Calibrating propane forge using K-type thermocouples and optical thermometer
- Thread starter zambezi
- Start date
I haven't experimented much with my optical pyrometer to see exactly where the laser dot is in relation to the sensor focus at different ranges...but I did presume they were aligned vertically...close quarters at 450mm the sensor looks about Ø9mm and just below the laser dot.
The curve could be caused by residual stress, or differential heating...one side being heated first or one side being quenched first...as long as the blade is held vertical in both furnace and quenchant it should not be caused by you.
Rather than faff with your “three bolt in the vice” straightening system, I would suggest that you try tapping any curved blades blades onto a slightly concave [just enough to cope with spring back] wood block with a full faced wooden mallet...much quicker and there is no risk of bruising the metal. Favourite to do this is while it is at tempering temperature. It is almost instant, and any over correction can be remedied equally fast.... When I am straightening small stuff with a mallet it is to preserve the texture and profile...done at red heat you do get a bit of wood smoke though!
I do have a number of versions of the three point bending/straightening tools for the fly press and hydraulic presses...but in the early days I welded pins to vice face cheeks so that I wasn't trying to juggle pins and workpiece and vice tommy bar. I used much larger diameter pins than your coach bolts and even ground them nearer to a crowned profile to reduce any likelihood of bruising. You could possiblly drill some holes in a bit of wood resting on the vice slide which would hold the bolts in place? But I would prefer to use a wood block and mallet.
An Australian industrial smith has a system for resetting the curve in leaf springs....he called it the greasy stick method...if you rub a bit of wood (commonly a hammer handle) on to the spring while you are heating it up, the wood suddenly slides easily as if it was greasy when the steel reaches around 480˚-500˚C which is the tempering temperature of spring steel. Too cold and it sticks, too hot and it chars.
A bit too hot for your steel though. But the principle of setting at tempering temperature is the same.
Alan
The curve could be caused by residual stress, or differential heating...one side being heated first or one side being quenched first...as long as the blade is held vertical in both furnace and quenchant it should not be caused by you.
Rather than faff with your “three bolt in the vice” straightening system, I would suggest that you try tapping any curved blades blades onto a slightly concave [just enough to cope with spring back] wood block with a full faced wooden mallet...much quicker and there is no risk of bruising the metal. Favourite to do this is while it is at tempering temperature. It is almost instant, and any over correction can be remedied equally fast.... When I am straightening small stuff with a mallet it is to preserve the texture and profile...done at red heat you do get a bit of wood smoke though!
I do have a number of versions of the three point bending/straightening tools for the fly press and hydraulic presses...but in the early days I welded pins to vice face cheeks so that I wasn't trying to juggle pins and workpiece and vice tommy bar. I used much larger diameter pins than your coach bolts and even ground them nearer to a crowned profile to reduce any likelihood of bruising. You could possiblly drill some holes in a bit of wood resting on the vice slide which would hold the bolts in place? But I would prefer to use a wood block and mallet.
An Australian industrial smith has a system for resetting the curve in leaf springs....he called it the greasy stick method...if you rub a bit of wood (commonly a hammer handle) on to the spring while you are heating it up, the wood suddenly slides easily as if it was greasy when the steel reaches around 480˚-500˚C which is the tempering temperature of spring steel. Too cold and it sticks, too hot and it chars.
A bit too hot for your steel though. But the principle of setting at tempering temperature is the same.
Alan
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The curve grew as the workpiece soaked held vertically in the forge at the 1053℃ HT mark. I.e. it was curved before quench.
It curved toward the burner. I would have thought the outside of a curve would be where the greater heat is arriving if it is a heat-induced bend.
So this looks to be an internal stress issue, not differential heating or cooling. It did not affect the last blade cut from same AEB-L barstock. Is there a work action that can induce stresses? Or just luck of the draw and be ready with a corrective press?
Stalker62
Well-Known Member
OK.
