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Question about bandsaw blades, hardening, and tempering...

Started by StephenRice, September 03, 2010, 08:31:51 PM

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StephenRice

Hello again everybody.  Newbie Stephen from NW Florida here again.

First of all, let me say how grateful I am that Woodmizer's web site turned me onto this forum!  I am blown away by the treasure trove of great information here.  Not only am I new to this forum, but I am also new to this industry, so I look forward to absorbing all of the knowledge I can from all of the experienced members here from all over.

Advanced Disclaimer
-- With the above in mind, I am fairly reluctant to even make this post.  PLEASE, everyone, do not take anything I say here personally as a slight or anything like that.  I pray that nobody is offended, because that is definitely not my intent.  That is why I have posted this in a new topic rather than in anyone else's thread.

Now, my question is in regards to the hardening and tempering of bandsaw blades.  Like I said, I am a complete newbie in this field.  I do not even have my own mill yet.  So, please take what I say with a grain of salt. 

Nonetheless, I have noticed a lot of people talking about how the cutting tips of new blades are harder than the metal underneath them, saying how much softer the tips are after the blades have been sharpened a number of times.  Some have seemed to have linked this with the idea that they have removed all of the tempered materials that were thought to be harder and now are sharpening into the softer parts of the blade.  I think that this is a common misconception as to the effects of the hardening and tempering process.

While I am not a metallurgical expert by any means, I am sure that there are some engineers or other at the blade manufacturers that may be able to step in here and explain things a little better than I can.  However, I will explain my understanding and ask everyone if I am perhaps mistaken about the hardening and tempering of bandsaw blades.

What I was taught in high school some 25 odd years ago or so is that, in order to obtain high grade spring steel, you must first start out with a good quality high carbon steel.  I assume this is the case with the bandsaw blades.  I am hypothesizing that they are a steel alloy with a very high carbon content and that they are cold rolled for density and strength.  Then, like we always did when forging any object that had to hold an edge, whether it be an ice spud, ax head, or a knife, after it was formed, we heated most or all of the steel up until it was completely red hot and then cooled it in oil to incorporate more carbon and to make sure that the metal cooled evenly.  That hardened the metal.  Items could also be cooled with water, but that cooled the material too quickly and hardened it too hard and in an unreliable manner.  This hardening process is how spring steel is obtained, or, it is how the "spring" is put into the steel.

After the metal was hardened, it would still not be ready for use.  It would be too hard and the cutting edge would simply shatter, chip, or crack right away when it was used if the next step of tempering was not accomplished.

I guess, the above hardening process was something akin to taking a limp hot dog or a rubber knife and dipping it into freezing liquid nitrogen.  It would harden the hot dog, but the hot dog would be so brittle that it would shatter into a thousand pieces if it was dropped onto the floor.

I believe that the same would be true with a bandsaw blade.

So, in order to keep the teeth from simply breaking off right away, I believe that the tempering process is a softening process that, if performed properly and with just the right amount of heat applied in just the right places only, would very slightly soften the metal only enough to prevent it from chipping, cracking, or breaking off.  So, where you see the "straw" or "blue" colors on the tips of the teeth, it should actually be just a little softer there than further down the blade into the gullet and beyond, not harder.

That is not to say that the heat generated from using the saw blades does not also temper the rest of the blades as time goes on.  Especially when the blades are dull, they will build up heat and that heat will soften the whole blade just a little bit, but mostly where the heat is building up.  So, it would stand to reason that the longer you use your blades, the more that they will get tempered (softened), especially along the cutting edges where the most heat is naturally built up.

Also, spring steel can lose some of its 'spring' with that continuous buildup of heat along with the constant flexing of the blade that naturally occurs while sawing.  Therefore, when you are re-sharpening your blades, if the teeth seem to get softer with time, it should not have anything to do with "cutting past the temper of the blades" because the tempered area is actually the softest part of the blade to begin with.  Rather, the softness and the accompanying tendency to dull more quickly and lose the tooth settings more easily is more likely a natural occurrence of the whole blade weakening over time.

