4140 Cracked when hardening

[jmccustomknives]

If you don't mind destroying the piece, cut into the cracks.  This might tell you when they formed.  If you forged at too high or low a heat you can get cracks.  These types of cracks and inclusions, in my experience will be black inside.  While the ones that happen during the quench will be clean.  Also, if you can, brake a piece and inspect the grain for growth that might indicate high quench heat or improper forge theory.

[Frank Turley]

Many things. Did you forge at above or below the recommended temps? Warm oil quenches faster than room temperature oil because of the viscosity change. Try not warming the oil before quenching. The critical temperature at slightly above non-magnetic works for plain carbon steels but not necessarily alloy steels. For 4140, normalize at 1600-1700F (bright red, just before orange incandescence); anneal at 1500-1550F (bright cherry red); harden at 1525-1575F (bright cherry red into bright red). Harden in oil.

 

Did it go into the quenchant vertically; it should. Did you agitate uniformly to get rid of the vapor blanket? On some tools, we figure-eight them in the quenchant, so that fresh liquid is making contact uniformly. Depends on the shape of the tool.

 

And tempering should be done immediately after hardening...no waiting period in between.

[Zachary]

Thanks for the replies guys I'll try to answer your questions. I'm pretty sure from what your saying it was quenching because I drove a drift in the handle hole and it split in half. I don't know much about grain structure but it looked nice and even to me and it was clean no black spots or anything. I did quench it vertically and agitated it I don't know for sure how uniformly. I probably did some forging a little below recommended temperature. It cracked right through the middle of the handle hole and out through the middle of the one head and kind of off to the side on the other one. I was polishing the head a little after the quench so I could put it right back in the fire to temper it. I'll see if I can get some pictures up a little later. Thanks again.

[Rashelle]

You drove the drift into the handle hole after hardening it? That would be the cause of the crack right there. If you are using a drift to temper just place it into the hole and let the heat radiate, don't drive it in.

[JNewman]

If the crack started at the eye I suspect you had a cold shut there.  Did you punch the hole or slit and drift?  Slitting and drifting is more likely to cause cold shuts especially if you don't get the 2 slits lined up perfectly.   Punching is a better way to go with higher carbon or alloy steels. 

[Zachary]

Here are some pictures. I didn't make that very plain after I saw that it was cracked I drove the drift in to split it open to see what it looked like. I did slit and drift the hole.

[anvil]

I always anneal before hardening. normalizing is not the same as an anneal.

when I make a handled tool, I anneal the whole tool, but only harden and temper the working ends. that way the eye area remains soft.

I also punch the eye instead of slitting and drifting or drill two small ~1/8" holes then slit between the two to keep a cold shut from happening.

by the way it broke, I suspect you hardened the whole tool and slitting caused a coldshut, thus the break.

you can still anneal the two halves, remove the broken material and reforge to make another tool, so all is not lost.

[Frank Turley]

I think Anvil is correct in talking about quenching the entire tool instead of doing the heads separately. Heated metal is in an expanded state. If the entire hammer head is quenched, the eye cheeks will harden slightly quicker than the heads. These different rates of cooling can cause unequal contraction, cracks, and breaks.

 

I have posted this before, but I use the "wet rag method." I heat the head only at the edge of a coke fire, quench, abrade the face and temper with a 7/8" square stock, turned eye heated to a welding heat. The head is above the vise jaws. A dark straw is my preferred temper for a forging hammer on 1045 steel. The tempered head is then wrapped with a wet rag to protect it from further heat. Using large bolt tongs, the other head or peen is heated and quenched. If a peen, I sometimes temper with the oxy torch in the vise keeping the wet rag in place. I have not tried using proprietary Heat Stop or Heat Block, but they might work as a wet rag replacement.

[Kal]

A correction, 4140 does not contain any nickel. It is a chrome and molybdenum alloy with some carbon and manganese thrown in for good measure.  

[JNewman]

A correction, 4140 does not contain any nickel. It is a chrome and molybdenum alloy with some carbon and manganese thrown in for good measure.  

 

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Thank you for pointing that out.  I just looked up the chemistry.  I knew it was similar to 4340 that does contain nickel (which is about 80% of what I forge).  Looks like the nickel content is the main  difference between the alloys. 

