Nickel-based hard alloys

For the properties of self-flowing nickel-based hard alloys, the characteristic alloying elements are chromium, boron, and silicon. The boron and silicon contents usually range between 2 and 4 per cent (mass contents); the chromium content ranges from 5 to 17 per cent. In industrial alloys, carbon and iron are usually present only because the elements chromium, boron, and silicon are added in the form of ferro-compounds during the fusion-metallurgical production of the alloys, for reasons of cost.

In contrast to iron, nickel is face-centred cubic over the entire temperature range; consequently, transformations with solubility jumps can be utilised. Since nickel, especially in solid solution with chromium, is characterised by excellent chemical stability, it is an important basis element for alloys which are resistant to wet and high-temperature corrosion. Since the solubility of the metalloids carbon, boron, and nitrogen is very low, these elements are not suited for solid-solution hardening. Hence, substitutionally alloyed elements such as chromium, silicon, molybdenum, and cobalt are primarily employed for this purpose. For the application of Ni-based hard alloys at elevated temperature, an increase in hardness as a function of the temperature by means of these elements is a plausible consideration.

The precipitation of primary and eutectic hard phases results only in a slight improvement in the wear resistance, as compared with the state which is free of hard phases. The metallic matrix is too soft. Nevertheless, these alloys are still suited for use as wear-resistant material, since the wear resistance does not change between 20 and 850 °C. The strain and hardening capability persist over the entire temperature range. Above 750 °C, the wear resistance is even higher than that of the fcc Fe-based materials with hard phases, since the high-temperature strength of the Ni-based matrix is higher.

During solid-solution hardening, Cr is effective at temperatures below 600 °C, whereas Si is effective below 600 °C. For this reason, Ni-based alloys with high high-temperature strength are usually alloyed with chromium. Moreover, the combination of chromium and silicon is especially effective and is utilised with the alloys of the system Ni-Cr-Si-B (self-flowing Ni-based hard alloys). Up to 8 per cent Cr and 4.5 per cent Si (mass contents) are present in the metal matrix, and the hardness at room temperature is thus increased up to 450 HV 0.5.

The alloying elements boron and silicon are responsible for the pronounced lowering of the melting point of nickel-based hard alloys. The melting ranges or temperatures of commercially available alloys range between 960 and 1220 °C (pure nickel: 1452 °C). Moreover, these elements impart the self-flowing character to the alloy.

Alloys with molybdenum as additional alloying element exhibit the highest wear resistance in this group. With the precipitation of σ phases, the supporting action of the metal matrix is more effective over the entire temperature range. Consequently, these alloys are more wear-resistant than high-speed steels at 700 °C. In view of the fact that the values of the wear resistance are close together at 900 °C, it can be assumed that the effect of the hard phases decreases with increasing temperature.

On the whole, the basis element Ni is characterised by several features which provide the Ni materials, among all high-temperature materials, with the most favourable combination of mechanical properties, corrosion resistance, and machinability:

• The lattice structure remains continuously fcc all the way to the melting point. Consequently, there is no need to add lattice-stabilising elements, as is the case with Fe and Co, and the associated disadvantages are thus avoided. The diffusion coefficient of the closest-packed fcc structure is lower than that of the bcc lattice.
• Sufficiently high Cr contents as well as Al contents can be attained for ensuring the necessary corrosion resistance up to very high homologous temperatures.
• No other basis element provides such a large increase in strength by alloying techniques in the high-temperature range.
• With a value of about 210 GPa at room temperature, the quasi-isotropic Young’s modulus is about the same as that of Fe and Co.