How Many Weld Cycles Can A Cr-Mo Take? 1

[tc7]

Is there a limit to how many weld cycles a Cr-Mo steel should be exposed to? Explanation:
On some development projects, where 4130 (normalized) is the standard for some engineers, we may alter a structure and reweld in the same vicinity of previous welds several times.
Now we have a frame which has been welded, PWHT'ed, had cracks repaired, stress relieved locally and put into test. Now on this same article we are asked to air arc remove the previous welds and reweld in same areas. Of course we will stress relieve it.

Should we be concerned about the number of times the same base material can be welded? and why/why not?

After air arcing (carbon arcing) is it wise to stress relieve again BEFORE welding if welding does not immediately follow?

Thanks

 

[metengr]

tc7;
If the 4130 has no required normalized and tempered or quenched and tempered heat treatment requirements or notch toughness concerns for the application, you can weld, reweld, PWHT, etc., as many times as necessary. Of course, if you happen to re-normalize after x repair cycles, you will return to a normalized state as if the material had no memory of previous weld, PWHT cycles.

 

[metengr]

After air arcing (carbon arcing) is it wise to stress relieve again BEFORE welding if welding does not immediately follow?

As long as preheat was used for arc gouging, no stress relief is required. Keep in mind that you need to remove (grind) the heat affected material along the entire edge of the excavation.

 

[tc7]

Thanks Met as always.

For the arc gouging process, we usually require a minimum of 1/16" to be mechanically ground beyond the final gouged surface, but I have no idea on how deep the heat affected material would be. How would I estimate the minimum depth of arc gouged HAZ?

 

[metengr]

tc7;
Based on expierence, it would be between 1/16" and 1/8". The depth really depends on the welder and preheat. You can also clean the area and use a hard etch (5% Nital) to determine if further grinding is necessary.

 

[davefitz]

I assume this above guidance applies specifically to 4130, and is not meant to be generally applied to other materials. Certainly not P22.

 

[metengr]

Correct.

 

[tc7]

Met-
Need to learn more about the Nital hard etch. How would I use this and what will it show? I am guessing it will emphasize grain structure under magnificaton? And then I'll compare it to a standard? Need more help on this.

Thanks

 

[metengr]

tc7;
The 5% Nital etch is used in this application as a macroetch to view any hardened material along the arc gouged surface. This is done visually on a surface that is ground and buffed without the aid of a portable microscope. The solution is common for macroetching steels and can be prepared in a chem lab.

I included a web site for information on etchants

http://www.metallographic.com/Etchants/Etchants.htm

The strength of the Nital etchant is determined by the amount of nitric acid that is added to ethanol.

 

[metengr]

tc7:
In addition, if you have any hardened material after grinding and buffing, it will show up as a darker region in comparison to the surrounding base material that was not affected by thermal cutting.

 

[GRoberts]

If you are subsequently welding over a carbon-arc gouged area, I don't think it is necessary to completely remove the HAZ. The weld will re-heat the previous carbon-arc HAZ and will generally be a wider HAZ than the carbon-arcing.

 

[metengr]

GRoberts;
I normally agree with your excellent responses, but in this case I would take exception to not removing the heat affected material.

 

[GRoberts]

metengr,
I feel the same way about your responses.

Having formerly worked in a foundry though, where 95% of all the welding is repair, I can tell you that except for some of the more exotic aloys like Hastelloy B2, or 254SMo, almost all foundry weld repair takes place without removing the HAZ from carbon arc gouging. (Unless it is customer specified) I seldom saw a problem resulting from this, even in alloys up to about 130-140ksi. Mainly because the HAZ of the arcing is so much thinner than the welding, that by the time you weld over the arcing HAZ, it is basically no longer there.

However, if you are doing a one time repair on a critical part, there is certainly no harm in removing it.