Powder Metallurgy Steels Research Articles

INVESTIGATION OF CORROSION AND ADHESION BEHAVIORS OF BORONIZED ASP® 2012 STEEL

Tool steels are extensively used in sheet metal forming dies in order to prolong the life of dies and reduce the cost of production. Powder metallurgy tool steels are also alternatively preferred because of their low-risk metallurgical behavior owing to their compositions by which bulk steel would be difficult to produce, such as high amount of carbides or high alloying elements. Such advantage is desired if the steel has a narrow range of fine grain sizes and is less metallurgically affected by the heat treatment process due to having large surface area of fine grains. In this study, ASP® 2012 powder metallurgy steel containing high amounts of Si, Cr, Mo and W was selected for boronizing by the boron-giving thermochemical process. The boride layers on the surface of ASP® 2012 tool steel were metallographically characterized by various characterization techniques including adhesion strength test, SEM and XRD. The effects of corrosion kinetics and chemical composition of steel on the amount and types of boride layers formed on P/M steel of ASP® 2012 after powder-pack boriding were investigated. The results indicate that, according to the Rockwell-C indentation test, the best adhesion in the boride layer was obtained in samples borided at 850°C for 4 h, while the worst adhesion was obtained in ASP® 2012 steel boronized at 950°C for 4 h. Corrosion rates of boronized ASP® 2012 steel were decreased compared to nonboronized ASP® 2012 steel.

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 (NG = 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.

Analysis of Wear Properties of Powder Metallurgy Steel in Abrasive Soil Mass.

This study presents the results of testing for abrasive wear of Vanadis 60 SuperClean powder metallurgy steel as compared to Hardox 600 steel and PMFe60P padding weld. The testing was conducted by the “rotating bowl” method using natural abrasive soil masses. Two types of abrasive masses with particle size distributions corresponding to light soil and medium soil were used. The obtained results enable the conclusion that the weight loss for Vanadis 60 SuperClean powder steel in both types of abrasive mass was approximately seven times lower than that for Hardox 600 steel and two times lower than PMFe60P padding weld. The high resistance of powder steel to abrasive wear in abrasive soil masses is related to the presence of a large number of fine M6C (tungsten-molybdenum) and MC (vanadium) carbide precipitates in its microstructure. The obtained test results indicate that the application of Vanadis 60 SuperClean steel may be extended to working elements operating in mineral abrasive environments.

Fatigue Behavior of Alloy Steels Sintered from Pre-Alloyed and Diffusion-Bonding Alloyed Powders

Porosity and phases are considered to be two key factors for the fatigue performance of powder metallurgy steels. In this paper, the fatigue strengths of the alloy steels sintered from two typical types of powders, pre-alloyed Fe-Cr-Mo (Astaloy CrM), and diffusion-bonding alloyed Fe-Cu-Mo-Ni (Distaloy AE), were comparatively analyzed in view of the geometry of porosity, the phases constitution, and fractography of fracture. Different modes of fatigue fracture were distinguished between the two materials. Namely, a trans-particle fracture is predominant in the Disitaloy AE steel due to the heterogeneous phases which consist of soft phases in powder interior and hard phases along powder borders. In contrast, the fatigue fracture of the Astaloy CrM steel with a homogeneous mono-phase of martensite is characterized by an inter-particle fracture at the sintering necks. Moreover, the fatigue endurance limit of the Distaloy AE steel was not pronouncedly improved by increasing sintering temperature in comparison with the Astaloy CrM steel. This was attributed to the softening of the network constructed by martensite at sintering necks. A modified Murakami model which considers micro-scale defect and micro-hardness is effective to predict the fatigue performance of the alloy steels sintered from pre-alloyed and diffusion-bonding alloyed powders, respectively.

