Let Weld Metal Match Base Metal

By Harry Ebert

The introduction to Section IX of the ASME Boiler and Pressure Vessel Code, which deals with welding, states that the purpose of a Welding Procedure Specification (WPS) is to meet the required properties of its intended application.  However, it does not make this mandatory and does not state or indicate how this is to be accomplished.  Interestingly enough, this Code has some provisions that permit or unintentionally encourage fabricators and erectors to ignore or overlook this good intention.

Here are two examples:

Many similar examples can be cited.  To overcome these potential problems, the following wording is proposed for specifications that involve welded components:  "Filler metals shall match the mechanical and chemical properties of the base metal as closely as possible.  Customer approval shall be obtained if this can't be achieved or for special conditions."

The review of filler metal selection can become even more involved and requires greater care and engineering judgment when joining two different metals.  It is important to consider not only strength and corrosion resistance, but also differences in metallurgy, in coefficients of expansion, and in required thermal treatments.