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Welding of Austenic Stainless Steels |
Welding of Austenic Stainless Steels -
1. When compared with common carbon steels, the austenitic stainless steels exhibit the following differences:
(i) Electrical resistance is about six times greater.
(ii) Melting point is about 93°C lower.
(iii) Thermal conductivity is about 50% lower.
(iv) Thermal expansion is about 50% greater.
The first three factors combine to make for lower welding current requirements. The greater expansion of stainless steel increases the tendency for warpage and distortion and may produce a higher incidence of weld cracking under restraint conditions, especially in the higher alloy types.
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The type of distortion encountered with thick sections differs from that of thin plate. In thick sections the methods of counteracting bowing and bending are similar to those employed with mild steel, except that greater allowance must be made for increased shrinkage.
Skip welding and back stepping are often indicated procedures. Other possible techniques include use of heavy and frequent tack welds, and positioning the sections, so that the contraction pulls them into alignment.
2. When austenitic stainless steels are heated within the temperature range of 427 to 870°C or cooled slowly through that range (as the weld cools) carbon is precipitated from solid solution mainly at the grain boundaries, where it unites intergranular with chromium to form chromium rich carbide.
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The formation of these carbides effectively eliminates much of the chromium (from the adjacent areas) which would otherwise be available to form the protective chromium oxide.
It is natural that the areas impoverished in chromium have a lower corrosion resistance and under corrosive conditions this results in a localized intergranular attack, the severity of which depends on the time and temperature of exposure, as well as the composition and prior heat treatment of steel.
The corrosion in austenitic stainless steels is generally localized in the heat affected zones adjacent to the welds where carbides are precipitated. When welding austenitic stainless steel, carbide precipitation can be minimized by:
(i) Reducing the time during which the temperature of steel is in the 427 to 870°C range. This is done by minimizing heat input and by rapid cooling through the critical temperatures. For this reason, intergranular corrosion is least in spot welds and greatest in gas welds.
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(ii) Selecting a stainless steel (for welding) with lower carbon content because the lower the amount of carbon, less is the effect, i.e., of forming chromium carbide. The effect increases as the % of carbon goes beyond 0.08%. Steels with small % of carbon take much longer time to form carbides.
(iii) Suitable addition of Titanium, Tantalum or Columbium. These elements tend to fix the carbon by forming titanium or columbium carbides, thereby preventing the formation of chromium carbide when the steel crosses the temperature range of 427 to 870°C.
(iv)An appropriate heat-treatment (as given below) of the welded structure also removes the susceptibility of austenitic stainless steel to intergranular corrosion because the heat treatment results in complete solution of the carbides. Such annealing heat treatment is a must where maximum corrosion resistance is required.
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Heat the weldment
Hold for atleast between 1040 and --> 1 Hr per 25 mm of maximum thickness; in no case 1120°C hold for less than 1/2 Hour. Cool rapidly through the range from 925 to 427°C to hold the carbon in solution.
(a) For thicknesses of 3 mm and less the weldment can be cooled in air.
(b) For thicknesses above 3 mm, the weldment should be plunged into water or sprayed with water.
The lower carbon alloys can be cooled at slower rates than the higher carbon grades. It should be taken care that the cooling method does not cause warpage of the weldment.
3. An intermetallic compound called the sigma phase has also been noticed in weld metals in the as welded condition. The sigma phase may cause loss in ductility, impact strength and corrosion resistance. Depending upon the alloy content of the stainless steel, sigma phase usually transforms preferentially from ferrite by holding the steel for long times at temperatures ranging from about 650 to 925°C.
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Welding Techniques
Various processes used for welding stainless steels are
1. Oxy acetylene welding
Shielded metal arc welding
Inert gas metal arc welding
2. Arc welding
Gas tungsten arc welding
Submerged arc welding
Plasma arc welding
3. Resistance welding
4. Brazing
It is suggested that the austenitic stainless steels should not be preheated prior to welding, since such a treatment would decrease the rate of cooling of the weld and heat affected zone and would, therefore, increase the amount of carbide precipitation in the weldment.
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