Heat treating high alloy and high-speed steel tools

[HennieL]

Hi everyone,
I was asked on another thread to describe my process of hardening HSS woodworking gauges (chisels) - I suggested that I rather open a new thread to do so here.

Some background - I've been making high-end knives using mostly high alloy martensitic stainless steel for more than ten years now, and in the process to make the best knives that I could, I did a lot of reading up on the subject of heat treating steel, and learned quite a lot about metallurgy - well, enough to know that I actually don't know much, but at least I can sound as if I do know something . So, after mastering the black art of using fire and water to magically turn soft medium carbon steel into hard knives, I went on to specializing in rather exotic " super steels" such as Bohler Elmax and M390, CPM S35VN and S90V, and various others. All these steels required higher austenitizing (hardening) temperatures than the normal carbon steels, and some very low temperature cryogenic treatments to complete the quenching process (down to soaking the knives in liquid nitrogen for 24 hours to transform retained austenite and to refine the grain of the steels (who knew that steel had grain...). Well to cut a long story short, I ended up looking for the next challenge, hardening high speed steels (HSS) - and this is where this post starts... Looking at the exorbitant prices charged (at least here in South Africa where EVERYTHING is imported (mostly from China...)), I thought that I could at least achieve the same, if not better, performance for much less money if I "rolled my own", so to speak.

So, I ordered some S600 (equivalent to M2 HSS) and S705 (equivalent to M41 Cobalt HSS) steels from Bohler South Africa (imported from Sweden, I think), and set of on a new quest...
Anyway, here's the outcome: The topmost " long and strong" gauge is made from the S705 (M45) steel, and the smaller diameter one is made from the S600 (M2) steel. The other chisels ahown here are two parting tools (the larger one made from D2 and the smaller one from an old high-carbon saw blade), and the little fingernail gauge on the bottom was made from O1.

OK - what equipment is needed to harden these steels?
I have a quite ordinary vertically loaded electric oven, used all over the world by knife makers. It runs on normal South African 220V household power, consuming about 9A of current (i.e. nominal 2KVA load). This oven is about 600mm (2') high, and has an internal diameter of 150mm (6") - ideal for knives and other skinny objects, impractical for anything larger than about 125mm diameter. As can be seen, I mounted the oven on small wheels, and push it to just outside my workshop when I need to do any hardening (right next to my quenching tank):

And here's a peek inside the oven:

I will post some more on the specifics of the heat treatment in my following post

 

[HennieL]

Before I get to the specifics of the heat treatment, some more pics...
I have three quenching tanks - one that is about 800mm high (30") made from a 300mm diameter pipe with the one end welded closed using a 2mm steel plate that also acts as a stand, and the other two made by cutting off the tops of two old fire extinguishers. Here's the large one:

And here's one of the small ones:

So, on to the specifics... I will first describe the process, and then answer any questions that anyone might have

Bohler S600 contains the following alloying elements:
C= 0.9%, Mn =0.3%, Cr = 4.1%, Mo = 5.0%, V = 1.8% and W = 6.4%

The recommended heat treatment (HT) process is as follow:

• Stress relieve at 600°C - 650°C (1110°F - 1200°F) for one to two hours.
• Heat to between 1190°C and 1230°C (2170°F - 2245°F).
• Soak at temperature for minimum 80 seconds and maximum 150 seconds, depending on heating temperature and cross-sectional area of tool.
• Quench in oil, vacuum or salt bath
• Temper twice at between 500°C and 550°C (930°F - 1020°F) for two hours per temper
• Stress relieve for a third temper at 30°C - 50°C (85°F - 120°F) lower than the first two tempers.

I followed this recipe as closely as I could, quenching into warm quenching oil (heated to 60°C), but cryo-treated the tools in liquid nitrogen for 24 hours after quenching, and before the first temper at 550°C. Hardness after tempering and stress relieving came out at 64.0 Rockwell C.

Bohler S705 contains the following alloying elements:
C= 0.92%, Mn =0.3%, Cr = 4.1%, Mo = 5.0%, V = 1.9%, W = 6.4%, and Co = 4.8%

The recommended heat treatment (HT) process is exactly the same as stated for the Bohler S600, and after my previously described process and a tempering temperature of 553°C I measured a hardness of 65.5 Rockwell C

Both gauges were covered in high-temperature stainless steel foil during the heating and quenching process to protect against de-carbonation.
As for shaping the gauges, I milled out the half-round slot in the back of the gauges using a ballmill in my little Optimum BF20L mill/drill, removing not more than about 0.2mm of steel in each pass... yes, it was @#$%^ slow going, but the mill stated to chatter with anything deeper, and I really did not want the gauges to work-harden on me.

