Thread: Heat treatment of 4140

Hi,
I am making a milling tool out of 4140 and I hardened and tempered it as per the advice in Tubal Cain's "Hardening, Tempering & Heat Treatment" book. However, I now have pitting on the faces. Looking for advice/wisdom.
The tool is the gear cutter hob that appeared in "Home Shop Machinist" magazine Sep/Oct 2010. I am making the cutting knurl tool that Michael Ward designed. The hob is to cut the small 80DP gears that control the angles of the two knurls that cut the workpiece. A good project... hope the tool works as well when I finish it.

To prevent scaling, Cain advises using powdered chalk mixed with water to form a paste and apply that to the surfaces of the tool. Of course I don't have powdered chalk at hand but I do have blackboard chalk which I crushed and made a paste from that.

The milling hob is 19mm diameter with 40 degree Vee grooves turned on the OD, pitched at the correct distance for 80DP and extending 30mm along the cutter. Then the straight axial flutes are milled to provide the cutting edges for milling the gears. The 80DP teeth are very small, about 1mm deep and pitch about 1mm.

To prevent scaling, Cain advises using powdered chalk mixed with water to form a paste and apply that to the surfaces of the tool. Of course I don't have powdered chalk at hand but I do have blackboard chalk which I crushed and made a paste from that.

My "furnace" comprises a few foundry bricks on the concrete garage floor, a MAPP torch, and a pyrometer with the temperature reading on my digital multimeter. I placed the tool along the back of the of the "furnace" cave and located the pyrometer near the teeth end.

I heated the tool to 810-870 Celcius by trying to apply the flame to the arbor of the tool and not the teeth, but found that I needed to play the flame over the teeth end to keep the temp reading within the band. I kept the temp more or less in range for 15mins, then quenched in water warmed to about 50C. When the tool was about 50C, I placed it in the kitchen oven and tempered it for 30mins at 210-230C (looking for a straw coloured temper).

Alas, when I scraped off the blackened paste I found that faces of the flutes were pitted as if corroded and that I could see (with magnification) that there was scaling on some of the teeth. In fact the points of a few teeth were eroded away. I suspect that the points of the damaged teeth were overheated and "burnt" away, and the pitting is either random scaling or perhaps the crushed blackboard chalk is not as pure as "powdered chalk".

My plan is to use the cutter to mill aluminium to attempt to clear the scale between the teeth and to see what the cut form is like.

In the meantime, any comments would be appreciated.
Bevin

Bevin, you might find that a 4140 cutter without heat treatment is good enough if you are only cutting a pair of gears on a soft material like brass, Al or plastic. You might even be able to cut a small amount of mild steel if you were gentle.
There are commercial scale prevention compounds around the place. The other one that I've heard of is a layer of soap. I'm wondering whether the chalk mix was not thick enough (or perhaps dry enough) and cracked while in the oven heating.

Michael

Just as a silly opening question - is the bar you bought not in hardened state as purchased? Ive only ever bought my rounds in hardened and tempered condition already and its hard, but not cutting tool hard. The local workshops that sold it to me have suggested that it can be heat treated to a harder state, but that there isnt a significant gain in hardness to be had (I think i recall the number 57 rockwell being mentioned). They were making pivot pins for excavator arms out of the stuff, but they were sending them off to be nitrided to get to the high 60's rockwell hardness. The context I was working from was making centre punches and I ended up going with S7 because it was hardenable with the tooling I had at hand (oxy / engineers kiln). I tried to harden some 4140, but I couldnt get any results that would suggest itd work long term as a punch.

Michael,
Yes I guess the gears could be made of brass since they only rotate when manually rotating the knurl holders to adjust the geometry of the knurls ground faces to the workpiece to be knurled, and then the knurl holders are locked in position. Therefore, there is no stress on the teeth, nor does any backlash have an effect in the cutting knurling operation. The design calls for mild steel but only for economy I guess.

Anyway I have already heat treated the cutter so using un-heattreated 4140 would require me to make a new cutter. Fortunately the amount of tooth circumference remaining after machining the flutes has allowed me to grind the cutting faces to achieve sharp points on most of the teeth. I then machined the end of an aluminium bar using the full length of the cutter and examined the results using a digital camera. Yes the cut teeth profiles vary and are not that good. But since the function of the gears is every basic, I will cut two gears to test how they perform the design intention.

