Why Do My AL-6XN Welds Look So Bad?

We’ve grown accustomed to judging the quality of a weld by its appearance. A good weld is shiny, smooth, and uniform in color, right? That may be the case when welding 304 or 316 stainless steel, but the rule doesn’t always apply when welding higher nickel alloys, such as AL-6XN.

A typical AL-6XN weld will have non-uniform freeze lines and slag islands in the weld bead. These slag islands are dull or blue-gray in color and adhere to the surface. The appearance of “light” and “dark” spots on both the inside and outside of the weld is common. The heat affected zone can also have discoloration and is generally a little darker than conventional 316L stainless steel welds. The image above is an example of a weld made with AL-6XN electropolished tubing.

Weld made with AL-6XN electropolished tubing

Example of a weld made with AL-6XN electropolished tubing

It’s a bad weld, right?

No. Metallurgical lab tests were used to identify the composition of slag islands, discoloration and the impact of such on the integrity of AL-6XN welds. The evaluation process employed the following analytical techniques:

  • Scanning Electron Microscopy (SEM) to determine what the surface “looks like” and to determine areas for evaluation with microprobe analysis
  • Energy Dispersive Spectroscopy (EDS), sometimes called microprobe analysis, to determine the approximate composition of any areas in question
  • X-ray Photoelectron Spectroscopy to determine the molecular composition of areas or compounds present and to provide light element detection
  • Accelerated corrosion testing in a modified ASTM G48 solution to identify areas of potential corrosion attack

Summary of the Weld Test and Analysis

  • The weld discoloration does not appear to have an effect on the corrosion resistance of the weld, and removal of the discoloration does not seem to be a requirement for good field performance.
  • Most of the discoloration observed originates from inclusions in the steel that are melted during welding and concentrated as slag on the weld. They originate in the steel making process or enter as tramp elements from the scrap used to make up the alloy.
  • It appears that little, if anything, can be done during the welding operation to eliminate the discoloration since it comes from the steel itself.
  • The white or silver areas on both surfaces of the weld are areas free from oxides or nitrides. They represent clean surfaces.
  • The dark areas are composed of a mixture of oxides, silicates and nitrides. They seem to come from the inclusions in the steel and possibly from the partial decomposition of the oxides in the slag. They appear to be stable and not attacked by the very aggressive corrosion test.

To learn more about AL-6XN alloy properties, specifications, applications, and corrosion studies, visit www.al6xn.com.

One comment

  1. very interesting and helpful articles

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