Flame cut 4145

[bpeirson]

We have been requested to flame cut an odd shape from a 4" thick 4145 plate for subsequent machining. What are the preheat and post heat requirements for this kind of operation?

Brad

 

[Metalguy]

While 4145 isn't a deep-hardening steel, you could easily end up with a hard untempered martensite layer just under the cut surfaces. Assuming no preheat, the quench rate from flame cutting can be pretty high.

Preheating can help, but the temperature where pearlite forms is way up ~11-1200 F, and if that layer cools fast enough through that temp. range, martensite and/or bainite could form down ~500F.

If the cut surface is to be machined away, there shouldn't be a problem under the hard layer--if it in fact develops.

"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

 

[arunmrao]

Flame cutting,might leave rough and uneven edges and a layer which has been affected by heat. Removing this layer during machining,might lead to lot of chatter and vibration. Perhaps you could grind this layer off,before machining.

 

[swall]

If you are talking small quantities, it might more sense to just temper (1000-1200F)after flame cutting and not worry about pre-heat.

 

[redpicker]

While 4145 isn't a deep-hardening steel,

I have to disagree with this. 4145 is one of the deepest hardening steels of the standard AISI low-alloy steels. Failure to preheat prior to flame cutting is sure to result in cracking, unless the parts are so small that the entire piece heats up to cutting temperature, and even those will crack if they aren't slow-cooled.


I would recommend a preheat of at least 600F, with a slow cool from cutting temperatures (wrap in ceramic blanket, or something similar) followed by a post-heat of at least 1000F; if the flame-cut surfaces are to be subsequently machined, a 1250F post heat would be in order.

rp

 

[Metalguy]

Compare 4145 with something like 4340 and you'll see what deep hardening is. Some heats of 4145, lean in the hardenability elements, don't transform well at all.

"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

 

[salmon2]

No argument please because there is no clear line between deep hardening and non-deep hardening steels. Depending on specific product's dimension, one chemistry can be viewed totally differently.

 

[dinjin]

What is the plate being used for and why was 4145 chosen initially?

 

[redpicker]

Not wanting to argue, just to defend my point.

Compare 4145 with something like 4340 and you'll see what deep hardening is. Some heats of 4145, lean in the hardenability elements, don't transform well at all.

OK. I compared the H-band of 4145H with that of 4340H. The h-band minimum for 4340H just about bisects the h-band for 4145H perfectly. It is entirely possible for some heats of 4145 to have deeper hardenability than 4340. So, even if you want to draw you line at "must exceed the hardenability of 4340 to be considered deep hardening" (which is a ridiculously high point to draw the line), you would need to consider 4145 to be deep hardening, particularly if you were setting up procedures for welding or flame cutting and the only information you had was the chemistry met 4145.


Just because a steel does not possess the same hardenability as 4340 does not mean is is not deep hardening. 4340 has the well-earned reputation of being the deepest hardening of the AISI low-alloy steels, but that does not mean it is the only one.

rp

 

[Metalguy]

Let's just say that all legal heats of 4340 are "deep hardening", while some heats of 4145 are too.

But some heats of 4145 are of "medium hardenability", but no heats of 4340 are. Keeping in mind that Cr and Mo are expensive, a steel purchaser may get a lean heat of 4145 if he doesn't know how to specify it.

"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