AWS D18.2:2020

AWS D18.2 PDF

This standard addresses factors that affect weld discoloration on the inside of austenitic stainless steel tube. The document contains a color illustration relating the discoloration to the oxygen content of the backing gas (purge gas or internal purge gas).

This standard provides a visual comparison guide that can be used to specify surface discoloration criteria for sanitary (hygienic) welds in austenitic stainless steel tube.

The tube sample was prepared using automatic orbital “bead-on-tube” gas tungsten arc welds on the outside diameter of a 2 in [50 mm] Type 304L (UNS S30403) stainless steel tube. The welds penetrated through the tube wall. The concentration of oxygen in parts per million (ppm or 0.0001%) added to the pure argon backing gas (purge gas or internal purge gas) for each weld was as follows:

No. 1—10 No. 3—50 No. 5—200 No. 7—1000 No. 9—12 500

No. 2—25 No. 4—100 No. 6—500 No. 8—5000 No. 10—25 000

The inside surface of the stainless steel tube was mechanically polished to an approximately 32 μin [0.8 μm] Ra finish prior to welding.

The illustration is most useful as a reference to identify the degree of discoloration rather than to specify oxygen limits in the backing gas. The amount of discoloration and its appearance can be influenced by factors other than oxygen such as:

• High levels of moisture in the backing gas will increase the degree of discoloration.

• Contaminants such as hydrocarbons, moisture, and some types of particulates on the surface prior to welding can influence discoloration levels.

• Hydrogen gas in the argon backing gas can significantly reduce the amount of discoloration.

• The metal’s surface finish can affect the appearance of discoloration.

NOTE: Another set of weld discoloration samples can be found in ASME BPE EP and ASME BPE MP. However, those samples were developed for lower levels of oxygen content in the backing gas (samples 1 through 3 above) and specifically for the biopharmaceutical industry. These may not be appropriate for other applications, and there should be agreement between the Owner, Contractor, and inspector prior to use.