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Hardened Silver Steel Shattered - How to Avoid?

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SillyOldDuffer13/03/2019 18:48:10
3857 forum posts
775 photos

Reporting back with another silver-steel under achievement:

dsc05953.jpg

The focus isn't quite right, but you can just about make out the crystalline look of another brittle failure.

After polishing this one went in the oven for 20 minutes at 220C. As it turned a nice straw yellow I was confident. With plenty of reversals for swarf removal it cut well into brass for about 3mm, then the end disintegrated without warning.

I believe my mistake was leaving the quenched bit cold overnight before tempering it. It seems quenching and tempering should be done at the same time, ideally while the metal is still uncomfortably hot to hold from the quench. Try again tomorrow...

Dave

Nigel McBurney 113/03/2019 20:21:08
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542 forum posts
3 photos

I think some of the problem is due to the change in section of the job,a big mass of steel too close to the the fine cutting part,heat transfers to the mass so the red hot end of the tool starts to cool before it has a chance to get quenched, also if the tool is plunged into water sharp end first there is a tendency for an airlock as the tool has a blind hole ,(a centrally drilled hole through the tool wood get rid of trapped air),so may tend not to quench so well on the ID of the cutter. My first employer used a lot of silver steel tools mainly pin cutter and large counterbores on the capstan lathe as most the work was brass and nickel silver,hardening was primitive ,paraffin blowlamp,and a bucket of water ,and we were taught to plunge vertically and keep the lamp flame on the tool until the tool was close to the surface of the water to maintain heat,crude but it works,and the tools wood last for years on thousands of the lens retaining parts of microscope eyepieces, resharpening was done on a cutter grinder.Tempering was done by polishing the tool wth emery until it chaged to straw . I still use a calor torch and water for hardening at home.

In those days nearly everyone one used Stubbs Silver steel ,which was very good,and always marked with the stubbs name at one end,and it was drummed into us,always cut the piece you want off the unmarked end,so that you know that the piece left was marked silver steel,so there would be no errors by using the wrong material.

Mick B113/03/2019 22:35:01
914 forum posts
55 photos

I wouldn't've thought leaving it before tempering would affect the issue.The change of section may have introduced stresses and microcracks on quenching, which tempering would obviously not remove. It might've been better to taper out the cutter diameter to the parent bar at 30 degrees included or suchlike, using a tool with a bit of a tip rad. You really only want the cutting surfaces of the teeth to be hard, so it might work just to get the tooth tips bright red and hold it for half a minute prior to quenching - then polish and temper light straw.

I can remember milling a stepped rectangular forming punch out of BS1407 on a Bridgeport when I was a (helluva) lot younger, heating and soaking it red hot, then tipping it into a bucket of oil and wondering why it cracked on the inside angle of the step. There was a lot more steel in that, though.

Hopper13/03/2019 22:49:15
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3288 forum posts
58 photos
Posted by SillyOldDuffer on 08/03/2019 16:48:24:

I heated the silver steel bright red and plunged it into cold water.

From your OP. Might be a source of some of the problem. Quenching in oil will tend to give a less brittle result than water. Anything form old motor oil to a tin full of cooking oil will do the job.

On your piece, I would heat the main shank with the torch and let the colour run up to the cutting tips then quench as soon as the colour hits the tips.

SillyOldDuffer14/03/2019 11:41:12
3857 forum posts
775 photos

Thanks chaps.

I would NEVER have thought of Nigel's point about airlocks during quenching - that's likely. I'm going to try again in water with air-lock counter-measures, and then try Hoppers' colour running tempering method.

I like the idea of quenching in oil too. Brine is recommended for a very hard finish, cold water for hard, but both risk cracking. Oil is gentler, less hard but tougher, and - of course - my steel tool doesn't have to be very hard to cut brass.

Cheers,

Dave

Hopper14/03/2019 11:51:16
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3288 forum posts
58 photos

I've always used oil for quenching. Never had a problem with it. Always hard enough to cut steel when making counter bore tools, D-bits etc. And no fractures that I remember. As your tool is very thin on the wall (1.6mm) I would aim for toughness over hardness -- as you say its for cutting brass.

As far as the plunging goes, yes hold the job vertically and plunge it into the oil/water in a large container such as a bucket then rapidly move the job up and down under the surface as you simultaneously run it around in a circle when viewed from above. This gets the job away from the air/vapor pockets that form.

