Autogenous welding, 3.37

Welding Without Filler Metal (Autogenous Welding). Weld joint geometry shall be square-butt single welded. 

Base Metals. Material specification, standard, P-No., subgroup number, and actual base metal thickness limits shall be supported by the PQR. For autogenous welding of P-8 materials where two heats with differing sulfur content (> 0.005%) are to be joined, consult Nonmandatory Appendix A, para. A-3.1.2, for guidance. 

Welder operator qualifications with or without filler material (autogenous welding) shall be limited to the mechanized WPS/PQR and ASME BPV Code Section IX, or API Standard 1104. Three consecutive, acceptable samples shall be required to support welder operator qualification testing. 

Fig. A-3.5 Weld Quality Illustrations for Autogenous Welded Pipe or Tube...

Unmixed zones. All methods of welding stainless steel with a filler metal produce a weld fusion boundary consisting of base metal that has been melted, but not mechanically mixed with filler metal and a partially melted zone in the base metal. An unmixed zone has the composition of base metal, but the microstructure of an autogenous weld. The microsegregation and precipitation phenomena characteristic of autogenous weldments decrease the corrosion resistance of an unmixed zone relative to the parent metal. Unmixed zones bordering welds made from overalloyed filler metals can be preferentially attacked when exposed on the weldment surface.14,61... 

Fig. 20. Autogenously welded sulfonab on vessel, especially for use with oleum.

Gamer, A., The Effect of Autogenous Welding on Chloride Pitting Corrosion in Austenitic Stainless Steels, Corrosion, Vol. 35, No. 3, 1979, pp. 108-113. 

FIG. 21—Intergranular attack on weldmant In ferritic stainless steel caused by high nitrogen content (5x). Autogenous weld In 28.5 %Cr<4.2 %Mo alloy with 22 ppm carbon and 388 ppm nitrogen after exposure In ferric sulfate-50 % sulfuric acid test. 

Flat and U-bend coupons may be supplied with an autogenous or butt weld [/3]. With a butt weld, the weld profile is retained and not machined ofi, to duplicate the condition of process equipment, except on U-bend coupons. Autogenous welding (application of a locally melted region on base metal that contains no filler metal) does not always simulate real-world conditions. If welding of the coupon with filler metal is not possible, for example with thin stock, a sensitizing heat treatment may be preferred. 

One of the most important corrosion properties of Type 439 is resistance to chloride stress corrosion cracking beyond the capabilities of conventional austenitic grades. This resistance is provided by the ferritic structure and low nickel content of Type 439. Results are equivalent for annealed, U-bent and autogenously welded Type 439. 

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