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This contribution is the follow-up draft article on Khukuri Steel, which was posted some time ago. I promised forumites I'd post this follow-up. It would be great if you could read & comment. Here goes!
Heat treating steel
The process of making sure one gets a khukuri of high quality does not end when high carbon leaf spring steel has been located by the kamis. It does not even end when the heated steel has been forged into shape. It is the heat treatment which effects certain changes in the steel, which gives one the desired end product. But, it seems, on the other hand, inferior heat treating of even the best steels will yield an inferior khukuri. "Hardness" is the cliche word here.
Now hardness of steel is generally measured by units known as Rockwells on the so-called Rockwell "C" scale. I found the following guidelines very informative:
20-25 Rc This is the hardness found in mild steel. Many fake Highland swords are not heat treated and are at this level. Edge retention is poor. In an article by Ron Hood it was reported that two Gurkha House khukuris (a Panawal and a Bhojpure) were tested for hardness. The spine on the blades (not the edges) read 22 and 25 Rc respectively, which puts the spine squarely in this category.
30-40 Rc Some say this is the minimum softness that should be allowed.
40-45 Rc This hardness level will have some spring qualities. Ron Hood reported that the midpoint of the bellies of the Panawal and Bhojpure khukuris mentioned above measured in at 45 and 46 Rc.
50-52 Rc This rating gives a good balance between toughness (slightly soft, enough for shock tolerance) and hardness (for edge holding). In the hardness testing done by Ron Hood, the edges came in at 54 and 55 Rc respectively, not bad for a kami squatting around a fire in sandals!
58-60 Rc Ideal hardness, they say, for the edge of differentially heat treated Chinese and Japanese swords. An edge of this hardness can cut into most materials that are compa rably softer. Anything over 60 ventures into brittle territory.
So, according to Ron Hood, it turned out that the khukuri blades that he tested were differentially tempered surprisingly well. I remember showing my newly acquired 15" Ang Khola khukuri to a knifemaker at a show in Bloemfontein last year. I mentioned that the blade was differentially heat treated. He doubted this, saying he didn't really trust the accuracy of the claim. But I soon gathered he did not have in-depth knowledge of khukuris.
Although nothing has been stated so far to contradict the general statement that "a knife with a hard edge is a good knife", Dan K. once wrote in Khukuri FAQ: "You shouldn't make too much over the relative hardness of the edge". However, he mentioned Gurkha Regiment blades with a hardness of Rc 58 on average, which, according to the table above, is high. Furthermore, I read that kami Lal Bahadur (Bura) has his edges at about Rc 60. But Dan K. says hardness does not make one blade better than the other. Harder edges are more prone to chipping, and softer blades tend to roll and dent, so there are disadvantages to both.
Ron Hood was impressed by the differential tempering of his pair of khukuri blades (spine 22 & 25 Rc, midpoint of the belly 45 & 46 Rc, and the edge 54 & 55 Rc). Bearing in mind that the blades are forged from truck springs, heated in a fire and quenched with water from a teakettle by a guy squatting in sandals, he thought at first that control over the tempering process might be a little spotty.
Hardness, heat treatment and tempering
I realised I would soon have to get into the nitty-gritties when it came to understanding hardness, heat treatment and tempering. I knew that the best way of going about it is to look at the basics again. The basic procedure of heat treating a knife is to anneal, harden and temper the steel. Certainly, one would expect these basics to be understood by the kamis!
What will be presented below is not yet the procedure of the kamis, but that of modern knifemakers, greatly condensed. We might use this three-part procedure below, and compare it with the kamis' way of doing things.
Annealing is the process used to soften steel and to relieve stress after sessions of hammer-forging. You do this by bringing the steel slightly above the so-called "transformation" temperature (also called the critical, austenizing point, found by using a magnet) and let it cool very slowly, preferably overnight. Annealing sets up the crystalline structure of the steel. I've read that accomplished knifemakers do this quite regularly after forging to keep the stresses to a minimum throughout the process. This procedure anneals the blade into a soft relaxed condition and the steel will be in its best condition for further forging the next day.
Now comes quenching or hardening the steel. The bright-red steel is submerged in water until cooled. Some Western bladesmiths use certain oils, like automatic transmission fluid. The blade should be fully hardened at this stage.
