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Advice on Heat Treating

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Dave S17/03/2021 22:16:46
257 forum posts
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The critical point is to get it hot enough, and heat it long enough to allow the transition from face centered to body centred (or the other way - I forget which).

I always quench to full cold then temper. Tubal Cain actually writes quench to not more than 50 degrees - I.e. cooler than 50.
Now I have a kiln I tend to use that - it’s got good control and I can bung the parts in and ignore them without risking overheating the tempering.

I did have a deep fat fryer - turned up to 190 and again little risk of over temperature. Not good for higher temperatures, but “yellow” for most things I do seems enough. The parts don’t change colour unless exposed to air as the colour is caused by the thickness of the oxide forming on the surface.

There is no problem with the length of time at temper temperature as far as I know, just that higher temps draw the temper more.

Dave

Ramon Wilson17/03/2021 23:17:35
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Well, as I said I spent quite a lot of time at work responsible for the heat treating and grinding of small parts from Uddeholm 'Arne' B01 as well as other tool steels from the same supplier.

That was twenty plus years ago but to my surprise the heat treatment sheet for Arne as provided by Uddeholm then was identical to the one I have just looked at now here - page 4

We followed this procedure to the letter heating in Wild Barfield ovens to very controlled temperatures before quenching to room temperature in (whale) oil and carried out the tempering in a separate oven designed for that express purpose. When we began this process at first the initial batch went into the tempering oven with residue oil on it which created a considerable amount of smoke. From that point, after the initial quenching, these parts - they were mainly punches and anvils for small press tools and would be done as batches of twenty hanging in a mild steel carrier, four carriers to a load so eighty parts in all - would be dropped into very hot water and a product called Citri Kleen. This was a potent degreaser, the oil being rapidly dispersed and the parts were allowed a short time to dry off then placed in the tempering oven. As previously said that would be done twice so the parts would see a Citri kleen bath three times before grinding.

Arne B01 is basically the same as guage plate and is ideal for home based heat treatment. What is difficult to obtain and be aware of when flame hardening is the actually temperature at which to quench. 'Cherry Red' is an oft quoted state but that can vary as much as the amount of different people attempting it. What I can say - with certainty - is that when these parts came out of the oven having been at the correct temperature 800-850 depending on harness required they came out a very dull red indeed. Over heating then to a bright red or leaning toward orange is too much and will, if overdone, lead to a crystaline and brittle metal structure.

As Dave S has just remarked, time at temperature does not affect the degree of temper but a variation of temperature most certainly does which brings me right back to the beginning - the best thing outside an oven in the home workshop is a tray of hot sand for about as best control as you can expect

Tug

Hopper18/03/2021 06:11:24
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Posted by Ramon Wilson on 17/03/2021 23:17:35:...

.... What is difficult to obtain and be aware of when flame hardening is the actually temperature at which to quench. 'Cherry Red' is an oft quoted state but that can vary as much as the amount of different people attempting it. What I can say - with certainty - is that when these parts came out of the oven having been at the correct temperature 800-850 depending on harness required they came out a very dull red indeed. Over heating then to a bright red or leaning toward orange is too much and will, if overdone, lead to a crystaline and brittle metal structure.

I read somewhere that there is confusion over "cherry red" because in the 19th century cherries were commonly a very dark, dull red colour. In the 20th century growers have selectively bred cherry trees to produce bright red fruit that customers are attracted to but amateur blacksmiths and heat treaters are much confused by.

SillyOldDuffer18/03/2021 09:18:57
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Posted by Dave S on 17/03/2021 22:16:46:

...

I always quench to full cold then temper. Tubal Cain actually writes quench to not more than 50 degrees - I.e. cooler than 50...

Dave

An apparent contradiction, easily explained! I said don't let the temperature drop below 50°C because William said he's using O1 which is an Oil Quenching steel.

I should have mentioned the type of steel matters, for example, I don't think Silver Steel (Water Quenching), is as fussy as O1. One source says of Silver Steel 'temper as soon as possible after hardening preferably before the tool reaches room temperature'. (My bold)

Hardening and Tempering are achieved by manipulating the metal's internal crystalline structure and the desired properties result from a careful mix of controlled temperature change and timing. The exact mix depends on what the steel is made of. Although getting consistent results to a specification requires rather careful control, home-workshop methods seem to work reasonably well for our unmeasured purposes.

There's skill and judgement in it; I've improved with practice, not because I'm doing anything clever, rather that I don't dither : heat, plunge, temper all done roughly at the right time and temperatures. Turning the lights down to judge the hardening temperature helps, as does turning them up again to judge edge tempering oxide colours.

Dave

Ramon Wilson18/03/2021 10:18:37
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Hopper, as you rightly infer there are as many shades of cherry in the basket as there are cherries! Knowing which one to pick is another matter wink

As Dave says there is skill and judgement required but it's not a difficult process in the home workshop. The main thing to be aware of is overheating the part. If it's felt taht is the case just let it cool slightly before plunging into the oil bath. Simply dropping parts in is not a good idea as if the part is thin and goes in at even a slight angle to the surface of the oil distortion usually occurs.

