The things that will destroy America are prosperity-at-any-price, peace-at-any-price, safety-first instead of duty-first, the love of soft living, and the get-rich-quick theory of life.Theodore Roosevelt
The terms White Steel and Blue Steel frequently pop up in discussions about Japanese woodworking tools and kitchen knives. The usual misunderstandings abound in those discussions and BS takes majestic wing. In this article your humble unworthy servant will try to share some accurate information sourced directly from the steel manufacturer, ancient blacksmiths that actually make and work these steels, and Japanese professional craftsmen paid to make sawdust using these steels instead of the usual soft-handed shopkeepers and self-proclaimed experts living in their Mom’s basement.
We will begin by studying some etymology of two of Japan’s most famous tool steels. We will then drop into history class to discuss ancient domestic Japanese steel, and then shift our attention to why these modern steels came into being. After that, we will go to metallurgy class, but without the technical jargon, to understand what chemicals these steels contain and why. We will also outline the defining performance characteristics of those same two steels in the case of woodworking tools.
For those who enjoy more technical details combined with pretty pictures, we have concluded with the results of a brief but very informative materials engineering study.
Please ready your BS shovel.
Product Designations: Yellow, White & Blue Label Steels
These terms refer to tool steels manufactured by Hitachi Metals, Ltd. (HML) in their plant located in Yasugi City in Shimane Prefecture, Japan. If you are into woodworking tools or Japanese cutlery you have heard of these steels.
Hitachi, Ltd., founded in 1910, is one of Japan’s most prestigious manufacturers. Its subsidiary, Hitachi Metals, Ltd., was established in 1956 primarily through acquisitions.
“White Steel” is an abbreviated translation of HML’s nomenclature of “Shirogamiko” 白紙鋼, which directly translates to “White Paper Steel.” Likewise, “Blue Steel” is an abbreviation of “Blue Paper Steel,” the translation of “Aogamiko” 青紙鋼.
Just as “Johnnie Walker Blue” is the commercial designation of a famous Scottish whiskey with a blue label pasted onto the bottle, Aogami is the designation of a particular formulation of high-carbon tool steel with a blue-colored paper label pasted onto it. It’s that simple.
While Johnny Walker may be kind sorta yellow in hue, it is definitely not tinted blue anymore than JW Red Label is sanguine. Likewise, the color of Hitachi Metal’s tool steels do not vary in color, only their labels do. If someone tells you they can tell the difference between these steels by simply looking at them, tell them to give you a nickle and pull the other one.
There are those who insist they can tell the difference between steels by licking them. Our feline masters see this as further evidence that some humans are just not right in the head.
Since your humble servant can read and write Japanese, I feel foolish calling these materials White Steel or Blue Steel as many in English-speaking countries do, so prefer to call them Yellow Label Steel, White Label Steel or Blue Label Steel in English, or Kigami, Aogami, or Shirogami steel. Please excuse this affectation.
Now that product nomenclature and feline psychoanalyses is out of the way, let’s go back in time a few hundred years. My tardis is that blue box just over there. A change into period-correct wardrobe will not be necessary, but please put away your iPhone and try to not embarrass me in front of the locals by holding it over your head and declaring out-loud that you can’t find a signal, if you will be so kind.
Traditional Japanese Steel: Tamahagane
Tamahagane, written 玉鋼 in Chinese characters, which translates to “Jewel Steel” and pronounced tah/mah/ha/gah/neh, is famous as the steel traditionally used to forge Japanese swords, but prior to the importation of steel from overseas, beginning with products from the Andrews Steel mill in England, it was once used for all steel production in Japan.
Before Admiral Perry’s black ships re-opened the many kingdoms and fiefdoms scattered across the islands that now comprise modern Japan, the only local source for natural iron was a material called Satetsu, a loose surface iron written 砂鉄 in Chinese characters, meaning ”sand iron,” and pronounced sah/teh/tsu. Satetsu looks exactly like black sand. It is quite common throughout the world, as you may discover if you drag a magnet through a sandy river.