The steel's hardness is sometimes checked with a fine-toothed file at this point. The file should skate across the edge without biting into the steel.
Tempering is performed after hardening to draw back the hardness of the steel into a tougher, less brittle form. Experienced knifemakers use color as a guideline - they go for an even light straw color and then let it cool in still air. They repeat the process until they get a very even bronze color in the edge. Then they bury the blade with the bronze coloured edge area in cool, fine sand with the rest of the blade exposed. Now comes an especially interesting part: some knifemakers use tongs brought up to an orange heat to grab the back or spine of the blade to transfer the heat into it and to further draw the hardness out of these selected areas for greater strength. They go over the entire back and spine area of the blade until they get an even solid blue color there.
So, in this way they end up with a blade with a dark blue spine going into a dusty gray in the tang with a bronze edge. This gives the blade a stiff, flexible spine and a hard sharp edge that is still possible to sharpen in the field.
Other considerations
A good solid heat treatment on a lesser steel will often result in a blade that outperforms a better steel with inferior heat treatment. Of the three most important properties (blade profile, steel type, and heat treatment), heat treatment is the one that is impossible to assess by eye, and as a result excessive attention is sometimes paid to the other two.
Assessing the work of the kamis
I have scrutinised a number of versions (including the video "Gurkha Steel") of the heat treatment procedure used by the kamis.
I managed to get hold of a post by a forumite in the HI Archive, who gave astounding information. He wanted to bring out the hardening line of his 18" AK by an etching process. He was not totally satisfied due to the fact that khukuris are edge-quenched, not clay hardened, which means the khukuri's hardening line doesn't look as good as in some Japanese sword blades. But the acid etch showed him exactly which part of the blade is hardened. When the blade is bathed in acid, it turns gray, but the hardened area becomes pitch black. As far as he could see, the hardened area is 8.5" long on the 12.5" blade. The boundary is well defined and straight. It starts 2" from the cho, and stops 1.5" from the tip. The hardened zone tapers at both ends and is generally as wide as the primary bevel (0.5"). The width is fairly consistant, with the widest point at 0.7" located 3" from the cho. In other words, only the relatively straight edge of the blade is hardened. The curved parts (the tip and the "neck") are not.
Basically what this means on the khukuri is that a small part of the blade is hardened (to about 58 Rc). The entire tip and "neck" (the areas that receive most stress) is probably the same hardness as the spine. This accounts for the khukuri tip and blade not breaking when it is used for digging and prying.
Another forumite remarked that if you look at how the kami's harden their khukuris, it makes sense that the transition in hardness is well defined.
Bill Martino read the forumite's post and declared that he was not at all surprised by the results. It was exactly how a high quality khukuri should be hardened.
Pertinent questions and answers about khukuri steel (in summary)
Q What kind of steel is my original khukuri made of?
A If you've got a khukuri made in Nepal by traditional kamis, and not a tourist grade knife mass-produced in India, and if you're lucky, it's made of Mercedes Benz truck or bus leaf spring steel. Kamis believe this type of leaf spring makes the best khukuris. Second in line of quality leaf spring manufacturers is Saab. But you might have a khukuri made from Lada springs. Whatever the case may be, the material is to all accounts 5160 and 1085 steel.
Q Is this a good steel to make knives from?
A 5160 and 1085 steels are both very popular for the making of knives and have, respectively, 0.60 and 0.95% carbon. 5160 has chromium added for hardenability, while 1085 has manganese as an alloying element.
Q I've heard kamis heat the steel till it's red-hot and then pound it to shape many times over. So when they have done with it, what is left of the 5160 or 1085 steel? Surely it undergoes some kind of change - loses it's good qualities or whatever?
A I'm going to try to be as non-technical as possible here. High carbon steel is considered a solid "solution" of iron and carbon. The iron atoms are arranged in a cubic configuration, within which the carbon atoms are tucked away in the interstices. The atoms are in a state of thermal agitation, even at room temperature, and are moving in small orbits or oscillating back and forth with more and more vigour depending on temperature. So when the kamis heat up the steel in the forging process, the state of agitation increases. Assuming they do not "burn" the steel by mistakenly pushing up the temperature too high above the so-called transformation temperature, no rearrangement of the configuration takes place, only the state of agitation is heightened. In other words, the alloys are in solution, and the carbon is fully dissolved in the mix. This makes the steel forgeable.
