History and Properties of 52100 Steel
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52100 is often a relatively simple steel with 1% carbon and 1.5% chromium, and small amounts of Mn and Si. 52100 steel has been doing use since no less than 1905 . It originated for utilization in bearings. High carbon steels (0.8-1.0% C) were primarily used prior to the late 1800’s or early 1900’s , then chromium inclusions in bearing steels were being made. 1% Cr steels are actually found in bearings since a minimum of 1903 . These early chromium-alloyed bearing steels were created in Germany by Fichtel & Sachs through Deutsche Waffen- und Munitionsfabrik . French-produced chromium steels were also employed in bearings inside a similar time period . 52100 remains the most used bearing steel , and so the steel design has certainly stood quality of your time. The steel passes a great many other names for example 100Cr6, 1.3505, GCr15, En31, and SUJ2.
Update 5/8/2019: Nick Dunham posted the subsequent about the good reputation for the SAE designation of 52100 (the name came later compared to steel, naturally):
It appears that in 1919, the SAE Iron & Steel Division decided to replace 5295 with 52100 as part of their seventh report .
5295, consequently, was introduced as 52-95 within the third report (1912) , and dashes were removed in the fifth report (1913-1914) . It was an impression steel in the first place – the third report says of 51- and 52- series chromium steels, “the utilization of this kind of steel is fixed almost entirely to ball and roller bearings.” 
The third report have also been the creation of the two-digit series prefix ; in the second and third reports (1911), only two-digit codes were used, numbered 1-23 (including certain). No chromium steels were listed . This is just not to convey that chromium steels failed to exist yet, but merely the SAE specifications didn't exist yet.
Ed Fowler is owed some credit in popularizing 52100 being a knife steel nowadays. He has produced many knives in 52100 and wrote extensively about its virtues in Knife Talk columns in blade magazine. Ed was introduced to 52100 within the kind of ball bearings sent to him by Wayne Goddard , another influential knifemaker who regularly wrote for Blade magazine. Because bearings were a comparatively common way of high carbon scrap steel, its utilization in knives extends back much further, obviously. Knives produced dating back the 1940’s in 52100 are already reported, including knives by William Scagel .
The obvious difference between 52100 as well as other high carbon steels utilized by forging bladesmiths is its high chromium content of 1.5%. The Cr addition is designed for several reasons, which I have described below.
One purpose of the Cr addition is good for “hardenability,” a measure of how fast steel has to be quenched from high temperature to attain full hardness. A simple carbon steel such as 1095 takes a very fast water quench to completely harden, in which a hard steel phase called martensite is created. 1095 has nearly 1% carbon like 52100 but devoid of the chromium addition. If quenched in slow oil or allowed to air cool, then some level of “pearlite” forms which cuts down on the hardness of steel in accordance with full martensite. Pearlite is often a blend of 0.02 wt% carbon ferrite and 6.67 wt% carbon cementite (Fe3C) that forms in alternating bands, so using a simple carbon steel the carbon must diffuse over the short distance for that bands of ferrite and cementite to form. Chromium is also enriched inside cementite, so in the chromium-alloyed steel the chromium must diffuse into the cementite to form pearlite. Chromium can be a bigger atom than carbon in order that it diffuses less quickly. Therefore having a chromium addition pearlite formation is suppressed and hardenability is increased.
This hardenability effect can result in seen using a Time-Temperature-Transformation (TTT), otherwise known as Isothermal Transformation (IT) graph the location where the “nose” from the transformation (labeled as ferrite+carbide) is pushed to longer times in 52100 in accordance with 1095. This allows bearings to get fully hardened so they have sufficient strength and so resist deformation during use. The core of an impression cools slower as opposed to surface during quenching, and so the higher hardenability allows larger bearings to get used. 52100 is still not a high hardenability steel, however, and is just not considered a genuine “oil hardening” steel like O1 (instead of water hardening). For large bearings requiring higher hardenability, modified versions of 52100 were developed. A higher Mn version was introduced inside the mid-1930s, as well as a Mo-alloyed version after WWII . However, neither of people versions have witnessed significant used in knives.
The “nose” from the curve within the TTT for 1095 actually extends off the chart because the time is indeed short. Very fast quenching is required to avoid soft pearlite
The “nose” from the 52100 TTT are at about 3 seconds, allowing more gentle quenching to attain full hardness
Effect of Chromium on Carbide Size
Carbides are hard particles in steel that improve wear resistance but reduce toughness or potential to deal with cracking. Therefore, greater amounts of carbides are desirable for applications that want high wear resistance. Applications requiring high toughness usually require carbides being as small as possible and to have a very small volume fraction ones. A typical high carbon steel like 1095 forms hard particles of iron carbides called cementite, with three iron atoms for every single carbon atom: Fe3C. High chromium steels form a chromium carbide for example Cr7C3 or Cr23C6. Some erroneously believe that 52100 forms one of people chromium carbide types. However, it doesn't have sufficient chromium in order to create those types of carbides. Some from the chromium is instead enriched inside the cementite, forming M3C where M can reference either iron or chromium. The cementite in 52100 contains about 9 wt% chromium .
