Hard alloys

The continuing technological development in many fields of industry, such as machine, equipment, and plant construction, marine technology, as well as environmental technology, has resulted in stringent demands for new materials. Besides high wear resistance, these materials must also exhibit high corrosion resistance and good thermal properties. In many industries, machine and tool components become severely worn during operation. As a result, the operating efficiency and the quality of the product are impaired, and equipment failure may be the final consequence. Wear and tear of this kind cause severe losses, and predictions indicate that these costs will continue to increase in the future. With the combination of abrasive wear, thermal stresses, and possibly corrosive attack, the demands imposed on the surface material of the components become more stringent. For protecting these functional surfaces, which are subject to combined wear, weld surfacing is applied to an increasing extent for reasons of cost and efficiency. In correspondence with the requirements, multiphase cladding materials are employed for the purpose, as specified in DIN 8555. These materials include especially hard alloys on an iron, nickel, and cobalt basis. Furthermore, intermetallic alloys and mixed powders are employed; these products consist of a ductile matrix alloy and highly wear-resistant hard material particles. With the use of these materials, the hardening behaviour and the associated properties and stability of the protective cladding can be appropriately adjusted to resist wear and corrosion, among other factors. For the components to be clad, the correct choice of welding filler implies finding the correct compromise between maximal wear resistance, corrosion resistance, and economically acceptable machining costs, since the wear-resistant claddings thus applied must be machined because of the required surface quality and dimensional tolerances.

The high-temperature materials include all products which can be employed on components for sustained service above 500 °C and which must therefore ensure adequate mechanical properties as well as corrosion resistance at high temperatures in the long term. This product category comprises metallic and ceramic materials as well as intermetallic phases, which are classified between metals and ceramics. For achieving sufficiently high long-term creep resistance, the melting point of the basis elements for high-temperature materials must be at least 1400 °C. With the use of intermetallic phases as base material, the melting point of the phase is decisive. Furthermore, the metals must be available in sufficient quantity, and this requirement is reflected in the price.

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.

FeCrVC alloy system

Alloy systems based on the iron-chromium-vanadium-carbon system are highly wear-resistant materials for satisfying today’s exacting requirements. The alloy versions X400Cr5MoV18 (FeV18) and X400Cr17MoV15 (FeCrV15) are characterised by a fine, homogeneous distribution of the vanadium carbides. The VC content by volume is about 30 per cent. Experience gained with the application of powder-metallurgical tool steels indicates that a multiple increase in the wear resistance is achieved under abrasive conditions with a carbide content of about 10 per cent in a hardenable matrix. By virtue of the fine microstructure and the high VC content, materials of this type are especially well suited for cladding of cutting-tool edges and component edges subject to severe wear. For this purpose, Fe-Cr-V-C-based alloys for PTA weld surfacing provide access to a completely new level of quality for wear-resistant alloys.

Alloy X400Cr5MoV18 (FeV18)

For alloy FeV18, the following chemical composition, as determined by standard analysis, is recommended:

C: 3.6-4.0 %; Mn: 0.7-1.0 %; Si: 0.8-1.2 %; V: 17-18 %; Cr: 3.5-4.5 %; Mo: 1.0-1.5 %, and Fe: remainder.

With this alloy, the essential material properties are more strongly affected by the carbon content than by the vanadium content. A difference of ΔC = 0.1 per cent in the carbon content causes a larger change in the cladding properties than a shift in the vanadium content by 1 per cent, for instance.

The C content must be adjusted to match the concrete V content (tolerance: +/- 1 %) with the use of the following formula: C(%) = 0.2V(%)+0.4 %

A very high ΔC value (>+0.5 %) is associated with favourable wear properties, of course, but is unsuited for industrial applications, since it favours a coarse and brittle structural constitution. Even at a ΔC value of +0.2 per cent, a maximal hardness value of about 65 HRC was measured. At the same time, very low mass erosion is associated with this high hardness value. Consequently, a specification of the free carbon content between limits of +0.2 and +0.5 per cent appears to be necessary. In any case, negative values of the free carbon content must be avoided.

Chromium improves the properties of the steel matrix. As indicated by investigations, there is no need to increase the chromium content. A decrease in the chromium content accelerates martensite formation during cooling after welding. The persistence of brittle martensite phases in the structure after postweld tempering is thus avoided. Even at a chromium content of about 4 per cent the desired hardness level and wear resistance have been demonstrated.

