Welding Procedure

A: Section IX of the ASME Boiler and Pressure Vessel Code is very complex, and the rules for qualifying welding procedures vary depending upon the welding processes used and the materials being welded. However, a few of the fundamental concepts are often misunderstood, and gaining a firm understanding of those concepts will make it easier for a new user of this code section to figure out how it works.

Section IX Part QW (which covers welding) is divided into four articles—I, II, III and IV:

• Article I covers general requirements.
• Article II covers requirements for qualifying welding procedures.
• Article III covers qualification of welders and welding operators (which is outside the scope of this article).
• Article IV covers welding data—variables, material groupings, test specimen requirements, etc.

A good way to begin is to read articles I and II. There is no point “reading” most of article IV because it is primarily dedicated to describing variables. However, the text at the beginning of each major section should be reviewed, including QW-401 (description of variables), QW-420 (material groupings), QW-424 (base metals for procedure qualification) and QW-431 (F-Numbers). Also, the definitions in QW-490 should be reviewed, as some of the terms used in the text may not mean what they seem to imply.

The end goal is a viable welding procedure specification (WPS). The WPS is a document that states requirements regarding the materials that may be welded, which welding process must be used, the filler metal used, the minimum preheat temperature, welding parameters such as current, voltage, travel speed, interpass temperature, PWHT requirements, etc.

The WPS is based upon the successful creation of a welded qualification test coupon. The procedure qualification record (PQR) is a document that lists all pertinent data associated with the creation and testing of the qualification specimen.

A common misconception is that the WPS is written first, and then the test coupon is created to “prove” that the WPS is valid. In actuality, the qualification coupon is created first. The PQR document is written, and then the WPS is created based upon the PQR data. The rules in Section IX govern how the parameters specified in the WPS are determined using the PQR data as a basis.

VARIABLES

In Section IX parlance, a variable is a parameter or factor that has been determined to have an effect upon the welding characteristics or the resulting weldment. Section IX includes many variables, and has categorized them for various welding situations as “essential,” “nonessential” and “supplementary essential.”

For a given welding situation, an essential variable is one that has been identified as critical to the success of the welding process or to the properties of the resulting weldment. A change in an essential variable requires requalification of the welding procedure.

A nonessential variable is a variable that must be documented in the WPS, but which may be changed during an editorial revision without requalification of the procedure.

A supplementary essential variable is a parameter or factor that becomes an essential variable when the base material is required to be impact tested.

Lists of essential, supplementary essential and non-essential variables for various welding processes are listed in tables immediately following paragraph QW-250. For example, QW-253 covers SMAW joining, and lists the paragraphs that describe the essential, supplementary essential and nonessential variables that apply. Two examples of variables listed in QW-253, and their impact are:

• QW-406.1 - Decrease > 100° F (55° C) is an essential variable. Paragraph QW-406.1 states: “QW-406.1 A decrease of more than 100° F (55° C) in the preheat temperature qualified. The minimum temperature for welding shall be specified in the WPS.”
  Therefore, assuming the PQR lists a preheat temperature of 150° F (65° C), the WPS can be written with a preheat temperature as low as 50° F (10° C). If creating or revising the WPS to utilize a preheat temperature below 50° F (10° C) is desired, the procedure would need to be requalified.

• QW-406.3 - Increase > 100° F (55° C) (IP) is a supplementary essential variable. Paragraph QW-406.3 states: “QW-406.3 An increase of more than 100° F (55° C) in the maximum interpass temperature recorded on the PQR. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic material is solution annealed after welding.”

Since this is a supplementary essential variable, it only applies if the base material is required to be impact tested (such as ASME SA352 LCC). Assuming that is so, and assuming the PQR lists a maximum interpass temperature of 500° F (260° C), the WPS can be written with a maximum interpass temperature of up to 600° F (315° C). If creating or revising a WPS to increase the maximum interpass temperature to higher than 600° F (315° C) is desired, the procedure would need to be requalified.

Note there are many QW-4XX paragraphs that describe variables; however, the only ones that apply to the welding procedure are those referenced in the QW-25X tables for the welding processes being used.

A valid WPS lists all essential and non-essential variables (and supplementary essential variables if the base metal is required to be impact tested), and provides appropriate values for each.

New users of Section IX sometimes think they can just use the example forms in Section IX and fill them in based upon what seems “obvious.” This approach is unlikely to result in a procedure that properly addresses the variables. The best way to ensure a WPS is complete and correct is to use the QW-250 tables as a type of “index” to ensure that all appropriate variables are included and properly addressed, and to review the definitions in QW-490 to ensure all terminology is interpreted and used correctly.

Don Bush is a principal materials engineer at Emerson Process Management-Fisher Valve Division (www.emersonprocess.com). Reach him at Don.Bush@Emerson.com.