Novel approaches for achieving full density powder metallurgy steels

Powder metallurgy (PM) is one of the most resource-efficient methods for manufacturing structural components with complex shapes. The utilisation of the metal powder to shape the components allows to minimise material waste and increase energy efficiency. However, with increased usage of PM parts in high-performance applications, there is a demand for components that can withstand extreme loading conditions with properties being equivalent or better than those of their wrought counterparts. The PM steel components fabricated through press and sinter route, even with all their advantages, have limitations due to the presence of residual porosity. Hence, it is desirable to reach full density to meet the highest performance demands. This study covers different powder consolidation approaches for water atomised steel powder with the aim of reaching near full density. This is achieved through the following processes: cold isostatic pressing (CIP) followed by sintering, liquid phase sintering (LPS), double pressing-double sintering (DPDS). These approaches were complimented by capsule free hot isostatic pressing (HIP) to reach full density.Densification and subsequent enhancement of mechanical properties are to a certain extent directly connected to the successful removal of the surface oxide layer, covering the metal particles. This behaviour is especially critical in the case of powder pre-alloyed with oxygen-sensitive elements as chromium. The hydrogen in the sintering atmosphere reduces most of the surface iron oxide layer and any oxide residues are transformed into more stable oxides rich in Cr and Mn. Vacuum sintering provides oxide reduction through the formation of better local microclimate in the pores. When the powder is encapsulated and processed using HIP, the initial surface oxide is transformed into stable oxide particles that decorate the particle boundaries. Based on these results a model of oxide transformation during powder consolidation is proposed with regards to the alloy composition, powder properties and processing conditions.In order to realise full density, CIP is utilised for consolidating iron powder and Cr-Mo pre-alloyed water atomised powder to reach a relative density of around 95% in sintered state to attain surface pore closure. This allows for subsequent HIP without capsule to reach full density. In case of Mo pre-alloyed powder, the LPS approach utilising Ni-Mn-B master alloy was established for enhanced sintering and densification. The best mechanical properties were then obtained with 0.12 wt.% of boron that allowed reaching as-sintered relative density of up to 96%. In addition, pore free surface was obtained after sintering that enabled capsule-free HIP to reach full density. Through the DPDS process, a pore free surface could also be achieved, which enabled reaching full-density through the subsequent HIP. Even though fine powder showed better densification, the density gradient in the compact persisted from the first pressing is there as the low-density region i.e., neutral zone, in the middle of the compact even after second pressing and HIP. Hence, optimisation during the first pressing is necessary to avoid this phenomenon.All the above approaches represent different methods of achieving full density and selection of the appropriate method depends on the required geometry, alloy composition and hence resulting properties, number of components, cost, etc. Based on the analysis of the different methods it can be concluded that the combination of the tailored alloy concepts and consolidation techniques allows manufacturing of complex-shaped full-density PM components for high-performance applications

Atom probe tomography characterisation of powder forged connecting rods alloyed with vanadium and copper

The precipitation of V and Cu in the powder forged connecting rods of Fe-V-Cu-C alloy, were studied by atom probe tomography (APT). The purpose of alloying with V was to further improve the mechanical properties of the existing powder forged materials based on Fe-Cu-C. In this study, materials tested at room temperature and 120 degrees C were investigated. It was found that Cu was unevenly distributed on a micrometer scale. The local Cu content affected the Cu precipitation; a higher Cu content resulted in a higher volume fraction of precipitates. The V was found to form very small nitrides. The N presumably originates from the sintering process. The vanadium nitrides act as nucleation points for the Cu precipitates during cooling of the material during fabrication. APT analysis of the deformed material close to the fracture surface of the tensile test samples showed a similar volume fraction of Cu precipitates, but statistical analysis of the data indicates that both Cu and VN precipitates are more diffuse than in the undeformed material

XPS Analysis of Oxide Transformation During Sintering of Chromium Alloyed PM Steels

Water atomized PM steels prealloyed with chromium are becoming more and more widely used due to their high performance characteristics and sinter-hardening capabilities. Due to its low cost and recyclability, chromium is a cost effective alloying alternative to replace expensive and toxic Ni and non-recyclable Cu that are traditionally used in PM. Chromium has higher affinity towards oxygen that brings risk of formation of stable oxides that are difficult to reduce and hence inhibit the development and the growth of inter-particle necks during sintering. This in turn directly influences the strength of the compacts. In this study, the analysis of the change in oxide state after delubrication and during heating stage until sintering temperature was performed by means of surface sensitive analytical techniques such as XPS and high resolution SEM combined with EDX analysis. Composition, morphology and distribution of the oxides was estimated on the fracture surface of the compacts, based on powder prealloyed with 1.8 wt. % Cr, in the as-delubricated condition and sampled during the heating stage at 700, 900 and 1120 \ub0C. Sintering atmosphere applied was dry 90% N2/10% H2 atmosphere (dew point ~-50 \ub0C). The results show enrichment of surface oxides in Cr and Mn at around 900 \ub0C during heating stage followed by their significant reduction close to the sintering temperature. This study provides the important details for tailoring the sintering process in terms of proper atmosphere and process control for chromium alloyed powder

XPS Analysis of Oxide Transformation During Sintering of Chromium Alloyed PM Steels

