High-temperature abrasive wear studies of high alloyed cast irons

Harald Rojacz, Markus Varga

AC2T research GmbH, Wiener Neustadt, AUT

INTRODUCTION: Several industrial applications operating under harsh environments require heat, creep and wear resistance of the utilized materials. By high temperatures (HT) increased abrasive wear of materials leads to high maintenance expenses and downtime costs by premature failure of core components.¹ In order to reduce wear at elevated temperatures, it is crucial to understand the ongoing damage mechanisms. The influence of different microstructures, and the resulting hardness, is of great importance to understand abrasive wear behaviour, which can be assessed with hot hardness measurements.^2,3 Since various parameters like load, temperature, abrasive type and shape and relative velocity influence the occurring wear mechanisms, several factors have to be considered to understand HT abrasive wear mechansims.⁴

Wear mechanisms, the formation of mechanically mixed layer ^5,6 and their change with temperature for high alloyed cast irons is still insufficiently addressed in literature. Therefore two ferritic, a duplex and an austenitic cast iron were chosen for investigations in abrasive environment up to 700° C.

MATERIALS AND METHODS: Four different high-temperature resistant cast irons were chosen for investigation: two nodular ferritic cast irons (EN-GJS-XSiMo 4-05 - Material 1 and EN-GJS-XSiMo 4-1 - Material 2), differing in Mo-content, one ferritic-austenitic cast iron (EN-GJS-XNiMn 13-7 – Material 3) with 13 wt.-% Ni and 6.5 wt.-% Mn and one austenitic cast iron (EN-GJS-XNi 22 – Material 4) with 22 wt.-% Ni. Prior to testing, samples were cut to 75×25×10 mm and plane parallel ground.

Hot hardness was measured with a novel hot hardness and scratch tester at 98.1 N load with Vickers-method (HV10).⁷ Abrasion tests were performed with a HT three-body abrasion test rig, which consist of a heated sample pressed onto a turning steel wheel a with a force of 45 N. Standard Ottawa quartz abrasive (212-300 µm –ASTM G65) is introduced into the wear gap between wheel and sample. Temperatures were set to 20, 500 and 700 °C.

Post-tests analysis comprise surface SEM of the wear tracks and SEM of metallographic cross-sections. Wear mechanisms of both, matrix and graphite, the covered surface area and the extent of eventually formed mechanically layers were investigated. To correlate materials’ parameters, the matrix hardness was measured with a nano indenter (Hysitron® Triboindenter T 900).

Figure 1 – Wear rates of selected materials investigated.

RESULTS: As given in Figure 1, wear rates of all materials investigated increase with rising temperatures. Starting with diverging wear rates at 20 °C, at 500 °C results are most divergent and finally reaching almost identical values at 700 °C for the cast materials investigated.

Figure 2 shows surface and cross-sectional analyses at 20 ° and 700 °C for Material 1 and 4, indicating, that ferritic cast irons tend to form mechanically mixed layers more easily than austenites. Statistically, the surface of the ferrites is covered with more particles, which can act as wear protection.

The wear rates are strongly dependent on hot hardness, elasto-plastic material’s behaviour and the in-situ formation of mechanically mixed layers ^4-9, as explained in the discussion and in the full paper of this study.

Figure 2 – Surface and cross-sectional analyses via SEM.

DISCUSSION: As given in the results, wear rates increase with temperature due to the increased plasticity and loss of hardness at elevated temperatures. This also promotes the formation of mechanically mixed layers and particle sticking to the surface.

A statistical analysis on the influence of hardness, toughness (apparent Young’s modulus) and temperature on wear behaviour and mechanically mixed layer formation was undertaken. It indicates that ferritic nodular cast irons form these protective mechanically mixed layers easier, while austenites tend to be abraded at a higher level due to less pronounced wear protective layers.

REFERENCES: 1. Osarenjen, CRC Press (2015), 2. Rojacz, Tribol. Int. (2017), 3. Varga, Wear (2017), 4. Zum Gahr, Elsevier (1987), 5. Varga, Tribol. Int. (2013), 6. Fischer, Wiley VCH. (2008) 7. Varga, J. Eng. Trib. (2006), 8. Antonov Wear (2007), 9. Rynio, Wear (2014).

ACKNOWLEDGEMENTS: This work was funded by the Austrian COMET Program (Project K2 XTribology, no. 849109) and carried out at the “Excellence Centre of Tribology”.