Pre heating

[nabeel3]

I am new to welding. I have a doubt, if any can help me to understand, will be thankful. I understand the reason for Preheating in welding as it slows down the cooling rate and prevent the formation of martinsite. But I am not able to understand how pre heating will slows the cooling. There is welding of A150 rail where pre heat temp is around 350 Degree. Process is Smaw. How a metal heated to 350,degree slows down cooling rate. Maybe the question is too silly, if then I am sorry.

 

[CWEng]

Preheated material doesn't have as much temperature differential between the hot weld and the cooler base metal, so the cooler base metal can't "suck" the heat out of the weld as fast when it's preheated. For steel, the critical range for cooling, in which microstructure transformation takes place for the most part is 500C to 800C, so slowing the cooling in that range has the most effect.

 

[nabeel3]

So, it's to influence the temperature differential. Thank you

 

[Dhurjati Sen]

Dear nabeel3,

Preheating prevents cracking of the HAZ / weld metal by reducing the temperature differential between the weld metal and the base metal. You would find thicker carbon steels and alloy steel (all thickness) are normally pre-heated before welding.

Regards.

DHURJATI SEN
Kolkata, India

 

[weldstan]

Effective preheating also slows the cooling rate in the lower temperature ranges (600 F to 400F to permit greater time for hydrogen to diffuse from the deposited weld metal and HAZ.