CPM S35VN Steel: Properties and Key Applications

CPM S35VN steel is a high-performance stainless steel grade that falls under the category of martensitic stainless steels. It is specifically designed for applications requiring excellent edge retention, corrosion resistance, and toughness. The primary alloying elements in CPM S35VN include carbon (C), chromium (Cr), vanadium (V), and molybdenum (Mo). These elements significantly influence the steel's microstructure and mechanical properties, making it particularly suitable for demanding applications such as knife blades and cutting tools.

Comprehensive Overview

CPM S35VN is a powder metallurgy steel developed by Crucible Industries, known for its superior performance characteristics compared to traditional stainless steels. The steel's composition typically includes around 1.4% carbon, 14% chromium, 3% vanadium, and 2% molybdenum, which contribute to its hardness and wear resistance. The powder metallurgy process enhances the uniformity of the microstructure, resulting in improved toughness and edge retention.

Key Characteristics:
- Edge Retention: The high carbon and vanadium content provide exceptional hardness, allowing for long-lasting sharpness in cutting applications.
- Corrosion Resistance: The chromium content offers good resistance to oxidation and corrosion, making it suitable for use in humid or corrosive environments.
- Toughness: The balanced alloying elements ensure that the steel maintains toughness, reducing the risk of chipping or breaking under stress.

Advantages:
- Excellent wear resistance and edge retention.
- Good corrosion resistance, suitable for various environments.
- Enhanced toughness compared to other high-carbon stainless steels.

Limitations:
- Higher cost compared to standard stainless steels.
- Requires careful heat treatment to achieve optimal properties.

Historically, CPM S35VN has gained popularity in the knife-making community and among manufacturers of high-performance tools due to its unique combination of properties, positioning it as a preferred choice for premium products.

Alternative Names, Standards, and Equivalents

Standard Organization	Designation/Grade	Country/Region of Origin	Notes/Remarks
UNS	S35VN	USA	Closest equivalent to AISI 440C with improved toughness.
AISI/SAE	35VN	USA	Designed for high-performance applications.
ASTM	A240	USA	Standard specification for stainless steel plates.
JIS	-	Japan	No direct equivalent, but similar to other high-performance steels.

While S35VN is often compared to AISI 440C, it offers improved toughness and wear resistance due to its unique composition and manufacturing process. This makes it a more suitable choice for applications where both edge retention and durability are critical.

Key Properties

Chemical Composition

Element (Symbol and Name)	Percentage Range (%)
C (Carbon)	1.35 - 1.45
Cr (Chromium)	14.00 - 15.00
V (Vanadium)	2.75 - 3.25
Mo (Molybdenum)	1.75 - 2.25
Mn (Manganese)	0.50 max
Si (Silicon)	0.50 max
P (Phosphorus)	0.03 max
S (Sulfur)	0.03 max

The primary role of the key alloying elements in CPM S35VN includes:
- Carbon (C): Increases hardness and wear resistance.
- Chromium (Cr): Enhances corrosion resistance and contributes to the formation of a protective oxide layer.
- Vanadium (V): Improves wear resistance and toughness by refining the grain structure.
- Molybdenum (Mo): Enhances corrosion resistance and contributes to strength at elevated temperatures.

Mechanical Properties

Property	Condition/Temper	Test Temperature	Typical Value/Range (Metric)	Typical Value/Range (Imperial)	Reference Standard for Test Method
Tensile Strength	Quenched & Tempered	Room Temp	2000 - 2200 MPa	290 - 320 ksi	ASTM E8
Yield Strength (0.2% offset)	Quenched & Tempered	Room Temp	1800 - 2000 MPa	260 - 290 ksi	ASTM E8
Elongation	Quenched & Tempered	Room Temp	4 - 6%	4 - 6%	ASTM E8
Hardness (HRC)	Quenched & Tempered	Room Temp	58 - 61 HRC	58 - 61 HRC	ASTM E18
Impact Strength	Quenched & Tempered	-40°C (-40°F)	30 - 40 J	22 - 30 ft-lbf	ASTM E23

The combination of high tensile and yield strength, along with good toughness and hardness, makes CPM S35VN suitable for applications that require high mechanical performance, such as knife blades and cutting tools. Its ability to maintain these properties under stress ensures structural integrity in demanding environments.

Physical Properties

Property	Condition/Temperature	Value (Metric)	Value (Imperial)
Density	-	7.75 g/cm³	0.28 lb/in³
Melting Point	-	1450 - 1500 °C	2642 - 2732 °F
Thermal Conductivity	20 °C	25 W/m·K	14.5 BTU·in/h·ft²·°F
Specific Heat Capacity	20 °C	460 J/kg·K	0.11 BTU/lb·°F
Electrical Resistivity	20 °C	0.7 µΩ·m	0.7 µΩ·in

The density and melting point of CPM S35VN indicate its robustness, while its thermal conductivity and specific heat capacity are crucial for applications involving heat treatment and thermal cycling. The electrical resistivity is relatively low, which can be beneficial in certain applications where conductivity is a factor.

