Charpy Differences between GTAW vs. SMAW 1

[tc7]

I recently had a rare opportunity to conduct two PQR’s for same mat’l (4130 Q&T to 135ksi UTS prior to welding) – one procedure was done by GTAW using ER120S-1 filler and the other by SMAW using E12018 rod. The plates used were cut from the same bar stock, the Q&T heat treatment was performed at the same time, both procedures done by the same welder, neither plate was PWHTed. Interesting differences in destructive testing occurred;
GTAW results:
Tensile Strength 140ksi & 138ksi, break in base mat’l
Yield Strength: 113ksi, 1113ksi
Side Bend tests satisfactory on 2.3 inch pin diameter.
HAZ Charpy results: 38, 32, 37 Ft-lbs at -60 deg F
Calculated welding heat input range: 40-50kJ/in (assumed efficiency of .7)
Total #passes to fill out the weld: 19

SMAW results:
Tensile Strength 128ksi & 130ksi, break in weld
Yield Strength: 104ksi, 111ksi
Side Bend tests satisfactory on 2.3 inch pin diameter.
HAZ Charpy results: 40, 44, 50 Ft-lbs at -60 deg F
Calculated welding heat input range: 24-30kJ/in (assumed efficiency of .8)
Total #passes to fill out the weld: 24

The GTAW tensile results surprised me because the weld filler is only advertised to have a UTS of 120ksi and it turned out to be stronger than the Q&T 4130. I did not know what to expect for the HAZ Charpy results but was satisfied.

The SMAW tensile results were very in line with expectations BUT the yield strength was much lower than expected. The HAZ Charpy results were very curious being ~25% higher than the GTAW results. HOW CAN THIS BE? My first thought was that the SMAW welding heat input may have contributed to some annealing and grain refinement of the HAZ but when calculated out, the SMAW welding heat input was significantly lower than the GTAW heat by about 50-70%.

Can anyone suggest why the two sets of HAZ Charpy data is so different?
Thanks

 

[metengr]

tc7;
Simple answer - GTAW provides HAZ grain refinement in comparison to SMAW. This one difference alone will affect toughness.

 

[metengr]

tc7;
Sorry, I hit the send button before I finished. Anyway, in my original response, the simple answer holds provided all other factors in welding are equal in making comparisons between welding processes. In your above scenario, you have a difference in heat inputs that also factors into your CVN impact differences.

Your heat input for SMAW was considerably lower and this was probably one of the reasons for higher CVN values, despite the greater number of passes. In essence, you have probably produced the same affect as grain refinement.

 

[stanweld]

In addition to metengr's comments, the quench rate is somewhat greater using the GTAW process, resulting in the higher yield and tensile values observed. You should also take a look at hardness in the weld metal and HAZ for both processes. I think that you will find that both weld metal and HAZ are harder in the coupons made with the GTAW process. This will also affect the toughness properties as you discovered.

 

[tc7]

So, rephrasing the simple answer:
GTAW = higher heat = grain refinement = lower ductility = lower toughness = lower CVN?

This has been eye opening, I thought grain refinement = higher ductility.

Why does GTAW = higher heat than SMAW? We melted in the same volume of weld metal in similar amount of time. Is the answer in difference of arc efficieny? I used GTAW efficiency = .7 & SMAW efficiency = .8; what values would you recommend?

 

[metengr]

tc7;
I would use the following arc efficiencies from AWS;

SMAW 0.9
GTAW 0.7
SAW 0.95
GMAW 0.9
FCAW 0.9

Now, getting back to your last post, smaller grain size improves toughness and increases strength with some loss in ductility.

 

[tc7]

I guess I had a different notion of what "grain refinement" meant. In the above context grain refinement should be replaced with "grain coarsening"?

 

[metengr]

Yep.