Stress-relieving

After Welding: Why and How.

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Residual Weld Stress Reduction

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Stress-relieving is the process generally specified after welding of most materials.

Removing or reducing the residual stresses generated by welding is required for improving the dimensional stability of weldments.

In certain applications, internal residual stresses can sum up with those generated by externally applied loads.

Then, if the yield strength of the material is exceeded, unacceptable plastic deformations will occur.

Non symmetrical machining may need to be performed on welded assemblies.

Then, following material removal, the redistribution of internal stresses may cause unwanted distortions.

Furthermore, without Stress-relieving, the material may suffer from service problems such as stress corrosion cracking.

Stress-relieving occurs by diffusion of atoms within solid materials.

Naturally, the material slowly approaches its equilibrium state.

Heat, if supplied, will increase the rate of diffusion by providing additional energy.

The movement of atoms has the effect of redistributing and eliminating linear imperfections, called dislocations, in crystalline arrays.

This alteration allows metals to deform more easily, so that their ductility is improved.

The relief of internal stresses is a thermodynamically spontaneous process.

However, at room temperatures, it is a very slow process.

Stress-relieving should always be considered

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Residual Internal Stresses can be reduced or removed either thermally or mechanically.

At higher temperatures the process will occur with an accelerated pace.

The treatment is not intended to produce significant changes in material structures or mechanical properties.

Therefore it is normally restricted to relatively low temperatures.

Most commonly thermal Stress-relieving is applied by heating the welded part in a furnace.

At elevated temperature the yield strength of materials is reduced, relative to that at room temperature.

Since the residual stress cannot exceed the yield stress, plastic flow will reduce the excess to the level of yield strength at that temperature.

In other words, a significant reduction of residual stresses is possible by heating weldments.

As materials are weaker at higher temperatures, (remember "Strike while the iron is hot"), plastic yield will occur.

For steels such a temperature is around 620 °C (1150 °F). Most Stress-relieving operations are carried out in air furnaces.

In air, alloys are subject to discoloration or scaling depending on the alloy and temperature used.

Post-treatment cleaning or scale removal treatments are therefore often required.

Time dependent stress relaxation will further reduce most of remaining residual stresses.

A rule of thumb requires that the time of the metal at temperature be at least one hour per 2.5 cm (1 inch) of thickness.

At higher temperatures the time required to reach a given lower stress level is shorter.

Stress-relieving may be skipped if other specialized thermal treatments are planned for other reasons.

For very large parts it may be unpractical and most expensive to find a suitable large enough furnace and the Stress-relieving treatment risks to become very costly.

In those cases heating may be performed locally near the welds with a gas flame torch, or using resistance heated blankets or by induction systems heating. (See Videos further down this page).

The local strains that produce high residual stresses can also be eliminated by plastic deformation at room temperature.

Sheet metal, plates and extrusions may be stretched beyond the yield stress to relieve by yielding the differential strains.

In other cases a working operation generating a different residual stress distribution may be superposed.

It will modify and correct the pattern of the original residual stresses to a more favorable equilibrium.

Such may be rolling or shot peening that are known to introduce compressive stresses.

In certain applications, like in the welding of tool steels, it is recommended to peen the weld while it is still hot to reduce the level of dangerous tensile stresses.

Manually applied mechanical systems are available, intended to peen welds in large constructions to reduce the residual stresses.
See Ultrasonic Impact Treatment at:
http://en.wikipedia.org/wiki/Ultrasonic_impact_treatment

Another form of mechanical Stress-relieving is called VSR or Vibratory Stress Relieving.

The induced vibration level, to be effective, should be in the lower, or sub-harmonic, portion of the harmonic curve, just before the amplitude quickly rises and reaches the part's natural resonance.

It is at the right frequency that the vibration has the greatest dampening effect, at which point it removes the stresses caused by welding.

A list of downloadable Articles on Vibratory Stress Relief is available at
http://www.vsrtechnology.net/resources/library-articles/

See an article showing typical applications of induction heating for Stress-relieving:

Welders Turn To Induction Heating For Preheating, Stress-Relieving
http://www.millerwelds.com/resources/articles/index?page=articles24.html

The Videos hereafter are proposed for illustration only.
No endorsement or recommendation intended.

Customers should check on their own the suitability of the solutions to their purposes and needs.

On this subject, an article introducing ASM Handbook Volume 4C on Induction Heating and Heat Treatment was published (2) in Issue 136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.

An Article on New issue of HTPro was published (7) in Issue 165 of Practical Welding Letter for May 2017.
Click on PWL#165.

Watch the following Videos:

1.-Heat Treatment Service Provider
2.- Induction Heating

1)- http://www.youtube.com/watch?v=4OS8_rCuu2w
2)- http://www.youtube.com/watch?v=DiOtM_pt7cY

A website page on Residual Stress is available to interested readers.

* * *

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