Edited By Hopper on 14/03/2019 11:55:33

Mike Poole14/03/2019 12:31:19
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1754 forum posts
44 photos

I remember a mate got a bollocking at tech college when quenching a fairly large bar of steel in oil, plunging the bar in the oil produced a cloud of oil vapour which could be ignited by pulling the still glowing bar out of the oil, while demonstrating this to us the instructor walked round the corner, even though the hint of a smile and the shake of his head in disbelief we were in no doubt that the game was over.

Mike

Phil Whitley14/03/2019 15:37:22
775 forum posts
102 photos

Duncan Webster is right about Tubal cain's book, it is excellent, and I think the MO is to heat till it loses its magnetism, and then hold at that heat for twenty minutes per inch of thickness before quenching. I too would quench in oil, you do not need glass hardness for brass, or anything much except perhaps files, and we know what happens when you drop a file dont we! It shatters!

SillyOldDuffer14/03/2019 16:06:31
3857 forum posts
775 photos

Success!

dsc05954.jpg

Can't be seen in the photo but the cutting diameter is chamfered into the main body as suggested by Nigel. Heated to bright red-heat in a dim room, then plunged vertically hole end-up to reduce chance of an airlock. Polished whilst still warm and blunt end heated on an insulating fire-brick until the yellow line reached the cutting tip as suggested by Hopper. Camera isn't capturing the colours accurately - the silver steel is rather bluer at the drill-chuck end than it appears.

I have to make another one to prove it's not a fluke!

Thanks all,

Dave

jaCK Hobson14/03/2019 18:07:51
160 forum posts
20 photos

The grain in the out of focus failure doesn't look that bad for silver steel but difficult to tell. If it is bad, then overheating would be the most likely cause. Use a magnet. For small silver steel like this, you don't need to soak for very long at all.

You need to cool to room temp before temper. Some steels, stainless in particular, need super cooling to get full hardness (liquid nitrogen). For silver steel, it doesn't really make much difference if you delay between quench and temper. (You should do super-cooling as soon as possible after quench).

If using a domestic oven then I would have thought that a single temper for one hour at 220C should do. However, the fail shows that the temper wasn't good enough. You could increase the temp a bit - especially if just cutting brass. So maybe 240. Some steels do need multiple tempers to get the best, but not silver steel.

Do you know if you did anything different for the successful version - maybe tempered for longer?

You can keep increasing temper temp without re-hardening but if you over temper then you will have to re-harden. You can repeat the cycle a lot - the risks are stress cracking or carbon depletion.

 

Edited By jaCK Hobson on 14/03/2019 18:11:55

Neil Lickfold14/03/2019 18:40:01
506 forum posts
89 photos

There is W1 and O1 in the silver steel range. The W1 is water quench. With the W1 , I was always taught that the water should be heated to at least 40C if using water quenching, then leave in ice water after the initial quench.

Polish, keep oil free, then temper 1st time at 150c and then slowly increase temper temp to the desired hardness. After the 1st temper at 150C for minimum of 1 hour per 1/2 inch of section, quench in boiling water, then retemper to the desired range. IE a ball race needs to be retempered at 150c , but cutters in the 180 to 210 c range.

The way the quenching is done is also important, it does require agitated oil or water to be effective.

Sometimes shell cutters fail due to no internal clearance , or outside clearance.

Neil

SillyOldDuffer14/03/2019 20:42:14
3857 forum posts
775 photos
Posted by jaCK Hobson on 14/03/2019 18:07:51:

The grain in the out of focus failure doesn't look that bad for silver steel but difficult to tell. If it is bad, then overheating would be the most likely cause. Use a magnet. For small silver steel like this, you don't need to soak for very long at all.

You need to cool to room temp before temper. Some steels, stainless in particular, need super cooling to get full hardness (liquid nitrogen). For silver steel, it doesn't really make much difference if you delay between quench and temper. (You should do super-cooling as soon as possible after quench).

If using a domestic oven then I would have thought that a single temper for one hour at 220C should do. However, the fail shows that the temper wasn't good enough. You could increase the temp a bit - especially if just cutting brass. So maybe 240. Some steels do need multiple tempers to get the best, but not silver steel.

Do you know if you did anything different for the successful version - maybe tempered for longer?

You can keep increasing temper temp without re-hardening but if you over temper then you will have to re-harden. You can repeat the cycle a lot - the risks are stress cracking or carbon depletion.

Edited By jaCK Hobson on 14/03/2019 18:11:55

Thanks Jack,

Truth is I don't know what made the difference. Obvious differences: I added a chamfer, plunged and agitated to avoid a bubble, and then tempered with a blow-lamp instead of the oven.

