4140 steel harden/temper suggestions... 1

[Crzyconvdude]

First post here thanks in advance for any assistance available.

2 3/16" dia round 4140.

I am trying to achieve HRC Value of 35-40 through hardened.
using the lower range of HRC-35, my numbers put a piece like this at:
158 ksi ultimate
140 ksi yield

and this required for my application.

Questions:
1. What heat treat/temper/quench temps and methods would be suggested to achieve these values.
2. I understand that I will get some drift in the hardness from center to surface...but how much is normal?

We have a sample that a client has had tested (from the treating source)that is only 40-38 for the first 3/64th then bam down to 24-27 for the next half inch. heat treat guy claims this "IS NORMAL AND EXPECTED". I have asked for a complete mechanical properties test to determine Ultimate Strength. if I run some numbers converted from say HRC25 this would be about 125 ksi (low end of acceptable) but, I would prefer to have real numbers.

Very happy to find this group!

 

[Lyrl]

At 2 3/16" diameter and the relatively low hardness (compared to max achievable) of 35-40 HRC I would expect 4140 material to be fairly consistent in hardness from surface to core - anything above 3 HRC difference would surprise me.

Typical heat treat would be 1550°F for at least 2 hours - depending on how much mass and how packed together the load is additional hours may be required - then oil quench. Atmosphere carbon potential needs to be well controlled to prevent carburizing or decarburizing. Surface hardness after quench should be within a point or two of 50 HRC. More than 53-54 HRC would indicate the parts were carburized. Less than 48 HRC could be decarburization, insufficient time at temperature, or a problem with the quench (oil quality, agitation, initial oil temperature, size of the tank relative to the mass being quenched, etc.)

Assuming quench hardness meets the target, temper would be about 950°F for at least 2 and a half hours. Again, a very high mass or a tightly packed load would require more time.

Your results might have been caused by wrong material (such as 1045), insufficient soak time at 1550°F (surface got up to temperature before quench but core didn't), or unintended carburization (so they had to temper very high to get the surface to come in, which over-softened the uncarburized material in the core).

 

[redpicker]

... is only 40-38 for the first 3/64th then bam down to 24-27 for the next half inch. heat treat guy claims this "IS NORMAL AND EXPECTED".

I would agree; although I would consider this to be at the extreme low end for normal. What I mean is that if you just send the material to a commercial heat treater and only specify a final surface hardness, don't be surprised at such a result. Unfortunately, the heat treat response of 4140 can vary widely if the Q&T condition and careful process control and routine testing is required to maintain quality.

If you take a look at Pratical Data for Metallurgists, Page 64 has the jominy curves for 4140H. According to it, you should expect the mid-radius of a 2" round to be approximately 55-42 HRC in the as quenched condition when given an oil quench. You may also be interested in the cooling correlations shown on Page 56 which suggest the surface should cool equivalent to J6 and the center at J10 with a good oil quench so if you know the Jominy curve for the material you are using, you can predict the hardness at various depths.

As for the heat treatment, you need to consider the heat treat condition the material is in originally. If you have an annealed structure, you might consider a normalizing treatment (1700F for two hours, air cool). I don't know what application you are using this for, but if it is going to be subjected to either shock loading or fatigue, I would recommend at least 1000F temper (personally, I would insist on 1050F minimum). Below this, and you are likely to be somewhat brittle and catastrophic failure is possible if the material is used in tension. Of course, if it is a critical application, then tensile and impact testing should be performed on each heat after heat treatment.

 

[CoryPad]

Here are example hardness variation curves for 3" OD 4140 rods:

Source:

http://baybridgeinfo.org/sites/defa...d Failure Fracture Analysis Report_5.7.13.pdf

 

[Crzyconvdude]

The order asked for was "4140 through hardened to HRC 35-40". Is this an incorrect callout? Do you have suggestions for better clarifying this next time?

To obtain the UTS needed for the application + SF, I used the lower end HRC35 to get my 158ksi value and the 5 points range would be gravy.

under normal running I max out at 37.2 ksi giving me a 4.2 safety factor (2.5 required) at under 200k cycles over 30 years.

Jam condition 138.6 ksi or 1.1sf protected with safety cables.<---this is the one I am worried about.

My hope is that the mechanical properties test comes back with a final UTS value that will be acceptable for the application but I doubt it. 500 of these were not cheap and I don't like the thought of trying to go back and "correct" something like this with a re-bake.

Thanks for your quick responses guys

 

[metengr]

The concern with your stated hardness range is two fold with 4140 steel - lowered toughness and potential for hydrogen embrittlement with hardness near 40 HRC scale. You really need to evaluate the service environment to determine if the upper end hardness creates a risk of failure in service.

My preference would be 30-35 HRC scale hardness where you have the blend of toughness and less risk of hydrogen embrittlement in service using 4140 steel. If this does not work, I would look at re-design to reduce loads to accommodate a lower strength range.