Overall Structure: A Progressive Unveiling of Stellite Alloys

The introduction follows a logical progression:
- [ ] What they are and where they came from.
  + [ ] Identification as Cobalt-base (Stellite) superalloys.
  + [ ] Core beneficial properties
        high strength, corrosion resistance, high-temperature hardness
  + [ ] Pioneering figure (Elwood Haynes) and seminal alloy
        Stellite 6 with nominal composition
  + [ ] Brief mention of other significant early alloys and their initial scope of applications.
- [ ] Fundamental Strengthening Mechanisms
  + [ ] Primary mechanism: Hard carbide precipitation (e.g., M$_{7}$C$_{3}$, M$_{23}$C$_{6}$), with dependence on carbon content and processing.
  + [ ] Secondary mechanism: Solid solution strengthening by specific elements (W, Mo, Cr), also linked to carbon content.
  + [ ] Additional mechanism: Stress-induced phase transformation (fcc to hcp) contributing to wear resistance via work hardening.
- [ ] How they are made and modified.
- [ ] Current research directions and future outlook.





Cobalt-base (Stellite) superalloys are valued for their high strength, corrosion resistance, and hardness, especially at high temperatures.

Originating in the early 1900s with Elwood Haynes's Stellite 6 (nominally Co–28Cr–4W–1.1C wt.%), these materials, alongside other early alloys like Vitallium and X-40, quickly found use in demanding applications, from industrial tools to aerospace components.

Stellite alloys derive their properties from hard carbides (e.g., M$_{7}$C$_{3}$, M$_{23}$C$_{6}$), whose formation depends on carbon content and processing, and from solid solution strengthening by elements like W, Mo, and Cr, whose effect is also carbon-dependent. A stress-induced face-centered cubic (fcc) to hexagonal close-packed (hcp) phase transformation further enhances wear resistance through work hardening.

Applications for Stellite alloys have expanded from traditional uses like machine tools and nuclear components to diverse sectors including oil and gas, chemical processing, and medical implants. This wider usage increases the demand for understanding their corrosion and tribo-corrosion performance in aggressive environments.

Manufacturing processes critically influence Stellite's microstructure (which can be hypoeutectic or hypereutectic) and, consequently, its performance. Beyond traditional casting, modern powder metallurgy techniques like Hot Isostatic Pressing (HIPing) are favored for producing dense, homogenous, near-net shape parts, minimizing internal defects and subsequent machining. Surface engineering methods, including plasma transferred arc (PTA) welding and laser surface melting, further tailor surfaces for specific wear resistance needs, though considerations like substrate dilution effects on corrosion properties are important.

Current research focuses on enhancing Stellite alloys through strategic alloying additions (e.g., Si, W, Mo) to tailor microstructure, mechanical properties, and corrosion behavior, partly by methods such as stabilizing the fcc phase. The integrity, protective qualities, and repassivation capability of their passive films are vital for resisting localized corrosion. Understanding the complex interplay between alloy composition, processing, microstructure, and performance in corrosive and wear-intensive conditions remains crucial for optimizing these alloys for existing and emerging industrial applications.








Cobalt-base (Stellite) alloys have seen extensive use in wear environments mainly due to their high strength, corrosion resistance and hardness Co-base superalloys rely primarily on carbides formed in the Co matrix and at grain boundaries for their strength and the distribution, size and shape of carbides depends on processing condition. Solid solution strengthening of Co-base alloy is normally provided by tantalum, tungsten, molybdenum, chromium and columbium [1]. Since these elements are all carbide formers their effectiveness in terms of solid solution strengthening is dependent on the C content of alloy. Stellite 6 with nominal composition Co–28Cr–4W–1.1C (wt.%) was the first Stellite alloy developed in 1900 by Elwood Haynes. In recent years, there have been investigations into the effect of additions of alloying elements [2] on the microstructure and mechanical properties of Stellite 6. Improved hardness through formation of intermetallic compounds and mixed carbides could be achieved in both cases. It was shown that W and Mo addition influences the corrosion behaviour by stabilising the fcc phase [1]. Recent work by Kuzucu et al. [3] has demonstrated how the addition of 6 wt.% Si can alter the microstructure and hardness of Stellite 6 alloy thus enabling the properties to be tailored towards a specific application.


