Stainless steels are classified according to their matrix structure.
(a) austenitic (b) ferritic (c) martensitic
(d) precipitation hardened and (e) duplex.
Special features of stainless steels related to welding.
1. Low thermal conductivity (50% of mild steel) results in less heat input for the job and 10% less current is needed for SS electrodes. higher electrode melt. off rates are also obtained. Melting point of stainless steel is 93°C lower.
2. Thermal expansion of Cr-Ni steels is about 50% greater than for mild steel. This increases the chances for warping and buckling. Thus suitable fixture must be used for welding stainless steels.
3. Electrical resistance is 6-12 times higher which causes overheating in the electrodes. Shorter electrodes are, therefore used to reduce electrode heating.
Austenitic stainless steels
1. These steels contain 16—26% chromium
2. Type 304 L and 316 L are low carbon grade (C < 0.03%).
3. Mo in type 316 improves corrosion resistance and high temperature properties.
4. Types 321 and 347 stainless steels are stabilized against carbide (Cr23C6) precipitation, weld decay and intergranular corrosion by addition of Ti and Nb. The strong carbide formers form TiC and NbC which impart creep resistance. Hence they are also used as creep resisting steels.
5. The 200 series s. s. sin lower Ni which is compensated by Mn and N2 for austenite formation.
6. Austenitic S. S. (except free machining grades) are easiest to weld and produced welds that are tough.
7. S. S. welding requires 20-30% less heat input than welds in carbon steels, because of low thermal conductivity and high electric resistance. Excess heat will cause distortion, reduce strength and corrosion resistance. Sulpher and Selenium added for free machining, makes the steel unweldable, also high carbon content inhibit weld serviceability. External sources of contamination include carbon nitrogen, oxygen, iron and water.
8. Contaminations and their effects.
• Carbon contamination may cause welds to cracks, change mechanical properties and reduce corrosion resistance in weld areas.
• Iron contamination lowers serviceability, flakes of iron on surface will rust, thus speeding localised corrosion.
• Contamination by copper, lead and zinc can lead to cracking in HAZ of the weld.
9. Welding current required is comparatively low.
10. When stainless steels are heated in the range of 427-870 C or cooled slowly through that range, carbon precipitates at grain boundaries.
11. Formation of these carbides effectively eliminates much of the chromium.
12. It will reduce corrosion resistance especially in HAZ.
13. This carbon precipitation can be minimized by :
(i) Reducing the time for which the temperature is between 427°-870°C range.
(ii) Selecting low carbon stainless steels to reduce carbide formation.
(tit) Addition of Ti, Ta, Columbium which form stable carbide preventing the formation of chromium carbide.