Hie All,

I needed to know that how can one decide on stress relieving of fabricated strucutures..??
Like to what extend does the below stated factors influence my decision to whether provide/not provide stress releiving-

1)The type of loading to be done on the strucuture.
2)The amount of welding (in terms of weight/volume).
3)How to measure the reduced stress level (most economical and easy way) after stress relieving.

Lets say we are using mild steel with metal arc welding for fabrication.

Thanks in advance.

regards,
DN :ugeek:

 

Dhanush
Stress relieving is often done to avoid or reduce warping, it is also done to reduce stress levels dangerously high that can cause surface cracking and much reduced service life do to fatigue failure.

In the late 70's a common technique was to vibrate the welded structure using a device that found the natural frequency of the beam suspended on rubber tires. This would normalize the stresses to a uniform level rather then relieve the stress to zero as heat treatment techniques will do. For large welded pieces this is not practical to do as the oven requirement can be very large.

I don't recall how the lowered stress was measured. I suspect the natural frequency would shift to a lowered frequency after the treatment compared to the initial but if this is used it is a very relative measurement.


I am not sure the unit of welded weight per volume is useful in general since weld stresses are local and can be very high.
Especially when there is a large difference in the two joined material thicknesses. The weld cools at different rates making the stress that more unpredictable.

 

There are differences between post weld heat treatment (PWHT) and stress relieving.
Stress relieving corrects fiber elongations due to bending or other cold working. Structural members rarely require stress relieving if fabricated properly. Thicker plates that have been bent and rolled may require a stress relive to realign metallic structures, reducing yield strength. Cold working will increase yield strength and may approach the tensile strength of a material. This is undesirable for several loading modes, fatigue being just one.
Welding results in precipitate hardening and not fiber elongation. This results in a localized hardening in the well known HAZ. This is undesirable for some purposes as a localized section of harder material will yield differently than others. If for fluid retention, there are a number of services that will attack hardened materials (stress corrosion cracking). The weld size, configuration and procedure can all impact hardness of the end product. Travel speeds & heat inputs can be controlled to limit this impact.
If you wish to determine the mechanical properties of the end product after heat treatment, a small sample piece, often called a mock-up, can be used that will receive the same thermal treatments. Physical tests of hardness, yield and tensile testing may be performed on this destructible sample that can be correlated to the finished product.