Hardness and Toughness 1

[augeoeng]

Is there a relationship between hardness and toughness in 4140 steel?
Many thanks in advance,

 

[Metalguy]

Yes. 4140 is almost always used in a heat treated condition, and the hardness/toughness tradeoff is determined by the tempering temp. and time (to a lesser extent)--assuming the hardening step was performed correctly for the particular heat and thickness.

A tempering temp. of 1,000 F min. gives a very good balance.

"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

 

[redpicker]

I agree with what Metalguy has stated, but feel the need to emphasize

assuming the hardening step was performed correctly for the particular heat and thickness

The hardness test provides evidence that the hardening step was performed, but it cannot, by itself, indicate that the hardening operation was performed correctly.

Also, there are other factors that can come into play, so one must be careful about any hardness:toughenss relationships one wants to make. In short, if toughness is important, you should probably perform a test to insure the steel has adequate toughness, particular with a steel that has been hardened by quench and tempering, and particularly with 4140.

rp

 

[arunmrao]

On the first instance,yes. hardness and toughness might appear to be connected in 4140. But if you look at it closely and start qualifying the properties,differences will emerge. For the same hardness, ,one can end up having different microstructures,thereby different toughness values. It is best advised,that you conduct a separate toughness evalution test and not rely on hardness data,which could be misleading.

 

[TemperedMartensite]

One thing to watch about 4140 is tempering in the "blue brittleness" range. I am working from memory, but I think toughness is severely impacted (no pun intended) if you temper 500-800F.