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Hard alloys - FeCrVC alloy system

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.

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.