When it comes to high-performance knife steels, K390 and M4 stand out as two of the most popular choices among knife enthusiasts and professionals. Both steels are renowned for their exceptional edge retention, toughness, and overall performance in demanding applications. However, they have distinct differences that make them suitable for different use cases.
This article will provide a detailed comparison of K390 and M4 based on scientific data and measurable properties, including chemical composition, edge retention, toughness, hardness, corrosion resistance, and more.
By the end of this article, you will have a clear understanding of which steel is better suited for your needs based on performance metrics, manufacturing processes, and real-world applications.
Composition Analysis
The chemical composition of a steel significantly influences its properties such as hardness, toughness, wear resistance, and corrosion resistance. Below is a comparison of the chemical makeup of K390 and M4:
| Element | K390 (%) | M4 (%) |
|---|---|---|
| Carbon (C) | 2.47 | 1.25-1.4 |
| Chromium (Cr) | 4.2 | 3.75-4.75 |
| Molybdenum (Mo) | 3.8 | 4.25-5.5 |
| Vanadium (V) | 9 | 3.75-4.5 |
| Tungsten (W) | 1 | 5.25-6.5 |
| Cobalt (Co) | 2 | – |
Explanation of Key Alloying Elements:
- Carbon (C): Higher carbon content in K390 (2.47%) compared to M4 (~1.3%) contributes to its superior hardness and wear resistance.
- Chromium (Cr): Both steels have moderate chromium content (~4%), but this is not enough to make them stainless steels.
- Vanadium (V): K390 contains significantly more vanadium (9%) than M4 (~4%), which enhances wear resistance by forming hard vanadium carbides.
- Tungsten (W): M4 has higher tungsten content (~6%), which improves its toughness and high-temperature strength.
- Cobalt (Co): Present only in K390 at 2%, cobalt increases hardness and edge stability under high-stress conditions.
Performance Metrics
Edge Retention
Edge retention is one of the most critical factors for knife users who need their tools to stay sharp for extended periods without frequent sharpening.
- K390: Known for its outstanding edge retention due to its high vanadium carbide content. In CATRA tests (which measure edge retention through repeated cutting of a standardized medium), K390 performs exceptionally well, often outperforming other non-stainless tool steels like M4.
- CATRA TCC Rating: ~950 mm
- M4: While M4 also has excellent edge retention due to its high carbon and vanadium content, it falls slightly behind K390 in this category.
- CATRA TCC Rating: ~800 mm
Toughness
Toughness refers to a steel’s ability to resist chipping or breaking under impact or stress.
- K390: Despite its high hardness and wear resistance, K390 maintains good toughness due to Bohler’s Microclean powder metallurgy process, which ensures a fine carbide distribution.
- Charpy Impact Test: ~20–25 ft-lbs at 63–64 HRC[1][5].
- M4: M4 is renowned for its balance between toughness and wear resistance, making it ideal for applications where both properties are critical.
- Charpy Impact Test: ~30–35 ft-lbs at similar hardness levels[3].
Corrosion Resistance
Neither K390 nor M4 are stainless steels due to their relatively low chromium content (<5%), meaning they are prone to rust if not properly maintained.
- K390: Offers very low corrosion resistance due to its high carbon content and lack of sufficient chromium.
- Corrosion Resistance Rating: Low
- M4: Slightly better corrosion resistance than K390 but still requires regular maintenance to prevent rusting.
- Corrosion Resistance Rating: Low-Medium[2].
Hardness Range
The Rockwell C scale (HRC) measures the hardness of knife steels.
- K390: Can achieve hardness levels between 62–65 HRC depending on heat treatment.
- Typical Hardness: ~64 HRC
- M4: Achieves similar hardness levels but with slightly more flexibility in heat treatment options.
- Typical Hardness: ~62–66 HRC[3].
Heat Treatment Considerations
Heat treatment plays a crucial role in optimizing the performance characteristics of both K390 and M4.
K390 Heat Treatment:
- Austenitizing Temperature: Typically around 1100–1150°C (2012–2102°F).
- Tempering Temperature: Between 500–550°C (932–1022°F) for optimal toughness without sacrificing too much hardness.