So, fire away with any questions...

Hennie

 

[trlvn]

Great writeup!

Curious about the stress relief and tempering...is the HSS hot enough to show colour at those temps (500-650 C)? Would love to see pictures when the pieces were at temperature but I'm sure you had more pressing issues to deal with!

Craig

 

[HennieL]

Great writeup!

Curious about the stress relief and tempering...is the HSS hot enough to show colour at those temps (500-650 C)? Would love to see pictures when the pieces were at temperature but I'm sure you had more pressing issues to deal with!

Thanks Craig,
Yes, unfortunately I do not have any photos of the hardening process.

I did my "high-heat" tempering using the same PID controlled electric oven that I used for the hardening heat, and am confident that the tempering temperature was correct to within 15°C (I check my oven regularly using an external thermometer). My regular toaster oven can only reliably heat up to 200°C, and it's quite rare that I need to go higher than that - basically only for tempering the HSS, and for tempering springs (also done in the "main" oven...).
I can confirm, though that at these high tempering temperatures the normal "tempering colours" have all disappeared, and one can only see the bright steel - no yellow, blue of purple colours...

 

[ajoeiam]

Mr HennieL

Thank you ever so much for sharing.
Was asking because I'm starting to think that it may have become cheaper to 'make' high quality tools that to buy them.
See - - - - that way I would get the fun of making the tool - - - the machining the heat treating etc - - - and then the fun of using the tool!!!

You liquid N - - - - where are you getting and where are you keeping it?
(Guessing you welding gas supplier and maybe a dewar?????)

Thanking you for your generous sharing!!!!!!!!!!!!!!!!

 

[HennieL]

First, a correction - I stated earlier that the Bohler S705 is equivalent to AISI M45... This is wrong. It is equivalent to AISI M41... apologies for this.

I was asked in another thread to comment on the possibility of re-hardening some HSS tools that had been over-heated in a fire. In my opinion (and I want to stress that I'm not a metallurgist, so it's just an opinion...) it should be possible to re-harden the tool, bearing in mind that either the original fire damage or the re-hardening could have resulted in some warping/deformation that one would have to live with - for HSS lathe cutters this won't be serious, but for e.g. a drill it might result in a useless tool that is out of round. Regardless, I think it would be worth a try, at least. I would do the following:

• Anneal the tool by heating to 770°C - 840°C (1418°F - 1544°F) (whilst protecting the tool from de-carbonation by wrapping it in stainless HT foil) and cooling it down very slowly (at a rate of 10°C - 20°C per hour (50°F - 68°F per hour) to approximately 600°C (1112°F), thereafter cool in still air. (This is per Bohler recommendation for their S705, but should be applicable to most HSS and HSS-Co steels). This is very slow cooling - dropping 200°C in 10 hours minimum, but can be done by either keeping the tools in an electrically (PID) controlled oven with a ramp function, or by simply heating up a good volume of cement or lime to the same ~800°C temperature and burying the tool in this, and just leaving everything in the oven with the door/lid closed.
• Stress relieving the annealed tool at 600°C for at least one hour (two would not hurt...), and then letting it cool inside the oven until it reaches air temperature (probably over night).
• Then follow the heating and quenching outlined in my second post.

 

[HennieL]

Thank you ever so much for sharing.

You liquid N - - - - where are you getting and where are you keeping it?
(Guessing you welding gas supplier and maybe a dewar?????)

It's only a pleasure

I get my liquid nitrogen (LN) from our local farming co-operative. Cattle farmers that do artificial insemination use it in large volumes, and this makes it much cheaper than buying it from anywhere else - at least here in South Africa.
I use a 1 liter stainless steel vacuum coffee flask to buy and store the LN - proper dewars are extremely expensive. Not sure if the same would be available and/or practical in your country, but it's worth asking around.

Two very important safety points to ALWAYS keep in mind when using and transporting LN:

• Never fully seal any container housing LN - as the liquid heats up, it changes into a gas that can build up enormous pressure in the sealed container, and it will turn into a bomb!
• LN is heavier than air, and although it is perfectly harmless, it will over time displace the air in your car or workshop, and in so doing also displace the oxygen you need to live. If a buildup of nitrogen gas occurs, and it reaches the level of your mouth and nose, you will pass out (faint) due to a lack of oxygen. When you then fall to the ground, you will be in a zone of absolutely no oxygen, and will die within a few minutes if you are not saved by someone else. Moral of the story - ALWAYS keep a window or (preferable) door open, and don't work or store LN in a basement. Always drive with your car windows open when transporting LN.
• Oh, and a third caveat - doctors use LN to "burn" off warts and other unwanted growths on your skin - it will flash freeze your flesh, and one can lose limbs, or be blinded, by spills... Always use common sense and appropriate PPE - I always wear a full face visor, an overall jacket, and welding gloves when working with LN.

