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can't get 4140 hard


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Posted · Report post


Sorry John, not my experience at all. Most references warn "NEVER water quench 4140". Water quenching is usually avoided in sections smaller that 2". In tools up to 1 to 1-1/2" I've never had trouble getting low to mid 50's Rc as-quenched and 46-48 Rc after 400 draw.

Grant


Grant,
I'll be happy to send you some "beater" hammer 4140 heads that did not harden in oil despite being heated to 1550 deg in a heat treat oven and quenched in 150 deg oil. Water is where it's at if you want to get 4140 hard. I don't care what most references say. I'm going with reality.
I made a number of hammer heads using the "referenced" method, only to find they were soft. As a last resort, I went to a water quench, and 4140 finally got hard...w/no cracks.
I feel that one has to go with what works for you. I'll take h2o.

JE

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Posted · Report post

Just as a comparison we harden tons and I mean tons of 4140 over 3" thickness in water from about 870 deg C and retemper to 575 deg C and can always reliably get a hardness of 302 hardness brinell. I reckon it's not 4140. For the app you describe I would tend to find 4140 a little soft, I'd use 5160 (spring) harden from 850 deg C in oil, temper by heating till the oil from quenching flashes into flame.

Phil

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Posted · Report post

Over 2", I agree completely with water. Even then it won't get very hard in the center.

As I was trying to tell John, I agree oil won't do it when you get above 1-1/2 or 2 inch. Which I believe is right where he's at unless he's making awful small hammers. Plus austenizing at 1600º as Phil is doing in important too.

I've made thousands and thousands of tools from 4140 and never have trouble getting full hardness in oil. But just like the original question in this thread I'm talking less than 1" thickness.

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Posted · Report post

Yeh I agree for 2" thickness or less I would recomend oil. Years ago I spent ages on making some fully forged axe heads from 4140, had a bright idea to harden them in water, still got 25 axe heads here years later, all with cracks running all over the blades. xxxx. I'll find a use for them one day. We also make a forged ring from 4140 normally 60 off at a time 185mm od x 120mm id x 75mm thick (or long depend on how you want to call it) gives about a 32.5mm wall thickness, we always harden them in oil from 870 deg C and temper to 580 deg C holding for a couple of hours, I can almost guarantee to get a minimum of 277HB every time.

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Posted · Report post

Try the salt based "superquench" formula. Even mild steel (A36) will get in the 35-40 range with a fast quench.

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Posted · Report post

I don't think it would matter if it was decarb steel, as he has heated it and forged it, hence you have reworked the surface and likely lost the decarb layer. I think he just doesn't have 4140, just mild steel. Also, there is no need at all to soak a 3/4" thick piece of steel in the forge at that high a temperature. Here is the data for that material.

Thermal Treatments. Annealing: 1550F (840C), hold 2 hours, slow cool 50F(30C)/hr. max. to 1200F (650C), then air or furnace cool. Hardness BHN 185/200.

Stress Relieving:

Annealed Material: 1100-1300F (595-740C), hold 2 hrs, air cool.

Hardened Material: 50-100F (30-55C) below last tempering temperature, hold 2 hrs, air cool.

Straightening: Best done warm 400-800F (205-425C)

Hardening: (Atmosphere or Vacuum Furnace)

Preheat: 1250-1300F (675-705C), equalize

High Heat: 1550-1600F (840-870C), soak 10 to 30 minutes. For vacuum hardening, use the high side of the high heat range and soak times.

Quench: Oil quench to hand warm, 150F (650C). Temper immediately. Water quenching from 1550F (840C) may be used for simple shapes and larger sections. Note vacuum furnaces must have oil quench capability to achieve comparable results.

Temper: Tempering at 400-1200F (205-650C) for 1 hour per inch (25mm) of thickness at temperature is recommended (2 hrs min). Air cool to room temperature.

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Posted · Report post

There are already many good responses to your original question, but I thought I'd share an URL that is loaded with good info about many steels, including heat treat procedures.

www.suppliersonline.com

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