Molybdenum likewise exerts a favourable effect on the martensite properties, as is the case with chromium. Molybdenum carbide of type M6C binds only 10 per cent of the carbon, in comparison with vanadium carbide. Since vanadium also exhibits a higher affinity for carbon, the molybdenum content is only of subordinate importance for the carbon adjustment in the alloy. The molybdenum content of 1.0 to 1.5 per cent employed for the test program should be retained, since an increased molybdenum content impairs the ductility because of the more pronounced formation of eutectic structural components. Significant effects of different silicon and manganese contents on the properties of the surfacing welds could not be detected within the scope of the test program. Hence, a change in the known contents of these elements in the alloys is not necessary. Nickel contents exceeding 1 per cent in the alloys must be avoided because of the austenite-stabilising effect.

Alloy X400Cr15MoV17(FeCrV15)

The FeCrV15 alloy versions have been developed on the basis of the Fe-Cr-V-C alloy system. In addition to increased wear resistance, the corrosion resistance of these materials is comparable with that of corrosion-resistant steels and hard alloys. Alloys of this type should be employed primarily for cladding of cutting-tool edges (synthetic-fibre- and plastics manufacture, food industry). In developing these alloys, the chromium content was specified between 17 and 20 per cent Cr, in order to provide a sufficient quantity of chromium for ensuring high corrosion resistance (≥12 % Cr dissolved in the matrix) as well as hardness of the matrix. Likewise, the molybdenum content was adjusted to 2 per cent Mo. For further enhancing the corrosion resistance, with a simultaneous improvement in ductility, the Ni content can be increased in steps of 3, 6, and 9 per cent. The standard analysis yields the following composition:

C:4.0-4.6 %, Si: 0.8 %, Mn: 0.7 %, Mo: 2.0 %, Cr: 17.0 %, V:15.0 %, Ni: 0-9 %

The carbon content is stoichiometrically adjusted to match the quantity of carbide formers; the formation of M₂C carbides (Cr, Mo) or MC carbides (V) is assumed for this purpose. In this context, special consideration was given to the fact that a minimum of 12 per cent Cr must be dissolved in the Fe matrix and therefore cannot form carbides. By varying the C-, Cr-, V-, and Ni contents, the alloy contents were appropriately adjusted in correspondence with the stress profile. For ensuring high resistance to wear, a V content of 15 per cent is necessary, whereas sufficiently high corrosion resistance is ensured at Cr contents from 17 to 20 per cent. Stepwise adjustment of the carbon (4.0 to 4.6 %) and nickel (2.0 to 9.0 %) contents results in the formation of austenitic or martensitic structures, respectively. This measure allows optimal adaptation of the cladding properties to match real wear conditions.

The metallurgical properties of this alloy (wetting, weld-pool formation) satisfy the prerequisites for the application of claddings with excellent welding quality and homogeneous structural constitution.

Weldability

Because of the metallurgical advantages offered by vanadium carbide, that is, no decomposition upon overheating and no formation of vanadium-rich mixed carbides, these alloys are especially well suited for weld surfacing. The claddings thus obtained are characterised by low dilution, even with large differences in properties with respect to the substrate material. These alloys can be applied without preheating to steel substrates with a volume content of carbide up to 60 per cent to produce crack-free claddings. Because of the similarity of the composition to that of the base metal, crack-free claddings can thus be obtained even on hardened steels.

With increasing vanadium carbide content, the viscosity of the melt increases, and the weld beads become increasingly uneven. Optimal adjustment of the weld-surfacing parameters is therefore absolutely necessary, in order to minimise expensive post-weld machining (grinding) – to the extent necessary.

Since the alloys are not self-flowing, exact mutual adjustment of the process parameters is necessary, in order to avoid “thinning” of the alloy by excessive dilution. The cladding substrates must be bare and free of grease. For the most stringent requirements on the cladding quality, single-pass weld surfacing is recommended, since a risk of forming pores is always associated with multi-pass operation as well as with the adjacent application of weld beads with these materials. A risk of cracking can be avoided by warm-in-warm operation (prevention of premature martensite formation at temperatures above 300 °C), even during multi-pass weld surfacing. Defective claddings can be repaired by overmelting without difficulty.

• FeV18 FeV18
• FeCrV15 martensitic FeCrV15 martensitic
• FeCrV15 austenitic FeCrV15 austenitic
•  

Alloying concept FeCrVCMn

The properties of hard manganese steel are due to a metastable quenching structure, which is associated with the high manganese content. The structure consists of austenite and ε-martensite, but is thermodynamically unstable under atmospheric conditions and consequently gives rise to stress-induced martensite formation of the type Υ->ε->α. As a result, the surface hardness increases, and the wear resistance is thus improved. The objective of this development was to impart properties of this kind to alloys with a high content of vanadium carbide for protection against abrasive wear. As dictated by the configuration of the alloying elements and heat treatment, such iron-based materials are characterised by a martensitic or chromium-martensitic structure, in which vanadium carbides are embedded as hard materials. Because of its great hardness, vanadium carbide is especially well suited for use in wear-resistant claddings.

