wood cutting properties comparable to those achieved with wrought and hipped HSSs of similar compositions.     

Analysis of reliability of powder metallurgy steels by means of weibull statistics

Porous sintered steels, because of porosity, present low tensile elongation and impact strength, suggesting reduced ductility and consequently poor reliability in structural applications. However, they present some typical features of ductile materials. To study the reliability of these materials, some Fe-C-P alloys were selected and tested by means of the Weibull analysis. The results are very encouraging, since they demonstrate that porous sintered steels may be compared to conventional structural steels. The values of the Weibull modulus are affected by the chemical composition, varying between 20 and 55. The aim of this paper is to correlate the microstructural characteristics and the fracture behaviour of the specimens, pointing out how the intrinsic properties of the microstructural constituents affect reliability.     

Effect of carbon content on workability of powder metallurgy steels

Workability behaviour of steel powder metallurgy performs containing 0%, 0.4% and 0.8% of carbon were completely investigated experimentally. Cold upsetting of abovementioned powder metallurgy sintered steels preforms with two different aspect ratios namely 0.39 and 0.59 was carried out with graphite as lubricant and the formability behaviour of the preforms under triaxial stress state condition was determined. The formability stress parameter was evaluated for all the above said preforms and its variation with respect to axial strain was plotted, studied and discussed relative to their as sintered microstructures. Also the characteristics of various stress ratio parameters with respect to axial strain were analyzed and presented.     

Corrosion fatigue resistance of powder metallurgy and forged duplex stainless steels

The high cycle corrosion fatigue resistance of powder metallurgically (PM) fabricated and hot isostatically pressed (hipped) duplex stainless steels (DSSs) was investigated and compared with that of a conventional forged DSS. Tests were performed in rotating bending fatigue in a chloride and sulphate containing aqueous solution at room temperature. The hipped PM DSSs studied had small grain size and homogeneous microstructure and isotropic properties and thus, their high cycle corrosion fatigue resistance was generally markedly higher than that of the corresponding forged DSS tested in the longitudinal direction. Fatigue cracks were observed to grow preferentially in the ferrite phase and a retarding effect of the austenite phase on corrosion fatigue crack growth was observed both in the hipped PM and wrought DSSs. Slow cooling after solution annealing improved the corrosion fatigue resistance, even though it induced slight second phase precipitation. Localised corrosion attack was observed at inclusion/matrix interfaces, when oxide inclusions were cerium modified (forged DSS). When oxide inclusions were either unmodified or calcium modified (hipped PM DSSs), no localised corrosion attack was observed.

MST/3429     

Preparation and properties of lead-free FeS/Cu-Bi composites by flake powder metallurgy via shift-speed ball milling

Lead-free FeS/Cu-Bi (FCB) self-lubricating composites were prepared via shift-speed ball milling (SSBM) and powder metallurgy. The microstructure and evolution of mixed powders were studied, and the mechanical and tribological properties of the FCB composites were tested. SSBM includes long-term low-speed ball milling (LSBM) and short-term high-speed ball milling (HSBM). In the early LSBM, the matrix phase CuSn10 powder was flattened into flakes, and the lubricating phases FeS and Bi were refined and gradually dispersed on the CuSn10 flakes. After short-term HSBM, FeS and Bi were further trapped on the CuSn10 flakes, which not only improved the bonding performance between the lubricating phase and copper alloy but also protected the continuity of the copper alloy matrix from additional damage. 6 + 1 h SSBM (6 h LSBM + 1 h HSBM) improved the mechanical properties, antifriction and wear resistance of FCB composites. Compared with traditional HSBM, SSBM provides a new way to prepare FCB self-lubricating composites with better comprehensive properties.     

