KB » Recommendations for Welding Iron Based Austenitics

Summary: This topic contains recommendations for cleaning, preheating, welding and postwelding of Iron Based Austenitics.
Categories: One, Three
Applies to: Test engineer, Design engineer

Prior Cleaning

Removal of contaminants such as oil, grease, waxes, cutting fluids from the top and bottom surface of the weldment shall be performed. This cleaning should be performed within approximately 1 in on both sides of the weld seams. Cleaning solutions can be acetone, alcohols, or other nontoxic solvents (Northwest Oil Company, IncExternal site, Pico Chemical, IncExternal site).

The prepared weld edges and the areas within approximately 0.25 in of the weld seam should be sanded or ground to remove mill scales and oxides. The weld seams then should be cleaned with clean cotton cloth and solvent prior to welding.

Abrasive weld surface modification should be used only on stainless steels. This avoids contamination by residues from carbon steels. Surface contamination from elements like copper, bronze, lead, or zinc must be avoided because they can lead to heat-affected zone cracking. Such contaminants can be present in various machine tools such as hammers.

Preheating

Under normal conditions, preheating is not required for welding. It might be required to prevent the formation of moisture when ambient temperatures are below approximately 60 F or for the removal of moisture associated with cleaning. In either case, warming of weldments should be minimized. If oxyacetylene warming is used, the heat should be applied evenly over the base metal rather than concentrated at the weld joint. Gas flames should be adjusted to neutral or slightly oxidizing rather than reducing or carburizing. Interpass temperatures should be maintained below approximately 175C.

Welding

Brushing between weld passes with clean stainless steel wire brushes (CS Unitec, IncExternal site, Viking Industrial CenterExternal site) should be performed to remove surface oxides. Caution is warranted because excessive pressure on power brushes should be avoided to prevent burnishing of surfaces. Deposited weld metal should be visually examined to identify and subsequently remove or fill any crater cracks, pinholes, lack-of-fusion defects, cold laps, or any other imperfections that could be considered detrimental to weld integrity. Note that certain welds must meet ASME accetance standards as defined int he ASME Code, Section III. Final cleaning should include brushing for oxide removal. Light grinding can also be used to remove any arc strikes or weld splatter. Caution is warranted because grinding of weld surfaces that come in contact with primary coolant are more suscetible to some forms of cracking becasue of the cold work imparted to the surface from the grinding process.

When inert gas purging is required, internal surfaces should be prepared as described previously. Assemblies that require purging should be sealed with gas plugs, purge plates, dissolvable purge paper and tape, foil, masking tape, purge bladders (COB Industries, IncExternal site, Ty-FlotExternal site) or other compatible materials capable of forming airtight seals. Purging barriers should be placed far enough from weld seam. It is suggested that the purge time be estimated as nominal pipe size x length of pipe in feet with disregard for dimensional units. Purge quality should be verified by welding a stainless steel strip thin enough for full penetration welding while the back side is shielded with gas exhausting from the purge vent. An acceptable color for a purge test weld ranges through silver, straw, gold, light blue, and then dark blue. Silver or straw colors are preferred for the best quality.

Austenitic stainless steels have a lower thermal conductivities and higher electrical resistivities than carbon steels. Consequently, they require typically lower welding currents currents or heat inputs than that used for carbon steels. Excessive heat input should be avoided because it promotes distortion and can contribute to weld or adjacent base metal cracking.

Post weld

Post-weld heat treatment of iron-based austenitics is not recommended and is generally not needed.

Further reading

  1. Metals Handbook, Tenth Edition, Vol. 6, "Welding, Brazing, and Soldering," ASM International, Materials Park, OH 1992

References

  1. Welding Guidelines for Advanced Austenitics, EPRI, Palo Alto, CA: 2009. 1015815

Questions and Answers

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Topic revision: r3 - 2012-09-28 - TWikiAdminUser
 

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