Unusual Weld Requirement: SS-to-CrMo Q&T 1

[tc7]

We have an assembly to build which requires that several bolted joints have the bolt heads tack welded to it's interfacing plate. The bolts are to be either a 304 or 316 stainless and the bolted plates are 4130 heat treated to a very high hardness (rockwell C32-35). I have no requirement to comply with any AWS, ASME Code or any mil spec as far as qualifying weld procedures.

Since this is just a single tack weld ( ~1/8" fillet x 1/8" long)on each bolt, should I be concerned about preheat, post weld stress relief or any other concerns about cracking in the CrMo plate? I will probably use a 308 or 310 SMAW rod. To be safe I will wait 48 hours an perform a dye check on each tack weld.

Any & all thoughts are welcome. Thanks.

 

[metengr]

tc7;
Only use 309 for this tack weld. Preheat locally to 350 deg F.

 

[tc7]

Met-
Do you consider FN when selecting a rod for this type of application? I calculate a FN of 1 for your suggestion of 309 but I also get a FN of 1 if using 308L (using WRC-1992 method).

If you don't use FN to determine the rod preference, what is your selection criteria?

Thankyou

 

[metengr]

tc7;
309 stainless is specifically formulated for dissimilar metal welds, and in this case you have more of a dilution problem with the Cr-Mo base material with using only tack welds. The alloy additions of Cr and Ni in 309 are such that the primary solidification mode would be ferrite. If you use 308, you may solidify in an austenitic mode that will result in possible hot cracking and more martensite formation, which could result in centerline cracking across the face of the tack weld.

 

[tc7]

Ok, based on my crude calculations and charting, I can see where the 308 would be very close to, and maybe in the austenitic part of the WRC chart whereas the 309 is very much higher in the "A-F" section of the chart.

But following your logic of "...additions of Cr and Ni in 309 are such that the primary solidification mode would be ferrite..." the why doesn't the selection of a 310 filler which is high in Cr & Ni serve that same purpose?

 

[metengr]

tc7;
My calculation shows that the typical 310 weld rod Cr and Ni equivalents would place you out into the austenite region using the standard WRC 1988 diagram for Cr and Ni equivalents.

I have a suggestion, locate the following NiDI paper on "Pay Attention to Dissimilar Metal Welds, Guidelines for Welding Dissimilar Materials" authored by Richard Avery. This is an excellent paper on practical guidelines for DMW design

http://www.nidi.org/index.cfm/ci_id/80.htm

 

[tc7]

That is a very excellent article, thanks for the link. I was surprised that the article cites two methods with far different assumptions on % base metal dilutions, i.e., the author cites 20-25% dilution for SMAW, 20-50% dilution for GTAW (that is a HUGE spread!!), etc, etc. But then also cites another method of calculation which assumes only 15% dilution, irrespective of process. The author provides no guidance on which method most nearly represents typical weld conditions, nor does he provide specific references to backup his idea for high dilution rates. Since Dr. Kotecki's text seems to only suggest 15% dilutions, that is what I am more comfortable with.

What is your preference and why?

 

[metengr]

tc7;
My preference is to go with higher dilution ratio for GTAW versus SMAW. Why? Typically, dilution ratios are not as simple as being process driven. You can manipulate heat input to either provide shallow penetration and more filler metal deposited or less filler metal deposited and high penetration. Both will affect the dilution ratio.

From studies I have seen between GTAW and SMAW processes, the dilution ratios were higher for GTAW than SMAW with all things being considered equally controlled. Why? Manual GTAW is a lower deposition welding process and this will tend to melt more base metal as filler metal is being added manually. I do not agree with 15% dilution ratio for GTAW, that is too low with all things being considered.

 

[tc7]

Thanks Met-
I think you have provided very good information on this thread (as you usually do!) and this will be a very good reference resource for others.

Best regards to you.