Consumables Corner: Figuring out FCAW Charpy V-notch failure
Q: We performed an AWS D1.1 procedure qualification test using gas-shielded FCAW. We used a 0.052-in. E71T-9C electrode with 100 percent CO2 shielding at 200 amps and 26 V on 1-in. thick plate. We used a preheat and interpass temperature of 70 degrees F. Our pass layer sequence was three full layers, three split-pass layers, and one triple-pass layer. We passed a radiograph test but failed the Charpy V-notch impact test with a combined average of 18 ft.-lbs. Can you guys shed some light on why we failed with a FCAW electrode that meets the AWS A5.20 requirements for a T-9?
A: There are several potential reasons for the Charpy V-notch impact testing failure. Let’s take a look at the differences between the AWS A5.20 specification for carbon steel FCAW electrodes and the actual test you conducted.
The A5.20 specification requires that the filler metal be tested in the flat position with an interpass temperature of 275 to 325 degrees F. If the interpass temperature falls below 275 degrees, the plate has to be reheated to the 275-325 range before you can continue to weld.
You didn’t specify what type of material you were welding on. Material type can affect the mechanical properties of the weld based on the amount of dilution in the weld metal.
Additionally, the A5.20 specification states the total number of allowable weld layers for 0.052-in.-dia. electrodes is five to eight. This specification is for a test plate that is ¾ in. thick. Your procedure test plate was 1 in. thick and you had a total of seven layers. This indicates that your weld layer thicknesses may have been excessive.
Here are some suggestions based on the information that you provided:
- While the 70-degree F preheat minimum is probably not affecting your results, the interpass temperature is very low and might be the cause of the failure. With such a low interpass temperature your quenching or cooling rate is very high, meaning the weld metal could be cooling too fast to allow for a finer grain size. Try increasing your interpass temperature to at least 250, but closer to 300 would be better.
- As for the pass layer sequence, getting an additional two layers will help the impacts as it would allow for more tempering of the existing weld metal, thereby increasing ductility. When you deposit “thick” weld beads, the tempering, as a result of subsequent passes, is minimized, which could have a detrimental effect on your impact toughness.
- While the Charpy V-notch impact test is used and accepted widely, it is known to produce varied results. While that probably is not the issue, it’s worth mentioning that the specimen should be machined to get consistent results.
- If possible, increase your amperage by 10 to 15 amps while keeping the voltage as close as possible to 26. Depending on the wire type, this may not be possible because of the product you’re using.
- Contact your welding supply distributor or the welding wire manufacturer for more suggestions. Although an E71T-9C wire should readily meet 20 ft.-lbs. at -20 degrees F, as you’ve experienced, the variables incorporated in doing a procedure qualification test can have a negative effect on the results. This is one of the reasons that the D1.1 and other codes have very strict limitations on welding variables. This ensures that acceptable results can be achieved consistently.
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