Instead of worrying about cutting past the tempered part of the teeth, perhaps it would be more important to make sure not to cut with a dull blade and resharpen more often instead of less.  Anther benefit of that would be that not as much metal would have to be taken off of the blade at a time, thus saving wear and tear on the sharpener, the blade, and even your mill.

Like I said, I don't even have a mill yet, so I am the last one on earth to be an authority on this subject.  I just kept seeing this topic come up after reading hundreds of posts and I kept thinking about this and wondering if maybe I posted something like this that it might help someone out.  So, what do you think?  Does this make sense  Or, am I just plain wrong?

Any WM blade experts out there willing to explain this better than I possibly could?
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

logwalker

Welcome to the forum first of all.

As to your hardening discussion I would have to say that you are probably wrong. It is my understanding that just the tips of the blade go through a separate heat treatment that hardens them more than the body. The reason the manufacturer does this is to make the band last as long as possible as it is constantly being flexed around the wheels. The tooth does not have the same flex occurring as the band. It may be an induction process that just heats the tips, I don't know exactly how they do it.

Also what you referred to as tempering I call annealing. It is a lower heat than the hardening process and can last for several hours and allows the molecular structure to form slowly into a more useful state of temper.

I also welcome someone with more accurate information than I to weigh in on this.

Joe
Let's all be careful out there tomorrow. Lt40hd, 22' Kenworth Flatbed rollback dump, MM45B Mitsubishi trackhoe, Clark5000lb Forklift, Kubota L2850 tractor

StephenRice

Quote from: logwalker on September 03, 2010, 10:13:35 PM
Welcome to the forum first of all.

As to your hardening discussion I would have to say that you are probably wrong. It is my understanding that just the tips of the blade go through a separate heat treatment that hardens them more than the body. The reason the manufacturer does this is to make the band last as long as possible as it is constantly being flexed around the wheels. The tooth does not have the same flex occurring as the band. It may be an induction process that just heats the tips, I don't know exactly how they do it.

Also what you referred to as tempering I call annealing. It is a lower heat than the hardening process and can last for several hours and allows the molecular structure to form slowly into a more useful state of temper.

I also welcome someone with more accurate information than I to weigh in on this.

Joe

Okay, I guess I was not specific enough.  And, again, there is a whole lot that I still have to learn about this, so, like you, I sure hope someone with a lot more accurate knowledge in this can weigh in as well.

From my understanding, annealing and tempering, although both use heat, are not direct equivalents.  To better explain what I was meant above, I believe that:


  • The initial "hardening" process I spoke of above is more scientifically described as austenitizing, or changing the metal into austenite through intense heating.  This may soften the metal temporarily until the completion of the next step.

  • The second step of cooling the metal with an oil bath is technically called "martenizing", or changing the metal from austenite to martenite, or a similar microstructure.

  • The actual "tempering" occurs when you evenly heat the metal (often just across the area of the cutting edge) back up to a temperature lower than the austenizing temperature.

At least, that is what I think I was taught way back in high school.  Of course, while I am still younger than a lot of the experienced folks on here, I do wear my salt n pepper hair and fart dust once in a while, so the cobwebs upstairs may have me just a bit cornfused!  I have been wrong more than once or twice before! (Even when I thought for sure I was right!)
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

StephenRice

Hey, to tell you the truth, I am just glad I got the list thingy on this forum to work for me!  Yay!
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

sgschwend

Perhaps the safer terminology is just heat treating.

There are hundreds if not thousands of steel formulas that most often have their own heat treatment process.  There is even a type of steel that is treated by just air cool.

Yes I believe tempering is the softening of steel, necessary to keep it from being so hard it will shatter on impact.

I do recall watching a saw making process that showed a flame heat treatment.  I would guess that process step was tempering.

It is a common issue in band saws that sharpened saws do not stay as sharp or hold their set as well.  Some brands are worse than others.
Steve Gschwend

sjgschwend@gmail.com

Gary_C

I too welcome you to the forum. And your memory of the heat treating processes and results is pretty accurate. It's the application that is incorrect.

