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Blacksmithing and metalworking questions answered.





metal work, blacksmithing, steel, harding, teempering, 4140, Rockwell, Brinell, metallurgy



Heat Treating AISI / SAE 4140 Steel

Hardening and tempering anvil and die steel


I'm looking for some advice on an anvil I'm constructing. I have used an forklift fork for the construction. I am at a point when I am trying to do some type of hardening to the face. The section that I am attempting to harden is approximately 2" x 5" x 18". It was cut from the upright portion of the fork and after heat treating will be welded onto the the tapered flat portion which will protrude from the front with a small section of a horn. My question concerns the hardening heat and quench. I am not certain as to the type of steel, but after doing some research, I am proceeding under the assumption I am working with 4140. For the sake of ease, will any attempt to air harden this steel work? If I were to get a good soak time and then pull the peice and cool the face with an air compressor, will the sheer mass of the piece prevent me from getting a good hard face? I know that this steel is best oil quenched, but I believe it would be dangerous for me to try and submerge that much steel in a drum full of oil. I really prefer not to use hardfacing electrodes, but will if I must. Is it possible that carburizing this peice would be a better option for me?
Don - Wednesday, 03/23/11 20:27:15 EDT
Don, Normally 4140 is an oil quench steel. In a section this large you cannot blow enough air to make difference.

You might get away with a water mist spray.

Carburizing also requires a quench to be hard and is VERY shallow, insufficient for an anvil.

4140 is pretty tough and those forks were already heat treated. I would use it as-is.
- guru - Wednesday, 03/23/11 21:21:32 EDT
4140 and oil quench It depends on the size. I spent about 3 years running the quench and temper line as the HT Metallurgist at Crucible Steel in Midland, PA. One of our bread and butter items was quenched and tempered 4140 to ASTM bolting specification sizes from 1/2 " round to 8" round, minimum tempering temperature was 1100 F. 2" round and under we almost always oil quenched (Sometimes with 2", if the Cr and Mo content were low we'd water quench.) Any diameter over 2", we water quenched in an extremely well agitated water quench that was coupled with a heat exchanger to maintain water temp. In the summer, we struggled to keep it below 100 F, which was where we needed it to be. The oil quench was also well agitated, and was heated to maintain a relatively constant temperature.
- Gavainh - Thursday, 03/24/11 22:10:58 EDT

Harden 4140 at 1550-1600°F Oil quench


4140 Power Hammer Dies:
I asked this question over on iforgeiron and was surprised with the answer, so I wanted to double check. I'm ready to heat treat the main flat dies for my homebrewed power hammer. The ram weight is 88 lbs, and the dies are 3" square 4140. After hardening (thanks for the timely tip on quench medium on large 4140, Gavainh!), where do I need to draw the temper to? I was thinking in the 500-600 degree range, but the answer I got was 350-400. I don't want the dies deforming during use, but I also don't want them cracking or chipping and sending out high-speed shrapnel.
- Stormcrow - Friday, 03/25/11 02:57:58 EDT
Hammer Die Hardness:
Stormcrow, the steel used and the heat treat are somewhat shape dependent on power hammer dies.

1) 4140 is an OK die steel but is not recommended for radical shaped dies such as narrow fullering, crown and so on.

2) Fully hardened 4140 ranges from 54 to 59 HRC. But it should be tempered for any heavy use.

3) Tempering recommendations from the ASM heat treaters guide for 4140-4142 is a minimum of 400°F. This leaves near full hardness. The following table is extrapolated from a hardness graph given in Brinell Hardness with nearest points taken from a conversion table for Rockwell hardness. Tensile values are generic, not specifically for 4140. Note that conversion tables rarely agree on Brinell to Rockwell.

SAE 4140 Steel Tempering
Tempering
Temperature
Hardness
Brinell
Hardness
Rockwell
Tensile
400°F 200°C 514 HB 55 HRc 297 KSI
500°F 260°C 477 HB 50 HRc 243 KSI
550°F 290°C 461 HB 48 HRc 235 KSI
600°F 320°C 444 HB 47 HRc 225 KSI
650°F 340°C 429 HB 46 HRc 217 KSI
700°F 370°C 415 HB 44 HRc 210 KSI
800°F 430°C 363 HB 39 HRc 182 KSI
900°F 480°C 331 HB 36 HRc 166 KSI
1000°F 540°C 293 HB 31 HRc 145 KSI
Data extrapolated from a hardness graph given in Brinell with nearest points taken from a conversion table for Rockwell hardness. Temperature conversion to Celcius rounded to nearest 10°. Tensile not specifically for 4140. All data +/- 10%

Die Steels:

The ASM Handbook on forging recommends 6G and 6F2 (??) steels for large dies and 4150 for small dies (4370 for higher carbon dies). In this case "small" is probably 300 pounds or less. . . Small die hardness in 4150 steel is recommended at only 277 to 321 BHn (HB = Bhn).

In our small blacksmith shops with hammer sizes of 50 to 150 pounds and dies of only about 5 pounds they are generally harder. All those fancy relatively aggressive dies and dies used for cold work that Big BLU sells are made of carefully heat treated S7. Early Big BLU dies were 4140, Bull and Phoenix dies are H13, Chinese hammers C45 (1045?). S7's carbon content is 0.55% and is a Chrome Vanadium steel. Very tough and wear resistant.

Conclusion:

So, your initial thought was right (a 500 to 600°F temper resulting in a 45 to 50 HRc).

Note that Rockwell 44-45 is just barely in the machinable range and that a grade of H13 is sold in this condition to avoid heat treatment after machining. This saves a lot of trouble in small shops. I have seen corner impact chips in this steel but would not recommend softer for small hammer dies. Corners should be rounded to avoid chipping.

The 350-400°F value commonly given is a MINIMUM starting point for almost all steels so is a safe answer but non specific. I often use this value but qualify it as a minimum AND because it can be achieved in a kitchen oven. It is recommended that you temper immediately after hardening and before the steel reaches room temperature to avoid cracking or warping. But you can always re-temper to a higher temperature - so a minimum temper is a good start if you do not know what your final temper should be.



The one thing I do not like about the ASM Heat Treaters Guide is that the data, collected from many sources is not given in a uniform form. Charts on the same page will give Rockwell and Brinell hardnesses and then differently on the next alloy.
- guru - Friday, 03/25/11 09:45:37 EDT

Stormcrow: one of the reasons for using a higher alloy (4340, S-7, etc) than 4140 is quench reliability. While large 4140 rounds can be water-quenched, squares are a different story. Each corner of a die is a three-sided pyramid and in quenching the corners will cool very quickly, often causing "finger-nail" cracking of the corners. That is the main reason for switching to an alloy that will give good results by oil quenching. If your dies survived the water-quench then you're good to go.
- grant - nakedanvil - Friday, 03/25/11 18:52:37 EDT

Good reason to round corners as much as possible before heat treating - guru


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