Available online at www.sciencedirect.com Wear 265 (2008) 772–779 Behaviour of iron-based hardfacing alloys under abrasion and impact M. Kirchgaßner a , E. Badisch b,∗ , F. Franek b,c a b Castolin Ges.m.b.H., Brunner Straße 69, 1230 Vienna, Austria research GmbH, Viktor Kaplan-Straße 2, 2700 Wiener Neustadt, Austria c Vienna University of Technology, Floragasse 7, 1040 Vienna, Austria AC2 T Received 21 December 2006; received in revised form 17 December 2007; accepted 9 January 2008 Available online 4 March 2008 Abstract Iron-based hardfacing alloys are widely used to protect machinery equipment exposed either to pure abrasion or to a combination of abrasion and impact. The specific wear behaviour of a welding alloy under these conditions depends on its chemical composition, the microstructure obtained after welding and finally the welding technology used to apply them respectively the parameter settings which strongly influence, for example, dilution with the base material or formation of metallurgically precipitated hard phases. The main objective of this study was to evaluate the wear behaviour for pure abrasion and for combined wear of iron-based alloys which are typically applied by gas metal arc welding (GMAW). A new complex Fe–Cr–W–Mo–Nb alloy with high boron content was set into comparison with lower alloyed materials on basis Fe–Cr–B–C, a synthetic multiphase alloy on iron base with around 50 wt.% tungsten carbides and a crack free martensitic Fe–Cr–C alloy containing finely precipitated Niobium carbides. Besides these a conventional hypereutectic Fe–Cr–Nb–C alloy was integrated in the program serving as standard which is already well described in literature. In order to simulate real field conditions on a lab scale, tests were performed with a standard ASTM G65 dry-sand rubber-wheel tester (3-body abrasion). A specially designed impeller-tumbler apparatus enabled investigation of impact abrasion wear tests (combined impact and abrasion wear). The evaluation of wear behaviour was supported by micro- and macrostructural investigations and by hardness tests. © 2008 Elsevier B.V. All rights reserved. Keywords: Fe-based; Hardfacing alloy; Abrasion; Impact; Wear 1. Introduction Many basic operations to process raw materials, among them crushing, classifying or delivering, are typical for mining, steel and many other industries. Core components such as crushers are exposed to heavy wear and require efficient surface protection measures to avoid costly downtimes and to reduce costs for expensive spare parts [1]. Wear resistance against abrasion and/or impact are often required to extend the lifetime of machinery equipment efficiently [2]. Welding is a key technology to fulfill these requirements and to apply hardfacing alloys. The most common processes are oxyacetylene welding (OAW), gas metal arc welding (GMAW), shielded metal arc welding (SMAW) and submerged arc welding ∗ Corresponding author. Tel.: +43 2622 8160024; fax: +43 2622 8160099. E-mail address: badisch@ac2t.at (E. Badisch). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.01.004 (SAW). State of the art hardfacing alloys comprise very cost efficient Fe–Cr–C or Fe–C–B systems on one hand, but on the other hand also more expensive synthetic multiphase composites reinforced with tungsten carbides for example are available. Above that, complex Fe-based alloys with niobium, titanium, molybdenum in combination with boron and carbon gained importance by achieving wear resistance due to the precipitation of different abrasion resistant hard phases and by optimized matrix properties [3,4]. The selection of the most effective wear protection solution, especially in case of combined wear, is either related to longterm practical experiences, in situ tests or to applying alloys according to their hardness or the content of specific hard phases such as tungsten carbide. The present research work has the main goal to evaluate the wear behaviour of six different Fe-based hardfacing alloys applied by gas metal arc welding under abrasion and also combined wear of abrasion and impact. The wear behaviour of M. Kirchgaßner et al. / Wear 265 (2008) 772–779 773 Table 1 Chemical composition of the Fe-based hardfacing alloys investigated Sample A B C D E F Chemical composition [wt.%] Fe C Cr Nb B Others (Mo, V, W, Ti, Ni) Basis Basis Basis Basis Basis Basis <1 5.5 2.5 <1 1.3 – 6 21 7 – 15.4 <1 3 7 – – 4.2 – – – <1 4 4.2 – 1.5 1 <1 1.5 11.5 1.8 complex metallurgically alloyed welding consumables versus synthetic multiphase composites and the relation of the results to conventional hardness tests are of major interest. 