Typically found in rivers and estuaries, for many centuries the area around Yasugi City in Shimane Prefecture was a prime source.
Satetsu was historically harvested in Japan using dredges and sluices creating horrendous environmental damage. Fortunately, the days of wholesale estuary destruction are in Japan’s past.
Although Aluminum is the most abundant metal on the third rock from the sun, iron is said to make up 34% of the earth’s mass. Japanese satetsu as harvested is a fairly pure form of iron lacking nearly all the impurities typically found in iron ore extracted from mines.
Historically, satetsu was refined in rather crude furnaces called ” tatara” to form clumps of brittle, excessively-high carbon steel. This technique is not unique to Japan, although many Japanese believe it is.
Steel produced this way in the West is called “bloom steel.” Blacksmiths hammer, fold, and re-hammer these crumbly lumps to remove impurities and reduce/distribute desirable carbon forming the more homogeneous Tamahagane steel. This webpage has some interesting photos of tamahagane.
Tatara furnaces are still operated today producing Tamahagane in limited quantities for use by registered sword smiths. Tool blacksmiths use Tamahagane occasionally too out of interest in traditional materials and methods. It is expensive and difficult to work, with lots of waste.
A sawsmith who was active both before and after the availability of British steel on the island of Shikoku in Japan is recorded as saying that imported Western steel increased saw production efficiency in his area tenfold. Clearly, Tamahagane was a very labor intensive material.
Mr. Kosuke Iwasaki, a famous metallurgist and blacksmith, described forging Tamahagane as being like “hammering butter” because it flattened and spread too quickly and unpredictably, at least compared to modern steels.
Besides its peculiar forging characteristics, compared to modern tool steels Tamahagane is a difficult material infamous for being easily ruined and extremely sensitive to temperature during all phases of forging and heat treatment. This tendency has created a traditional sensitivity among Japanese blacksmiths regarding precise temperature control.
In use, tools made from Tamahagane behave differently from modern commercial steel, or so I am told. I own and use a straight razor custom forged from Tamahagane for me many years ago by Mr. Iwasaki. I also own antique Scheffield and German razors, but my Iwasaki razor puts them all to shame in terms of sharpness, edge retention, and ease of sharpening. I also own a couple of antique Tamahagane saws, but I have not used them much, nor have I used Tamahagane chisels, planes or knives, so my experience is limited to this one wickedly sharp little blade.
Why do I bother Gentle Reader with this story of ancient techniques and obscure products no longer viable? Simply because Tamahagane and the cutting tools and weapons it was once used to produce had a huge practical and cultural influence on both Japan’s history and the Japanese people’s attitude towards weapons and cutting tools, in your humble servant’s opinion.
Although imported Western steel served Japan well during its ramp-up to modernity, the memory of the performance of cutting tools made from Tamahagane remained alive in the national memory. Indeed, I am convinced the Japanese people’s love and fear of sharp things is not only psychological but genetic, although I have not seen any studies on the “sharpness gene.” But that is a story I will save for the next time we are enjoying a mug of hot coco together around the iori fire on a moonlit Autumn night. May that evening come soon.
Steel Production in Modern Japan
Enough ancient history. Let’s jump back into the tardis and travel to the late 1950’s so we can shift our focus to more modern steels. Yes, you can turn your mobile phone back on. No you can’t bring back souvenirs. I don’t care what Doctor Whatsit did with his tardis, we are responsible time travelers and will avoid creating casual conundrums. Besides, the import taxes are pure murder. And please, do be careful no little children slip inside with you.
When Japan began to mass-produce commercial steel from imported pig iron using modern techniques, the first tool steel made was identical to Western steels, including the impurities. These are still available today as the “SK” series of steels as defined by Japan Industrial Standards (JIS).
