Alternative Alloys for Grade B7M Heat Exchanger Studs in Sour Service? 1

[Guest102023]

I'm now completing my Nth failure investigation of ASTM A193 Gr. B7 studs that suffered sulfide stress cracking. Client will of course go to grade B7M to avoid SSC, but that doesn't address the issue of corrosion pitting severe enough after one year to render the studs unserviceable.

Any suggestions that preferably avoid the austenitic stainless grades? I'm not necessarily looking for infinite service life; 3~5 years would be nice.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[CoryPad]

A duplex stainless steel, e.g. UNS S31803 included in ASTM A1082/A1082M Standard Specification for High Strength Precipitation Hardening and Duplex Stainless Steel Bolting for Special Purpose Applications, is an option. Is your objection to austenitic stainless steel based on cost?

 

[Guest102023]

My objection to austenitic stainless steel is based on their unsuitability as fastener material, although some are better than others. Thermal expansion difference is also a concern.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[CoryPad]

If you do not want to use austenitic steel (either full or duplex), then other options include nickel alloys 625 and 718 (UNS N06625 and N07718) and the multiphase alloy MP35N (R30035).

 

[EdStainless]

Cold finished (thread rolled) superaustenitic would be a good option.
Duplex should either be lightly cold worked or annealed (it looses ductility when heavily cold worked)

If I went beyond those I would go straight to MP35N
I have has some poor experience with 718 fasteners.

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P.E. Metallurgy, Plymouth Tube

 

[Guest102023]

I had to google MP35N, as superalloys are a gap in my experience. Very impressive, but complete overkill for the application - 28 studs would probably cost more than the rest of the exchanger! A1082 duplex SS sounds appropriate though.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[SJones]

Just a clarification: the bolts in question are apparently suffering from both SSC and pitting; would you elaborate upon how the bolts are actually becoming exposed to a corrosive, H2S-containing environment having the propensity to cause such damage?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Guest102023]

SJ,

The exchanger is laid up seasonally in a Great Lakes climate. Not sure of the details of the protective cover but I suspect it isn't much different than a tent. I believe it is bad actors dissolved in condensing atmospheric moisture that is causing most of the attack. I view the two damage mechanisms as separate although having probably a common corrodent. The sulphide spot test on the cleaned threads was positive for both colour and odour.

.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[SJones]

Thanks. The scenario appears to swing it for duplex. Just a thought: are the installers of the fasteners using a lubricant containing moly disulphide? Shell did some work to show that it can react with atmospheric moisture to generate H2S and crack LAS bolts.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Guest102023]

No moly disulphide as far as I have ever seen, nor any other compound in this particular case. But many times I have encountered copper-coloured 'never-seize' which the mechanics don't seem to understand is a lubricant.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[PVPguy]

You would be better off getting your B7M bolts coated to avoid the pitting, a combination of electroless nickel and PTFE (i.e.: any of the proprietary coatings like Zylan). If you wanted overkill then you could get them hot-dip spun galvanized. It doesn't sound like your climate is that aggressive, so just accepting that some corrosion will occur on the B7M bolts is also an option (many use a guideline of 10% or area loss being acceptable). This is particularly the case since you only want a few years out of them.

Another option (assuming you don't have chlorides) is A453-660. It is listed in NACE MR0103, but not in MR0175 (the difference being chlorides). It will be cheaper than a lot of the other materials and actually has an ASME bolt spec, which makes it easier.

SJ, I would be interested to know of any published references regarding Moly Disulphide causing SCC. The basis for that statement is usually work done by the nuclear industry in the US and has been subsequently questioned by other researchers who looked at the same failure. A lot of "Moly" anti-seizes don't actually contain much Moly Disulphide and they have been very widely used in industry (refining in particular) around the world without identified incidents of late.

 

[SJones]

Sorry, nothing in the public domain that I have. It was an internal failure analysis report, the results of which then found their way into Shell standards as a requirement.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.