The weld joint strength reduction factors added in the 2004 edition were based on those provided in ASME BPVC Section IH, Subsection NH. Weld metal, base metal and cross-weld creep rupture data had been used to develop weld joint strength reduction factors in Subsection NH for limited material combinations. The basic weld joint strength reduction factors provided in ASME B31.3 were developed based on evaluating the factors provided in Subsection NH. Factors for 100,000 hour durations were used. These factors are plotted in the attached charts.
Figure 3.3 shows the data for 2-l/4Cr-lMo, 800H, 9Cr-lMo, and austenitic stainless steel with at least 0.04% minimum carbon content weld metal as well as the basic weld joint strength reduction factor, W, provided for use in the absence of more applicable data. Figure 3.4 shows data for type 304 and 316 base material, required by Subsection NH to have a minimum 0.04% carbon content, welded with lower creep strength low carbon (<0.04%) weld metal. Note that the sudden drop in the weld joint strength reduction factor for Type 316 stainless steel between 510°C (950°F) and 538°C (1000°F) is due to a shift from tensile property to creep property control of the allowable stress. The weld joint strength reduction factor is 1.0 when tensile properties govern the allowable stress.
The data in Figure 3.4 were considered to represent a poor choice of weld material for high temperature service and were not included in the data used to develop the weld joint strength reduction factors. Selection of an appropriate weld material for elevated temperature service remains the responsibility of the Designer. The weld joint strength reduction factors will not protect against poor material selection, but are a step towards addressing creep effects in high temperature welds.
Generally good experience with the strength of carbon steel weldments relative to carbon steel base material indicated that there was no need to use the strength reduction factor at temperatures below 510°C (950°F) for carbon steel. This is independent of other concerns, such as the potential for graphitization, that may limit use of carbon steel at elevated temperatures.