Effect of V addition on tribocorrosion wear behavior of boride layer produced on AISI 1040

In this study, the effect of the addition of vanadium (V) on the densification, microstructure, and wear behavior of borosintered powder metallurgy steels was investigated. Ball-milled powders containing 0, 1, 5 and 10 wt.% V were cold-compacted under 200 MPa pressure and then subjected to borosintering at 1100°C for 2 h in ambient air. The characterization of the boride layers formed after borosintering was conducted by scanning electron microscopy, X-ray diffraction, microhardness tests, nanoindentation, surface roughness measurements, and ball-on-disk-type wear tests. Wear testing was carried out under 10 N load for a total sliding distance of 250 m in ambient air and in a 3.5% sodium chloride (NaCl) solution. While the thickness of the boride layer decreased with increasing V content, its hardness and modulus of elasticity also increased, owing to the formation of additional vanadium boride (VB) and vanadium carbide (V2C) phases. Wear resistance increased with increasing V content, and the samples tested in 3.5% NaCl displayed lower coefficients of friction and higher wear resistance compared to the samples tested in ambient air. Reduced wear in 3.5% NaCl was attributed to the cooling and lubricant effects provided by the 3.5% NaCl solution.

Anisotropic size change during continuous quenching and deep freezing of tool steel *

Abstract The size change of a low carbon maraging steel and of a ledeburitic cold work tool steel produced by ingot (IN) or powder metallurgy (PM) was measured by dilatometry using specimens taken in longitudinal or transverse direction of hot working. The anisotropy of the size change during martensitic transformation ΔεM was negative for the maraging and the PM steel, but positive for the IN steel and the more so, if the cooling rate was raised by injecting liquid nitrogen during quenching to −150 °C. The microstructure and texture was investigated by high resolution SEM equipped with microprobe analysis and respective imaging as well as by EBSD. The results show that Δεis not related to a texture M of the martensitic matrix but to a microstructural banding caused by microsegregation and hot working. The alignment of the eutectic M7 C3 carbides in the direction of hot working coincides with their texture.

The investigation of the effect of cu addition on the Nb-V microalloyed steel produced by powder metallurgy

In this work, the effect of Cu content on the microstructures, mechanical properties and electrical conductivity of Nb-V added microalloyed powder metallurgy (PM) steels were investigated. Microalloyed steel samples were pressed at 750 MPa and sintered at 1400oC in argon atmosphere for 1 h. The grain size and phase distribution of the microalloy steels were determined by optical microscope. The precipitates and fracture surface of samples were analyzed with the help of SEM and EDS analyses. Tensile test, hardness test and electrical conductivity measurement were carried out for the Nb-V added microalloyed steel with different Cu content. Results indicated that 10 wt.% Cu added PM microalloyed steel showed the highest values in yield strength (YS) and ultimate tensile strength (UTS). However, when the amount of Cu content increased from 10 to 15 wt.%, YS and UTS decreased. Elongation also tends to decrease with increasing Cu content. Although the electrical conductivity in general increased with the addition of Cu, a decrease in some conductivity was observed in the addition of 15 wt.% Cu.

Mechanical and Physical Properties of Differently Alloyed Sintered Steels as a Function of the Sintering Temperature

In this paper, the effect of processes occurring during the sintering of four powder metallurgy steel grades on the resulting properties were investigated. This included three grades prepared from plain iron powder with admixed graphite, one grade alloyed also with elemental copper and another with Fe-Mn-Si masteralloy. One further grade was prepared from Cr-Mo pre-alloyed powder with admixed graphite. The effect of the sintering processes was examined in the temperature range of 700–1300 °C in an inert atmosphere (Ar). In order to study oxygen removal, DTA/TG runs linked with mass spectrometry (MS) as well as C/O elemental analysis were performed. Charpy impact tests and fractography studies were performed to study the effect of the temperature on the formation and growth of sintering contacts. Characterization also included metallography, dimensional change, sintered density, and hardness measurements to describe the dissolution of carbon and alloying elements during the process. Physical properties that were measured were electrical conductivity and coercive force. The results showed that, in all steels, the reduction of oxides that occur during the heating stage plays a key role in the formation and growth of the sintering contacts as well as in the completion of alloying processes. In the chromium alloy steel, the presence of the stable chromium oxides delays these processes up to higher temperatures, while in the other steels that are based on plain iron powder, these processes take place earlier in the heating stage, at lower temperatures. Compared to the standard Fe-C and Fe-Cu-C grades, the Cr-Mo steel requires more sophisticated sintering to ensure oxygen removal, but on the other hand it offers the best properties. The masteralloy variant, finally, can be regarded as a highly attractive compromise between manufacturing requirements, alloy element content, and product properties.

