Welding technology has been for long employed in various engineering application like construction and manufacturing of various components. It is used to join metal pieces together to meet the required application properties and prevent structural failures. This is because it is generally cost-effective and easy to use. Unfortunately, welding joints are susceptible to various failures thus limiting the performance of the components.
Generally, failure in welded structures and components are mostly as a result of fatigue even though other causes like poor welding practices and inappropriate parameters may also be considered. To this end, a proper understanding of the behavior of high cycle fatigue and very high cycle fatigue is very significant. They are the key consideration for setting appropriate welding parameters as well as designing structural components against fatigue failures.
Unlike uniform materials and structures, predicting fatigue failure in welded joints is complicated. It comprises of mismatched materials, weld flaws, discontinuous microstructures and residual stresses. Fatigue performance of welds does not depend on either the base material or the initial strength distribution. However, fatigue resistance, in one way or the other, relates to the microstructure from the time of damage development to the occurrence of the full failure. For instance, micro-defect like cracking is the dominant cause of failure in very high cycle fatigue. Therefore, it is necessary to balance the microstructures and micro-defects so as to properly design high-quality welded joints in the very high cycle fatigue regime.
In a recent publication in International Journal of Fatigue , East China University of Science and Technology scientists: Wei-Chang Zhang, Professor Ming-Liang Zhu, Kai Wang and Professor Fu-Zhen Xuan investigated tensile and high cycle fatigue behaviors of dissimilar weld joint comprising of 9%Cr steel, CrMoV steel and 9%Cr-CrMoV at 500°C. They elucidated the failure mechanism, their prevention and applications in the practical design of high-quality weld joints.
The authors observed that the fatigue strength of the welds was significantly weakened much lower than the base metals owing to the difference in the failure mechanisms. Consequently, the variation of the microstructure along the welds correlated with micro-hardness distribution in the weld metals and tensile behavior. Soft zone of HAZ of CrMoV was the origin of fatigue failure particularly for the cross-weld samples where cracks were initiated by inclusion and microstructural heterogeneities. However, for pure base metals, cracks nucleated from microstructure heterogeneities. Furthermore, geometrical discontinuities were associated with the weaker weld joints.
According to the authors, fatigue design for high-quality and long-lasting welded joints must impose a strict optimization of the welding parameters as well as a thorough knowledge on the significance of the filler metal relative to the base metal. The procedure also has to take into consideration the soft-zone and micro defects that are resulted from welding. Therefore, the study provides a platform and a basis for the next generation high-quality welding for the production of longer life structures and components.
Zhang, W., Zhu, M., Wang, K., & Xuan, F. (2018). Failure mechanisms and design of dissimilar welds of 9%Cr and CrMoV steels up to very high cycle fatigue regime. International Journal of Fatigue, 113, 367-376.Go To International Journal of Fatigue