Stainless tubes are for the most part oversized, and the weld factor seldom decides the issue in calculations. The main significance of the weld factor is in other contexts:

 – A weld factor of 1.0 calls for 100% non-destructive testing of the weld. The customer specifies a weld factor of 1.0 in order to have a guarantee that the test will be carried out.

– Government authorities stipulate that only tubes with a specific weld factor may be used for certain operating conditions.

Welded tubes are supplied either with specification of the weld factor 1.0, or with a lower weld factor, or with no weld factor at all.

The specification of a weld factor imposes demands on production methods and also on non-destructive and destructive testing. The demands vary widely from one country to another.

Different weld-factor approvals are therefore not comparable. If information as to the weld factor is to be definite, it must be accompanied by an identification of the licensing authority that granted the approval.

It is important to know that all pipes and tubes, irrespective of the weld factor, are made in the same way and with same preconditions as to quality. The difference is in the scope of the inspection.
 
Europe:
Today the weld-factor/joint coefficient, z is regulated in EN 13480-3 §4.5

Fusion welded tube and pipe, manufactured by a qualified and approved manufacturer and inspected according to EN 10217-7, receive welding factor z =1.0. The testing consists of

• Leak test with 100% ET or HT
• NDT of the weld seam with 100% ET or RT

USA: 
The Joint quality factor, Ej for welded tube and pipe is regulated in ASME B31.3, table 302.3.4
Pipes and tubes fusion welded (EFW) with or without filler metal manufactured and inspected according to any of the ASTM standards A269, A 312, A358, A 409, A790 and A928, receive a ”Joint quality factor”, Ej, depending on welding method and type and amount of NDT.

Germany: 
Approval by TÜV and inspection to DIN 17457
NDT according to SEP 1914
Weld factor: v = 1.0