Transient Liquid Phase Sintering of PM Steel—A Matter of the Heating Rate

Powder metallurgy (PM) offers several variants to introduce alloying elements for establishing the desired final composition. One route is the master alloy (MA) approach. The composition and the elements contained in the MA can be adjusted to obtain a liquid phase that penetrates through the interconnected pore network and thus enhances the distribution of the alloying elements and the homogenization of the microstructure. Such a liquid phase is often of a transient character, and therefore the amount of liquid formed and the time the liquid is present during the sintering are highly dependent on the heating rates. The heating rate has also an impact on the reaction temperatures, and therefore, by properly adjusting the heating rate, it is possible to sinter PM-steels alloyed with Fe-Cr-Si-C-MA at temperatures below 1250 °C. The present study shows the dependence of the melting regimes on the heating rate (5, 10, 20, 120 K/min) represented by “Kissinger plots”. For this purpose, liquid phase formation and distribution were monitored in quenching dilatometer experiments with defined heating up to different temperatures (1120 °C, 1180 °C, 1250 °C, 1300 °C) and subsequent quenching. Optimum sintering conditions for the materials were identified, and the concept was corroborated by C and O analysis, CCT diagrams, metallographic sections, and hardness measurements.

Microstructure engineering for superior wear and impact toughness strength of hypereutectoid powder metallurgy steel

ABSTRACT In this study, powder metallurgical steel samples containing 1.5% C by weight were produced by sintering in a vacuum atmosphere at 1200°C. The impact test and dry sliding wear test specimens were prepared in accordance with ISO 5754 and ASTM G-99 standards, respectively. The wear and impact toughness properties of spherical cementite in the bainitic matrix (SCBM) microstructured specimens produced with different heat treatment routes were compared with pearlite plus primary cementite, fully tempered martensitic, fully bainitic and spherical cementite microstructured specimens in ferritic matrix (SCFM). Microstructural characterisations of all specimens were performed by SEM, EBSD and XRD analyses methods. Compared to fully tempered martensitic and fully bainitic microstructured specimens with similar hardness, both the dry sliding wear and the impact toughness properties of the specimens having SCBM microstructures were improved with the increased austempering time after spheroidisation treatment.

Properties of a powder metallurgy low-alloy steel with varying fractions of large iron particles

ABSTRACT Interiors of coarse iron particles in blends used in powder metallurgy may not be adequately alloyed during sintering. Relatively low concentrations of alloying elements reduce the hardenability of these regions, which may not transform to martensite during quenching. Since an actual microstructure (martensite, pores and pearlite) may differ from the desired microstructure (martensite and voids), it is reasonable to speculate that large and relatively soft pearlitic inclusions may worsen mechanical properties. This conjecture was tested, and it was experimentally found that unless the fraction of coarse particles exceeded a certain threshold, the properties of a medium-density powder metallurgy steel did not vitiate significantly. It also was observed that when the fraction of large particles dropped below a certain limit, apparent density, compressibility and flowability degraded to an extent rendering the corresponding mixtures unusable.A contradiction between a visual perception and a verdict of a statistical analysis is commented upon.

Capsule-free hot isostatic pressing of sintered steel to full density using water atomised iron and Cr-alloyed powder consolidated by cold isostatic pressing

ABSTRACT Powder metallurgy (PM) steel with enhanced density is essential for meeting the demands on high strength and high-performance applications. In this study, cold isostatic pressing (CIP) is applied for consolidating water-atomised iron and Cr-alloyed steel powder to reach about 95% densification before sintering. Additionally, the effect of the lubricant and graphite addition on the compressibility of the different base powder types was evaluated. The maximum green densities of ≈7.55 g cm−3 for iron powder and ≈7.45 g cm−3 for Cr-alloyed powder were reached after CIP at 800 MPa. The sintering at 1250°C with added graphite allowed to achieve increased densification to ≈7.60 g cm−3 for the iron and ≈7.50 g cm−3 for Cr-alloyed grades, respectively. These levels were sufficient for surface pore closure, which allowed the application of the capsule-free hot isostatic pressing (HIP) after sintering to reach full density. This approach paves the way for manufacturing large-sized PM components.

