Charpy V-Notch Testing Heat Affected Zone

[pbc825]

I've received a specification from a client asking for CVN testing of the heat affected zone. The parent material is 350WT category 1. By my thinking, if the parent material passes CVN (by definition of the material specification) and the weld has as-welded properties that are tough, then the HAZ would likely pass as well (unless someone spilled a drink during the slow-cool period). Sounds like an overzealous specification to me, but interested to have some commentary from those with experience.

 

[SJones]

You might think it, but your client wants to see the hard evidence. Does the HAZ have the same grain structure as the parent material or the weld metal?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[pbc825]

SJones:
"Does the HAZ have the same grain structure as the parent material or the weld metal?"

I'm no expert, but thought the grain structure HAZ would be largely similar to the parent material if it was slow cooled. Am I off-base?

 

[weldstan]

You are off base. Very slow cooling in the transformation range will only increase the width of the HAZ and very likely adversely affect toughness. The grain structure will vary through the width of the HAZ. Essentially slow cooling does not occur in the transformation zone with normal welding processes used in structural welding.

And how do you intend to very slow cool in this temp range (1600F to 1200F)? By preheating or maintaining interpass temperatures above 900F? In this case you will be assured to adversely affect toughness in both the HAZ and the deposited weld metal.

 

Don't worry, HAZ impact testing is one of the biggest unintended loopholes in the entire B&PV Code. Geometrically, the coarse-grained region is almost never properly captured in the test, mostly because the sloping profile of the fusion line does not align with the perpendicular aspect of the specimen notch. And the impact toughness of the regions on either side of the CG-HAZ invariably captured by the notch is usually superior.
Unless your weld deposit and/or base metal are totally brittle crap, you will generally meet the minimum requirement if those other zones do.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[SJmetallurgist]

I would suggest you to read through a few articles of ASM manual on welding to get a hint and details of how HAZ behaves and how the same can be restricted, or expanded, by temperatures of preheat/pre-heat maintenance, etc.

Indeed it remains to be the most sensitive part of the weldmment, which is the reason of specifications, that look overzealous to some of us. But HAZ testing (hardness/impact) really is practically relevant.

Yes, it's difficult to have an impact test specimens from HAZ only.. but inside test lab rooms (behind the scenes) they do, ALWAYS, macro-etch the specimens to identify the HAZ and locate their notches (CVN shall have a V notch) within HAZ to the extent possible, or rather, let's say they try to keep as much HAZ in the notch, as practically possible.

It is relevant and this requirement is NOT over-speficied in the specification you are dealing with.

Shashank

 

[weldstan]

Actually it is not that difficult to test the HAZ if the contract spec requires the bevel to be limited to not greater than 7 degrees on the side to be impact tested. The other side bevel could be up to 45 degrees (determined by the Manufacturer)to permit full penetration on the near square bevel. We required such in our specifications over 40 years ago and had our inspectors or welding engineers witness testing including placement of the notch.

 

[gtaw]

I was going to suggest a single bevel groove, with the samples removed from the square cut side of the groove. The groove detail is a nonessential variable per Section IX. Unless there is something contrary in the construction code as to the groove detail, single bevel is the way to go.

Best regards - Al

 

Shashank,

Of course HAZ impact resistance is relevant and important. What I am saying is that in commercial practice it is almost never captured effectively, for reasons I previously outlined. No amount of care and etching will bring a sloping fusion line into alignment with the notch of a Charpy coupon. And given the low levels of welding metallurgy knowledge among average fabricators, they don't know enough to even ask the right questions.

As gtaw recommends, it requires a square-bevel groove to give it the best chance. Even then it requires a lot of care and attention to capture the CG-HAZ. Typically it is only researchers who go to this kind of trouble. If regulators took this property more seriously they would enforce this more rigorous approach (which I would if I were a regulator). As it stands, I know that I can run a WPQ on any plate with a decent pedigree; i.e., normalized, low impurities) and be confident of passing Cv testing to temperatures at least as low as those on the base metal MTR.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."