Eventually, to satisfy the irrepressible sharpness gene of their domestic customers, Japanese blacksmiths and tool manufacturers pressured Japanese steel companies to develop products with fewer impurities and with performance characteristics approaching traditional Tamahagane.
Rising to the challenge, Hitachi Metals endeavored to replicate the performance of Tamagane using modern smelting techniques and imported pig iron and scrap metal instead of expensive and environmentally unsustainable satetsu.
Hitachi purchased and modernized an old steel plant in Yasugi for this purpose. They formulated the best steel they could make using the best pig iron they could find, mostly from Sweden, an area famous for hundreds of years for producing especially pure iron ore. The results were Shirogami Steel (pronounced she/roh/gah/mee/koh 白紙鋼), Aogami Steel (pronounced aoh/gah/mee/koh 青紙鋼), and Kigami Steel (pronounced kee/gah/me/koh and written黄紙鋼) meaning “Yellow Label Steel.” Later, they developed Aogami Super steel (青紙スパー ) and Silver Label Steel (stainless steel). Each of these products are available in various subgroups, each having a unique chemical formulation.
For a time, Hitachi marketed these steels with the “Tamahagane” designation. Problematic, that. Indeed, many saws and knives were stamped “Tamahagane” when these steels were first introduced.
With the increased popularity of Japanese knives overseas, several Japanese manufacturers have once again adopted this problematic practice of labeling the steel their products are made from as “Tamahagane” despite being made of common steels and even stainless steels. Because these spurious representations were and continue to be made for the purpose of increasing profits for companies that clearly know better, in your humble servant’s opinion even the stinky label of BS is too good for them.
Caveat emptor, baby.
We tend to think of steel as a hard metallic thing, but lo and behold, ’tis a chemical compound.
Few chemicals humans make are absolutely pure, and while White Label, Blue Label, and Yellow Label steels contain exceptionally low amounts of undesirable contaminants, they do exist. Dealing with the negative impacts of these impurities has been the bane of blacksmiths since the iron age.
The most common undesirable impurities include Phosphorus (reduces ductility, increases brittleness, and messes with heat treating), Silicon (a useful chemical that increases strength, but too much decreases impact resistance), and Sulfur (a demonic chemical that reduces strength, increases brittleness and gleefully promotes warping). Obviously, something must be done about these bad boys.
Some people imagine that, through the Alchemy of Science, impurities are simply “disappeared” from steel during smelting. While some impurities can be eliminated through heat and chemical reactions, it is not possible to reduce the content of those listed above to insignificant levels through smelting alone.
Undesirable chemicals, including those listed above, can be tolerated in steel to some degree because, like arsenic in drinking water and carbon monoxide in air, below certain levels they cause no significant harm. The best solution we have discovered is to reduce the concentration of impurities to acceptable levels by using ore and scrap that contains low levels of impurities to begin with, and constantly test, and either reject or dilute the ”pot” as necessary to keep impurities below acceptable levels. This practice is known as “Solution by Dilution.”
White Label steel is plain high-carbon steel without other additives, while Blue Label, Silver Label, and Aogami Super steels have various chemicals added to achieve specific performance criteria. Please see the flowchart below.
Production Flowchart of Yellow Label, White Label, Blue Label, and Super Aogami Steels
Another technique used to mitigate the negative effects of impurities found in steel ore is to add chemicals to the mix. Chrome, molybdenum, vanadium, tungsten and other chemicals are added to create “high-alloy” steels that can be more predictably forged and heat-treated, are less likely to crack and warp, and will reliably develop useful crystalline structures despite detrimental impurities. Such high-alloy steels can reliably produce useful tools in mass-production situations by untrained labor and with minimal manpower spent on quality control. But regardless of the hype, such chemicals do not improve sharpness or make sharpening easier. Indeed, the exact opposite is true.