Sintering of bimodal micrometre/nanometre iron powder compacts - A master sintering curve approach

Though press and sinter powder metallurgy (PM) steel offers cost-effective solutions for structural applications, there is a constant drive for improvement in their density. Addition of nanopowder to the conventional micrometre-sized metal powder is explored to improve the sinter density. In this study, the effect of nanopowder addition in varying amounts has been studied. Carbonyl iron powder (<5 μm) and water atomized iron powder (<45 μm) were used as the base powder to which varying amounts of iron nanopowder (<100 nm) was added. Dilatometric sintering studies under pure hydrogen atmosphere were carried out to analyze the densification behavior. The results revealed that the bimodal powder mixture containing 25% nanopowder exhibited the highest green density for both carbonyl and ASC 300 compacts. Master sinter curve for compacts was developed based on the dilatometer data. The apparent activation energy for sintering decreased with an increase in nanopowder content. This is reflected in the values of work of sintering.

The Influence of Holding Time to Mechanical Properties of Steel Processed with Powder Metallurgy

Steel with very small grain has high strength but still have ductility. Heating the steel until reach to austenite temperature can make the grain of the steel will growth and reduce the hardness of the steel, to prevent this it is necessary to know the effect of holding time on steel hardness. The hardness is indicating of grain growth of the steel. In this research, we investigated the effect of holding time to the hardness of very small-grained steel. Heat treatment was done at 1100 °C using an electric furnace. Micro-hardness testing used a micro-Vickers method using 300 gram forces and indentation time 10 minutes. Microstructure observation was done with metallurgical microstructure. The etching solution for microstructure observation was nital solution. The heat treatment process of steel powder metallurgy with a holding time of 60 minutes will cause hardness to decrease from 245 in holding 15 minutes to 207 and grain size grow from 1 µm to 4 µm.

The Influence of Aluminum Content on the Porosity, Microstructure, and Mechanical Properties of Powder Metallurgy Steels

The Influence of Aluminum Content on the Porosity, Microstructure, and Mechanical Properties of Powder Metallurgy Steels

Wear performance and mechanisms of PCBN tools in boring of powder metallurgy steels

Wear performance and mechanisms of PCBN tools with different CBN content (80% vol. and 95% vol.) and cutting edge geometry (honed edge radius, 0.05 mm chamfered edge width, and 0.10 mm chamfered edge width) in boring powder metallurgy (PM) steels had been evaluated. Post-cutting examination of the worn cutting inserts was performed using focus variation microscopy (FVM), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS). The advanced three-dimensional wear characterization parameters WRM, WMD, WAM, WMH, and WAA are used to measure the wear of PCBN tools. A novel characterization method of removed material (η1 = WMD/WRM) and adhered material (η2 = WMH/WAM) was used to measure the trend of tool wear in this paper. With the increasing of CBN content, the PCBN tool wear parameters WRM, WMD, and WAA were decreased; adhesion was increased by WAM and WMH parameter characterization. For the variation of cutting edge geometry, chamfered edge showed a higher tool life and better wear performance than honed edge radius. With the increasing of chamfered edge width from 0.05 to 0.10 mm, the three-dimensional wear WRM, WMD, WAM, and WMH were increased. WAA showed an opposite trend and parameters η1 and η2 showed a not convergence trend. Besides typical wear mechanisms such as diffusion, adhesion, abrasive, microchipping was observed on PCBN tool; the tool protection layer (TPL) formation and peeling and impact load could be the reasons to explain the microchipping that occurred on cutting edge. Tool life is measured by surface roughness and calculated according to the number of workpieces processed. The results showed that PCBN tool with 95% vol. CBN content and 0.10 mm chamfered edge width achieves higher tool life.

Sinter Hardening PM Steels Prepared through Hybrid Alloying

Abstract In powder metallurgy (PM), there are several ways of introducing alloying elements into a PM material in order to adjust a certain alloying element content. Each alloying route has its advantages and disadvantages. Master alloys (MA), powders with a high content of typically several alloying elements, can be added in small amounts to a base powder, especially to introduce oxygen sensitive elements such as Cr, Mn, and Si. In addition, the master alloy can be designed in such a way that a liquid phase is formed intermediately during the sintering process to improve the distribution of alloying elements in the material and to accelerate homogenization. In this study, such master alloys were combined with pre-alloyed base powders to form hybrid alloyed mixtures with the aim of improving the material‘s sinter hardenability. The hybrid alloys were compared with mixtures of master alloy and plain Fe as reference material. The sinter hardenability of all materials was determined by generating CCT diagrams recorded with 13 different cooling rates. These were verified by metallographic cross-sections of specimens treated at common cooling rates of 3 and 1.5 K/s and subsequent hardness measurements of the microhardness (HV 0.1) of the microstructural constituents and the apparent hardness (HV 30). ◼

