SCC of 416 SS in Demineralized water

I don't have time to search my references today, but I would bet that there were some pre-existing microfissures from the forming (forging?) or heat treatment.
Ken

Still short on time, but will try to give a more complete answer.

Stainless steels can be subject to IGSCC (InterGranular Stress Corrosion Cracking) in high purity water in the sensitized condition (where improper heating has allowed chromium carbide and/or nitride formation in the grain boundaries). Fontana’s book, Corrosion Engineering, 3rd Edn., p. 383, has a figure showing ranges of various types of SCC for sensitized austenitic 304 SS. And, a general rule is that the higher the strength, the more susceptible SS is to SCC.

The martensitic 416 SS is behind the 8-ball because
1) Its normal heat treatment doesn’t give good corrosion resistance to begin with, and people frequently [in my opinion – no statistics] try to achieve a greater hardness by faster-than-recommended quenching. E.g., a water quench is used rather than the recommended oil quench (which only yields R[sub]C[/sub] in the range 36-41 – Carpenter Technologies). This creates greater stresses and possible flaws. Also, forgings should be air-cooled (max. R[sub]C[/sub] 39), which allows sensitization, so either way, corrosion resistance suffers.
2) The tempering temperature is in the range for sensitization.
3) It is a free-machining SS containing 0.15 % sulfur. The sulfides in the grain boundaries increase corrosion susceptibility, especially if elongated rather than globular. Further, these sulfides can create defects if forged. These free-machining SS alloys require a modified passivation procedure (frequently not done), and passivation is sometimes entirely omitted. A bit of conjecture, but the sulfide phase may be an internal impurity contributing to IGSCC. It can react with hot water to create sulfate, sulfuric acid, hydrogen sulfide, etc. depending upon oxygen potential.

So far, I haven’t found any references to IGA or SCC of 416 SS in high purity water. Maybe mcguire will help out.

Many years ago I did a failure analysis of IGSCC in some 403 SS steam turbine blades. 403 is the low S version of 410. These blades had been welded at the outer shrouds, and the cracks ran right down the HAZ's, which were ~Rc 41, IIRC.

At a meeting with the "major" mfg. of large steam turbines, the chief engineer answered my questions about how they had welded them. They used an austenitic filler (TIG), w/o preheat or PWHT. When I asked, in amazement, why, they gave me the usual BS answer-"We've always done it this way, and you're the first to have problems".

Metalguy,
I'll bet that they just happened to have an improved version ready.

LCE,
I'll have to concur with "kenvlach" about 416 S/S in high purity water. I have no references to it at all. In fact I've seen very few parts other than some valve stems (steam service) made from 416 S/S in what could be considered a corrosive media.

I don't think it is SCC, though I'm at a loss as to the mechanism.

Does the crack have multiple branching?

A search shows that someone else experienced 416 SS rusting in clean steam:
The advice was to use a martensitic precipitation hardenable SS such as 17-4PH.

I was going to suggest switching to 410 SS (the 0.03%S, non-free machining analog of 416), but based on Metalguy’s experience, maybe not.

Re my earlier post, the figure in Fontana’s book is for SCC of sensitized 304 in pure water.

unclesyd,
Yes the cracking does have multiple braching.

kenvlach,
The manganese sulfides are elongated and plentiful in the microstructure. I definitely see the potential for having some sulfur leach into solution and thus developing a corrosive media like you mentioned. I am currently running more SEM scans to look for contaminents in crack tips.