What I mean by that is a band saw blade that has to flex without cracking would never be hardened as a whole. In fact dead soft (annealed) or a low carbon steel would be best for long flex life. But the tips of course have different requirements so something like an induction or flame hardening process is used on the tips only. I'm not sure just how those two processes are done but would guess it would be heating and rapid cooling of just the tips or heating the tips in the presence of a lot of carbon to actually increase the carbon content of the tips only.
Never take life seriously. Nobody gets out alive anyway.

StephenRice

Quote from: sgschwend on September 04, 2010, 12:22:38 AM
Perhaps the safer terminology is just heat treating.

There are hundreds if not thousands of steel formulas that most often have their own heat treatment process.  There is even a type of steel that is treated by just air cool.

Yes I believe tempering is the softening of steel, necessary to keep it from being so hard it will shatter on impact.

I do recall watching a saw making process that showed a flame heat treatment.  I would guess that process step was tempering.

It is a common issue in band saws that sharpened saws do not stay as sharp or hold their set as well.  Some brands are worse than others.

Thanks for the input, Steve!  Since I was taught a process in high school that did not include the scientific aspects, but just the overall process of forging and flame hardening and tempering (I did most of the forging in the shop... loved it), I spent a little more time this evening doing some web research to try to educate myself a little bit.  Lord knows I have a lot to learn, and I am always trying to do just that. :P

Anyways, that is where I got the information about the austenitizing and martenizing processes that explained what was going on at the molecular level when I was flame hardening, adding carbon to the surface layer through forging in soot, quenching, and then tempering.  I did not know the exact science back then, just the basics and the end result that I achieved when I was careful with the temperatures that I used.

Now that I understand it a little better, I still come to the same conclusion as before.  However, I would really love the WM pro to pipe in here sometime because I went back to their web site and was a little confused about the language used on their site. ???

http://www.woodmizerblades.com/whyBlades/bestProcess/bestProcess.aspx

Here is what they say regarding the steel that they use:

Quote
"Wood-Mizer blades combine two different metallurgical techniques that result in superior hardness and toughness not found in other blades. The Wood-Mizer DoubleHard blades use high-quality steel, and the teeth are induction hardened so they stay sharp and can be re-sharpened multiple times. The performance of these blades means higher productivity and lower cost per board foot."

Then, under the next section entitled, Punching vs. Milling / Broaching

Quote"Wood-Mizer's punching process allows us to manufacture blades from pre-hardened, rolled material that provides superior flex-life. The punching process also allows us to provide a wider range of blade profiles with more accurate tooth spacing."

Okay, great.  I am not sure if the two different metallurgical techniques are referring to:


  • the initial pre-hardening process of the cold rolled banding as WM receives it plus the "induction hardening" of the teeth for the Double Hard blades as two separate metallurgical processes, or
  • the entire process listed above as only one process for the Double Hard blades plus a second metallurgical process for the Stellite blades, which of course have a different base alloy metal and, I believe, slightly different hardening and tempering process.  (Don't know much about that except that Stellite is a really hard alloy that resists heat and wear.  I used to be an auto mechanic years ago and Stellite intake and exhaust valves were upgrades that were used in remanufacturing engine heads, especially racing engines.)

I have a tendency to think that the two processes mentioned were for the two different kinds of blade metals, but who knows?  I am not a mind reader and don;t know what the writer meant.

Anyways, my confusion about the web site wording is this.  If the banding that WM uses for their Double Hard blades are already pre-hardened high carbon spring steel, then why would the teeth need to be re-hardened?  It seems more likely to me that the induction hardening ("induction" simply meaning that the heat used to harden the metal came from electricity rather than direct flame) is rather an induction tempering process, one that simply slightly softens the teeth themselves to keep them from being too hard and brittle like the main blade and helps them to bend without breaking.

In fact, this slight softening process does not weaken the blade, but in practical terms, strengthens it.  It is sort of like the old country song about an oak tree that is strong enough to bend.  Do you guys remember that song?  If not, then surely those in this forum in high wind areas and familiar with wood types can appreciate the difference between a gnarly live oak and a cedar tree.  While of course, hurricanes topple many a live oak, many grand troopers out there have stood strong in the face of dozens of hurricanes over the years while the cedars often would snap in two.  Why?  Because the live oaks with their high water content and twisted grain were strong enough to bend and sway in the wind while the brittle cedar would simply crack in two.  Maybe not the best illustration, but I hope it made it's point.

