Preheat for welding 1

[Sam Low]

Hi All,

As per my understanding, preheat is to achieve slow cooling rate and remove all the moisture in the material to avoid the hydrogen induced cracking.
Preheat is required when the thickness of material is more than 32 mm (As per ASME Section VIII Div 1). Since preheat is so important why it is only required for material/welding thickness above 32 mm ?

Is this because the hydrogen molecules will retain in the weld in the thicker material ?

Thank you!

 

[tygerdawg]

(beware my answer: I am not a Weld Engineer) I thought preheating's purpose is to reduce temperature gradients during the weld process. Thus reducing inherent residual stresses and possible cracks in the work piece.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[weldstan]

The thicker the part, the faster the cooling rate and residual welding stress increases with thickness as well. Preheat can slow the cooling rate enough to reduce the potential for martensite formation and allow hydrogen to diffuse from the weld metal and HAZ.

 

[Sam Low]

HI tygerdawg ,
Yes, it is used to reduce the temperature gradient as well as the expansion and contraction rate with residual stress. I was wondering that it should be done for the thinner plate too since it is a good practice.

Hi Weldstan,
Is slow cooling rate the reason of no preheat is required for thinner plate in normal industrial practice?

Thanks in advance!

 

[weldstan]

To a large degree, yes. Cooling rate affects the production of martensite. The slower the cooling rate the better to eliminate martensite production.

 

[Sam Low]

Hi Weldstan,

Thanks a lot for your info!

 

[gtaw]

No one addressed the need to remove the moisture in the material being welded. Likewise, no one addressed the thermal shock the molecules experience when the arc is struck on cold steel!

These wives' tales persist. Two of the answers I often see when the question is: "Why are carbon and low alloy steels preheated before welding?"
1) "To drive moisture out of the steel." My response is that if there was moisture in the steel, there must be some holes in the steel where the moisture resides. I would not want to be in a submarine if there were small holes in the steel. Once submerge, it would unlikely surface. It might take longer, but storage tanks would leak and ships would sink.
2) "To eliminate thermal shock to the steel." If preheat is necessary to prevent thermal shock, why don't we have to preheat all metals before welding?

It is good to see so many people know that preheating carbon and low alloy steels reduces the cooling rate and reduces the tendency to form hard martensite in the HAZ, thus mitigating the problem of delayed cold cracking. High preheat also allows the diffusible hydrogen to escape from the steel as it slowly cools thereby reducing the amount of hydrogen that can lead to delayed cold cracking




Best regards - Al

 

[Tmoose]

I can think of reason for the "To drive moisture out of the steel" to exist.

When a propane or mapp gas flame is first played on a piece of steel, liquid forms on the surface. When the steel gets a little warmer there is no more liquid.
I always figured the liquid was water vapor products of combustion condensing on the cool steel , but I see how the timing of the phenomenon might lead to other interpretations.