For the record.
From the title of this thread, through every post, every post - I have no clue.
Not a single clue what any of this means.
Surely I cannot be the only one...surely?
No?
Really, no?
Just me then...(thrown out of metal work class in 1976 and with good reason).
For the record.
From the title of this thread, through every post, every post - I have no clue.
Not a single clue what any of this means.
Surely I cannot be the only one...surely?
No?
Really, no?
Just me then...(thrown out of metal work class in 1976 and with good reason).
Fascinating, lots going on here.
Re internal stresses in the plate stock of your steel, yes I expect that there will be. Since you are grinding your blades to shape by stock removal, rather than blacksmithing them to shape, that will be relieving some, but maybe not entirely symmetrically.
As an early on apprenticeship exercise we were handed a piece of cold-rolled bar, then told to hacksaw it halfway down it's length. It opened up like a banana being unpeeled.
It may be that you do not need to get it up to austenizing temperature, indeed that might even be counter-productive. Maybe all it needs is to be is to be taken to normalisation temperature, around 850C. It may have been austenized sufficiently during the manufacturing process and needs no more.
However you do it, a slow transition between the two might be preferable.
It also may not even need an oil quench. Air quenching can also work on a thin section like this, even as simple as blasting it with air from a compressor, evenly applied.
Providing it gets over-hard, for your application, you can then anneal it down to where you want it.
Even insert a cryo treatment during the process. My "domestic" freezer gets down to -23C. Dry ice much lower, if you can get it conveniently in the UK. Liquid nitrogen better (and yes, I can get that from my old contacts, just up the road, If I wanted some).
Some very wordy and detailed background on AEB-L. Seems largely credible:
knifesteelnerds.com
I have never made a knife. But have been involved, mostly as a specifier, observer and customer, with some very different applications of steels, so am intrigued to observe this work in progress. From those that are doing it (not me).
Re internal stresses in the plate stock of your steel, yes I expect that there will be. Since you are grinding your blades to shape by stock removal, rather than blacksmithing them to shape, that will be relieving some, but maybe not entirely symmetrically.
As an early on apprenticeship exercise we were handed a piece of cold-rolled bar, then told to hacksaw it halfway down it's length. It opened up like a banana being unpeeled.
It may be that you do not need to get it up to austenizing temperature, indeed that might even be counter-productive. Maybe all it needs is to be is to be taken to normalisation temperature, around 850C. It may have been austenized sufficiently during the manufacturing process and needs no more.
However you do it, a slow transition between the two might be preferable.
It also may not even need an oil quench. Air quenching can also work on a thin section like this, even as simple as blasting it with air from a compressor, evenly applied.
Providing it gets over-hard, for your application, you can then anneal it down to where you want it.
Even insert a cryo treatment during the process. My "domestic" freezer gets down to -23C. Dry ice much lower, if you can get it conveniently in the UK. Liquid nitrogen better (and yes, I can get that from my old contacts, just up the road, If I wanted some).
Some very wordy and detailed background on AEB-L. Seems largely credible:
All About AEB-L - Knife Steel Nerds
I've gathered all of the information I have on AEB-L steel, including its history, toughness, edge retention, ease in sharpening, and other properties.
knifesteelnerds.com
I have never made a knife. But have been involved, mostly as a specifier, observer and customer, with some very different applications of steels, so am intrigued to observe this work in progress. From those that are doing it (not me).
To be fair, some of the technical literature in the links supplied by some members can be opaque to those of us who are lay persons. Much goes over my head. But finding practical solutions to measurable challenges is do-able.
The simple proposition is how to bake steel till it becomes reliably hard? The quest of this thread is how can I know when the gas-powered forge is cantering at the pace I need for the work that is in it.
That is a link I had already bookmarked and referenced. A good mix of high level theory and practical steps:
There may be something in that. I checked back on my last AEB-L heat treat strategy and it included a slower and steadier progression to full temp with a pause at the normalising temp en route. That blade did not bend.