Because Stellite alloys are often used to combat wear there have been numerous studies in which surface engineering strategies to functionalise the surface for a specific application have been assessed. These have included plasma transferred arc (PTA) welding [4], laser surface melting [5] and plasma diffusion treatments [6]. Currently use of Stellite alloys has extended into various industrial sectors (e.g. pulp and paper processing, oil and gas processing, pharmaceuticals, chemical processing) and the need for improving information regarding corrosion (and often tribo-corrosion) of Stellite alloys has increased. It has been recognised that processing changes, which affect the microstructure of Stellite alloy, will most probably affect the corrosion performance [7]. Mohamed et al. [7] in 1999 used dc electrochemical techniques to investigate the corrosion behaviour of crevice-containing and crevice-free Cast and Hot Isostatically Pressed (HIPed) Stellite 6 in 3% NaCl at ambient temperature. They concluded that Hot Isostatic Pressing (HIPing) could potentially improve the localised crevice and pitting corrosion resistance and related their findings to the crevice corrosion models developed by Oldfield and Sutton [8]. In a study by Kim and Kim [9], the corrosion resistance of PTA-welded surfaces was compared to spray-fused and open arc-welded surfaces. It was concluded that dilution effects, which change the composition due to mixing of the coating and the underlying substrate, are key issues in the corrosion resistance of welded layers and the PTA process in this respect was superior to the other two processes.


The compositional roots of contemporary cobalt-base superalloys stem from the early 1900s when patents covering the cobalt–chromium and cobalt–chromium–tungsten system were issued. Consequently, the Stellite alloys of E. Haynes became important industrial materials for cutlery, machine tools and wear-resistant hardfacing applications [1,2]. The cobalt–chromium–molybdenum casting alloy Vitallium was developed in the 1930s for dental prosthetics, and derivative HS-21 soon became an important material for turbocharger and gas turbine applications during the 1940s. Similarly, wrought cobalt–nickel–chromium alloy S816 was used extensively for both gas turbine blades and vanes during this period. Another key alloy, invented in about 1943 by R.H.  ∗ Corresponding author. Present address: School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK. Tel.: +44 113 343 6812; fax: +44 113 242 4611. E-mail address: a.neville@leeds.ac.uk (A. Neville). 1 Present address: School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK.  Thielemann, was cast cobalt–nickel–chromium–tungsten alloy X-40. This alloy is still used in gas turbine vanes and subsea drill bits, and it has served extensively as a model for newer generations of cobalt-base superalloys. Hot isostatic pressing (HIPing) is rapidly becoming an industry standard as a processing method. Due to increasingly complex engineering shape specifications, higher demands on quality and lowering costs, HIPing has matured to a stage where it is recognized universally and is used on an industrial scale. HIPing requires a high-pressure vessel and consists of applying high isostatic pressure, using an inert gas, to the surface of the piece being processed or on the surface of a can filled with powder. A resistance heater inside the pressure vessel provides the necessary heat for the treatment. The microstructures of Stellite alloys vary considerably with composition, manufacturing process and post treatment. They may either be in the form of hypoeutectic structure consisting of a Co-rich solid solution surrounded by eutectic carbides, or of the hypereutectic type containing large idiomorphic primary chromium-rich carbides and a eutectic [1]

It is generally acknowledged that the susceptibility of passive metals to localised corrosion (including pitting) and the rate at which this corrosion process occurs are closely related to the ability of the passive film to resist breakdown and to repassivate once corrosion has initiated [2]. The chemical composition of the passive film, its structure, physical properties, coherence and thickness are of paramount importance in the nucleation and propagation of localised corrosion. Investigations into the composition and structure of passive oxide films on stainless steels and other related passive alloys are much more difficult than in the case of iron because the films are thinner, their chemical composition is complicated, and they cannot be reduced cathodically. A major part of the information available on composition and structure of passive films on stainless steels has been obtained with spectroscopic techniques, particularly X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). Other methods such as ion scattering spectroscopy (ISS) and secondary ion mass spectrometry (SIMS) also provide valuable data [3].

