Carbon vs Stainless Steel in Knives

Thanks to Matt Davidson and Barton Smith for becoming Knife Steel Nerds Patreon supporters! I put in the order for the small impact tester for knife edges. The purchase of the impact tester was decided by Patreon voting members and purchased with money from Patreon supporters. 

Thanks to A.G. Russell, Devin Thomas, Bill Bagwell, Gil Hibben, Wes Hibben, Linda Hibben, and Jimmy Fikes for answering my many questions for this article. 

Stainless vs Carbon Steel

The great debate! I think the arguments have died down somewhat but they won’t be dying any time soon. Which is better? Carbon steel or stainless steel? Let’s get down to the basics.

Definitions

Steel is defined as iron with carbon added (we will ignore carbon-free steel for now). So of course stainless steels used in knives also have carbon. They wouldn’t achieve sufficient hardness otherwise. The term “carbon steel” refers to steels that are primarily alloyed with carbon but manganese and silicon are generally added. The most common types of the 10XX series of steels such as 1060, 1075, 1084, 1095, etc.

The amount of each element is given in “weight percent,” so in 1095, 0.95% of the steel is carbon based on weight. Impurities such as phosphorous (P) and sulfur (S) are inevitable in all steels so I won’t be including those in any of the other tables. When some more elements are added the steels are referred to as “alloy steels” because alloy has been added to them. Typical alloy additions include higher manganese, nickel, molybdenum, or chromium additions. Common grades include 4340, 5160, 15N20, and 52100:

Another category is called “tool steels” which is somewhat more difficult to define. Any steel used for tools, dies, etc. can be defined as a tool steel. They vary in composition from W2 which is essentially a simple carbon steel to high speed steels with high contents of molybdenum, tungsten, and vanadium. Sometimes I use another made up category called “high alloy” steels. Common tool steels include W2, O1, D2, A2, and M2 though also includes more recent steels such as 10V, 3V, or Vanadis 4 Extra:

Stainless steels are those that have sufficient chromium to have high corrosion resistance relative to carbon steels. This is also a somewhat fuzzy line, sometimes given as a cutoff such as 10.5, 11, or 12% minimum chromium to be stainless. However, that must be balanced against other elements such as carbon. The most common example is D2 which has ~12% chromium but is not classified as stainless and indeed has much lower chromium in solution to contribute to corrosion resistance than a stainless steel. I wrote about that in this article on corrosion resistance of D2. I previously wrote about the development/invention of stainless steels in the early 20th century and how the early stainless steels were in fact developed specifically for knives: you can read about that here. By the early 1930’s, the basic common stainless steels 420, 440A, and 440C had already been invented and 420 and 440A were being used frequently in knives:

However sometimes the term “carbon steel” is used simply as the opposite of stainless; in other words, carbon steel means non-stainless. That is often the way the term “carbon steel” is used when debating the merits of carbon steel and stainless steel. In this article I will try to be consistent in the way that I use these terms, and use “carbon steel” when referring to simple carbon steels, and “non-stainless” when referring to the broad group of steels that are not considered to be stainless steels.

Early Stainless Steel Knives

In the approximate timeframe of 1920-1960 stainless steel was used in many production knives but did not have a good reputation. Most of these knives were produced with 420 steel and later with 440A and were typically low in hardness. Being low in hardness means that the edge easily rolls and wears quickly. To avoid edge rolls a heavier edge geometry is required, which also reduces cutting ability and edge retention. Those factors mean that the knives in stainless steel often had a poor reputation relative to carbon steel. There was of course also a perception issue as carbon steel was “traditional” while stainless steel was new and less known. A.G. Russell reports [1] that he bought a beautiful Case drop point sailor’s knife in stainless steel in 1957 and liked the look of it so much he put elephant ivory handles on it. However, after cutting with it he learned that the edge wouldn’t last more than 5 minutes during cutting and threw it away. Bill Bagwell says that none of his WWII-veteran uncles liked stainless steel knives and thought they wouldn’t hold an edge [2]. A.G. did say that in the 1960’s Victorinox and Schrade began making some higher quality knives in stainless steel.

The Rise of Modern Custom Knifemaking

Custom knifemakers were becoming more common in the USA in the 1950’s and 1960’s. This led to the start of the Knifemaker’s Guild in 1970 to promote custom knives and to sponsor knife shows, of which Bob Loveless was the first secretary and A.G. Russell the Honorary President [3].

Gil Hibben is the first custom knifemaker known to use 440C, he began using it around 1964 [4][5]. 440C was only available as round stock and therefore Gil had to forge it out to be used in knives. Gil also was one of the first to make mirror polished knives which was a distinct look. Between the stainless steel and the striking appearance of the mirror polish his knives were very popular. Gil still thinks that 440C is his favorite and has a good balance of properties, which he has evaluated through use with fish and game. Unlike the soft stainless steel used in many commercially-produced knives of the time, 440C could be heat treated to relatively high hardness and therefore held an edge well.

