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Update 10/22/2020: I now have an article with how to heat treat D2, PSF27, and CPM-D2 and it also includes toughness testing of each steel and edge retention testing of D2. https://knifesteelnerds.com/2020/08/31/how-to-heat-treat-d2-psf27-and-cpm-d2/
D2 is a common tool steel and knife steel. It is also known by other names such as the Japanese designation SKD11, German designation 1.2379, Hitachi SLD, Uddeholm Sverker 21, and many others. How long has it been around? Where did it come from? Who started using it in knives? How do its properties compare to other steels? Find your answers here!
Early Chromium Steels
The development of D2 steel coincides in part with the invention of stainless steel as well as high speed steel. You can read an article about the history of stainless steel here or the history of high speed steel here. D2 is part of a tool steel category called “high carbon, high chromium” steels. The production of chromium-alloyed steel wasn’t practical until ferrochromium was developed in 1821 and more practically in 1895 with the development of low carbon ferrochromium. The first commercially produced steel with a chromium addition was in 1861 by Robert Mushet, the inventor of the first tool steel . A patent on chromium steel was granted to Julius Baur in New York in 1865 . Robert Hadfield reported on the properties of chromium-alloyed steels in 1892 and also covered high carbon, high chromium steels which were in their infancy . However, he concluded that the forgeability of the alloys was poor and often cracked, and said that a steel with 1.27% C and 11.13% Cr was at the limit.
Development of High Carbon, High Chromium Steels
After 1900 the number of people experimenting with chromium steels and tool steels in general exploded . That date coincides with the discovery of high speed steels that I linked above. Also early in that period they developed high speed steels which used Cr-alloying rather than Mn-alloying for hardenability, where they used about 4% Cr. They also added large amounts of tungsten for hot hardness. The period of rapid development that occurred in the period shortly after 1900 is very difficult to nail down. Many companies and people were developing steel, and there was also widespread copying. James Gill (read about him here) writing in 1929 reported that he could not find which company was the first to produce high-carbon high-chromium steel . In Becker’s High Speed Steel book in 1910 he reported that a steel with 2.25% C and 15% Cr was being used in Europe, particularly in France. In the USA a patent was granted in 1916 to Richard Patch and Radclyffe Furness for steel with 1-2% carbon and 15-20% chromium . They gave an example composition of 1.35% C and 19.5% Cr which looks like it would be a stainless steel but was not patented as such. In the patent they stated that they had only heard of steels with carbon above 2% and chromium between 12-16%. High carbon, high chromium steels were frequently used in England during World War I for a range of applications including dies and cutting tools . Cutting tools were more typically produced with high tungsten high speed steel at the time because of the superior hot hardness with high speed steel. You can read about hot hardness in the article on high speed steel. However, tungsten was expensive and difficult to obtain leading to the use of high chromium steel as an alternative. Those early high carbon high chromium steels were more similar to the modern D3 or D4 steels rather than D2 because their carbon content was higher, around 2.2-2.4%.
Development of D2
In 1918 a patent was filed in England by Paul Kuehnrich  for a high carbon high chromium steel modified with cobalt, approximately 3.5%. The cobalt addition was to improve the hot hardness of the steels so that they were closer to high speed steel. You can read more about what cobalt does to steel in this article. The patent has fairly broad chemistry ranges: 1.2-3.5% carbon, 8-20% chromium, and 1-6% cobalt. However, interestingly the example alloy given had 1.5% C, 12% Cr, and 3.5% cobalt which without the cobalt would be very close to modern D2.
While in the USA the high carbon high chromium steels were not used as a replacement of high speed steel, it did gain in popularity with die steels. Die steels required high wear resistance which was gained through the large amounts of chromium carbide present in those steels. These were initially the D3-type 2.2-2.4% chromium steels which had relatively poor toughness and machinability. These steels also did not typically contain vanadium or molybdenum. A composition consistent with D2 was not reported by Gill in 1929  so even if it existed by that point it was likely not in widespread use.
Update 4/11/2019:I finally found the patent for D2, the application was filed June 30, 1927 by Gregory Comstock of Firth-Sterling Steel company. Comstock, Gregory J. “Alloy steel.” U.S. Patent 1,695,916, issued December 18, 1928.
By 1934 a composition consistent with D2 was discussed with 1.55% C, 12% Cr, 0.25% V, and 0.8% Mo . It wasn’t yet named D2, of course. The molybdenum was added to make it a true “air hardening” steel which allows the steel to fully harden in thick sections or without oil. Without Mo, the high Cr did make the steel quite hardenable but not enough to make it truly air hardening. The vanadium addition was made to improve toughness which it does by refining both the grain size and also the carbide structure. This new D2-type steel was gaining in popularity because of its “air hardening property, low distortion and better machining quality than the other [high carbon, high chromium steels]” . It was also reported to be, “the most universally adaptable of the…high carbon high chromium steels” . And as mentioned previously the lower carbon meant much greater toughness than the earlier D3-like steel which you can see in the figure below. Vanadium and nickel additions had been experimented with the D3-type, 2.2% carbon steel, but while that improved toughness, the lower carbon D2 was much tougher.From that point D2 became one of the most popular tool steels, particularly in dies. New “better” steels made for dies continue to be compared to D2 because of its ubiquity.