In the chemical, petrochemical and pump industries, machine parts often work in conditions where erosion and corrosion processes, acting together, are the main failure mechanisms. The deleterious synergistic effect of the erosion removes either corrosion product or the passive layer that is protecting the underlying surface. When a passive layer is removed, the time taken for it to repassivate is an important consideration in the assessment of wear rates. The rate of repassivation determines the amount of charge that can transfer when the surface is activated by an erosion event (e.g. impact of sand). The rate of repassivation in relation to the frequency of impact is an important consideration in erosion–corrosion. On stainless steels [1] it was shown that higher amounts of key elements (chromium, molybdenum) can lead a faster repassivation during erosion impacts.  ∗ Corresponding author. Tel.: +44 113 343 6812; fax: +44 113 242 4611. E-mail address: a.neville@leeds.ac.uk (A. Neville).  Cobalt-based alloys have enjoyed extensive use in wearrelated engineering applications for well over 50 years because of their inherent high-strength, corrosion resistance and ability to retain hardness at elevated temperatures [2]. In recent years a concentrated effort has been made to understand the deformation characteristics of cobalt-based alloys exposed to erosion–corrosion environments in order to optimize those factors contributing to their erosion resistance [3–5]. Alloying cobalt with chromium and various quantities of carbon, tungsten and molybdenum produces a family of alloys which can have excellent resistance to corrosion and/or erosion. Understanding how microstructural changes, as a result of alloying, affect corrosion and erosion resistance is critical to optimising the alloy for a particular purpose. In cobalt-based alloys, the key element chromium is added in the range of 20–30 wt.% to improve corrosion and impart some measure of solid-solution strengthening. Where carbide precipitation strengthening is a desirable feature, chromium also plays a strong role through the formation of a series of varying chromium–carbon ratio carbides such as M7C3 and M23C6. Alloying elements like tungsten, molybdenum and  0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.02.038 tantalum are added to cobalt for solid solution strengthening. If these metals are added in excess of their solubility, formation of carbides like MC and M6C is likely to occur. Shin et al. [6] investigated the effect of molybdenum on the microstructure and wear resistance properties of Stellite 6 hardfacing alloy. They showed that with an increase in molybdenum content, the M23C6 and M6C type carbides were formed instead of chromium-rich M7C3. They concluded that this microstructural change was responsible for the improvement of the mechanical properties such as hardness and wear resistance of molybdenum-modified Stellite 6 hardfacing alloy. Most cobalt-based alloys possess outstanding cavitation resistance compared to stainless steels which has been shown to be independent of the carbon content (hence hardness), and has been attributed by Crook [7] to crystallographic transformation, under stress, from the face centred cubic (fcc) to hexagonal close packed (hcp) structure by twinning. Heathcock and Ball [8] studied the cavitation resistance of a number of Stellite alloys, cemented carbides and surface-treated alloy steels and showed that in Stellite alloys the cobalt-rich solid solution, incorporating elements such as chromium, tungsten and molybdenum is highly resistant to erosion due to a rapid increase in the work-hardening rate and the strain to fracture which are caused by deformation twinning. Lee et al. [9] compared the liquid impact erosion resistance of 12 Cr steel with a Vickers hardness of 380 kg/mm2 (∼39 MPa) and Stellite 6B with a hardness value of 420 kg/mm2 (∼43 MPa). The liquid impact erosion resistance of Stellite 6B was at least six times greater than that of 12 Cr steel, implying that hardness is not the governing factor for liquid erosion. Stellite 6B also showed very different behaviour in liquid impact erosion in comparison with 12 Cr steel. They concluded that the superior erosion resistance of Stellite 6B results from the cobalt matrix whose deformation appeared mostly as mechanical twins and the material removal was more dominant in the hard carbide precipitates than in the ductile cobalt matrix. Wong-Kian [10] showed that Stellite coatings were advantageous for use in erosion–corrosion environments and can even function at relatively high temperatures. They reported that this is because wear resistance is promoted by the harder complex carbides of chromium and tungsten, while corrosion resistance is enhanced by the presence of cobalt in the matrix.

