4140 heat treating

jermfab · Nov 5, 2019

Hello all,

So. I had some fall-off of 4140 pre-hardened at work. I decided to make the pieces into a couple of lathe chuck keys. I made the shank longer and knurled it. The factory chuck key doesn’t clear the lathe headstock, plus, with only an 8” chuck it seems I’m forever changing the jaws from inside to outside. Hoping the knurling will help spin the screws and make flipping the jaws a little faster.

Anyhow, as I’m spending a bit of time on these keys I’m wondering what are/if there are any advantages to heat treating these chuck keys? I have access to a glass blowing kiln, as well as propane and oxy-fuel torches, I don’t have “real” heat treating facilities.

What are the chances of me improving the current hardness?

Is there any way to do so without a proper oven?

Should I instead consider tempering from the current hardness?

This is a new realm for me, I’ve watched the YouTube’s and read what I can find open source. Hoping someone here has some practical knowledge of either heat treating 4140 with a torch, or tell me it’s a stupid idea... I’m receptive to either.

Jeremy

Hardplates · Nov 5, 2019

Heat it till it reaches its austenizing point (no longer magnetic), dunk it in oil, draw back to a light tan. The beginning of your post says its pre-hard..... so it should already be heat treated. 4140 doesn't get super hard, I've played around with quenching it in water but at the end of them day if you need something harder you need a different material or to have it case hardened.

johnoder · Nov 5, 2019

Mass Effect data on 4142 from EMJ catalog. Chart on page 14 will give some idea of tempering temps

Doug W · Nov 5, 2019

I was told by the metallurgist at the heat treat place that 4140 prehard should be sub annealed before rehardening to a higher hardness.

rklopp · Nov 5, 2019

I have made a few chucked keys from 4140 pre-hard over the years. I did not do anything to the existing heat treatment, and they have held up better than OEM keys IMHO.

eKretz · Nov 6, 2019

I've made a few of these for pretty good sized machines from 4140 Q&T and torch heat treated the business end with damn good results. Quenched in oil and tempered to medium straw to brown. They all still look near brand new after a decade of daily use. And they have all been leaned on with a pipe more than a few times I'd guess.

gbent · Nov 6, 2019

If it's already heat treated, use as is. If it ever wears out, ask us then about better materials (in 40 years).

If you are changing the jaws often, get an old school mechanics speed handle.
Better yet, an air ratchet..

eKretz · Nov 6, 2019

We ran a few without altering Q&T HT - they wear out. If you don't want to mess with torching them, make sure they are a good snug fit to start and they will last years of daily use before loosening up - IF your socket is in good shape. That's the snag. With additional HT they will hold up either way.

GregSY · Nov 6, 2019

You don't say whether your chuck keys are square of hex or what but in any case, consider tossing the 4140 in the scrap pile. Then, buy a socket that fits the chuck and use a speed wrench to run your jaws in and out. Way faster.

HuFlungDung · Nov 6, 2019

Agree with eKretz: better to harden them in oil than to use as is. Make sure you get them hot enough, it is my impression that, compared to a plain carbon steel, it takes just a tad longer at high heat for the steel crystal to 'convert' to the martensitic state. So when it is orange hot, just give it an extra 30 seconds (increase the distance from the torch just a couple of inches so you don't start to burn it) then quench it and stir swiftly, but do not agitate aggressively. I'd then temper it a couple of times just to be sure, to the dark straw color. Take your time tempering while getting it up to temperature, there is no hurry, you want the color just to peak at dark straw brown.

Another thing when machining the square drive on the end: use the side of the endmill when you mill the flats. This will create a nice radius blend into the sturdier shank, and will vastly improve the strength and reduce the potential for cracking at a sharp intersecting corner next to the shoulder.

jermfab · Nov 6, 2019

If this was my personal machine I would the dedicated driver solution. Additionally the square measures .410, so too big for 3/8” and too small for 1/2” driver. As it’s the work machine and I’m hourly I don’t want to speed things up too much. Mostly wanted a wrench I could spin a full rotation without banging the paint off the headstock. This one accomplishes that from 9-12 o’clock. At this point I’m leaning most towards... doing nothing. I’m not convinced I can improve the current temper substantially. I’ve got enough of the fall off to hold on to a couple pieces if this experiment fails and I need to make another.

Here she is:

Thanks all for the advice.

Jeremy

gbent · Nov 6, 2019

From the pictures that is a very nice looking knurl, especially on pre heat treated stock.

HuFlungDung · Nov 6, 2019

jermfab said:
If this was my personal machine I would the dedicated driver solution. Additionally the square measures .410, so too big for 3/8” and too small for 1/2” driver. As it’s the work machine and I’m hourly I don’t want to speed things up too much. Mostly wanted a wrench I could spin a full rotation without banging the paint off the headstock. This one accomplishes that from 9-12 o’clock. At this point I’m leaning most towards... doing nothing. I’m not convinced I can improve the current temper substantially. I’ve got enough of the fall off to hold on to a couple pieces if this experiment fails and I need to make another.

Here she is:

Thanks all for the advice.

Jeremy

T handles are a lot more comfortable to use if you put about a 3/4" diameter tube for the handle. Helps to minimize hand injury.

dgfoster · Nov 7, 2019

HuFlungDung said:
T handles are a lot more comfortable to use if you put about a 3/4" diameter tube for the handle. Helps to minimize hand injury.

So, for the OP: get a hold of some grey 3/4" delrin round and bore it with a 3/8" hole or whatever it takes to fit snugly over the steel cross arms you have. Glue or pin the sleeves in place on the cross arms. That tool will fill your hand comfortably so that when you need to crank on the key or bump it you will thank yourself. (I made a deluxe 4-arm handle for my Kurt vise. In that case I tightly stacked leather washers on the cross arms sort of like Estwing does on their hammers. No varnish or finish, I just just smoothed the leather edges by turning them on a lathe to make the edges blend smoothly. Yes, that is definitely going a bit overboard, but that is one handsome and very comfortable tool.) Delrin plastic will hold up well, does not feel cold, and machines very nicely and is more practical.

What you have made looks mighty good and should work well for a long time. And it could make sense to allow the key to be a bit on the soft side. That way the key wears sacrificially and the chuck pinion sockets don't wear out. Much cheaper and easier to make a new key tip than to buy and replace the chuck pinions.

Denis

ryanbcnc · Nov 8, 2019

If it is pre-hard for the application I would not bother trying to harden it further. If your having issues it likely is not pre-hard .. or like some people I work with you need to learn the difference between a chuck key and a hammer .. haha.

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