General guidance on hardening steel required

[Russo08]

Hi

I'm looking for info from the old hands that knows their stuff. First of all I'm not a mchinist and are totally new and green in what I'm doing, I really do need a lot of help.

I'am building a roll former and turning the rolls (dies) myself (CNC lathe). I really take great care to get them as close to tollerances I need etc. I spent hours polishing them, they really looked nice. Will I save a lot of time with a burnishing head? Is burnishing really what the videos say, answer to all my prayers.

So, I send off the rolls to get hardened, and all my shiny rolls turned into these bloody black mongrels (I do not have the money to get them vacuum hardened). The guy doing the hardening said vacuum hardening does not leave them black -but @ 6 x the price though. (My total weight of all dies are 4 tons!)

How can I get this black off again? I've read of muriatic acid (hydrocloric) but I do not want to enhance their rusting. SO, what to do? I've sanded a few on the lathe (hours) and yes they do get back a little shine. Which brings me to another topic, the rolls are "warped" - meaning they are deffenetely not as concentric as when I send them off. So, do I need to grind them back in shape? What do you experts do? If I get diamond tips for my lathe, can I turn off say 0,01mm or so, to get them back into shape?

Last question for now. How much "meat" do you guys leave on your parts before hardening? With other words, if I have to "take" material off after hardening, how much should I leave on.

I don't know the importance but the steel I'm using is AISI 1040 / EN8. The dies/rolls are variable diameters from 120mm - 200mm. The shaft hole is 40mm.

I would really appreciate your guidance, help and suggestions if we need to alter the process in any aspect.

Thanx a million in advance

[RICHARD ZASTROW]

Rough turn then stress relieve before pre-harden turning will help.

For heat treating, if you hang them vertically, they will not warp as much.

If you finish turn them after hardening, use CBN, not diamond. With the right feed and speed, you should need very little polishing.

Dick Z

[diecutter]

There are several ways to reduce and/or eliminate the oxidizing scale that regular heat treating produces. Hardening in a liquid salt bath eliminates the problem, but may be as expensive as a controlled atmosphere furnace.

A regular gas furnace with a highly experienced operator can have it's air/fuel ratio adjusted to a slightly reducing atmosphere which greatly reduces scale.

The parts can be wrapped in special stainless steel foil and hardened in a regular furnace without any scale resulting.


Pack hardening the items in a regular furnace with neutral compounds such as cast iron chips can also do wonders.

I have used a powdered product called KeepBryte available from MSC to coat the parts ( it's mainly boxax and iron oxide powder I am told) but this was only on small parts. It did completely eliminate scale, but can only be used if the heat is below 1650F. The stainless foil has higher ratings and a couple choices. Hope this helps.

[Jhjr]

Generally we usually leave about .04"-.08" (1-2mm) on the part to turn after hardening. Depending on part size & shape. In your case id say 1mm would be good. If you don't let enough material on the part it can ruin your cutter tip if you don't get under the crust. Not as bad as rough steel but it can still happen. You may be able to leave less material on it but I usually stay on the conservative side till I know how much the part will warp or shrink. 1040 isn't super hard so I'd think you could use ceramic or cbn after it's hardened. Diamond sounds like overkill for 1040.

[Cmailco]

Just a 'for the record' but, diamond cannot be used on ferrous materials; the friction and high heat in the presence of air causes the diamond structure to decompose into carbon dioxide. It basically dissolves when machining iron.

[roninB4]

I've been a toolmaker for 25 years but don't know everything so I'll just offer my opinion. First of all, bringing a piece that has to be hardened to finish size is asking for trouble. Sometimes it can't be helped and it can be done if you approach it knowing the pitfalls/limitations. I usually prefer to leave stock on a piece (stress relieving does help as Mr. Zastrow pointed out) and plan on finishing AFTER hardening/tempering. The results of hardening can vary greatly (the steel, furnace, and the process itself) and can even create more problems, as it has for you by leaving the finished surfaces with a layer of burnt carbon from the furnace atmosphere. When I had a die block/insert/punch that required hardening I've had good results from using the SS wrap and sealing it against the atmosphere. The pack hardening methods are suppoed to yield good results but I've no experience with them. Just wrap it like you would a sandwich with folded over seams and rolled up ends. Do be careful with the wrap as it will cut quite deeply if you handle it carelessly. For A-2 the surface comes out an even gray tint and very little de-carb (black) that's easily removed with subsequent finishing. Oil hardening steel can be done in a similar manner but I've not used wrap on this type of steel for precision toolmaking. The amount of material left on will depend upon the workpiece geometry and the level of precision required. For stamping die work the standard is about .010-.015 (.25-.4mm) in holes and up to .020 on outside dimensions on a larger plate. Most of that is about how much warp on a plate needs to be accounted for. It's a good idea to count on removing some stock as the de-carbed surface will be softer than the base metal you're wanting. Some warp is almost unavoidable if your part is larger than a pack of playing cards. This will depend upon the shape and how much material has been removed from the original chunk of stock. I've noticed that if the hardened surface is free of burnt carbon (black scaly gunk) then I only need about .0025 (.0635mm) to clean up and leave shiny metal surfaces but prefer to take a bit more when possible. If the part has to have a good appearance or surface finish then I prefer to grind the surface before moving on the polishing with emery or diamond paste. The short stint I had in roll forming tooling isn't enough to advise on your job but if you have a surface grinder and a Harig spin fixture it should be easy to bring your surfaces back to form and may be faster than gumming up a lot of abrasive belts with The Black Gunk. My opinion isn't the last word but I'd do the wrap and plan on grinding off a few .001's before polishing next time. You didn't do anything wrong, it's just what happens and you didn't know. Heat treating has ruined work for me too despite the precautions I've taken, it happens sometimes. Even with good heat treating methods your part will NOT retain a shiny reflective surface like you would hope. Another method is to harden, polish, and then draw it back (tempering). This can leave a shiny, reflective surface that is colored the temperature you used in tempering (straw to full royal blue) Hope this helped, if not I'll apologize for being long winded.