Cobalt-base alloys are now progressively used in many industrial and other applications. This is basically due to their high-temperature mechanical strength and their corrosion resistance in many environments. Some categories of these alloys may be used as high-temperature structural materials, wear resistant materials in aggressive media or for orthopedic implants. The main alloying elements are usually Cr, MO, W and Ni. Additionally, wear resistant alloys normally contain relatively high levels of carbon (0.25 to 2.5 wt.%) needed for carbide formation, while alloys used for structural applications are normally low in carbon.  Alloy Stellite-6 is a Co—Cr—W—C alloy which exhibits an outstanding oxidation and corrosion resistance, hightemperature strength as well as resistance to thermal fatigue. The wrought alloy is used in nuclear and other industrial engineering purposes [1 to 4]. In nuclear industry, Stellite-6 is one of the most popular alloys used in manufacturing control and safety valve components in pressurized water reactors (PWR). It has also been used for control shaft guide bushings in sodium cooled reactors (LMFBR). In chemical industry, the alloy is used in the form of weld overlay for catalytic reactor valves for various liquids to resist the effect of corrosive wear and for surfacing of combustion engine valves and steam turbine valves. Powder metallurgy (PIM) is a fabrication technology capable of producing reasonably complex designs at relatively high rates of production. Using P/M technology, segregation problems (normally associated with conventional casting techniques) are minimized, especially for small parts. The metallurgical characteristics of the end product are primarily developed during the sintering cycle [5, 6]. Powder metallurgy techniques have been applied for the production of cobalt-base alloys. Two basic P/M techniques, namely hot isostatic pressing (HIP) and wet powder pouring (WPP) have been used for production of alloy Stellite-6. Details of these processes are described elsewhere [7, 8].

Cobalt-base (Stellite) alloys have seen extensive use in wear environments mainly due to their high strength, corrosion resistance and hardness. Co-base superalloys rely primarily on carbides, formed in the Co matrix and at grain boundaries, for their strength and wear resistance. The distribution, size and shape of carbides depend on processing conditions. Solid solution strengthening of Co-base alloys is normally provided by tantalum, tungsten, molybdenum, chromium and niobium [1]. Since these elements are all carbide formers, their effectiveness in terms of solid solution strengthening is dependent on the C content of the alloy. Stellite 6 with nominal composition Co–28Cr–4.5W–1.2C (wt%) was the first Stellite alloy developed in the early 1900s by Elwood Haynes. In recent years there have been investigations into the effect of alloying elements additions on the microstructure and mechanical properties of Stellite 6 [2]. Improved hardness through formation of intermetallic compounds and mixed carbides was achieved in both cases. It has been shown that adding W and Mo influences corrosion behaviour by stabilizing the face-centred cubic (fcc) phase [1].  ∗ Corresponding author. Tel.: +39 011 0904641; fax: +39 0110904699. E-mail address: francesco.rosalbino@polito.it (F. Rosalbino).  Because Stellite alloys are often used to combat wear, there have been numerous studies into surface engineering strategies to functionalize the surface for a specific application. These have included plasma transferred arc (PTA) welding [3], laser surface melting [4] and plasma diffusion treatments [5] all involving Stellite alloys. Application of Co-base superalloys was traditionally most prevalent in the nuclear industry in the 1960s and 1970s and, for this reason, much research into corrosion of Stellite focused on conditions relating to nuclear power applications such as simulated PWR primary heat transfer conditions [6,7]. Currently, use of Stellite alloys has extended into various industrial sectors (e.g. pulp and paper processing, oil and gas processing, pharmaceuticals, chemical processing) and the need for improved information regarding corrosion (and often tribo-corrosion) of Stellite has increased. It has been recognized that processing changes, which affect the microstructure of Stellite alloys, most affect corrosion performance [8]. Hot isostatic pressing (HIPing) is a thermo-mechanical process [9] in which components or a contained powder is subjected to simultaneous applications of heat and high pressure in an inert medium. HIPing removes internal void cavities thus consolidating the structure making it homogenous, segregation free, dense, nearnet shape and requiring little or no machining.