CPM REX 45 vs K390 Knife Steel Comparison

CPM REX 45 vs. K390: A Comprehensive Knife Steel Comparison

When it comes to high-performance knife steels, few comparisons generate as much excitement among enthusiasts as CPM REX 45 vs. K390. Both are powder metallurgy (PM) steels renowned for their excellent wear resistance, high hardness capabilities, and impressive edge retention. Yet each steel also brings its own unique blend of elements, heat-treatment nuances, and real-world performance characteristics. In this comprehensive guide, we’ll introduce each steel, break down their chemical compositions, explore ideal use cases, and compare how they stack up in everyday carry (EDC), bushcraft, kitchen, and tactical applications. Whether you’re a collector, an outdoors enthusiast, or a casual user looking to invest in a premium blade, this overview will help you make an informed choice.

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1. CPM REX 45: Overview

CPM REX 45 is a high-speed tool steel produced using Crucible Industries’ powder metallurgy process. It's often compared to steels like CPM M4 but includes notable cobalt (8%) content, which boosts its hardness and “hot hardness” (ability to maintain hardness at high temperatures). REX 45 typically provides:

• Very high hardness potential (approximately 64–66+ HRC)
• Exceptional wear resistance and edge retention, on par with—or exceeding—that of M4
• Lower corrosion resistance than stainless steels (due to minimal chromium at ~4%)

Key Alloy Elements (Approximate Highlights)

• Carbon (1.3%): Contributes to hardness and edge retention.
• Chromium (4.05%): Slightly aids wear resistance but too low for stainless classification.
• Cobalt (8%): Enhances hardness and red hardness, making the steel more stable at higher temperatures.
• Molybdenum (5%) and Tungsten (6.25%): Improve wear resistance and toughness.
• Vanadium (3.05%): Forms very hard vanadium carbides, boosting abrasion resistance.

Why Choose REX 45?

Knife makers and collectors prize CPM REX 45 because it can be hardened to extremely high levels while retaining a fine edge. Though it’s not stainless, it’s sought after by those who want superior wear resistance and excellent cutting performance over a wide range of tasks—from slicing cardboard to more intensive bushcraft chores—provided the user is willing to maintain the blade to prevent corrosion.

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2. K390: Overview

Developed by Böhler-Uddeholm, K390 is another powder metallurgy steel that stands out for its exceptionally high wear resistance, owing to its very high carbon (2.47%) and vanadium (9%) content. It strikes a balance between toughness and hardness, making it a favorite among those who demand a blade that can hold a razor-sharp edge for long periods.

Key Performance Ratings (From Reference Data)

• Corrosion Resistance: 4 (low, not stainless)
• Toughness: 5 (moderate)
• Edge Retention: 9 (extremely high)
• Ease of Sharpening: 3 (challenging, but not the worst among “super steels”)

Key Alloy Elements (Approximate Highlights)

• Carbon (2.47%): Extremely high carbon for maximum hardness and outstanding wear resistance.
• Chromium (4.2%): Helps slightly with wear resistance but too low for stainless performance.
• Vanadium (9%): Contributes to abundant vanadium carbides, dramatically increasing edge retention.
• Molybdenum (3.8%) and Tungsten (1%): Further enhance wear resistance and hardness.
• Cobalt (2%): Provides additional hardness and temperature stability, though not as high as in REX 45.

Why Choose K390?

K390 is for users who prioritize extraordinary edge retention above all else. From extended wood carving sessions in bushcraft to repeated cardboard breakdown in industrial settings, K390 is known for its ability to stay sharp for extended periods. Like REX 45, it’s not stainless and needs care to prevent rust.

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3. Typical Hardness (Rockwell C) Ranges

• CPM REX 45: Most makers target around 64–66 HRC. It can be pushed higher, but 64–65 HRC is common to balance durability and high wear resistance.
• K390: Heat treats typically range 62–64 HRC, though some custom treatments can nudge it higher. Its sweet spot for daily and outdoor use is usually in the mid-60s HRC.