My original question really was about how to harden and temper any tool steel and avoid scaling, which I now think is cause of the pitting I found. In future I will try using a commercial anti-scaling product. Just another expense in pursuing the hobby, just a minor cost as I will explain to my dearly beloved (she is very good to me).
Bevin

Originally Posted by scottyd

Just as a silly opening question - is the bar you bought not in hardened state as purchased? Ive only ever bought my rounds in hardened and tempered condition already and its hard, but not cutting tool hard.

Hi Scott,
No I don't think the stock I used was hardened. I cut the grooves using just a HSS cutter without any problem or detectable wear after the 25 or so grooves. I bought it from a local engineering company (Canberra) and based upon the advice he gave me re his scrap metal (that I ogled), I think he would have told me if it was hardened or other than annealed. But it is a question I will ask in future.
Thanks for the advice.
Bevin

I forgot to add that I have ordered the "Heat Treatment, Selection and Application of Tool Steels" by William E. Bryson, from Book Depository. Therefore I will become an instant expert in the topic.
Bevin

Hi All ,many years ago i read of old technology of the finer gunsmiths used to use a muffle furnace for hardening gun parts.Apparently consists of a muffle ie cast iron box with a tight lid.The parts were submerged in bone dust.then heated to the required temp then quenched then polished then reheated to the straw colour.I always wanted to experiment with that using blood and bone garden supplies would stink.I have a cast iron switch here with a lid actually an old junction box with a tight lid.
Never got round to it [that word'] as still had a qty of casenite that have been guarding the health hazards have taken it out of sales as the risk of cyanide poisoning. done lots of it here and am still alive but do it away from confined areas and plenty of ventilation. Cheers John.

I've never hardened 4140 but looking up the BH website for heat treatment of 4140 at http://buau.com.au/english/files/4140.pdf your hardening and tempering method has a couple of differences

1) oil rather than water quenching.
2) tempering ranges of 540 to 680º C which are well outside the range of a domestic oven.

Indirect heating of the object with as small a flame as a standard MAPP torch is not likely to be successful at raising the temperature uniformly or quickly enough even if the furnace/forge is a small one. Getting around this by applying any part of a MAPP flame directly on the object edges can contribute to scaling because MAPP flames are usually oxidizing. Reducing the oxygen content of the flame requires playing around with the burner which may then further reduce the heat output and is not a advisable unless you have some experience with this sort of thing.

Originally Posted by bevinp

I forgot to add that I have ordered the "Heat Treatment, Selection and Application of Tool Steels" by William E. Bryson, from Book Depository. Therefore I will become an instant expert in the topic.
Bevin

Hi Bevin,

For instant expert status you can also get the "Heat Treaters Guide Practices and Procedures for Iron and Steels" there are 7 pages on heat treating 4140.

Regards scaling, you can always wrap the part in stainless steel foil and seal it with a bit of paper inside, the paper burns when the temperature rises above the auto ignition point and removes the oxygen from inside the stainless steel envelope.. and so prevents scaling.

Trivia... the self ignition point for paper is Fahrenheit 451... I only remember that because of the Kurt Vonnegurt book.


Whenever you have sharp pointy bits on the part being heat treated, like the cutter you are making, makes quenching a bit tricky. As Bob said, 4140 should be oil quenched.

What about some pictures of the cutter...

Regards
Ray

Originally Posted by bevinp

My original question really was about how to harden and temper any tool steel and avoid scaling, which I now think is cause of the pitting I found. In future I will try using a commercial anti-scaling product. Just another expense in pursuing the hobby, just a minor cost as I will explain to my dearly beloved (she is very good to me).
Bevin

Home and commercial anti-scaling treatments, usually have borax as the major ingredient. If you want to try one of the commercial ones I have small containers of Rose Mill's PBC anti scaling compound available.

PBC Anti-Scaling Compound

Hi Bevin,
Did the steel come from baxters? I have used clay and ground charcoal mix in the past as a barrier.
Cheers,
Ew

Thanks all for the advice. My responses are:
John,
I don't think I could get any bone dust, which I guess is baked bones that are then finely crushed. Perhaps I could get something from a crematorium!!!
A commercial paste sounds much easier/more reliable and certainly not like grave robbing.


BobL,
The temper I was trying to achieve was for a hardness in the high 50s Rockwell C. Googled info seems to confirm a temperature of about 230C for that hardness. The Bohr document you pointed me to, doesn't say what hardness would result from such high temps. Probably the was for the best compromise between toughness and wear resistance.