Synthesis of Cu-W nanocomposite by high-energy ball milling

The Cu-W bulk nanocomposites of different compositions were successfully synthesized by high-energy ball milling of elemental powders. The nanocrystalline nature of the Cu-W composite powder is confirmed by X-ray diffraction analysis, transmission electron microscopy, and atomic force microscopy. The Cu-W nanocomposite powder could be sintered at 300-400 degrees C below the sintering temperature of the un-milled Cu-W powders. The Cu-W nanocomposites showed superior densification and hardness than that of un-milled Cu-W composites. The nanocomposites also have three times higher hardness to resistivity ratio in comparison to Oxygen free high conductivity copper.     

The sintering process of a high permeability Ni-Fe-Cu-Mo alloy made by powder metallurgy

The magnetic permeability of a 77 Ni-14 Fe-5 Cu-4 Mo wt % alloy made by powder metallurgy is known to be improved by extending the sintering time considerably beyond that normally used. The room temperature measurement of resistivity during the sintering cycle of such an alloy clearly shows the overlapping stages of change that occur during the sintering process. The variation in resistivity and its relationship with the changes in density, in weight and in bend strength of compacts shows that de-oxidation of the constituent element powders occurs initially. De-oxidation is followed by sintering and alloying of the nickel and iron which is followed in turn by alloying of the molybdenum. The final stage involves the alloying of the copper and the elimination of pores.Electron microprobe analysis has shown that the copper does not alloy substantially until the copper particles melt, and that alloying is hindered if copper powder of large particle size is used. Sintering occurs more rapidly than alloying, but the rate of alloying is the most important factor in determining the electrical and magnetic properties of the alloy.     

The nanostructure evolution of Ag powder synthesized by high energy ball milling

In this paper, nanostructured silver with an average crystallite size of 28 nm and internal strain of 0.44% was synthesized by mechanochemical reduction of Ag₂O using graphite in a high energy planetary ball mill. XRD, SEM and TEM techniques were used to characterize the structural evolution and morphological changes of products. The results showed that the reaction is progressed by a nucleation and growth mechanism process. Although the changes of crystallite size and internal strain in Ag₂O were regular during the milling, there was an irregularity in the aforementioned parameters of Ag particles. This irregularity was probably owing to the progressive generation of silver during the milling.     

高能球磨对雾化磁粉结构与磁性能的影响

采用高能球磨的方法处理水雾化Fe-Si-Al软磁合金粉末,运用SEM、XRD分析球磨粉末的显微结构和晶体结构,测试粉末的静磁参数及其2～18GHz范围的复磁导率(μr=μ′-jμ″).结果表明,球磨时间增加,粉末的扁平率增大、粒度减小;平均晶粒尺寸减小、内应变增大导致结晶有序度降低;粉末的饱和磁感应强度减小、矫顽力增大;球磨粉末的复磁导率实部和自然共振频率均相应提高,但过度球磨不利于材料磁损耗性能的提高.     

High performance stainless steel via powder metallurgy hot isostatic pressing

Abstract

Argon atomised austenitic stainless steel (AISI 304) powder was characterized for its physical properties such as particle shape, microstructure, median particle size, particle size distribution, apparent density, tap density, and jlowrate. Subsequently, the tap density of the as received powder was improved to the desired level by adjusting the powder distribution followed by mixing and blending. This powder was subjected to hot isostatic pressing (hipping) at two different combinations of temperature and pressure to optimise the microstructure and mechanical properties. Mechanical properties of the stainless steel obtained by the powder metallurgy (PM) hipping route were found to be superior to those of conventionally processed wrought steel. The superior performance of PM hipped steel is attributed to its low oxygen content, fine grain size, and high degree of chemical homogeneity. Although the production of billets by the hipping route does not appear to be economical owing to the high capital cost of the hot isostatic press, the added advantage of obtaining a nearnet shape makes the process economically viable for production of intricate shapes.     

