Cobalt-based hard alloys

Applications at temperatures above 700 °C frequently demand the use of Co-based matrices, whose high-temperature strength is even higher. The high-temperature strength of the element cobalt is due to its very low SFE. Consequently, a high degree of hardening is thus achieved, on the one hand, and the beginning of recovery and recrystallisation is shifted toward higher temperatures, on the other hand. As a result, processes of cohesion loss are effectively hindered. In this manner, hardening processes are effective even at elevated temperatures.

Cast Co-based hard alloys are employed especially for guide blades in aircraft engines and stationary gas turbines. Moreover, Co alloys are extensively employed for furnace baffles in the glass, ceramics, and metallurgical industries.

The long-term stability and oxidation resistance of these materials are situated between the values for austenitic steels and those for γ’-hardened Ni alloys, whereas the strength values are decidedly closer to those for steels. The essential advantages and disadvantages of Co-based hard alloys are the following:

+ Since the usual Co alloys do not contain highly reactive elements, such as Ti and Al, they can be cast in air. Moreover, the size of the components is not limited by the capacity of vacuum-casting equipment. There is no need for an elaborate heat treatment; hence, the components can be manufactured more economically than comparable γ’-hardened Ni alloys.

+ The weldability of Co alloys is comparable to that of austenitic steels.

+ Because of the low SFE of Co, the matrix of Co exhibits relatively high (creep) strength.

+/- Under certain conditions, the hot-gas corrosion resistance may be higher than that of Ni alloys. The liquid Co-S phase cannot occur below 877 °C (Ni-S: 637 °C). Furthermore, the Cr content of Co alloys is decidedly higher than that of most Ni versions. However, the low-temperature hot-gas corrosion behaviour tends to be worse in the case of Co materials, unless the Cr content is extremely high.

- It is not possible to obtain especially high contents of hardening phases, unless other disadvantages are tolerated.

- In a manner similar to that of iron, cobalt undergoes a reversible allotropic phase transformation upon heating and cooling. As with iron, the face-centred cubic phase (α Co) is stable at high temperature; upon cooling, this phase transforms to the hexagonal dense phase (ε-Co) at about 420 °C. The associated changes in properties are undesirable; consequently, the more ductile fcc phase must be stabilised with the addition of Ni and Fe, which results in a higher SFE.

Since industrial Co-based alloys contain up to 30 per cent chromium, up to 15 per cent tungsten, and up to 8 per cent molybdenum (mass contents), their metal matrices are usually phase mixtures of α and ε cobalt. Molybdenum and tungsten exert a favourable effect on the high-temperature strength, since their atomic radii are very much larger than that of the Co atom. In this manner, they prevent the motion of dislocations; consequently, recovery can occur only at considerably higher temperatures.

The structure of the carbide phases is determined by the primary crystallisation – notwithstanding exceptions. For the use of metal powders in welding processes, this structure can be altered by the previous addition of various further carbides or other hard materials. In the weld metal, the hard material phases thus formed with carbon, boron, and in part silicon are often thermodynamically unstable, but are of interest for increasing the wear resistance.

The melting point or melting range is determined exclusively by the chemical composition of the alloys. Commercially available alloys for powder-welding technology contain additions of nickel, manganese, iron, silicon, and in some cases boron. The melting points of these multi-component alloys usually range between 1050 and 1400 °C.

The more ductile α phase is frequently desired for components subject to wear, since it is metastable and can undergo a stress-induced transformation. The transformation temperature (M_S, A_S) is shifted by the addition of alloying elements. Consequently, all phase mixtures are possible between the pure α and the pure ε phase in industrial alloys. Hard alloys and hard composite materials on a Co basis are derived primarily from the Co-Cr-W-C system. In these alloys, which are designated by the trade name Stellite, the metal matrix is a Co-Cr-W solid solution, which can also contain precipitated WC because of the decrease in the solubility of WC with increasing temperature. The metal matrices of industrial alloys can thus attain a microhardness value up to 450 HV0.05. In surfaces subject to friction, metal matrices of this type can attain a hardness value of 650 HV0.05 by cold-hardening and transformation of the metastable α phase. This hardness level is otherwise restricted to martensitic Fe matrices. Besides solid-solution hardening, precipitation hardening by intermetallic phases is also important. With appropriate contents of tungsten and molybdenum in α as well as in ε Co alloys, intermetallic phases of type Co₃(W, Mo) can precipitate after solution heat treatment by postweld ageing at 850 °C for 70 hours. These metal matrices are well suited for application even up to 1000 °C, since the loss of hardness associated with overageing is very small.

The thermal fatigue strength is sometimes claimed to be higher, in comparison with that of Ni materials, because of the relatively high thermal conductivity of pure Co; however, such a result cannot be confirmed for Co alloys. Thus, a value of about 11 Wm^-1K^-1 is indicated for the wrought alloy, “Haynes 188” as well as for the cast alloys X 40 and X 45 at room temperature; this value is the same as that for Ni alloys and austenitic steels. The temperature dependence of λ is also similar. Furthermore, no appreciable difference in thermal expansion coefficient has been detected; the values are in the range from 16 to 17.10^-6 K^-1. A slight tendency toward thermal fatigue cracking of Co-based hard alloys likewise does not correspond to practical experience under comparable operating conditions. In view of the lower strength value in comparison with that of the competing Ni alloys, the opposite trend is more likely to be observed.

corrosion behavior of NT^® - Cobalt-based hard alloys

RT: Room temperature; TE: Temperature of ebullition