The purpose of this thread, if my understandings are correct, is to show that the teeth on band saws are not harder than the rest of the blade, but really the opposite.  So then, if I am correct, it would be impossible to grind off the hardened portion of the teeth.  However, this does NOT mean that the entire blade itself (or especially the cutting edge) does not soften and weaken over time and with use.  It definitely will.  That is from the continual flexing the blade undergoes as it winds around the wheels combined with the heat generated while sawing, ESPECIALLY with a dull blade.  Also, it does not mean that many repeated sharpenings will not narrow the blade until its use is questionable, especially when the teeth are hitting the guide rollers.

In short, if I am correct, do not worry so much about cutting the hardness of the teeth off, but worry more about getting the blade too hot (which makes it soften up) and worry more about other blade defects as the blade ages and also watch that you are not running the teeth between the guide rollers.  Naturally, as the blade gets hot and ages, the cutting edge will get softer, but that is from the blade getting too hot and and age.  All it means is that you may have to sharpen and reset the blade more often than a new one.  But it also shows that refraining from using dull blades and using a Lube-Mizer does more than keep you sawing faster and the blade clean... it helps your blades last longer as well!
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

StephenRice

Quote from: Gary_C on September 04, 2010, 02:24:55 AM
What I mean by that is a band saw blade that has to flex without cracking would never be hardened as a whole.  In fact dead soft (annealed) or a low carbon steel would be best for long flex life.  But the tips of course have different requirements so something like an induction or flame hardening process is used on the tips only. I'm not sure just how those two processes are done but would guess it would be heating and rapid cooling of just the tips or heating the tips in the presence of a lot of carbon to actually increase the carbon content of the tips only.

Well, I have to question that, Gary.  And, here is why...

The reason being is that I believe that spring steel is made of a high carbon hardened steel or alloy.  Soft low carbon steel will roll around the wheels of a mill without falling off... for a little while.  The soft metal would stretch out so badly from the heat that it would quickly be falling off the drive wheels. 

Additionally, compare a spring to a soft steel wire.  Bend them both back and forth a few times and what happens?  The soft steel breaks and the hardened steel spring does just that... it springs back to its original shape.

If you look at the WM web page that I quoted above, you will see that WM uses only pre-hardened high carbon steel alloy banding for its Double Hard blades.  So, in fact, the whole blade is hardened at the mill that produced the raw banding, not just the tips or teeth.

As far as the tips or teeth being induction hardened, here is where I would like to hear it from WM.  I personally think that it was a simple unintentional miswording on their web site, but one that may be unintentionally misleading.  I love these guys to death and I am convinced that they would never intentionally mislead anyone.  I just doubt it was a metallurgical scientist that wrote the web pages, that is all.  I think that it should have said, induction tempering, but tht is what I would like WM to clarify if possible.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

Magicman

Welcome StephenRice.

I'll just leave the R&D to WM.  I can assure you that they have never intentionally or unintentionally mislead me with any of their advertising or other printed material.

I understand that the band teeth are harder than the band for obvious reasons, and that's good enough for me.
Knothole Sawmill, LLC     '98 Wood-Mizer LT40SuperHydraulic   WM Million BF Club Member   WM Pro Sawyer Network

It's Weird being the Same Age as Old People

Never allow your "need" to make money to exceed your "desire" to provide quality service.....The Magicman

sgschwend

Hardness can be measured.  So you could actually measure the saw and see the hardness values.

I am sure you will find the teeth are harder.  As I mentioned before there are steel formulas that harden in air cooling.
Steve Gschwend

sjgschwend@gmail.com

Gary_C

Quote from: StephenRice on September 04, 2010, 03:16:37 AM

Well, I have to question that, Gary.  And, here is why...

The reason being is that I believe that spring steel is made of a high carbon hardened steel or alloy. 

Again you are correct though the term "spring steel" is a generic one that could encompass many different alloys of steel and hardening processes. In my defense, I did not say that the steel used in WM's blades is a low carbon or dead soft steel, only that it would be the prefered steel for long flex life.