Q What about hardening? Have I got a khukuri with a properly tempered edge?
A Most folks do not use or understand these terms "hardened" and "tempered" correctly. Hardening and tempering are two different and separate stages in the process of heat treatment of knife blades. During quenching a structure, called martensite, is formed, which is the hardest and most brittle condition of steel. If a quenched steel is moderately reheated, it loses some of its hardness and acquires a measure of toughness. This action of moderately reheating the steel is called tempering or drawing.
Q So has my khukuri been properly heat treated?
A Yes and no. If by properly heat treated you mean, have modern bladesmiths equipped with the most modern smithies used the most up- to-date methods to heat treat your khukuri, I'd say most definitely no. There are indications that khukuri blades are not actually tempered. But that does not mean your khukuri is inferior. An original khukuri made in the traditional way by kamis who have been handed down their craft through numbers of generations, have more going for it than simply the Rockwell hardness assessment of the edge. Your khukuri will have been heat treated, but then with an ancient procedure likened to none. It's all part of the lore of a khukuri, if that's what you're interested in. You don't want it done differently.
Q How has the edge of my khukuri been sharpened?
A If you're looking for a hollow-ground blade because you are accus tomed to this kind of edge, you'll be disappointed. Hollow-grinding is common in quality knives, but the edges are said to be so delicate they can't hold their sharpness under heavy usage. The edge seems flimsy, as can be seen under a microscope. Under hard usage, the edge can be expected to chip rapidly. Hollow-ground edges occur naturally when a knife blank is clamped in a jig and pushed against the wheel of a belt sander. It's not done that way with khukuris. A good convex sabre- grind (the type put on khukuris) gives an edge that can be expected to work very hard.
Q How much evidence is there that I have a blade that can do an honest job of work?
A I believe there's ample evidence. In a nutshell, there's the hardness of the edge. In tests for hardness, khukuri edges have come in at between 54 and 55 Rc. Some have even been reported as being 60 Rc. Then there is the so-called edge geometry, which simply refers to the robust construction of the edge. These factors, viewed in harmony, introduce you properly to khukuris, objects of workmanship, culture, art, history and tradition which are collectable in the extreme!
*****
Heat treating steel
The process of making sure one gets a khukuri of high quality does not end when high carbon leaf spring steel has been located by the kamis. It does not even end when the heated steel has been forged into shape. It is the heat treatment which effects certain changes in the steel, which gives one the desired end product. But, it seems, on the other hand, inferior heat treating of even the best steels will yield an inferior khukuri. "Hardness" is the cliche word here.
Now hardness of steel is generally measured by units known as Rockwells on the so-called Rockwell "C" scale. I found the following guidelines very informative:
20-25 Rc This is the hardness found in mild steel. Many fake Highland swords are not heat treated and are at this level. Edge retention is poor. In an article by Ron Hood it was reported that two Gurkha House khukuris (a Panawal and a Bhojpure) were tested for hardness. The spine on the blades (not the edges) read 22 and 25 Rc respectively, which puts the spine squarely in this category.
30-40 Rc Some say this is the minimum softness that should be allowed.
40-45 Rc This hardness level will have some spring qualities. Ron Hood reported that the midpoint of the bellies of the Panawal and Bhojpure khukuris mentioned above measured in at 45 and 46 Rc.
50-52 Rc This rating gives a good balance between toughness (slightly soft, enough for shock tolerance) and hardness (for edge holding). In the hardness testing done by Ron Hood, the edges came in at 54 and 55 Rc respectively, not bad for a kami squatting around a fire in sandals!
58-60 Rc Ideal hardness, they say, for the edge of differentially heat treated Chinese and Japanese swords. An edge of this hardness can cut into most materials that are compa rably softer. Anything over 60 ventures into brittle territory.
So, according to Ron Hood, it turned out that the khukuri blades that he tested were differentially tempered surprisingly well. I remember showing my newly acquired 15" Ang Khola khukuri to a knifemaker at a show in Bloemfontein last year. I mentioned that the blade was differentially heat treated. He doubted this, saying he didn't really trust the accuracy of the claim. But I soon gathered he did not have in-depth knowledge of khukuris.
