Extra low carbon grades

Selective corrosion in the heat-affected zone of a weld occurs most commonly when unstabilised stainless steels are used in certain environments. The obvious answer is to use an extra-low-carbon grade of stainless steel, e.g. types 304L, 316L or a stabilised grade of steel, e.g. types 321 and 347. Knifeline attack at the edge of a weld is not commonly encountered and is seldom predictable, and it must be hoped that it is revealed during preliminary corrosion testing. 

The molten ferroalloy is regularly tapped by tilting the arc furnace and pouring it into ladles while the silicate slag is also tapped but on the other side and disposed of in landfill. The sili-cothermic process yields a low-carbon ferrochrome or LC ferrochrome (0.05 to 0.50 wt.% C) but with a silicon content of 8 to 12 wt.% Si. Further purification of ferrochrome can be performed by the Simplex process, which consists in reacting, in the solid state, high-carbon with oxidized ferrochromium to produce the extra-low-carbon grade (0.01 wt% C). 

Another method used to overcome intergranular corrosion is to use the extra low carbon grades such as grades 316L and 304L these have extremely low carbon levels (generally less than 0.03%) and are therefore considerably more resistant to the precipitation of carbide. 

Sensitization temperatures (under -320°F). Room temperature embrittlement is nominal. and vessels. stabilized (Types 321,347) and extra low carbon (304 L, 316L) grades. into grain boundaries, with depletion of chromium in contiguous grain boundary areas. austenitic stainless steels. 

Stainless steel type 304L is an extra-low-carbon stainless steel. Grade 304L can be used for applications involving service temperatures up to 450°C, and under certain conditions up to 800°C. The following data were obtained from Ref. G8 (p.B10). 

Austenitic stainless steels, except the stabilized (Types 321,347) and extra low carbon (304L, 316L) grades. 

Use stabilized or extra low carbon stainless steels, or if steel has become sensitized in service apply full solution heat treatment to regular nonstabilized grades. 

The extra low-carbon L grades), the chemically-stabilized (Types 347 or 321) or the controlled-ferrite (centrifugally-cast, usually) varieties of austenitic stainless steels cannot be expected to solve these potential cracking problems. They may help, but success is unpredictable. 

---