In comparison with other hard materials, a particular advantage of vanadium carbide is the simplicity of operation in welding applications. The material is thermally stable, und fused vanadium carbide re-precipitates from the melt primarily as vanadium carbide because of the strong affinity of vanadium for carbon. In this process, the composition of the matrix is hardly affected at all. There is no solution tendency. The result is a structure in which the vanadium carbide is homogeneously distributed and fine-grained. Moreover, the properties of mixed carbides and the associated adverse effects are almost completely absent. The structure of this alloy is illustrated in figure 3.

The purpose of creating a metastable austenitic structure with the use of manganese is to improve the post-weld machinability of the materials by means of the associated decrease in hardness. At the same time, the stress-induced martensite formation ensures high wear resistance in applications and generates a self-sharpening effect, for instance, on machine-knife claddings.

With the addition of manganese to the FeCrVC alloys, an austenitic structure has been obtained. The hardness has been decreased by about 30 per cent in comparison with manganese-free steels, and post-weld machining has thus been simplified. The carbide configuration is not adversely affected by the manganese: Only the vanadium carbides are present. No mixed carbides are formed; associated adverse effects therefore do not occur. The resistance to purely abrasive wear was also improved significantly with the addition of hard material, in comparison with the hard manganese alloys under investigation. As a matter of principle, a higher content of vanadium carbide is associated with decreased wear. In the block-disc test, which simulates a complex tribo-system consisting of wear caused by sliding of grains and impact stress, the steels with a high manganese content yielded results similar to those obtained with the starting materials. No clear-cut dependence on the content of hard material could be detected.

The results of the block-disc test have proved that the alloys with a high manganese content exhibit very good wear properties for certain applications, despite the low initial hardness value.

Unfortunately, no phase-transformation effect could be demonstrated on any of the specimens under investigation. It is presumed that the effective force applied during the tests was not sufficient for unambiguously demonstrating a phase-transformation effect. Nevertheless, the results are sufficiently encouraging for performing further tests, such as a fatigue-wear test, in which high punctual loads occur. This material concept should therefore be investigated in more detail and subjected to continuing development. Further investigations are also necessary for achieving a self-sharpening effect.

Wear-resistant claddings with a high carbide content for applications under severely corrosive conditions

In comprehensive investigations, the corrosion behaviour of various alloy versions has been determined in pure corrosion tests, and the corresponding corrosion components have been determined in a combined test and calculated for the alloys. These results demonstrate that sufficiently high corrosion resistance is achieved with the use of such target materials. As indicated by these results, the corrosion resistance depends on the respective alloy type in combination with the corresponding corrosive medium.

In the combined test on materials of this kind, the resistance values are higher by a factor of up to 7 in synthetic sea water and up to 15 in organic acids, as compared to those of conventional cobalt-based alloys. The vanadium carbide content up to 50 per cent is reasonable from a technical as well as economic standpoint. In addition, the powder cost advantage ranges up to 20 per cent.

As dictated by the corrosive medium and alloy composition, the Fe-based materials provide sufficiently high corrosion resistance (synthetic sea water, 30 % citric acid).

With Inconel 625 + 30 per cent VC, the user has an excellent nickel-based alloy with high corrosion and wear resistance (20 % sulphuric acid) at his disposal.

Hard-material composite alloys

Hard alloys and hard composite materials are defined as metallic materials on an iron, nickel, or cobalt basis with a volume content up to ≈ 50 per cent of hard particles, such as carbides, borides, and nitrides for protecting against wear. Hard alloys are obtained by solidification of a melt with the precipitation of hard phases. The alloy and composite material can constitute mixed forms, as is the case with thermal spraying with a non-fused hard material component.

Metal carbides, borides, and nitrides are especially well suited for use as hard phases for the following reasons:

(a) The high solubility of the components in the melt is in contrast to the low solubility of these components in the solid state; consequently, solidification results in a high yield of hard phases.

(b) An increase in the share of covalent bonding results in a several-fold increase in the hardness of the hard phases, in comparison with that of the metallic matrix, and thus provides effective protection against wear by abrasive particles.

(c) Nevertheless, the metallic bonding component provides higher ductility for these brittle hard phases, in comparison with that of abrasive oxide mineral particles. Consequently, fracture of the mineral is more probable upon contact than that of the hard phase.

