Cobalt-bearing high-speed steel represents a specialized category of high-speed steel (HSS) that incorporates cobalt as a key alloying element. Its fundamental composition includes primary alloying elements such as tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V), with a typical cobalt (Co) content ranging from 5% to 8%. This specific formulation is engineered to significantly enhance the material's performance under demanding high-temperature conditions. Representative steel grades within this family include W6Mo5Cr4V2Co5 (commonly designated as M35), W6Mo5Cr4V2Co8 (M36), and W18Cr4VCo5 (T4), each tailored for specific application requirements and performance benchmarks.

Manufacturing Process and Microstructural Advantages
The production of cobalt-bearing high-speed steel often involves advanced powder metallurgy techniques, notably gas atomization. In this sophisticated process, molten metal is transformed into fine, spherical particles. These powder particles are subsequently consolidated into a dense steel ingot through Hot Isostatic Pressing (HIP). A significant advantage of this manufacturing route is the substantial reduction of non-metallic inclusions and the achievement of a highly uniform microstructure [1]. The application of powder metallurgy technology facilitates the refinement of carbide grains and ensures their even distribution throughout the steel matrix. This refined and homogeneous microstructure is crucial as it directly contributes to enhanced wear resistance and improved toughness, mitigating the brittleness often associated with coarse, irregular carbides found in conventionally cast high-speed steels.

Exceptional Performance Characteristics
A defining property of cobalt-bearing high-speed steel is its superior red hardness (hot hardness). This refers to the material's exceptional ability to retain a high degree of hardness and strength even when exposed to elevated temperatures encountered during high-speed cutting operations. In its annealed state, this steel exhibits a relatively high baseline hardness. After undergoing appropriate heat treatment processes (including quenching and multiple tempering cycles), it can achieve an impressive final hardness ranging from 64 to 68 HRC. The specific quenching hardness typically reaches 65-66 HRC. The role of cobalt in this context is pivotal; it enhances the thermal stability of the steel matrix and increases the dissolution temperature of the hard carbides, thereby allowing the material to maintain its cutting edge for longer durations under thermal stress.

                                                                                              Jimmy M35 HSS tap

Primary Industrial Applications
Owing to its combination of high hot hardness, excellent wear resistance, and sufficient toughness, cobalt-bearing HSS is predominantly used in the manufacture of high-performance cutting tools. These include gear hobs, broaches, end mills, and other tools designed for machining difficult-to-cut materials. Its application spectrum covers the processing of iron-based superalloys, titanium alloys, and ultra-high-strength steels, where standard high-speed steels would rapidly soften and fail [1-2]. A notable practical example is the prevalent use of the S6-5-2-5 steel (a grade containing 5% cobalt) by European tool manufacturers for producing gear hobs intended for small and medium module gears, underscoring its reliability in precision machining applications [2].

Supply Considerations and Alternative Surface Engineering
Despite its superior properties, the widespread adoption of cobalt-bearing high-speed steel faces challenges related to the availability and cost of cobalt. Cobalt resources are relatively scarce and geographically concentrated, leading to supply chain vulnerabilities and higher costs. Consequently, the production of cobalt-containing super-hard high-speed steel is somewhat limited in regions like China, where cobalt is not abundantly available. To address this constraint while still leveraging the benefits of cobalt, advanced surface metallurgy technologies have been developed. These innovative processes, such as laser cladding, plasma transfer arc welding, or diffusion coating, enable the formation of a cobalt-enriched, super-hard surface layer on a substrate made from more common or less expensive grades of steel. This approach provides a cost-effective solution by imparting the desired high surface properties (wear resistance, red hardness) exactly where they are needed—at the cutting edge or working surface—while utilizing a more economical material for the tool body, thus optimizing both performance and cost-efficiency.

References
[1] Source on manufacturing process and general applications.
[2] Source on European tooling practices and specific material use.