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Jeremy, the time you leave the blade in the quench is only a huge factor if you are attempting an interrupt, a slightly more advanced technique that is not entirely necessary for everybody. If all you want to do is quench your blade fully keep it in the oil until it is the same temperature as the oil and then temper as soon as possible. IF you have clay on the blade you will want to leave it in a couple minutes longer.

The interrupted quench is an improvised version of what industry calls mar-quenching. Actual mar-quenching involves quench mediums engineered to handle being heated to 400F+ and still handle heat extraction; most oils drop off drastically in cooling ability above 250F and like to burst into huge walls of fire when quenched into at those temps. Marquenching takes advantage of the fact that steel does not actually harden until it reaches 400F to 500F and no longer needs the severe cooling speeds to avoid the soft stuff. In my heat treating video that the ABS offers, I demonstrate the principles behind the interrupted quench.

The idea is that since you no longer need to fear the soft stuff, you can stop the fast stressful cooling before the blade starts to harden and allow it to cool in the air instead. The advantages of this are that the blade cools more evenly,without added stress, and you can also spot any warps as they begin and gently guide them back straight before the blade cools. The slower cool allows the blades own thermal mass to get a head start on the tempering process with the new hardened structures within the blade providing some gains in toughness.

However, as I said, it is a more advanced technique that does have some pitfalls to be avoided. It is important that you interrupt as close to the hardening start temperature for your specific steel as possible without under shooting. This requires practice with an oil, that you are familiar with, which has consistent performance over time. Clay will add at least three to five seconds to the time before interrupt. If the interrupt is too high the auto-tempering effect can be overdone and if it is too low you lose many of the benefits or even create some full hardening issues with more alloyed steel. The cooling should also be continuous after the interrupt to avoid the same hardening issues.

Kevin, I've never been sure if the commonly heard "interrupt at 7 to 9 seconds" was directly related to Ms (martensite start)... is yes or no?

Mike

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So rigging an aquarium type pump in the oil pan (a large steam table tray I got from salvage) isn't really needed? I did that and set it so the flow goes in the direction of the blade.

I know its a little off topic but doesn't the design also affect the bending test, by both how the steel hardens (thickness mostly I guess) and the actual design in how it redistributes stress?

An air pump would be bad but a pump that pushes liquid is excellent, it was some extra work but you can just relax and let the quenchant do the work.

I am not sure what design you mean? If you are referring to the quench circulation, then it would only have an affect on differential hardening. For through hardening with a differential temper it should make things much more consistent and predictable.

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Kevin, I've never been sure if the commonly heard "interrupt at 7 to 9 seconds" was directly related to MS (martensite start)... is yes or no?

Mike

When I say 7 to 10 seconds, that would be yes. The main reason to practice an interrupt is to approximate a marquench. To be get the most success from that technique you want to interrupt the cooling as close to Ms as possible without going under it. This will be different for every steel to some degree, and I have found it changes from shop to shop even with the same oil. Clay really messes with it! That is the nature of that beast, it is not true marquenching where the quench can be controlled at the exact temperature but rather our best improvisation, so we need to work at it and be ready to adapt or settle with some compromises.

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When I say 7 to 10 seconds, that would be yes. The main reason to practice an interrupt is to approximate a marquench. To be get the most success from that technique you want to interrupt the cooling as close to Ms as possible without going under it. This will be different for every steel to some degree, and I have found it changes from shop to shop even with the same oil. Clay really messes with it! That is the nature of that beast, it is not true marquenching where the quench can be controlled at the exact temperature but rather our best improvisation, so we need to work at it and be ready to adapt or settle with some compromises.

How did you teach it to yourself?

Mike

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How did you teach it to yourself?

Mike

Which part?

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Which part?

The part about knowing when the porridge is not too hot and not too cold, but just right...