Update 9/11/2018: D.E. Henry is reported to have begun using 440C in either 1960 [9] or 1962 [10]. He also had to have it forged from rounds to flats [9]. 

In the 1970’s there was also the rise of the American Bladesmith Society which was dedicated to promoting the virtues of forged knives [6] as opposed to stock removal. The rise of the ABS was helped by the excitement over pattern-welded damascus steel as promoted by Bill Moran. Part of the rise of the ABS was the development of performance tests designed to demonstrate that forged blades are superior to stock removal blades. Stainless steels and high alloy tool steels are much more difficult to forge because they have high hot hardness (difficult to move under the hammer), cannot be annealed easily for grinding and working without temperature-controlled furnaces, and have much more tendency to crack during forging or cooling from high temperature. Therefore, virtually all knives made by ABS bladesmiths were, and are, produced with carbon steel, alloy steel, or low alloy tool steels.

This led to there being two primary camps: the knifemakers who used stainless steel with stock removal methods and knifemakers who used non-stainless steel with forging. There are of course exceptions, those knifemakers who used non-stainless steel with stock removal and the few forging smiths who used stainless. However, many of the “carbon vs stainless” debates came out of the forging vs stock removal debate and therefore it is difficult to separate one from the other. Forging bladesmiths found it necessary to promote the virtues of carbon steels in part because that was what they used; while stock removal makers were promoting 154CM as a a new super steel designed for the latest in jet turbines. In that way the stainless vs carbon debate was also about tradition vs new steels and new ways of doing things. Some knife customers were slow to change their minds about stainless steel as well. A.G. Russell reports [1] that in 1970 a customer in his store told A.G. that the stainless steel knife he was promoting was worthless and that stainless steel knives would never be as good as non-stainless.

Carbon vs Stainless Steel Properties 

While perceptions of stainless steel were often based on 420 or 440A knives heat treated to relatively low hardness, the 440C and 154CM used in stock removal knives in the 70’s were a different breed. They were capable of high hardness (>58 Rc) and had high wear resistance. Due to the high carbon and chromium content, 440C and 154CM had large amounts of chromium carbides, whereas carbon steels like 1095 had a small amount of iron carbides. Here are micrographs comparing 440C (stainless) and O1 (low alloy tool steel):

You can see the stark difference in carbide structure by comparing the white particles between 440C and O1. 440C and 154CM have relatively large carbides, often greater than 10 microns across, while carbides in steels like O1, 52100, and 1095, are usually less than 2 microns. Chromium carbides are also harder than the iron carbides in carbon and low-alloy steels. That means that the common stainless steels 440C and 154CM used with stock removal in the 1970’s had much higher wear resistance than the common carbon and alloy steels used in forging. Higher wear resistance means better slicing edge retention but more difficulty in sharpening. It also meant more difficulty in finishing which was particularly an issue with mirror polished blades. The large volume fraction of large carbides also means that the toughness of 440C and 154CM is somewhat less than 1095, O1, and 52100, and significantly less than high toughness steels like 5160. Therefore those that promote carbon steels often talk about ease in sharpening and superior toughness. There are low alloy steels with higher wear resistance, notably the tungsten-alloyed steels such as Blue Super, 1.2562, and F2, but those steels have had poor availability in the USA, and have seen little use by American bladesmiths. You can read more about tungsten-alloyed steels in this article.  

The focus on carbon steel-specific performance advantages are seen by the development of ABS performance tests. One of the required tests is the 90° bend test. That test is done with knives that have had a “differential heat treatment” where a blade is typically produced by one of several methods:

1) Edge quench – a blade is heated and then only the edge is quenched so that the edge is hard and the spine that is more slowly cooled is soft and ductile.

2) Torch hardening – only the edge is heated to the hardening temperature so that the spine is never hardened.

3) Tempering the spine to a lower hardness than the edge, usually with a torch. The entire blade is heated and quenched and then the spine is softened later.

All three of these methods are difficult to do with a stainless steel. The edge quench doesn’t work because stainless steels are “air hardening” meaning even though only the edge is quenched the spine would still harden. Torch hardening isn’t a realistic option because stainless steels require higher temperatures and a prolonged soak prior to quenching. And tempering the spine with a torch is difficult because stainless steels have better “tempering resistance,” so it is difficult to achieve sufficiently low hardness with a torch. Therefore tests like the 90° bend test were used to demonstrate the superiority of carbon steels, or at least forged blades. In fact the rules of the Journeyman Smith test state that only a “carbon steel” can be used [7]. 