(d) Strong bonding is present between carbides, borides, and the surrounding metal matrix and thus enhances the mutual adhesion of these structural components. When subjected to stress, oxides of comparable hardness become more readily detached from the metal matrix at the interfaces because of the weaker bonding.

If hard phases with ceramic properties are embedded in a matrix with metallic properties, the resulting materials provide a favourable combination of wear resistance and fracture resistance. By appropriately adjusting the quantity, type, size, shape, and distribution of the structural components, the properties of components can be varied from metallic-ductile to ceramic-hard within wide limits and adapted to match the particular application. Thus, this material group is well suited for protecting against wear in a wide range of applications, /Bürgel98/.

In contrast to the hard alloys, particles of hard materials are added in the solid state in the case of hard composite materials, rather than being formed in-situ from a melt. To an increasing extent, alloyed metal powder is mixed with powdered hard material, and the resulting mixture is subsequently employed by powder-metallurgical methods. In this manner, structural building blocks can be arbitrarily combined and arranged. Excellent wear-resistant properties are obtained at both room temperature and elevated temperature with the use of tungsten carbides (eutectic WC/W₂C) in a high-temperature-resistant steel matrix as well as in a precipitation-hardenable Ni matrix.

Physical and mechanical properties of a few hard materials suitable for use as weld fillers are compiled in table 2.

Table 2: Physical and mechanical properties of hard materials

Intermetallic alloys

Intermetallic alloys are formed because of special bonding properties between at least two types of atoms in a certain stoichiometric ratio; in the case of binary phases, the general designation is thus A_mB_n. In the structure of pure metals and solid solutions, on the one hand, and that of ceramics, on the other hand, intermetallic alloys are distinguished by the fact that their bonding character is neither purely metallic nor completely covalent or ionic. A certain share of metallic bonding is always present, however.

The limit for the application of conventional Fe-, Co-, and Ni-based materials is around 1100 °C; for very low mechanical stress, the limit is slightly higher. The disadvantages of ceramic materials are their low ductility and fault tolerance. In view of these aspects, materials with an intermetallic matrix have been developed. High-strength Ni alloys consist predominantly of the intermetallic phase γ’-Ni₃Al, of course. However, the creep strain at low stress occurs in the softer coherent γ’ structure of the solid solution. With a basic intermetallic mass, however, the strength and strain within this phase are decisive for the mechanical properties. The purposes of intermetallic alloys are to raise the temperature limit for applications beyond that of conventional superalloys and to ensure sufficient corrosion resistance at the same time. The ductility and fault tolerance should be sufficient for manufacturing components with reasonable effort and expense, and for ensuring reliability in operation. These alloys are intended for closing the gap between classical high-temperature alloys and ceramics.

Intermetallic phases are characterised by strong bonding between the unlike types of atoms. In the ideal case, the preferential A-B bonding in the superlattice phases results in the maximal possible number of unlike neighbours, which is designated as structural disorder. As dictated by the phase type, covalent as well as ionic bonding components can occur; nevertheless, a certain measure of metallic bonding character always persists. These structural features give rise to high values of Young’s modulus and high Peierls stresses; consequently, the strength values are very high. Because of the remaining metallic bonding component, the expected brittleness is at least lower than that of ceramics.

For employing intermetallic phases, they must be solid-solution-hardened with the addition of foreign elements and particle-hardened by second phases. The strain and ductility characteristics are usually similar to those of ceramics, at least at low temperatures. The brittle fracture range can extend to about 0.5 T_s.

The commercially available intermetallic alloys are designated as Triballoys. As indicated by the manufacturers, these alloys are characterised by “excellent resistance to abrasive and adhesive wear as well as corrosion, even at high temperatures”, /Delo99/. They consist of a hard intermetallic (Laves) phase which is dispersed in a softer matrix. The hard Laves phase, which imparts strength to the Triballoys, is stable up to 900 °C. These Laves phases have the following properties:

• Stoichiometric ratio AB₂
• Very high packing density of the atoms; maximal volume filling of 71 per cent at an atomic radius ratio r_A : r_B = √3/2 = 1.225; actual fluctuation width: 1.05 to 1.68; Laves phases classified among the TCP phases (topologically close-packed)
• Cubic or hexagonal lattice structure
• Predominantly metallic bonding

The intermetallic Co-based alloy Triballoy 400 combines excellent wear resistance with high corrosion resistance. The intermetallic Ni-based alloy Triballoy 700 has a higher Cr content than Triballoy 400 for improved oxidation and corrosion resistance. The intermetallic Co-based alloy Triballoy 800 likewise has a higher Cr content than T 400; it is harder and has a higher wear resistance than T 400 and T 700.

