Page 12 - Stainless Steel Solutions

JOINING

Welding guidelines

This loss of corrosion resistance typically occurs in the HAZ, as shown in figure 2. When a sensitized
weldment is put into service, the corrosion will occur just beside the weld HAZ and often penetrates through
the base metal. Sensitized stainless steel weldments are sometimes referred to as “peppered weld metal” as
the precipitate is dark in colour and heavily oxidized.

Figure 2
Sensitization reduces the free
chromium level in the HAZ

16%
Free Chromium 10.5%

Base Metal
HAZ Weld Metal

Position

The three most common methods available to avoid sensitization are:
1. Post weld heat treatment
2. Low carbon materials
JOINING 1. Use a post weld heat treatment
3. Stabilized grades of materials and fillers

A postweld heat treatment entails heating the entire weldment to about 1900 ºF (1038 ºC) allowing the
carbides to go back into solution in the matrix. The weldment is then cooled very rapidly to allow very
little time for another precipitation reaction to occur. This technique does have problems because at
1900 ºF (1038 ºC), stainless steels oxidize rapidly and so must be heated in an inert atmosphere to prevent
oxides from forming. Another problem is that it is sometimes very difficult to cool a large weldment very
quickly to prevent carbide precipitation. Because of these problems, this technique is not used very often.
2. Use low carbon materials
By reducing the level of carbon in the material, a low carbon or “L”
grade material is produced. Typical examples are 304L, 308 ELC
and 316L. If the available carbon is limited, the problem of
sensitization is limited. Production of low carbon stainless steel
is done using a steel making process known as argon/oxygen
decarburization (AOD), patented by Praxair, Inc.
3. Use stabilized grades of materials and fillers
The stabilized grades of stainless steels contain titanium and
niobium (columbium). These elements have a higher affinity for
carbon than chromium does. This means if precipitation occurs,
the carbon will preferentially combine with these elements so that
the free chromium in the matrix is not reduced.

9

praxair.ca/stores praxair.ca/stainless