If you look at the table below, you will notice that White Label and Blue Label steels both have the same minute allowable amounts of harmful impurities such as Silicon, Phosphorus, and Sulfur.
The table below is a summary of a few tool steels in Hitachi Metal’s Japanese-language catalogue. A PDF can be found at this LINK.
Chemical Table of White Label, Blue Label and Aogami Super Steels
|Product Designation||Shirogami 1 (White Label 1)||Shirogami 2 (White Label 2)||Aogami 1 (Blue Label1)||Aogami 2|
|Annealing Temp °C||740~770°cooled slowly||740~770°cooled slowly||750~780°cooled slowly||750~780°cooled slowly||750~780°cooled slowly|
|Quench Temp°C||760~800°water||760~800°water||760~830°water or oil||760~830°water or oil||760~830°water or oil|
|Primary Usages||Highest-quality cutlery, chisels, planes||High-quality cutlery, chisels, saws, axes, sickles||Highest-quality cutlery, planes, knives||High-quality cutlery, planes, knives,saws, sickles||High-quality cutlery, planes, knives|
Carbon of course is the element that changes soft iron into hardenable steel, so all five steels listed in the table above contain carbon, but you will notice that White Label No.1 has more carbon than White Label No.2. Likewise, Blue Label No.1 has more carbon than Blue Label No.2.
The greater the carbon content, the harder the steel can be made, but with increased hardness comes increased brittleness, so White Label No.1 is likely to produce a chisel with a harder, more brittle blade than one made of White Label No.2.
Accordingly, White Label No.2 steel makes a wonderful saw, but the plates and teeth of saws forged from White Label No.1 tend to be fragile unless the blacksmith removes excess carbon during forging to improve toughness. This is entirely within the skillset of an experienced blacksmith, and can even occur by accident.
In the case of chisels, plane blades, and kitchen knives intended for professional use, White Label No.1 is the first choice of Japanese professionals followed by Blue Label No.1 steel.
Where high performance at less cost is required, Blue Label No.1 is often preferred.
With impurities and carbon content the same, the chemical difference between White Label No.1 and Blue Label No. 1 then is the addition of chrome and tungsten, elements which make the steel much easier to heat treat, and reduces warping and cracking, thereby yielding fewer defects with less work. Chrome, and especially tungsten are expensive chemicals that make Blue Label steel costlier than White Label steel, but with easier quality control and fewer rejects, overall production costs are reduced.
All things considered, and this is a critical point to understand, compared to White Label steel, Blue Label steel is easier to use, and more productive despite being a more expensive material. Indeed, many blacksmiths and all mass-producers prefer Blue Label steel over White Label steel, when given a choice, because it is easier to use and more profitable, not because it makes a superior blade.
Many wholesalers and retailers insist that Blue Label steel is superior to White Label steel because it is costlier and contains elements that make it more resistant to wear and abrasion intimating that it will stay sharper longer. To the easily deceived and those who do not follow this blog this may make perfect sense. But when wise Gentle Readers hear this sort of tripe they will know to quickly gird up their loins and take up their BS shovels in time to keep their heads above the stinky, brown flood.
Wise Gentle Readers who choose blades forged from Blue Label steel will do so because they know that Blue Label steel makes a fine blade at less cost than White Label steel, not because Blue Label steel blades are superior in performance. Moreover, regardless of the steel used, they will always purchase blades forged by blacksmiths that possess the requisite dedication and have mastered the skills and QC procedures necessary to routinely produce high-quality blades from the more temperamental White Label steel. The reasons are made clear in the Technical Example below.
Quenching & Tempering
The process of hardening steel, called “heat treatment,” (in Japanese “netsu shori” 熱処理）is key to making useful tools.
High-alloy steels vary in this regard, but in the case of plain high-carbon steels, the two primary stages (with various intermediate steps we won’t touch on) of heat treatment are called “quenching” and “tempering.”