2. Experimental 2.1. Materials and welding parameters Six different hardfacing alloys produced as flux cored wires on iron basis were selected and welded onto 1.0038 mild steel plates with a dimension of 195 mm × 125 mm × 6 mm. The chemical composition of the alloys can be seen in Table 1. Table 2 shows the welding parameters, which are optimized related to the welding behaviour and the composition of the different flux cored wires concerning to practical welding procedures done, e.g. on crusher systems. The welding was carried out in flat position in 2 layers. For all hardfacing alloys welding gas and wire diameter were kept constant at Ar + 2.5% CO2 and 1.6 mm, respectively. The interpass temperature was kept in a level of about 150–200 ◦ C (see Table 2). The final samples were cut out of the original plates by water jet cutting to avoid any heat effect on the final overlay. 2.2. Wear testing apparatus To simulate field condition in lab-scale as realistic as possible, wear tests were performed with a special impeller-tumbler apparatus (combined impact and abrasion wear) and a standard ASTM G65 dry-sand rubber-wheel tester (3-body abrasion). The impeller-tumbler testing device consists of a slowly rotating outer tumbler and a fast rotating inner impeller at a rotation speed of 60 and 650 rpm, respectively, where the testing specimens are mounted on (see Figs. 1 and 2) [2,5,6]. Fig. 1. Total view of the impeller-tumbler testing device (combined impact and abrasion wear). The tumbler is filled with a defined amount of abrasive, and is responsible for a controlled flow of abrasive particles hitting the fast moving testing specimens (see Fig. 2). Due to the kinematical situation the particles get in contact with the specimen (surface exposed to abrasive particles, 2.5 cm × 1.0 cm) at an impact velocity of approximately 10 m/s. For the experiments two different kinds of abrasives were used. At first, 5 kg of finegrained silica sand (1.6–2.2 mm) and secondly 1 kg of coarse corundum particles (5–10 mm) for higher impact loading was used. The duration of the runs was given by 60 min for finegrained silica sand and by 20 min for coarse corundum particles. Each test has been repeated 3 times for statistic calculation. To make a direct comparison between impeller-tumbler tests with low and high impact loading possible, the wear rate is given in volume loss divided by testing time and mass of abrasive Table 2 Welding parameters of the Fe-based hardfacing alloys investigated Sample Current [A] Voltage [V] Interpass temperature [◦ C] Wire speed [m/min] A B C D E F 175 247 246 188 244 113 22.9 27.5 21.7 23.3 23.6 21.0 168 197 205 182 175 151 4.4 8.4 5.4 4.7 9.3 3.0 Fig. 2. Impeller-tumbler testing chamber (visualization of particle flow). 774 M. Kirchgaßner et al. / Wear 265 (2008) 772–779 Table 3 Summary of the hard phases (HP) of the Fe-based hardfacing alloys investigated Sample Type of HP A Nb carbides B Fe/Cr carbides Nb carbides C D Content of HP [vol.%] Size of HP [␮m] Shape of HP <3 Spattered 57.1 5.4 30–200 <7 Columnar Granular Fe/Cr carbides Fe carbo-borides 48.3 18.5 – 20-80 Closed skeleton Columnar E Fe/Cr carbo-borides Nb carbides Mo/W carbo-borides 52 4.6 5.3 10–100 5–10 10–25 Columnar Blocky Blocky F Synthetic added WC 38.7 65–250 Rounded 7 particles used. The fine-grained silica sand corresponds to low wear level, whereas the high energy level of coarse corundum particles is resulting in a severe wear regime [7]. Abrasion tests were carried out on a dry-sand rubber-wheel tester with 3-body abrasion condition under low stress according to ASTM G65 procedure A. Rotation speed, normal load and sliding distance were kept constant at 200 rpm, 130 N and 4309 m, respectively. Ottawa silica sand at grain size of 212–300 ␮m was used as abrasive. 2.3. Characterization of microstructure and wear behaviour Characterization of microstructure has been done with optical microscopy and scanning electron microscopy (SEM + EDS). Quantitative analysis of the microstructure was carried out by the use of Intronic Image C Software. Hardness measurements were carried out with a standard Vickers hardness technique HV5 for macroscopic hardness. To determine hardness of each phase in microstructure, e.g. hard particles and metallic matrix, HV0.1 was used. Quantitative wear characterization has been done by gravimetric mass loss of the testing specimen during wear testing. Qualitative characterization of worn surfaces and worn edges has been carried out by evaluating of macroscopic and cross-section images and by SEM investigations. 3. Results 3.1. Microstructure and hardness The characterization of microstructure has been done with optical microscopy after etching with 5 vol.% alcoholic nitric acid. Typical microstructures of the welded deposits are shown in Fig. 3. Alloy A is mainly martensitic with some islands of austenite. Fine primary Niobium carbides are well distributed throughout the micro-section. The hardness of the martensite is about 800 HV0.1. The content of Nb carbides is given in Table 3 to 7% at a size of <3 ␮m in a spattered shape. Alloy B consists of primary Fe/Cr carbides with a micro-hardness of roughly 1600 HV0.1 in a ledeburitic matrix. The content of Fe/Cr carbides is listed in Table 3 to 57.1% at a size of 30–200 ␮m. The chemistry of the Fe/Cr carbides is reported for hypereutectic FeCrC Fig. 3. Microstructures of the Fe-based hardfacing alloys investigated: (A) martensitic FeCrNbC alloy, (B) hypereutectic FeCrCNb alloy, (C) hypoeutectic FeCrBC alloy, (D) hypereutectic FeCB alloy, (E) complex high alloyed FeCrWMoNbBC type and (F) synthetic multiphase Fe alloy with roughly 50 wt.