City water dept. finds 316 SS Butterfly valves 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.

A. Vinu,

Regarding the rust on outside of 316 castings, This can be caused from blasting with steel shot. Where the steel becomes embedded in surface. If no further machining is done, then this embedded iron will rust over time. For internal surfaces which are most likely machined after casting + blast cleaning, the embedded iron would be removed.

There are stainless steel shots and stainless steel cut wire shots available, which will prevent embedding of Free iron in the first place. The next time a unit rusting in the field is removed, have it sectioned + evaluated metallurgically for the depth of rust, and give you peace of mind for other units in service are ok. You will then be able to confirm if rust is limited to as cast surfaces.

A. Vinu,

I've just about reached the point where I'd need to write a book to take the whole matter further, because there are so many combinations of possible variations in processing details during manufacture, and they are all just that ... possibilities.

Even if the surface was contaminated by non-stainless steel, that doesn't constitute non-compliance with ASTM material specifications than include CF8M, and I suspect that your organization didn't set out any requirements on the subject.

If you really want to know what has caused the rust, you need to take the valve out of service and submit it to a metallurgical lab for investigation.

But my best advice now is to try Lee Kramer's clean-up if you don't like the look of the rust, then print this whole thread and give it to your valve supplier with a request that they don't let it happen again.

A. My final post on this thread.

In general terms:
Pickling is only needed if the castings are blasted with steel or iron abrasive, thus contaminating the surface. If they are blasted with stainless shot or garnet, there is no need to pickle them afterwards but no harm is done if in fact they are pickled.

For your castings:
If they are NOT rusting, either (1) they were blasted with an appropriate abrasive and might or might not have subsequently been pickled, or (2) they were blasted with an inappropriate abrasive and were subsequently correctly pickled. It doesn't matter which, because they're not rusting and that's what you want.

If they ARE rusting, there are multiple possible causes, including (but not limited to) any one of, or combination of: inappropriate abrasive and subsequent pickling, inappropriate element balance within specification, incorrect heat treatment, composition out-of-specification, unsuitable specification in the first place. Examination by a metallurgical laboratory would be needed to determine the actual cause(s) in a particular case. I'd give you a price myself to do the investigation, but you're in USA and I'm in Australia so it really wouldn't work.

A. Check out the moly percentage,

A. The first thing to ask of the 316 valve casting is what casting process was used. Sand casting or investment casting. Looking at the picture it looks like a sand casting! This would be the start of the rusting problem!
In sand casting, particles of sand are attached to the surface of the SS and leave pockets on the surface when removed with grit or sand blasting method. In these pockets will hold contaminations and cause rusting. In passivation or pickling the solution is tied up in these pockets and dried and when exposed to moisture again rusting will seem to come from these pockets in the SS surface. If the surface is machined to remove a thin layer you remove these pockets and remove this problem.

The grit blasting with steel will cause this rusting but what casting house would use steel shot blast on SS casting?

A. Hi, Omkar. As you can see, we appended your inquiry to a thread that hopefully answers it. If you have remaining questions, please post again.

Regards,