EFFECT OF PREHEATING ON MECHANCIAL PROPERTIES OF MATERIAL T91

Weld preheating in P91 steel, as in many others, is applied to reduce the cooling rate after welding (the temperature gradient is lower) and thus trying to minimize embrittlement of HAZ by martensite formation.

Preheating by itself, usually performed at relatively low temperature doesn't affect mechanical properties.

when we are preheating the T 91 one material & other T 91 material with nil preheating then both the pieces are given to PWHT.

what will the effects on mechanical properties. pwht will changes the effects on both material and shows same effect on mechanical properties

with regards

ganesh

When you weld metals that have a high creep cracking problem, preheat is the best preventive.

When you weld on P91 and don't use a good preheat there is a very narrow heat affected zone, there are extreme stresses developed across this heat affected zone that are not completely removed with PWHT.

Preheat helps to make this heat affected zonal area wider and lets the stresses build or rather dissipate over a larger area. There are the same stresses but now there are not as concentrated in this very narrow band.

P91!

p91 Has been my bane in the past, so keep the comments coming. Preheat, inprocess heat, and postweld heat are all important here. And I have found cleanliness is of more importance than some other alloys.

When using P91 steel the preheat is as important as the PWHT.

It is also important to do a hydrogen bake-out and then cool slowly to ambient before PWHT.

PWHT must be performed in less than ten days after welding.

Of coarse you must have a 100% oxygen free purge.

AND a good, qualified welder.

Amen and more to all of those comments. Too bad that a lot of people take shortcuts. I remember seeing Pipefitters trying to weld at a dirty and dusty field site when the wind was blowing creating a wind chill of minus 0 Degrees F. A Most weld continued to fail as they were using a flame thrower for heat instead of using a blanket. Interesting, this Gas Turbine Powerplant never got commissioned.

when we are preheating the T 91 one material & other T 91 material with nil preheating then both the pieces are given to PWHT.

what will the effects on mechanical properties. pwht will changes the effects on both material and shows same effect on mechanical properties

with regards

ganesh

PREHEATING: As its name says is not a POSTweld heat treating.

It's necessary as I've said in welding P91 material to lowering the cooling rate as in many weld-hardenable steels, but it's done at relatively low temperatures, so the preheating itself, even necessary, doesn't change the structure of P91 which had been tempered at more than 700ºC and therefore, doesn't change the mechanical properties.

Postweld heat treating of P91 is independent of the preheating or interpass temperatures or the convenience of a soaking time. It's a Code requirement technically based on the nature of the material.