Hypereutectoid steel

69_knives · Feb 24, 2009

Thank you, Mete.

I'll be using 1095 for the first time this weekend.

Kevin R. Cashen · Feb 25, 2009

Mike Krall said:
"The book" would spheroidize 1200F to 1250F, minimum 1 hour. What is happening with "several cycles in the 1300F range" that is different than that? (I'm assuming you mean with air cools between... and so you know, Sarah says, "Several can be more than two and often is more than three, but not many, in any case.".)...

My usual is to stress relieve (1200F-1250F, min. one hour) before heating to quench temp., soaking, and quenching. I feel doing that makes even bigger spheroids and makes them further apart. In this instance, that seems wrong-headed... that I want the softness of spheroidizing but want the spheroides to be as small as possible and not clumped up... to go back into solution, well dispursed, easily. Am I getting it, Kevin?

Mike

yes you are getting it,

Kevin R. Cashen · Feb 25, 2009

me2 said:
I was kind of taking the "austenizing temperature controls cabide volume and carbon in solution" idea to an extreme. I figure if toughness were needed and 1095, for whatever reason, were the only steel around, higher tempering temperatures would be the best way to go.

It isto ness in the quench and then temper back to your desired level, not only is it good for the steel it gives you the maker the maximum level of control. I have actually heard otherwise knowledgeable makers ask -why bother hardening beyond 59HRC if that is what no taller t always better to nail it and get maximum hardyou are going to get after tethe point it grows mpering anyhow, why not save some time. Such a display of lack of consideration is rather discouraging. The concept is very much like a farmer neglecting his crop han 6" since that is what he is going to cut it to at harvest anyhow

.

Kevin R. Cashen · Feb 25, 2009

69_knives said:
Thank you, Mete.

I'll be using 1095 for the first time this weekend.

69_knives, I love your avatar, everytime I hear claims of the consequences of global warming I look around me at a full Michigan winter and reply "promises, promises"

.

Mike Krall · Feb 27, 2009

God, already back to the middle of page three!!!

The system I'm using is a still-air kiln with computer contol. I've been looking for a reference I've seen, a graph, on thermocouple temperature relation to steel temperature for various heating mediums and can't find it. Thought it was in Moniz' book, then looked in my links... nope. I'm looking because if a person is going to spheroidize at 1300F "several" times they don't need to be having the steel sit at 1300F any length of time. I understand thin sections will heat with the rise in kiln temp. but I'm not sure I don't want to run the kiln up then put the blade in.

Does anyone have references to this information?

Mike

okzj · Feb 28, 2009

Mike Krall said:
God, already back to the middle of page three!!!

The system I'm using is a still-air kiln with computer contol. I've been looking for a reference I've seen, a graph, on thermocouple temperature relation to steel temperature for various heating mediums and can't find it. Thought it was in Moniz' book, then looked in my links... nope. I'm looking because if a person is going to spheroidize at 1300F "several" times they don't need to be having the steel sit at 1300F any length of time. I understand thin sections will heat with the rise in kiln temp. but I'm not sure I don't want to run the kiln up then put the blade in.

Does anyone have references to this information?

Mike

Here's something that I can maybe help on...

You have three things going on at any one moment in your kiln. #1 is the heating elements are cycling on and off trying to maintain a constant temp. #2 is the termocouple is reading some temp within the kiln. #3 is your steel in the kiln is searching for equilibrium. The thermocouple is absorbing radiant heat (unless it is shielded), and so is the steel (unless it is shielded). The thermocouple is also being heated by convection, and so is the steel.

At low temperatures, the steel is still pretty shiny. It reflects a lot of the radiant heat, so it's not so big a deal. At austenizing temps, the oxide layer has built up on the steel, so it no longer reflects radiant heat, it absorbs it. Now, you have a problem. If you put the steel in you kiln when both are cold and turn on your kiln, the coils immediately ramp to 2600° and stay there. The steel is shiny, so it reflects the radiation, and it heats by convection at a rate that is roughly eqivalent to the rest of the kiln. Once you hit ~900°, you have a black oxide layer on the steel, and its emissivity in the modified Stefan-Boltzman equation has changed. Now it absorbs much more energy that it reflects, not only because of its black color, but because of the matte surface. Since the thermocouple does not accumulate an oxide layer, its emissivity does not change, and does not absorb any more radiation. Therefore, the thermocouple is oblivious the the fact that your steel is heating much faster than itself and that unless it shuts down the coils the steel may very well overshoot the desired temperature.