% tungsten carbides. M. Kirchgaßner et al. / Wear 265 (2008) 772–779 Fig. 4. SEM image of the eutectic matrix of sample C. alloys in literature to M7 C3 structure [8–11]. The hardness values of the ledeburitic matrix which are determined to about 800 HV0.1 are close to previous investigations from Fischer [12] and Buytoz [13]. Besides small and evenly distributed primary Nb carbides (light grey in image B of Fig. 3) at a volume content of approximately 5% can be detected, which are supposed be of major importance for increasing the resistance against erosion and abrasion due to their high hardness. Alloy C has solidified in hypoeutectic ␥-dendrites with about 920 HV0.1, which are embedded in an eutectic matrix of about 1000 HV0.1 (see Fig. 3). A closed net or skeleton of brittle Fe/Cr carbides at a volume content of 48.3% (see Table 3) is clearly surrounding the primary dendrites which can be seen in detail in the SEM image of Fig. 4. This appearance comes close to a solidification of the finally solidifying eutectic melt described as N-type [12]. Alloy D is built up of Fe carbo-borides in columnar structure with a hardness of about 1500 HV0.1 in a hard eutectic matrix of about 1000 HV0.1 (see Fig. 3). The distribution of hard phases in this alloy is quite uniform. Volume content and size of the Fe carbo-borides are listed in Table 3 to 18.5% and 20–80 ␮m, respectively. The complex alloy E which contains an amount of Boron similar to alloy D, but a much higher level of other elements like W, Mo, Nb and Cr and shows a dense and uniform distribution of very hard complex carbides and carbo-borides (see Fig. 3) with hardness values between 1200 and 1900 HV0.1. Type and content of the HP were determined to Fe/Cr carbo-borides at a volume content of 52% at a size of 10–100 ␮m, Nb carbides and Mo/W carbo-borides at a volume content of approximately 5% in blocky shape (see Fig. 5). In Ref. [14] hard phases of a very similar alloy are described as M23 (BC)6 and M7 (CB)3 carbo-borides phases in a matrix exhibiting high fracture toughness up to 73.3 MPa m1/2 due to an effective distribution of fine carbide and boride phases in ductile dendrites/cells. The synthetic multiphase alloy F shows the original fused and crushed tungsten carbides (2500–2700 HV0.1) which are extensively dissolved in the Fe-based matrix and lead to well distributed reprecipitated carbides with a decreased hardness of 1200–1600 HV0.1. Content and size of the synthetically added tungsten car- 775 Fig. 5. Detailed SEM + EDS investigation of the microstructure of sample E. bides is determined to 38.7% and 65–250 ␮m, respectively. The matrix has a hardness between 800 and 1100 HV0.1. Higher welding amperage increases the rate of tungsten carbide dissolution. Overall the tungsten carbides are irregularly distributed. There is a higher density of original carbides close to the fusion line, whereas at the surface only rests of carbides are visible. 3.2. Dry-sand rubber-wheel tests (3-body abrasion) Wear tests with a standard ASTM G65 dry-sand rubber-wheel tester according to procedure A were carried out to simulate 3-body abrasion similar to practical applications. Quantitative wear analysis was done by volume loss, and the results are given in Fig. 6. It can be seen, that there is a correlation of abrasive wear and hardness. In general, better performance against 3body abrasion can be obtained by increasing material hardness [15]. High abrasive wear can be observed for alloy A which is in good agreement with the relative low hardness (see Fig. 6). The lowest abrasive wear resistance of the hardfacing alloys investigated was observed for alloy C. It can be seen, that alloys A and C are given in a high wear level, whereas the other materials investigated are situated in a low level of abrasive wear. Fig. 6. Abrasive wear behaviour (dry-sand rubber-wheel test, ASTM G65 procedure A) and hardness (HV5) of the Fe-based hardfacing alloys investigated. 776 M. Kirchgaßner et al. / Wear 265 (2008) 772–779 4. Discussion 4.1. Abrasive wear behaviour Fig. 7. Impeller-tumbler wear rate at low impact level (testing abrasive, finegrained silica sand) and hardness of the Fe-based hardfacing alloys investigated. The influence of coarse primary precipitations on abrasive wear resistance can be seen for alloys C and D. These alloys exhibit similar macroscopic hardness, however wear behaviour differs by a factor of about 5 (see Fig. 6). Very good wear resistance can be obtained for alloy F (synthetic multiphase alloy with tungsten carbides), where the macro-hardness is given to about 750 HV5. Alloy E, showing highest hardness, performed best wear behaviour in dry-sand rubber-wheel testing. 