Stainless/Carbon Weld & Thermal 3

[jt12]

I've had 3 or 4 jobs in the past year where I've had some sort of a stainless steel plate application where the pressures on the plate are supported by carbon steel stiffeners. The stiffeners are welded to the SS using intermittant welds on each side. One of the applications got up to 350 degrees F. I allowed this design through some careful detailing, but don't think the numbers exactly worked to prevent one or both (plate / stiffener) from elastically yielding under the thermal stress. I wouldn't allow this arrangement another applcation got up to 950 degrees F. I told them they needed to use plate and stiffener metals of more matching thermal expansion properties. The stainless and carbon steels expand differently under heat and since they are attached stress each other.

Does anyone have any experience with this type of thing to know what operating temperatures are OK and which cause problems? Also, is anyone familiar with the welding process involved to prevent the welding heat to thermally stress the connecting metals? The 350 degree application has been operating for close to 6 months now, and I haven't heard of any problems with it, but I'd like to know more about this topic for the next time this situation occurs.

 

[metengr]

Using external carbon steel stiffeners on stainless ductwork is not that uncommon provided you do not have galvanic corrosion concerns (structure is immersed in an electrolyte or severe external condensation).

The linear coefficient of expansion difference between carbon steel (which expands at ~6.7 X 10^-6 in/in/deg F) austenitic stainless steel (which expands at ~9.8 X 10^-6 in/in/deg F) is indeed a concern as service temperature increases. Since the stainless expands at a greater rate in comparison to carbon steel, the stainless duct (which depends on wall thickness) may try to overload the fillet welds that joint it to the carbon steel stiffeners resulting in fillet weld tears. You can calculate the thermal strain at the rigid attachment points (fillet welds) based on knowing the linear coefficient of expansion and metal temperature. Most of the applications I have seen with heavy wall stainless steel duct and external carbon steel stiffeners were flue gas temperatures at or below 400 deg F.

 

[EdStainless]

jt, you have struck a good balance. You need to consider both tehmperature range and the stiffness of the structure. The stiffer that it is the worse that the problem can be. I have never seen problems in equipment operated at less than 250F.
You need to be carful since a lot of these cases have the carbon steel external, it has both the lower expansion coef and a lower temperature rise.
Before I went above 350-400F with one of these I would do some careful analysis.

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[unclesyd]

Is the weldment subject to any thermal cycles?

Is the weldment insulated?

 

[jt12]

Typically not subject to thermal cycles except in extreme events or shut down. Some weldments were insulated and some were not.

 

[unclesyd]

If you do have a thermal cycle on such an arraignment you probably have trouble with differential thermal fatigue. It doesn't take many cycles to initiate cracking in the SS. In your situation the highest stress will come on cool down.
Insulation doesn't prevent this from occurring it only extends the initiation period.