Cobalt - Based Carbide - Hardened Alloys - Nature and Properties

Topics Covered

Background

Composition

Properties

Creep Resistance

Stress Rupture Strengths

Room Temperature Properties

Thermal Properties

Oxidation Resistance

Background

While cobalt is primarily used as an additive to iron and nickel-based alloys, there are a number of cobalt-based alloys. Of these, the main group is the carbide-hardened cobalt-chrome alloys, which have cobalt contents between 40 and 70%.

These alloys are mainly used as casting alloys, although there are grades that can be hot and cold worked and forged to produce semi-finished products such as rod and sheet.

Composition

The compositions of some casting and wrought cobalt-based carbide hardened alloys are given in tables 1 and 2 respectively. All these alloys contain approximately 20% chromium which provides resistance to high temperature corrosion. Carbide forming elements such as niobium, tantalum, zirconium or vanadium are present in amounts usually less 9%, while the carbon content lies in the range 0.25-1.0% for casting alloys and slightly lower at 0.05-0.4% for wrought alloys. The lower carbon content aids ductility for processes such as hot working.

Table 1. Compositions of some cobalt-base carbide-hardened casting alloys.

Table 1. Compositions of some cobalt-base carbide-hardened wrought alloys.

Properties

Creep Resistance

One of the main attractions of cobalt-based alloys is their excellent creep resistance. Materials creep due to thermally-activated movement of dislocations through a crystalline matrix. These alloys possess a matrix that is resistant to this as cobalt has a good tolerance for other elements in solid solution. These elements can effectively strengthen the matrix. Their ability to do this depends on factors such as:

•         The difference in atomic size between cobalt and the solute

•         The effect of the solute on the stacking fault energy

•         The diffusion rate of the solute into the cobalt matrix

It has also been found that a matrix containing a larger number of solutes is often better than one containing a fewer number, hence the strengthening of the matrix is also dependent on the amount of alloying elements available to the go into solid solution, that have not formed carbides, or for that matter intermetallics. The key elements for this process are chromium, tungsten, niobium and tantalum.

A second strengthening mechanism also exists and involves the formation of carbides and carbonitrides forming with chromium (primarily), tungsten, molybdenum, niobium, tantalum, zirconium, vanadium and titanium. Carbides formed include MC, M₆C, M₇C₃, M₂₃C₆ and sometimes M₂C₃, with the amount of each depending on factors such as availability of elements to form carbides, carbon content and thermal history. It is also possible for nitrogen to substitute for carbon in these structures. Optimum properties are produced when carbides precipitate both intergranularly and intragranularly. Intergranular precipitation prevents gross sliding and grain boundary migration and can form a skeleton if present in sufficient quantities, while intragranular precipitation strengthens the matrix by inhibiting the motion of dislocations.

Carbide distribution by solidifaction parameters such as pouring temperature and cooling rate. As cast alloys are rarely heat treated, carbides will generally only form during prolonged exposure to operating temperatures. Wrought materials on the other hand may be hot worked. Further strengthening can be induced by solution heat treatment between 1175-1230°C and rapid cooling.

Stress-Rupture Strengths

Since many of the applications that these materials are used for involve high operating temperatures, the stress rupture strength is of interest to designers and engineers (table 3).

Table 3. Stress Rupture values for cast and wrought cobalt-based carbide-hardened alloys.

Room Temperature Properties

As these alloys are generally used at elevated temperatures, the room temperature properties are not relevant to the service conditions. They do however, play a role for manufacturers e.g. tensile strength and ductility can influence how much hot or cold working the material can withstand and hardness influences machinablity.

It should also be noted that room temperature properties such as elongation can be effected by the thermal history of the material, i.e. amount of carbide precipitation, with more precipitation leading to lower ductility. Also increased exposure to high temperatures increases the hardness of higher carbon alloys more so than lower carbon content alloys.

Thermal Properties

Thermal expansion properties are similar to those of nickel-based alloys (table 4).

Table 4. Typical Co-Efficient of Thermal expansion data for cobalt-based carbide-hardened alloys.

Thermal conductivity values for cobalt-based carbide-hardened alloys such as HS 21 are typically about 15% of those for pure cobalt (table 5).

Table 5. Thermal conductivity data for HS 21.

Oxidation Resistance

This property is almost entirely dictated by the chromium content. Chromium contents in the range 20-25% are usually sufficient to protect the alloy up to temperatures of 1100°C.

Although the chromium is responsible for the formation of a protective oxide layer, it is susceptible to attack from elements such as sulphur, vanadium and alkali metal halides or oxides. These commonly come from contaminatyed fuels and other sources. Sulphur penetration caqn lead to the formation of sulphides within the alloy, forming low melting point eutectics such as Co₄S₃ (melting point 877°C). Strengthening carbides may also be preferentially attacked in some alloys.

Primary author: AZoM.com