Understanding Medical Grade Stainless Steel: Properties and Applications

Although stainless steel is widely used in non-industrial fields like food and healthcare due to its corrosion resistance and high strength, it is not completely rust-proof, and different grades of stainless steel vary significantly in performance. 

Thus, not every type of stainless steel is appropriate for healthcare and surgical settings. Currently, only a select few grades are recommended for use in the medical field. 

Please continue reading our article to learn about the characteristics, types, and most popular grades of medical-grade stainless steel, as well as how to choose the right one for medical applications.

What is Medical Grade Stainless Steel?

Medical stainless steel, including surgical stainless steel, is composed of 60-70% iron, at least 16% chromium, 8-10% nickel, and other elements. It is typically classified according to AISI standards, such as AISI 304 and 316L, while ASTM defines its performance. With an austenitic microstructure commonly used for medical applications. Known for its corrosion resistance, durability, and biocompatibility, this material is often shaped using techniques like hot rolling to achieve its final form.

Properties of Medical Stainless Steel

Medical stainless steel is designed to meet the strict demands of healthcare, ensuring safety, durability, and performance. Let’s examine its chemical makeup, as well as its physical and mechanical characteristics.

Chemical Properties

Medical stainless steel’s chemical composition is designed for optimal corrosion resistance, strength, and biocompatibility, ideal for medical use.

Corrosion Resistance

Healthcare stainless steel possesses outstanding corrosion resistance. This is crucial for keeping surgical tools and implants sterile. The high chromium content creates a protective layer that prevents damage from bodily fluids and cleaning agents.

Composition Requirements

1. Chromium (Cr): Must be above 16% to provide excellent corrosion resistance, essential for maintaining sterility in medical environments.
2. Nickel (Ni): Typically around 8-10%, contributing to both strength and enhanced corrosion resistance.
3. Molybdenum (Mo): Present at 2-3%, improving protection against pitting and crevice corrosion.
4. Carbon (C): Must be kept below 0.08% to prevent brittleness and maintain ductility.
5. Phosphorus (P): Limited to below 0.045% to ensure durability and avoid embrittlement.
6. Sulfur (S): Restricted to below 0.03% to preserve the material’s overall durability and biocompatibility.

These regulated amounts ensure the material is secure and efficient for application in healthcare settings.

Physical Properties

The physical properties of medical stainless steel determine its suitability for medical applications. The following ranges for physical properties are typical for medical-grade stainless steel, though they may vary slightly by grade.

• Density: Ranges from 7.93 to 7.98 g/cm³. Higher density improves the durability and feel of medical devices.
• Melting Point: 1400-1450°C. Ensures the material can handle high temperatures during sterilization.
• Thermal Conductivity: About 10-30 W/m·K. Affects how the material handles temperature changes.
• Modulus of Elasticity: Approximately 200 GPa. Reflects the rigidity of the material, crucial for providing support and resisting deformation.
• Specific Heat Capacity: 500 J/kg·K. Shows how well the material maintains stability under temperature fluctuations.

Medical stainless steel’s density affects durability. High melting point resists sterilization. Thermal conductivity impacts temperature response. Elastic modulus shows stiffness. Specific heat capacity indicates temperature stability. These ensure reliable medical performance.

Mechanical Properties

The mechanical properties of medical stainless steel are essential for ensuring durability and functionality under stress in medical applications.

Strength and Hardness

Medical stainless steel offers high strength and hardness, which are crucial for withstanding mechanical stress and wear. Its robust properties ensure that surgical instruments and implants maintain their shape and function under pressure. This strength helps prevent deformation and damage during use, while hardness contributes to the material’s durability and resistance to abrasion.

Density

Medical stainless steel has a relatively high density, typically ranging from 7.7 to 8.0 g/cm³. This contributes to its strength and stability, ensuring that medical devices and implants are durable and provide a solid feel, which is important for their performance and reliability in medical applications.

Temperature Resistance

Medical stainless steel has good temperature resistance, allowing it to withstand high and low temperatures without losing its properties. This makes it suitable for autoclaving and other sterilization processes that involve extreme heat. Its stability under varying temperatures ensures reliable performance in different medical environments.

