Study on Microstructure and Properties of Powder Metallurgy High Speed Steel by High-throughput

Xiang Xiong*, Junhao Yang, Rutie Liu

State Key Laboratory of Powder Metallurgy, Central South University, Changsha,410083, China

ABSTRACT: Powder metallurgy technology can add more alloying elements to high speed steel (HSS) to give it better machinability without damaging strength and toughness. The Powder metallurgy high speed steel (PM HSS) can not only process ordinary steel, but also be used for the treatment of hard-cutting materials such as nickel-based alloys and titanium alloys. It has significant advantages in impact-resistant and precision tools. For the characteristics of multiple components and complex preparation process of PM HSS, the research on high-throughput preparation technology is carried out to quickly obtain various samples to explore the influence of powder characteristics, preparation process, alloy components on material properties, which effectively reduce the research and development cost, and then shorten the development cycle of new products, and obtain high-performance PM HSS.

The carbonyl iron powder, graphite powder, element powder (W、Mo、Cr、V) , carbide powder (WC、Mo₂C、Cr₃C₂、VC) , etc. were used as raw materials. The key steps such as composition design, powder mixing and sample densification were focused on. The multi-component powder synchronous mixing and the multi-sample multi-temperature zone synchronous gradient sintering device were designed and tested. This method achieved high throughput preparation of discrete multi-samples of powder bulk materials. In addition, this research studied the evolution of the microstructure of PM HSS, discussed the influence of powder characteristics on the carbothermal reduction and densification mechanism during the sintering process.

PM HSS prepared by element powders as raw materials had high oxygen content. During the sintering process, the O in the powder was internally transferred from low-oxygen affinity elements (Fe, W, Mo) to high-oxygen affinity elements (V, Cr). A small amount of metal oxide of V in the sample was not reduced after the carbothermic reduction at 1100°C, resulting in some pores distributed near the MC-type carbide in the structure, and poor bonding between the particles and the matrix. Therefore, the residual oxygen content of sintered specimen was close to 400 ppm, which limited the further improvement of the material properties. PM HSS prepared by using FeV pre-alloyed powder instead of V element powder can reduce the residual oxygen content to 220 ppm, thereby reducing the material porosity. Simultaneously, the FeV can form In-situ VC during the sintering process to promote the precipitation of fine MC carbide and improve the hardness of the material. PM HSS prepared by carbide powders as raw materials had low oxygen content. The metal oxides (mainly iron oxides) were fully reduced after carbothermic reduction, and the residual oxygen content was less than 10 ppm, which met the industry requirements for high-performance PM HSS. During the sintering process, the original carbide particles gradually transformed into stable complex carbides, which acted as hard phases to hinder the grain growth. After the liquid phase was generated, the sample was densified and accompanied by the formation of brittle eutectic carbides at the grain boundaries. Increasing the powder ball milling time can achieve low-temperature solid-phase sintering densification, avoid the generation of eutectic carbides, and improve the properties of sintered sample. Since increasing the time of ball milling can promote powder deformation and breakage, reduce powder particle size, increase powder distortion energy and surface energy, and increase the driving force for sintering. On the other hand, the fine powder with higher oxygen content can promote the carbothermic reduction, therefore, the activation sintering effect is remarkable. At the same time, the high oxygen content of the powder can reduce the carbon content of the sample and increase the solidus temperature. The samples prepared by powder which milling for 72 h have excellent properties, the relative density is 99.3%, and the hardness and the bending strength are 50.6 HRC and 2852 MPa, respectively.

PM HSS was prepared by the method of multi-component powder synchronous mixing and multi-temperature zone synchronous gradient sintering. This research explored the evolution of the microstructure of PM HSS, studied the influence of powder type, powder particle size and powder oxygen content on the subsequent sintering process. It transformed the traditional single-threaded & single-variable experimental method into multi-threaded & multivariable parallel experiments, which saved experimental batches and greatly improved research efficiency.

Keywords: PM HSS; High throughput; Residual oxygen; Carbothermic reduction; Mechanical properties