The Inbetweeners

There were a few steels that were promoted as having some properties of both steel types, most notably D2, which is sometimes called a “semi-stainless” steel. D2 has a high chromium content (~12%) so it is often claimed that it is nearly stainless. And while it does have much higher stain resistance than a simple carbon steel there are other non-stainless steels with more chromium in solution for better corrosion resistance, such as Vasco Wear which is a steel that has also been around a long time. However, even though D2 is not a stainless steel, its carbide structure is actually very similar to 440C and 154CM, if anything the carbides in D2 are even bigger:

D2 steel micrograph

154CM steel micrograph

D2 is also an “air hardening” steel and has high tempering resistance meaning it is just as difficult to achieve a differentially hardened blade as it is with stainless steels. D2 shares many characteristics in common with stainless steels like 440C and 154CM in terms of carbide structure and mechanical properties and is a good example of how there isn’t always much difference between stainless and non-stainless steels when it comes to mechanical properties. And while D2 is perhaps an obvious example with its high chromium content, there are other non-stainless steels with similar characteristics. 

The Recent Debate

Since the 1970’s and 1980’s there has been the rise of many more stainless steels and non-stainless tool steels. This has perhaps not changed the debate as much as one might suppose. The forging bladesmiths continue to primarily use carbon and alloy steels which have good toughness, great ease in sharpening, and low wear resistance. Knife companies and stock removal makers primarily use stainless steel with some use of high alloy tool steels such as 3V or 10V. The new powder metallurgy stainless steels such as Crucible S30V have smaller carbides than the conventional steels but still have a large volume fraction of carbides which limits the potential toughness:

S30V steel micrograph

These powder metallurgy stainless steels have even higher wear resistance than 440C and 154CM which gives them even greater slicing edge retention, and even more difficulty in finishing and sharpening. So in some ways the divide in the carbon vs stainless debate has gotten even bigger. There are now many powder metallurgy knife steels available from companies like Crucible, Carpenter, Bohler, and Uddeholm. The primary exception has been the increase in popularity of low carbide stainless steels like AEB-L, 13C26, and 14C28N which have a small volume of carbides that are just as small as those found in carbon steels:

13C26 micrograph

Therefore these stainless steels have the same advantages of carbon steels when it comes to a fine carbide distribution, ease in finishing, and high toughness, though reduced slicing edge retention compared to the higher carbide stainless steels and high alloy tool steels. They still have the same difficulties in forging as other air hardening steels, however, and therefore have seen little or no use among the forging bladesmiths. Here is a table summarizing the composition of some of these more recent stainless steels:

Many forging bladesmiths use low alloy steels simply because they work well with forging and do not feel the need to claim that their steel choices are necessarily “superior.” And overall the debate has died down; stock removal knifemakers don’t typically spend much time defending the good performance of their chosen steels, stainless or not. There are still those who feel that carbon steels show superior performance, however. Jimmy Fikes, who helped develop the ABS performance tests, tells me that one of his forged carbon steel blades will make 100 cuts on a rope and be stropped back to shaving sharp while a high quality production stainless pocket knife he owns will only make 15 and needs to be fully resharpened [8]. Perhaps that is due to superior edge geometry rather than strictly steel choice. Regardless of the reasons for the performance discrepancy, Jimmy continues to believe that carbon steel is the way to go.

Conclusions

“Carbon steel” can refer to either simple carbon steels or broadly to “non-stainless” steel. Non-stainless tool steels can share a lot in terms of mechanical properties with stainless steels which can make the carbon vs stainless debate more murky. Early distrust of stainless steel came from production stainless steel knives with low hardness that had poor performance. The continued debate about carbon vs stainless steel came out of the forging vs stock removal arguments that led to the start of the ABS and forging bladesmiths that promoted their way of knifemaking. Many stainless steels have a large volume of carbides which gives them superior wear resistance and slicing edge retention, but reduced toughness and ease in sharpening relative to simple carbon and low alloy steels. Low carbide volume stainless steels like AEB-L and 13C26 have become more popular which have the high toughness and ease in sharpening found in simple carbon steels. In general I would say that knife enthusiasts and knifemakers have more of an appreciation that there are many types of steels with different properties. There are steels with low wear resistance and very high wear resistance, steels that are good for forging, sharpening, finishing, etc. More expensive does not necessarily equal better, though certain combinations of properties are only achieved with more expensive steel. Because of the larger number of steels and better understanding of steel properties there is not such a strong division between non-stainless and stainless steel. There are too many categories of steel now to focus on just “carbon vs stainless.” However, I still see the discussion come up now and then and I have the feeling that the debate may never fully die.

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[1] Russell, A.G. (2018, July 20). Phone interview.

[2] Bagwell, Bill. (2018, July 20). Phone interview.

[3] https://knifemakersguild.com/history.php

[4] Hibben, Gil and Wes. (2018, July 21). Phone interview.

[5] http://www.hibbenknives.com/Gil-Hibben

[6] http://www.americanbladesmith.com/index.php?section=pages&id=117

[7] http://www.americanbladesmith.com/uploads/file/Testing/JS%20Test%20FINAL%204-24-2010.pdf

[8] Fikes, Jimmy. (2018, July 23). Phone interview.

[9] Warner, Ken. Knives,’84. DBI Books, 1983.

[10] Henry, D.E. Collins Machetes and Bowies, 1845-1965. Krause Publications, 1995.