S90V vs M390 Steel

S90V and M390 are two premium stainless blade steels used in high-end knives and tools.

S90V is produced by Crucible Industries as part of their CPM (Crucible Particle Metallurgy) line of tool steels.

It is an ultra-high vanadium alloy originally called CPM-420V with 9% vanadium added to increase wear resistance. 

M390 was developed by Bohler and contains lower vanadium at 4% but higher chromium for better corrosion resistance.

Both steels achieve excellent edge retention and have found favor among manufacturers and knifemakers looking to improve on previous CPM alloys like S30V, S35VN and S45VN.

The key difference between CPM S90V and Bohler M390 lies in their alloying strategies.

S90V prioritizes wear resistance through the addition of high vanadium while M390 focuses more on corrosion resistance with its high chromium content. 

This results in CPM S90V demonstrating superior edge holding and abrasion resistance compared to Bohler M390, though M390 offers better corrosion resistance.

S90V vs M390 Steel At A Glance

Corrosion Resistance

M390 demonstrates superior corrosion resistance compared to S90V due to its higher chromium and molybdenum content.

M390 contains 20% chromium and 1% molybdenum while S90V has 14% chromium and 1% molybdenum.

The higher chromium content of M390 enables it to form a more protective and stable passive oxide layer that resists corrosion.

In corrosion testing, M390 performed similarly to S110V steel which is known for its excellent corrosion resistance.

On the other hand, S90V was more comparable to S30V and S35VN steels, with adequate but not exceptional corrosion resistance.

The lower chromium content of S90V is beneficial for wear resistance by enabling more vanadium carbide formation, but it reduces corrosion resistance compared to M390.

While S90V has moderate corrosion resistance sufficient for most knife applications, M390 offers superior performance approaching that of stainless steels optimized specifically for corrosion resistance like LC200N.

This enables better resistance to pitting and rusting in harsh, wet environments.

Hardness

S90V and M390 steels are capable of achieving similar maximum hardness values of around 64+ HRC when optimally heat treated.

However, S90V generally reaches slightly higher hardness than M390, S90V achieved ~2 HRC points higher than M390 based on data from Crucible and Bohler-Uddeholm.

The higher hardness of S90V is attributed to its higher vanadium content (9% vs 4% in M390) which forms hard vanadium carbides.

Though both steels can theoretically attain 64+ HRC, knife makers often aim for 56-62 HRC for S90V and M390 to balance hardness with toughness and sharpenability. 

The marginally higher hardness contributes to its superior edge retention compared to M390 in knife usage, though it comes at the expense of being more difficult to sharpen. 

Edge Retention

S90V steel demonstrates superior edge retention and wear resistance compared to M390 steel in knife usage and testing.

In CATRA edge retention testing, S90V showed 15-20% higher values than M390 when compared at equal hardness levels.

The higher wear resistance of S90V is attributed to its higher volume of vanadium carbides which are harder than the chromium carbides that predominantly form in M390.

S90V contains 9% vanadium while M390 has only 4% vanadium, and S90V also has a lower chromium content of 14% vs 20% in M390.

The lower chromium in S90V enables more vanadium carbide formation. The high-hardness vanadium carbides impart excellent abrasion resistance and edge holding to S90V.

In contrast, the M390 relies more on its high carbide volume rather than carbide hardness to achieve good edge retention. 

While both steels exhibit excellent edge holding, S90V is superior due to the hardness and higher volume of vanadium carbides that develop during heat treatment.

This gives S90V the maximum edge retention and wear resistance of any stainless steel, though it comes at the cost of being more difficult to sharpen than M390.

Toughness

The toughness of S90V and M390 stainless steels is fairly comparable despite differences in alloying.

In toughness testing of 1/4 size unnotched Charpy specimens, S90V achieved toughness values of 5-6 ft-lbs at 61-62 HRC while M390 was in the range of 4-5 ft-lbs.

The toughness of both steels is considered relatively low, which is attributed to their high volume of carbides – approximately 22% for M390 and 20% for S90V. 

M390 forms primarily chromium carbides which are larger than the vanadium carbides in S90V, resulting in its slightly lower toughness. 

However, the overall toughness of S90V and M390 is similar and lower than other powder metallurgy stainless steels.

The relatively low toughness of S90V and M390 can be acceptable in knives as long as the knives do not have very thin edge geometry and are not subjected to high stresses or impacts during use.

Care must be taken to avoid brittle behavior in very thin or highly stressed designs. But with appropriate geometry, the comparable toughness of S90V and M390 allows their use in most knives.

Ease of Sharpening

S90V is slightly more difficult to sharpen and grind than M390 due to the difference in carbide types between the steels.

S90V knife steel forms hard vanadium carbides while M390 knife steel predominantly forms chromium carbides. 

The vanadium carbides in S90V are harder than common sharpening abrasives like aluminum oxide, making the steel more difficult to abrade and sharpen.

In contrast, M390 gets its good edge retention from a high volume of chromium carbides rather than extremely hard carbides.

The chromium carbides are softer than vanadium carbides, so M390 offers a somewhat better ease of sharpening compared to steels like S90V with similar edge retention. 

This advantage in sharpenability only applies when using abrasives like aluminum oxide that are softer than vanadium carbide.

When sharpening with diamond or CBN abrasives that are harder than both carbide types, S90V and M390 sharpen comparably.

Final Thoughts on S90V vs M390

Having used both M390 and S90V extensively, I can certainly attest to the impressive edge retention and wear resistance of S90V steel.

The high vanadium content enables it to achieve the best abrasion resistance of any stainless blade steel.

However, for my preferences, I generally favor M390 over S90V. While S90V offers better edge holding, I find M390 easier to sharpen and grind.

The balance of edge retention, corrosion resistance, and toughness in M390 premium steel allows me to use thinner edge geometries than I would with S90V.

Though S90V undoubtedly has superior abrasion resistance, I love the M390 for its well-rounded performance. 

The 4% vanadium provides substantial improvement in edge retention over steels like S30V without compromising ease of sharpening.

M390 also has excellent corrosion resistance on par with premium alloys like S110V.

While both are fantastic steels, the sharpenability, thinner edge capability, and balanced properties of M390 make it my top choice between the two.