I know I can look at TTT & CCC curves, maybe Jominy end tests, for a steel. I can look at cooling times based on the quenchants I use (Houghto-quench "K" & "G"), maybe holler at Houghton directly, to get a rough time. I've also thought about IR temp guns and/or Tempil sticks. I guess what I'm wondering is if there are other ways... maybe ones less equipment oriented... and what kinds of things a person sees if the the interrupt time was too high, too low, just right.

Mike

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The part about knowing when the porridge is not too hot and not too cold, but just right...

I know I can look at TTT & CCC curves, maybe Jominy end tests, for a steel. I can look at cooling times based on the quenchants I use (Houghto-quench "K" & "G"), maybe holler at Houghton directly, to get a rough time. I've also thought about IR temp guns and/or Tempil sticks. I guess what I'm wondering is if there are other ways... maybe ones less equipment oriented... and what kinds of things a person sees if the the interrupt time was too high, too low, just right.

Mike

To ball park it the oils will give you some key hints. Ms is right around the vapor point of many oils so your blade should have a light coating of oil that is giving off some wisps of vapor. If the blade is really smoking or goes dry, it is way too hot, if the blade is dripping with oil but no vapors are present, you are probably too cool. For better calibration of the technique your IR thermometer idea is pretty good <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' />. In the end you do have to realize that it is never going to be as spot on as a full marquenching setup, but then many folks feel that it is not bladesmithing if we don't leave some of the human factor in the equation. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

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To ball park it the oils will give you some key hints. Ms is right around the vapor point of many oils so your blade should have a light coating of oil that is giving off some wisps of vapor. If the blade is really smoking or goes dry, it is way too hot, if the blade is dripping with oil but no vapors are present, you are probably too cool. For better calibration of the technique your IR thermometer idea is pretty good <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' />. In the end you do have to realize that it is never going to be as spot on as a full marquenching setup, but then many folks feel that it is not bladesmithing if we don't leave some of the human factor in the equation. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//smile.gi f' class='bbc_emoticon' alt=':)' />

My little wheels are turning now... =] Thank you!

I don't think I missed it, but... the down sides of cooling a ways past Ms before interrupting the quench?

Mike

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My little wheels are turning now... =] Thank you!

I don't think I missed it but... the down sides of cooling a ways past Ms before interrupting the quench?

Mike

Some studies published not too long ago by ASM confirmed exactly some of the things I had observed in martempered steels. At Ms the shear driven transformation finally achieves enough driving force (the Bain strain) to initiate. It has been found that once initiated if it is halted again it apparently facilitates the stabilization of austenite. Also if you allow the edge to cool well into martensite while the spine is above 450F the interrupt will allow the edge to be tempered but the thermal mass of the spine- at temperatures you probably do not want.

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Some studies published not too long ago by ASM confirmed exactly some of the things I had observed in martempered steels. At Ms the shear driven transformation finally achieves enough driving force (the Bain strain) to initiate. It has been found that once initiated if it is halted again it apparently facilitates the stabilization of austenite. Also if you allow the edge to cool well into martensite while the spine is above 450F the interrupt will allow the edge to be tempered but the thermal mass of the spine- at temperatures you probably do not want.

In "if it is halted again...", I don't see how an interrupted quench actually halts martensite formation, once it is started.

And along that line... Given 1070 through 1090 have Ms temps a little above 400F (not until 1095 is Ms less than 400F), how can either martempering or interrupting not temper the edge more than ideally? There is an assumption in there... that Ms happens at the edge and other corners sooner than other areas of the blade do to heat-loss-for-form characteristics... and I don't know that is right.

And Kevin, if you'd like to take your holiday, I can put off pecking at you with my dimwittedness until afterwards...

Mike

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In "if it is halted again...", I don't see how an interrupted quench actually halts martensite formation, once it is started.

And along that line... Given 1070 through 1090 have Ms temps a little above 400F (not until 1095 is Ms less than 400F), how can either martempering or interrupting not temper the edge more than ideally? There is an assumption in there... that Ms happens at the edge and other corners sooner than other areas of the blade do to heat-loss-for-form characteristics... and I don't know that is right.

