Suitability of SAE 4140 for Low Temperature 2

[syedsamer]

Dear All,
We wish to use SAE 4140 steel for pressure vessel operating at -25 deg C. As i understand SAE/AISI 4140 is a low alloy austenitic steel. Can anyone tell me will SAE 4140 can withstand -25 degree C of temperature without any structural deformation?
Thanks and regards
Amer

 

[metengr]

syedsamer;
4140 is not a low alloy austenitic steel, it is a low alloy steel that contains 0.8-1.1 w% Cr and 0.15-0.25 w% Mo. This material contains 0.4w% C and is not really suitable for "low temperature' service. Your concern is not really structural deformation, it is notch toughness at low service temperature.

For pressure vessel applications, you need to look at ASME SA 516 grades of pressure vessel quality steel plate, that is for moderate to low temperature service.

 

[syedsamer]

thank you metengr.... Yeah that was mistake on my part.. it is NOT an austenitic steel.... I have to look for an austenitic steel (Am I right?). Thank you for your suggession.. How about ASTM A 351,352 487 or 516
Actually we do have the equipment(Pressure vessel made of SAE/AISI 4140 steel) and now we want to keep it in a freezer at -25 deg C. If you could let me know whether this would be fine or we need to do the get an equipment with changed material. Thanks

 

[metengr]

Amer;
If this is indeed a pressure vessel, I would use material that is at a minimum endorsed by either ASME Boiler and Pressure Vessel Code, or an equivalent construction code for pressure vessels.

At -25 deg C, normalized and tempered ASME SA 516 Grade 70 would work just fine.

Just curious, what is this pressure vessel? What is the rated pressure?

 

[israelkk]

For low wall thickness AISI 4130 is used for pressure vessels for aerospace use and 10000psi end even more. The temperature range in aerospace is usually -54 to 71 Celsius. It is weldable without the need for preheat. Fully (gas) Compressed pressure vessel made of 4130 are subject to gun fire shot and stay in one piece if correctly designed.

I have seen 4140 used for thick wall very high pressure vessels for aerospace use too.

However, I am not familiar with the requirements according to ASME.

 

[metengr]

israelkk;
No doubt about what you stated above especially if the material is supplied in a quenched and tempered heat treatment condition. However, pressure vessels used in military or space applications do not equate to pressure vessels that are designed for public use. In the US, pressure vessels that are rated for 15 psig or higher pressure must be built to ASME B&PV Code or an accepted pressure vessel code, this is the law.

At 0.40 w/%C, notch toughness is marginal in low alloy steel unless it is supplied in a quenched and tempered condition. Also, hardenability of 4130 and 4140 is limited in assuring a uniform quenched and tempered microstructure for low temperature toughness. When you have quenched and tempered heat treatment, additional welding controls come into play to prevent untempered martensite in the base metal HAZ, and to prevent over tempering to reduce strength. You also have temper embrittlement issues to deal with these materials. ASME Boiler and Pressure Vessel Code is conservative and trying to fabricate pressure vessels out of 4130, 4140 materials are not favorable.

ASME B&PV Code deals with all sorts of pressure vessels that are designed based on past experience and to assure public safety. Using lower alloy grades of steel with a fine grained structure and a N&T heat treatment is much easier to work with, especially regarding weldability because of the lower carbon content (less than 0.35%). Also, temper embrittlement is not a concern with most pressure vessel quality steel plates.

 

[israelkk]

metengr

I fully agree with you. I just wanted to show that there are some cases where 4140 can be used. This is mainly for small size pressure vessels (~ up to 30-40 cm in diameter) and wall thickness that will fully harden during quenching and tempering. The vessel is usually welded before quenching and tempering so there is no much loss of strength in the welded HAZ zones.