Innovative Densification Process of a Fe-Cr-C Powder Metallurgy Steel

In this study, the efficacy of an innovative ultra-fast sintering technique called electro-sinter-forging (ESF) was evaluated in the densification of Fe-Cr-C steel. Although ESF proved to be effective in densifying several different metallic materials and composites, it has not yet been applied to powder metallurgy Fe-Cr-C steels. Pre-alloyed Astaloy CrM powders have been ad-mixed with either graphite or graphene and then processed by ESF. By properly tuning the process parameters, final densities higher than 99% were obtained. Mechanical properties such as hardness and transverse rupture strength (TRS) were tested on samples produced by employing different process parameters and then submitted to different post-treatments (machining, heat treatment). A final transverse rupture strength up to 1340 ± 147 MPa was achieved after heat treatment, corresponding to a hardness of 852 ± 41 HV. The experimental characterization highlighted that porosity is the main factor affecting the samples’ mechanical resistance, correlating linearly with the transverse rupture strength. Conversely, it is not possible to establish a similar interdependency between hardness and mechanical resistance, since porosity has a higher effect on the final properties.

Investigation of microstructure, mechanical and machinability properties of Mo-added steel produced by powder metallurgy method

This study presents the impact of molybdenum (Mo) inclusion on microstructure, mechanical, and machinability behavior of steels manufactured with powder metallurgy (PM) approach. PM steel samples with different molybdenum ratios were pressed at 750-MPa pressing pressure and sintered in the atmosphere-controlled tube furnace at 1400°C for 1 h. While particle size and distribution of phases of PM steels with different molybdenum ratios were determined by optical microscope, mechanical properties were determined by applying tensile test. The results were observed that 3% Mo weight-added steel displayed the maximum yield and tensile strength. In addition, the machinability properties of 3% Mo-added steel, which has the highest yield and tensile strength, were investigated. In this work, we researched the thrust force and surface roughness as machinability output, and drilling parameters on the output were determined by utilizing analysis of variance. Finally, SEM images were taken from the inner surfaces of the machined holes and the fractured surfaces from tensile test, and information about the machinability of this alloy produced with PM was presented. As a result, an increase in yield and tensile strength and a reduction in strain were identified with the increase in Mo content. Moreover, the coated cutting tools are better on the machining output than the uncoated cutting tools in terms of green environment. The most important factors on the Fz and Ra are the coating condition and the feed rate with 56.53% and 43.62% PCR, respectively.

Mechanical properties and fracture mechanism of boron-containing 304L austenitic stainless steel densified by liquid phase sintering

Abstract Boron (B) is an effective element for facilitating liquid phase sintering (LPS) of powder metallurgy (PM) steels. However, the roles of B in the mechanical properties and fracture behaviors of PM steels have not yet been fully clarified. The purpose of this study was to investigate the effects of B on the LPS, microstructure, mechanical properties, and fracture mechanism of 304L PM stainless steel. The deformation and fracture behaviors were analyzed by in-situ tensile tests, digital image correction, and observation of the fracture surfaces. The results showed that adding 0.6 wt% B to 304L significantly increased the sintered density from 6.86 g/cm3 to 7.47 g/cm3 and facilitated pore spheroidization after 1250 °C sintering. Networked eutectics consisting of austenite and M2B borides rich in chromium formed along the grain boundaries, as identified by electron backscatter diffraction and electron probe micro-analysis. Moreover, the apparent hardness improved by 86%, the yield strength by 64%, and the ultimate tensile strength by 63% without obvious sacrifices to tensile elongation. The fracture analyses in this study indicated that, in pure 304L, the main fracture mode was ductile fracture at sintered necks. In contrast, in 304L+0.6B, the predominant fracture mode was transgranular cleavage through the austenite grains. Intergranular eutectics and pores were not the primary fracture sites. The intergranular eutectics were not fully continuous along the grain boundaries and did not dominate the fracture, although the eutectics were the regions with strain concentration or cracking before fracture. The correlations among the microstructure, mechanical properties, and fracture mechanisms of B-alloyed 304L stainless steel are discussed in this study.