And Kevin, if you'd like to take your holiday, I can put off pecking at you with my dimwittedness until afterwards...

Mike

And along the lines of merely interrupting an oil quench to continue in air, you are correct, as long as the cooling is continuous, but in the case of a much thicker spine that could put its above Ms heat back into an edge that has begun transformation there would be a possibility. Also bladesmiths, being the creative folks that we are, often get “ideas” about “improving” on the process and adjusting slow cooling rates after the interrupt. It must also be conceeded that the differential in temepr would also have to be great enough to voercoem the airs ability to cool the eddge. But for the most part the issue would be of greater concern in actual martempering where the quenching hold temp can be set before the final cooling.

On the over tempering, it is a matter of conduction values. As long as the blade is in a liquid cooling medium the thermal extraction should be able to overcome the steels ability to choose its own thermal equilibrium, this is, after all, what we do with quenching, particularly with clay backed blades and edge quenching. That the heat-loss-for-form is a factor is self-evident when we see sori, intentional or unintentional, particularly on un-clayed blades, or even on a blade that formed a striking hamon without clay. The edge blasted through the pearlite region while the bulk of the blade slowly dragged through it. And it is common to see secondary hamon like activity at the corners of the spine, just as you predicted.

If soak times are appropriate, Ms being around 400F should not be a problem in the auto-tempering as actual tempering temperatures could be 50 degrees or more higher in order to dip below useable Rc values. Soak 1084 or 1095 thoroughly before a full quench and then start your tempering at 400F, bumping it up between each Rc test and you will be almost annoyed at how much heating it takes to get it down to 57-58 Rc.

On the hypoeutectoid side it gets even trickier. I have had some very frustrating experiences dealing with steels like 6150 where Ms is around 550F. These steels really can’t even be marquenched if you want any significant hardness left. In order to avoid massive auto-tempering you need to actually cool the steel to its Mf, which is no longer marquenching, but just plain old quenching!

Fortunately this is one case where the universe actually cuts us some slack, simple alloys low enough in carbon to have that high Mf normally do not suffer from retained austenite, and those that have high enough carbon to have RA also have a nice low Ms.

But once again all this may be splitting hairs considering the number of impressive blades that have been made with no consideration of it all. It is just another one of those things that fall under the category of what you are willing to lose sleep over. Some of us are just wound too tight not to obsess over these things. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' />

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And along the lines of merely interrupting an oil quench to continue in air, you are correct, as long as the cooling is continuous, but in the case of a much thicker spine that could put its above Ms heat back into an edge that has begun transformation there would be a possibility. Also bladesmiths, being the creative folks that we are, often get “ideas” about “improving” on the process and adjusting slow cooling rates after the interrupt. It must also be conceeded that the differential in temepr would also have to be great enough to voercoem the airs ability to cool the eddge. But for the most part the issue would be of greater concern in actual martempering where the quenching hold temp can be set before the final cooling.

On the over tempering, it is a matter of conduction values. As long as the blade is in a liquid cooling medium the thermal extraction should be able to overcome the steels ability to choose its own thermal equilibrium, this is, after all, what we do with quenching, particularly with clay backed blades and edge quenching. That the heat-loss-for-form is a factor is self-evident when we see sori, intentional or unintentional, particularly on un-clayed blades, or even on a blade that formed a striking hamon without clay. The edge blasted through the pearlite region while the bulk of the blade slowly dragged through it. And it is common to see secondary hamon like activity at the corners of the spine, just as you predicted.

If soak times are appropriate, Ms being around 400F should not be a problem in the auto-tempering as actual tempering temperatures could be 50 degrees or more higher in order to dip below useable Rc values. Soak 1084 or 1095 thoroughly before a full quench and then start your tempering at 400F, bumping it up between each Rc test and you will be almost annoyed at how much heating it takes to get it down to 57-58 Rc.

