316 SS / CF8M Is Rusting

A. CF8M is the casting alloy that corresponds to the wrought 316 material. It has slightly different chemical composition, which has several consequences. 1. Foundries usually balance the relative amounts of the various elements (while maintaining each within its own specified range) so that the microstructure is predominantly austenitic, but with typically 5-8 percent ferrite. The ferrite phase is attracted to a magnet (if it is present to say 5% of the structure, the strength of the attraction will be in the vicinity of 5% of that of mild steel). It is by no means essential to induce this small amount of ferrite, and indeed the CF8M composition can be balanced so that there is no ferrite and hence no magnetic attraction, but knowledgeable foundries do not do this unless these is a special need for low magnetic permeability that was specified by the purchaser. The wrought 316 has its composition deliberately balanced so that ferrite does not form, because it is much easier to roll the material into its final form (plate or strip) if there is no ferrite in the microstructure. 2. A consequence of the ferrite is an increase in strength during and immediately after solidification, which greatly reduces the possibility of hot-tear defects occurring which would require subsequent repair by the foundry - such a repair would not detract from the integrity of the component, but is an avoidable additional manufacturing cost. So the ferrite is induced to give this protection against hot tearing. 3. An incidental benefit is that the room temperature yield strength is increased, although this benefit is rarely exploited by design engineers. 4. An incidental benefit is enhanced corrosion resistance, because the chromium specification range for CF8M is higher than that for 316, and furthermore it is usual to manufacture CF8M towards the top end of its chromium range as part of the ferrite-inducing element balance but to manufacture 316 towards the bottom end of its chromium range as part of the ferrite-avoiding element balance. The higher chromium content of CF8M confers better corrosion resistance. 5. All of the above relies on the manufacturer heat treating the product appropriately, so as to avoid the presence of any of carbides, or sigma phase, or chi phase, in the microstructure because any of these will detract from corrosion resistance. Additional note: Having written the above, it now occurs to me that perhaps you are referring only to rust on visible external surfaces of the valves, and not to corrosion on internal surfaces. If that is the case, it is possible that a manufacturer shot-blasted the castings with steel shot rather than stainless shot, thus contaminating the surface with non-stainless debris. There are many postings on this site that refer to pickling (or passivation) to clean such contamination from stainless surfaces. You need to talk with the suppliers of the valves to clarify their manufacturing procedures. Bear in mind that any one particular valve supplier might source his component castings from different foundries, so even if you are only using one valve supplier you might have castings from several different manufacturers, and some might be using better procedures than others.