Dissimilar Metal Welds Between ASTM A709 Grade 50CR and Other Bridge Steels

About the report

Corrosion is a prevailing cause of bridge deterioration for the Virginia Department of Transportation (VDOT) and leads to expensive maintenance actions. During the past decade, ASTM A709 Grade 50CR steel has been used on bridges to provide additional corrosion resistance, although its higher material and fabrication costs present implementation challenges. Dissimilar metal welds (DMWs) using 50CR steel in targeted, highly corrosive locations can provide enhanced corrosion protection while minimizing initial costs.

This project investigated the feasibility of using DMWs made between Grade 50CR and other bridge steels in terms of their weldability, mechanical properties, galvanic corrosion performance, and compatibility with standard nondestructive evaluation techniques. In addition, the project also sought to identify an alternative to 50CR steel because of challenges with its long lead time and quality in recent VDOT projects. ATI 412 steel was identified as a potential alternative to 50CR steel based on its similar corrosion resistance and mechanical properties. DMW specimens were fabricated by welding either 50CR or 412 steel to either ASTM A36 or ASTM A709 Grade 50W steel. A steel bridge fabricator conducted all welding by using a variety of welding parameters.

The weldability evaluation showed that DMWs made between 50CR/412 steel and A36/50W steel are susceptible to solidification cracking and cold cracking. Both types of cracking can be eliminated using lower heat input and a single-V with backgouge weld joint. Crack-free DMWs were found to meet the mechanical property requirements of the American Association of State Highway and Transportation Officials and American Welding Society (AASHTO/AWS) D1.5 Bridge Welding Code. DMWs experienced uniform, pitting, galvanic, and intergranular corrosion. Galvanic corrosion was present in the heat-affected zones on both sides of the stainless steel weld interface. The A36/50W steel heat-affected zones experienced accelerated uniform corrosion compared with the A36/50W steel base metal. The 50CR steel experienced accelerated pitting and intergranular corrosion compared with the 50CR steel base metal.

The nondestructive evaluation results showed challenges with radiographic testing, ultrasonic testing, and phased array ultrasonic testing of DMWs. Some radiographic testing conducted at the fabricator displayed artifacts that could mask potential defects in the welds. DMWs also exhibited ultrasonic anisotropic behavior and ultrasonic wave refraction, causing typical ultrasonic and phased array ultrasonic testing procedures to mislocate or completely miss defects in DMWs.

Based on the results, the Virginia Transportation Research Council recommends that VDOT not pursue using 50CR/412 steel in DMWs due to remaining challenges with their corrosion performance and nondestructive evaluation. The Virginia Transportation Research Council also recommends that VDOT continue to exercise caution with specifying 50CR steel because of recent poor quality and long lead times. Both recommendations allow VDOT to proactively manage risk on future projects and help to ensure structural integrity and on-time, on-budget project delivery.

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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