M390 vs D2 Steel

D2 is a conventional air-hardening, high-carbon, high-chromium tool steel produced by various manufacturers, including Niagara Specialty Metals.

M390 is a martensitic chromium steel produced through the powder metallurgy process by Bohler Uddeholm, a leading steel manufacturer. 

M390 is also known as 20CV (produced by Latrobe/Crucible) or CTS-204P (produced by Carpenter).

The key difference between D2 and M390 lies in their manufacturing processes and resulting microstructures.

While D2 is conventionally produced, M390 is a premium powder metallurgy steel that offers a finer and more uniform distribution of carbides, including hard vanadium-rich carbides. 

This microstructural advantage of M390 translates into superior wear resistance, edge retention, and corrosion resistance compared to D2, making it a preferred choice for premium knives.

M390 vs D2 Steel At A Glance

Hardness

M390 steel is capable of achieving higher hardness values compared to D2 steel.

According to the tempering charts in the M390 data sheet, M390 can reach hardness levels up to around 66 HRC after proper heat treatment and subzero treatment.

In contrast, the D2 data sheet shows that D2 typically achieves hardness in the range of 55-62 HRC after tempering.

The higher attainable hardness of M390 makes it more suitable for applications that require superior wear resistance and edge retention in knives.

Additionally, the powder metallurgy manufacturing process used for M390 results in a finer and more uniform carbide distribution, further enhancing its hardness compared to the relatively large carbides present in conventionally produced D2 steel.

Toughness 

D2 and M390 steels exhibit comparable levels of toughness, but D2 may have a slight advantage, which can be beneficial for knives.

The marginally higher toughness of D2 steel translates to a reduced risk of chipping or breaking under impact or high-stress conditions compared to knives made from M390 steel.

D2 steel exhibits Charpy impact toughness values in the range of 19-23 ft-lbs. While M390 steel also demonstrates adequate toughness, its values are somewhat lower.

The better toughness of D2 steel can be attributed to its relatively lower carbide volume fraction compared to the high carbide content of M390.

The presence of large chromium carbides in D2, although softer than the vanadium-rich carbides in M390, potentially contributes to its slightly better impact resistance.

Edge Retention

M390 steel exhibits significantly better edge retention and wear resistance compared to D2 steel, making it a superior choice for knives where edge retention is critical.

In a CATRA edge retention testing carried out by Bohler, M390 steel at a hardness of 61+ HRC was able to achieve a Total Cards Cut (TCC) edge retention of 958.6, while AISI D2 at 61 HRC was able to achieve a TCC edge retention of 665.8. 

The excellent edge retention of M390 can be attributed to its very high carbide volume fraction containing hard vanadium-rich carbides.

In contrast, D2 has a lower overall carbide content with relatively large chromium carbides that are softer.

The superior edge retention of M390 translates to blades that can maintain a sharp cutting edge for much longer periods compared to D2 blades, especially when cutting through abrasive materials.

Corrosion Resistance

M390 steel is a stainless steel while D2 steel is a non-stainless steel.

M390 steel exhibits significantly better corrosion resistance compared to D2 steel, making it a more suitable choice for knives that may be exposed to corrosive environments.

In corrosion resistance testing conducted by Knife Steel Nerds, M390 (tested as CTS-204P) scored very well, performing better than stainless steels like S30V, S35VN, and S90V.

While D2 steel has good corrosion resistance for a tool steel due to its high chromium content, approximately half of that chromium is tied up in carbides and does not contribute to corrosion resistance.

M390 allows the blades to better withstand exposure to moisture, salts, acids, and other corrosive elements without staining, pitting, or rusting.

Ease of Sharpening

D2 steel offers better ease of sharpening and grinding compared to M390 steel.

M390 steel has a higher volume fraction of hard vanadium-rich carbides compared to D2.

These extremely hard carbides in M390 make it more challenging to remove material during sharpening or grinding processes that use conventional aluminum oxide abrasives.

In contrast, D2 steel contains relatively large but softer chromium carbides, which are more easily abraded and removed during sharpening. 

Final thoughts on M390 vs D2 Steel

Having experience working with both D2 and M390 steels, I can confidently say that M390 emerges as the superior choice for most high-performance knife applications due to its outstanding corrosion resistance and unparalleled edge retention capabilities.

M390, being a premium powder metallurgy steel offers a finer and more uniform distribution of hard vanadium-rich carbides, contributing to its exceptional wear resistance and edge retention properties.

Additionally, its stainless characteristics and high chromium content in solution make M390 knives highly resistant to corrosion, ensuring longevity and minimal maintenance requirements.

While D2 is an excellent conventional air-hardening tool steel that provides very good edge retention and wear resistance, its non-stainless nature means that it requires more care and maintenance to prevent rusting, especially in environments with moisture or exposure to corrosive elements.

Personally, I have a D2 knife that I’ve owned for ages, and I absolutely love it.

Over time, I’ve mastered the art of sharpening D2 steel, which can be a bit challenging due to its large chromium carbides. 

However, it’s important to note that while M390 steel can attain a sharper edge, it is generally more difficult to sharpen and grind due to its high content of hard vanadium carbides.

It’s also worth mentioning that M390 is typically more expensive than D2 due to its premium powder metallurgy production process.

For anyone seeking the best in edge retention and corrosion resistance in a knife, I highly recommend considering M390 steel.

However, if cost is a concern and you don’t mind the extra maintenance required to prevent rust, D2 steel can still be an excellent choice.