There is conflicting advice on the net about oil vs water quenching, with aspects such as cracking, speed of quenching, cost and other considerations. I opted for a cheap compromise: warm water as advised by someone in a similar 2010 thread in this forum.


Ray,
I'll start with the book I have just purchased, and once "mastered" that, I will have two books in my CV so I am sure I will be able to pontificate at will on all forums (as some seem to do with less knowledge).


Where can I get stainless steel foil? And what is the type of stainless or does this make no difference? The idea of not only stopping scaling but also preventing the nasty side effects of a MAPP flame sounds great. Also the wrapping would minimise the over-heating effect of the direct flame on the points of the teeth. And since I use a pyrometer I don't need to see colour to determine temp. I see you are in Aust so please advise a good supplier. I am in Canberra, which as the politicians believe, is the heart of Aust, but it has bugger all engineering/manufacturing.


I have attached a copy of the page from Home Shop Machinist that shows good photos of the cutter and of Michaels Ward's hardening using what appears to be a MAPP torch and without any covering of the tool to prevent scaling. And on subsequent pages (not asttached his results look really good without any erosion of the teeth. Perhaos, if I were cynical, I would say that he photoshopped the images. Only joking.


Bob,
Your product sounds interesting, but with the cost of shipping 5lbs from the US and the declining value of our dollar, I will just try the stainless steel foil first. Thanks very much for the info. If I need to heat treat a number of tools I will remember your site. By the way, is your compond a paste that will adhere to the workpiece or a powder in which one submerges the tool? Your wesite doesn't make it clear how to apply it and consequently how much one needs. And I'd love to hear your comments about the use of SS foil versus your compound, use of MAPP torch and any other issues relevant to hobbyists, particularly since you must be very much involved in solving the problem of scaling.


Ew,
No it was not Baxters and I would prefer not to publicise the firm as having just found him, I don't want him swamped with other hobbyists. Being selfish I know, but he was friendly and I don't want to use up his patience/generousity/good will.


And my next actions are to use the cutter as is and cut two gears to see how the backlash, consistency and smoothness of movement is and whether I need to make another cutter from scratch.


I will post my progress, or otherwise.
Bevin
PS. The diameter of the cutter in the attachment is 19mm, so the teeth are really small

Hi Bevan,

If you can find an Australian supplier I'd be interested.. I got mine from Travers Tool Stainless Steel Tool Wrap | Travers Tool

There is a video on this page showing how to seal the edges. 50003 50'X24" 309 TOOLWRAP SHOPAID | Travers Tool

It can be pretty expensive stuff if you have to buy a full roll, just for one part, but, If you can give me an idea of the size you'd need I can cut off a bit and post it to you. I got 50' of 24" 2 thou thick 321 grade

Regards
Ray

PS the cutter looks impressive, what's it going to be used for?

Hi Ray,
Can you re use the foil, or does it go hard and brittle with the hardening process?

Hi Bevin,
No worries on the supplier, Baxters is the only place i know to get it locally, i have a stack of old backhoe pins to keep me going anyway. Personally i would have used O-1 but thats just cause it is so much easier to machine and tempers with little or no distortion.

Cheers,
Ew

Originally Posted by RayG

Hi Bevan,

If you can find an Australian supplier I'd be interested.. I got mine from Travers Tool Stainless Steel Tool Wrap | Travers Tool

Ray
PS the cutter looks impressive, what's it going to be used for?

Ray,
Yes I would like to have some of the SS wrap. I will send you a private mail with the details. I haven't found any Aussie outlet so far. A few eBay US sources selling 12"x12" sheets at $9/sheet.

Travers has an excellent website and range of tools, but their minimum overseas order is US$100 as you probably know. Their HQ is in New York, but that guy on the video was from the deep south "Sure was Pally".

Regarding info about using SS wrap, I found this excellent QA on the following link to a US producer of SS everything. Actually the range of SS wrap they make the same range as Travers sells.

Precision Brand
I think you will have to copy it and paste in your web browser.

I explained the purpose of the cutter my original post but to add more info, the gears to be cut are approx 7mm dia (20 teeth) and 14mm dia (40 teeth). Very small teeth.

Uee,
According to Precision Brand's QA, you cannot reuse the wrap due brittleness and titanium depletion. See, already I am beginning to sound like an expert!
Bevin