In the case of quenching, the steel is heated to a specific temperature, maintained at that temperature for a set amount of time, and then plunged into either water or oil, locking the dissolved carbon in the steel into a rigid crystalline structure containing hard carbide particles. After this process the steel is brittle enough to shatter if dropped onto a concrete floor, for instance; Basically useless.
To make the steel useful for tools it needs to go through the next step in the heat-treatment process, called “tempering,” to adjust the rigid crystalline structures created during the quench, losing some carbides, but making the steel less brittle and much tougher.
This is achieved by reheating the steel to a set temperature for a set period of time and then cooling it in a specific way. This heating and cooling process can happen in air (e.g oven), water, oil, or even lead. All that really matters is the temperature curve applied. Every blacksmith has their own preferences and procedures.
With that ridiculously overly-simplified explanation out of the way, let’s next take a gander at the “Quench Temp” row in the table above which indicates the acceptable range of temperatures within which each steel can be quenched to successfully achieve proper hardness. If quenching is attempted outside these ranges, hardening will fail and the blade may be ruined.
In the case of White Label steel, Gentle Reader will observe that the quenching temperature range is 760～800°C, or 40°C. Please note that this is a very narrow range to both judge and maintain in the case of yellow-hot steel, demanding a sharp, well-trained eye, a good thermometer, proper preparation, and speedy, decisive action, not to mention a good purging of iron pixies from the workplace.
Just to make things worse, even within this allowable range, a shift of temperature too far one way or the other will significantly impact the quality of the resulting crystalline structure, so the actual temperature variation within the recommended quench temp range an excellent blacksmith must aim for is more like ± 10˚C.
In the modern world with uniform gas fires, consistent electric blowers, and reliable infrared thermometers, this target can be hit through training and diligent attention, but not that long ago it was seen as a supernatural achievement.
Compare this range of quenching temps to those for Blue Label steel with an acceptable quenching temperature range of 760～830°C, or 70°C of range, a 75% increase over White Label steel. That’s huge.
Let’s next consider the recommended tempering temperatures.
For White Label steel, the tempering temperatures are 180～220°C, or 40°C of range. Blue Label steel’s temperatures are 160～230°C, or 70°C of range, once again, a 75% greater safety margin.
The practical temperature range for quenching and tempering Blue Label steel is still quite narrow, but this increase in the allowable margin of error makes the job a lot easier, making Blue Label Steel much less risky to heat-treat successfully than White Label steel.
Judging and maintaining proper temperatures during forging, quenching and tempering operations is where all blacksmiths, without exception, fail when they first begin working plain high-carbon steel. The guidance of a patient master, time and perseverance are necessary to develop the knack. Experience matters.
I hope this partially brings into focus the challenges these two steels present to the blacksmith.
If you seek greater light and knowledge, please look online to find similar data for many of the popular high-alloy tool steels. Comparing those numbers to White Label steel and Blue Label steel will help you understand why mass-producers of tools, with their lowest-possible-cost mindset, minimal quality control efforts, and workforce of uneducated peasant farmers instead of trained blacksmiths, prefer them for making the sharpened screwdrivers represented as chisels nowadays.
Warping & Cracking
A huge advantage of chrome and tungsten additives is that they reduce warping and cracking significantly. This matters because a blacksmith using a plain high-carbon steel like White Label steel must anticipate the amount of warpage that will occur during quenching and shape the chisel, knife, or plane blade in the opposite direction so that the blade straightens out when quenched. This exercise requires a lot of experience to get right consistently, making White Label steel steel totally unsuitable for mass-production.
Steel is a magical material. When yellow hot, the carbon is dissolved and moves relatively freely within the iron matrix. Anneal the steel by heating it and then cooling it slowly and the carbon molecules will migrate and gather into relatively isolated clumps with little crystalline structure leaving the steel soft.