The addition of Cr reduces the carbide size. Smaller carbides means better toughness and capacity fracture. 52100 is acknowledged for its small carbide size and high density of carbides, regardless if in comparison with other carbon and alloy steels like 1095. The carbide dimension is reduced by way of a similar mechanism for the surge in hardenability. Prior to delivering steel for the end customer, the steel is annealed to become soft for machining and to set it up for final heat treating. One method for annealing is to slow cool the steel from temperature in order to create pearlite, as well as an intermediate temperature treatment in which the pearlite structure is “spheroidized” to form small round carbides :
Because Cr is an element of the carbides which diffuses less quickly than carbon, the spacing between cementite in pearlite is smaller, therefore the rate of “spheroidization” and growth in the round carbides is reduced. Here are images  comparing 52100 (top) with 1095 (bottom), the location where the white particles are carbides. The 1095 is fairly fine, nevertheless the 52100 has a greater density of carbides along with the maximum carbide size is smaller than 1095.
Carbide Fraction and Carbon in Solution
When comparing steels in the same high hardening temperature but increasing carbon content, the amount of carbon in solution remains constant but the quantity of carbide increases. You can see that by looking at the iron-carbon phase diagram below; the black circle at risk represents the carbon in solution which does not change with increasing carbon content. However, with higher carbon the fishing line extends further in the “austenite + cementite” field indicating more cementite is present.
The phase diagram represents the microstructure of steel at different carbon contents and temperatures. At a temperature of 1400°F, in a carbon content between about 0.55-0.7% the steel is inside the “austenite” region where no carbides/cementite exists. If quenched from that temperature a final microstructure is tough martensite without carbides. If the carbon content is increased above 0.7% then carbides can be found at the high temperature, resulting inside a final microstructure of martensite with carbides. The carbides help with wear resistance. The more carbon is added above 0.7% the higher the volume of carbide is present:
The amount of carbon “in solution” to bring about hardness remains the same in a fixed temperature inspite of the increasing bulk carbon content, for the reason that carbon is leading to carbide formation. However, when the temperature is increased then this carbon in solution goes up along the queue. If we look at a 1% carbon steel at 1400°F (point 1) you will find the same 0.7% carbon in solution being a steel with every other steel with carbon more than 0.7%. Dotted lines show the carbon in solution vs the majority composition in the steel. At 1450°F there exists 0.8% carbon (point 2), and 1% carbon in solution at about 1570°F (point 3). The length from the dotted line shortens with increasing temperature indicating how the quantity of carbide is decreasing, until point 3 where no more carbide is found possesses reached the “austenite” field:
The addition of 1.5% Cr shifts the positioning of the iron-carbon phase diagram, to higher temperatures and lower carbon contents:
The shift in the phase diagram ensures that for the same bulk carbon content, there's less carbon in solution and a greater volume fraction of carbide. This is why the recommended hardening/austenitizing temperatures of 52100 is above 1095, usually 1550°F in lieu of 1475°F. The reduction in carbon in solution vs 1095 helps improve toughness, as carbon above about 0.6% in solution results in plate martensite which reduces toughness. Experimentally, 52100 has about 0.63% carbon in solution using a hardening treatment from 1550°F  which gives maximum hardness without forming plate martensite. Lower hardening temperatures further slow up the carbon in solution for better toughness. You can read more on this page about the hardness of steel. The surge in carbide fraction also raises the wear resistance of 52100, where heat treated 52100 has around 6-10% carbide volume , and 1095 has approximately half that.
Ease in Forging, Quenching, and Heat Treating
With its low chromium content relative to air hardening steels like A2 or D2, 52100 is often a good option for forging. It won't have carbides present at forging temperatures like those air hardening steels which suggests it moves with less effort underneath the hammer. Its medium-low hardenability also causes it to be a great choice. The low hardenability of 1095 means water or quickly oil is required for quenching, while 52100 is much more forgiving with slower quenches. Slower quenches lessen the chance of warping and quench cracking. A more hardenable steel like O1, or air hardening steels, are extremely forgiving out of this standpoint, but which makes them tough to anneal without a controlled temperature furnace. Those steels can also be difficult or impossible to normalize since they will harden when cooled in air, as opposed to forming the required pearlite. High hardenability steels may also be more prone to crack when forging at lower temperatures, or just when cooling to room temperature after forging. Therefore, the amount of hardenability in 52100 is a good compromise for flexibility in quenching while still being possible to normalize and anneal with simple cycling. The increased temperature and time essential for austenitizing in accordance with simple carbon steel, however, makes austenitizing more challenging when heat treating in a forge or which has a torch as opposed to a PID-controlled furnace.
Heat Treatment of 52100
We now have a separate article about how precisely to best heat treat 52100.
