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 ,4610 218 atm at 1120uC 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 10 216 atm) due to favourable conditions locally in the material. Sintering of the powder grade with 0?35 wt-% graphite added at 1120uC for 30 min at reducing conditions leads to high mechanical properties, although kinetics is insufficient for complete reduction of chromium oxides. At 1250uC, kinetics is faster and practically all oxides are reduced after sintering for 30 min, with enhanced mechanical performance as result.

Critical aspects of sinter-hardening of prealloyed Cr–Mo steel

“…Shortly, it should however be mentioned that the equilibrium partial pressure of oxygen needs to be below about 10 −18 atmosphere at 1120 • C as shown by e.g. (Berg et al, 2008;Bergman, 2007;Frykholm et al, 2005;Hatami et al, 2008;Yu, 2000). Having such controlled atmospheres it is also possible to sinter Cr-alloyed steel powder and arrive at good mechanical properties Engström et al, 2006).…”

“…Numerous investigations have been published on the atmosphere control for successful sintering of this grade, see e.g. (Berg et al, 2008;Bergman, 2007;Hryha et al, 2007). Thermodynamic simulations and calculations have been carried out to estimate the allowable oxygen partial pressure, dew point, and the CO/CO 2 partial pressure ratio of the atmosphere for successful sintering.…”

Experimental and thermodynamic investigations on the chlorine-induced corrosion of HVOF thermal sprayed NiAl coatings and 304 stainless steels at 700 °C

“…It concerns not only compositions, but also O 2 and Cl 2 partial pressures, phase, and temperatures, etc. Thermo-Calc ® calculation is a useful tool to study the selective oxidation behaviour of alloys, especially those complex multicomponent alloys [37]. The application of Thermo-Calc ® to provide insight into the alumina-forming and chromia-forming alloys systems has been validated by our previous work [28] and others [38][39][40][41][42][43][44][45].…”

Effect of Austempering Conditions on the Microstructure and Tensile Properties of Low Alloyed Sintered Steel

“…At the same time, for materials processes at high sintering temperature, the oxygen content decreased, due to the more effective oxide reduction attained up to 1473 K (1200°C); these behaviors agreed with the findings of others authors. [20][21][22][23][24] A combined carbon level greater than 0.5 pct in weight and low oxygen content indicate that samples were correctly sintered and pressed. Thus, good performance during the heat treatment is expected.…”