Heat Treating 4150 Stee l- Help please

[bes5318]

Hey all, brand new to the forum with a question that I hope hasn't been beaten to death.

Short version: How do I properly harden and temper 4150 Steel?

Long version: I'm getting married in May and me and my groomsmen are all in the Army. For their groomsman gifts I'm taking a bunch of old burned up M249 barrels and forging them into combat knives. I'll detail my process below. I've been making knives for 7 years now, but largely backyard stuff for fun; rebar, RR spikes, lawnmower blades, etc. I've only recently been getting into the "good" steels and I've had a lot of cracked blades.

US military barrels are made almost exclusively with 4150 Steel (or so I've read). I tried quenching a test piece in oil, but it didn't harden. Then water, still didn't harden. I think I'm going going to try next with a cold brine to really get a fast quench, coming down from 1500. I've only got so many of these blades and I'd really like to not crack any. Any insight as to what I'm doing right or wrong would be greatly appreciated.

 

[bes5318]

Images as promised:
Original barrels- cut of the barrel extension, front sight/gas block/ and flash hider.

Cut barrels lengthwise to avoid the issue of forge welding the bore, and making the blanks a little easier to hammer out.

Hammered out into blanks, drew with the stencil and did stock removal

Cleaned the blades up with an Angle grinder

Now I'm doing a lot of file work before the rest of the heat treat. Intending to paint them black with a heavy duty epoxy paint and do a paracord handle wrap.

 

[Stacy E. Apelt - Bladesmith]

I'll check my books when I get home, but my thoughts are to harden them as if they were 5160.

 

[Bo T]

You could try hammering out a few test coupons from your cut offs. Increase the temperature by 50 f each time above 1500 f, and test hardness right after quench to see if the coupon will easily break. I read somewhere that the alloy might be slightly different to handle the higher sustained barrel temperature.

 

[Drew Riley]

The following is from http://www.steelforge.com/alloy-steel4150/

HEAT TREATMENT
The alloy is heat treated to render it suitable for machining, and to meet the mechanical property ranges specified for its particular applications.

ANNEALING
Annealing of 4150 forgings may be carried out by transferring the part straight from the forging operation to a furnace held at a suitable temperature, around 1500 º F (815 º C), for annealing, holding for a suitable time then furnace cooling, forming a structure suitable for machining. This kind of treatment is best used for parts with simple shapes. If some areas of a forging will finish much colder than others then a uniform structure will not be obtained, in which case a spheroidizing anneal at around 1380 º F (750 º C) might be used. It is safe to say that experience alone will decide the best type of annealing treatment to be used prior to machining.

NORMALIZING 4150 ALLOY STEEL
Normalizing may be defined as heating a steel to a temperature above the ferrite to austenite transformation range and then cooling in air to a temperature well below this transformation range. This treatment may be carried out on forged or rolled products as a conditioning treatment prior to final heat treatment. Normalizing also serves to refine the structure of forgings that might have cooled non-uniformly from their forging operation. The nominal normalizing temperature range for 4150 grade is 1600 to 1700 º F (870 to 925 º C). followed by air cooling. In fact when forgings are normalized before, say, carburizing or hardening and tempering, the upper range of normalizing temperatures is used. When normalizing is the final heat treatment, the lower temperature range is used.

HARDENING
This heat treatment results in the formation of martensite after quenching and hence an increase in strength and hardness together with a significant loss of ductility. The steel should be austenitized at 1500 to 1550 º F (815 to 845 º C), the actual temperature being a function of chemical composition within the allowed range, section size and cooling method. Smaller sections of 4150 might be quenched in oil, heavier sections in water.

TEMPERING
Tempering is carried out to relieve stresses from the hardening process, but primarily to obtain the required mechanical properties. The actual tempering temperature will be chosen to meet the required properties, and in many cases will be a matter of trial and error. This alloy steel should not be tempered between 400-790 º F (200-420 º C) to prevent embrittlement.

This grade may also be hardened by nitriding or flame or induction hardening.

Maybe try warming your oil quench just a bit?