The powder metallurgical (PM) process chain is a great opportunity for gear manufacturing regarding both material and energy consumption compared to conventional wrought steel gear production and could potentially also reduce manufacturing cost. However, the remaining porosity after pressing and sintering results in a lower load capacity of the tooth flank and tooth root. Therefore, PM gears for high performance applications are surface densified to increase the density of the tooth surface.This paper presents a method for modelling the surface densification rolling process and an experimental validation of the model. The results are evaluated and discussed regarding geometry, process parameters and material behavior. With the validated model, a method for process design is developed and presented, and it is shown how the process development time can be shortened through simulation.

Astaloy® CrS is a sustainable alternative to Fe-Cu-C PM steels that can also be heat treated in a conventional way with gas carburization and oil quenching. When heat treated this material has the potential to reach performance levels of Mo-prealloyed PM steels. To achieve improved fatigue performances, the evaluation was performed in two stages. First by varying the heat treatment parameters such as carburisation temperature and holding time. Secondly, Ni and graphite amounts were varied, applying carburisation at low temperature, to realise the full potential of this material. The performance was benchmarked against other materials. By optimising heat treatment and mix compositions, fatigue performance has significantly improved, making Astaloy CrS superior to Fe-Cu-C and equivalent to that of Mo-prealloyed PM steels.

A lean Cr-alloy with 0.85wt.% Cr and 0.15wt.% Mo was recently introduced as a Cu-free sustainable alternative PM material. It is always necessary to ensure sufficient hardenability for a selected component and heat treatment process, and with leaner alloys more precautions are necessary. Low-pressure carburising (LPC) combined with gas quenching is preferred for carburising Cr materials, as it avoids oxidation and provides better control over case depth. However, the nature of the gas quenching process requires more alloying to achieve better hardenability compared to oil quenching. In this study to investigate the influence of alloying element on the hardenability, both Ni and graphite were varied between 0 and 2 wt.% and 0.25-0.45 wt.%, respectively for the lean Cr samples. LPC was performed after sintering, and the mechanical performance and hardenability were evaluated. In addition, using simulations, geometry aspects necessary to fulfil the hardenability requirement for high performances were explored to be able to optimise the alloying content for a given application.

Porosity and interparticle neck size are microstructural parameters that play an important role for pressed and sintered materials. To understand the effect of sintering parameters such as time and temperature on the microstructure of a pre-alloyed sintered steel (Astaloy® 85 Mo), a mean-field modeling approach tracking the neck size and geometry evolution during sintering is developed in combination with experimental studies. Building upon a mathematical framework describing the geometrical changes in a presently developed two-particle model, due to the diffusion mechanisms active during solid-state sintering, the influence of sintering conditions and of the initial microstructure on various aspects of the geometry is investigated. In addition, the predicted effects of each diffusion mechanism on different geometrical parameters are presented. To calibrate the model, the green-to-sintered dimensional change as well as experimentally observed microstructures of Astaloy® 85 Mo are also studied.

The powder metallurgical (PM) process chain is a great opportunity for gear manufacturing regarding both material and energy consumption when compared to the conventional gear production. However, the remaining porosity after pressing and sintering results in a lower load capacity of the tooth flank and tooth root. Therefore, PM gears for high performance applications are surface densified to increase the density of the surface of the tooth. \n This paper presents a new general approach of modelling the surface densification rolling process of PM gears. Using the finite element analysis, the material behavior, geometry, and process kinematics are modelled. The method leads to a prediction of the densification profile as well as geometry and can be used as a tool to optimize the rolling process. The results are evaluated and discussed in relation to the process parameters and material behavior.

One of the characteristic features of sintered steels is the porosity in their microstructure resulting from the compaction and sintering process. This porosity strongly influences the mechanical properties. To enhance the understanding for the structure–property relationship of sintered Astaloy®85Mo with 0.4 wt.%C, a micromechanical modelling approach based on face-centred cubic (fcc) representative volume elements (RVE) is proposed. The fcc-like periodic arrangement of the sintered particles in the RVE enables the consideration of a realistic non-spherical pore morphology. To compare the predictions with experimental results, accompanying uniaxial tensile tests are considered at different pore volume fractions after initial microstructure characterisation. In addition to the effect of pore volume fraction, the influence of sinter necks on the predicted overall strength is also systematically investigated. Despite the fairly simple nature of the underlying fcc structure, the RVE simulations are perfectly capable of reproducing the experimental trend, showing that the elasto-plastic properties decrease with increasing porosity. This is in contrast to analytical predictions, which underestimate the decrease in properties due to spherical pore assumptions. Moreover, the finite element-based simulations reveal a less pronounced influence of the sinter neck shape on the macroscopic behaviour, even though substantial differences in plastic strain localisation are discernible at the microscopic scale.

