When it comes to high-performance knife steels, K390 and M390 are often at the top of the list for enthusiasts and professionals alike.
Both steels are products of Böhler’s advanced powder metallurgy processes, but they serve different purposes and excel in distinct areas.
While K390 is renowned for its extreme wear resistance and toughness, M390 is celebrated for its excellent balance of edge retention, corrosion resistance, and ease of sharpening.
This article delves into the scientific data behind these steels, comparing their chemical compositions, performance metrics, heat treatment requirements, and real-world applications to help you make an informed decision.
Key Differences at a Glance
- K390: High wear resistance, excellent toughness, but low corrosion resistance.
- M390: Superior corrosion resistance with a balanced combination of edge retention and toughness.
Composition Analysis
The chemical composition of a steel determines its mechanical properties such as hardness, toughness, wear resistance, and corrosion resistance. Below is a comparison of the chemical compositions of K390 and M390:
| Element | K390 (% by weight) | M390 (% by weight) |
|---|---|---|
| Carbon (C) | 2.45 | 1.90 |
| Chromium (Cr) | 4.20 | 20.00 |
| Vanadium (V) | 9.00 | 4.00 |
| Molybdenum (Mo) | 3.80 | 1.00 |
| Tungsten (W) | 1.00 | 0.60 |
| Cobalt (Co) | 2.00 | – |
Impact of Key Alloying Elements
- Carbon (C): Higher carbon content in K390 contributes to its superior hardness and wear resistance but can make it more prone to brittleness.
- Chromium (Cr): M390 has significantly more chromium, which enhances its corrosion resistance by forming a protective oxide layer.
- Vanadium (V): Both steels contain high levels of vanadium, which forms hard vanadium carbides that contribute to wear resistance.
- Molybdenum (Mo) and Tungsten (W): These elements improve both steels’ ability to maintain hardness at high temperatures (hot hardness), but K390 has higher levels, making it more suitable for extreme wear environments.
Performance Metrics
Edge Retention
Edge retention is one of the most critical factors for knife users, especially in demanding tasks like cutting abrasive materials.
- K390: Known for its exceptional edge retention due to its high vanadium carbide content (~9%), K390 outperforms many other tool steels in this category. In CATRA tests (a standardized test for measuring edge retention), K390 scores significantly higher than M390 when both are heat-treated to similar hardness levels[1].
- M390: While M390 also excels in edge retention due to its mix of vanadium and chromium carbides (~22% carbide volume), it generally falls slightly behind K390 in CATRA tests[3]. However, M390’s edge retention is still considered excellent for a stainless steel.
Toughness
Toughness refers to the steel’s ability to resist chipping or breaking under stress.
- K390: Despite its high hardness and wear resistance, K390 maintains impressive toughness compared to other high-carbide tool steels like CPM-10V[1]. Its toughness is comparable to stainless steels like S35VN and CPM-154 but with much better wear resistance[2].
- M390: M390 offers good toughness for a stainless steel but is generally less tough than K390 due to its higher chromium content and larger carbide structures[3]. It performs well in applications where both corrosion resistance and moderate toughness are required.
Corrosion Resistance
Corrosion resistance is essential for knives exposed to moisture or harsh environments.
- K390: As a non-stainless tool steel with only 4.2% chromium, K390 has poor corrosion resistance[2]. It requires regular maintenance to prevent rusting.
- M390: With 20% chromium content, M390 provides excellent corrosion resistance[7]. This makes it ideal for outdoor use or environments where the blade may be exposed to moisture or corrosive substances.
Hardness Range
Hardness is measured on the Rockwell C scale (HRC) and indicates how well the steel can resist deformation.
- K390: Typically hardened between 62–65 HRC[4], K390 can reach even higher hardness levels (~66 HRC) without sacrificing too much toughness.
- M390: M390 usually falls within the 60–62 HRC range[7], balancing hardness with ease of sharpening and toughness.
Summary Table of Performance Metrics
| Property | K390 | M390 |
|---|---|---|
| Edge Retention | Excellent | Very Good |
| Toughness | Very Good | Good |
| Corrosion Resistance | Poor | Excellent |
| Maximum Hardness | 62–65 HRC | 60–62 HRC |
Heat Treatment Considerations
Heat treatment plays a crucial role in optimizing a steel’s properties like hardness, toughness, and wear resistance.
K390 Heat Treatment
For optimal performance:
- Austenitizing Temperature: 1800–2100°F (980–1150°C), with shorter times at higher temperatures (~10 minutes at 2150°F)[4].