Ductility and Toughness

Medical stainless steel exhibits high ductility and toughness. It can deform without breaking, which helps it absorb impact and stress. This property ensures that surgical instruments and implants can endure the demands of medical procedures and remain functional over time.

Medical stainless steel is corrosion-resistant, highly biocompatible, and has excellent mechanical properties, including high strength, hardness, and temperature resistance. This makes it essential in the medical industry.

Other Important Properties of Medical Grade Stainless Steel

Biocompatibility

Medical stainless steel is highly biocompatible. It is designed to interact safely with human tissues without causing adverse reactions such as inflammation, allergic reactions, or toxicity. This renders it appropriate for implants and devices that stay inside the body for long durations.

The material’s impact on the body depends on the following factors:

• Composition: The specific alloy and its chemical elements.
• Surface Finish: The smoothness and cleanliness of the material’s surface.
• Usage Duration: The length of time the material remains in contact with tissues.
• Interaction with Body Fluids: How the material interacts with bodily fluids and conditions.

To ensure safety, these specifications must be followed closely. Additionally, proper sterilization and handling during manufacturing and use play a crucial role in maintaining biocompatibility and preventing potential issues.

How Medical Grade Stainless Steel is Produced

Medical-grade stainless steel undergoes a precise production process to meet the strict standards required for medical applications. Unlike standard stainless steel, medical grade variants require additional steps to ensure biocompatibility and performance.

Production Process:

1. Raw Material Selection: Only high-purity raw materials are used to minimize contaminants that could affect biocompatibility.
2. Controlled Melting: The steel is melted in a vacuum or argon-oxygen decarburization (AOD) furnace to reduce carbon and other impurities, ensuring a high-quality alloy.
3. Alloying and Mixing: Specific alloying elements are added in precise amounts to achieve the desired mechanical properties and corrosion resistance.
4. Casting and Forming: The steel is cast into shapes and then formed using methods like hot rolling or forging. This step is closely monitored to maintain uniform properties.
5. Heat Treatment: Medical grade stainless steel undergoes heat treatment processes to enhance its mechanical properties and remove residual stresses. This ensures optimal performance and stability.
6. Surface Finishing: The steel’s surface is polished and cleaned to remove any contaminants and achieve a smooth finish, crucial for hygiene and biocompatibility.
7. Quality Testing: Rigorous testing is conducted to ensure the steel meets medical standards, including tests for corrosion resistance, mechanical properties, and biocompatibility.

These additional steps ensure that medical stainless steel is safe, reliable, and suitable for use in medical devices and implants.

Medical Stainless Steel Processing Properties

Medical-grade stainless steel can undergo heat treatment, cold working, machining, welding, forming, and surface treatments, thanks to its excellent ductility and formability. These processes ensure the material meets the high standards required for medical devices.

Heat Treatment

• Purpose: To adjust hardness, strength, and corrosion resistance.
• Methods: Solution annealing, followed by rapid cooling, is commonly used for grades like 316L to restore ductility and enhance corrosion resistance.
• Considerations: Care must be taken to avoid carbide precipitation, which can reduce corrosion resistance.

Cold Working

• Purpose:
  • Increase strength and hardness, essential for implants.
  • Improve mechanical properties such as ductility.
• Methods:
  • Cold Rolling: Produces high-strength sheets with about 20% deformation.
  • Cold Drawing: Improves strength in rods or tubes.
  • Cold Forging: Used to shape complex medical components.
• Considerations:
  • Control deformation to around 20% to balance strength and ductility.
  • Avoid stress concentration to prevent corrosion.
  • Post-process heat treatment may be needed to relieve stress and enhance corrosion resistance.

Machinability

• Purpose: To shape the material into precise medical components.
• Methods: Sharp cutting tools with low cutting speeds are essential, especially for 316L, which is harder to machine than 304.
• Considerations: Avoid excessive work hardening; maintain sharp tools to prevent tool wear.

Weldability

• Purpose: To join parts for medical devices like implants or surgical instruments.
• Methods: TIG and laser welding are common, especially for 316L stainless steel.
• Considerations: Post-weld heat treatment may be necessary to relieve stress and prevent corrosion in welded joints.

