purpose of preheating before welding

Strive★Silentl

I have a question to need your help
Q345R that is  similar to S355J2 do not need preheat before welding when the base metal thickness less than 30mm according to PQR and our experience，recently my colleagues，a Frenchman，from Germany headquarters ，tell me that we should preheat befor welding，he said preheating is better，he also tell tack welding is no problem without preheat，it confuse me. In my opinion,preheat can prevent weld from
delayed crack due to quick cooling,so we preheat when we weld more thick plate,I think that tack welding have a much quickly cooling,however he told me you can no preheat before tack welding.
Can you help to explain why he so say?
Thanks in advance!

hejing0412

please read BS EN 1101-1 & 2, BS EN ISO 13916, you will know the preheat requirement.

wuqitao0000

真牛，英文提问题，英文解答。

孙俪

向楼主学习。                  

niat8888

对于一切材料，预热是有好处的，降低温度梯度差别，降低焊接应力。。
预热是有好处，但是不一定必要。。
他说的，不能说错，但是不一定有必要。

Strive★Silentl

hejing0412 发表于 2016-9-10 23:47
please read BS EN 1101-1 & 2, BS EN ISO 13916, you will know the preheat requirement.

the three standard describe：
1.EN1011-1 require that tack welding and welding have same preheat requirement
2.ISO13916 give operational requirement of preheat ，including preheat area and so on
3.EN1011-2 give some suggestion and method which decide preheat  temperature

tcctianzw

非常感谢资料的共享，，，，，

hejing0412

If your job follow U.S CODE, maybe AWS D1.1 charpter 3 preqaulify preheat tempature also can be followed.

If you want save time and cost to avoid preheat, you have to qualify that:

PQR withourt preheat and thickness equal to the product and the stress also same, then, your hardness test value lower than the requirment at BS EN ISO 15614, your owner should be accept.

So, hardness test on the product also help, normally HAZ hardness value will be highest area.

Such as AH 36 to FH 36, 350 HV10 was required normally, for offshore product hardness lower than 325 HV10 much better.

hejing0412

You should follow the standard system your Spec. mensioned.

BS , EN, ISO, ASME, JIS, AS , AWS, something like this.