Spring Failure 3

[Pras]

Hi,

We are facing failure problems of a springs made up of SS316L material ( 1.0 mm wire diamter, 6.3mm outer diamter, 9.5mm length)which are used in our special pipe fittings. The spring is exposed to the fluid and supposed to work in 'compressed condition'. It is clear from our observations that the fluid ( Saturated steam at 200degC in this case) is attacking the spring material and causing it to break in pieces. The microscopic examination of the spring pieces indicate the presence of 'brittle cracks' within the piece, which we believe could be due to corrosion attack. It has also been confirmed by the customer that he is adding some additives like chemicals / powders as ‘descaling agents’ in the boiler feed water. This could be the cause of ‘Corrosion’

I am planning to change the material of construction of these springs to Hastelloy –C ( Grade C-276) to increase the corrosion resistance.
Now my questions are :-

(1) Is Hastelloy –C ( C-276) material suitable for springs ??

(2) Do we need to provide any heat treatment (oil hardening and tempering ) to the spring wire before coiling ? One of the spring manufacturers informed us that this heat treatment would be required to avoid problems like ‘Permanent set’ in hastelloy-C springs.

(3) Any other material of construction for this application ???

Please advise.

Thanks in advance.

Pras

 

[butelja]

Hastelloy C276 is used often for springs in mechanical seals. 316SS is very poor for springs, because of chloride stress cracking problems and relatively low strength compared to other spring materials. Most stainless springs use 302SS, which when cold worked is stronger than 316. However, it is even more succeptable to cracking.

 

[TVP]

(1) As butelja mentioned, Hastelloy C276 is suitable for use as a spring material, provided that it has been processed to a "spring temper". I am not that familiar with this alloy, and could not find any standards or specifications for a spring temper, so you should start with your spring vendor for more information. Permanent set (or compression set) at 200 C may be an issue with this alloy, depending on the stresses in the spring.

(2) Oil hardening and tempering are used during the manufacture of steel springs, in order to produce a tempered martensite microstructure (very high strength). Nickel alloys like C-276 will not respond to this type of process. In order to produce high strength, the wire would need to be strain hardened by cold drawing/rolling prior to the coiling operation. Stress relieving would likely follow the coiling operation. Additional information is available at the following website:

http://www.ropa-stahl.de/276.HTM

(3) You might also consider a titanium alloy like Beta-C (Ti 3-8-6-4-4) for this application. It should provide good corrosion resistance, as well as resistance to compression set at 200 C. More information on the alloy can be found at the following two websites:

http://www.allvac.com/allvac/pages/PDF/tech/TI-50938-644.pdf

http://www.rti-intl.com/

Renton Coil Spring in Renton, WA, USA is the leader in titanium spring production. You can obtain more information from their website at

http://www.rentoncoilspring.com/index.html

Another option would be the nickel-based superalloy X-750. Special Metals produces this alloy as Inconel X-750 (

http://www.specialmetals.com/publication/tech_bulletin_x750.pdf),

while Haynes International just calls it Alloy X-750 (

http://www.haynesintl.com/pdf/h3131.pdf

). This alloy would provide outstanding corrosion resistance, and is frequently specified for spring applications involving high temperatures. It is likely to be more expensive than either the Hastelloy C-276 or Ti Beta-C options.

 

[mcguire]

You didn't mention whether the 316 is used in the annealed state or in some higher strength version. Knowing that would help us give you a specific alternative.
You could immediately alleviate the cracking by switching to 2205 or 2507 duplex alloys. These have significantly higher annealed yield strength. Or at the next higher level of cost you could use a superaustenitic like AL6XN.
Lastly, the most expensive fix is to go to a superalloy like C276 or Inconel 718.
If cost is a major consideration, try to make the 2205 work. It should do the job.

 

[rustbuster1]

Mcguire has given you excellent advice>

 

[TVP]

mcguire/rustbuster1,

Do you have experience using duplex or superaustenitic alloys in a high stress application like a helical compression spring? ASTM A 313 covers a variety of stainless steel spring wire materials, but I am not aware of any of these alloys being similarly used. My limited experience is that these alloys are most frequently used in the annealed condition, thereby offering limited scope for competing against typical spring materials. Can you provide some additional information, such as the availability of these grades in wire form, spring manufacturers who use these grades, etc.?

 

[TVP]

I did some additional investigation, and found that Sandvik produces alloy 2205 (UNS S32205 or DIN 1.4462) wire in a spring temper. More data is available at the following website:

http://www2.sandvik.com/sandvik/014...5f1d47d402a6646c4125685c0029a50d?OpenDocument

And Elgiloy Special Metals in Elgin, IL, USA apparently produces wire out of practically any type of material, including highly alloyed stainless steels like 2205 and AL6XN. More information is available at the folllowing website:

http://www.elgiloy.com/esm.html

 

[mcguire]

The beauty of 2205 in this application is that you're above the temperature at which ferrite experiences SCC, so the cold work won't hurt your SCC resistance whereas in the superaustenitic, if you happen to get pitting, then SCC might ensue even though the annealed alloy may be highly resistant.
Elgiloy and Combined are good sources.

 

[Pras]

butelja / TVP / Mcguire,

Thank you for your advice. I would like to have more information on the following :-

(1) 'Spring temper' process applicable to C-276.

(2) Permanent set of Hastelloy C276 springs (Please note that the operating temperature in this case is 250 degC continuous)

Thanks.

Pras

 

[TVP]

Pras,

You really need to discuss these questions with 1) a spring vendor, and 2) the supplier of the hastelloy wire, because they are both interrelated. The amount of permanent set will depend on the stress that the spring experiences, which is dependent on the actual spring geometry (free length, solid length, coil diameter, wire diameter, etc.). Temperature affects the material properties (strength and stiffness), as does the wire processing (amount of cold work/strain hardening, etc.). In order to properly engineer a spring for this application, you really need to have the input from both of these vendors, and possibly the actual material melting source, which would probably be Haynes for Hastelloy C-276 (

http://www.haynesintl.com/).

 

[Pras]

TVP / Mcguire,

I need to know the applicable international standards ASTM / EN for procurement of Hastelloy C-276 spring wires.I would like the spring suppliers to deliver the wires with other general specs as per this standard.

I understand that standard ASTM B471 was applicable to nickel alloy spring wires , but has been withdrawn without replacement.

Thanks.

Pras

 

[TVP]

As I mentioned previously, I could find no standards or specifications for Hastelloy C-276 spring-temper wire. You should consult with someone from Haynes and the wire vendor to define the requirements. I would suggest reviewing other spring wire specifications for guidance on important items, like surface characteristics, inclusion ratings, etc. Some standards you may consider for reference include the following:

SAE AMS 5961: Nickel Alloy, Corrosion and Heat Resistant, Wire 74ni 15.5cr 8.0fe Cold Reduced, Spring Temper

SAE AMS 5699: Nickel Alloy, Corrosion and Heat Resistant, Wire 72ni 15.5cr 0.95cb 2.5ti 0.70al 7.0fe, Spring Temper, Precipitation Hardenable

A list of other standards related to nickel spring wire include:

http://global.ihs.com/search_res.cf...'%(?,K (K[0 ' &country_code=US&lang_code=ENGL