But if the steel is heated to the right temperature and suddenly cooled by quenching, the carbon is denied the time and freedom it had during the annealing process, instead becoming locked into the iron matrix forming a hard, rigid crystalline structure. This iron/carbon crystalline structure has a significantly greater volume than pure iron, which is why the blade wants to warp when quenched.
Adding chrome and tungsten and other chemicals reduces this tendency to warp.
Sword blades are an interesting example. A Japanese sword blade is typically shaped either straight or curved towards the cutting edge before quenching, but during quenching the blade warps and curves without encouragement from the blacksmith. The skill and experience required to pre-judge the amount of this warpage and the resulting curvature of the blade, and then compensate while shaping the blade before quenching to achieve the desired curvature post-quench is not something one learns in just a few months or even years.
Unlike Tamahagane, however, modern commercial steels containing alloys like chrome and tungsten warp much less, and suffer far fewer shrinkage cracks.
Aogami Super is another HML product listed in the table and flowchart above. It’s an interesting steel, containing more carbon than both White Label steel and Blue Label steel and a lot more tungsten than regular Blue Label steel. Consequently, it is even more expensive. Aogami Super was originally developed as a high-speed tool steel especially resistant to wear. There are much better steels available for this role now, but Aogami Super is still hanging in there.
But all is not blue bunnies and fairy farts because high-alloy steels have some disadvantages too.
Those who hype high-alloy steels always praise to the heavens the “wear-resistant” properties Chrome and Tungsten additives afford. When the subject is woodworking handtool blades, however, please understand that the meaning of “wear resistance” includes “a bitch to sharpen,” and/or “not very sharp.”
Tungsten makes the steel warp less and expands the heat-treat and tempering temperature ranges significantly leading to fewer defects during production. But the addition of tungsten also produces larger, tougher crystals that simply can’t be made as sharp as White Label No.1, and that makes the blade much more difficult, unpleasant, and time consuming to sharpen, all while wasting more sharpening stone material in the process.
White Label steel has no additives other than carbon. It does not need additives to compensate for or to dilute impurities because its production begins with exceptionally pure pig iron, mostly from Sweden (for many centuries the source of the purest iron in the world), and carefully tested and sorted scrap metal. Both White Label and Blue Label steels, if properly hand-forged and heat treated by an experienced blacksmith with high quality standards, will have many more and much smaller carbide clumps distributed more evenly throughout the iron crystalline matrix producing a ” fine-grained” steel of the sort coveted since ancient times.
Nearly all the tool steel available nowadays contains high percentages of scrap metal content. Scrap metal is simply too cost effective to ignore. Careful testing is the key to using scrap metal advantageously.
Gentle Reader may have found the historical and chemical information presented above interesting, but they do not really answer questions you may have about the performance differences between these steels, and when presented a choice, which one you should purchase. Your humble servant has been asked and answered these questions hundreds of times, and while only you can decide which steel is best for you, I will be so bold as to share with you the viewpoint of the Japanese blacksmith and woodworking professional.
Long story short, in the case of planes and chisels, the typical choices of steel are still White Label No.1, White Label No.2 or Blue Label No.1. These steels will not be available much longer.
If you are dealing with honest blacksmiths and honest/knowledgeable retailers with experience actually using, not just talking about and selling, tools, you will have observed that a specific plane blade, for instance one made from Blue Label steel, will cost less than the same blade made from White Label steel, despite Blue Label steel being a more costly material.
At C&S Tools a 70mm White Label No.1 steel plane blade cost 77% more than one made from Blue Label No.1. This means that the blacksmith’s average cost in terms of his labor (overhead, forging and shaping cost being equal) is also around 77% greater than Blue Label steel, a direct reflection of his potential additional time expenditure due to risk of failure.
White Label steel simply warps and cracks more, but when failure occurs it only becomes apparent after all the work of laminating, forging and shaping are complete. Ruined steel cannot be reliably re-forged or re-used, so all the material and labor costs up to the point of failure are simply wasted like an expectation of moral behavior in a California politician. It is not a material for careless people or newbies.