 

[HennieL]

OK, here's a bit more technical information on the Bohler S705 steel (again, from the Bohler data sheet... The CCT (Continuous Cooling Transformation) chart:

Charts such as this provides a wealth of information on the hardening process, with the various slanting lines representing various cooling rates, and the figures in the circles showing the Vickers hardness that one can expect if it was possible to cool the steel at the rate of that particular slanting line. As an example, the highest hardness obtainable is indicated by the cooling rate of the second slanting line, with the Vickers number 882 in the circle. Now, using normal oil quenching (or air quenching, salt bath quenching, etc) one would not have a continuous uniform cooling rate, but rather a rate that starts off very high, and then slows down as the temperature drops. This is depicted in the so-called TTT (Time Temperature Transformation) curve, which looks like this:

To the top right of this graph is a region marked "P", , which represents Pearlite - a soft phase of steel. With a bit of imagination this P-region looks like a nose, and one should ensure that the quenching cooling rate is rapid enough to not cross the Pearlite nose, else the steel will not harden as it will transform into Pearlite, and not into Martensite (the good, hard stuff). Each steel has it's own particular curves, and that makes it such a gamble if one tries to harden an unknown steel.
Anyway, with reference to the TTT curve above, one can see that we need to reach a temperature of below 600°C in less than about 200 seconds - quite doable even in air... A steel such as O1 has a time to reach 600°C of only about 8 seconds, and something like 1095 only has about 1.0 seconds - really challenging.
Of course, one needs to consult both the TTT and the CCT charts to get the best picture, as one would still gain a substantial increase in hardness by quenching closer to the optimum cooling rate indicated by the CCT - i.e. closer to the second curved line on that graph.

OK - enough metallurgy...

Here's something easier to get one's teeth into - the tempering graph for this Bohler S705 steel:

Interestingly, unlike normal tool steels, the hardness increases significantly when the tempering temperature increases, up to the ideal maximum temperature of about 550°C, after which it drops quite rapidly.

These graphs should hopefully assist anyone else who wants to try his/her hand at hardening HSS.

 

[clockworkcheval]

Our resident metallurgist has the practice of wrapping a piece of paper around the tool before wrapping it in stainless foil. This will carbonize in heat and so offer a tiny bit of addtional protection all over the tool inside the foil.

 

[Chiptosser]

From my Experience of heat treating of tool steels, from over exposure of high temperatures. The sample loses carbon from the steel! This changes the surface properties. If you can grind the surface .040-.060 thou. you may be able to salvage. The surface will have a grainy appearance after grinding.

 

[L98fiero]

From my Experience of heat treating of tool steels, from over exposure of high temperatures. The sample loses carbon from the steel! This changes the surface properties. If you can grind the surface .040-.060 thou. you may be able to salvage. The surface will have a grainy appearance after grinding.

I think that's from excessive over heating, kind of overheating overheating and referred to as burning where you get intergranular corrosion/oxidation and that can't be rectified by retreating as some of the constituent elements are 'burned out'.

 

[HennieL]

From my Experience of heat treating of tool steels, from over exposure of high temperatures. The sample loses carbon from the steel! This changes the surface properties.

It's true that any elevated heat (e.g. when hardening) coupled with oxygen will result in decarbonation - I have experienced it many times when hardening normal medium- and high-carbon steels that were not wrapped in a protective foil. Chiptosser is also correct that grinding away the de-carbonated outer skin will result in one again achieving the design hardness, as testing the very outer layer will give one a false low hardness reading. I don't believe, however, that it's necessary to remove this outer lower carbon "skin" just because it does not have the correct hardness - it is at worse still mild steel, and will still contribute some strength to the part. Also, it would defeat the object of trying to recover (say) a twist drill if one has to reduce it's diameter because the outer edge has lost some hardness - just use it until wear and tear has done that job for you, and then throw it away (or just use it on wood...).

Grain growth, however, is something else entirely. Larger grain makes for more brittle steel, and that's not good. Fortunately, if one has not lost too much carbon, one can refine large grain down into smaller grain again by multiple quenches, and by cryo-treatment, before one does a final hardening heat - this is "old school" for the top forgers out there, but is more appropriate to lower alloy and simple carbon steels. Given all the above, I would still try to recover at least some of the tools that were damaged in the fire by annealing and re-hardening as previously suggested.

