W
Way-Barney
I read the paper,
Mujahid and his colleagues at the Drexel University in Philadelphia studied the microstructure of a stainless steel designed for special applications such as pumps used to move media that has more corrosiveness and abrasiveness then typical applications thus requiring a more carbide rich and stain resistant stainless steel then what is typically used which he examines the microstructure of.
(Toughness was not a requirement)
Mujahid and his team received an as cast piece of steel from a pump manufacturer; a steel designed for pumps with the following chemistry.
1.6% Carbon, 26% Chromium, 2% Molybdenum 1% silicon, 1% Manganese.
(This would not be a good knife steel)
Mujahid observed large, dendritic, eutectic primary carbides (M7C3) formed around the grain boundaries in a matrix of ferrite of the as received cast condition. He further examines the morphology of these different large primary carbide structures, it should be noted that we do not commonly see, desire or use as cast steel for knives.
(David Boyle does like as cast 440C and Cobalt alloys; it's a trade off)
Knife steels are normally hot rolled and annealed so we wouldn't see these large dendritic structures and large grain boundary carbides which will bottom out the toughness at the same hardness if compared to a material that does not have those structures after hot rolling and annealing of course there are limits which is why there are special processes like particle metallurgy etc.
(This pump steel would not be desirable for knives regardless due to its chemistry regardless of special processing)
Mujahid then heat treats the material in the as cast condition, He observes and identifies porosity that's remaining from the casting along with seeing secondary carbide formation which he identifies by chemically dissolving the steel to extract the carbides.
Worst of all, due to the chemistry and coarse structure of this material, he still has ferrite, austenite and large primary carbide networks remaining in the structure after heat treatment. While austenitizing is obviously not going to be breaking up any of these primary carbides. The chemistry of this material will not help get the structure we like for knife steel.
(We like tempered martensite with spherical undissolved carbides and low RA)
High volumes of residual austenite and ferrite would make a very low performance quality cutting tool and It would also be difficult to get good finish on.
(Special pump steel chemistry not ideal for knives)
Again, this paper is not focused on what is pertinent to knives, the main focus of Mujahid's paper is that he identifies the the microstructures in the as cast material designed for special pumps in the as received condition and after austenitizing as range of temperatures.
He identified M23C6 as the secondary carbide and M7C3 as the primary carbide but this isn't a unique discovery, it is well understood by non lay people we can expect those structures in any high carbon stainless steel.
He does talk about the the presence of metallic elements "M" inside of the carbides and how they are mostly rich in chromium, However, he does not share what the measured chemistry of the M7C3 or M23C6
(It's not important for his paper/audience)
Mujahid's paper is also not intended to be showing some revolutionary breakthrough in the discovery of carbide structure identification.
Keep in mind this paper came out in 1998 and all these types of carbides had been well identified over 80 years before this paper with work that was done by Bragg, Westgren and Phragmén in the early days of x-ray crystallography.
Mujahid does not discuss in detail M3C (nor does he need to) he only brings it up for discussing the generalized Fe-Cr-C system diagram at the beginning of the paper to show how with increasing amounts of chromium how the different types of carbides will take preference of formation.
In summary,
The only thing that is pertinent to knife steel in this paper is that you will have the presence of M7C3 and M23C6 in the knife steels that we use but the paper does not discuss, measure or give the context of the importance between these structures That is not a poor reflection of the author. Mujahid and his colleagues did a good job for their audience. It is simply not a paper designed to educate you about the elaborate details of carbides.
Knowing about these carbide structures(M3C, M7C3, M23C6) will not translate directly into creating high performance knives and heat treatment, but it does give some interesting perspective when we look at steels like AEB-L and 440C and when we ask ourselves why one looks finer than the other if we examine the microstructure. Especially when we see AEB-L can be finer than even PM 440C. How? Why?
You can find more reading on the subject of primary and secondary carbides in Dr Verhoeven's famous book "Steel Metallurgy for The Non-metallurgist" printed in 2007 which is a complete classic.
I would more strongly recommend reading Dr. Larrin Thomas's book "Knife Engineering: Steel, Heat Treating and Geometry" made in 2020. This book will also get you into further detail about carbides, austenitizing, martensite, ferrite, etc then what you can find in that paper you shared.
Most importantly, Dr Larrin's book will keep content more focused towards knives so you can develop a good base before running off and reading different papers and scratching your head about trying to find what's actually relevant to knives.
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