Now, consider this scenario; you start the kiln, and all this stuff happens without the steel inside. Once the kiln reaches equilibrium, you can be pretty much assured that what the thermocouple is reading is the actual temp of everything in the kiln. The coils are cycling, unlike before, when they are just pegged at "afterburn". Since modern PID controllers strive to maintain very tight temp control, I would bet it cycles every 2 seconds or so. If you're using an infinite switch, even better. Cycling coils will not have a chance to get to ~2600°. They may cycle around ~100-150° (Don't hold me to these numbers. The point is the coils have to be hotter than the rest of the kiln for heat transfer to take place) hotter than what the thermocouple is reading, then the average temp in the kiln goes up enough and the controller shuts the coils. Coils on an infinite switch may be ~75-150° hotter than the thermocouple reading, and will be a consistent temp since they don't cycle to maintain an average temp. Now you put your steel in. It's getting heated by convection and radiation. It builds an oxide layer and heats faster. But, since the hottest thing in the kiln is now only ~150° hotter than the setpoint, there is much less tendency to overshoot your desired steel temperature.
Before, the hottest thing in the kiln was 2600°, which would be 1100° hotter than your setpoint if you were shooting for 1500°. That temperature difference is what drives the steel to overshoot your setpoint temp.

In either case, your steel is going to be hotter than what your thermocouple is reading unless your thermocouple is laying on top of a coil. How much hotter would require a transient heat transfer analysis that would make your head spin. I personally think that if you are letting the steel sit in the kiln while it heats you could see steel temps that could be in the ballpark of 300° hotter than what you are expecting UNTIL the entire kiln come us to temp. As that happens, the average temperature of the coils comes down and so does the steel temp. In other words, radiative heat transfer from the coils is the controlling factor in steel temp in a cold kiln. In a hot kiln, radaitive heat transfer from the entire kiln will be the controlling factor. Since the temperature of the entire kiln is right at the setpoint and the only thing over the setpoint is the coils(and them only by ~150° or so), the temperature of the steel will be much closer to the setpoint; it could be within 15-50° or so. It may be much closer, but I would have to have an IR thermometer and take quick readings to know for sure.

You can get around all of this in 3 ways... heat treat with foil to shield the steel from the radiative heat transfer, make a firebrick box inside your kiln and put the steel in it to shield it, or wait until the kiln comes up to temp. I build a box AND wait until it comes up to temp, but I'm anal like that.

Now, if your thermocouple is inaccurate or improperly placed, that's a whole 'nother can o' worms.

Kevin R. Cashen · Feb 28, 2009

Just in case there is some confusion (I don't know how that could be, after all this topic is so simple

), we need to be careful not to confuse normalizing with annealing, as many people often do. I will always attempt to addres them seperately, thus I may stress the importance of high heat in at least one or more normalizing cycles, but then stress the importance of subcritical temperatures in spheroidizing operations.

Kevin R. Cashen · Feb 28, 2009

If the information is useful to people I guess I am alright with the sticky, but if we are going to keep it at the top of the forum I would request a Moderator clean it up a bit. Most forums are not keen on deleting posts, and for good reason, but would it be possible to make a copy of the thread with posts 40-43 and 49-64 removed so that the first impression a person gets of this forum is how much information we can share and not how much disharmony we can display?

Mike Krall · Feb 28, 2009

Kevin R. Cashen said:
If the information is useful to people I guess I am alright with the sticky, but if we are going to keep it at the top of the forum I would request a Moderator clean it up a bit. Most forums are not keen on deleting posts, and for good reason, but would it be possible to make a copy of the thread with posts 40-43 and 49-64 removed so that the first impression a person gets of this forum is how much information we can share and not how much disharmony we can display?

Or a note with a link to those posts in addendum form... so the reality of real is preserved?

I mean, I understand... from the point of education, anything unecessary to understanding the topic is in the way of understanding the topic. But I easily find the aspects in those posts of individuals caring about folks understanding and making personal effort to further that.

Mike

Mike Krall · Feb 28, 2009

okzj said:
Here's something that I can maybe help on...

You have three things going on at any one moment in your kiln. #1 is the heating elements are cycling on and off trying to maintain a constant temp. #2 is the termocouple is reading some temp within the kiln. #3 is your steel in the kiln is searching for equilibrium. The thermocouple is absorbing radiant heat (unless it is shielded), and so is the steel (unless it is shielded). The thermocouple is also being heated by convection, and so is the steel.

