Answer: There is tremendous interest today in Mn-Cr steels from both a cost and a performance standpoint (Fig. 1). Until fairly recently, however, the high affinity for oxygen of chromium and manganese made adding Mn or Cr difficult; it was very difficult to suppress oxide formation during primary powder production or reduce these oxides during sintering. Today, the introduction of chromium and manganese into PM steels from high-carbon ferroalloys allows more compatibility with powder production over previous prealloying techniques. Also, improvements in atmosphere control during the sintering process have resulted in high-temperature sintering furnaces capable of sintering chromium and manganese alloy steels effectively.
Ideally, the mechanical properties of a PM steel component should equal those of wrought steels for a given application. For example, wrought grades such as 41XX and 86XX require only a simple heat treatment to produce property improvements via a fully martensitic microstructure, even in large section sizes. By contrast, mechanical-property improvement in PM traditionally has been attained through increasing part density by such processes as double press/double sinter or even powder forging. However, the extra cost severely limits the competitiveness of these PM materials.
In wrought ferrous metallurgy, a microstructure of tempered martensite produces the best combination of high strength, hardness, wear and fatigue resistance. In PM steels, common alloying additions such as copper and nickel result in lower hardenability than, say, chromium or manganese additions. PM producers have compensated for this by increasing carbon contents to achieve better hardness and tensile strength. However, such alloys require high quench rates to produce fully martensitic microstructures, and the residual stresses produced create a myriad of problems (e.g., cracking). The need for high quench rates also limits the section size that can be hardened efficiently.
Part Two will talk about hardenability issues.