Water atomized PM steels prealloyed with chromium are becoming more and more widely used due to their high performance characteristics and sinter-hardening capabilities. Due to its low cost and recyclability, chromium is a cost effective alloying alternative to replace expensive and toxic Ni and non-recyclable Cu that are traditionally used in PM. Chromium has higher affinity towards oxygen that brings risk of formation of stable oxides that are difficult to reduce and hence inhibit the development and the growth of inter-particle necks during sintering. This in turn directly influences the strength of the compacts. In this study, the analysis of the change in oxide state after delubrication and during heating stage until sintering temperature was performed by means of surface sensitive analytical techniques such as XPS and high resolution SEM combined with EDX analysis. Composition, morphology and distribution of the oxides was estimated on the fracture surface of the compacts, based on powder prealloyed with 1.8 wt. % Cr, in the as-delubricated condition and sampled during the heating stage at 700, 900 and 1120 \ub0C. Sintering atmosphere applied was dry 90% N2/10% H2 atmosphere (dew point ~-50 \ub0C). The results show enrichment of surface oxides in Cr and Mn at around 900 \ub0C during heating stage followed by their significant reduction close to the sintering temperature. This study provides the important details for tailoring the sintering process in terms of proper atmosphere and process control for chromium alloyed powder

Comparative study on the densification of chromium pre-alloyed powder metallurgy steel through nanopowder addition using design of experiments

There is a constant demand for high density press and sinter powder metallurgical components for automotiveapplications. Steel powder pre-alloyed with chromium is an attractive material for such applications, but newways to further increase the sinter density are required for successful processing of these powders to high density.Nanopowder could be used as a potential sintering aid in order to boost the densification of the steel powdercompact. In this study, steel powder pre-alloyed with chromium, without and with admixed nickel, is used as basepowder, to which nanopowder was added. Surface oxide removal, crucial for successful sintering of such mate-rials, was studied by thermogravimetry analysis in order to understand the influence of nanopowder addition onthe oxide reduction. Powder compacts containing nanopowder showed higher mass loss in comparison to the oneswithout nanopowder. Linear shrinkage obtained from dilatometric curves increased with the addition of nano-powder. To depict the influence of the critical parameters; sintering temperature, powder size, addition ofnanopowder and composition (with or without nickel), a design of experiment approach was applied. The criticalparameters were then adjusted at 2 different values (categorical parameters) and a‘full factorial design model’was used involving 16 experiments, with sinter density and hardness as output measures of the experimentsdetermined. The results were analyzed using polynomialfit to determine which of the parameter exerts themaximum influence. Presence of nickel increased the hardness whereas sintering temperature and presence ofnanopowder enhanced the sinter density. This led to the tentative design of optimum conditions that resulted inincrease in sinter density from 7.25 g/cm3(92.5% of the theoretical density) to 7.4 g/cm3(94% of the theoreticaldensity) with an addition of 5% nanopowder to Ni-containing grade when sintered at 1350\ua0​\ub0C instead of 1250\ua0​\ub0C

Experimental and finite element simulation study of capsule-free hot isostatic pressing of sintered gears

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

Effect of Alloying Type and Lean Sintering Atmosphere on the Performance of PM Components

In order to be cost effective and to meet increasing performance demands, powder metallurgy steel components require continuous improvement in terms of materials and process development. This study demonstrates the feasibility of manufacturing structural components using two different alloys systems, i.e. lean Cr-prealloyed and diffusion bonded water atomised powders with different processing conditions. The components were sintered at two different temperatures, i.e. 1120 and 1250 \ub0C for 30 minutes in three different atmospheres: vacuum, N2-10%H2 atmosphere as well as lean N2-5%H2-0.5%CO-(0.1-0.4)%CH4 sintering atmosphere. Components after sintering were further processed by either low pressure carburizing, sinterhardening or case hardening. All trails were performed in the industrial furnaces to simulate the actual production of the components. Microstructure, fractography, apparent and micro hardness analyses were performed close to the surface and in the middle of the sample to characterize the degree of sintering (temperature and atmosphere) and the effect of heat treatment. In all cases, components possess mostly martensitic microstructure with a few bainitic regions. The fracture surface shows well developed sinter necks. Inter- and trans-granular ductile and cleavage fracture modes are dominant and their fraction is determined by alloy and processing rout

Manufacturing of Valve Bridge Component Utilizing Lean Alloyed Powders and Vacuum Sintering

Increasing the application area of powder metallurgy (PM) steels for\ua0manufacturing of high-performance structural components results in\ua0material saving, reduction in energy consumption, etc. In this study,\ua0feasibility of the manufacturing of valve bridge component for heavy duty\ua0engine utilizing lean alloyed powders and novel vacuum sintering\ua0approach, followed by low pressure carburizing, is studied. Three low\ua0alloyed steel powders were processed by conventional uniaxial pressing\ua0and sintering at 1120 and 1250\ub0C in industrial vacuum furnace. The\ua0components were tested under high cycle fatigue testing, simulating real\ua0conditions of operation. Fatigue properties did not show significant\ua0dependence on the sintering temperature and were comparable to\ua0currently used reference cast material. Fracture surfaces of broken\ua0samples were analyzed to detect crack initiations and fracture\ua0mechanisms as well as quality of sintering. Results showed preferentially\ua0ductile failure, well developed sintering necks and clean pore surfaces,\ua0indicating good sintering. Tested material in combination with novel\ua0vacuum sintering process show to be an attractive alternative for\ua0manufacturing of this type of components for heavy duty engineapplications

Investigation of Low Temperature Creep Behaviour of PM Steels

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\ub0C) and high static loading may experience dimensional instability. Hence, high performance diffusion-alloyed powder grade was investigated for low temperature creep/relaxation at 120\ub0C 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