Powder metallurgy (PM) offers numerous advantages as a manufacturing technique. Minimal raw material waste and near-net shape components strengthen 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 with wrought steels. There are two ways to increase PM performance: increasing the density by sintering or altering the alloying elements to tailor the microstructure. These two ways can also be combined. There are several ways to introduce alloying elements, including premixing with different powders, pre-alloying, diffusion bonding and admixing.

Hardenability is a key property that determines the utilisation of PM steels in high-performance applications. This property is mainly influenced by the alloying elements in PM steel. Enhanced hardenability can be obtained by using alloying elements such as Cr, Mn, Si and B. 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 during sintering, is through the master alloy (MA) method. The concept of master alloys has been known for decades but is still not widely implemented in the PM industry due to requirements for conditions not presently met by conventional industrial practice.

This work focuses on the development and application of master alloys to improve the properties of PM steels. Design and optimisation of MA comprising Cr, Mn, Si and B were performed with the help of thermodynamic simulations, using low melting temperature as the main criterion. 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 master alloy properties. Additionally, the influence of MA particle size fractions was investigated using fine and coarse MA powders. To understand the role of sintering parameters in the MA route, different sintering temperatures were used while evaluating the resultant microstructure and final sinter properties.

The results show that adding MA into base powders significantly improved the steel’s hardenability. Continuous cooling transformation results showed an increase in martensite formation at lower temperatures due to elements from the MA, especially with B. Similar results were 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 microstructure. 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. Finer MA powder fractions accelerated homogenisation process and left smaller pores after sintering. Gas atomisation provided better control of oxygen in the MA, whereas water atomisation is a more economical and robust process. Overall, MA addition yielded mechanical properties comparable to or better than Ni, suggesting that Ni free master alloys are potentially a sustainable replacement in PM steels.