Sinter hardening is a cost efficient manufacturing process for press and sinter. At the same time material selection is crucial to make sure sufficient hardenability is reached. By selecting a higher alloyed material, hardenability can easily be satisfied, however this also increases the material cost. Thus, it’s interesting to be able to optimize the composition. In this paper the potential to use Astaloyâ CrS, a newly developed lean chromium grade, is investigated for sinter hardening with different combinations with nickel and graphite. \nA model is presented, where the cooling rates for sinter hardening conditions are calculated and combined with a calculation of the martensite transformation from a Time Temperature Transformation diagram (TTT). It’s demonstrated how the martensite content after heat treatment can be estimated by taking factors such as material composition, component size and quenching conditions into account. The calculated results are also compared with experiments in the paper. In the end, the tool can help making sure a component is properly hardened while optimizing the material composition cost.

When processing powder metallurgy (PM) steels, the conventional press and sinter route can reach a relative density up to 95%, which is insufficient for applications when dynamic mechanical performance is critical. In this study, a novel route is demonstrated consisting of cold isostatic pressing (CIP) followed by sintering and capsule-free hot isostatic pressing (HIP), allowing to achieve full density PM steels. Water-atomized steel powder admixed with 2 wt.% Ni was subjected to CIP and followed by sintering in 90N 2 /10H 2 atmosphere at 1120 and 1250°C, and in vacuum (10 −2 mbar) at 1250 and 1350°C, respectively. At the highest explored CIP pressure of 600 MPa, the three high-temperature sintering runs at 1250°C in 90N 2 /10H 2 atmosphere and vacuum, and 1350°C in vacuum resulted in relative density of ∼94% and closed surface pores. This condition with necessary closed porosity then allowed subsequent capsule-free HIP after sintering, resulting in full densification of the components.

The performance of PM steels is directly related to the material properties which is a consequence of the input alloying addition and processes involved. However, it is of significance to understand the hardenability requirements, specific to the component size/dimensions and the selected material for the specific heat treatment (HT) processes where gas and oil are used for quenching. Processes such as sinter-hardening, casehardening, and through-hardening are commonly performed to enhance the performances. In this work, quenching simulations were performed and a tool for hardenability calculation is developed with respect to the materials, components size, and different quenching mediums. For given component dimensions this tool predicts the suitable material and the cooling rate required to transform the microstructure into fully martensitic. This allows for minimising the number of trials required for optimising the HT process for PM steels.

In this study, compacts of Cr-prealloyed steel with admixed nickel and graphite, fabricated through cold isostatic pressing (CIP), were sintered in low vacuum at 1150 °C and at 1250 °C in a HIP furnace, followed by the capsule-free hot isostatic pressing (HIP) at 1150 °C in the same HIP furnace using argon at 100 MPa. Microstructures of these compacts sintered at 1250 °C revealed the complete closure of interconnected porosity, after which densification to full bulk density was achieved by final HIP stage. Hardness measurements and chemical analysis were also employed. Carbon as a reducing agent played a crucial part to reach very low oxygen content of 0.02% after sintering and capsule-free HIP. This study demonstrates the possibility of achieving full density in high performance powder metallurgy (PM) steels through novel approach of CIP and in-situ vacuum sintering in combination with capsule-free HIP using a two-stage densification process.

The properties and the performance of PM steels can be tailored and enhanced by performing an appropriate heat treatment process after sintering. Conventional gas carburising (CQT) is the most used process for PM steels, whereas for Cr-alloyed PM steels, casehardening by low-pressure carburising (LPC) is gaining interest due to its oxide free process and better control over case depth. Also, when combined with high-pressure gas quenching (GQ) it provides less distortion in the components. In this work, Cr-alloyed PM steel with and without Ni additions were evaluated after both the CQT and LPC heat treatment process. The mechanical properties and performances were evaluated, and the results obtained indicated a strong correlation between the obtained properties and resulting microstructures based on the alloying elements and optimised heat treatment process.