[Kal]

Back in the day I used to make steel at an electric furnace shop for USS.  Most of it was plain carbon but we also made alloy steel.  The 41xx, 51xx, and other grades were poured through a vacutum into another ladle to improve the grain/purity before being poured into a final ingot.  Wish I had been smithing back then, we had all kinds of steels in the scrap yard.

[SReynolds]

I think Anvil is correct in talking about quenching the entire tool instead of doing the heads separately. Heated metal is in an expanded state. If the entire hammer head is quenched, the eye cheeks will harden slightly quicker than the heads. These different rates of cooling can cause unequal contraction, cracks, and breaks.

 

I have posted this before, but I use the "wet rag method." I heat the head only at the edge of a coke fire, quench, abrade the face and temper with a 7/8" square stock, turned eye heated to a welding heat. The head is above the vise jaws. A dark straw is my preferred temper for a forging hammer on 1045 steel. The tempered head is then wrapped with a wet rag to protect it from further heat. Using large bolt tongs, the other head or peen is heated and quenched. If a peen, I sometimes temper with the oxy torch in the vise keeping the wet rag in place. I have not tried using proprietary Heat Stop or Heat Block, but they might work as a wet rag replacement.

 

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What does;

 

Abrade the face mean?

Temper with a 7/8 inch square stock mean?

Turned eye heated......mean?

Head above vice jaw refer to?

What is heat stop or heat block?

 

Your description was like a forign language to me. You probabaly know what it all means, but to others, who are studying this is is perplexing.

[EricJergensen]

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What does;

 

Abrade the face mean?

Temper with a 7/8 inch square stock mean?

Turned eye heated......mean?

Head above vice jaw refer to?

What is heat stop or heat block?

 

Your description was like a forign language to me. You probabaly know what it all means, but to others, who are studying this is is perplexing.

 

Abrade the face: file or sand or grind off the scale layer so you can see temper colors

 

Temper with a 7/8 inch square stock / turned eye / heated: he uses 7/8" square stock to form an eye-shaped bar that can be heated to a high heat and inserted into the eye of the hammer as a source of tempering heat. It needs to be fairly close to the eye shape so that there is enough contact for the heat to move into the hammer.

 

Head above vice jaw: not sure where you got that, but probably from the part where he describes an alternative for tempering using a torch as a heat source with the already tempered hammer face covered with a wet rag and clamped in his vise.

 

Heat stop / block are commercial pastes that absorb heat (presumably by melting). Haven't used them, but they would be handy for shapes not easily covered with a rag.

[Frank Turley]

The internal diameter of the turned, circular eye gives a snug fit OVER the hammer head, the hammer face being upward. The head to be tempered is held in the vise and above the vise jaws. The temper is drawn by the heat conduction from the hot stock. After bending the eye, I leave enough straight stock beyond the eye to act as shank and handle, so I don't cook my hand. The old Plumb hammer company used to advertise their "exclusive rim temper." The idea was to temper the head from the outside inward, which is what I attempt with the 7/8" square. The theory is that by the time one reaches a straw (hard) temper in the middle of the hammer face, the rim of the hammer face will be a softer temper and less subject to spalling.

[patrick]

Back in the day I used to make steel at an electric furnace shop for USS.  Most of it was plain carbon but we also made alloy steel.  The 41xx, 51xx, and other grades were poured through a vacutum into another ladle to improve the grain/purity before being poured into a final ingot.  Wish I had been smithing back then, we had all kinds of steels in the scrap yard.

The process described above is called vacuum stream degassing. It's function is to removed dissoveld gases from liquid steel, mostly hydrogen. Hydrogen in too high a concentration can result in internal cracking. Today, most steels used for open die forging have a maximum hydrogen content of 2 parts per million.

[Rashelle]

Thank you Frank. That just lit a lightbulb inside my head. I really like the idea of tempering a hammer face that way and will do a few to see how well it works for me. I might even try a variation where I put a heated drift in the hammer eye, then the snug ring around the face, and see how that also works.

