Improving fatigue properties of welded structural steels

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Motor Coach Industries (MCI) makes buses and their parts for various transit agencies in North America. Electric arc welding is used in the manufacturing of main vehicle parts at MCI due to its effectiveness, productivity and economical benefits. However, fatigue failures of welded steel parts, at the company, originate directly form the weld or in its immediate vicinity. Damage to fatigue resistance of welded materials can ensue from detrimental microstructural features formed in weld fusion zone and in the heat affected zone (HAZ) during welding. These include cracks, inclusions, porosity and deleterious secondary phases caused by the rapid welding thermal cycle. These microstructural features are significantly influenced by welding parameters, unfortunately welding parameters influence the microstructure and fatigue properties of welded steel materials. Therefore, the objective of this proposed research is to systematically study how electric arc welding parameters alter the microstructure in and around welds and how this influences fatigue properties in a steel material that is used in the production of buses. Detailed microstructural analyses of welds and HAZs produced by different welding parameters will be performed both prior to and after fatigue testing to determine welding parameters dependent microstructural features that are responsible for fatigue crack initiation and propagation. This would be valuable in the proper selection of welding parameters for improving fatigue resistance of electric-arc welded steel structures. The production of welded coaches with superior fatigue resistance will enable safer bus coaches with increased service life and reduced repair services. The research outcome would be beneficial not only to the automotive industry but also to other Canadian industries, where welded structural steels are subjected to repeated loading conditions. This will enhance safety and aid the economy of the country by facilitating Canadian businesses.