Astaloy™ 85 Mo is a pre-alloyed, water-atomized 0.85% Mo steel powder. The aim of the present investigation is to study the influence of porosity, controlled by both mechanical and thermal processing, on the mechanical properties in a bainitic microstructure of a pressed and sintered steel. To achieve this, uniaxial tensile and compression testing is performed, together with Vickers macro- and microhardness experiments. Microhardness testing is carried out in order to determine the behavior of the matrix material at a scale where porosity influence is minimized. Both the influence from size and shape of the pores is investigated and compared with relevant mechanical analyses for porous solids. Such mechanical analyses are pertinent to both elastic and plastic properties, where in the latter case the well-known Gurson-Tvergaard model for solids with spherical pores is relied upon. It is shown that assuming a spherical pore shape is not sufficient in order to achieve good agreement between predictions and experimental results and will be further investigated in future studies.

Metal Powder ReportVol. 74, No. 4 Special FeatureManufacturing full density powder metallurgy gears through HIP:ingMichael Andersson, Magnus Bergendahl, Ulf Bjarre, Anders Eklund, Staffan Gunnarsson, Sven Haglund, Hans Hansson, Irma Heikkilä, Alireza Khodaee, Arne Melander, Harald Nyberg, Lars Nyborg, Annika Strondl, Maheswaran Vattur SundaramMichael Andersson⁎Corresponding author. email: E-mail Address: [email protected]aHöganäs AB, SwedenSearch for more papers by this author, Magnus BergendahlbVolvo AB, SwedenSearch for more papers by this author, Ulf BjarrecScania CV AB, SwedenSearch for more papers by this author, Anders EklunddQuintus Technologies AB, SwedenSearch for more papers by this author, Staffan GunnarssoneUddeholm AB, SwedenSearch for more papers by this author, Sven HaglundfSwerim AB, SwedenSearch for more papers by this author, Hans HanssongSwepart Transmission AB, SwedenSearch for more papers by this author, Irma HeikkiläfSwerim AB, SwedenSearch for more papers by this author, Alireza KhodaeehKTH Royal Institute of Technology, SwedenSearch for more papers by this author, Arne MelanderiSwerea KIMAB, SwedenSearch for more papers by this author, Harald NybergcScania CV AB, SwedenSearch for more papers by this author, Lars NyborgjChalmers University of Technology, SwedenSearch for more papers by this author, Annika StrondlfSwerim AB, SwedenSearch for more papers by this author, Maheswaran Vattur SundaramjChalmers University of Technology, SwedenSearch for more papers by this authorMichael Andersson; Magnus Bergendahl; Ulf Bjarre; Anders Eklund; Staffan Gunnarsson; Sven Haglund; Hans Hansson; Irma Heikkilä; Alireza Khodaee; Arne Melander; Harald Nyberg; Lars Nyborg; Annika Strondl; Maheswaran Vattur SundaramPublished Online:30 Sep 2021https://doi.org/10.1016/j.mprp.2018.12.076AboutSectionsView articleView Full TextPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareShare onFacebookTwitterLinked InEmail View article References [1] A. Flodin et al. Metal Powder Report; 72(2); 2017: 107–110. Google Scholar[2] Sundaram M. Vattur et al. Int. J. Adv. Manuf. Technol.; 99(5–8); 2018: 1725–1733. Google Scholar FiguresReferencesRelatedDetails 1 January 2019Volume 74Issue 4ISSN (print): 0026-0657ISSN (online): 1873-4065 Metrics History Published online 30 September 2021 Published in print 1 January 2019 Information© MA Business Ltd.AcknowledgementsThis work was financed by the national agency VINNOVA under contract 2013-05594 within the Programme FFI. The project consisted of the following partners: Chalmers University of Technology, Höganäs AB, KTH Royal Institute of Technology, Leax AB, Quintus Technologies AB, Scania CV AB, Swepart AB, Swerea KIMAB AB, Uddeholm AB and Volvo AB.PDF download