At low temperatures, the steel is still pretty shiny. It reflects a lot of the radiant heat, so it's not so big a deal. At austenizing temps, the oxide layer has built up on the steel, so it no longer reflects radiant heat, it absorbs it. Now, you have a problem. If you put the steel in you kiln when both are cold and turn on your kiln, the coils immediately ramp to 2600° and stay there. The steel is shiny, so it reflects the radiation, and it heats by convection at a rate that is roughly eqivalent to the rest of the kiln. Once you hit ~900°, you have a black oxide layer on the steel, and its emissivity in the modified Stefan-Boltzman equation has changed. Now it absorbs much more energy that it reflects, not only because of its black color, but because of the matte surface. Since the thermocouple does not accumulate an oxide layer, its emissivity does not change, and does not absorb any more radiation. Therefore, the thermocouple is oblivious the the fact that your steel is heating much faster than itself and that unless it shuts down the coils the steel may very well overshoot the desired temperature.

Now, consider this scenario; you start the kiln, and all this stuff happens without the steel inside. Once the kiln reaches equilibrium, you can be pretty much assured that what the thermocouple is reading is the actual temp of everything in the kiln. The coils are cycling, unlike before, when they are just pegged at "afterburn". Since modern PID controllers strive to maintain very tight temp control, I would bet it cycles every 2 seconds or so. If you're using an infinite switch, even better. Cycling coils will not have a chance to get to ~2600°. They may cycle around ~100-150° (Don't hold me to these numbers. The point is the coils have to be hotter than the rest of the kiln for heat transfer to take place) hotter than what the thermocouple is reading, then the average temp in the kiln goes up enough and the controller shuts the coils. Coils on an infinite switch may be ~75-150° hotter than the thermocouple reading, and will be a consistent temp since they don't cycle to maintain an average temp. Now you put your steel in. It's getting heated by convection and radiation. It builds an oxide layer and heats faster. But, since the hottest thing in the kiln is now only ~150° hotter than the setpoint, there is much less tendency to overshoot your desired steel temperature.
Before, the hottest thing in the kiln was 2600°, which would be 1100° hotter than your setpoint if you were shooting for 1500°. That temperature difference is what drives the steel to overshoot your setpoint temp.

In either case, your steel is going to be hotter than what your thermocouple is reading unless your thermocouple is laying on top of a coil. How much hotter would require a transient heat transfer analysis that would make your head spin. I personally think that if you are letting the steel sit in the kiln while it heats you could see steel temps that could be in the ballpark of 300° hotter than what you are expecting UNTIL the entire kiln come us to temp. As that happens, the average temperature of the coils comes down and so does the steel temp. In other words, radiative heat transfer from the coils is the controlling factor in steel temp in a cold kiln. In a hot kiln, radaitive heat transfer from the entire kiln will be the controlling factor. Since the temperature of the entire kiln is right at the setpoint and the only thing over the setpoint is the coils(and them only by ~150° or so), the temperature of the steel will be much closer to the setpoint; it could be within 15-50° or so. It may be much closer, but I would have to have an IR thermometer and take quick readings to know for sure.

You can get around all of this in 3 ways... heat treat with foil to shield the steel from the radiative heat transfer, make a firebrick box inside your kiln and put the steel in it to shield it, or wait until the kiln comes up to temp. I build a box AND wait until it comes up to temp, but I'm anal like that.

Now, if your thermocouple is inaccurate or improperly placed, that's a whole 'nother can o' worms.

Taylor,

Could easily be I'm not getting something here and it would be likely I asked the question in an unclear way.

I'll say it differently... The steel has both equal and fine sized grain. That is, I have nomalized it. I have also quenched it and the nature of hypereutectoid steel requires a spheroidized anneal. I accept (only marginally understand the entire story, though) Kevin's reccommendation to spheroidize anneal by running the temp. to 1300F a number of times to spheroidize anneal as opposed to running at 1200F-1250F and holding an hour. It needs to be done quickly or there is more time in the spheroidizing temperature range (900F through just below AC1). The "more time" will cause larger and further apart spheroides of carbon (all of why this is problematic, I don't get). So, I want to run to 1300F fast and get the blade into air-cooling as soon as it gets there... not hang out at 1300F at all as that is "more time" (it's a higher temp/faster spheroidizing thing, too).

I understand the blade temp being more equal to the thermocouple temp by putting the blade into a stabilized kiln at the desired temperature. But there is nothing about the funtion of the kiln that indicates when the blade has stabilized with the kiln/thermocouple.

I mentioned a graph I've seen showing amount of time for heat transfer in still air. The graph showed more time for less heat transfer as the object got closer to the set temperature. If I can find it, the way I was going to work with it was noting the amount increase in time and accounting for it.

The only other way I know of dealing with this is by watching difference in color between the blade and the thermocouple. Doing that is not impossible but is problematic in an unmodified HT kiln because the two have to be viewed as overlapping each other directly (no glass in between) so there needs to be a pluggable viewing port in the right place or the door needs to be opened to see when the thermocouple and the blade are the same color (same heat).