Optimal heat treatment for K390 involves careful control over tempering temperatures to balance wear resistance with toughness.
M4 Heat Treatment:
- Austenitizing Temperature: Around 1200°C (2192°F).
- Tempering Temperature: Typically at around 565°C (1050°F) for achieving peak hardness (~62–66 HRC).
M4’s heat treatment process allows for some flexibility depending on whether the user prioritizes toughness or edge retention.
Real-World Performance
Edge Stability
Edge stability refers to how well a steel maintains a sharp edge under heavy use without chipping or rolling.
- K390: Due to its high vanadium carbide content and fine microstructure, K390 offers excellent edge stability even under heavy use.
- M4: While also providing good edge stability, M4 may be slightly more prone to chipping compared to K390 when used in thin-edge geometries.
Sharpening Behavior
Sharpening ease is an important consideration for users who regularly maintain their knives.
- K390: Due to its extreme hardness and wear resistance, sharpening K390 can be challenging without proper equipment such as diamond stones.
- M4: Easier to sharpen than K390 but still requires more effort compared to lower alloyed steels like AEB-L or VG10.
Wear Patterns
Both steels exhibit excellent wear resistance but differ slightly in how they wear over time:
- K390: Tends to develop micro-chipping rather than rolling under heavy use due to its high carbide volume.
- M4: More likely to roll or deform slightly before chipping, giving it an advantage in applications where toughness is prioritized over wear resistance.
Manufacturing Considerations
Production Method
Both K390 and M4 are produced using powder metallurgy techniques, which ensure fine carbide distribution and uniformity throughout the steel.
- K390: Manufactured using Bohler’s proprietary Microclean technology, which results in superior cleanliness and homogeneity compared to conventional ingot-cast steels[1].
- M4: Produced using Crucible’s CPM process, which also ensures fine carbide distribution but may not match the purity levels achieved by Bohler’s Microclean process[3].
Cost Implications
The cost of manufacturing knives from these steels can vary significantly based on factors such as raw material costs and machining difficulty.
- K390: Generally more expensive due to its complex alloy composition and difficulty in machining.
- M4: Slightly less expensive than K390 but still considered a premium steel due to its high alloy content.
Direct Comparison Table
| Property | K390 | M4 |
|---|---|---|
| Edge Retention | Superior (~950 mm) | Excellent (~800 mm) |
| Toughness | Good (~20–25 ft-lbs) | Excellent (~30–35 ft-lbs) |
| Corrosion Resistance | Low | Low-Medium |
| Maximum Hardness | ~64 HRC | ~66 HRC |
| Cost Factor | Higher | Moderate |
Conclusion
Both K390 and M4 are exceptional knife steels that offer unique advantages depending on the application:
- Choose K390 if you prioritize maximum edge retention and wear resistance at the expense of ease of sharpening and corrosion resistance.
- Choose M4 if you need a well-balanced steel that offers excellent toughness with good edge retention while being slightly easier to sharpen than K390.
In summary:
- For outdoor knives that may be exposed to moisture: M4 with proper maintenance is preferable.
- For industrial cutting tools requiring long-lasting edges: K390 is the superior choice.
FAQ Section
- Which steel holds an edge longer?
- K390 holds an edge longer due to its higher vanadium carbide content.
- Is either steel stainless?
- No, both are non-stainless tool steels with low corrosion resistance.
- Which steel is tougher?
- M4 is tougher overall, making it better suited for impact-heavy tasks.
- Which steel is harder?
- Both can reach similar maximum hardness levels (~64–66 HRC).
Citations:
[1] https://nemoknivesreview.com/2019/06/14/steel-k390-bohlers-microclean-uncanny-blade-material/
[2] https://www.bohler.in/en/products/k390-microclean/
[3] https://www.suppliersonline.com/propertypages/M4.asp
[4] https://www.bladeforums.com/threads/k390-toughness-data-or-comparisons-any-charpy-info.1091253/
[5] https://knifesteelnerds.com/2023/04/11/k390-best-high-wear-resistance-steel/
[6] https://knifesteelnerds.com/2021/10/19/knife-steels-rated-by-a-metallurgist-toughness-edge-retention-and-corrosion-resistance/
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