Acceptable Heat Affected Zone Toughness of A709 Bridge Steel
I. RESEARCH PROBLEM STATEMENT
Research must be performed to establish the acceptable limits of heat affected zone (HAZ) toughness for all welding processes currently allowed by the AASHTO/AWS D1.5 Bridge Welding Code (SMAW, SAW, FCAW, GMAW, and ESW) using ASTM A709 bridge steels.
It is well established that the most brittle part of a ferritic steel weld is in the HAZ due to its coarse grained structure and carbide dissolution due to the weld thermal cycle. The American Welding Society (AWS) has never prescribed a HAZ toughness requirement for welds used in bridge fabrication. The only qualifications of weld metal prescribed in the AWS Bridge Welding Code (AWS D1.5) are macroetches, side-bend tests, reduced section tension test, Charpy V-notch (CVN) and all-weld-metal tension tests. While the side bend and reduced section tension test do include the HAZ, each is just a statically loaded test. Since bridges are subject to variable loading, fatigue and fracture become a concern and AWS currently does not prescribe a dynamic test that includes the weld HAZ.
Other industries outside of bridge commonly perform HAZ toughness testing as part of their welding procedures. The CVN test may be used; this entails cutting a longer than normal specimen, macroetching it to identify the HAZ location, then cutting out a CVN specimen such that the notch is in the HAZ. While the test does nominally increase the cost to the qualification of a welding procedure, it is recognized in other industries to be integral to the development of sound welding procedures. The use of crack-tip opening displacement fracture testing has also been successfully implemented to measure HAZ toughness.
HAZ toughness measurement was performed in development of the Narrow Gap Improved Electroslag Welding (NGI-ESW) process sponsored by FHWA in the 1980’s and 90’s. As plates became thicker and higher strength they required higher heat inputs and this adversely affected the HAZ toughness. In some cases, depending if it was a fracture-critical application or not, the resulting HAZ toughness was inadequate when using the AASHTO requirement for base and weld metal. Unfortunately, NGI-ESW welding process may be unnecessarily penalized because some owners require HAZ testing of ESW but not other processes. Some ESW research has shown in certain situations it may have inadequate HAZ toughness leading owners to specify this additional testing, though AWS and AASHTO are both silent on what the HAZ toughness should be. HAZ properties are a function of the base metal, the welding process, and its execution.
II. LITERATURE SEARCH SUMMARY
A TRID search was performed using the search terms “HAZ” AND “toughness”. There were multiple hits that discussed the testing of HAZ toughness in the marine and offshore industries. While this research maybe of relevance in a literature review for the proposed problem statement, these other research did not use ASTM A709 steels.
The most relevant literature that was identified was a FHWA sponsored project entitled “Fracture Toughness and Weldability Tests for Submerged Arc Welded Joints, FHWA/RD-87/020”. This project did investigate the HAZ toughness issue in A514 and A588 steels that were welded with SMAW and SAW processes. This research did identify that HAZ toughness can be lower than the weld metal, however a recommendation of the project was further research to determine the significance of the lower values. It should also be noted that since this research was performed, HPS steels have been introduced to the market, the A514 material has been removed from the A709 specification, and the project did not evaluate FCAW, GMAW, and ESW welding processes.
FHWA also sponsored a project entitled, “Heat-Affected Zone Toughness of Electroslag Weldments, FHWA-RD-93-014” to investigate the HAZ toughness of electroslag welds. It showed that ESW welds in A36 and A588 could achieve Zone 2 non-fracture-critical toughness, but highlighted the sensitivity of making this measurement and urged the need to minimize the heat input to attain desirable CVN toughness. Again, this project was limited to the testing of A36 and A588 steel and just one welding process.
III. RESEARCH OBJECTIVE
The objective of the research project is three-fold;
· Experimentally determine the HAZ toughness of many types of A709 steels using the five allowable welding processes of AWS D1.5. This should explore maximum heat inputs that are allowed by AWS through the use of existing qualified weld procedures for a variety of different types of A709 steel and thicknesses.
· Since the first objective focuses on current acceptable welding techniques, analyze the HAZ toughness criteria to determine an acceptable target of HAZ toughness, and
· Recommended changes to relevant AASHTO or AWS documents to enforce HAZ toughness requirements.
IV. ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD
Recommended Funding: $500,000
Research Period: 30 months
V. URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
The electroslag welding process is the most efficient welding process for the joining of thick plates, often used in the splicing of girder flange plates. This single step welding process would reduce plate handling and as a single-pass process it reduces the incidences of weld defects that multi-pass welds are susceptible to; both would make steel bridge fabrication faster and therefore cheaper. This research could affect the fabrication of all future steel bridges and the fabrication cost savings alone in a handful of projects could pay for this research. The results of this research may show that both the AASHTO/AWS D1.5 Bridge Welding Code and AASHTO LRFD Bridge Design Specification may have to be revised to take a position on HAZ toughness requirements.
This research meets the AASHTO Grand Challenge 4 in the area of “Advancing the AASHTO Specifications.”