So if White Label steel blades are riskier to make, with more wastage, and therefore more expensive, what are the performance characteristics that make White Label steel blades a favorite with professional Japanese craftsmen? Two primary reasons: First, properly made White Label steel blades can be made sharper. This makes the craftsman’s work go quicker and more precisely.
Second, properly made White Label steel blades are quicker and more pleasant to sharpen. That sums it up.
To some people, especially those that use edged tools professionally all day long, these differences matter a great deal; To others, not so much.
Is White Label steel worth the extra cost? I think so, but the performance differential is not huge, and only someone with advanced sharpening skills will be able to take full advantage of the difference. For most people on a tight budget, or in the case of woodworking situations where sharpness is not critical, and sharpening speed and pleasure are not driving factors, then a less-expensive Blue Label steel blade is perhaps a better choice. It absolutely makes a fine tool that does a great job of cutting wood.
The Wise Man’s Q&A
Let’s shovel some more BS out of the way by performing the mandatory experiment of taking a high-quality White Label steel blade and a high-quality Blue Label steel blade, sharpening them identically using the best stones and advanced technique, test them to cut some wood, and then consider the answers to the following two important questions:
Question 1: Will the additional sharpness of a White Label steel plane blade create a smoother, shinier finish surface on wood than a Blue Label steel blade?
Answer 1: Definitely no. But since the blade started out a little sharper, it will cut good wood a little better a little longer. These results will depend on the skills of the user, of course.
Question 2: In the case where edge-retention, cutting speed, and cutting precision are more important than a shiny finish, which absolutely applies to chisels and knives, will the additional sharpness of a properly made and proficiently sharpened White Label steel blade improve a woodworking tool’s cutting speed, edge-retention, precision and control?
Answer 2: Absolutely yes. On condition that the user possesses the skills to achieve and maintain that extra degree of sharpness. There is a reason sharpening has always been the first essential skill in woodworking.
These are the reasons why we don’t even offer chisels made from Blue Label steel, or even White Label No.2 with its lower-carbon content, and resulting reduced hardness.
But whether plane blade, chisel or knife, a properly forged and heat-treated blade made by an experienced professional blacksmith from simple White Label steel will always be quicker and more pleasant to sharpen than one made of Blue Label steel with its added sticky chrome and hard tungsten. To the professional that has the need for the additional sharpness as well as the skills necessary to produce and maintain it, that’s a difference many find worth the extra cost.
I daresay many of our Beloved Customers agree.
A Technical Example
You may find all the technical stuff below a bit obscure, but perhaps an example with pretty pictures will help bring things into focus. Please see this informative article by Niigata Prefecture’s Prefectural Central Technical Support Center. If you input the URL into Google and use the translate feature a decent English-language version may magically appear. Or not. Some of the key results are copied below.
The steel being tested in the study outlined below is White Label No.2 steel (row 2 on page 4 of the Hitachi catalogue pdf). They heat-treated seven samples and listed the results. In each case, the quench temp varied from 750˚~900˚C (1382˚~1652˚F) in water, but the tempering temp was kept constant at 180˚C (356˚F).
Figure 4 below at 775˚C (1427˚F) shows the best, finest, most uniform crystalline (Austentite) structure. Lower temps are not as good. Higher temps are worse. A 25˚ variation one way or the other made a big difference.
The photographs below tell the story graphically. The white stuff visible in the photographs is Ferrite (iron), while the black stuff is spherical carbide (Cementite). When Ferrite and Cementite meld, a desireable hard crystalline structure called Martensite is formed, although there are several steps in between we will not touch on. This subtle change is the essence of the ancient Mystery of Steel, and the keystone to modern civilization.
Fig.1 shows the steel before quench. Notice how the soft iron Ferrite and spherical carbon Cementite are isolated from each other indicative of little crystalline structure and a soft metal. No significant Martensite is visible.
