Post quench stopgap temper?

[petek]

This is my first post after weeks of reading through this wonderful resource - I've learned a ton and want to thank all of you for sharing your knowledge!

I've worked with metal all of my adult life and have decided that it's time to learn more about the heat treatment process. My goal is to make the process as repeatable and accurate as possible - so that I can keep notes, control the variables (as much as possible), and learn from my mistakes (and sucesses, down the road!). I have a evenheat programable heat treating oven to work with. I would like to both austenize and temper using the oven, the problem being that it's not really possible to cool the oven from hardening temp to tempering temp quickly enough to prevent post quench stresses from building up to an unacceptable level (or just cracking the tool).

The question is this, and any alternative suggestions are certainly welcome. Is it good practice to heat and quench the tools using the oven and appropriate quench medium, then immediately place the items in the kitchen oven for a pre-temper - to minimize stresses until the heat treating oven can be cooled and reset to the proper tempering temperatures? If so, what temperature would get the items stabilized until proper tempering can take place? The steels of interest are: 5160, 4140, H13, A2, S7, and W1.

Thanks!
Pete

[Timothy Miller]

You can temper in your kitchen oven why not just use that.

[pkrankow]

One of those materials wants a higher temper temperature than a typical oven provides (H13), but I agree with Tim for materials being tempered to 500F or less.

Phil

[IronAlchemy]

Pete - Your suggested approach sounds sensible. However, my experience with 4140, H13 and S7 is that it depends on the temp control in your kitchen oven and what hardness you are shooting for in the final product. My kitchen oven swings +/- 30F so it is quite difficult to hit, for example, a 450F temper for S7 to hit hardness in the upper 50s Rc. Also, the amount of radiative heat gain from the oven elements may affect the metal temperature especially at the surface. I see this as a soft surface hardness that needs to be ground away on hammer dies to get full hardness.

Until I get a tempering oven with decent control, I just take the risk and wait for my heat treating oven to cool. Typically, you let your piece cool to 150F before tempering. However, the quench is largely completed at a much higher temperature. For heavier sections, I quench to a level below the key transitions from the TTT curve and let it cool slowly to 150 while my oven cools. For small tools like drifts, punches and chisels, I just don't worry about the stress in the tools and temper later at some convenient time.

- Doug

[petek]

Thanks for your input, it's great to have such a wonderful resource to draw from... Now I just need to gain more experience and knowledge so that I might have some thoughtful replies in the future :)

[rthibeau]

if I understand this right, you want to heat the metal in the heat treating oven, quench the metal, then put it back into that oven to temper after the oven cools down some. The metals you mention are oil quench (A2 is air quench, not sure on oil results), so I would just leave them in the oil until the oven is at the temper temp.

[Frank Turley]

Most all specs say to temper immediately after hardening. The metallurgists must be correct, but I have fudged at times on their recommendations. W1 is a high distortion, shallow hardening steel, and I am careful with it. Before taking a forging heat, I put W1 at the edge of the fire to put a few hundred degrees in it. When water quenching, I withdraw it from the quenchant when the underwater 'cush' sound stops. It is then usually at 150F to 200F, and that is supposed to prevent cracks. If you need to wait a few minutes before tempering, I would lay the tool on a non reactive surface. I have a block of graphite, but a fire brick or pile of coke would work as well.

In my pritchel making days, I would let S7 air quench and draw no temper.

The steels you mention all have different specs and properties, so just lumping them as a group and putting them in a kitchen oven is not the ticket.

I'm a coal forge smith, and I still use the heat rainbow and incandescent colors for heat treatment. In thinking about chasing color on say, a W1 tool, whatever you temper the business end to, there will be a color band or more of 'softer' temper behind it. The oldtimers say that this acts as a cushion or shock absorber for the tool, especially if it is a tool of percussion. This feature is not obtained in an oven.

[ThomasPowers]

Not to mention that you generally want the striking end softer anyway! (there is an exception for tooling with hardened striking ends used with a dead soft hammer...)

[petek]

Thanks for your further thoughts on this. I've been focusing on S7 lately and have had pretty good luck austenizing at 1800 degrees (book says 1725, but it wasn't hardening at that temperature), and then using a forced air quench in front of an old furnace blower converted to a shop fan. For some reason it wouldn't harden on a firebrick in still air. Read some previous posts mentioning forced air, and that seems to get it hard. The advice regarding the hardened striking surface was a very good point. Would heating just the striking surface to 1500 or so with the torch and letting it air cool draw enough temper off to make a good striking surface? Advice much welcomed and appreciated!

-- Pete

[Rich Hale]

Heating the striking end with a troch and letting it air cool will allow it to harden again. I have helped makeing over a hundred punches from S-7..including a few that were for my use and have seen a lot of use. I forged the punches then annealed the whole tool. Next day stick the punch end in a coal forge and bring it up to a good red. i then let them air harden in still air. You can tell with a file how hard they are in relationship to each other. You wish to do this in a programmable oven and I understand that. There is s simple option: Use a brass hammer to strike with.

[Ten Hammers]

PeteK, welcome aboard. You have 150 years + experience in 6 of the prior posts. I deeply respect these posters. These are not online smiths. They are experienced men and I read what they type. thank you for the question. I like to learn as well. I heat treat by eye and coal forge and I make my own tools only (with fair success). Good luck.

[petek]

Thank you much for your input, I'll give that a shot. The aircraft mechanic in me wants to control the variables to a degree of perfection, but it appears in this case a forge and experience / practice may be just the ticket!

[Jaz]

Forgive me if I just completely misunderstand and am being ignorant. Are you saying that you're wondering what to do once you quench your steel, in terms of keeping it at some "safety" temperature before you temper it (giving the oven time to cool down)? (or instead just use the kitchen oven as the heat treatment oven cools)

I haven't really heard of this, from the information I've looked at, you don't need to keep your quenched steel at any temperature after you've quenched it, and you can wait as long as you want before tempering it.

If cracking is going to occur, it will occur during the quench, not during the tempering process, and this is due to the stresses you mentioned, thermal stresses, etc.

Now as far as I know when you temper your steel you are probably going to be moving into the recovery temperature range for the steel, where the residual stresses etc will be mostly eliminated from the quench, along with the carbon diffusion which makes the cementite grow coarser. So you shouldn't have to worry about what to do between the quench and temper stage.

I may be completely wrong here, in which case I look forward to the learning experience haha, I know some of those steels you listed have "special" quirks with heat treatment.

[petek]

I was reading this piece: http://tidewaterblacksmiths.net/2.html from Dave Smucker of the Tidewater Blacksmiths Guild along with the books on the subject that have been recommended here and other blacksmithing sites. The above article distills a lot of the information down to the practical level for the blacksmith. About 2/3 of the way through the article he speaks of quickly transferring the quenched piece to the tempering oven to lessen the chance of retained stresses from the quench relieving themselves in the form of cracks.

Thanks you all for your thoughts and bearing with my beginner questions!

-- Pete