Formability

• Purpose: To create complex shapes needed for medical tools and implants.
• Methods: Deep drawing, bending, and stamping are common techniques.
• Considerations: Ensure uniform deformation to avoid weakening the structure.

Surface Treatment Characteristics

• Purpose: Enhance corrosion resistance, biocompatibility, and smoothness for medical applications.
• Methods:
  • Electropolishing: Smoothens the surface and increases corrosion resistance.
  • Passivation: Removes free iron and enhances the chromium oxide layer, improving corrosion resistance.
  • Blasting: Grit or bead blasting can be used to achieve a matte finish by removing surface impurities.
• Considerations: Surface treatments must be free from contaminants to ensure biocompatibility and sterility. Electropolishing also reduces surface roughness, which is critical for implants to prevent bacterial adhesion.

The Difference Between Medical Grade Stainless Steel (Surgical Steel) and Ordinary Stainless Steel

Medical-grade stainless steel and ordinary stainless steel may seem similar at first glance, but they differ significantly in both composition and properties.

So, what are the most notable differences between medical-grade stainless steel and ordinary stainless steel?

Composition:

1. Corrosion Resistance: Medical-grade stainless steel minimizes metal ion release and prevents intergranular and stress corrosion, ensuring implant safety in the human body.
2. Elemental Composition: It contains higher levels of nickel (Ni) and chromium (Cr), reaching the upper limits of ordinary stainless steel to enhance corrosion resistance.
3. Impurities: Medical stainless steel has stricter limits on sulfur (S) and phosphorus (P), while ordinary stainless steel has relatively higher levels of these impurities.
4. Non-metallic Inclusions: Medical stainless steel requires strict control on inclusion size (<115-grade for fine, <1-grade for coarse), whereas ordinary stainless steel does not impose strict standards.
5. Ordinary Stainless Steel: Contains lower levels of chromium and nickel, with no strict control over inclusion size and higher impurity levels than medical-grade steel.

Corrosion Resistance:

• Medical-grade stainless steel offers superior resistance to corrosion, crucial for sterile environments.
• Ordinary stainless steel may corrode more easily in harsh conditions.

Biocompatibility:

• Healthcare-grade stainless steel is engineered to be compatible with biological tissues, indicating that it can safely engage with human tissue.
• Ordinary stainless steel is not necessarily biocompatible and may cause adverse reactions if used in medical settings.

Processing:

• Healthcare-grade stainless steel goes through strict processing to guarantee it complies with demanding criteria for cleanliness and effectiveness.
• Ordinary stainless steel is processed for general use and does not require the same level of refinement.

Applications:

• Healthcare-grade stainless steel is utilized in medical tools, implants, and other essential healthcare devices.
• Ordinary stainless steel is used in everyday items like kitchenware, construction materials, and industrial equipment.

Cost:

• Medical-grade stainless steel is typically more expensive due to its specialized properties and manufacturing process.
• Ordinary stainless steel is more cost-effective for non-medical applications.

Type of Medical Stainless Steels

Medical stainless steels come in various types, each tailored to specific applications in the medical field. Understanding these types helps ensure the right material is chosen for optimal performance and safety in medical devices and implants.

Implant Grade Stainless Steel

Used for implants that remain in the body long-term, such as with 316L stainless steel. It offers high biocompatibility and resistance to body fluids, ensuring safety and durability within the body.

Surgical Instrument Grade Stainless Steel

Designed for tools used in surgeries, like 420 stainless steel. This grade provides exceptional strength, sharpness, and corrosion resistance, ensuring precision and reliability during medical procedures.

General Medical Device Grade Stainless Steel

Suitable for various medical devices, with 304 stainless steel being a common choice. It balances cost and performance with its corrosion resistance and durability, making it versatile for different medical applications.

Dental Instrument Grade Stainless Steel

Specifically for dental tools, often using 440C stainless steel. This type provides excellent hardness and corrosion resistance, ensuring that the tools remain effective and reliable in dental procedures.

Medical stainless steels are designed for specific uses. Each type is selected for its special qualities to guarantee the best performance and safety in medical uses. For more information on grades and their applications, please continue reading.

Read this article to learn about the full classification of stainless steel!