3.3. Impeller-tumbler tests at low and high energy The impeller-tumbler wear rates obtained for the six different Fe-based hardfacing alloys tested under low impact loading (low energy level) are given in Fig. 7. There, for the softest material, alloy A, highest wear rate is observed. The better performance of alloy C compared to alloy A can be explained by the higher hardness (see Fig. 7). Best wear resistance can be seen for alloys D and E, which is in good agreement with their high hardness. The impeller-tumbler wear rates obtained for high impact loading (high energy level) are given in Fig. 8. Very good correlation of wear rate and material hardness can be seen. For alloy F, the synthetic multiphase alloy, a higher wear level can be detected. Fig. 8. Impeller-tumbler wear rate at upper impact level (testing abrasive, coarse corundum particles) and hardness of the Fe-based hardfacing alloys investigated. To evaluate the abrasive wear behaviour of the different Fe-based hardfacing alloys it is important to regard the macrohardness and the microstructure and to set them into relation. The hardness, which is determined by both, the hardness of the matrix and the hard phases, respectively, is a general measure for the wear resistance which is comprisingly described in Refs. [16,17]. Type, content, size and morphology of the hard phases occurring play a major role in the final wear properties of the individual alloys [18–20]. The alloys A and C, which are mainly supposed for being used under combined wear, show high wear rates under pure 3-body abrasion. The wear resistance of alloy A is based both on a martensitic microstructure and uniformly spread primary Nb carbides. Compared to alloy C which exhibits higher macrohardness, the performance under 3-body abrasion is better due to the hard primary Nb carbides of alloy A. The eutectic Fe/Cr carbides of alloy C have a minor effect on wear resistance and besides the brittle eutectic net around the primary ␥-dendrites can be damaged by micro-cracking. Berns describes this behaviour for FeCrC alloys to become relevant for hardness values above 830 HV [4]. The higher hardness of alloy C compared to B is not reflected by a higher abrasion resistance which shows the positive effect of hypereutectic solidification with primary carbides respectively borides as in case of alloy D which reduces the wear rates significantly (Fig. 6). Alloy D is slightly superior compared to alloy B even though its Fe borides show a lower hardness than the primary Fe/Cr and Nb carbides of alloy B and despite the fact that hard primary phases in alloy B are generally distributed more densely than in alloy D. That means that the very uniform and fine eutectic matrix of alloy D combined with a higher hardness provides better resistance against washing out. Cracking of large primary carbides in alloy B can have an additional influence. The lowest wear rates are obtained for alloys E and F. Alloy E combines high density of carbo-borides with a very hard and tough matrix. This microstructure is related to the complex composition, to the high boron level, a sufficient content of carbide and boride forming elements like Cr and V, and finally in a reduction in microstructure scale to the nanometer regime and super saturation of transition metal alloying elements significantly above their equilibrium solubility limits leading to a partially amorphous matrix [21]. The synthetic multiphase alloy F exhibits significant wear resistance and comes close to alloy E due to the presence of eutectic fused and crushed tungsten carbides (W2 C/WC) added as filling to the starting flux cored welding wire. The macro-hardness is significantly lower than for alloy E, but both carbides which are not or only partially dissolved and the dissolved carbides that are re-precipitated afterwards shows an extremely high hardness which contributes to their abrasion resistance. The original carbides are spread quite irregular over the weld overlay whereas dissolved ones can be found throughout the micro-section giving a uniform protection. M. Kirchgaßner et al. / Wear 265 (2008) 772–779 Fig. 9. SEM image of a typical worn surface of sample F after dry-sand rubberwheel testing (ASTM G65, procedure A). The typical wear mechanism for alloy F can be observed on the SEM image in Fig. 9. There it can be seen that the hard tungsten carbides are not worn significantly during dry-sand rubber-wheel test, only the Fe-based martensitic matrix shows a washing out effect. For the synthetic alloy F, the correlation with macro-hardness and wear resistance is not valid in contrary to the alloys B, D and E, where this correlation can be found to be independent from complexity of composition. 