Piping Welding - How to know, Which metals can be welded together and which can not.? 3

[AlfaMike5886]

Hi All,
as i understand P no. has something to do with it. but is there any strict criteria where it states that some metals can be welded together and some can not be.? i still do not clearly understand the P no. and Group no. how they are used to determine welding compatibility with other metal? does metal need to have same P no and group no in order to be weldable together?

i have same question for selection of electrodes. How to decide which electrode should be selected for joining specific metals? what if the joining metals are not same?

 

[weldstan]

I think you need to hire a welding engineer/metallurgist to answer your questions. Many different alloys can be welded together. The P-No. is simply a grouping of alloys that are highly comparable to each other from a weldability standpointe; e.g., P-No.1 includes low carbon steels with restrictions on maximums of specific alloying elements. P-No 8 includes numerous austenitic stainless steel alloys like 304, 316, 321, 347, etc. because their weldability is highly similar.

 

[Rputvin]

P numbers are used to classify metals into groups. The P number is one of the variables in a Weld Procedure Specification that you cannot change without rewriting and requalifying the WPS to the new materials. Same thing for electrodes.

So a procedure written with the correct filler rod, methods, settings, etc for a general carbon steel in the P1 group can be used to weld any P1 material without having to re-qualify the procedure. If you want to weld a different material with a different P number your filler rod will be different, your weld machine settings will change, your weld method will change, so the procedure has to be re-written with that material in mind and qualified to the new version welding a new material.

ASME BPVC Section IX covers welding, welders, weld procedures, qualifying, how the variables are handled, etc.

I don't believe it ever states things in terms of "you cannot weld this material to this material" - it only prescribes that to do the welding you need to do the analysis, write a WPS, qualify that WPS as valid through testing, train your welders to that WPS and qualify their performance to the WPS through testing as well. A WPS with a PQR and a welder with a WQR to the WPS is needed before you can start melting metal on an ASME project - and adherence to that WPS is not up for debate or interpretation - if you want to weld on ASME, it needs to follow a properly qualified WPS (or a purchased pre-qualified SWPS).

 

[weldstan]

The P-Nos permit qualification on one alloy within that P-No. to weld all alloys within that P-No.; for example, qualifying on 304 base metal permits writing a WPS to weld 316, 317, 321, 347 etc. without requalification within the ranges qualified of course.

 

[r6155]

It´s simple: P Number is to reduce the quantity of tests.

Regards