a fabrication job has to be reheated, why should no rework be carried out after heat treatment?

Question

in terms of shipbuilding or fabricationa dn welding

Answer 1

The technique is called Post Weld Heat Treatment (PWHT for short).

Adam the Engineer has got it wrong (sorry Adam) and BikerwookieJim has got it almost right.

The only flaw in his answer is that you CAN rework a piece after PWHT, but you have to repeat the PWHT after the rework. The cost of doing PWHT is high and can be very high for large pieces (like in ship building), so you try to do it only once.

Pieces that have undergone PWHT are usually marked as such so that nobody does any rework which (as BikerwookieJim said) would leave a week point prone to failure.

Answer 2

As you know, a weld messes around with the molecules of the metal thus changing its strength and characteristics. sometimes this can make the areas around the weld weaker.

reheating or "heat treatment" heats all the metal up to a temperature that "realigns" all the molecules in the whole piece, making the whole piece uniform in strength and characteristic.

i.e the whole piece is as Strong as one solid piece, not two welded together pieces.

You cannot rework a reheated piece because the rework will be weaker than the original piece and could fail with lethal consequences.

Also the piece that has been heat treated can only stand so much, another rework and reheat may seriously affect the molecular structure of the piece making the whole piece weak.

hope that helps.

Answer 3

Others have mentioned changes in molecular structure of the metal. It may be easier to think in terms of changes in the crystaline structure of the metal which may also be involved. The length of time and exact high temperature at which steel is maintained and then the rate (fast or slow) that the steel is cooled down changes crystal structure such as size of grains and concentration of carbon, etc in the grain boundaries. Proper heat treatment enhances desirable properties (hardness or ductility) while avoiding unwanted properties (brittleness?) depending on the alloying agents in the particular metal. Rework after heat treament may lose the useful properties that have been produced and they may have to be recovered by additional heat treatment (when that is practicable). Hammering on steel (cold working) may cause slippage of lattice surfaces within grains producing residual stress and strain that can be reduced by heat treatment. Welding results in "heat affected zones" that may not have the desired properties required of the steel until heat treatment. Uneven or poorly controlled cooling of a weld joint may cause distortion and leave stress levels in the parts joined that may need to be stress relieved by heat treatment. Each industry (shipbuilding?) usually has set standards and techniques for welding and heat treatment that avoid past problems by reflecting all lessons learned. Hope that helps a little.

Answer 4

When steel is heated and then re-cooled, the molecular structure changes to a weaker state. If you mess with it after you've already re-heatede it, you could have failure at less-than-expected limits.

Answer 5

because it will input stresses into the structure... and will create a weak point where the heat was applied.and strees fractures will probably occur there. the whole thing will need to be reheattreated...afterwards cost? high