The Effect of Weld Metal Strength Mismatch on the Deformation and Fracture Behavior of Steel Butt Weldments

Considerable Naval and industrial experience dating from the explosion bulge studies of Pellini and Hartbower in the early 1950s has indicated the engineering utility of using weld metal having strength greater than the plates being joined (overmatching). This practice shields the weld region, which typically has lower toughness than the plate and is often the site of defects, from the high strains that develop during an overload. This practical advantage, coupled with the ease of achieving overmatch in lower strength steel alloys (80 ksi yield strength or less) has led to codification of overmatching as a requirement in most structural design codes and fabrication specifications. However, overmatching has certain economic and technical disadvantages which undermatched (weld metal strength less than plate strength) systems might alleviate. Examples of undermatch benefits discussed in the literature include reduction of the preheat needed to avoid hydrogen cracking and increase of weld metal deposition rate relative to overmatched practice. Such changes could reduce the need to hold electrodes at an elevated temperature prior to use, extend the welder's duty cycle, reduce the lack of fusion / lack of penetration defect rate, reduce restraint stresses, and increase weld metal toughness. This information suggests that overmatched welds, while quite effective for low strength steel construction, may not be as advantageous when fabricating structures from higher strength grades. However, undermatched welds cannot be immediately adopted for use due to the much greater strains that would have to be borne by the weld metal. Undermatched welds will require greater toughness to provide the same safety margin against fracture as overmatched construction. To quantify the toughness / matching combinations which provide acceptable service performance, information regarding how over and under matching influences the stresses and strains in a weld joint is needed. Additionally, procedures for quantifying the fracture toughness of candidate weld metals and their heat affected zones will be required. In support of the further development of such information and procedures, this report reviews investigations concerning the deformation and fracture characteristics of Simple mechanical test specimens containing butt welds, focusing on how the relative strength of the weld deposit and the plate influences these characteristics.David Taylor Research Center, Metals and Welding Divisio