As discussed above, enhancing the hardening/austenitizing temperature of 52100 leads to an increase in carbon in solution and a decline in carbide fraction. That is seen experimentally at the same time, though numbers are somewhat distinct from those predicted through the phase diagrams, as those predictions are on an infinite hold time at temperature, instead of the 10-30 minutes employed in heat treating. As the carbon in solution increases, the amount of retained austenite after quenching also increases. You can read about why in the following paragraphs about cryogenic processing of steel. The peak in hardness arises from an austenitizing temperature of around 1650°F; above that excessive retained austenite forms which reduces hardness. Here is retained austenite and carbide volume vs austenitizing temperature :
With lower tempering temperatures and higher austenitizing temperatures, hardness is increased. Using 1650°F and 300°F ends in approximately 66 Rc , though that condition likely also results in relatively low toughness. A typical heat management of 1550°F austenitizing and 400°F tempering results in about 61.5 Rc. Many knifemakers use 1475°F and 400°F, which would result in about 59.5 Rc. I’m accomplishment sure why they'll use 1475°F, perhaps it comes from copying recommended heat treatments from 1095. Knifemakers, like a great many other people, like round numbers, so an austenitizing temperature which leads to the round number of 60 Rc after having a nice round number temper of 400°F may perhaps be appealing.
Using lower austenitizing temperatures can lead to improved toughness, which you'll want to find about in this article on austentiizing. Typically, it is advisable to relieve both austenitizing temperature and the tempering temperature, in lieu of to maintain a similar austenitizing temperature and increasing the tempering temperature. One reason is as the carbon in solution is reduced when the austenitizing temperature is lower, as described above. Another problem is the “tempered martensite embrittlement” (TME) range when tempering too high, you will see a drop in toughness in the figure below when you use a tempering temperature of 230°C (450°F) You can read more about TME on this page on silicon additions, a component that minimizes embrittlement. You can see the improved toughness of 52100 with lower austenitizing temperature on this figure :
Increasing the austenitizing temperature also increases hardness, but regardless if the toughness is plotted vs hardness, the advance with lower austenitizing temperature still holds. I removed the as-quenched and 230°C tempered conditions because those conditions had poor toughness:
Ed Fowler also popularized “triple quenching” of 52100, an activity through which the steel is austenitized and quenched multiple times for grain refinement and improved toughness. 52100 isn’t particularly any further perfect for triple quenching than other low-alloy steels but 52100 is frequently associated with it in order that it is worth mentioning. I wrote regarding how multiple quenching works and its potential benefits in this post. We also performed triple quenching on CruForgeV and tested its toughness but didn't find a noticable difference, which you can learn about in the following paragraphs.
Austempering and Bainite
52100 is comparatively well suited for austempering to create bainite, instead of forming martensite with a quench and temper heat treatment. Austempering involves quenching for an intermediate temperature, such as 500°F and holding there for minutes or hours, which leads on the formation of bainite which can be a phase that is certainly just like tempered martensite though somewhat different properties. There is some evidence to point that bainite has greater toughness than tempered martensite. You can read a little more about bainite plus some experiments that happen to be performed on 52100 on this page on austempering. When steels have high hardenability, austempering takes too much time to be feasible. To reach high hardness levels, relatively high carbon content articles are necessary with austempering. So 52100 has a good blend of high carbon and medium hardenability for ease in austempering.
Toughness of 52100
Despite all of the studies on 52100, it's somewhat challenging to find good comparisons when it comes to toughness compared to other steels. Many in the studies give attention to 52100 itself, as it is the kick off point being probably the most frequently used bearing steel. Tool Steels  rates 52100 like a “4” from 10, which is much like A2, and more than O1, M2, and D2, and lower than L6 and shock resisting steels, according to the book. We will be testing a sample of 52100 soon to compare with our current toughness dataset. And if someone knows anything good published comparative toughness numbers please send these to me. Using the Tool Steels ratings we can position 52100 within other steels with reported toughness values from Crucible :
Edge Retention of 52100
Edge retention of 52100 is not particularly high, much like other carbon and low alloy steels. The relatively low level of carbide, plus the low hardness of cementite, means there are other steels with higher wear resistance and slicing edge retention. In CATRA tests by Verhoeven , 52100 is discovered to possess superior edge retention to 1086 and Wootz damascus, though less good as AEB-L, a stainless steel. 1086 is really a lower carbon steel for lower carbide volume, and AEB-L has harder chromium carbides, so the result is smart. You can read more about how exactly good the slicing edge retention of 52100 is regards to other steels within the articles on CATRA edge retention: Part 1 and Part 2.
In rope cutting tests by Wayne Goddard , 52100 was found to possess similar slicing edge retention to other 60 Rc steels; there is less effect of steel in their testing and hardness was the primary factor, though Vascowear (CruWear) was somewhat better:
52100 was created in the early 1900’s, and first employed in 1905. It was created for use in bearings. It has been employed in many knives, to some extent due to its good properties in forging and in part because bearings are a simple supply of scrap steel. The chromium addition improves hardenability, and decreases the carbide size to have an improvement in toughness. The chromium addition also means that 52100 requires higher austenitizing temperatures, and has a greater level of carbide in accordance with a straightforward carbon steel for improved wear resistance. The blend of reduced carbide size but increased carbide volume fraction gives 52100 a fantastic mixture of toughness and wear resistance relative to other carbon and alloy steels. Lower austenitizing temperatures bring about improved toughness. The medium hardenability of 52100 means it is suitable for forging, and also a good candidate for austempering to create bainite.