Practical Implication: At these elevated hardness levels, both steels may be more prone to chipping if the edge geometry is extremely thin or if the blade is subjected to sudden impacts. Users should ensure the edge geometry and heat treat match the intended task.

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4. Detailed Alloy Element Effects

Below is a brief overview of how some key elements affect these steels:

• Carbon

  • REX 45 (1.3%), K390 (2.47%)
  • Higher carbon content increases potential hardness and wear resistance. K390’s higher carbon significantly boosts its edge retention.

• Chromium

  • REX 45 (4.05%), K390 (4.2%)
  • Both steels are far below the ~13% chromium needed for stainless classification, so they require consistent maintenance to prevent rust.

• Vanadium

  • REX 45 (3.05%), K390 (9%)
  • Responsible for forming tough vanadium carbides. K390’s remarkable 9% vanadium content is a main reason for its excellent edge retention.

• Tungsten (W)

  • REX 45 (6.25%), K390 (1%)
  • Tungsten refines the grain structure and boosts hardness. REX 45’s higher tungsten content contributes to its superior hot hardness.

• Cobalt (Co)

  • REX 45 (8%), K390 (2%)
  • Cobalt intensifies the effects of other elements, helping achieve higher hardness. REX 45 can therefore be run very hard without significantly compromising toughness.

• Molybdenum (Mo)

  • REX 45 (5%), K390 (3.8%)
  • Enhances strength, wear resistance, and hardenability in both steels.

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5. Heat-Treatment Nuances and Forging

Both CPM REX 45 and K390 are produced via powder metallurgy, and neither is easy to forge in a traditional blacksmith sense due to their complexity and high alloy content. Most producers rely on carefully controlled processes involving:

• Preheat or normalizing cycles: Minimizes warping or cracking.
• Precise austenitizing temperatures: Typically in the 2100–2200°F (1150–1200°C) range to achieve the best combination of hardness and toughness.
• Multiple tempering cycles: Helps reduce brittleness while maintaining high hardness.

REX 45 requires specialized steps to manage its cobalt content and reduce the risk of decarburization or warping. K390, with its high carbon and vanadium content, also requires precise soak times. These steels are best left to experienced heat-treating professionals.

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6. Patina Formation and Protective Coatings

Because both steels have relatively low chromium levels (~4%), they are not considered stainless. As a result, they can develop a patina over time, particularly when exposed to acidic or corrosive environments (e.g., cutting citrus fruits or onions). Some users enjoy this patina for its appearance and mild protective effect.

• Forced Patina: Users may apply vinegar, mustard, or other mild acids to create an accelerated patina for a rustic look and a bit of added corrosion resistance.
• Coatings/Finishes: Knife makers often offer coatings like DLC, Cerakote, or black oxide on REX 45 or K390 blades to reduce corrosion risk, especially for tactical or survival applications.

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7. Key Performance Characteristics

CPM REX 45

• Edge Retention: Excellent—rivals or surpasses other high-speed steels like M4.
• Toughness: Good for a steel that can reach such high hardness, though not as tough as some lower-alloy steels like 3V.
• Corrosion Resistance: Low—must be cleaned and dried to prevent rust.
• Ease of Sharpening: Challenging—diamond plates or premium abrasives work best.

K390

• Edge Retention: Outstanding (often referenced as 9 out of 10).
• Toughness: Moderate (5/10)—above average for such a high-hardness steel.
• Corrosion Resistance: Low (4/10)—susceptible to rust without proper maintenance.
• Ease of Sharpening: Difficult (3/10)—due to abundant vanadium carbides, diamond or high-quality ceramic stones are recommended.