The graph in Fig.2 below shows Vickers Hardness on the vertical axis and quench temperature (with a 20 minute soak) on the horizontal axis. Notice how hardness makes a big jump between 750˚C and 775˚C. This 25˚ range is the sweet spot.
Fig. 3 below shows the crystalline structure at a quench temp in water of 750˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. This is 10˚C below the manufacturer’s recommended quench temp. Notice how the iron Ferrite and spherical carbon Cementite are mixing, forming some gray-colored Martensite, but there are still big lakes of Ferrite visible. Better, but not yet good.
Fig. 4 below shows the crystalline structure at a quench temp in water of 775˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. Notice how the iron Ferrite and spherical carbon Cementite are well-mixed forming pretty grey Martensite, indicating that this is close to the ideal quench and tempering protocol; The sweet spot. The crystalline structure shows few lakes of iron Ferrite or islands of spherical carbon typical of durable, hard, fine-grained steel. A mere 25˚C increase in quench temp has yielded a large improvement.
Fig. 5 below shows the crystalline structure at a quench temp in water of 800˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. This is still within the quench temp range recommended by Hitachi. Notice how the Ferrite and spherical carbon Cementite are still fairly well-mixed, but the dark spherical carbon is becoming a bit more isolated from the Ferrite forming more, darker groupings. While the Martensite formed is still quite adequate, the performance of this steel may not be as ideal as that in Fig. 4. Notice also that the hardness of the steel has dropped slightly.
Fig. 6 below shows the crystalline structure at a quench temp in water of 825˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. Notice how the crystalline structure has become less uniform than in Fig 5 after only a 25˚ increase in quenching temp.
Fig. 7 below shows the crystalline structure at a quench temp in water of 850˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. Once again, only a 25˚ increase in quenching temp has resulted in significant degradation in the uniformity of the crystalline structure as well as reduced hardness.
Fig. 8 below shows the crystalline structure at a quench temp in water of 875˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. Once again, significant degradation in the uniformity of the crystalline structure and loss of Martensite is apparent.
Fig. 9 below shows the crystalline structure at a quench temp in water of 900˚C, after a 20 min. soak, followed by tempering at 180˚C for one hour, followed by air cooling. Gentle Reader will notice the many white tissues that have developed in addition to tempered martensite. The fibrous-appearing white stuff is considered retained Austenite, a formation that can later be converted into hard Martensite. Once again, only a 25˚ increase in quenching temp has resulted in significant degradation in the uniformity of the crystalline structure as well as reduced hardness.
Clearly, Shirogami No.2 steel can be a very good tool steel, but it is also obvious that it is extremely sensitive to heat-treatment technique, requiring knowledge, experience and care to produce good results.
What should Gentle Reader take away from this technical presentation?
The first thing to understand is that plain, high-purity, high-carbon steel that has been skillfully forged, quenched and tempered will exhibit the finest, most evenly-distributed hard carbides in a uniform crystalline steel structure mankind can produce. Such steel will become sharper than any other metal from which a practical chisel or plane blade can be forged.
This fact has not changed since ancient times, regardless of the hype and marketing of the mass-producers who can at best achieve comparatively mediocre results using modern high-alloy steels.
The second thing to understand is that, while it is not difficult to make high-carbon steel hard, nor to temper it to make a durable product, producing a uniform crystalline structure that will become very sharp, will be especially resistant to dulling, and can be sharpened quickly requires serious skills of the sort that only result from many years of study under a master, and dogged commitment to quality control, especially temperature control and timing.
If the quality of the steel the blacksmith uses is the lock, then the crystalline structure he produces through skill and dedication is the key to the Mystery of Steel. It’s a lock and key mankind has used since ancient times, but it’s only been a handful of decades since we developed the technology to really understand it. Rejoice for you live in enlightened times!
I hope this discussion has been more helpful than confusing.
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