heat treating 2

[mielke]

We have an application with bent stainless steel tubes that we need heat treated. but the tubes will be used in an operational temperature of about 1000F. Is it necessary to heat treat them or will the application of 1000F take care of them?

 

[TVP]

You need to more precisely define what the heat treatment of the stainless steel tubes is supposed to be. Annealing? Hardening by quenching and tempering? Something else? What is the end use? Mechanical property requirements?

 

[mielke]

Its 304 stainless steel 3/8" od tubes bent to u-tubes. We just want to stress relieve them after bending. They will be used in a pressure vessel about 1000F and 50psi.

 

[CastMetal]

With the information TVP asked for in hand, you can find your answers in
PRC-0001 (REV.F), "PROCESS SPECIFICATION FOR THE HEAT TREATMENT OF STEEL ALLOYS"
which you can get here for free

http://www.everyspec.com/specifications-standards-search.php

 

[mielke]

anyone have an opinion from what I said above if heat treatment (stress relieving) is even necessary. The tubes will be roller expanded into a tube sheet and not welded.

 

[mfgenggear]

mielke

if the 304 SS is in the annealed condition when bending
then it is normally recommended to re-anneal them after bending. stress relieving in my opinion is not enough.
because the parent material will work harden during bending.

MfgEngGear

 

[EngineerTex]

I'm going to dissent from mfgenggear's recommendation. The cold working process will strain harden the tubing, but to what extent? Starting with annealed tubing, I don't think you have the first thing to worry about at 50 psi. I don't know what your wall thickness is, but calculate the burst pressure on your tubing and I'll bet you're looking at somewhere in the 10,000 psi neighborhood even with relatively moderate wall thickness. And even it it's not that thick, where would your burst be? 5,000 psi? 1,000 psi? Even at a ridiculously low 1,000 psi, you're looking at a 20:1 safety factor against burst.

So now you're wondering about how the cold working is going to affect the useage of the tubing. Remember that all of your ductility occurs between the yield strength and the ultimate strength. When you bend it -- once -- you will modify the yield strength, but you will not be changing the ultimate strength. Burst is calculated based on ultimate strength and not yield strength. So when you bend it, your ultimate is unchanged so your burst SF remains unchanged somewhere between 20:1 and 200:1.

But what about cyclic loading? Even after you have made the bend, you would have to determine if your stress is above the material's endurance limit at service temperature. If it is close, then you would probably be good to anneal it, but at 1,000F, I can't imagine there being a problem.

Now, it's time to talk about creep. If you're going to have a problem, I doubt it will be because you didn't anneal it. It will be because of the creep properties of metals at elevated temperatures. Even then, at the (I assume) very low stresses you're dealing with, I don't think it's going to be a problem. If you want to know more about high temperature creep, do some research on the internet. I would imagine that you're going to find that you're still just fine.

Here's some good reading on the subject of 304 from a steel supplier:

http://www.sandmeyersteel.com/304.html

I vote to bend it and be done with it. If you're still worried, talk to your vendor.

Engineering is not the science behind building. It is the science behind not building.

 

[mfgenggear]

It is true what EngineerTex said he would be more authorities on the subject, however what is not stated is how much more post work is involved. I assumed there was machining & welding of some type required. in that case it is best to have parts in the annealed condition. no weld stress cracks. & I was assuming the tubes where chosen for the burst, yield & ultimate reqmt's as purchased.

Cheers
MfgEnggear