Ola Bergman

Influence of oxygen partial pressure in sintering atmosphere on properties of Cr–Mo prealloyed powder metallurgy steel

Abstract Chromium is an attractive alloying element in low alloyed steels since it gives good hardenability at low cost. The drawback with using chromium in powder metallurgy (PM) grades is its high affinity for oxygen. Thermodynamic calculations show that the oxygen partial pressure should be <4 × 10−18 atm at 1120°C in order to have reducing conditions for a powder grade prealloyed with 3 wt-%Cr and 0·5 wt-%Mo, which is supported by sintering experiments. With graphite added to the powder grade, conditions are reducing during sintering at higher partial pressures of oxygen (up to 10−16 atm) due to favourable conditions locally in the material. Sintering of the powder grade with 0·35 wt-% graphite added at 1120°C for 30 min at reducing conditions leads to high mechanical properties, although kinetics is insufficient for complete reduction of chromium oxides. At 1250°C, kinetics is faster and practically all oxides are reduced after sintering for 30 min, with enhanced mechanical performance as result.

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.

Cu redistribution during sintering of Fe–2Cu and Fe–2Cu–0·5C compacts

Abstract The effective use of alloying elements in powder metallurgical steels requires a deep understanding of their redistribution kinetics during sintering. In this work, interrupted sintering trials of Fe–2Cu and Fe–2Cu–0·5C compacts were performed. Moreover, diffusion simulations of Cu in γ-Fe using Dictra were performed. It is found that transient liquid phase penetrates the Fe interparticle and grain boundaries in less than 3 min of holding time. However, C addition limits the penetration of liquid Cu, particularly into grain boundaries of large Fe particles. The results also show that the mean diffusion distance of Cu in γ-Fe in the C added system is slightly lower than that in the C-free system at 3 min of holding time; however, after 33 min, the mean diffusion distance is similar in both systems. The diffusion distances of Cu in γ-Fe, predicted by Dictra, are in good agreement with the measured values.

Modified powders ‘point the way to designer alloys’

Early results from Hoganas researchers clearly show that modified specification powders, outside normal standard compositions, can be manufactured and subsequently processed into components by readily available processes. They say that although its work in progress, it opens up possibilities for ‘designer’ alloys…

Behaviour of master alloy during sintering of PM steels: redistribution and dimensional variations

Abstract The addition of alloying elements in low alloyed PM steels in the form of a master alloy gives the advantage of introducing oxidation sensitive but less expensive elements and also allows manipulation in composition adjustment to achieve desired properties. In this work, interrupted sintering trials of the Fe–2MA–0.5C (%) (MA = Cu based master alloy) are performed. The behaviour of the liquid forming master alloy, for instance in terms of liquid phase formation, alloying element redistribution and effect on the dimensional changes, is investigated. The results show that master alloy particles melt over a range of temperature, which is also supported by the thermodynamic calculations. The low swelling in the master alloy system, compared to a reference system of Fe–2Cu–0.5C, is attributed to the progressive melting of the master alloy. The mean diffusion distance of Cu in Fe at the interparticle boundaries is 5.8 μm after 34 min of isothermal holding.

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.