Processing, Microstructures and Properties of a Dual Phase Precipitation-Hardening PM Stainless Steel

To improve the mechanical properties of PM stainless steels in comparison with their wrought counterparts, a PM stainless steel alloy was developed which combines a dual-phase microstructure with precipitation-hardening. The use of a mixed microstructure of martensite and ferrite results in an alloy with a combination of the optimum properties of each phase, namely strength and ductility. The use of precipitation hardening via the addition of copper results in additional strength and hardness. A range of compositions was studied in combination with various sintering conditions to determine the optimal thermal processing to achieve the desired microstructure. The microstructure could be varied from predominately ferrite to one containing a high percentage of martensite by additions of copper and a variation of the sintering temperature before rapid cooling. Mechanical properties (transverse rupture strength (TRS), yield strength, tensile strength, ductility and impact toughness) were measured as a function of the v/o ferrite in the microstructure. A dual phase alloy with the optimal combination of properties served as the base for introducing precipitation hardening. Copper was added to the base alloy at various levels and its effect on the microstructure and mechanical properties was quantified. Processing at various sintering temperatures led to a range of microstructures; dilatometry was used utilized to monitor and understand the transformations and the formation of the two phases.;The aging process was studied as a function of temperature and time by measuring TRS, yield strength, tensile strength, ductility, impact toughness and apparent hardness. It was determined that optimum aging was achieved at 538°C for 1h. Aging at slightly lower temperatures led to the formation of carbides, which contributed to reduced hardness and tensile strength. As expected, at the peak aging temperature, an increase in yield strength and ultimate tensile strength as well as apparent hardness was found. Aging also lead to an unexpected and concurrent increase in ductility and impact toughness. The alloys also showed an increase in strain hardening on aging. The increase in ductility varied with the v/o martensite in the microstructure and was shown to occur after short time intervals at the optimum aging temperature. Compressive strength measurements revealed that the increase in ductility was due to the relaxation of residuals stresses that occur when the high temperature austenite transforms to martensite in the dual phase microstructure. The specific volume of martensite is much larger than that of austenite so that when the transformation takes place, a compressive stress is induced in the ferrite. In the sintered state, the residual stress leads to a higher work hardening rate in tension. When the alloy is aged, the work hardening rate is reduced and the ductility is increased compared with the sintered state, even though aging increases the strength and apparent hardness.