U.S. Department of Transportation
Federal Highway Administration
|Subject:||INFORMATION: Narrow-Gap Electroslag Welding for Bridges||Date:||March 20, 2000|
|From:||/s/ Original signed by:
David H. Densmore
Director of Bridge Technology
|Reply to Attn. of:||HIBT-10|
Resource Center Directors
Federal Lands Highway Division Engineers
Improving the reliability and toughness of electroslag welds has been the subject of long-term research sponsored by the Federal Highway Administration. As a result of this research and together with the ongoing FHWA sponsored demonstrations across the United States, a new modified version, "Narrow-Gap Electroslag Welding Process" has been extensively tested. As described in the FHWA publications mentioned in the attachment, Narrow-Gap Improved Electroslag Welding (NGI-ESW) practices and procedures will be allowed on Federal-aid projects for electroslag welding. This applies to electroslag welding that joins non-fracture critical tension and/or reversal stress members in AASHTO temperature Zones 1 and 2 with material up to 3 inches thick.
As of the date of this memorandum, the moratorium imposed by FHWA Notice N 5040.23 dated February 23, 1977, on electroslag welds in tension and/or reversal stress loaded members on Federal-aid projects, is rescinded. Please see attachment concerning application of the modified electroslag (NGI-ESW) process.
Narrow-Gap Improved Electroslag Welding
Improving the reliability and toughness of electroslag welds has been the subject of long-term research sponsored by the Federal Highway Administration. As a result of this research and together with the continued success of the FHWA sponsored demonstrations conducted across the United States, the moratorium on electroslag welds in tension and/or reversal stress loaded members on bridge structures is being lifted. (The moratorium was imposed by FHWA Notice N 5040.23 dated February 16, 1977.) Narrow-Gap Improved Electroslag Welding (NGI-ESW) practices and procedures will be allowed on Federal-aid projects for electroslag welding for joining non-fracture critical tension and/or reversal stress loading members in Zones 1 and 2 with material up to 3 inches thick.
The electroslag welding process as used through the early 1970's was unable to consistently produce defect-free welds with adequate weld metal toughness. The high incidence of defects required major repairs which further complicated inspection. (Note: Nonredundant main load carrying tension members in existing structures that are known to have been welded with electroslag process should continue to be subjected to rigorous NDT (radiographic and ultrasonic) inspection.)) Furthermore, examination of weld metal toughness revealed very low Charpy V-Notch (CVN) impact toughness values. Federal Highway Administration sponsored research has addressed these issues and resulted in producing uniformly acceptable reliability, consistency, toughness, and fatigue performance when the NGI-ESW modified practices and procedures are used. These practices and procedures are embodied in a modified process designated as NGI-ESW by the Federal Highway Administration. The following FHWA publications should be referenced for the NGI-ESW process:
- Report no. FHWA-SA-96-053
Technical Information Guide for Narrow-Gap Improved Electroslag Welding
- Report No. FHWA-SA-96-052
Process Operational Guide for Narrow-Gap Improved Electroslag Welding
- Report No. FHWA-SA-96-051
Training Manual for Narrow-Gap Improved Electroslag Welding for Bridges
- Report No. FHWA-SA-96-050
D1.5 Bridge Welding Code Proposed Revisions to Include Narrow-Gap Improved Electroslag Welding
Synopsis: NGI-ESW Major Improvements
Three major improvements included (1) consistent defect-free welds, (2) fatigue performance, and (3) impact toughness in the weld and heat-affected zones. The results are summarized as follows:
1. Consistent Defect Free Welds
As a result of many procedural and consumable changes, the NGI-ESW process has virtually eliminated the occurrence of internal weld defects, including slag inclusions, ferrite vein cracking, hot cracking, and lack of fusion. To confirm the consistent lack of defects when the NGI-ESW process is used, at least one weld from each demonstration workshop has been subjected to visual (VT), radiographic (RT), and ultrasonic (UT) testing. No internal defects have been found by either UT or RT inspection. Subsequent sectioning for physical testing has further confirmed the lack of any defects.
2. Fatigue Performance
In order to evaluate the fatigue behavior of NGI-ESW welds, large scale 22 ft. long girders containing 2 in. thick flange NGI-ESW welds were fabricated and tested. Of the 14 NGI-ESW welds fatigue tested, none initiated any fatigue cracks. Included in these tests were NGI-ESW welds that were fatigue tested and then subjected to a variety of weld repairs based on simulated defects, using the Shielded Metal Arc Welding (SMAW) process. Since the welds did not contain any defects, excavations typical of those for real defects were prepared. The repairs were completed using SMAW and standard AISI/AASHTO/AWS D1.5 Bridge Welding Code repair procedures. These welds were then re-fatigue tested in some cases for up to three times the required fatigue life cycles. No NGI-ESW welds containing weld repairs of simulated defects developed fatigue cracks. In summary, under no circumstances did any of the NGI-ESW welds develop fatigue cracks.
3. Impact Toughness
The FHWA moratorium was initiated largely due to extremely low CVN values that were obtained from NGI-ESW weld metal. The practices and procedures of the research program have resulted in major improvement of the weld metal CVN. By substantially reducing heat input and by designing the weld metal alloy chemistry to produce a uniformly tough microstructure, the NGI-ESW weld metal CVN properties taken at the weld centerline, the region of lowest toughness, exceed the proposed CVN values of 20 ft-lb. (27 joules) at 0° F (-20° C).
A secondary area of CVN property concern was identified at the heat-affected zone. While not required for any other welding process, the proposed criteria for NGI-ESW welds in members subject to tension and/or reversed loading conditions is 15 ft-lb. at +40° F (20J at +4° C), tested with the notch at the quarter thickness (1/4t) location. The results obtained during the research phase and during industrial field trials demonstrated adequate impact properties using NGI-ESW practices.
NGI-ESW has resulted in improved weld reliability and toughness and virtually eliminated internal flaws, which in turn virtually eliminates major repairs. It also demonstrates fatigue resistance, and greatly improves impact toughness. The NGI-ESW process is well documented, from both fundamental and practical considerations, and the procedures can be readily adopted by commercial fabricators. The development of the technology, as evidenced by NGI-ESW, successfully eliminates the conditions and concerns that led to the moratorium. Detailed research data is available from the FHWA report FHWA/RD-87/026 titled "Improved Fracture Toughness and Fatigue Characteristics of Electroslag Welds."
For these reasons, the moratorium on electroslag welding is being rescinded permitting NGI-ESW for joining non-fracture critical members subject to tension and/or reversal stresses in temperature Zones 1 and 2 with material up to 3 inches thick on Federal-aid projects. All NGI-ESW welds subject to tension and reversal stresses shall be 100 % inspected by both RT and UT.