Powder metallurgy (PM) offers numerous advantages as a manufacturing technique. Minimal raw material waste and near net shapes strengthens PM as a competitive industrial production method. There is, however, a need to enhance the properties of PM steels as they have inherent porosity of up to 10% compared to wrought steels. There are two ways to increase PM performance; increase the density by sintering or altering the alloying elements for tailoring the microstructure. These two ways can also be combined.  There are several ways to introduce alloying elements, premixing with different powders, prealloying, diffusion bonding and admixing. 

Hardenability is a key property that determines utilization of PM steels in high performance applications. This property is mainly influenced by the alloying elements in PM steel. Better hardenability can be obtained by using alloying elements such as Cr, Si, B and Mn. Moreover, these elements have a lower carbon footprint compared to Ni and Cu which are the traditional PM alloying elements. One way to introduce these oxygen-sensitive elements while overcoming potential drawbacks of sintering is the master alloy (MA) method. The concept of master alloy has been known for decades but is still not widely implemented in PM industry due to requirements for conditions beyond the conventional ones. 

This work focuses on development and application of master alloys to improve the properties of PM steels. Design and optimization of MA comprising Cr, Mn, Si and B have been performed with the help of thermodynamic simulations using low solidus temperature as the main criteria. This is to enable liquid phase sintering. Different atomisation techniques, namely water, gas and gas-water atomisation, were evaluated to establish their effects on the optimised MAs. Additionally, influence of particle size fractions of MA was carried out using fine and course MA powders. To understand the role of sintering parameters on the MA route, different sintering temperatures were used while evaluating the resultant microstructure and final properties.

The results show that adding MA into base powders significantly improved the steel’s hardenability. Continuous cooling transformations showed increase in martensite formation at lower temperatures due to elements from the MA especially with B. The same result was obtained after sintering experiments where bainitic and martensitic transformations were evident in the microstructure. Better final mechanical properties after sintering were obtained due to martensitic structure. This was reflected in higher tensile strength and apparent hardness with MA. Higher sintering temperatures facilitated homogenisation of alloying elements, thus leading to better properties. Fine size fraction MA powder speeded up homogenisation process and left smaller pores after sintering. In as much as gas atomisation gives better control of oxygen in the MA, water atomisation is a more economical and robust process. Overall, addition of MA yields similar or better results than Ni hence MA is potentially a sustainable viable replacement in PM steels.