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In terms of welding think of 3CR12 as similar to a grade 50 strength carbon steel. It is a ferritic grade of stainless steel and so has similar physical properties as corbon steel (density, coefficient of thermal expansion etc). Typically shearing should be restricted to a maximum 1/4" thick, as thicker materials can suffer from shear break. Plasma cutting for thicker sections works very well.
As has been commented ER 308L consumables are good, and ER 316L and Er309L have also been used successfully. LSi versions of these wires are also good and can help with weld pool fluidity.
The shielding gas quoted is a good choice, Argon Helium mixes can also be used, and offer a better wetting action, but are generally more expensive.
Shrinkage should be similar to carbon steels as coeficients of thermal expansion are about the same.
Forming again is comparable to grade 50 type carbon steels. Forming radius' should not be less than equal to the material thickness up to and including 1/4" thick, above this the forming radius should be at least twice the thickness of the material.
While rings as described have been regularly made by rolling and welding, the size described may be better cut whole from a plate due to the smaller diameter and wide flange.