I do not know what steel alloy WM uses in their bands and I am sure they would not disclose that for obvious reasons. And you may be correct in surmising that an unscientific person may have written that promotional literature that says "hardened" about the bands as they are purchased. Most likely they are hard and stiff because of the alloy and carbon content plus the results of the rolling process that was used to manufacture the bands and not any hardening process.

But regardless of the specific alloy and hardness of the base bands, it is clear that WM uses some type of flame or induction hardening process after stamping, setting and then sharpening so the teeth are harder than the base band material. And yes, that means that you can sharpen away the harder material in the teeth.
Never take life seriously. Nobody gets out alive anyway.

Hilltop366

Hi and welcome!

I don't see any reason why the entire band stock can't be hardened before they get it when it is made, and the tips made even harder after the profile is cut.

When I see the word "hardened" when talking about the band body I think "hardened to the best compromise of rigid enough to hold it's shape yet as flexable as possible to allow it to go around the band wheels as many times as usually needed to use up the available amount of tooth that can sharpened.

When I see the word "hardened" when talking about the tooth I think " as hard as can be with out breaking off and can still be sharpened with a stone with out deforming the stone on every tooth, another compromise.

I see no reason why the word hardened can't be used to describe both the blade body and the tooth, after all they are not saying the body is hardened as hard as that type of steel can be leaving no room to make the tooth even harder.

As far as who wrote the info on their site I don't know, however I think the term "hardened" is a good way to describe it to the largest amount of people, where if more technical terms were used a lot of people would be left behind including me.

Talking about using a blade too long an making the tooth softer, I can't see how it could happen in wood with out starting to burn the wood first, I'm guessing that most people will stop long before that and change the blade.

Gary_C

Quote from: Hilltop366 on September 04, 2010, 12:53:37 PM

I don't see any reason why the entire band stock can't be hardened before they get it when it is made,

You are right, there is no reason other than hardness for the body of the band is not a desired property. It needs to be strong enough to resist the tension placed on it plus flexible enough to bend repeatedly without breaking. It is only the teeth that are desired to be hard or hardenable to hold an edge.

This discussion is getting too far into physical chemistry and lattice structure for me. Let's just say that WM knows what they are doing when it comes to bands and little we say here will influence them. Suffice it to say they buy a semi hard and strong band material that has the ability to have the formed tips hardened beyond the hardness of the band and yes if you sharpen them enough times or get them hot thru friction, you can loose that hardness in the tips.  ;D
Never take life seriously. Nobody gets out alive anyway.

Hilltop366

I think we are saying the same thing in a different way Gary C.

I kinda think that WM is using the term "hardened" as a generic term for the proper level of hardness for the required task. Neither the body nor the teeth are soft just not he same.
Quote from: Gary_C on September 04, 2010, 01:59:34 PM
and yes if you sharpen them enough times or get them hot thru friction, you can loose that hardness in the tips.  ;D

The "voice" of experience?

StephenRice

Quote from: Hilltop366 on September 04, 2010, 07:00:16 PM
I think we are saying the same thing in a different way Gary C.

I kinda think that WM is using the term "hardened" as a generic term for the proper level of hardness for the required task. Neither the body nor the teeth are soft just not he same.
Quote from: Gary_C on September 04, 2010, 01:59:34 PM
and yes if you sharpen them enough times or get them hot thru friction, you can loose that hardness in the tips.  ;D

It is the same thing on a chainsaw or any other edge that you are sharpening.  If, when you are sharpening, you start to see the metal change colors such as straw brown or blue gray, then you are tempering or softening the metal, and probably too much so.  That is why it is better to take only a little bit off at a time and cool the metal with water or something between grindings if taking a lot of metal off to prevent excessive tempering of the metal.  A little tempering is necessary to prevent chipping and breakage on a sharpened edge, but too much tempering and the metal will just get soft on you.  There is a delicate balance there.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

John Mc

For almost 25 years, I have worked for a company that makes steel wire (round, flat and other shapes), heat treats it and sells it to spring makers (among other customers).

A few terms to get things straightened out:

Martenizing is I believe a process used by dry cleaners. It has nothing to do with heat treating steel. But Stephen is pretty close to the correct term... I'd have never remembered it that close if I weren't dealing with it on a regular basis)

Martensite is a steel microstructure formed by heating high carbon steel (or some other steel alloys) over a critical temperature to form Austenite (the temp is somewhere in the 1400's F, I forget exactly where, but most people heat up well over that to assure they rapidly get a fully austenitic structure - depending on the process, people will heat to anywhere from 1550 or so, or even a couple hundred degrees higher than that). From there, the steel is rapidly quenched to form martensite (traditionally in oil). At this point, it is very brittle. If you smacked a piece of wire on something hard, it would likely shatter. The wire is then reheated to "draw out" or "Temper" the martensitic structure (metallurgists call the microstructure "tempered martensite"). This makes it less brittle, and slightly softer, but still heard and springy. (This reheat is typically in the 700 F to 1000 or 1100 F range, depending on the final properties desired).

Many people refer to this whole process as "Tempering". This is actually shorthand for "Hardened and Tempered" or "Quenched and Tempered". Sometimes it's referred to as "Oil Tempering" (due to the fact that oil was the traditional quench medium). Technically, it is really only that final reheat that is Tempering, but the single word makes a convenient shorthand.

The term "induction heating" by itself doesn't tell you much about what is going on. It just refers to the method used to heat the steel. It does not tell you what type of heat treatment was done. It could mean annealing (which softens the steel making it easy to bend, but making it much less wear resistant) - not likely for the heat treatment on a saw blade. It could also mean a heating process that mimics the heat-quench-reheat described above, resulting in a tempered martensitic structure. With the right equipment, one thing that induction heat treating does is let you highly focus the area that is being heat treated - so it is possible for example to harden just the teeth of a saw blade for good wear characteristics while leaving the body of the blade in some other condition.

You can also "work harden" steel by cold forming it (in the case of a band saw, this would mean taking strip steel and rolling it down to the final desired thickness. The more you roll it, the harder and springier it gets -- up to a point. If you push it beyong the limits of the steel's ductility it will get brittle and start to break up.

My guess is that Woodmizer is buying "cold-rolled" (also known as "hard-rolled") strip steel, and this is where the hardening of the body of the blade comes from. They then induction heat-treat the teeth to further harden them. The hard-rolled body would have a very good cycle life, and would take the repeated flexing of a band saw blade very well. The heat treated teeth would have very good wear characteristics, and you might be able to get away with a harder (and therefore more brittle) tooth, since the body of the blade could have different properties more suited to the continual flexing. You could heat treat the whole thing, but one of the problems of a martensitic structure is that it is very "notch sensitive" - a small nick or imperfection will grow and crack resulting in failure of the blade. Hard rolled steel is more tolerant of these imperfections.

If they are going for a banitic structure instead of martensite, that's a whole different ball of wax. It might make a good saw tooth... it's very wear and impact resistant... but can be a significantly more expensive heat treatment. And I don't know enough about it to say with any certainty that it's suited for saw blades.

BTW... there is not much point in WoodMizer keeping the particular alloy of their blades a secret. Any competitor who wanted to know would just have to buy one of their blades and send it to a lab to have an analysis done. They'd get a complete report of the chemical makeup of the steel.

Sorry for the long ramble. Not often I get to tie in work with forestry/sawing...

John Mc
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

DouginUtah

-Doug
When you hang around with good people, good things happen. -Darrell Waltrip

There is no need to say 'unleaded regular gas'. It's all unleaded. Just say 'regular gas'. It's not the 70s anymore. (At least that's what my wife tells me.)

---

Gary_C

Very good explanation John. That should answer every question about blades that anyone could have.   ::)

Using just the word "tempering" to describe the whole "quenched and tempered" process is probably why most people incorrectly believe tempering to be a hardening process.
Never take life seriously. Nobody gets out alive anyway.

StephenRice

Quote from: John Mc on September 04, 2010, 09:50:19 PM
Martensite is a steel microstructure formed by heating high carbon steel (or some other steel alloys) over a critical temperature to form Austenite (the temp is somewhere in the 1400's F, I forget exactly where, but most people heat up well over that to assure they rapidly get a fully austenitic structure - depending on the process, people will heat to anywhere from 1550 or so, or even a couple hundred degrees higher than that). From there, the steel is rapidly quenched to form martensite (traditionally in oil). At this point, it is very brittle. If you smacked a piece of wire on something hard, it would likely shatter. The wire is then reheated to "draw out" or "Temper" the martensitic structure (metallurgists call the microstructure "tempered martensite"). This makes it less brittle, and slightly softer, but still heard and springy. (This reheat is typically in the 700 F to 1000 or 1100 F range, depending on the final properties desired).

Many people refer to this whole process as "Tempering". This is actually shorthand for "Hardened and Tempered" or "Quenched and Tempered". Sometimes it's referred to as "Oil Tempering" (due to the fact that oil was the traditional quench medium). Technically, it is really only that final reheat that is Tempering, but the single word makes a convenient shorthand.

The term "induction heating" by itself doesn't tell you much about what is going on. It just refers to the method used to heat the steel. It does not tell you what type of heat treatment was done. It could mean annealing (which softens the steel making it easy to bend, but making it much less wear resistant) - not likely for the heat treatment on a saw blade. It could also mean a heating process that mimics the heat-quench-reheat described above, resulting in a tempered martensitic structure. With the right equipment, one thing that induction heat treating does is let you highly focus the area that is being heat treated - so it is possible for example to harden just the teeth of a saw blade for good wear characteristics while leaving the body of the blade in some other condition.

You can also "work harden" steel by cold forming it (in the case of a band saw, this would mean taking strip steel and rolling it down to the final desired thickness. The more you roll it, the harder and springier it gets -- up to a point. If you push it beyong the limits of the steel's ductility it will get brittle and start to break up.

My guess is that Woodmizer is buying "cold-rolled" (also known as "hard-rolled") strip steel, and this is where the hardening of the body of the blade comes from. They then induction heat-treat the teeth to further harden them. The hard-rolled body would have a very good cycle life, and would take the repeated flexing of a band saw blade very well. The heat treated teeth would have very good wear characteristics, and you might be able to get away with a harder (and therefore more brittle) tooth, since the body of the blade could have different properties more suited to the continual flexing. You could heat treat the whole thing, but one of the problems of a martensitic structure is that it is very "notch sensitive" - a small nick or imperfection will grow and crack resulting in failure of the blade. Hard rolled steel is more tolerant of these imperfections.

Thank you, John.  Your reply (and no, it was not too long of a ramble) much more eloquently described the process that I was talking about.  And, your hypothesis that WM is using a cold rolled steel banding as their "pre-hardened" blade material makes sense to me.  If it is true that they simply use strapping pre-hardened by cold rolling rather than by martensitic transformation, then the whole process makes a lot more sense to me.  They could be induction hardening just the tips by passing the top edge of the blades through an electrical current to heat it and then immediately quenching the blade teeth by spraying most likely oil or possibly even water on it to cool it.

This is what I was trying to find out by my original question.  IF the whole blade had already undergone a martensitic pre-hardening, then it seems to me that running just the teeth  through an induction heating would only temper the steel and thus make the teeth slightly softer than the banding.  The only advantage to that would be to prevent the teeth from chipping and breaking off if they were too hard and brittle, much like you often see on carbide teeth on circular saw blades that are run hard.  If the above were the case and the blade was pre-hardened as martensite, then the colors you see on the blade teeth would most likely be evidence of a tempering process designed to prevent chipping an breakage of the teeth and therefore the teeth would be slightly softer than the martensite blade.  So, if that were the case, you could not cut the hardness out of the tooth by grinding it too deep.  You would only be grinding into harder metal.  However, if the "pre-hardened steel" that WM is talking about is simply cold rolled steel as you suggested (and I, quite frankly, had not thought of), then a pass of the teeth alone through an induction hardening process with a spray of oil immediately afterwards could turn it into martensite and would indeed harden the teeth to a point that exceeded that of the rest of the blade.  In that scenario, which after your post now seems likely to me, you could grind the teeth past their point of hardening.