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8. Side-by-Side Comparison Table

Property	CPM REX 45	K390
Typical Hardness Range (HRC)	64–66	62–64 (can go higher)
Edge Retention (Qualitative)	Excellent	Outstanding (9/10)
Toughness (Qualitative)	Good (for HSS)	Moderate (5/10)
Corrosion Resistance	Low	Low (4/10)
Ease of Sharpening	Difficult	Difficult (3/10)
Cobalt Content	8%	2%
Vanadium Content	3.05%	9%

Note: Numerical ratings for K390 are from the reference dataset. REX 45 is presented qualitatively, as no precise ratings were listed.

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9. Ideal Uses and Application Insights

Everyday Carry (EDC)

• REX 45 and K390 both excel if you want a knife that maintains long-lasting sharpness for slicing, opening packages, or light food prep.
• Their low corrosion resistance means you’ll have to wipe and oil regularly, unlike stainless steels.

Bushcraft/Outdoor/Tactical

• K390 offers superb edge retention for long sessions of wood carving or repetitive cutting.
• REX 45 is also highly capable, especially if you prefer a steel that can be hardened to the mid- to upper-60s HRC range.
• Neither is recommended for extremely wet or salty environments without extra maintenance or coatings.

Kitchen Use

• Both steels can deliver impressive edge retention in a kitchen setting; however, they’re prone to corrosion, so you must dry them thoroughly.
• Enthusiasts who appreciate high-carbon steels may find them appealing for specialized kitchen knives, but routine care is essential.

Survival

• Steels like 3V or AEB-L might be better for heavy chopping and batoning because of their higher toughness and better corrosion resistance.
• K390 or REX 45 can still work if your tasks focus on cutting efficiency and if you can keep the knife clean and dry.

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10. Cost, Availability, and Maintenance

• Cost & Availability

  • Both REX 45 and K390 generally command premium prices.
  • K390 is often used by select manufacturers (e.g., Spyderco) in specialized runs.
  • REX 45 similarly appears in limited-edition or sprint-run knives, which can elevate prices in secondary markets.

• Maintenance

  • Corrosion Prevention: Apply a light coat of oil or rust inhibitor after contact with moisture or acidic materials.
  • Sharpening: Use diamond plates or high-quality ceramic stones; these steels are quite abrasion-resistant due to their high carbide content.

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11. Pros and Cons

CPM REX 45

Pros

• Exceptional wear resistance and edge retention.
• High hardness potential (64–66+ HRC).
• Excellent for fine-edged cutting tasks.

Cons

• Low corrosion resistance—requires diligent maintenance.
• Can be tough to sharpen without specialized abrasives.
• Availability may be limited to specific runs or custom makers.

K390

Pros

• Extreme edge retention (9/10 rating).
• Good balance of hardness and moderate toughness for a “super steel.”
• Excels in heavy or repetitive cutting applications.

Cons

• Not stainless—prone to rust if neglected.
• Sharpening difficulty is high due to its vanadium content.
• Often available in premium or limited-edition lines, driving up cost.

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12. Conclusion and Recommendations

Choosing between CPM REX 45 and K390 largely depends on how much you value absolute edge retention, the hardness range you prefer, and whether you’re willing to maintain a non-stainless steel. Both are high-end powdered metallurgy steels that reward users with outstanding performance when properly heat-treated and cared for.

• K390: Best for those seeking the longest possible edge life, making it ideal for extended wood carving, industrial cardboard breakdown, or other repetitive tasks. Its moderate toughness (5/10) is adequate for most EDC and bushcraft chores.
• CPM REX 45: An excellent alternative if you want a steel that can comfortably be pushed into the higher Rockwell range. Its cobalt content helps achieve a crisp, stable edge, and it still delivers superb wear resistance.

Whichever you choose, keeping the blade clean, dry, and lightly oiled—and sharpening with the right abrasives—will ensure both steels remain top performers. For collectors, they represent some of the most advanced tool steel technology available. For everyday users, their durability and sustained sharpness can be game-changers, provided you’re up for the maintenance they require.

Whether you pick REX 45 or K390, you’ll be investing in a premium material that demonstrates the cutting-edge capabilities of modern metallurgy—pun intended!