Hardening and Tempering 4140 Steel from Rifle Shoppe

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FlinterNick

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Building some locks from the Rifle Shoppe.

I noticed that the Rifle Shoppe case hardens the internal parts (bridal, tumbler and sear).

After speaking to the Rifle Shoppe, they said they carburize and then quench their parts in water mixed with potassium nitrate to achieve a mild case hardening.

I’m not an expert on steel, but from what I’ve read online 4140 steel is very brittle once hardened, would additional tempering be needed for stress relief ?
 

dave_person

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Hi,
It has 40 points of carbon and is a through hardening steel so you should temper it. The minimum I would go is 400 degrees for at least 30 minutes. That will produce about 55 RC hardness. I usually temper it at 580-600, which produces about 48-49 RC hardness in the interior of the parts. In the case hardening process, you are adding carbon to the surface so the surface is no longer 4140, it is something higher in carbon. Hence, at a temper of 580-600 degrees, the interior of the part may be 48 RC but the exterior may be as high as 58 RC depending on the points of carbon added by casing. That provides good wear resistance but is not brittle because the interior of the part is softer.

dave
 

FlinterNick

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Hi,
It has 40 points of carbon and is a through hardening steel so you should temper it. The minimum I would go is 400 degrees for at least 30 minutes. That will produce about 55 RC hardness. I usually temper it at 580-600, which produces about 48-49 RC hardness in the interior of the parts. In the case hardening process, you are adding carbon to the surface so the surface is no longer 4140, it is something higher in carbon. Hence, at a temper of 580-600 degrees, the interior of the part may be 48 RC but the exterior may be as high as 58 RC depending on the points of carbon added by casing. That provides good wear resistance but is not brittle because the interior of the part is softer.

dave
Thanks Dave ! I was around 500-560 for 30 - 40 min. The surface doesn’t catch on a file, I think its good.

Have you ever quenched using motor oil ?
 

FlinterNick

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"Have you ever quenched using motor oil ?"
Hi,
Yes, before I knew better. It stinks and does not cool as fast as transmission oil, quenching oil, or canola oil. I use canola because it smells like fried food. :)

dave
I was reading that motor oil can case harden 4140 steel, but just as you had mentioned I figured it might have some type of hazard Lol.
 

dave_person

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Hi,
I guess it can develop a surface skin quenching in motor oil but it will be damn thin. The depth of the skin depends on amount of time it is heat soaked in the presence of the carbon source before quenching.

dave
 

LRB

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There is not enough carbon in motor oil to even notice as far case hardening. As Dave said. ATF or canola. With 4140, any cooking oil should work. 4140 does not require a super fast cooling quench, but heat whatever oil you use to around 120/130 F. The warmed oil will produce a lesser vapor jacket around the part than if cold. Warmed oil actually cools a little faster and more evenly in the quench. Once in the oil, move the part around. Don't hold it one place.
 

FlinterNick

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There is not enough carbon in motor oil to even notice as far case hardening. As Dave said. ATF or canola. With 4140, any cooking oil should work. 4140 does not require a super fast cooling quench, but heat whatever oil you use to around 120/130 F. The warmed oil will produce a lesser vapor jacket around the part than if cold. Warmed oil actually cools a little faster and more evenly in the quench. Once in the oil, move the part around. Don't hold it one place.
what oils have carbon in them ? I always thought you had to introduce the carbon via a compound or mixture of coal and bone meal ? Then quench.
 

dave_person

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Hi,
Nick you answered your own question about case hardening with motor oil. Let me clarify this case hardening stuff so you all are on the same page. Wick, please correct me if I get anything wrong. In the old days before 1850 or so, steel was an expensive commodity because it was expensive and slow to make in any large quantity. You had blister and crucible steel and that was it. In fact, during the 17th and early 18th centuries, steel was as valuable as silver. In 1700, gun makers mounted their high-end guns in steel, not silver. So lock makers reserved their steel for springs, frizzens, and frizzen soles. All of the other parts in the lock were made from wrought iron with little or no carbon so it could not be hardened. To get around that, the wrought iron tumblers, sears bridles, lock plates, flintcocks, and top jaws were case hardened. So the wrought iron parts were packed in an iron box filled with a source of carbon like burnt leather, bone, or wood charcoal, and heated to some temp above 1375 degrees. It was soaked at that heat so the surface of the iron could absorb carbon and become steel. The longer the soak, the deeper the skin of steel. Then, the contents of the box were dumped into room temperature water, which caused the steel on the surfaces of the parts to harden. Because the interior of the parts was wrought iron, it could not harden, so the part was like an egg, hard on the outside, soft in the middle. Consequently, it did not require tempering to make less brittle.

