Question about long term/short term stress in 316SS...

[someguy2]

This webpage says that 316H has a "short time" tensile strength of 120MPA at 900C.

It also says that for a creep of less than 1% in 10,000 hours at 800C the pressure should be kept below 20 MPA.

http://www.askzn.co.za/tech/tech_grade_316.htm

Could someone explain the relevance of these numbers in the context of a pressure vessel.

Thanks.

 

[metengr]

someguy2;
In your previous post regarding ASME and pressure vessels you have the necessary information to seek outside expertise. Without getting too in depth, the suitability of materials for use in pressure vessels must consider the following;

-tensile/yield strength at service temperature
-time dependent properties like creep deformation or stress rupture
-fatigue
and corrosion

The ASME B&PV Code evaluates and endorses materials for use at maximum service temperatures. These materials are permitted to be used in boiler and pressure vessels. In general, ASME B&PV Code provides rules for design, and also one can design by analysis for pressure vessels.

 

[CalvinKelly]

For 1100F (593C) and above, 316H design stresses for pressure vessel use are based on the long-time creep-rupture properties. Tensile strength cannot be used here.
The reason is at this temperature and above the metal will deform slowly with time, and at stresses much lower than the short time tensile or yield strength. Stress is no longer proportional to strain, time is now involved.
One might say Hooke's Law was repealed above roughly 540-600C.
The good news, which I presume you already know, is that ASME publishes tables of allowable design stresses for 316H and other alloys, for different temperatures.
Your next problem will be actually finding 316H available somewhere.

James Kelly

http://www.rolledalloys.com

 

[someguy2]

Thanks for the reply, James. I got copies of Table 1A and I now understand why there is a difference.

As far as 316H and 316, I've been told that we can safely substitute some 316s for 316H, it depends on the composition. Care to comment on that ?

 

[metengr]

someguy2;
I looked at the allowable stress values for 316 and 316H in plate form under SA-240. Both have comparable allowable stresses at 1500 deg F. Keep in mind that this is the upper limit for design use of this material (1.3 Ksi) and I would not recommend using this material unless you are willing to substantially increase the wall thickness to reduce your service stresses to well below 1.3 Ksi. This will increase the cost of material. I believe you need to re-evaluate your use of 316 material for this application.

 

[someguy2]

It is true that one needs very thick walls to use 316 or 316H. I've looked at other materials but have yet to find one that allows a more economic vessel. While other materials have marginally better design stresses, their incremental cost is way higher than their incremental pressure.

The material cost isn't an issue for the test vessel, but it will be for the real vessel. See my post on insulating the vessel walls and water cooling it.

 

[someguy2]

err... "incremental pressure" above should read "incremental design stress".

If I am wrong on this, please tell me.

 

[EdStainless]

Just a word of caution. When you are using 304L (or 316) in palce of straight grade, or straight grade in place of H, be careful.
I know that the lower C material usually makes the mechanical properties at room temp, and even short term high temp. But without the C you will not get the same creep/stress rupture results. Often alloys have higher strength because of nitrogen, this does not help out at high temp.

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