I've said on many occasions I still get a buzz out of making a cutting tool and seeing it work as hoped for.

Like most I use silver steel and for most small cutters never bother to temper so as to have maximum hardness. Though it is intended as water hardening steel I always quench in oil basing this on an experience at work. At one time we produced a largish (for us) batch of small 12 mm diameter 'cams' with a 4 mm hex socket broached in one end. Quenched to manufacturer recommendations a very high percentage of them cracked on use around the hex socket.

The new batch were quenched in oil with a 100% success rate. Since then I have always quenched in oil for that reason. Yes the resultant hardness factor is not quite the same but it has always proved more than enough to do the job in hand. It has to be said that usually said tools are only for the one job so the risk of cracking is outweighed by the hardness of the cutting edge - for me. Were it to be something intended for long time use then it would get tempered.

Besides that occurance with Silver Steel, in the many thousands of tool steel parts heat treated over several years I only witnessed cracking of one batch of components. Because that was only one of the four batches out of the oven treated it was quickly realised that this was the first batch quenched into oil that was at ambient temperature. It had been a very cold night and though a modern factory the oil temp was well below what it would have been normally. From that point on a cube of steel was heated at the same time and dropped ito the oil to take the chill off before that first batch went in. Obviously that oil increased in temperature as each batch was done but no variation in hardness was experienced as a result.

Most, though not all of our steels but including silver steel came from Uddeholm and all our heat treatment followed their procedures as closely as possible. Heat treatment was a regular occurence in the machine shop that I ran not so much daily but certainly weekly - along time ago now but still quite fresh in my minds eye smiley

Happy days - certainly miss the grinding facilities!

Tug

Rod Renshaw18/03/2021 10:28:41
347 forum posts
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I read somewhere that back in the day when the colour of "Cherry Red" was originally used as a description of metal temperature, most of the apprentices being taught would only have been familiar with the glace cherries they saw on the Sunday tea table. Apparently fresh cherries would not have been seen in industrial cities at that time.

Rod

ega18/03/2021 10:28:43
2323 forum posts
190 photos

Has the use of a magnet to test for temperature been mentioned? This avoids the uncertainty of cherries, etc.

Tubal Cain's little book has excellent photos of the various temperatures and it just happens to be number one in the series.

Martin Kyte18/03/2021 10:51:43
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Niel once made the usefull alternative as cooked carrots to cherry red which may even have started out as cheery red, who knows.?

With most propane torches and hearths it equates to as hot as you can get it. I can't say I have ever managed to 'over cook' but then I'm not sure I would know if I had. Everything always comes out hard except the odd time I've picked up a lump of stainless thinking it was silver steel.

Probably more important to hold or soak at the top temperature. An hour for each inch of thickness rings a bell.

regards Martin

Hopper18/03/2021 11:07:33
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Posted by Ramon Wilson on 18/03/2021 10:18:37:

.. silver steel ... Though it is intended as water hardening steel I always quench in oil basing this on an experience at work.

I'm glad to hear I'm not the only one who does this. I have always used oil when making cutters etc out of silver steel, in line with what we did at work many years ago. Been told by well informed types since that silver steel is water hardening and oil will make it brittle etc etc etc but that has never happened to me.

One thing I see some people doing is using a tiny cup of oil (or water) for quenching, and bubbling and boiling and smoking going on as a result. I use a large bucket, or at least a half gallon tin, and don't just drop the job in. Plunge it under the surface held in a pair of pliers and move it quickly up and down under the surface while moving it around in a circle near the outer edge of the container. This helps stop vapor bubbles forming on the hot job and slowing down the heat transfer so vital for quenching.

Hopper18/03/2021 11:11:57
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Posted by Martin Kyte on 18/03/2021 10:51:43:

Niel once made the usefull alternative as cooked carrots to cherry red which may even have started out as cheery red, who knows.?

Carrots I cook are orange, another colour again from dark cherry red or even bright glaced cherry red. As ega says, maybe best to use a magnet as Tubal Cain suggests. And like Martin, I find with a propane torch and firebricks and largish lumps (eg stress relieving my fabricated steel dividing head body) I will be happy with whatever shade of red I can struggle to achieve. No choice but a nice dull red usually.

Andrew Johnston18/03/2021 11:28:18
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Posted by Hopper on 18/03/2021 11:07:33:
Plunge it under the surface held in a pair of pliers and move it quickly up and down under the surface while moving it around in a circle near the outer edge of the container.

Likewise I use a large bucket. For expediency I use brine for quenching both silver steel and gauge plate. I'd agree that agitation is vital. To achieve maximum hardness, mid sixties Rc, the agitation needs to be vigorous. Anything less will result in a measured hardness around the mid forties Rc. I always temper, but don't worry about keeping the part above a certain temperature after quenching. I temper within an hour or two of hardening; basically the time taken for the electric furnace to cool down to tempering temperature. These are what I use to measure hardness:

hardness_sized.jpg

Andrew

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