Corrosion Resistance

Corrosive Agent	Concentration (%)	Temperature (°C)	Resistance Rating	Notes
Saltwater	3.5	25	Good	Risk of pitting in high salinity.
Acetic Acid	10	25	Fair	Susceptible to localized corrosion.
Chlorides	1	25	Fair	Risk of stress corrosion cracking.
Sulfuric Acid	5	25	Poor	Not recommended for use.

CPM S35VN exhibits good corrosion resistance in various environments, particularly against atmospheric conditions and mild chlorides. However, it is susceptible to pitting and stress corrosion cracking in high-salinity environments and certain acidic conditions. Compared to other stainless steels like AISI 440C and D2, S35VN offers better toughness and edge retention, making it more suitable for applications where both corrosion resistance and mechanical performance are critical.

Heat Resistance

Property/Limit	Temperature (°C)	Temperature (°F)	Remarks
Max Continuous Service Temp	350	662	Suitable for prolonged exposure.
Max Intermittent Service Temp	400	752	Short-term exposure acceptable.
Scaling Temperature	600	1112	Risk of oxidation beyond this temp.

At elevated temperatures, CPM S35VN maintains its mechanical properties well, but prolonged exposure above 350 °C (662 °F) can lead to oxidation and scaling. It is essential to consider these limits in applications involving heat, as they can affect the steel's performance and longevity.

Fabrication Properties

Weldability

Welding Process	Recommended Filler Metal (AWS Classification)	Typical Shielding Gas/Flux	Notes
TIG	ER308L	Argon	Preheat recommended.
MIG	ER308L	Argon + CO2 mix	Post-weld heat treatment may be necessary.

Welding CPM S35VN requires careful consideration due to its high carbon content, which can lead to cracking if not properly managed. Preheating before welding and post-weld heat treatment are recommended to relieve stresses and improve toughness.

Machinability

Machining Parameter	CPM S35VN	AISI 1212	Notes/Tips
Relative Machinability Index	60%	100%	Requires high-quality tooling.
Typical Cutting Speed (Turning)	30 m/min	60 m/min	Use carbide tools for best results.

Machining CPM S35VN can be challenging due to its hardness. It is advisable to use high-speed steel or carbide tools and to maintain optimal cutting speeds to achieve the best results.

Formability

CPM S35VN is not particularly known for its formability due to its high hardness and strength. Cold forming is limited, and hot forming is generally preferred to avoid cracking. The steel exhibits work hardening, which can complicate forming processes.

Heat Treatment

Treatment Process	Temperature Range (°C/°F)	Typical Soaking Time	Cooling Method	Primary Purpose / Expected Result
Annealing	800 - 900 / 1472 - 1652	1 - 2 hours	Air	Reduce hardness, improve toughness.
Quenching	1000 - 1100 / 1832 - 2012	30 minutes	Oil	Achieve maximum hardness.
Tempering	200 - 300 / 392 - 572	1 hour	Air	Reduce brittleness, enhance toughness.

The heat treatment process for CPM S35VN involves austenitizing, quenching, and tempering. This sequence is crucial for achieving the desired balance of hardness and toughness. The metallurgical transformations during these treatments significantly affect the microstructure, leading to improved performance characteristics.

Typical Applications and End Uses

Industry/Sector	Specific Application Example	Key Steel Properties Utilized in this Application	Reason for Selection
Knife Manufacturing	High-end folding knives	Excellent edge retention, corrosion resistance	Performance and durability
Tool Making	Cutting tools	High hardness, wear resistance	Longevity and effectiveness
Aerospace	Component parts	Toughness, heat resistance	Safety and reliability

Other applications include:
- Surgical instruments
- Industrial blades
- High-performance outdoor gear

CPM S35VN is chosen for applications where a combination of edge retention, toughness, and corrosion resistance is critical. Its performance in demanding environments makes it a preferred choice for high-end products.

Important Considerations, Selection Criteria, and Further Insights

Feature/Property	CPM S35VN	AISI 440C	D2	Brief Pro/Con or Trade-off Note
Key Mechanical Property	High hardness	Moderate hardness	High hardness	S35VN offers better toughness.
Key Corrosion Aspect	Good	Fair	Poor	S35VN is more corrosion resistant.
Weldability	Moderate	Good	Poor	S35VN requires careful welding.
Machinability	Moderate	Good	Poor	S35VN is harder to machine.
Formability	Poor	Fair	Poor	Limited forming capabilities.
Approx. Relative Cost	High	Moderate	Low	S35VN is more expensive.
Typical Availability	Moderate	High	High	S35VN may be less available.

When selecting CPM S35VN, considerations include its cost-effectiveness, availability, and specific application requirements. While it may be more expensive than alternatives, its superior performance in edge retention and corrosion resistance justifies the investment for high-performance applications. Additionally, its unique properties make it suitable for niche uses where traditional steels may fall short.

In summary, CPM S35VN steel stands out for its exceptional balance of hardness, toughness, and corrosion resistance, making it a top choice for demanding applications in various industries.