Mike

Tai Goo · Feb 28, 2009

Kevin R. Cashen said:
If the information is useful to people I guess I am alright with the sticky, but if we are going to keep it at the top of the forum I would request a Moderator clean it up a bit. Most forums are not keen on deleting posts, and for good reason, but would it be possible to make a copy of the thread with posts 40-43 and 49-64 removed so that the first impression a person gets of this forum is how much information we can share and not how much disharmony we can display?

Something along those lines would be fine with me.

Kevin R. Cashen · Feb 28, 2009

Mike Krall said:
...The "more time" will cause larger and further apart spheroides of carbon (all of why this is problematic, I don't get)...

A chronic problem with miscommunication ....

The focus on finer carbide distribution is for the benefit of the guys who may find holding at precisely 1475F for 10 minutes or more a bit daunting, and thus is one way to prepare the steel ahead of time to kick over suffiencient austenite in a much shorter time.

Mike Krall · Feb 28, 2009

Kevin R. Cashen said:
A chronic problem with miscommunication or misinterpretation, are my attempts to present to topic on two levels at the same time. One with exacting temperatures and methods like those that industry provides us with under the assumption that the treater is working with well controlled equipment, and the other level dealing with folks trying to approximate or get as close as they can with simpler equipment. If we exclude all the folks not working with digital controllers and precise heating equipment, then things like the coarser spheroids are not a problem, since we can just wait it out at exactly the austenitizing temp we set things for.

Although some may see me as rigidly locked into the high tech method I actually have the greatest desire to make this information useful to the average smith working with a forge as well, it is they who have probably found the most frustration with the quirks of hypereutectoids and could use these underlying principles the most in overcoming so much misunderstanding and resulting misinformation.

The focus on finer carbide distribution is for the benefit of the guys who may find holding at precisely 1475F for 10 minutes or more a bit daunting, and thus is one way to prepare the steel ahead of time to kick over suffiencient austenite in a much shorter time.

I don't know why I didn't get that because it is what I have been doing with a forge since you described it for me last time (6/06... KN Forums... http://www.knifenetwork.com/forum/showthread.php?t=36392). That is, quench at the end of normalize so a person can actually spheroidize anneal with a forge by running up quickly and very evenly from room temperature to 1300F a number of times (I had been doing three)... thereby avoiding long hold times. I didn't understand doing that decreased the time at austenitizing temp (typical 10 minute soak) and had been struggling to get that done in the forge... took a special made holding tool, a lot of work on eveness and stroking to maintain color.

Thanks Kevin... little by little you drive the data into the granite of my head...

Mike

okzj · Feb 28, 2009

Mike Krall said:
Taylor,

Could easily be I'm not getting something here and it would be likely I asked the question in an unclear way.

I'll say it differently... The steel has both equal and fine sized grain. That is, I have nomalized it. I have also quenched it and the nature of hypereutectoid steel requires a spheroidized anneal. I accept (only marginally understand the entire story, though) Kevin's reccommendation to spheroidize anneal by running the temp. to 1300F a number of times to spheroidize anneal as opposed to running at 1200F-1250F and holding an hour. It needs to be done quickly or there is more time in the spheroidizing temperature range (900F through just below AC1). The "more time" will cause larger and further apart spheroides of carbon (all of why this is problematic, I don't get). So, I want to run to 1300F fast and get the blade into air-cooling as soon as it gets there... not hang out at 1300F at all as that is "more time" (it's a higher temp/faster spheroidizing thing, too).

I understand the blade temp being more equal to the thermocouple temp by putting the blade into a stabilized kiln at the desired temperature. But there is nothing about the funtion of the kiln that indicates when the blade has stabilized with the kiln/thermocouple.

I mentioned a graph I've seen showing amount of time for heat transfer in still air. The graph showed more time for less heat transfer as the object got closer to the set temperature. If I can find it, the way I was going to work with it was noting the amount increase in time and accounting for it.

The only other way I know of dealing with this is by watching difference in color between the blade and the thermocouple. Doing that is not impossible but is problematic in an unmodified HT kiln because the two have to be viewed as overlapping each other directly (no glass in between) so there needs to be a pluggable viewing port in the right place or the door needs to be opened to see when the thermocouple and the blade are the same color (same heat).

Mike

Mike,

Yeah, I guess I only answered the last part of your question about heating with the kiln vs waiting for it to get hot. I was more focused on it since that's my area of expertise. I would definitely wait for it to heat up. That way, you can be resonably certain that you're only getting to 1300° instead of overshooting to 1500-1600° because of radiant heat transfer, which would take you well above critical and ruin what you are trying to accomplish in your spheroidizing.