If you can see it bending while you are heating it...turn it over and see if it bends the other way. That will tell you if it is being caused by the heat source or by the process stresses being released.
But you might also just whip it out and give a dap with the mallet on a wood block before taking it up to quenching temperature.
Alan
But you might also just whip it out and give a dap with the mallet on a wood block before taking it up to quenching temperature.
Alan
Nope...you and me both...
But in view of the absence of any other knife makers helping out, I have been stepping sideways from my experience of similar struggles. Some of the issues are applicable but the main one of problem solving is fairly universal.
Try something, if it don't work, maybe try something different rather than repeat the same mistake just to be sure!
Alan
I would, politely, suggest, that that profile is backwards, at the beginning.
If you are going to do a full on heat-treat, then the normalisation step should come after the highest temp. part. Followed by quench, cryo if wanted, then temper.
Highest temperature first, then normalise. Not the other way around. Any normalisation prior to that will have been defeated by the subsequent higher temperatures.
Further reading, from the horse's mouth (albeit discussing tool steels, not those for knives.)
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Your link explains normalisation precedes highest temp:
However, what I had not previously done is taken the piece heated to normalising temp [850℃] back to room temp slowly before ramping back up to HT. I may add that cooling sequence...
But if Alantoo's dap with a wooden mallet en route to HT temp delivers the same outcome, that is time preferable. Certainly I cannot entertain as extended a normalisation cycle as suggested in that link.
I did say that my linky was about how to deal with specialist tool steels. The time consuming stress-relieving procedure is a quite different thing, necessary to avoid large expensive chunks of roughed out steel blocks from cracking disastrously before the following procedures.
The suggested soak at 850C for 15 minutes on the way up the profile, for thin sections, is there to allow the start of the conversion of the soft steel into Austenite, during which phase transition extreme stresses, and strains will be occurring within the crystal lattice. At this sort of temperature the steel is, for want of a better word, sufficiently ductile to take up these (considerable) internal forces. If you rush through this phase you could risk invisibly micro-cracking it, or even warping it as you proceed rapidly to the full Austenising temperature, 1100 degrees plus. This is why there is a suggested pause there. However if you start your forge coolly, then slowly ramp it up, It may not be completely necessary.
If you were dealing with a big block of steel that could be quite another matter.
I expect that your steel will not be Austenitic when you get it. It will be in a raw soft state, so that you can easily work it, grind it, drill it, punch out blanks from it. Even bend it. If it was Austenitic a magnet would not stick to it.
On that point, the Curie point of many steels is 770C. I expect that yours will be too, given its high chromium content.
This is where the unscientific "heat it up until a magnet doesn't stick anymore, then quench it" brigade are probably falling short, never getting it hot enough to fully Austenitise, so missing out on the benefit of much potential Martensite during the quench.
My suggestion of pausing again, on the way down, at around 850C, is that the subsequent quench should be equally as effective as one done from over 1000 degrees, but far less stressful to the blade.
The quench is to convert Austenite to Martensite. Once the steel is Martensitic after quenching it becomes hard, brittle and difficult to work in this condition. Then must come the magic of the tempering, to change the Martensite to reduce the hardness to desired levels.
The knifesteelnerd certainly knows his stuff, I hit upon his treatise on this whilst researching this AM.
https://knifesteelnerds.com/2019/03/18/magnet-heat-treating/#:~:text=Steel%20Magnetism,-Pure%20iron%20is&text=The%20Curie%20temperature%20is%20different,670°F)%20for%20nickel.
And, in retrospect, so might the Aussie chap in the video. I wondered about what he seemed to be blathering on about when disparaging the magnet check, but it seems that he was making a purely decorative knife from "rope Damascus" which I take to be old wire rope forged together. If that was e.g. marine SS rope then it would certainly be Austenitic so a magnet was never going to stick anyway.