Preparation of nitinol by non-conventional powder metallurgy techniques

The aim of the present paper was to compare the evolution of Ni–Ti intermetallics in two non-conventional production techniques for the synthesis of NiTi shape memory alloy. Short term ultrahigh energy mechanical alloying is proposed to be able to describe the early stages of the milling process, which was not described in the literature previously, and to obtain intermetallics in shorter process durations. The reactive sintering using high heating rate (>300°C?min^??1) is a process designed to suppress the formation of secondary intermetallics and to reduce the porosity of the product. The same phases' formation sequence was determined for both processes. The detrimental Ti₂Ni phase forms preferentially, and therefore, its presence cannot be avoided in any of the investigated techniques.     

Properties of porous magnesium prepared by powder metallurgy

Porous magnesium-based materials are biodegradable and promising for use in orthopaedic applications, but their applications are hampered by their difficult fabrication. This work reports the preparation of porous magnesium materials by a powder metallurgy technique using ammonium bicarbonate as spacer particles. The porosity of the materials depended on the amount of ammonium bicarbonate and was found to have strong negative effects on flexural strength and corrosion behaviour. However, the flexural strength of materials with porosities of up to 28 vol.% was higher than the flexural strength of non-metallic biomaterials and comparable with that of natural bone.     

Nanostructure cathode materials prepared by high-energy ball milling method

We prepare Li₂MnO₃ and LiNi_0.5Mn_0.3Co_0.2O₂ as nanostructure cathode materials using a high-energy ball milling method with bulk-type electrodes. The nanostructure electrodes prepared by the ball milling exhibit much smaller particle sizes in diameter than those of bulk-type electrodes. The 1st charge–discharge capacitance and efficiency of the nanostructure cathode materials are superior to those of the bulk-type electrodes.     

Mechanical properties of high performance lightweight steels

Abstract

The incorporation of low density, high modulus ceramic particles into a steel matrix is a potential route to improve the mechanical performance of steels. A powder metallurgy, mechanical blending route has been adopted to produce a homogeneous distribution of TiB₂ particles in both pure Fe and 316L stainless steel matrices. This approach gave large increases in both the static and fatigue strength with increasing TiB₂ volume fractions, in comparison with the matrix material. Additions of TiB₂ also resulted in reduced density and increased stiffness in the composite or lightweight steel materials, giving a specific stiffness increase of 52%with a fraction of 30 vol.-%TiB₂ in Fe, compared with the matrix.     

Development of TiB and nanocrystalline Ti-reinforced novel hybrid Ti nanocomposite produced by powder metallurgy

Journal of Materials Science - Titanium monoboride (TiB) whisker-reinforced titanium (Ti) matrix composites were produced by powder metallurgy, through vacuum sintering. TiB is formed by thermal...     

Metallography of powder metallurgy materials

The primary distinction between the microstructure of an ingot metallurgy/wrought material and one fabricated by the powder metallurgy route of pressing followed by sintering is the presence of porosity in the latter. In its various morphologies, porosity affects the mechanical, physical, chemical, electrical and thermal properties of the material. Thus, it is important to be able to characterize quantitatively the microstructure of powder metallurgy parts and components. Metallographic procedures necessary for the reliable characterization of microstructures in powder metallurgy materials are reviewed, with emphasis on the intrinsic challenges presented by the presence of porosity. To illustrate the utility of these techniques, five case studies are presented involving powder metallurgy materials. These case studies demonstrate problem solving via metallography in diverse situations: failure of a tungsten carbide-coated precipitation hardening stainless steel, failure of a steel pump gear, quantification of the degree of sinter (DOS), simulation of performance of a porous filter using automated image analysis, and analysis of failure in a sinter brazed part assembly.     

Modeling of powder metallurgy processes

The purpose of this paper is to present the theory of powder metallurgy (PM) materials in cold and hot forming, and its application to computer modeling of powder technology processes. The study is both a review and a research article. The theory presented can be treated as a review paper. The experimental as well as numerical investigations can be treated as a research paper. In the example a simple compression of a cylindrical PM specimen is considered. Numerical calculations are performed by using the finite element method. Iron PM specimens have been used for experimental studies.