Fast forward to today. Most of our lock parts are cast from some steel alloy and most will harden without casing and they will harden all the way through. They contain enough carbon to harden when brought to a high temp and then quenched in water, oil, brine, or whatever. So, when you case harden modern lock parts, the interior of the part is not soft and requires tempering to reduce brittleness, unlike the old parts from wrought iron. Most modern parts do not require casing. They can be successfully hardened just by heating them red hot and quenching. However, during that process, carbon may be burned off the surface of the steel. That is why, modern steel frizzens often spark better when case hardened than when just heated and quenched because the casing process adds carbon lost during heating. Moreover the case hardening process will add carbon to the surfaces of parts made from 5160 and 4140 steel commonly used for cast lock parts today, giving the surface a higher carbon content. That gives them a stronger surface to resist wear. But they must be tempered before use unlike the old wrought iron parts. The depth and degree of carbon added during case hardening depends on temperature and time at that temperature. No case hardening from coating with a compound like Kasenit, heating for a few minutes and then quenching will be very deep, and any case hardening resulting from the few seconds of quenching in motor oil can possibly be deep. We measure time for casing in hours, not seconds.

dave

dave
 

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Hi,
Nick you answered your own question about case hardening with motor oil. Let me clarify this case hardening stuff so you all are on the same page. Wick, please correct me if I get anything wrong. In the old days before 1850 or so, steel was an expensive commodity because it was expensive and slow to make in any large quantity. You had blister and crucible steel and that was it. In fact, during the 17th and early 18th centuries, steel was as valuable as silver. In 1700, gun makers mounted their high-end guns in steel, not silver. So lock makers reserved their steel for springs, frizzens, and frizzen soles. All of the other parts in the lock were made from wrought iron with little or no carbon so it could not be hardened. To get around that, the wrought iron tumblers, sears bridles, lock plates, flintcocks, and top jaws were case hardened. So the wrought iron parts were packed in an iron box filled with a source of carbon like burnt leather, bone, or wood charcoal, and heated to some temp above 1375 degrees. It was soaked at that heat so the surface of the iron could absorb carbon and become steel. The longer the soak, the deeper the skin of steel. Then, the contents of the box were dumped into room temperature water, which caused the steel on the surfaces of the parts to harden. Because the interior of the parts was wrought iron, it could not harden, so the part was like an egg, hard on the outside, soft in the middle. Consequently, it did not require tempering to make less brittle.

Fast forward to today. Most of our lock parts are cast from some steel alloy and most will harden without casing and they will harden all the way through. They contain enough carbon to harden when brought to a high temp and then quenched in water, oil, brine, or whatever. So, when you case harden modern lock parts, the interior of the part is not soft and requires tempering to reduce brittleness, unlike the old parts from wrought iron. Most modern parts do not require casing. They can be successfully hardened just by heating them red hot and quenching. However, during that process, carbon may be burned off the surface of the steel. That is why, modern steel frizzens often spark better when case hardened than when just heated and quenched because the casing process adds carbon lost during heating. Moreover the case hardening process will add carbon to the surfaces of parts made from 5160 and 4140 steel commonly used for cast lock parts today, giving the surface a higher carbon content. That gives them a stronger surface to resist wear. But they must be tempered before use unlike the old wrought iron parts. The depth and degree of carbon added during case hardening depends on temperature and time at that temperature. No case hardening from coating with a compound like Kasenit, heating for a few minutes and then quenching will be very deep, and any case hardening resulting from the few seconds of quenching in motor oil can possibly be deep. We measure time for casing in hours, not seconds.

dave

dave
A SUPERB explanation, Sir.

BTW, what we today call a Frizzen was called "The Steel" by British Ordnance during the late 17th through and a little beyond the middle 18th century, in part because it was the only part other than the Springs that was made from Steel.

