Fatigue Properties and Life Prediction of 8630 Cast Steel in the Presence of Weld Repair and Porosities

Abstract

The objective of the this work is to study the effect of weld repair and macro-porosities on fatigue properties and fatigue life of AISI 8630 cast steel subjected to bending cyclic loads. Test specimens were cut from a cast steel component that contains macro-porosities. To regain the structural integrity, the component is typically excavated and weld repaired. In this study, weld repairs are simulated by machining a cylindrical groove across the width of the specimen then the groove is welded using different weld rods and various pre and post weld heat treatment conditions. All specimens were examined by radiography (X-Ray) in accordance with ASTM E446 before depositing the weld in order to verify the quality of the samples. After welding, the quality of the welded grooves was examined using ultrasonic testing (UT) and magnetic particle inspection (MPI) in accordance with ASTM A609 and ASTM E709 standards, respectively. Qualified samples were then machines to the final dimensions. The fatigue test was performed under pure bending conditions using four-point bend set up. This set up allows localizing stresses at the sites where weld repairs were applied. Experimental results show that specimens with stronger weld material tested under no heat treatment conditions have comparable fatigue performance to those heat treated specimens with lower strength weld material. Furthermore, it was found that the fatigue test results are highly affected by the presence of micro-porosities within the cast steel material. Therefore, an analytical approach to predict the fatigue life is also presented in this work. A good agreement is achieved between the predicted fatigue lives and the experimental results. Generally, the results show that for porosities with width to depth ratios between 0.25 to 1 and width size smaller than or equal to 1 mm the life is reduced by up to 1.5 orders of magnitude while the endurance limit was reduced by a factor of 1.57. Similarly for porosities with ratios between 1.5 and 2.5 and width greater than 1.5 mm the life was reduced by 2.5 orders of magnitude and the endurance limit was reduced by a factor of 2.2.