Loading, heat treatment and welding parameters influence on wear resistance: martensitic tool steel weld metal for reclaiming

A. GUALCO, H. G. SVOBODA, E. SURIAN, L. DE VEDIA

Chicago, USA

Conferencia; Fabtech International and AWS Annual Convention; 2009

American Welding Society

Introduction:The annual expenditure by wear losses in the USA amounts approximately to 1-2% of the National Gross Product. Of this amount, 22% is related to wear due to metal-metal sliding. In order to reduce these losses welding is employed for reclaiming surfaces that undergo wear during service in order to extend the useful life of the affected components. The present work is oriented to rationalize under which conditions post weld heat treatment should be applied and which is the most suitable welding procedure for the optimization of the resulting weld deposits, since the present available information on the subject is rather scarce.:The annual expenditure by wear losses in the USA amounts approximately to 1-2% of the National Gross Product. Of this amount, 22% is related to wear due to metal-metal sliding. In order to reduce these losses welding is employed for reclaiming surfaces that undergo wear during service in order to extend the useful life of the affected components. The present work is oriented to rationalize under which conditions post weld heat treatment should be applied and which is the most suitable welding procedure for the optimization of the resulting weld deposits, since the present available information on the subject is rather scarce. Technical Approach:Martensitic tool steel weld metal alloyed with Cr, Mn, Mo, V y W, deposited with the FCAW process, under Ar-2%CO2 shielding, with two heat inputs (2 kJ/mm and 3 kJ/mm, respectively identified as RC and RH) were considered. A post weld heat treatment at 550ºC during 2 hours was employed. From each welded coupon, cross sections were obtained for microstructural characterization, hardness measurements and chemical composition determination. In order to reproduce the conditions of the material during service, wear tests (metal-metal) were carried out using an Amsler machine under conditions of pure sliding, using applied loads of 500 and 2000 N. Martensitic tool steel weld metal alloyed with Cr, Mn, Mo, V y W, deposited with the FCAW process, under Ar-2%CO2 shielding, with two heat inputs (2 kJ/mm and 3 kJ/mm, respectively identified as RC and RH) were considered. A post weld heat treatment at 550ºC during 2 hours was employed. From each welded coupon, cross sections were obtained for microstructural characterization, hardness measurements and chemical composition determination. In order to reproduce the conditions of the material during service, wear tests (metal-metal) were carried out using an Amsler machine under conditions of pure sliding, using applied loads of 500 and 2000 N. Results/Discussion:The sample RH presented, for the same number of weld beads than the RC, a weld deposit 40% thicker than the RC sample. RC sample chemical composition exhibited higher values of C, Mn, Si, Cr, Mo, V and W than RH specimen. Both samples as welded microstructure presented martensite with 16% of retained austenite in the RC sample and 10% of retained austenite in the RH sample. The observed differences were probably related to the higher alloy content in the RC specimen that resulted in a reduction in the Ms Temperature. Heat treatment led to a reduction in the amount of retained austenite to 4 % (RH) and 6 % (RC). Microhardness was 640 HV for RC and 690 HV for RH; the heat treatment produced secondary hardening raising hardness to 720 HV and 740 HV respectively. The wear rates for tests conducted with an applied load of 500 N were 1.49 10-05g/m for RC and 1.63 10- 05g/m for RH; heat treatment increased these values (RC: 1.94 10-05g/m and RH: 1.93 10-05g/m). A linear relationship was found between hardness and wear rate. It was concluded that samples without heat treatment exhibited better wear resistance, which was an unexpected result, since it is generally accepted that higher hardness corresponds to higher wear resistance. The wear rates for tests conducted with an applied load of 2000 N were 11.88 10-05g/m (RC) and 10.09 10-05g/m (RH); in this case heat treatment reduced these values to (9.59 10-05g/m and 9.61 10-05g/m respectively). It was also found that a linear relationship holds between hardness and wear rate, but in this case increasing hardness improved wear resistance. The main wear mechanism was soft oxidative for the 500 N samples and severe oxidative for the 2000 N samples. The average temperature generated by friction during testing was 110ºC for 500 N and 530ºC for 2000 N. Heat input did not show any significant effect on wear resistance. Conclusions: For the conditions studied, under moderate service loads it seems preferable not to recur to post weld heat treatment since this reduced wear resistance in about 30%. On the other hand, for more extreme conditions where temperature generated by friction reached 530 ºC, heat treatment should benefit leading to about 20% increase in wear resistance. As far as heat input is concerned, no differences were detected in wear resistance, although it is worth noting that due to the higher weld thickness obtained with higher heat input, it is possible to obtain the same useful life with fewer beads when this heat input is employed.