Sven Bengtsson

Influence of powder properties on compressibility of prealloyed atomised powders

Abstract The compressibility of metal powders depends on many factors, including morphological and mechanical properties of particles. Alloying elements increasing ferrite solid solution strengthening can influence the compressibility of prealloyed steel powders. The contribution deals with compressibility of Fe–Mn–Cr–[Mo–Ni] prealloyed and premixed powders with total alloying ∼2·0%. Quantification of powders compaction was performed using equation P=P 0exp (−Kp11); proposed by Parilak et al. (1994), where P is the porosity at pressure p; P 0 is apparent porosity; K and n are parameters related to morphology and plasticity of powder particles. The equation enables to study the compressibility in relation to geometry and mechanical properties of powder particles, through development of K and n parameters on pressing pressure p. The tested powders exhibited low varieties in the density at pressing pressures higher than ∼400 MPa, but some differences were identified during the first stage of compaction, at pressing pressures up to ∼200 MPa.

Comparison of high performance PM gears manufactured by conventional and warm compaction and surface densification

Abstract The size of planetary gears in heavy truck gearboxes means that PM can be a cost effective alternative to conventional manufacturing from forged blanks. Warm compaction and surface densification offer effective routes to reach high density, and thereby high strength and fatigue properties. Characteristics for PM gears manufactured by these methods are outlined and compared with wrought and machined gears.

Influence of sintering parameters on the mechanical performance of PM steels pre-alloyed with chromium

Powder grades pre-alloyed with 1.5-3 wt% chromium are suitable for PM steel components in high performance applications. These materials can be successfully sintered at the conventional temperature 1120 °C, although well-monitored sintering atmospheres with low oxygen partial pressures (<10-17-10-18 atm) are required to avoid oxidation. Mechanical properties of the Cralloyed PM grades are enhanced by a higher sintering temperature in the range 1120-1250 °C, due to positive effects from pore rounding, increased density and more effective oxide reduction. A material consisting of Astaloy CrM, which is pre-alloyed with 3 wt% Cr and 0.5 wt% Mo, and 0.6 wt% graphite obtains an ultimate tensile strength of 1470 MPa combined with an impact strength of 31 J at density 7.1 g/cm3, after sintering at 1250 °C followed by cooling at 2.5 °C/s and tempering.

Study of dilution in laser cladding of a carbon steel substrate with Co alloy powders

High power diode laser with coaxial powder injection was used to deposit single tracks of cobalt alloy on to a carbon steel plate in order to study dilution. Two different methods to evaluate dilution are proposed and validated: dilution results to be proportional to the average percent or iron present in the clad. To study the correlations between dilution and processing parameters, clads were produced in different processing conditions. Dilution is correlated with the specific energy, and equation to estimate the average iron contamination of the clads was found. ‘Trial and error’ method was applied to improve this estimation. A statistically better prediction of the iron contamination is obtained when the combined parameter P2.5/F4 is used. Dilution influences clad microstructure and thus hardness of the final coating, which decreases on increasing dilution. Phase distribution is also affected by dilution, Fe and C contamination stabilises α-fcc phase.

Chemical Reactions During Sintering of Mn and Mn-Cr Prealloyed Steels in Inert versus Reducing Atmospheres

In this work the reduction during sintering of Mn and/or Cr prealloyed steels has been studied. In inert atmospheres, the reduction of surface and especially of internal oxides in these steels is shifted to markedly higher temperatures than in Fe-C or conventionally alloyed PM steel grades, reduction being not yet completed at 1300°C. At least at higher alloy element contents, the main reduction starts about 1000°C compared to 700°C for Fe-C, in both cases removal of surface oxides being the first major process. Sintering in high purity H2 results in partial reduction in the 400°C range, H2O being formed as the product, but removal of the remaining oxides requires more or less the same high temperatures as in inert atmospheres, and the reaction product is mainly CO, which confirms that at temperatures >1000°C carbon is the reducing agent in any sintering atmosphere.

PM takes on truck test in the gearbox

Truck gear boxes take some of the heaviest strains in automotive engineering, but it looks as though PM is up to the challenge…

Reaction of Unalloyed and Cr-Mo Alloyed Steels with Nitrogen from the Sintering Atmosphere

Abstract Nitrogen is usually regarded as an inert sintering atmosphere for PM steels; however, this cannot be taken for granted in particular for steels alloyed with nitride forming elements. Among those elements, chromium has become more and more important as an alloying element in sintered low alloy structural steels in the last decade due to the moderate alloying cost and the excellent mechanical properties obtainable, in particular when sinter hardening is applied. The high affinity of Cr to oxygen and the possible ways to overcome related problems have been the subject of numerous studies, while the fact that chromium is also a fairly strong nitride forming element has largely been neglected at least for low alloy steel grades, although frequently used materials like steels from Cr and Cr-Mo prealloyed powders are commonly sintered in atmospheres consisting mainly of nitrogen. In the present study, nitrogen pickup during sintering at different temperatures and for varying times has been studied for Cr-Mo prealloyed steel grades as well as for unalloyed carbon steel. Also the effect of the cooling rate and its influence on the properties, of the microstructure and the composition have been investigated. It showed that the main nitrogen uptake occurs not during isothermal sintering but rather during cooling. It could be demonstrated that a critical temperature range exists within which the investigated CrM-based steel is particularly sensitive to nitrogen pickup.

Microstructure of High Cr-Alloyed Sintered Steel – Prediction and Analysis

Study of microstructure of high Cr-alloyed sintered austenitic stainless steel was performed in few stages XPS analysis of powder surface, theoretical prediction of microstructure by Thermo-Calc and JMatPro software and metallographic observation of sintered material. XPS analysis showed presence of thin iron oxide layer on the surface of powder particles and oxide islands formed by Si, Mn and Cr. Theoretical prediction made by Thermo-Calc and JMatPro calculations showed presence of austenite with chromium carbides and carbonitrides in equilibrium state. Both predictions are in good agreement. Metallographic observation of sintered material showed that microstructure contains small austenitic grains with size of 3-5 μm with fine carbides (1-2 μm) and carbonitrides distributed mostly on grain boundaries. Metallographic study of material confirmed theoretical predictions.

Local Mechanical Properties of Cast and Sintered High Cr-Alloyed Steel

Local mechanical properties of high Cr-alloyed sintered and cast steels with the same chemical composition were investigated using instrumented indentation method. Standard loading/unloading mode was applied, the measurements were done in load range 1 – 500 mN. Load size effect was observed and its parameters were evaluated. Indentation hardness and elastic modulus were found slightly higher for the sintered material. Differences in indentation parameters were explained based on microstructure of materials.

High Performance Planetary Gears for Heavy Duty Automotive Transmissions

Planetary gears in heavy truck gearboxes are normally manufactured by forging a blank, turning, hobbing, shaving and heat-treatment followed by grinding. Due to the size of the gear the net shape capability of PM methods can be cost effective alternatively to conventional manufacturing. Warm compaction and surface densification are two PM methods to reach high density and thereby high strength and fatigue properties. Typical characteristics for PM gears manufactured by these methods are outlined.