- Tempering Temperature: Typically tempered between 400–500°F (205–260°C). Higher tempering temperatures (~1000°F) reduce hardness significantly[4].
- Quenching Method: Plate quenching or rapid gas quenching is recommended for thin sections like knives[4].
M390 Heat Treatment
For optimal performance:
- Austenitizing Temperature: Around 2100°F (1150°C), followed by cryogenic treatment to reduce retained austenite[3].
- Tempering Temperature: Usually tempered at around 400°F (205°C) for maximum hardness without sacrificing too much toughness[3].
- Quenching Method: Air or oil quenching works well due to M390’s high chromium content.
Real-World Performance
Edge Stability
Edge stability refers to how well the knife retains its sharpness under heavy use without chipping or rolling.
- K390: With its high vanadium carbide content and fine grain structure from powder metallurgy processing, K390 offers excellent edge stability even under heavy use[1][2].
- M390: While not as stable as K390 under extreme conditions due to its larger carbide structures, M390 still holds up well in everyday cutting tasks thanks to its balance between carbide size and distribution[7].
Sharpening Behavior
Sharpening behavior depends on the steel’s carbide content and distribution.
- K390: Due to its high vanadium carbide content (~9%), sharpening K390 can be challenging without diamond or ceramic abrasives[2]. However, once sharpened, it holds an edge for an extended period.
- M390: Easier to sharpen than K390 because its chromium carbides are softer than vanadium carbides. Standard sharpening stones work well on M390 blades[7].
Wear Patterns
Wear patterns refer to how evenly the blade wears down during use.
- K390: Tends to wear more evenly due to its fine microstructure from powder metallurgy processing. The high volume of hard vanadium carbides ensures that the edge remains sharp even after prolonged use on abrasive materials[1][2].
- M390: Exhibits slightly less uniform wear patterns because of its larger chromium carbides but still performs admirably in most cutting tasks[7].
Manufacturing Considerations
Both K390 and M390 are produced using advanced powder metallurgy processes that ensure fine carbide distribution and uniformity across the steel’s structure.
Powder Metallurgy Process
Powder metallurgy involves atomizing molten steel into fine particles, which are then compacted under high pressure and heat-treated. This process results in:
- Uniform carbide distribution
- Improved toughness compared to conventional steels
- Better machinability
Cost Implications
Due to their complex manufacturing processes:
- K390 tends to be more expensive than conventional tool steels but is often priced similarly to other high-end non-stainless steels like CPM-M4 or CPM-S110V.
- M390, being stainless with excellent all-around properties, commands a premium price among stainless steels but is generally more affordable than exotic options like Maxamet or S90V.
Conclusion
In summary:
- Choose K390 if you need extreme wear resistance and toughness but can manage corrosion through regular maintenance.
- Opt for M390 if you prioritize corrosion resistance along with good edge retention and easier sharpening characteristics.
Both steels excel in specific areas:
- For outdoor knives exposed to moisture or corrosive environments, go with M390.
- For cutting tools that require superior wear resistance in dry conditions, K390 is unbeatable.
FAQ
- Which steel holds an edge longer?
- K390 generally holds an edge longer due to its higher vanadium carbide content.
- Is M390 easier to sharpen than K390?
- Yes, M390’s softer chromium carbides make it easier to sharpen compared to K390’s harder vanadium carbides.
- Which steel is better for outdoor use?
- M390 is better suited for outdoor use due to its superior corrosion resistance.
- Can I use standard sharpening stones on K390?
- No, you will need diamond or ceramic abrasives due to the hardness of vanadium carbides in K390.
- Which steel offers better toughness?
- While both offer good toughness, K390 edges out slightly over M390 in this department due to its lower chromium content.
Citations:
[1] https://knifesteelnerds.com/2023/04/11/k390-best-high-wear-resistance-steel/
[2] https://nemoknivesreview.com/2019/06/14/steel-k390-bohlers-microclean-uncanny-blade-material/
[3] https://knifesteelnerds.com/2020/06/01/m390-steel-history-and-properties-and-20cv-and-204p/
[4] https://knifesteelnerds.com/2023/09/23/how-to-heat-treat-k390/
[5] https://www.youtube.com/watch?v=UhfBLjuyqsU
[6] https://www.youtube.com/watch?v=CxlkSRc3YRA
[7] https://nobliecustomknives.com/m390-steel/
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