Although nothing has been stated so far to contradict the general statement that "a knife with a hard edge is a good knife", Dan K. once wrote in Khukuri FAQ: "You shouldn't make too much over the relative hardness of the edge". However, he mentioned Gurkha Regiment blades with a hardness of Rc 58 on average, which, according to the table above, is high. Furthermore, I read that kami Lal Bahadur (Bura) has his edges at about Rc 60. But Dan K. says hardness does not make one blade better than the other. Harder edges are more prone to chipping, and softer blades tend to roll and dent, so there are disadvantages to both.
Ron Hood was impressed by the differential tempering of his pair of khukuri blades (spine 22 & 25 Rc, midpoint of the belly 45 & 46 Rc, and the edge 54 & 55 Rc). Bearing in mind that the blades are forged from truck springs, heated in a fire and quenched with water from a teakettle by a guy squatting in sandals, he thought at first that control over the tempering process might be a little spotty.
Hardness, heat treatment and tempering
I realised I would soon have to get into the nitty-gritties when it came to understanding hardness, heat treatment and tempering. I knew that the best way of going about it is to look at the basics again. The basic procedure of heat treating a knife is to anneal, harden and temper the steel. Certainly, one would expect these basics to be understood by the kamis!
What will be presented below is not yet the procedure of the kamis, but that of modern knifemakers, greatly condensed. We might use this three-part procedure below, and compare it with the kamis' way of doing things.
Annealing is the process used to soften steel and to relieve stress after sessions of hammer-forging. You do this by bringing the steel slightly above the so-called "transformation" temperature (also called the critical, austenizing point, found by using a magnet) and let it cool very slowly, preferably overnight. Annealing sets up the crystalline structure of the steel. I've read that accomplished knifemakers do this quite regularly after forging to keep the stresses to a minimum throughout the process. This procedure anneals the blade into a soft relaxed condition and the steel will be in its best condition for further forging the next day.
Now comes quenching or hardening the steel. The bright-red steel is submerged in water until cooled. Some Western bladesmiths use certain oils, like automatic transmission fluid. The blade should be fully hardened at this stage.
The steel's hardness is sometimes checked with a fine-toothed file at this point. The file should skate across the edge without biting into the steel.
Tempering is performed after hardening to draw back the hardness of the steel into a tougher, less brittle form. Experienced knifemakers use color as a guideline - they go for an even light straw color and then let it cool in still air. They repeat the process until they get a very even bronze color in the edge. Then they bury the blade with the bronze coloured edge area in cool, fine sand with the rest of the blade exposed. Now comes an especially interesting part: some knifemakers use tongs brought up to an orange heat to grab the back or spine of the blade to transfer the heat into it and to further draw the hardness out of these selected areas for greater strength. They go over the entire back and spine area of the blade until they get an even solid blue color there.
So, in this way they end up with a blade with a dark blue spine going into a dusty gray in the tang with a bronze edge. This gives the blade a stiff, flexible spine and a hard sharp edge that is still possible to sharpen in the field.
Other considerations
A good solid heat treatment on a lesser steel will often result in a blade that outperforms a better steel with inferior heat treatment. Of the three most important properties (blade profile, steel type, and heat treatment), heat treatment is the one that is impossible to assess by eye, and as a result excessive attention is sometimes paid to the other two.
Assessing the work of the kamis
I have scrutinised a number of versions (including the video "Gurkha Steel") of the heat treatment procedure used by the kamis.
I managed to get hold of a post by a forumite in the HI Archive, who gave astounding information. He wanted to bring out the hardening line of his 18" AK by an etching process. He was not totally satisfied due to the fact that khukuris are edge-quenched, not clay hardened, which means the khukuri's hardening line doesn't look as good as in some Japanese sword blades. But the acid etch showed him exactly which part of the blade is hardened. When the blade is bathed in acid, it turns gray, but the hardened area becomes pitch black. As far as he could see, the hardened area is 8.5" long on the 12.5" blade. The boundary is well defined and straight. It starts 2" from the cho, and stops 1.5" from the tip. The hardened zone tapers at both ends and is generally as wide as the primary bevel (0.5"). The width is fairly consistant, with the widest point at 0.7" located 3" from the cho. In other words, only the relatively straight edge of the blade is hardened. The curved parts (the tip and the "neck") are not.