 

[abby]

Makes one wonder how our simple tools such as D bits made from silver steel ever managed to work !
Dan.

 

[SmithDoor]

At time I use oven follow all data on heating.

Today I just use torch. It seems to work as just good as using a oven.

I like tampering use a toaster oven so hold temperatures for hours.

It just makes life simpler
Dave

Hi everyone,
I was asked on another thread to describe my process of hardening HSS woodworking gauges (chisels) - I suggested that I rather open a new thread to do so here.

Some background - I've been making high-end knives using mostly high alloy martensitic stainless steel for more than ten years now, and in the process to make the best knives that I could, I did a lot of reading up on the subject of heat treating steel, and learned quite a lot about metallurgy - well, enough to know that I actually don't know much, but at least I can sound as if I do know something . So, after mastering the black art of using fire and water to magically turn soft medium carbon steel into hard knives, I went on to specializing in rather exotic " super steels" such as Bohler Elmax and M390, CPM S35VN and S90V, and various others. All these steels required higher austenitizing (hardening) temperatures than the normal carbon steels, and some very low temperature cryogenic treatments to complete the quenching process (down to soaking the knives in liquid nitrogen for 24 hours to transform retained austenite and to refine the grain of the steels (who knew that steel had grain...). Well to cut a long story short, I ended up looking for the next challenge, hardening high speed steels (HSS) - and this is where this post starts... Looking at the exorbitant prices charged (at least here in South Africa where EVERYTHING is imported (mostly from China...)), I thought that I could at least achieve the same, if not better, performance for much less money if I "rolled my own", so to speak.

So, I ordered some S600 (equivalent to M2 HSS) and S705 (equivalent to M41 Cobalt HSS) steels from Bohler South Africa (imported from Sweden, I think), and set of on a new quest...
Anyway, here's the outcome: The topmost " long and strong" gauge is made from the S705 (M45) steel, and the smaller diameter one is made from the S600 (M2) steel. The other chisels ahown here are two parting tools (the larger one made from D2 and the smaller one from an old high-carbon saw blade), and the little fingernail gauge on the bottom was made from O1.
View attachment 125555

OK - what equipment is needed to harden these steels?
I have a quite ordinary vertically loaded electric oven, used all over the world by knife makers. It runs on normal South African 220V household power, consuming about 9A of current (i.e. nominal 2KVA load). This oven is about 600mm (2') high, and has an internal diameter of 150mm (6") - ideal for knives and other skinny objects, impractical for anything larger than about 125mm diameter. As can be seen, I mounted the oven on small wheels, and push it to just outside my workshop when I need to do any hardening (right next to my quenching tank):
View attachment 125556

And here's a peek inside the oven:
View attachment 125558

I will post some more on the specifics of the heat treatment in my following post

 

[vk7krj]

To add to Henniel's safety warnings re. liquid nitrogen, suffocation by nitrogen gas is quite insidious as the brain only reacts to an increase in carbon dioxide, NOT a reduction in oxygen- hence there is absolutely no warning of suffocation by excess nitrogen in the air- one moment you are awake, the next you are unconscious on the floor. And as the nitrogen that put you there is heavier than air, without outside help you will rapidly become an ex- model engineer......

 

[willray]

I was asked in another thread to comment on the possibility of re-hardening some HSS tools that had been over-heated in a fire. In my opinion (and I want to stress that I'm not a metallurgist, so it's just an opinion...) it should be possible to re-harden the tool, bearing in mind that either the original fire damage or the re-hardening could have resulted in some warping/deformation that one would have to live with - for HSS lathe cutters this won't be serious, but for e.g. a drill it might result in a useless tool that is out of round. Regardless, I think it would be worth a try, at least. I would do the following:...

For thread context, I was the asker in Brian's thread on his heat-treat oven installation.

I am hoping to salvage as much (any!) usable value out of a batch of HSS tooling that went through my shop fire.

Yes, HSS can stand a lot of heat without losing hardness. Yes, I had a hot shop fire. In some spots, cast-iron machine tools turned to puddles on the floor. Barrels of silica flour, turned into big glass plugs. The (aluminum) engines and transmissions on 3 cars parked about 10 feet away from the wall of the shop, melted and ran down the hill. A pallet full of fire brick parked against the outside wall of the shop turned to powder. Amusingly, the freezer half-full of frozen pizzas and burgers turned into an oven and the local raccoons had a week-long banquet.