However I think overheating in the earlier attempts is possible - I don't quite know what I'm doing in terms of required redness and time held at temperature! Also, in the flame, the thin part of the cutter tends to heat up faster than the body, and the outer rim nudged yellow briefly.

Might be smart for me to have an extended session heat treating silver-steel in various ways. The core drill seems an excellent way of exposing any weakness in the metal. My earlier attempts hardening home-made drills seemed OK, perhaps they were imperfect too, and I was lucky they lasted long enough.

Cheers,

Dave

Neil Lickfold15/03/2019 07:26:14
506 forum posts
89 photos

For hardening a part like your cutter, I would be heating the solid shank, and allow the heat to transfer to the thinner part. It would need a hold time of about 5 minutes. With a gas torch that seems an eternity . Don,t over heat and get it all scaley, and I always use a carburising flame for heat treatment.

Practice is a good idea. Real daylight or no light is best to get the idea of the heat when getting the steel hot. There are colour charts on the net, but getting them nicely printed I have found is best from a photo centre.

Neil

jaCK Hobson15/03/2019 08:10:39
160 forum posts
20 photos
Posted by Neil Lickfold on 14/03/2019 18:40:01:

There is W1 and O1 in the silver steel range.

I would agree that 'silver steel' may cover a broad range of alloys but the composition of modern silver steel should be better understood and should not include W1 or O1.

W1 is good for small edged tools, Not that easy to get hold of in UK in small quantities. Easy to heat treat and anneal. Very fine grained. Old files are often similar alloy.

O1 easy to heat treat, not so easy to anneal. Can get it hard for larger sections than W1. Fine grain. Ground flat stock is often O1. 'Shim steel' may also be ground flat stock and can often be something with less carbon and won't get as hard.

Bright round bar in small diameters is often silver steel. I think 'pivot steel' is similar, if not silver steel. I wonder if., as silver steel is usually round bar, it is expected it might be turned in a lathe, and therefore has the composition been designed to give larger grain in order to make it easier to machine on a lathe.? That question obviously exceeds my knowledge of metallurgy but would be easy to answer for someone who knows.

Edited By jaCK Hobson on 15/03/2019 08:19:30

jaCK Hobson15/03/2019 08:17:47
160 forum posts
20 photos

Posted by SillyOldDuffer on 14/03/2019 20:42:14:

tempered with a blow-lamp instead of the oven.

Dave

My guess would be that the blow lamp got it hotter than the oven. Oxidising colours are a poor judge of absolute temp. They can be reliable for indicating relative temps if all conditions are the same - same material, same heat source, same atmosphere, same surface preparation. I think you have done a perfect demonstration of this - it looks like both success and fail are 'dark straw' but seem to behave differently.

Mick B115/03/2019 09:22:32
914 forum posts
55 photos

I made this little cutter to slice out the radiused groove for the cascabel ring on my carronade. It was turned, then offhand ground. I used a butane blowlamp to heat the tooth tips bright red and quenched in the grinder's diptank. On the second attempt when I held the red heat for half-a-minute or so, it worked - the teeth resisted filing. Then I polished the end face and played the blowlamp on it carefully with pauses between quick heating passes until I saw the light straw oxide, then quenched again and finished the tooth surfaces with an India oilstone.

OK, so it only had <10 mm3 of EN1A to remove, but the teeth are still hard and the shank fileable. It's about 11mm. diameter, plain silver steel.

cascabelring2.jpg

Edited By Mick B1 on 15/03/2019 09:25:20

SillyOldDuffer17/03/2019 16:26:23
3857 forum posts
775 photos

Just to finish this one off, I've made progress! I can now make tools that are too soft, too hard, or just right to order!

I think my main problem was the mix of thin and thick metal involved in making a core drill, plus the construction is likely to trap steam inside and spoil the quench. Judging the temperature and holding it for a suitable amount of time is a little tricky: I was working on soak time calculated on the thin part of the metal (ie not very long), and have found that the time needed for the full thickness gives better results. Better control of the light in the workshop helps judge temperatures; full brilliance needed tp see the straw oxide layer during tempering, and rather dim for seeing cherry red before quenching.

Heating the silver-steel whilst it's laid on a fire-brick rather than holding it in a pair of pliers produces more consistent results. It's easier to avoid overheating the thin end while the main body is still warming up. Laying on a brick also makes it easier to accurately chase the straw-colour up to the sharp end during tempering.

Thanks again,

Dave

PS I like Mick's cannon!

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