Mike Krall · Mar 1, 2009

okzj said:
Mike,

Yeah, I guess I only answered the last part of your question about heating with the kiln vs waiting for it to get hot. I was more focused on it since that's my area of expertise. I would definitely wait for it to heat up. That way, you can be resonably certain that you're only getting to 1300° instead of overshooting to 1500-1600° because of radiant heat transfer, which would take you well above critical and ruin what you are trying to accomplish in your spheroidizing.

Taylor,

After I made the post in response and had gone out the door, I thought I should have thanked you. What you put up is a good lesson in kiln operation and it's going to be a good tool for me.

If the thermocouple has a grounded or ungrounded tip (sheath over the probe), is that shielded? Either way, how does a shielded thermocouple change what you said?

Mike

69_knives · Mar 1, 2009

Kevin R. Cashen said:
69_knives, I love your avatar, everytime I hear claims of the consequences of global warming I look around me at a full Michigan winter and reply "promises, promises" .

On the brine quench- good luck. But regardless of the outcome I would like to be denied any credit for advising you to take that route. If it works, it is all yours, be proud. If what I think will mostly likely happen, happens- I didn't tell you to do it.

Well, I learned something today; don't overheat 1095 and quench in brine.

I built a muffle for my forge...

But had a bit of a brainfart and didn't weld the end shut and since my burner fires from the end of the forge I was getting flame inside the muffle, and getting the tip too hot. And yes, the tip cracked. However, I did get a nice utensil that I will use for shaving.

I also forgot to pick up some salt so I used the next best thing, we have tanks of caustic solution (sodium hydroxide instead of sodium chloride).

I quenched the blade a total of 4 times, the first time I dipped it in the brine and pulled it out as fast as I could then into my oil quenchant. the blade only hardened about 1/4" from the edge and not at all on the spine. the next time I dipped it in the brine for maybe 1 or 2 seconds then into the oil, and the blade got completely hard (this is where I overheated the tip and it cracked, it also warped so I went for quench # 3 and while it was heating is when I noticed that it was cracked so I said what the hell and quenched it directly into my oil, did not harden at all. So, on to quench #4, to try to get some more data and try to get at least something that I could salvage out of 3 hours of work..

Will be fixing my muffle tomorrow.

As a side note, I did successfully HT an O1 blade I made a few weeks ago. Yay me.

okzj · Mar 1, 2009

Mike Krall said:
Taylor,

After I made the post in response and had gone out the door, I thought I should have thanked you. What you put up is a good lesson in kiln operation and it's going to be a good tool for me.

If the thermocouple has a grounded or ungrounded tip (sheath over the probe), is that shielded? Either way, how does a shielded thermocouple change what you said?

Mike

No problem. Just trying to help.

When I speak of shielded, I am talking about a physical heat shield, whether it be a stainless or inconel sheath or a piece of firebrick. Basically anything that keeps the thermocouple or blade out of the line of sight of the coils. Of course, once the shield heat up, you will have largely the same problem, but I expect the effect to be de minimis.

A shielded thermocoupld will read the temperature of the fluid around it, which is the actual temperature inside the kiln, which is what we want to measure. You just have to be aware that an unshielded thermocouple in transient operation (i.e. when the kiln is heating) is not going to give a true reading of the kiln temp.

For our knifemaking purposes, convective heat transfer is our friend and radiative heat transfer can ba a detriment. In industry it is eaxctly the opposite. 90-95% of the heat transfer in a fired process furnace is through radiative heat transfer. I'm having to think backwards when doing knifemaking stuff, so if I say something that is contradictory just call me on it, because I might have slipped.

Hope you guys benefit from my ramblings.

Mike Krall · Mar 1, 2009

okzj said:
Hope you guys benefit from my ramblings.

Thanks... I am...

Mike

Mike Krall · Mar 2, 2009

Here's a link with more information on hypereutectoid/eutectoid steel... http://www.bladeforums.com/forums/showthread.php?p=6566699#post6566699

Mike

69_knives · Mar 5, 2009

OK, so I found out by research and by first hand data that sodium hydroxide solution is the WORST thing you can quench 1095 steel into. so, if I were to have two choices of oil available to me, one being vet grade mineral oil and the other being a really light (thin) low operating temperature hydraulic oil, would one of these be advisable for 1095? I'd like to buy something locally if possible.

I'm open to suggestion, can an 'almost' thin enough oil be thinned with mineral spirits or diesel. (is there a tongue in cheek emoticon)?