4140 charecteristics and welding 2

[shanelang]

today had to weld 70 dia shaft into 16 mm thick flange preped back weld only it is a mounting pin for mine service tanker.
we preheat to 100 c then two run mig weld and cool in lime.

my questions are -why preheatand why cool in lime my understanding so far is ,moisture in metal, difference in thickness of two components, cooling slow is that for grain growth issues, or for different cooling rates of two coponents due to size/thickness.
4140 cromium /molybdenum steel is it low med high carbon
i am used to stainless steel and matching as close as possible fillers when i ask about mig wire got an answer it is 70 tonne wire i am trying to understand mild steels more relationship off grades and fillers er70s6 i think

any info in regards to these or even more factors would be greatly aprreciated

 

[Metalguy]

Very briefly, what you are trying to avoid is making martensite in the HAZ of the weld. Your preheating temp. is probably too low for 4140. Preheating slows the rate of cooling after welding, and is more important than cooling in lime. Martensite will try to form if the high temp. FCC austenite doesn't have time to form around 1100-1250F.

Depending on the stresses involved, your weld wire may be OK.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[metengr]

Agree with above. You should increase the preheat to 150 deg C.

 

[Metalguy]

Martensite will try to form if the high temp. FCC austenite doesn't have time to form PEARLITE around 1100-1250F.

Sorry, I left out "pearlite" from my first reply. Sure wish there was an edit function.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein