overheating steel

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toddhill

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I have read quite extensively about the dangers of overheating steel. Article after article in M.T. Richardson's books talk about this and I recently read the same thing in Fowler's Knife Talk II. Apparently overheating hurts grain size and I'm not sure what else it does. Anyway, I was wondering, if this is true, how it affects damascus and welded blades. The steel in these is definitely "overheated." Does this ruin the steel? Is a knife made of regular homogenous steel better for not having been overheated? Thanks, ToddH
 
Overheating causes grain growth which makes it brittle, but that can be corrected good as new with thermal cycling. Overheating also causes decarb which while it can't be fixed, it can be accounted for by having high enough carbon to start with. Overheating also speeds up carbon diffusion, but if both your damascus steels are tool steels then that really doesn't matter. As long as you don't actually burn the steel, and you account for everything, overheating early in your process isn't a big deal.

A perfectly done damascus blade should perform equally well as an identical blade made from homogenous steel, but due to the complexity of making damascus, it might not always be perfectly done damascus unless you're really good.
 
When we get above the critical temperature the structure becomes austenite. What happens then is time and temperature dependant , temperature being much more critical than time.The austenite grains grow.The larger the grains the more brittle the blade.We can correct this by normalizing an rehardening.Decarb also becomes more of a problem....At even higher temperatures [ above normal forging temps] we get burning . There is no cure for that.
 
If overheating is so "correctable" by thermal cycling then why do some smiths make such a big deal about it? It seems like we shouldn't be worried about it as long as we include some thermal cycling at the end of our forging. Fowler said, "If during the forging process I allow the temp of the blade to exceed 1,625 degrees F., the blade will not perform as well as one forged at consistently lower temps." Is this solely because of decarburization? Or is there something else to it?
 
When a maker wants every bit of performance from the steel he uses, an overheat is just a variable that he can avoid to even slightly decrease said performance. It's like plucking one hair off your head. Your not bald , but you still have less hair than you could have.


A question for mete.

If you keep two pieces of steel at carburizing temp; one in pack carb media, and the other in a neutral flame for the same amount if time, is the carbon gain and loss rate of the respective pieces about the same ?
 
What happens in carburizing/decarburizing is that the steel and environment tend to equalize as far as carbon. So if the steel is richer in carbon than the environment you get decarburization. If the environment is richer in carbon you get carburization. The rate of carburization/ decarburization depends on temperature[ the higher the faster] and relative difference between environment and steel as far as carbon content [greater difference -greater rate.
 
There are some excellent thoughts in this thread. The finer the grain of the steel you have developed, the lower the temperature at which the grain can grow.
The first seminar for the High Performance Blade on the Willow Bow ended last week. Four men forged blades from 5160 load control shafts, all at low temp. Multiple Quenched and tempered they all were capable of stabing the pedestal on my burr king without deforming the tip, all made 20 edge flexes with no chip. All made 300 cuts on some agressive rope with edge to spare.
We were pushed for time so the thermal cycles in heat treating had to be abbreviated. Still my blade made 4 1/2 180 degree flexes before the edge cracked, no one could flex the blade with the tip in my vice with their bare hands, Bill Burke flexed her using a 20 inch length of pipe for leverage on the tang.
All those who attneded were experienced knife makers and the three days were about 16 hours long.
High Performance blades are within the grasp of any knife maker who does his homework.
 
Thanks mete . I was just wondering if the decarb was close to the same as carburizing rate. I read .08" per hour carburizing depth, sound about right at 1700 degrees?
 
I'm curious to know if anyone has done comparative testing. Has anyone taken a knife forged at higher temps and then thermal cycled and compared it molecularly or by cut testing with a knife forged at lower temps. How significant are the differences we're talking about? I realize there are some variables such as how high the higher temps were.

Thank you all for your thoughts. Mr. Fowler, I just bought and finished reading your Knife Talk II. I ordered Knife Talk from Amazon and have been tracking it. It arrived last night so I should be getting it today. I'm very excited. I thoroughly enjoy your thoughts and writing style. It's hard to argue with someone who has done the extensive testing you do to confirm your theories about bladesmithing.

For anyone interested: I grew up in the Amazon jungles of Brazil. My parents were missionaries there. We lived with knives from the time we could walk. We always had a knife with us. Lacking belts or sheaths we just stuck them handle first at an angle in the back of our shorts. This worked fine even walking and hunting through thick jungle. We used these knives for so many varied tasks, and they weren't special by any means. The knives were made by a Brazilian company called Tramontina. They averaged 12 inches in length. The steel was simple carbon and the handles were wooden. We sharpened the blades with files. They stained and gradually became dark gray to black in color. The only time I had complaints about a knife was when I got a stainless one. It chipped badly the first time I used it for chopping.

Anyway, I got into blacksmithing a couple years ago and recently bladesmithing has become my passion. I am learning so much, and have so much yet to learn.
 
todhill: Thanks for the kind thoughts, it was interesting to read about folks who use knives extensively.
As to the differences between high temp and low temp forged blades. There was a time when the load control shafts yielded blades that could make 60 cuts at best as our talents increased so did performance. Now with total dedication to performace we can get 400 cuts from the same steel, along with tough and strong. You will read all about the techniques in my Knife Talk Books.

Blades that come from poor forging technique can be as bad as it gets, I have tested blades that would not do one cut, could not be sharpened and would chip our with little stress being appleid.

I forgot to mention that Bill Burke has been doing some highly interesting experiments using layered damascus blades. He feels he can nurture layered Damascus blades into the lower end of High Performance levels.

Damascus means pattern. I call our etched 52100 blades homogenous Damascus. We are only kidding ourselves by believing we can make steel better in our shops than the big outfits can do with all the technology the steel industry has developed for their high performance steels.
We can make beautiful blades using methods of pattern development, but performance is another proposition.
 
This is what I thought. So it really boils down to performance or looks when it comes to homogenous vs. damascus. Damascus can approach performance with great care and skill, but the best choice for performance is homogenous with low temp. forging. Thanks for clearing this up for me. Those test results are pretty dramatic and impressive. Todd Hill
 
The main thing that has been left out is dynamic recrystalization, which essentially says that as you mechanically forge a peice of steel, new austenite grains are formed.
 
Forging through the use of heat and hammer refines grain, multiple thermal cycles within the desirable heat ranges is beneficial to the development of the qualities sought.

Inorder for the bladesmith to develop the skills to take the steel to the highest potential it is absolutely necessary that he test his blades as they are completed. The edge flex is a very reliable indicator of edge toughness. Testing the tip for strength by stabing in to a piece of steel is another test that results in immediate feedback. The rope cut requires some skill, some feel this test is too demanding of skill, I believe this is a skill of great value to the bladesmith who wants to make the best knife he can make. The man who has not developed the skills necessary to test his blades will never make his best potential knife.

Etching blades is another event that will educate the bladesmith as he builds his skills. With a little experience the bladesmith will soon be able to identify blades that have been overheated as well as the overall development of the hardening process. All events in the development of the blade are obvious to the man who works with one steel and seeks to know it completely. The nature of the blade is obvious to maker and client, very few secrets can be hidden from view through the use of an honest etch. Bladesmiths who etch their blades will know what they have developed and the story is open to any who look at the blades. Etching all my blades taught me a lot about the forged blade and led the way to the high performace blade.

There are trails around every fence, but honest, dedicated development of the etched blade is very rewarding when supported by testing blades for tough, strong and cut. All these evaluations are readilly available to every bladesmth in his shop.
 
Because of the way I grew up I have a definite bias toward the practical. I don't see the good of a knife if it's not being put to honest hard use. I know this isn't the only right way to view knives. I have good friends with a collector's mindset. They have dozens of great knives--all nicely displayed or stored. Something inside me always questions this. I immediately picture how the knife would function at this or that task and cringe at the thought that it will never be put to use. But I have come to accept it. I guess that's why I like your [Mr. Fowler] advice so much. It's all about testing and comparing and results. It's all practical. The world I grew up in was so different than this one. Nothing was wasted. Resources were scarce so everything used had a specific purpose--nothing frivolous. Life depended on tools and their ability to work hard and well. That's what I think a knife should be: simple yet elegant, up to its intended task. This can't be accomplished without the extensive testing you challenge us to do.
 
I'll second that, while I do apreciate an art knife, to me it's not realy a knife unless it's capable of being used as a knife, or will at least hold an edge. If there's any dought, test and test some more.

Also while I'm not in nearly the same leage as Ed, hear him out, my blades have been vastly improved by his writings, advice and experiments.
 
Thank you William and Todhill:
Many years ago a man I know well was a boy and survived a winter in the Rockies with only a 22 pistol and a knife to provide food. They nearly starved due to a very tough winter, got down to eating mice to survive. His experience taught me many things, I have a great fear of running out of essentials, and constantly seek tools that can be counted on.

I consider absolute dedication to function to be the most viable aspect of art. Today many have no idea as to the requirements of the high performance knife totally dedicated to function. I would have loved to have been around this country I live in back in the 1800's when man lived as a hunter and depended on his companion, lady knife, for his needs.

Naturally I would want to take with me a good supply of our 52100.

The methods we speak of were probably known by great knife makers who lived many years before, their knowledge and methods were unable to survive the industrial revolution and mass marketing. They did not have the internet to share information and the marketing methods and traditions of the day also restricted communication.

The best knife makers of the past served agriculture and gained reputations by making the supreme instruments of cut. This was the arena where cut prevailed, not in the cities where jewlery for men was the demand. The cutlers who made knives for the gentry became known as great simply because of the dress their ladies of cut wore, not by cut. To my eye, any aspect of a knife that is not dedicated to performance is a distraction rather than attribute beauty.

As long as we keep the two realms of lady knive well delineated and serve our clients honestly we are providing our best product. When those who have lived and loved lady knife for what she can do rather than what she looks like speak at every opportunity we can maintain a balance to her nature.

Thank you for speaking out!
 
This is a great thread! Like most makers I first desire to make a tool that will cut exceptionally well. For what it's worth most of the knives I make never have that opportunity, so I'm thrilled whenever one does see service. I've learned a lot from feedback on those knives that are working, and from discussions here on heat treating.

Ed can you describe how you "read" the etch of your knife blades? I make very few knives and so have only tested a couple to destruction - nowhere near enough to learn all that I need to about my steel. Maybe if I knew how to see what the etch is telling me this would add to my basket of non-destructive testing methods. Any ideas you would provide are greatly appreciated.

toddhill I too really appreciate your life using knives. I've carried a knife all my life to use for simple everyday tasks and am always amazed when someone is surprised that I have one; what the heck do they use to cut stuff? (Scissors and steak knives - pffft!) So few people actually understand the utility value of a good knife. I think it's a tragic consequence of urban life that people so misunderstand this most basic tool.
 
Dave: Etching blades is like opening a book to the nature of the steel. When you etch your first blades you will have many questions, when I first started etching 52100 steel I was using ball bearings, many times I would find swirls in the grain when I failed to align the grain flow of the steel perfectly. It took some time to know what I was seeing; at first I considered it simply a beautiful manifestation of the steel, later I found it was actual grain flow. If you do a lot of proper forging on your steel you will be able to actually see the grain flow. If it is a stock removal steel you can still see the grain flow.

Soft steel will etch deeper than hard steel and if everything was done correctly also reveal a larger grain. Scratches are always more difficult to remove from soft steel than hard steel, this may be because they are deeper.
We go into heat treat with a grain that is 3 to 4 points finer in the lower half of the blade that that of the spine simply because we never hammer on the spine, only the future citting edge and sides of the blade. We can see the grain size difference by etching, this was also and first revealed by photomicrographs. I had never considered etching a blade before hardening, had I done so I would have known earlier.

When you see some deviation from the expected pattern, this is where destructive testing may become very revealing, don['t be bashful about having a lab examine it for you, then do your destructive testing on what is left over.

Etching blades reveals obvious characteristics of the steel. It also reveals the nature and extent of your heat treat. Differentially hardened blades are very interesting. Etching blades was absolutely critical to my development as a knife maker. Many who do not etch their blades have no idea what lies behind the mirror. I etched some old Richtig knives and immediately gained great respect for his talents.

Etching and reading the etch comes with practice, a lot of practice. It starts with the first blade etched and continues as long as your curiosity pulls you into the nature of your blades. Etching is cheaper than sending blades to a lab. but the thousands of dollars worth of lab work Rex, Bill and I have invested have all been well worth every penny and more.

Overheated steel will ususally look like someone threw a handfull of grapes into a plate of spaggetti. Stress risers induced during the forging will show up as waves, kind of like ladder pattern damascus, but conforming to the nature of the forging error.

Should you seek a lab. to work with you, you need to find a metalurgist who dreams of steel and great knives. Many men with degrees have never ventured beyond regurgitating the material in their textbooks, avoid this kind of science or you will remain like barnyard chickens picking grain out of what the cows leave behind. You can learn from text books, but study what they are making, use the information where it may apply, questing its value as knife and don't let yourself get hooked into making knives made on the science of fence posts.

Once you find a man of science you can communicate with treat him like gold and work together. No man can make great knives by himself, it takes team work with history, science and dreams of perfection.

Use only the best steel available, many knives fail because they started as poor steel. These failures lead you down paths of deception. You feel what you did was wrong and try to correct it while the fault was there long before you started. When you find a steel you can call friend, buy tons of it if necessary but never run out. If you find something better you can always use your old steel for paper weights or ballast or to keep your bacement from blowing away.

The etching of blades is like learning to dance with your lady, each tune requires a little learning, your practice, dance, etch and the affair continues.

There is more, a lot more but this should provide you with some idea of why and what for. Don't be afraid to call me when you have questions, we all learn through questions and each failure is an opportunity for learning.
 
Mr. Fowler, in your article "The Best Knifemaker Nobody Remembers" you talk about Richtig. You mention getting the opportunity to test his knives. You don't mention etching the blades as you did in the last post. What did etching reveal on his blades? I assume they were differentially hardened. Thanks, ToddHill
 
Tod Hill: You asked about reading the etch, I did a search and found two nice etches, one in a gallery thread titled Best Hamons by Dylan d. 6/6/2004 The knife by Steve Filicietti, third photo down I believe appears to be a good honest etch, you can see the hardened portion on the bottom of the blade, then what appears to be a transition zone, then a soft back. When you can forge a blade with an honest etch that shows what this one does, you have done well.
Another thread by me shows a very complex etch. The title was for the Oregon Show A special grain structure.
There are a lot of ways to etch blades and prepare them for etching, for me the honest etch that lets me see the actual structure of the steel is most rewarding. The other stuff is pretty and beautiful, but not what you want to describe the steel itself, in my opinion.
What did I see in the etch of Richtig's blades, a nice grain that had been subject to what appears to be a supreme heat treat. Rex figures his tempering process was very sophisticated on the blades he did photo micrographs of.
When I get time I am going to try to duplicate it. Richtig's blades were excellent and better than most made today. I believe he had a very accurate and simple way to test his blades based on how they perform. As far as I know they were all stock removal.
 
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