Powder metallurgy (PM) for bearing steel manufacturing was introduced several decades ago and mainly aimed at limiting segregation effects in high-alloy grades. Despite the significant potential of this relatively new process for producing high-performance bearing steels, its use in commercial applications is still very limited today. It is thought that the slow acceptance of this promising technology is partially due to a lack of understanding of how modern PM steels compare to conventional ingot metallurgical steels. Most of the comparative studies published on this topic have only focused on a few key mechanical properties, which are rarely related to the microstructure. For this study, several variants of M50 were produced using ingot metallurgical and PM processes. This grade was chosen as its performance is well known to be limited by segregation, and it could therefore benefit from a PM process route. The evolution of the microstructure during manufacturing, from solidification to tempering, was carefully investigated. After heat treatment, toughness, hardness, and rolling contact fatigue (RCF) life were measured. RCF tests were performed using a ball-on-rod configuration to compare the performance of the alloys as well as to evaluate the microstructural changes during testing. Differences in the populations of stress raisers (primary carbides and nonmetallic inclusions) were also assessed and used to explain the variations in RCF fatigue lives.

For tomorrow’s mechanical systems—for example, in aerospace engines—bearings must meet more and more demanding requirements, such as weight savings and increased reliability. For these reasons, bearing materials must have an increased load capacity that today’s conventional metallurgy can only answer with dedicated lengthy and expensive heat treatment or a technological breakthrough. As powder metallurgy (PM) enables the realization of steel grades not obtainable by conventional metallurgy (high alloy and carbide contents, a very fine microstructure with low segregation, and above all a high hardness), and because technological improvements realized in recent years make this technology suitable for high reliability bearings, PM steel grades can be an answer to these more and more demanding requirements. ASP®2055 grade steel, with a hardened Rockwell hardness of 68HRC, was selected because of its good hardness/toughness compromise. Even though the inclusion cleanliness evaluations show that this PM steel is still not as clean as vacuum induction melted-vacuum arc remelted (known as VIM-VAR) and the best quality electric arc furnace-melted steels, the fatigue behavior rolling contact fatigue tests at 4.2 GPa yielded results with life durations similar to VIM-VAR steels. The reasons why it performed so well under elastohydrodynamic conditions, despite a limited cleanliness, were linked to both the high intrinsic microyield stress of the matrix and the presence of mixed oxide inclusions, leading to limited stress concentrations. These results were very promising for the use of PM ASP®2055 in high reliability bearings.

The load-carrying demands for rolling bearing components are constantly increasing and the bearing materials are required to be progressively durable. The industry is familiar with incremental rolling bearing material improvements and these are well documented. The testing of powder metallurgy high-speed steel (HSS) grades started in the 1970s. The introduction of high cleanliness steel melting, refining, and powder atomization, with consolidation by cold and hot isostatic pressing, resulted in a significant improvement in the metallurgical integrity of powder metallurgy steel HSSs. Cobalt-free HSS based on M62 composition steel (VIM CRU 20) yielded improvements in the rolling bearing function properties. The push for higher load capacities, particularly to support the use of ceramic rolling elements, meant that a new ultra-high hardness class of steel is desired. Cobalt alloying of HSSs is well established for tooling but not for bearings. An HSS with a 10 % cobalt addition has been tested with the aim to use it for demanding aerospace applications. In this paper, the microstructure of the steel (Aerospace Material Specification 6560) has been investigated together with its rolling bearing functional properties. The obtained results are compared to the industry standard vacuum induction melting–vacuum arc refining M50 (Aerospace Material Specification 6490).

The evolution of aerospace engines has led to a significant reduction in polluting emissions and fuel consumption, while increasing the demands placed on bearings. Hence, meeting these increasing demands for tomorrow's bearings raises the issue of producing a new material that will ensure the fatigue life and reliability. The research on high-performance material is a good opportunity to promote the introduction of powder metallurgy steel. Powder metallurgy for bearing steel manufacturing was introduced several decades ago, but the effective use in aerospace engines is still very limited. This is probably the consequence of a lack of understanding of how modern powder metallurgy steels could be used to manufacture bearings. The main interest in powder metallurgy is that it is very well adapted to increase hardness after heat treatment as a consequence of the high addition of alloying elements. Due to the fast solidification conditions, there is no specific problem concerning segregation and carbide size. In a previous symposium, we presented some data concerning ASP 2055 and the interesting metallurgy associated with this material. This article will focus on additional results concerning the mechanical and rolling contact fatigue behavior, with emphasis on toughness and microcleanliness. Thermomechanical treatment and the metallurgical evolution after each manufacturing step will be described. The main parameters concerning the formability of ASP 2055 will also be detailed. In this article, the methodology used for the metallurgical characterization will be described in detail. The mechanical results and rolling contact fatigue test results will be discussed in comparison with the well-known reference M50 vacuum induction melting/vacuum arc refining (VIM/VAR). Emphasis will be put mainly on the toughness and cleanliness assessment results. The interest in using new techniques such as microwave microscopy to reveal subsurface defects is highlighted, underlining how promising these techniques are.

Selective laser melting (SLM) is a commonly used laser powder bed technique where the final properties are influenced by many different powder related properties, such as particle size distribution, chemical composition and flowability. In applications where high hardness, wear resistance, strength and good heat properties are required, high speed steels (HSS) are widely used today. HSS has high carbon content and are therefore considered as unweldable. The rapidly growing implementation of AM technologies has led to a growing range of new applications and demands for new alloys and properties. The interest in being able to manufacture HSS by SLM without cracking is therefore increasing. In SLM, it is possible to preheat the base plate to a few hundred degrees Celsius which has been used for HSS and proved successful due to reduced thermal gradients. In this study, the properties of SLM produced high speed steel PEARL Micro®2012 with a carbon content of 0.61 wt.-% have been investigated and compared to those of a forged and rolled PM-HIP counterpart ASP®2012.