FWIW I came across this tempering chart for AEB-L, which might be of interest. The plateaux seems to be around 625 Vickers. Which would be around 55.5 Rockwell C. Rather too soft for a fine knife I think. So to keep it harder you must try to keep it on the slippery slope around 200C.
And never ever get the edge hotter. No powered sharpeners or grinders, gentle treatment.
Now, when making razor blades out of it, back in 1928, I don't suppose they tempered them, or cryo treated, but used them directly from quench. Maybe 750 Vickers, 60.5 Rockwell C. Perfect for a disposable shaving blade.
For some esoteric educational material about this, see:
For his more populist vids for the Cambridge undergrads, start here, and check out his channel. Dry and dusty it is not:
And yes, this is all way over my head as well, but trying to keep my mind active whilst giving the CV jab a chance to kick in before venturing out once more.
The suggested soak at 850C for 15 minutes on the way up the profile, for thin sections, is there to allow the start of the conversion of the soft steel into Austenite, during which phase transition extreme stresses, and strains will be occurring within the crystal lattice. At this sort of temperature the steel is, for want of a better word, sufficiently ductile to take up these (considerable) internal forces. If you rush through this phase you could risk invisibly micro-cracking it, or even warping it as you proceed rapidly to the full Austenising temperature, 1100 degrees plus. This is why there is a suggested pause there. However if you start your forge coolly, then slowly ramp it up, It may not be completely necessary.
If you were dealing with a big block of steel that could be quite another matter.
I expect that your steel will not be Austenitic when you get it. It will be in a raw soft state, so that you can easily work it, grind it, drill it, punch out blanks from it. Even bend it. If it was Austenitic a magnet would not stick to it.
On that point, the Curie point of many steels is 770C. I expect that yours will be too, given its high chromium content.
This is where the unscientific "heat it up until a magnet doesn't stick anymore, then quench it" brigade are probably falling short, never getting it hot enough to fully Austenitise, so missing out on the benefit of much potential Martensite during the quench.
My suggestion of pausing again, on the way down, at around 850C, is that the subsequent quench should be equally as effective as one done from over 1000 degrees, but far less stressful to the blade.
The quench is to convert Austenite to Martensite. Once the steel is Martensitic after quenching it becomes hard, brittle and difficult to work in this condition. Then must come the magic of the tempering, to change the Martensite to reduce the hardness to desired levels.
The knifesteelnerd certainly knows his stuff, I hit upon his treatise on this whilst researching this AM.
https://knifesteelnerds.com/2019/03/18/magnet-heat-treating/#:~:text=Steel%20Magnetism,-Pure%20iron%20is&text=The%20Curie%20temperature%20is%20different,670°F)%20for%20nickel.
And, in retrospect, so might the Aussie chap in the video. I wondered about what he seemed to be blathering on about when disparaging the magnet check, but it seems that he was making a purely decorative knife from "rope Damascus" which I take to be old wire rope forged together. If that was e.g. marine SS rope then it would certainly be Austenitic so a magnet was never going to stick anyway.
FWIW I came across this tempering chart for AEB-L, which might be of interest. The plateaux seems to be around 625 Vickers. Which would be around 55.5 Rockwell C. Rather too soft for a fine knife I think. So to keep it harder you must try to keep it on the slippery slope around 200C.
And never ever get the edge hotter. No powered sharpeners or grinders, gentle treatment.
Now, when making razor blades out of it, back in 1928, I don't suppose they tempered them, or cryo treated, but used them directly from quench. Maybe 750 Vickers, 60.5 Rockwell C. Perfect for a disposable shaving blade.
For some esoteric educational material about this, see:
For his more populist vids for the Cambridge undergrads, start here, and check out his channel. Dry and dusty it is not:
And yes, this is all way over my head as well, but trying to keep my mind active whilst giving the CV jab a chance to kick in before venturing out once more.
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