The following tips assume you have an existing piece of shafting in 4140 or 4150 alloy steel, that is presently at a through hardness of approx. 30RC. And, that you require an increased hardness of approximately high 40’s to low 50’s RC.

PREFERRED METHOD: Long cycle anneal
Anneal at 1525F, one hour per inch of greatest cross section.
Cool in a furnace to 800F, at a rate of about 20F per/hour.
Re-heat to 1525F, one hour per inch of greatest cross section.
NOTE: In the event you have to move the material a significant distance between the heat temp and the oil quench, take precautions not to let the temperature drop below 1525F between the heat and quench. Cool in oil until smoking but no flame, approximately 250F to 300F.

NOTE: Alternate is a “sub-critical anneal”. Heat the bar to 1350F – 1400F, hold for one hour per inch of greatest cross section, atmosphere cool (in a protected atmosphere) to 350F. If the carbon is closer to .50, temper at approx. 700F to 800F for three hours per inch of greatest cross section (even for small diameters) should yield about 50 to 54RC. If carbon is closer to .40, lower the temp to approximately 400F to 600F.

NOTE: These are intended to be, and should be taken to be, suggestions for consideration, not instructions. This is not exact. If the material is too hard, repeat the cycle and try a higher tempering temperature. Repeated thermal cycling is not detrimental to the material, provided excessive temperatures are not encountered. If you do have to try several times, you may develop significant surface de-carb (powdery surface coat), which you will have to remove if you are not getting the results expected. CAUTION: Do not overlook removing all decarb in the area you will be checking hardness, no matter if using Rockwell “RC” or Brinell (bhn) equipment. Just grinding to “bright metal” does not mean you have removed all decarb. If your tests indicate you are only getting 18RC or lower, you are probably still into decarb. Even in an annealed condition those alloys would likely register a higher hardness than that.

Simply reaching for a higher hardness is not always the answer. If you try to go too high, brittleness may become a factor. In shaft applications where 28 to 32RC are deemed to be insufficient, you may want to try 32RC to 38Rc. Years ago, I was advised by an old friend; “With 4140 and 4150, “funny things begin to occur when you exceed 40RC (they tend to spit carbides). Don’t ask me what that means. I just got a strong negative visual and have been happy to follow that little gem for many years. (Real hard things can shatter like glass).

The above “tips” are just that; TIPS, as in “Hey, my brother in law’s uncle’s wife tried this once.” Plus, they are reduced to the simplest form. These basically relate to shaft stock and do not take into account a host of variables that may affect your safety and results. Part configuration, cross sectional differences, experience, etc. In all instances it is our recommendation that you utilize experienced thermal treatment personnel familiar with thermal conditioning alloys, who make use of all appropriate safety gear.


-Howard Thomas, April 8th, 2022