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

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.

Nickel supperalloys

The driving force for the development of Ni-based superalloys originated from the construction of gas turbines for aircraft and power-generating stations. In order to increase the efficiency of these machines, the combustion temperature must be raised. For this application, the components must simultaneously withstand high temperatures and high mechanical stress in sustained operation. However, these materials have also become firmly established in the manufacture of hot-forming tools for some applications, for instance, in extruding presses for heavy metals, especially extrusion dies, where operating temperatures up to 1000 °C can be attained. Commercially available Ni superalloys offer decided advantages, both technological and economical, in comparison with conventional hot-working steels.

However, optimal properties are achieved only by an appropriate solution heat treatment and subsequent artificial ageing. These heat treatments are of paramount importance for attaining the maximal hardness and strength of the material.

The excellent fatigue behaviour of the nickel alloys is affected by five essential parameters:

• Solid-solution hardening
• Precipitation hardening
• Carbide hardening
• Microstructure
• Trace elements

An exact mutual adjustment of these parameters is a prerequisite for achieving the intended properties for a particular application.

Especially the elements Mo, W, Cr, and Co are involved in solid-solution hardening; after addition to the nickel-rich solid solution, these elements hinder the motion of the dislocations by distortion of the atomic lattice. At high temperatures above 0.6 TS, the high-temperature strength properties, especially the time-dependent creep, are controlled predominantly by diffusion. For this case, the diffusion-inert elements Mo and W are especially well suited for solid-solution hardening. For this purpose, Mo is preferred because of its lower atomic mass.

The addition of Fe to the alloy results from purely economic considerations; that is, the alloys can be produced more economically if iron is present. However, the resistance to oxidation is thus lowered, and the σ-phase, which impairs the properties, is also formed. A considerable increase in the creep strength of Ni superalloys is achieved by precipitation hardening. With the addition of the elements Ti, Al, and Nb, the finely distributed intermetallic γ’ phase Ni₃(Al, Ti) can be precipitated coherently from a supersaturated solid solution by an appropriate annealing treatment. Above 4 per cent (mass content), Nb forms the intermetallic γ’’ phase Ni₃Nb. Below this limit, Nb is substituted for Al and Ti in the γ’ phase. The hindrance to the motion of dislocations in a matrix with finely distributed precipitates is most pronounced if the grain size of the precipitates is between 20 and 50 nm, /Decker72/. At a carbon content between 0.05 and 0.2 per cent (mass contents), various carbide types, such as MC, M₆C, and M₂₃C₆, are already formed during solidification, and during the heat treatment at the latest; these carbides depend on the composition (table 1). Small, globular, non-coherent carbides, especially the primary precipitated carbides of types MC and M₆C, are well suited for stabilising the grain boundaries. Because of the high Cr content of the Ni superalloys, the formation of M₂₃C₆ carbides is unavoidable. Furthermore, carbides of types MC and M₆C tend to transform to carbide of the M₂₃C₆ type during sustained annealing. This carbide has a tendency to form coherent grain-boundary precipitates and thus increases the susceptibility toward brittle fracture. Hafnium is a grain-boundary-active element and prevents brittle fracture by the early formation of carbides which are highly stable and finely distributed in the structure. Boron and zirconium segregate at the grain boundaries because of the great difference between their atomic diameters and that of nickel. Consequently, they occupy vacant sites there and thus hinder the diffusion of other elements. As a result, not only the sliding of grains is hindered; the creation of γ’-depleted grain-boundary edges as well as continuous carbide films are also prevented in this manner.

Protection against high-temperature corrosion is provided by Al and Cr, which form firmly bonded oxide films. The protective action of Al₂O₃ is superior to that of Cr₂O₃. Up to about 1000 °C, Cr₂O₃ is insoluble; above 1100 °C, however, Al₂O₃ alone must assume the protective function. The ratios of the alloying elements employed in Ni superalloys are summarised in figure 1.

Besides the three classes of γ-, γ’-stabilising and grain-boundary-active elements, two subclasses must be distinguished: carbide-forming elements and those which form a protective film. The strength is enhanced by the microstructures located in the interior of the grains and at the grain boundaries; these microstructures in turn depend on the grain geometry, of course. However, it is important to realise that the mechanical properties also depend on the grain geometry, especially at high temperature. The creep strength is affected by strain processes at the grain boundaries. With a coarse-grained structure, considerably fewer possibilities are available for sliding processes than with a fine-grained structure. With an appropriate increase in grain size, the creep rupture strength can be substantially increased, especially at very high temperatures.

Figure 1: Metallurgical effects of the main alloying elements of Ni superalloys