D2 in Knives
It took some time before D2 was used in knives. The first recorded use I can find is by D.E. Henry in 1965 or 1966 . He tried the higher carbon D3 first followed by D2, unintentionally mimicking the order in which they were developed. Due to its popularity as a tool steel, it was only a matter of time before someone used D2. Its relatively high wear resistance along with good hardness and toughness made it work well as a knife steel. With its high chromium content it had a unique position in the stainless vs carbon steel debate. D2 has somewhat better wear resistance and toughness than 440C, the most commonly used stainless steel in the 70’s, so for makers who felt that the stain resistance of D2 was “good enough” it could offer superior properties. You can read more about how corrosion resistant D2 is in this article. It also had much greater wear resistance than the carbon steels commonly used by forging bladesmiths, so was used by some knife makers that wanted a high wear resistance steel.D2 has since been used in many knives, famously by makers such as Bob Dozier.
Carbide Structure of D2
The large carbides in D2 limits its toughness and also its edge stability. A powder metallurgy version, CPM-D2, was released around 2007  to reduce the carbide size, which is reported to improve the toughness, corrosion resistance, and heat treatment response. You can read more about why D2 has large carbides andthe powder metallurgy process in this article. Sprayform is a somewhat similar technology that leads to a somewhat larger carbide size than powder metallurgy. There is a sprayform version of D2 called PSF27 produced by Dan Spray in Denmark, made at least since 2002 . You can see the decreasing carbide size in conventional (well, ESR anyway), spray form, and PM D2 in the images below . Note the PM is at a higher magnification.
Those are pretty low resolution micrographs. I took micrographs of D2, PSF27, and CPM-D2 which are shown below:
Properties of D2
Bohler Uddeholm measured the edge retention of D2 along with other steels with CATRA testing and found it to be somewhat better than N690, ATS-34/154CM, and 440C, on par with 3V, but worse than S35VN, Vanadis 4 Extra, Elmax, S30V, M4, and M390 . I also calculated the edge retention relative to 440C which is a value that Crucible has reported in the past.
Crucible reports that D2 has toughness roughly equivalent to 10V, better than 440C and S90V, but worse than 3V, CruWear, and A2 .
In our toughness testing D2 was not very impressive though we have only tested one heat treatment and have not compared to many other low toughness steels like 10V, 440C, and S90V:
I previously wrote about the potential corrosion resistance of D2 in this article. Its corrosion resistance has been somewhat over-promoted in some cases because of its high chromium content. Approximately half of that chromium is tied up in carbides where it doesn’t contribute to corrosion resistance. Therefore, it has good corrosion resistance for a tool steel, though there are some non-stainless steels that potentially have better corrosion resistance, particularly many of the 8% Cr steels like 3V or CruWear. Here is the chart from that article with the steels ranked by “chromium in solution” which is approximately equal to the corrosion resistance of each steel:
D2 in Knives Today
D2 continues to see use in knives; a search on BladeHQ brings up 1,690 available knives in D2. Knifemakers like Bob Dozier have built their reputation on making a superior knife with D2. With the rise of powder metallurgy vanadium-containing steels, there are now other options with both higher wear resistance and toughness. Or powder metallurgy stainless steels that can match or exceed its wear resistance and toughness but with better corrosion resistance. Powder metallurgy steels are much more expensive than D2, as D2 is conventionally produced and widely available from virtually every tool steel company. Therefore from a cost perspective D2 still has an advantage over many newer steels. The newer sprayform and PM versions of D2 help to make up some of the difference in properties relative to other powder metallurgy steels. Due to its good properties and reputation built over decades, D2 will likely continue to be seen in knives.
High carbon, high chromium steel was developed as an alternative to high speed steel in England in the early 20th century. These steels were similar to the modern D3 tool steel with very high carbon (2.2%). The carbon was reduced to 1.5%, and additions of Mo and V were made to improve the toughness and hardenability of the steel which was in use by 1934. This steel became what we know as D2, which is popular as a die steel. The steel was first used in knives by D.E. Henry in 1965 or 1966 and became popular in knives. Sprayform and powder metallurgy version have been produced to improve the toughness and refine the microstructure of D2. D2 has good wear resistance, hardness, and adequate toughness.
Gill, J. P. “High-carbon high chromium steels.”Trans. ASST15 (1929): 387-400.
Hadfield, Robert Abbott. “Alloys of Iron and Chromium, Including a Report by F. Osmond.”J. Iron Steel Inst.42 (1892): 49.
Patch, Richard H., and Radclyffe Furness. “Tool-steel alloy.” U.S. Patent 1,206,902, issued December 5, 1916.
Gill, James Presley, Robert Steadman Rose, George Adam Roberts, Harry Grant Johnstin, and Robert Burns George.Tool steels. American Society for Metals, 1944.
Kuehnrich, Paul Richard. “Steel.” U.S. Patent 1,277,431, issued September 3, 1918.
Wills, W. H. “Practical Observations on High-Carbon High-Chromium Tool Steels.”Trans. ASM23 (1935): 469.
Warner, Ken.Knives,’84. DBI Books, 1983.
Henry, D.E.Collins Machetes and Bowies, 1845-1965. Krause Publications, 1995.
Schruff, I., V. Schüler, and C. Spiegelhauer. “Advanced tool steels produced via spray forming.”The Use of Tool Steels: Experience and Research2 (2002): 973-990.