Introduction
Welding austenitic stainless steels to carbon and low alloy steels are established methods in the process and construction industries.
Dissimilar metal welds involving stainless steels can be done using most full fusion weld methods, including TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas).
Weld procedures using filler (consumable) enable better control of joint corrosion resistance and mechanical properties. In selecting the weld filler, the joint is considered as being stainless, rather than the carbon steel. Over-alloyed fillers are used to avoid dilution of the alloying elements in the fusion zone of the parent stainless steel.
Dissimilar metal combinations
The most common combinations of dissimilar steels involving stainless steel are plain carbon or low alloy structural grades and austenitic stainless steel grades such as 1.4301 (304) or 1.4401 (316).
Welding conditions
Carbon and alloy steels containing less than 0.20%C do not normally need any preheat when being welded to austenitic stainless steels. Carbon and alloy steels with carbon levels over 0.20% may require preheat. High restraint joints, where material thickness is over 30mm, should also be preheated. Temperatures of 150 C are usually adequate.
Carbon steels may be more prone to hydrogen associated defects than austenitic stainless steels and so careful drying of welding consumables is advisable.
When welding stainless steels to galvanised steel, the zinc coating around the area to be joined should be removed before welding. Molten zinc if present in the weld fusion zone can result in embrittlement or reduced corrosion resistance of the finished weld.
Selection of welding consumables (filler)
Ambient service temperatures
Although a standard 308 type filler can be used for joining a 304 type stainless steel to carbon steel, more highly alloyed fillers, such as the 309 type (23 12L to BS EN 12072) are preferable.
This is should help avoid cracking in the weld dilution zone that can be a problem if a 308 type (19 9L to BS EN 12072) filler is used, where there can be too low a ferrite level and martensite may also be formed on cooling.
Elevated temperature services temperatures (over 425�C)
If the welded parts are for higher temperature service, then the differences in thermal expansion rates of the steels and filler can lead to thermal fatigue cracking. Long exposure times at these temperatures of welds with enhanced ferrite levels can result in embrittlement due to sigma phase formation.
Nickel based fillers (Inconel) produce welds with lower thermal expansion rates than the stainless steel fillers and so may be preferable.
Risk of bimetallic corrosion at welds between carbon and stainless steel
Unprotected welds subject to aggressive environments such as immersion in seawater could result in sacrificial corrosion to the less noble carbon steel part. However, post weld surface coating repair would normally be needed to the carbon steel to 'restore' the corrosion protection.
If this re-coating / painting is allowed to lap over onto the weld bead then the joint should not be at risk to bimetallic corrosion in any environment so long as the coating is sound. Ideally the weld bead should be covered so that only the 'parent' stainless steel is ultimately exposed. This ensures that galvanic corrosion cells cannot be set up across the joint, where there is a composition 'gradient'.
Source: http://www.bssa.org.uk/
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