Filler metal selection

[engineer2385]

Hello,
I am attempting to learn welding engineering by myself. When it comes to selecting filler metals, I am aware of the considerations such as base metal matching strength, welding process and welding position do matter. I recently came across a wps where ER-347 was used in the TIG welding of F91 and A105. ER-347 contains about 19-21% of Chromium whereas base metal, this case, F91 contains 8-9.5% Chromium. How this filler selection can be justified in this case? So my question is what would be the best way to start learning this aspect of welding? Knowing composition of base metal and electrodes would be a great start. Also, Is there an article in section IX that can give a tell us which electrode/filler to use for certain materials? I appreciate your advise.

 

[metengr]

Filler metal selection is based on several factors as you mentioned. Most codes and standards do not specify filler metals only test conditions to ensure ductility and strength via qualification of s welding procedure. With that said you can search the Internet for recommended filler metal/base metal combinations. However, final selection is based on expertise. There is no equation or formula to rely on for optimum performance in service.

Second, I would recommend you search for a Schaeffler DeLong diagram to understand what phases can be present based on anticipated filler metal and base metal combinations.

 

[engineer2385]

metengr, I am studying Schaeffler diagram to understand if the weld microstructure is Austenitic or Ferritic based on its composition. Can you please elaborate a little on how this diagram can be used to select filler metal?

 

[metengr]

The diagram is used to establish what phase constituents will form based on base metal and filler metal Cr and Ni equivalents. You can read about this diagram by doing a Google search. I am too lazy at this point because it is a nice day outside.

 

[weldstan]

Because codes require PWHT for this weld and the service conditions more than likely involve relatively high temperature (> 650 F), ER 347 is a dubious selection. The weld design itself is dubious and I have seen cracks develop in similar welds in < 10 thermal cycles. So it would appear that the blind are leading the blind but at least you have the sence to rightly question this WPS - good job!

 

[engineer2385]

weldstan, thanks for your reply. F91 to CS does not require pwht as per WPS. However, it's a fact that F91 welds undergo cracking if not treated well. I am trying to understand the reason why such a High Cr filler is used for F91 which has 8-9.5% Chromium?

 

[weldstan]

B31.1, B31.3, and ASME VIII all require PWHT for this weld. Do not expect this weld to last.

 

[metengr]

The dissimilar metal welding of Grade 91 to carbon steel is really a poor design overall, regardless of filler metal selection. The mismatch in strength and elevated temperature creep strength between Grade 91 and CS is just too large.

Austenitic, nickel-based filler metals which have higher chromium and nickel have been used with success on Grade 91 base material because they exhibit greater ductility over Grade 91 base material. Using a standard 9 Cr -1 Mo filler (undermatched strength) or matching 91 filler metal are your best choices for welding Grade 91 base materials.

 

[weldstan]

I have prohibited the carbon steel to P91 weld for 20 + years. The use of 347 filler is nearly the worst possible filler metal that could be thought of. The WPS should be rejected!

 

[Guest102023]

Not to discourage more people practising welding engineering, but proper understanding of an alloy such as P91 requires an advanced course in physical metallurgy.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"