Iron-based hard alloys

For iron-based hard alloys, chromium and carbon are characteristic alloying elements. The mass contents of these elements usually range from 10 to 35 per cent and from 2 to 6 per cent, respectively. Moreover, tungsten, molybdenum, and vanadium are frequently employed as alloying partners for the production of carbidic hard material phases. Further matrix metals include silicon, manganese, nickel, and cobalt.

At application temperatures above 600 °C, a basic fcc lattice is advantageous. The low SFE of the austenite hinders the progress of cohesion-loss processes and thus shifts the recrystallisation toward higher temperatures. Since the abrasion resistance of the austenitic metal matrix is decidedly lower than that of the hard phases, alloys with high contents of hard phases are frequently employed.

In comparison with ferritic steels, the austenitic, heat-resistant steels are characterised by higher microstructural stability, higher high-temperature strength, and lower tendency toward embrittlement. The mass content of nickel ranges up to 35 per cent and must be sufficiently high for ensuring that the structure remains completely austenitic over the entire temperature range. The corrosion resistance of the austenite depends on the Cr content, which varies between 18 and 30 per cent by mass. Under oxidising conditions, spinels of the Ni(Cr, Fe)₂O₄ type as well as nearly pure Cr₂O₃ form surface layers. The carburisation resistance of the heat-resistant austenitic steels increases with the Ni content. For the usual austenitic steels, therefore, the upper temperature limit for application is specified by the properties of the chromium oxide layers. This limit lies between 900 and 1150 °C and depends on the atmosphere, the cyclic conditions, as well as the required lifetime. Heat-resistant steels compete with Ni alloys which have a high Cr content; comparable or higher high-temperature corrosion resistance in combination with higher high-temperature strength can be achieved with the latter.

In conclusion it should be pointed out that austenitic steels must be employed above 600 °C. These materials offer the following advantages in comparison to ferrites:

• In the closest-packed fcc γ-lattice, the diffusion coefficient is only about 1/350 of the value for the bcc α-lattice. Likewise, the creep rate decreases, and the diffusion-controlled grain coarsening is also less severe.
• The SFE of about 50 mJ/m² is decidedly lower than that for the bcc alloys, whose value is about 300 mJ/m². With the addition of certain alloying elements, the SFE can be further decreased, with a corresponding increase in creep strength. However, the effect of Ni on the SFE is in the opposite direction.
• The contents of the alloying elements, especially Ni and Cr, are adjusted in such a way that a stable, completely austenitic structure is present over the entire temperature range of interest; consequently, lattice transformations do not occur.
• The Cr content can be increased for improving the corrosion resistance, usually to a value between 20 and 30 per cent by mass.
• As a whole, the alloying contents for solid solution hardening are higher, since the solubility is higher in the austenite lattice.
• The ductility is considerably higher, especially in the low-temperature range, which can be quite critical for starting processes in equipment.

Moreover, the creep strength of austenitic steels results from particle hardening with carbides, nitrides, or carbonitrides, as well as with intermetallic phases in a few cases. Two types of carbides are dominant: MC and M₂₃C₆. A general survey of the important carbide types in high-temperature alloys and their essential features, which are also applicable to the other basis element groups, is presented in table 1. Carbides which preferentially precipitate at the grain boundaries prevent sliding of grain boundaries there. With the use of the refractory alloying elements, Mo, W, and Nb, at mass contents beyond approximately 1 per cent, the hexagonal Laves phase Fe₂(Mo, W, Nb) can precipitate in austenitic alloys; as a result, the creep strength increases.