K390 vs M390 Steel

M390 and K390 are powder metallurgy steel produced by the renowned Austrian manufacturer Bohler.

K390 is a non-stainless, high-vanadium tool steel designed to provide ultimate wear resistance and edge retention for knives and industrial cutting applications.

K390 steel was developed as an alternative to Crucible’s popular CPM-10V steel. K390 contains 9% vanadium along with increased molybdenum, tungsten, and cobalt compared to 10V.

In recent years, K390 has been hugely popularized by Spyderco using it in several of their premium knife models.

M390 is a martensitic stainless steel containing 20% chromium to provide excellent corrosion resistance in addition to superb wear resistance from its 4% vanadium content. 

The key difference is that K390 is a non-stainless ultra-high vanadium tool steel focused solely on maximizing wear resistance.

While M390 is a stainless steel that trades off a small amount of wear resistance for much improved corrosion resistance compared to K390. 

K390 vs M390 Steel At A Glance 

Hardness

K390 steel has a distinct advantage over M390 steel when it comes to achievable hardness.

According to the tempering charts provided by Böhler-Uddeholm, K390 can reach hardness values up to 66 HRC after proper heat treatment.

In contrast, M390 steel typically tops out at around 64 HRC, even with subzero treatment.

This difference in achievable hardness makes K390 a better choice for applications that require exceptional wear resistance and edge retention, such as knives.

While K390 has the potential to achieve higher hardness values than M390, the aim hardness for knives is typically in the low range of 60-62 HRC for both steels. 

At this hardness level a balance between toughness, edge stability and ease of sharpening is often achieved.

Toughness

Toughness is the ability to resist fracture and deformation under impact or stress, it is a crucial property for knife steels, as knives are often subjected to demanding conditions during use. 

According to the impact energy charts provided by Böhler-Uddeholm, K390 exhibits slightly better toughness compared to M390.

At a hardness of around 61 HRC, K390 achieves an impact energy of approximately 44J, while M390 falls slightly lower, with an impact energy of around 35J at a similar hardness level. 

This difference in toughness can be attributed to the unique alloying concept and microstructure of K390.

K390 is designed to have an excellent balance of wear resistance and toughness, achieved through its alloying elements.

The steel contains molybdenum, tungsten, and cobalt, which contribute to its “secondary hardening” and “hot hardness” properties.

These elements help maintain the steel’s hardness and toughness at elevated temperatures, a critical factor in demanding cutting applications.

Furthermore, K390 steel microstructure, characterized by a uniform distribution of vanadium carbides, plays a crucial role in its toughness. 

The vanadium carbides in K390 are finer and more evenly dispersed compared to the carbides in M390, which are primarily chromium carbides.

This finer carbide structure contributes to improved toughness while maintaining excellent wear resistance.

For knife applications, the slightly better toughness of K390 may provide an edge over M390, especially for larger or thicker blades where fracture resistance is critical.

Edge Retention

Edge retention and wear resistance are among the most critical properties for knife steels, as they directly impact the cutting performance and longevity of the blade.

K390 and M390 are both exceptional steels at edge retention, however, K390 steel exhibits a better edge retention over M390 steel.

According to the wear resistance data provided by Böhler-Uddeholm, K390 outperforms M390 in terms of abrasive wear resistance. 

The superior edge retention of K390 can be attributed to its alloying elements and higher volume of hard vanadium carbides.

K390 steel contains 9% vanadium, which is higher than M390 steel 4% vanadium content contributing to  higher vanadium carbides.

M390 steel is designed to strike a balance between wear resistance and corrosion resistance.

While M390 edge retention is excellent, it falls slightly behind K390 due to its lower vanadium content and higher chromium carbide volume. 

However, M390 steel edge retention is still superior to many other stainless steels like S35VN and S45VN.

Corrosion Resistance

M390 holds an advantage over K390 in corrosion resistance due to its stainless steel composition, while K390 is a non-stainless tool steel.

M390 is classified as a martensitic chromium steel produced through powder metallurgy, with a nominal chromium content of 20%.

This high chromium content, combined with the addition of molybdenum (1%), contributes to M390’s excellent corrosion resistance.

The chromium content in M390 allows for the formation of a protective passive oxide layer on the steel’s surface, which acts as a barrier against corrosive agents.

In contrast, K390 is a non-stainless tool steel designed primarily for exceptional wear resistance and toughness.

While it contains chromium (4.2%) and molybdenum (3.8%), these alloying elements are primarily intended to enhance the steel’s mechanical properties rather than corrosion resistance.

As a result, K390 lacks the high chromium content necessary to form a protective passive layer, making it more susceptible to corrosion compared to stainless steels like M390.

Ease of Sharpening

M390 blade steel offers an advantage in terms of ease of sharpening compared to K390 blade steel.

This advantage stems from the nature of the carbides present in each steel microstructure.

M390 is a stainless steel that derives its excellent wear resistance from a combination of vanadium carbides (approximately 2.5%) and chromium carbides (approximately 18%).

Chromium carbides are softer than common sharpening abrasives like aluminum oxide, which makes them easier to abrade and reshape during the sharpening process.

On the other hand, K390 steel exceptional wear resistance is primarily attributed to its high volume of vanadium carbides (approximately 17%). 

Vanadium carbides are among the hardest carbides found in tool steels, surpassing the hardness of common sharpening abrasives.

This high hardness of the vanadium carbides in K390 makes the steel more challenging to sharpen, as the abrasives have a harder time cutting and reshaping the carbide structure.

It’s important to note that while M390 has an advantage in ease of sharpening with aluminum oxide abrasives.

This advantage may not hold true when using diamond or cubic boron nitride (CBN) abrasives.

These superabrasives are harder than both chromium and vanadium carbides, and their effectiveness in sharpening would be more dependent on the overall carbide volume rather than the specific types of carbides present.

Final Thoughts on K390 vs M390 Steel

In conclusion, both K390 and M390 are truly superb powder metallurgy knife steels. 

From a personal perspective, I am very impressed by the properties of K390.

Its extremely high 9% vanadium content gives it simply outstanding edge retention and slicing ability that few other steels can match.

The toughness is also remarkably good for such a highly wear-resistant tool steel.

K390 would make an exceptional high-performance knife blade.

However, for most knife users, I would recommend M390 stainless steel. 

Knives made from M390 can still achieve high edge retention while providing robust stain and corrosion resistance.

K390 knife steel is however non stainless and thus requires extra care to prevent rusting and corrosion particularly when exposed to moisture.

Another potential drawback of K390 steel knives is that its high vanadium carbide content makes it incredibly difficult to sharpen freehand compared to M390’s chromium carbides. 

For beginner knifemakers or those without diamond abrasives, sharpening K390 knives could be a real struggle.