[roninB4]

Originally Posted by Cmailco Just a 'for the record' but, diamond cannot be used on ferrous materials; the friction and high heat in the presence of air causes the diamond structure to decompose into carbon dioxide. It basically dissolves when machining iron.

-This post further proves that after 25+ years of shop work I still don't know everything. Thanks for posting this, I've not used diamond tooling (CBN yes...) and found this to be an interesting tidbit to file away for further reference.

[Cmailco]

Originally Posted by roninB4 -This post further proves that after 25+ years of shop work I still don't know everything. Thanks for posting this, I've not used diamond tooling (CBN yes...) and found this to be an interesting tidbit to file away for further reference.

Learning is what keeps this stuff interesting. I'm a Manufacturing Engineer but tend to lean heavily towards the machine trades, do a great deal of programming, toolmaking, automation, general design work and thoroughly enjoy working with machinery and building things. I have a great deal of respect for machinist, toolmakers etc., probably because my father worked his whole life as a mold & die shop owner and toolmaker. Never a dull moment, and always lots of new things to learn.

Oh... and another tidbit about iron and diamonds; use red markers, dykem, etc., on any surface to be diamond machined. A lot of the black markers and some marking dyes in general.. have iron in them. If in doubt, call the manufacturer to find out. I've never ruined a diamond turned lens mold because of this... but I learned it from a toolmaker who did.

Regards,
Chuck

[roninB4]

Originally Posted by Cmailco Learning is what keeps this stuff interesting. I'm a Manufacturing Engineer but tend to lean heavily towards the machine trades, do a great deal of programming, toolmaking, automation, general design work and thoroughly enjoy working with machinery and building things. I have a great deal of respect for machinist, toolmakers etc., probably because my father worked his whole life as a mold & die shop owner and toolmaker. Never a dull moment, and always lots of new things to learn. Oh... and another tidbit about iron and diamonds; use red markers, dykem, etc., on any surface to be diamond machined. A lot of the black markers and some marking dyes in general.. have iron in them. If in doubt, call the manufacturer to find out. I've never ruined a diamond turned lens mold because of this... but I learned it from a toolmaker who did. Regards, Chuck

-Nice of you to share your experiences with those of us (me in particular) that didn't know. It's especially gratifying to gain the experience without incurring the cost of, for example........a lens mold. Ouch.

I still enjoy learning things, I seem to find that I know less than I did the day before. I'm a product/project engineer now but find myself wanting to just build a shop out back. Fascinating bit about a diamond decomposing, thanks.

[neilw20]

I have a diamond dust dresser (plated bar) with a few high spots.
Does that mean if I stick a bit of cast iron in the lathe I can dress off the high spots?

[Cmailco]

Welcome ronin and glad to be in the learning boat as well... the day I stop will be the day I draw my last breath!

For ****s and giggles:

Prosit,
Chuck

[Russo08]

Hi

Thanks a lot for all your suggestions. I know so little. I did not even knew of CBN inserts. Anyway, the CBN seems to be all I need. Hopefully my lathe will be good enough to handle it. It is an Okuma LSN 10. Big and heavy. But I do not know if there is any viabration or not. How do you guys test vibration on your lathes?

I've read that they are using ultrasonic viabrators on diamond tooling to be able to machine steels. Apparently, when the diamond move away from the steel it gets enough time to cool down. Thought that was very interesting. But in this world I've seen that interesting = mega $'s.

I've been trying to get hold of the SS foil in South Africa. And you won't believe me, nobody even knows of its existence.......(I've started phoning the company that does our hardening). SO, I'll probably end up buying from the US and ship to South Africa.

I've learned a lot from you already. If you do have any other tips (even irrelevant ones) please post. I'm sure there's a lot of others who can also benefit.

Thanks again.

Regards

R