Basically what this means on the khukuri is that a small part of the blade is hardened (to about 58 Rc). The entire tip and "neck" (the areas that receive most stress) is probably the same hardness as the spine. This accounts for the khukuri tip and blade not breaking when it is used for digging and prying.
Another forumite remarked that if you look at how the kami's harden their khukuris, it makes sense that the transition in hardness is well defined.
Bill Martino read the forumite's post and declared that he was not at all surprised by the results. It was exactly how a high quality khukuri should be hardened.
Pertinent questions and answers about khukuri steel (in summary)
Q What kind of steel is my original khukuri made of?
A If you've got a khukuri made in Nepal by traditional kamis, and not a tourist grade knife mass-produced in India, and if you're lucky, it's made of Mercedes Benz truck or bus leaf spring steel. Kamis believe this type of leaf spring makes the best khukuris. Second in line of quality leaf spring manufacturers is Saab. But you might have a khukuri made from Lada springs. Whatever the case may be, the material is to all accounts 5160 and 1085 steel.
Q Is this a good steel to make knives from?
A 5160 and 1085 steels are both very popular for the making of knives and have, respectively, 0.60 and 0.95% carbon. 5160 has chromium added for hardenability, while 1085 has manganese as an alloying element.
Q I've heard kamis heat the steel till it's red-hot and then pound it to shape many times over. So when they have done with it, what is left of the 5160 or 1085 steel? Surely it undergoes some kind of change - loses it's good qualities or whatever?
A I'm going to try to be as non-technical as possible here. High carbon steel is considered a solid "solution" of iron and carbon. The iron atoms are arranged in a cubic configuration, within which the carbon atoms are tucked away in the interstices. The atoms are in a state of thermal agitation, even at room temperature, and are moving in small orbits or oscillating back and forth with more and more vigour depending on temperature. So when the kamis heat up the steel in the forging process, the state of agitation increases. Assuming they do not "burn" the steel by mistakenly pushing up the temperature too high above the so-called transformation temperature, no rearrangement of the configuration takes place, only the state of agitation is heightened. In other words, the alloys are in solution, and the carbon is fully dissolved in the mix. This makes the steel forgeable.
Q What about hardening? Have I got a khukuri with a properly tempered edge?
A Most folks do not use or understand these terms "hardened" and "tempered" correctly. Hardening and tempering are two different and separate stages in the process of heat treatment of knife blades. During quenching a structure, called martensite, is formed, which is the hardest and most brittle condition of steel. If a quenched steel is moderately reheated, it loses some of its hardness and acquires a measure of toughness. This action of moderately reheating the steel is called tempering or drawing.
Q So has my khukuri been properly heat treated?
A Yes and no. If by properly heat treated you mean, have modern bladesmiths equipped with the most modern smithies used the most up- to-date methods to heat treat your khukuri, I'd say most definitely no. There are indications that khukuri blades are not actually tempered. But that does not mean your khukuri is inferior. An original khukuri made in the traditional way by kamis who have been handed down their craft through numbers of generations, have more going for it than simply the Rockwell hardness assessment of the edge. Your khukuri will have been heat treated, but then with an ancient procedure likened to none. It's all part of the lore of a khukuri, if that's what you're interested in. You don't want it done differently.
Q How has the edge of my khukuri been sharpened?
A If you're looking for a hollow-ground blade because you are accus tomed to this kind of edge, you'll be disappointed. Hollow-grinding is common in quality knives, but the edges are said to be so delicate they can't hold their sharpness under heavy usage. The edge seems flimsy, as can be seen under a microscope. Under hard usage, the edge can be expected to chip rapidly. Hollow-ground edges occur naturally when a knife blank is clamped in a jig and pushed against the wheel of a belt sander. It's not done that way with khukuris. A good convex sabre- grind (the type put on khukuris) gives an edge that can be expected to work very hard.
Q How much evidence is there that I have a blade that can do an honest job of work?
A I believe there's ample evidence. In a nutshell, there's the hardness of the edge. In tests for hardness, khukuri edges have come in at between 54 and 55 Rc. Some have even been reported as being 60 Rc. Then there is the so-called edge geometry, which simply refers to the robust construction of the edge. These factors, viewed in harmony, introduce you properly to khukuris, objects of workmanship, culture, art, history and tradition which are collectable in the extreme!
*****