4.2. Behaviour under impact loading The low energy impeller-tumbler procedure using finegrained silica sand gives only low impact which can be described mainly as an erosive process. For this procedure there is stronger correlation between HV5 hardness and wear rates for all tested materials (Fig. 7). Still alloy C underperforms compared to alloy B due to the lack of hypereutectic primary hard phases. Except this and alloy F which behaves differently due to the above mentioned reasons, higher hardness results in lower erosive wear (Fig. 7). Alloy C shows better wear performance than alloy A compared to pure abrasion. The skeleton of hard eutectic Fe/Cr carbides is not cracking and is such more effective in reducing wear than the primary Nb carbides of alloy A which leaves a large volume of material only poorly protected. The edges of the samples wear rather evenly. Neither breaking of larger volumes of material on a macro-scale nor cracking of hard phases can be found (see Fig. 10, image a). For the high energy impeller-tumbler procedure using corundum, impact becomes dominant and the wear behaviour turns completely compared to abrasion and erosion. The combination of hardness and toughness of alloy A exhibits the lowest wear rates followed by alloy B which is beating the hypoeutectic alloy C as in the case of abrasion and erosion (see Fig. 8). This can be explained by its lower matrix hardness. A typical worn cross-section of the high loaded edge of ally B after testing is given in Fig. 11. There the plastic deformation of the ledeburite can be seen clearly which is an indicator for a ductile material behaviour, whereas the coarse Cr carbides 777 Fig. 10. Macro-image of the high loaded edge of sample E after impeller-tumbler testing: (a) low impact loading (low energy), and (b) high impact loading (high energy). are not worn significantly. Alloys C–E show loss and breaking of larger areas along the edge, in case of C and D this is related to the hard eutectic matrix cracking constantly during load. Alloys A and B wear rather evenly without showing large single material losses (Fig. 12). The dependence of the impact sensitivity for alloys A, B and E (see Fig. 12) shows impact sensitive behaviour for alloy E, whereas alloys A and B perform uncritical at high impact loads. This behaviour can be seen on the macro-image of the sample edge of alloy E in Fig. 10 (image b) where breaking of larger volumes of materials on a macro-scale can be observed. The synthetic compound F shows the highest wear rate of all materials and especially performs significantly worse than the hardest alloy E. The matrix of alloy E obviously offers more strength and toughness and gives a much better embedding of the hard phases. The synthetically added coarse tungsten carbides at grain size of 65–250 ␮m into the martensitic Fe-based matrix mostly suffer from cracking of tungsten carbides (see Fig. 13). There it can be seen that the hard and brittle tungsten carbides break under cyclic Fig. 11. A cross-section image of sample B after impeller-tumbler testing at high impact loading (high energy). 778 M. Kirchgaßner et al. / Wear 265 (2008) 772–779 where they are an economical alternative to higher alloyed materials. For high impact applications, martensitic materials behave best, whereas alloying elements as Nb which solidify primarily are a possible solution to obtain better performance under 3-body abrasion. Acknowledgements This work was founded from the “Austrian Kplus-Program” and has been carried out within the “Austrian Center of Competence for Tribology”. The authors are also grateful to the IAESTE student L. Ekres for helpful work at testing procedure and wear quantification. Fig. 12. Dependence of the impact sensitivity of alloys A, B and E in impellertumbler wear testing with fine-grained silica sand and coarse corundum. Fig. 13. SEM image of a typical worn tungsten carbide of sample F after impeller-tumbler testing at high impact loading (high energy). loading situation in the impeller-tumbler procedure resulting in an high wear regime which makes this alloy least useful for this type of application which was shown earlier in Refs. [22,23]. 5. Conclusions Though very important, hardness is only one factor to be evaluated when comparing the wear resistance of Fe-based hardfacing welding alloys. For pure abrasion either synthetic materials using tungsten carbides or complex alloys providing both hard phases, and a hard and tough nanostructured matrix at the same time are performing best and very similar. Under erosive wear or high impact loads, the tungsten carbide-containing hardfacing alloy underperforms significantly compared to this new complex type of Fe-based alloy. State of the art hypereutectic FeCrCNb alloys provide reasonably good behaviour under all test conditions but never being the leading edge. 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