On the hypoeutectoid side it gets even trickier. I have had some very frustrating experiences dealing with steels like 6150 where Ms is around 550F. These steels really can’t even be marquenched if you want any significant hardness left. In order to avoid massive auto-tempering you need to actually cool the steel to its Mf, which is no longer marquenching, but just plain old quenching!

Fortunately this is one case where the universe actually cuts us some slack, simple alloys low enough in carbon to have that high Mf normally do not suffer from retained austenite, and those that have high enough carbon to have RA also have a nice low Ms.

But once again all this may be splitting hairs considering the number of impressive blades that have been made with no consideration of it all. It is just another one of those things that fall under the category of what you are willing to lose sleep over. Some of us are just wound too tight not to obsess over these things. <img src=' http://www.americanbladesmith.com/ipboard/public/style_emoticons//wink.gi f' class='bbc_emoticon' alt=';)' />

Well that helps... I was starting to obsess over not obsessing enough... =]

I guess I'll keep messing with interrupting for a while. Maybe I can teach myself either to control it well, or to not need to mess with it.

Thanks again for the willing help, Kevin.

Mike

Mike, I was drawn back to this thread and your questions to take time to give you recognition and applause for something that is all too easy to take for granted but should be valued and recognized as essential to the growth of knowledge for all of us. I wanted to compliment and thank you for asking the most important questions of all- the ones that cause us to examine how and why. Sure I have read a few books and have made years worth of mistakes to learn from in the shop, but if all I, or anybody else, need do is dictate our own recipes to be unquestioningly followed with no explanations as to why we recommend these things, nobody really learns. I myself don’t want to be a writer of recipes; I want to be a provider of tools for everybody to write their own recipes. That is why my writings are so verbose; I feel a need to explain the hows and whys. If I, or others, fall short of providing those details (those tools) I feel it is not only your prerogative but your duty to our craft to ask for more details. My word on what I will do is golden, my word on what steel will do is worthless compared to the actual facts and that is what we need to work with. I have had folks fear they may insult me by saying they researched something to double check me or to prove me wrong, to their surprise I am quick to inform them that I consider that the greatest honor one can give an information provider. Your excellent questions get to the heart of that, so I wanted to thank you for digging deeper and helping me examine the hows and whys in the process.

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Mike, I was drawn back to this thread and your questions to take time to give you recognition and applause for something that is all too easy to take for granted but should be valued and recognized as essential to the growth of knowledge for all of us. I wanted to compliment and thank you for asking the most important questions of all- the ones that cause us to examine how and why. Sure I have read a few books and have made years worth of mistakes to learn from in the shop, but if all I, or anybody else, need do is dictate our own recipes to be unquestioningly followed with no explanations as to why we recommend these things, nobody really learns. I myself don’t want to be a writer of recipes; I want to be a provider of tools for everybody to write their own recipes. That is why my writings are so verbose; I feel a need to explain the hows and whys. If I, or others, fall short of providing those details (those tools) I feel it is not only your prerogative but your duty to our craft to ask for more details. My word on what I will do is golden, my word on what steel will do is worthless compared to the actual facts and that is what we need to work with. I have had folks fear they may insult me by saying they researched something to double check me or to prove me wrong, to their surprise I am quick to inform them that I consider that the greatest honor one can give an information provider. Your excellent questions get to the heart of that, so I wanted to thank you for digging deeper and helping me examine the hows and whys in the process.

Yow! I'm red in the cheeks! In that anything I say helps you do what you are doing, you are certainly welcome.

So... tit for tat (not looking to get you red in the cheeks, mind you). I'd thank you for what you have been doing on the knife forums for over 10 years now... "the provider of tools". Literally, if you had not posted the definitions, processes, and relationships of steel metallurgy and related them to the questions and considerations other posters had, I would never have gotten a glimpse of anything real about metallurgy. For me, it was your consistency and depth in answering or adding-to discussions that allowed me to learn enough to see questions. It's a great and good work you have there, Kevin.

Mike