Hi all, i shall be greatly thankful to all the knowledgeable authorities here on this forum who shed some light on welding distortions while considering me a novice. I have little understanding of it, like it is caused by non uniform heating which makes the different portions of base material to expand depending upon the temperature exposure. Areas near to weld is subjected to high temperature so it expands more, area farther from weld expands relatively less, so high temperature area wants a tension but cooler rest of the base metal resists it. This causes compression stresses in hot portions and tensile in cooler. After heating is gone, reverse happens, hotter wants to contract( it wants compression) but cooler base metal resists it so this causes reversals of stresses.

I know all this but confusion comes when yield strength exceeding is brought into discussion and its effect on distortion.

Thanx all in advance for spending your precious time in reading the post and giving feedback.

Regards

Waqas Malik

Hi
The key thing is in simple terms about distortion is you are putting a lot of heat is a small place. When this happens the heat needs to go somewhere but if the base or parent material has low thermal conductivity (does not want to let heat go anywhere) BUT high thermal expansion (moves a lot) then things start getting stressed. Buckling, moving the metal just does not like the heat in a small area as it can't cope with the stress so so it has to bend and that is what you see in the distortion of the metal.
Hope this helps
cheers
mike

As the base metal is heated in a localized area, it expands. However, the surrounding base metal resists that expansion, so the heated region exerts a stress on the surrounding colder base metal. When the stress introduced by the delta T exceeds the yield point of the base metal, permanent deformation results.

The delta T required is not as high as one might suspect. A simple equation says it all:

E = (stress)/(strain) = (yield strength)/(delta T)(Coefficient of expansion)

delta T = (yield strength)/(E)(Ce)

Example:

delta T = (40 ksi) / 29x10^6)(6.5 X 10^-6) = 212 degrees F

That would indicate a delta T of just a little above 212 degrees is sufficient to cause permanent distortion.

Assuming it is 75 degrees (ambient temperature), heating a localized spot to about 300 degrees F is sufficient to produce permanent distortion.

While the YS of the base metal decreases with temperature, as would the Modulus of Elasticity, the temperature range of interest is relative small, so the values can be considered constant and provides a reasonable approximation.

Good luck

Al

I can't add to the educational example given by Al, but let me use a practical example that often turns the light on for those who are struggling with the practical application of heat and distortion:

Take a 2" diameter carbon steel cold rolled round bar and cut it exactly 6" long (reasonably close to 2" but measure it with a micrometer or dial caliper for research purposes, same with the length- use a dial caliper so you can get accurate results).  Put it in a vise so that both ends are under pressure from the jaws of the vise, not the sides.  Put heat on it, say, 500°F for the purposes of our example (you can do less, more, and change the times held at heat just for your own curiosity).  After holding at heat for a short time take the torch away and let it cool.  More than likely, it will fall out of the vise.

Why?  As it was heated under restraint and the bar wanted to expand we restricted that expansion so it could only go one direction, sideways.  Thus, the bar got fatter while at the same time stayed the same length.  But, as it cooled down and shrunk in all directions the length became shorter than it was originally.  So, when returned to room temperature the bar is now over 2" in diameter but thinner than it would have been while at maximum temperature (you can really see this if you take the bar clear up to a bright yellow/orange in color) and it is also shorter because all the growth was forced to the sides and when it cooled it still had to shrink.

You have the same volume of material as when you started but it is distributed differently. 

Now, picture that but without the vise.  The other material on your member is providing the restraint unless you were to put the whole thing in an oven and heat it evenly in it's entire volume.  So as you heat one area and the material expands things have to move and give and how much depends upon amount of heat, time at heat, size of material, grade of material and many other factors. 

Hopefully my feeble example will help you picture what is happening when added to the expert explanations and equations given by Al and others.

He Is In Control, Have a Great Day,  Brent

Thanx all especially 803056 and Welderbrent for excellent examples to make things easy for novice.

803056

When the stress exceeds the yield strength of the material then permanent deformation occurs. Whether this happens while heating( restrained heating) or cooling ( restrained shrinkage)?

Thank you.