Gears made of PM steel are of interest for the automotive industry because they can be produced to near net shape with only a few processing steps. Automotive gears experience a complex contact situation with rolling as well as combined rolling and sliding. They also have to be able to withstand high loads and fairly high temperature variations. Earlier work show that the main limiting parameter of the contact fatigue life of PM steel is the porosity. A higher porosity/lower density will decrease the fatigue life of the PM component. In the present study, the pure rolling contact fatigue life of PM steel with different density and surface finish has been investigated. A rolling contact fatigue test rig, where rods of the tested material are mounted between two rolling wheels, was used. Two densities of PM steel, 6.8 and 7.15 g/cm(3) and a full density reference steel with two different surface finishes, centerless grinded and superfinished, were tested. The tests were run for a given number of load cycles or until failure (fatigue life). SEM was used to study the surfaces and cross-sections to reveal the mechanisms of crack initiation and propagation. The higher density PM steel (7.15 g/cm(3)) outperformed the lower density steel (6.18 g/cm(3)) by a factor of around 4 in fatigue life at the same surface pressure, regardless of surface finish. Cracks are initiated at a depth of around 100 mu m. These cracks propagate and eventually they reach the surface, causing surface damage and failure. For the low density PM steel, both sub surface crack initiation and failure occurred earlier (at a lower number of load cycles) than for the higher density PM steel. Severe surface damage or wear were not found until failure occurred. Still, some initial alteration of the surfaces was seen already after 0.5 million load cycles, in the form of removal of the highest asperity peaks on the centerless grinded surfaces, and opening of the surface pores on the superfinished surface. No effect of surface finish was found on the fatigue life. The difference in surface roughness could induce a difference in local stress concentrations at the surface, but in this test the cracks causing fatigue failure are initiated at a depth where the stress distribution is not affected by local surface stress concentrations. This means that for fairly smooth surfaces roughness, the surface of PM steel is not important when it comes to pure rolling fatigue life.

A novel approach to reach full density in powder metallurgy (PM) components is demonstrated in this work. Water-atomised Mo-prealloyed steel powder is utilised for manufacturing cylindrical and gear samples through double pressing and double sintering (DPDS) process route. The effect of sample geometry and powder size fraction on densification is investigated and it is found that the DPDS route enables a density level of >95% which is sufficient to eliminate the surface open pores. Reaching such high density is necessary, in order to perform capsule-free hot isostatic pressing (HIP). After HIP, full densification is achieved for the cylindrical samples and only near full density is realised for the gears resulting in neutral zone formation due to the density gradient. In order to predict the densification behaviour during the compaction, FEM simulations considering the gear geometry are performed for both the pressing stages and HIP. The simulation predicted a similar densification behaviour with the formation of the neutral zone. The proposed DPDS route with capsule-free HIP in combination with FEM simulation is demonstrated as a potential route for manufacturing full-density PM steel components, e.g. gears, suitable for high-performance applications.

The automotive industry accounts for almost 70% of the total use of water atomized steel powder for powder metallurgical structural components. Nowadays, such components are increasingly used for high demanding applications, where good tolerances and high mechanical properties are combined. However, it has been found that some PM-steel components at low temperature (100-150°C) and high static loading may experience dimensional instability. Hence, high performance diffusion-alloyed powder grade was investigated for low temperature creep/relaxation at 120°C and 20 kN tensile loading (corresponding to 90% of the yield strength of the material). The materials investigated were sinter-hardened and subsequently tempered at different temperatures. Characterization using different techniques (optical microscopy, dedicated testing, X-ray analysis, hardness testing, etc.) was carried out before and after creep testing and it was revealed that each kind of sample exhibited creep/relaxation behaviour correlated to the tempering temperature. The results were compared to test results of components under similar conditions and a good correlation between the test bars and components were found. Moreover, it was found that selecting proper tempering considerably lowered the creep/relaxation response. Hence, the dimensional instability at high static loading conditions for the studied powder metallurgical could be reduced.

This study addresses the sinter-hardening response of sintered steel based on Astaloy Mo grade (prealloyed with 1.5 wt.% Mo) and its derivatives Distaloy DH- diffusion bonded with 2 wt.% copper and Distaloy DC- diffusion bonded with 2 wt.% Ni, respectively. Compacts of these powder grades were sintered at 1120°C for 30 minutes and cooled at a rate of 2.5 °C/s within the range of 800 to 300 °C followed by stress relieving at 200 °C for 60 minutes. In order to understand the relation between microstructure and the obtained mechanical properties, tensile testing and metallographic investigations were performed. It is shown that the distribution of the alloying elements as expected has a crucial impact on the microstructure development where regions lean in Cu or Ni become bainitic while regions with Cu or Ni become martensitic. Also, despite of the fact that Cu is more homogeneously distributed into the DH material, while Ni partly remains in Ni-rich regions in the DC material, both alloys show similar sinter-hardenability. The compensating effects by carbon redistribution upon bainite formation and Ni on C activity are supposed to be key factors in this context.