17-7 PH disc springs failure!!! 13

[zztop]

I had disc-springs made of "equivalent" material to 17-7 PH called 1.4568 (european standard).

They are now completely broken and I don't know why. The machine has been out of use for two years near the sea... till they were going to use it.

The springs were loaded, but no vibration and no shocks at all.

Does anybody know if this material shows any type of "stress cracking" with H2S or marine environments?

ZZtop

 

[metengr]

There could be several possible causes of failure - one of which could be exposure of the high strength "equivalent" steel to chlorides causing stress corrosion cracking under static load conditions.

I would suggest you have a metallurgical analysis performed to establish the root cause.

 

[unclesyd]

Yes to both your questions. As with all PH steels the heat treatment has a lot of influence on the corrosion/failure of 17/7.

If you can come back with the heat treatment used for the springs and their enviroment at prior to the failure.

Make sure you get a sample for metallurgical analysis.

 

[TVP]

17-7 PH (Type 631) is indeed equivalent to X7CrNiAl17-7 (1.4568). Spring-like components made from this grade must be cleaned and passivated after coiling to insure maximum corrosion resistance of the stainless steel. One procedure is as follows:

a. Remove drawing compounds from the wire surface by a 5 min dip in alkaline cleaner at approximately 90 °C (190 °F), followed by a water rinse.
b. Remove metallic and most nonmetallic coatings from the wire surface and passivate the surface by immersing parts in a nitric acid solution of 15 to 25% at 60 to 70 °C (140 to 160 °F) for 5 min or until clean. Follow with a water rinse.

After passivating, parts made from this this grade must be heated at 477 to 488 °C (890 to 900 °F) for 1 h and air cooled. Heating should be performed in a protective atmosphere. No forming should be done to the parts after heat treating. See SAE J217 and SAE AMS 2759/3C for more information.

 

[israelkk]

TVP

I think zztop talks about disk springs (belleville springs) which are made of flat strip or sheet and not from wire. I noticed that you recommend to passivate before the the heat treatment why it can not be the other way as long as during cleaning after heat treatment the cleaning process does not produce hydrogen (for example a mechanical cleaning.

 

[EdStainless]

Springs need to be stress relieved after forming. If these weren't then CSCC could be a real issue.
This alloy does not have great pitting and CSCC resistance anyway. These parts should never be exposed to a marine environment.
I always liked:
-form
-degrease
-heat treat (some types of springs are heat treated first and then formed)
-passivate
-stress relief

I also prefer the RH threatment for springs, it has better toughness.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[NickE]

I use the CH900 heat treat for disk springs. I gives the highest resistanc to set, and provides the highest UTS for fatigue resistance.

nick

 

[israelkk]

EdStainless

I think that if you heat treat 17-7PH at 900F which is just aging-precipitation hardening(there is no quenching) you get the stress relief built in this process. could do a stress relief after forming and before heat treating to minimize deformations.

 

[YLWEA]

Several years ago when I was working with 17-4PH in hot brine with some H2S the standard guidance was that the preferred heat treatment was H1150M. Not sure if that is still true, but we did have failure of valve stems at lower temperature treatments (such as H900, I believe). If I was in your position I would refer to the current guidance in NACE MR0175 as a start, at least for the H2S portion of the problem.

 

[TVP]

The procedure that I outlined is the one recommended in SAE J217, which is a standard for 17-7 PH wire. It pertains to flat components as well. The important thing to remember is that the component needs to be clean before exposure to elevated temperature. Lubricants will degrade at the high temperature required for aging, and this can severely affect subsequent corrosion resistance. Also, the microstructure that develops after aging is susceptible to cracking and embrittlement when exposed to concentrated acids. Cleaning followed aging, grit blasting to remove scale, and then passivation is another procedure that works.

 

[unclesyd]

Here is the website of the daddy of 17/7 PH with recommendations for using 17/17 around chlorides. The heat tint formed during the ageing process should be left on if possible, it will be better than any chemical passivation that you can do.

http://www.aksteel.com/markets_products/stainless_precipitation.asp

Come back with a little more information as to the enviroment and things like are the washers stacked.

 

[zztop]

Hello to everybody,

we have done some microstructure analisys using an electronic microscope, and there was a nice trace of Chlorides on the cracked surface.We also found carbides. So we think that the heat treatment wasn't also good.

I think that from now on we'll use 304csp. I am sure inconnel x-750 will do this work ok, but before we'll test in salt spray cabin with both materials. Just because of the price difference between inconnel and 304csp. Money...you know.

zztop

 

[unclesyd]

zztop,

If Chlorides are suspected in the original failure I would not use 304 SS which will almost guarantee a SCC failure.

 

[metengr]

I would recommend MP35N alloy as a suitable replacement material for the spring.

 

[TVP]

zztop,

I agree with the others-- 304 stainless is not a good choice for Cl environment. MP35N is an excellent choice.

 

[SMF1964]

The NRC came out with an advisory a number of years ago about SCC of 17/4 PH stainless when heat treated improperly. If I recall accurately, it was an issue with the ageing treatment... Anyone have a similar recollection?

 

[unclesyd]

SMF1964,
There has been several papers and internal reports on 17-7 that relate the SCC resistance to the PH treatment.
ARMCO (AK Steel) literature states that CH900 or TH1050 are the preferred treatments to resist SCC. This is the same treatments, mentioned in the reports.
In the literature I have on Disc Springs all list the hardness of their 17-7 PH @ 40-45 Rc. Given the nature of the PH alloys this doesn't really tell you anything about the PH treatment. Warrants a call to the tech rep.

metengr,
Several times we have attempted to procure disc springs from Multiphase (MP35N) and had to back off due to price and availability, nobody wanted to touch the material and we couldn't afford it. It is probably better now as there are several more manufactures of disc springs than there were when we were in the hunt.

zztop,
You had better sit down when you get a quote on MP35N under the present materials upheavel. Noticed all the metal companies stock went up yesterday and I think Carpenter lead the way. If you need the springs immediately I would start on ordering as soon as possible since there is no prediction on the lead time and availability you will get.

 

[israelkk]

unclesyd

To my best knowledge all 17-7 PH disk springs (and any other spring type) are heat treated to CH900.

http://www.webspawner.com/users/israelkk/

 

[NickelMet]

This is sort-of related, though not entirely. We just had some valve plate (on a compressor) springs fail prematurely (<3 weeks cycle time) due to what we thought was fatigue. However, the analysis showed Chloride Stress Corrosion Cracking. The springs were Nimonic 90, supposedly made per NACE specifications, being used in a hydrogen compressor. In the middle of all this, the distributor tells us that they have had an issue with the supplier concerning improper heat treatments (one of those V-8 moments I suppose). And, the distributor now wants us to use 17-7PH springs! (There's the tie-in for this thread...)

Any critique on using this material or another?

~NiM

 

[zztop]

NickelMet,

you mean that for a disc spring working under stress cycles, and chlorides, your distributor prefers 17-7PH with CH900, don't you?

I think that the right solution means a reasonable cost, but no failure in a short time. I mean that I don't want a lifetime-disc-spring like the one suggested by Unclesyd, MP35N. We can accept maintenance works and no critical or dangerous aplication. That's why I told you 304csp. I just want to improve the behaviour of these springs. Maybe I have expressed myself, since the beginning, in a wrong way. The question should be:

Which are the good spring materials, gradually, for chloride environments? As I understand, the top one is MP35N. But what about the materials between this one and the 17-7PH? Which is your experience?

Thanks to all. This is a very interesting thread...at least for me!