Gus
 

FlinterNick

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Hi,
Nick you answered your own question about case hardening with motor oil. Let me clarify this case hardening stuff so you all are on the same page. Wick, please correct me if I get anything wrong. In the old days before 1850 or so, steel was an expensive commodity because it was expensive and slow to make in any large quantity. You had blister and crucible steel and that was it. In fact, during the 17th and early 18th centuries, steel was as valuable as silver. In 1700, gun makers mounted their high-end guns in steel, not silver. So lock makers reserved their steel for springs, frizzens, and frizzen soles. All of the other parts in the lock were made from wrought iron with little or no carbon so it could not be hardened. To get around that, the wrought iron tumblers, sears bridles, lock plates, flintcocks, and top jaws were case hardened. So the wrought iron parts were packed in an iron box filled with a source of carbon like burnt leather, bone, or wood charcoal, and heated to some temp above 1375 degrees. It was soaked at that heat so the surface of the iron could absorb carbon and become steel. The longer the soak, the deeper the skin of steel. Then, the contents of the box were dumped into room temperature water, which caused the steel on the surfaces of the parts to harden. Because the interior of the parts was wrought iron, it could not harden, so the part was like an egg, hard on the outside, soft in the middle. Consequently, it did not require tempering to make less brittle.

Fast forward to today. Most of our lock parts are cast from some steel alloy and most will harden without casing and they will harden all the way through. They contain enough carbon to harden when brought to a high temp and then quenched in water, oil, brine, or whatever. So, when you case harden modern lock parts, the interior of the part is not soft and requires tempering to reduce brittleness, unlike the old parts from wrought iron. Most modern parts do not require casing. They can be successfully hardened just by heating them red hot and quenching. However, during that process, carbon may be burned off the surface of the steel. That is why, modern steel frizzens often spark better when case hardened than when just heated and quenched because the casing process adds carbon lost during heating. Moreover the case hardening process will add carbon to the surfaces of parts made from 5160 and 4140 steel commonly used for cast lock parts today, giving the surface a higher carbon content. That gives them a stronger surface to resist wear. But they must be tempered before use unlike the old wrought iron parts. The depth and degree of carbon added during case hardening depends on temperature and time at that temperature. No case hardening from coating with a compound like Kasenit, heating for a few minutes and then quenching will be very deep, and any case hardening resulting from the few seconds of quenching in motor oil can possibly be deep. We measure time for casing in hours, not seconds.

dave

dave
Thanks Dave, that was a wonderful explanation. I did some experimenting today with hardening internal parts. I used an oxy propane torch to heat and carburized with kastinite, and then quenched in oil. The parts didnt catch a file and I tempered at around 400-500 like you suggested, it worked well. This might sound but I took them outside and let my kids bounce them around on the pavement, not a scratch or dent. Kids are the most destructive force on the planet, if they can’t break it, its good to Go.

You’ve been a wonderful mentor Dave ! Thanks for all the help and support, if I could recommend it, you ought to put your instructions in an instructional biography !
 

dave_person

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Hi Nick,
Apparently, I am not very effective as a mentor because you did not realize that your hardening with Kasenite will be very superficial. Let me repeat this, the depth of a hardened case is determined by the temperature and time at that temperature. Using kasenite, unless you heat soaked the parts for an hour or more, will be paper thin. Adequate depth of casing requires hours of heating not seconds. You have a heat treating oven. USE IT !! Don't use a damn torch.

dave
 

FlinterNick

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Hi Nick,
Apparently, I am not very effective as a mentor because you did not realize that your hardening with Kasenite will be very superficial. Let me repeat this, the depth of a hardened case is determined by the temperature and time at that temperature. Using kasenite, unless you heat soaked the parts for an hour or more, will be paper thin. Adequate depth of casing requires hours of heating not seconds. You have a heat treating oven. USE IT !! Don't use a damn torch.

dave
Thanks Dave I understand now. I’m still learning how to use this oven too ! I just go the pyrometer yesterday in the mail.
 
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Get a gunsmith, you are not a gunsmith. You will never be one. Sell all that crap you have. stick to what ever it is you’re good at. do everyone a favor.
Rude at best,

Many of us are hobbyist types and love a good challenge, I gleaned the above information many years ago and have used it many times with great success, I learned to build flintlocks and the associated processes because I could not afford to pay for the quality of muzzle loaders that I prefer.

Curious just how much this is cutting into your profits?
 
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