A couple tooling cabinets full of mostly-new-unopened Chicago Latrobe, Cleveland Twist Drill and Morse HSS tooling - mostly drills and milling cutters - baked well enough to anneal.

Since HennieL has accomplished what the sages of industrial HSS treatment have long-claimed is impossible, I am hoping that maybe his approach might let me recapture some use out of the remains. Even if all I can get is woodworking capability, it will still be far better than throwing it away or trying to find the funds to replace it - insurance certainly isn't going to help with that one!

Will Ray

 

[willray]

...

• Oh, and a third caveat - doctors use LN to "burn" off warts and other unwanted growths on your skin - it will flash freeze your flesh, and one can lose limbs, or be blinded, by spills... Always use common sense and appropriate PPE - I always wear a full face visor, an overall jacket, and welding gloves when working with LN.

While I will third HennieL's warning about the breathing hazards of LN, I will offer a somewhat different opinion regarding the flash-freezing dangers.

This is mostly offered as a public-service-announcement in the vein of "you can do this (use LN safely in the shop), you won't kill yourself", so please, standard caveats apply, don't try this at home, professional ***** physics students performed all stunts, etc, etc, etc...

None of what HennieL said is untrue, but at the same time, you don't need to tiptoe around LN to be safe using it, AND, using the inappropriate personal protective equipment to try to be safe is more dangerous than not using any PPE at all.

The thing you have to avoid, is contact with LN, but coming into contact with LN isn't as easy as touching it. As long as you are warm, LN will flash-boil on contact with you, and the layer of vapor created is a quite effective insulator. You can carry a spoonful of LN around in your hand, and it will feel cool, but as long as you keep it moving, it will just roll around sputtering in a ball, much like if you put a drop of water in a hot frying pan. We had one ***** in the lab that liked to impress the interns by "gargling" with mouthfuls of LN. You can find youtube videos of people pulling this stunt.

What you can't do, is let the LN sit in contact with you for any extended period of time. Give it a few seconds, and it'll cool the skin its bouncing around on sufficiently that the LN is no-longer boiling violently, and then it can make better contact and drag your skin down to -320 F.

As a result, you are actually in more danger using inappropriate personal protective equipment with LN, than just doing it bare-handed. Light, permeable materials don't have much heat capacity, and so they cool relatively quickly to the point the LN no-longer boils off of them, and instead soaks in. They then can hold it still so that it can't dance around on its own vapors, allowing the LN to cool the underlying skin far faster than would have happened if you just dumped the LN on bare skin. If you don't have PPE that's actually designed for use with LN, there's a good chance that using something inappropriate increases your chance of hurting yourself, over just handling the LN like it's water, and letting your "oh, that's cold" reflexes keep you out of trouble.

And keep in mind - that chunk of steel that's been soaking in LN, is at -320F, and it doesn't boil off and protect you from the temperature when you pick it up in your bare fingers... DAMHIKT...

 

[HennieL]

Thanks Will - very true...

I did not try to create the impression that one must not use LN, or that it's very dangerous - from personal experience I can attest that I have been using LN regularly (around once per month or two) for the past 9+ years without any incident... but then I AM aware of it's potential to do harm, and don't give it the opportunity (much as in my younger days when I used to catch and keep all kinds of snakes... but ran away whenever there was a Mamba involved )

 

[willray]

Thanks Will - very true...

I did not try to create the impression that one must not use LN, or that it's very dangerous - from personal experience I can attest that I have been using LN regularly (around once per month or two) for the past 9+ years without any incident... but then I AM aware of it's potential to do harm, and don't give it the opportunity (much as in my younger days when I used to catch and keep all kinds of snakes... but ran away whenever there was a Mamba involved )

Ah, my apologies - if my writing appeared to imply that I thought you were trying to overplay the dangers, that was not my intent.

I just find that most people who have been given a little information on the dangers of something, frequently tend to badly overestimate, or badly underestimate the actual danger and appropriate safety precautions. Since the "flash freeze" danger from LN is one of those non-intuitive situations where a novice without proper protective gear is usually safer just with their reflexes (and safety glasses) than with cobbled-inappropriate protective gear, I thought it was worth expanding a bit.

Now a dewar full of methanol and dry ice, that's a different beast all together. -109F, ought to be balmy by comparison to LN. Only the methanol doesn't evaporate... Damn that hurts!

 

[awake]

Hennie, is this a case of buy-and-use-immediately? Or can you store it for a (presumably short) time?