Thank you again, John, for your insight.  If WM would jump in and confirm what you suggested might be the case, then that would definitely end the question in my mind and explain a lot.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

John Mc

Another bit of industry terminology: when talking about cold-rolled steel, there are some terms referring to the amount of "work hardening that has been done to it. "Soft" or dead soft" refers to something that was annealed (a heat treatement to soften the still and give it a lot of ductility) at the finish size. They also refer to "quarter-hard", "half hard", and "full hard" each category refers to a range of hardness achieved by increasing amounts of cold work. Full hard feels pretty springy, even though it has not had a hardening heat treatment.

If Woodmizer is heat treating just the teeth, there are some alloys which will "air harden". If you heat them up enough to austenitize them, they can quench in air. I don't have a lot of experience with those, however. I do know that the hardening process doesn't go very deep on large chunks of the alloy, but something as thin as a bandsaw blade would probably work OK...
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

StephenRice

I believe that the depth of hardening depends a lot upon the process used for heating and cooling.

I remember one thing my old shop teacher would do to me after I made a tool or punch or whatever.  It kinda shocked me the first time...  He would come up, take whatever it ws that I had made, hold it up high to the light to inspect it closely and then, CLANG! he would drop it straight to the concrete floor!  Thankfully, my stuff never chipped or cracked or deformed.

You could audibly tell a well hardened tool or object from one that was softer by the noise that it made when it hit the ground.  A softer piece of steel would make a sort of dull clanking sound, while a well hardened one would make a ringing sound.  Also, when you picked it back up, the well hardened tool appeared non the worse for wear.  The overhardened tool would often chip or crack an edge, where a tool that had been over tempered and softened would dull or dent the sharpened edge.  The trick was to find that perfect middle ground.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

John Mc

Quote from: StephenRice on September 05, 2010, 11:39:19 AM
I believe that the depth of hardening depends a lot upon the process used for heating and cooling.

Yes. It depends on the alloy of steel used as well. Some alloys lend themselves better to surface hardening, and some are more "through hardening" alloys. The size of the piece to be hardened makes a difference as well. The bigger/thicker the piece, the tougher it is to through-harden (it's hard to suck the heat out of the interior of a big piece fast enough to get a good quench on it).

QuoteI remember one thing my old shop teacher would do to me after I made a tool or punch or whatever.  It kinda shocked me the first time...  He would come up, take whatever it ws that I had made, hold it up high to the light to inspect it closely and then, CLANG! he would drop it straight to the concrete floor!  Thankfully, my stuff never chipped or cracked or deformed.

You could audibly tell a well hardened tool or object from one that was softer by the noise that it made when it hit the ground.  A softer piece of steel would make a sort of dull clanking sound, while a well hardened one would make a ringing sound.  Also, when you picked it back up, the well hardened tool appeared non the worse for wear.  The overhardened tool would often chip or crack an edge, where a tool that had been over tempered and softened would dull or dent the sharpened edge.  The trick was to find that perfect middle ground.

Some of our maintenance guys at work were real artists at making various tools and knives. They'd wait until we were running a job making something close to the size, shape and alloy they wanted. Then they'd grab some scraps and form them into what they wanted. Next time one of our heat treating furnaces was running temperatures they wanted, they'd be right there with their tools and a bucket of quench oil to dip them in.

We used to do a 1/4" square wire from a chrome-vanadium alloy that was quite popular for screwdrivers and similar tools. Another guy was quite an artist at knife making. He'd grab some pieces when we were making 1/2" or 5/8" wide X 1/8" thick flat wire. He used different alloys and a different heat treat than the screwdriver guys.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

StephenRice

Most of the forging and heat treating that I did came from salvage materials.  My favorite materials were often from discs, springs, and blades from old salvaged farm equipment.  Other favorites that I used often were leaf springs from cars and trucks, used blade tips / edges from bulldozer blades and loader buckets and, especially for punches, the rods cut off of old worn out shock absorbers and struts.  Those were generally where I found the best quality high carbon steel to work with. Plus, it was usually free.  I was too poor to buy a bunch of new steel, but the older stuff was generally better anyways.  Otherwise, for regular repairs and projects, I would try to round up somedrops and other scrap stock somewhere.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

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