Surface finish for thermal shock cracking

[howkers]

We manufacture nuclear valves to ASME III.
Some of our older designs have surface finishes on internal surfaces of 16 micro inch, which is hard to achieve.
It has been suggested that this was introduced to prevent cracking due to thermal shock and that this was derived from a requirement in ASME III. I have had a look but can find no such clause in ASME III.
Does anyone know if there is such a clause in ASME?
Is it good enginering practice anyway?

Bill H

 

[metengr]

Surface finish does play some role in resistance to thermal shock (low cycle fatigue crack) of materials because it reduces local surface stress risers. However, how much of a role is the important question, and is it worth the added cost to obtain this added resistance.

Maybe to better help your situation, I had found an equation below in a technical publication that shows the essential variables that can be used to evaluate thermal fatigue resistance between various materials;

Thermal fatigue resistance parameter = yield strength x thermal conductivity/Elastic Modulus x coefficient of thermal expansion.

Obviously, the higher the resistance parameter, the better off you are for a given thermal stress. As you can see, surface finish really does not play a significant role. In most of the components we have purchased for cyclic service, the key is to reduce stress concentration by introducing a radius at any sharp corners or openings, and removing by blend grinding any sharp tool marks.

 

[4fletch]

metengr, do you believe the formula you provided is applicable in evaluating boiler tube materials and overlays for use in waterwalls serviced by water cannons? Sorry to get off topic.

 

[metengr]

Yes, I don't see why it could not be used to evaluate weld overlay materials for boiler tubing. I don't believe it would help to evaluate differences between boiler tube base materials because other more important design factors come into play. In pure thermal fatigue damage assessment, the tube material doesn't care where the source of the temperature differential comes from.

As a side note, if you are experiencing thermal fatigue damage to the fireside waterwall tube surfaces it may be that your dwell time or cycling duty is too high. You really should not be experiencing any TF damage. We have installed several water cannons on our PC, T-fired boilers with no adverse effects.