[SReynolds]

Sears 40 oz cross piens chip easily. Buy one and all others are free, but still. They should have gotten the temper correct. Don't care to use them when they chip, cuz then they crack.

[NMcCartney]

Hey everyone. A gentleman from my local blacksmithing guild donated a piece of 4140 steel for me to use as my first anvil, but it is needing to be heat treated. This will actually be my first time doing ANY kind of HT and I am wanting to know exactly what I am doing before putting it into a fire. The steel is 4" round diameter, about 12-13" long and I will be using a gas forge.

I've posed this question to some other places but you can never have to much advice to draw from.

[BIGGUNDOCTOR]

Check with local heat treating facilities, it may be cheaper than you think to have this done by a professional who can do this the right way.

That is a big chunk of steel that is going to take a lot of fuel to get heated properly. Even if you are just wanting to heat treat the end. It is also going to take a lot of cooling oil,water. Then you will also be tempering it, so more fuel, and cooling. Added up, it may be more than a pro would charge.

Now if you know someone doing a bonfire on a beach.........

[JNewman]

I would not want to try hardening that big a chunk of 4140 in oil with less then 30-40gallons of oil at a bare minimum. Any less any you are asking for a fire. You could quench it in water but you risk cracking it as 4140 is an oil hardening steel. You might get away with it on that big a piece because it will take a while to cool.

 

Try using it without any heat treatment. It should work well as is. Do you have a way of handling a hot piece of steel that big?  Even with 4" tongs a 4" piece of steel is a workout to handle when hot. 

 

If you find it is too soft I would be inclined to heat one end about 1" long in a coal forge and then quench it by placing the hot face on another heavy piece of steel. The cold steel will quench the face only not too violently you could pour some water on the bottom steel to keep it cool. I did a stake anvil about 6" across made of 4340 this way and I got about 45RC on the face. 4340 or 4140 will still harden with a slower quench than a straight carbon steel.

[navasky]

Check this thread '?do=embed' frameborder='0' data-embedContent>>

[CKillgore]

I bought a foot long piece of 5" square 4140. It cost me about $100 to have it professionally heat treated and they got it up to 55 HRC.

[will52100]

Wish I'd thought of that CKillgore, I did a 4"x4"x12" block myself.  Between fuel cost, time, and mixing up 30 gallons of super quench it wouldn't have been much more to send it out.

I made a blade smith post anvil out of said 4140.  First quench was done in 50 gallons of water with a hose running full blast in it.  Got hard, but not nearly as hard as I wanted.  I'd been using a harbor freight russian cast steel anvil with the horn cut off for blade work and it was too soft.  So I tried the super quench and it worked very well, I'd estimate the RC to be about 52-54.  Unfortunately it also developed a couple small cracks following the sides about 1/2" to 3/4" from the edge.  I finished it up and mounted it in a tube filled with sand and it works great for blades, but I'm not going to be doing heavy work on it, got a good anvil for that.

[will52100]

Here's my blade smith anvil, I used it a bit while straighten up the facets on a hammer I forged, solid as a rock.  Good rebound, but no ring like a traditional anvil and should work good for blades.  Idealy I'd love to have about a 12" long x 5" wide by about 10" high anvil for blades, just a big hard rectangular block of steel for blade work.

[j.w.s.]

I would not want to try hardening that big a chunk of 4140 in oil with less then 30-40gallons of oil at a bare minimum. Any less any you are asking for a fire. You could quench it in water but you risk cracking it as 4140 is an oil hardening steel. You might get away with it on that big a piece because it will take a while to cool.

 

From my experience, as a shop that uses 4130 by the ton every single year, as much as I was told that 41xx was oil hardening, I've never gotten better results than with water hardening. We do several hundred theatrical combat swords a year ranging from 3/16th down to 1/8th and have been quenching in warm water for the past 4 years with awesome results. These are pieces that get used and abused by actors in stunt shows, we offer a lifetime warranty, and we get back maybe 2 or 3 pieces a year that have broken, most of them older pieces from when we used to send them off for ht, or a rare brittle weld at the crossguard or pommel. That being said, I've done 2 - 4 lb hammers out of 4140 the same way with equal results, but for a block that size, I'd make sure I had a lot of water!

J