4140 Heat Treat 1

[bryanround]

I'm not incredibly familiar with steel, and less familiar with how high I can heat treat steel before it becomes too brittle. That being said, I have some 4140 steel that I want to heat treat to 180-200 ksi for a loading clevis. From what I understand this is a tool steel and is typically treated up to 300 ksi. I just wanted to confirm that heat treating 4140 up to 200 ksi isn't asking for a miracle or that the heat treat will make the material so brittle it will be useless in a load application.

 

[OGMetEngr]

Your question depends on many things, like thickness of the part, where the strength is needed (throughout or only partially along cross-section), how the part is loaded (steady and slow, impact loaded, etc.) and temperature of loading if impact loaded, if brittle failure vs ductile failure can be tolerated.

For example, a cold-formed 4140 round bar can be hardened to 42 HRC (approx. 194 ksi ultimate tensile) at the center of a 1.75" oil-quenched bar. If water quenching is used a 2.4" bar can be fully hardened to the same strength and hardness at it's center. The hardness and strength will only increase as you move from center to surface. (all from ASM Vol 1)

For either of these bars, due to the high strength and relatively high carbon content, notched impact strength will be low. Tempering will help, but strength will decrease as well.

As with most questions on here, more details will be needed to provide better guidance.

 

[redpicker]

4140 is not a "tool steel". It is a "low-alloy steel". I would not use it in this application. The maximum useable ultimate strength level is around 140 KSI, and that is with very controlled heat treating. With "Commercial Heat Treating", I would not want a loading part made from 4140 to be much over 120 KSI tensile (24 HRC) unless I had tensile and impact testing performed on a sacrificial part from the same HT batch to demonstrate that the HT batch met the design requirements.

rp

 

[TVP]

180-200 ksi ultimate tensile strength is approximately 39-43 HRC. 4140 can be quenched and tempered to this range, provided that the section size is not too great. I have made small clevises out of 4140 for testing purposes, and I used about the same hardness for them.