An Experimental Investigation of the Hardenabilities Tensile and Fracture Properties of Powdered Metal Steels

Abstract

Powder metallurgy (PM) steel is produced by near net shape manufacturing, which is used to fabricate alloy steels for many purposes. Designing new powder metal steels that can form a significant fraction of martensite relies on hardenability calculations developed for wrought steels. These proven tools are built upon assumptions for wrought steels that do not hold true for PM steels. One assumption is that the alloying elements are homogenized throughout the material. In admixed powder blends that are industrially sintered this is not the case. Using prealloyed powder is a solution to this issue, yet it places restrictions on alloy design and compressibility. There are tools available to computationally optimize diffusion problems, yet the complexity during the sintering of PM steel is such that a robust model has yet been produced. It is intuitive that with smaller particles of Fe sintering time can be reduced. A direct experimental investigation linking Fe-powders’ sizes and hardenability on Fe-C-Cr-Mn-Mo-Ni PM steel was subject to microstructure analysis and mechanical properties (Jominy test) for comparative analysis.

Another assumption that is made for wrought steel is a consistent density of 7.87g/cm3. This is not the case for PM steel as the press and sinter method produces pores, decreasing the density. This directly affects the thermal conductivity and phase transformation of the steel. In an effort to understand how these differences affect Grossmann’s predictions of hardenability, a direct experimental investigation linking the density to hardenability was launched on prealloyed FL-4605 and FL-4605+2%Cu. Specifically the Jominy test was completed on a range of densities, as well as compared to software predictions.

The chemical variations in admixed and sintered PM steel produce a unique system where one TTT diagram cannot predict the entire final microstructure. PM steel such as this is observed in industry, and can be created through incorporating larger Fe-particles such that less alloying constituents have a chance to fully alloy these regions. Since the large particles will not have the chance to be alloyed, they will not have the ability to form martensite. Since the regions between large particles will be alloyed, martensite will form, creating a hard matrix surrounding softer particles. This structure is characteristic of a metal matrix composite (MMC), and therefore should be treated as such. There are methods of MMC design that involve numerical methods of predicting strength and toughness. These methods, along with experimental data (tensile and Charpy testing) of Fe-C-Cr-Mn-Mo-Ni PM steels with ranging volume fractions of pearlitic inclusions were compared.