Common Medical Stainless Steel Grade

For medical use, stainless steel is available in specific grades, with 316L and 304 being the most common due to their corrosion resistance and biocompatibility. The 440 and 420 series are also used in certain instruments for their hardness and wear resistance.

SAE 304 and 304L

SAE 304: Known for its excellent corrosion resistance, SAE 304 is chemically inert, meaning it does not react during sanitation or sterilization processes. It is durable and can withstand the rigors of both surgical procedures and everyday use, making it a reliable choice for medical devices.

SAE 304L: This low-carbon version of SAE 304 has better corrosion resistance and reduced sensitization. It is ideal for medical devices and implants that contact the body. SAE 304L can be welded without annealing, making it suitable for bone screws and plates. This minimizes harmful carbide precipitation during welding.

SAE 316 and 316L

SAE 316: This grade has excellent corrosion resistance, especially in chloride environments. It is used in medical implants and surgical tools for its durability and resistance to chemicals. SAE 316 can withstand autoclaving, chemical disinfectants, and other sterilization methods, ensuring device safety and effectiveness.

SAE 316L: The low-carbon version of SAE 316, SAE 316L, offers even greater corrosion resistance and fewer welding defects. Besides its high strength and resistance to sterilization, SAE 316L also maintains stability in high-chloride environments, making it ideal for demanding medical applications.

SAE 410

SAE 410: This type of stainless steel is recognized for its toughness and durability. SAE 410 is used where high wear resistance and moderate corrosion resistance are needed, such as in some surgical tools and dental instruments. It also offers good machinability and cost-effectiveness.

SAE 420

SAE 420: This martensitic stainless steel offers higher hardness and wear resistance compared to SAE 410. It is used in areas needing both strong corrosion resistance and high durability, like surgical blades and dental tools. SAE 420 also features excellent cleanliness and uniformity, enhancing its performance and reliability.

Implant Grades in Different Countries

Implant-grade stainless steel is regulated by various standards in different countries to ensure safety and reliability. The table below outlines key standards and applicable grades from the U.S., Japan, the EU, and China.

Common Applications of Medical Stainless Steel

Medical stainless steels are chosen for specific applications based on their unique properties. Here’s a breakdown of their common uses.

Future Developments in Medical Stainless Steel

The area of medical stainless steel is changing quickly due to new technologies and rising global demand. Key future trends include:

1. Market Growth and Demand: The global market for medical stainless steel is expected to grow significantly, driven by increased demand for medical devices, particularly after the COVID-19 pandemic.
2. Advancements in 3D Printing and Customization: The integration of stainless steel with 3D printing technology is facilitating the production of customized and complex medical implants, improving patient-specific solutions.
3. Enhanced Materials and Properties: Research is focusing on developing stainless steels with nano-structures for improved strength, corrosion resistance, and biocompatibility, as well as exploring nickel-free alloys to address allergy concerns.
4. Sustainable Production: Efforts are underway to make production more sustainable by reducing energy consumption, minimizing waste, and using recycled materials.
5. Smart and Functional Materials: The incorporation of smart technologies and responsive materials into stainless steel is being explored to enhance device functionality and patient outcomes.

These developments will enhance the performance, safety, and sustainability of medical stainless steel, benefiting the healthcare industry and patients.

What Is Surgical Steel Made Of?

Surgical steel typically contains 60-70% iron, at least 16% chromium, 8-10% nickel, and sometimes molybdenum for corrosion resistance.

Is Stainless Steel The Same As Surgical Steel?

No, surgical steel is a subset of stainless steel specifically refined for medical applications with enhanced corrosion resistance and biocompatibility.

What Grade Is Surgical Stainless Steel?

Common grades are 316L and 304, with 316L being preferred for implants due to its higher corrosion resistance.

Is Medical Grade Stainless Steel Magnetic?

316L stainless steel is non-magnetic, while some other medical-grade steels, like 400 series, can be slightly magnetic.

What Is The Difference Between 304 And 316 Medical Stainless Steel?

The main difference is that 316 contains molybdenum (2-3%), which provides better corrosion resistance, especially against chlorides, making it more suitable for implants. 304 is more cost-effective but has lower corrosion resistance.

Final Thoughts on Medical Stainless Steel

Choosing the right stainless steel for medical applications is crucial. Understanding the unique properties and grades of medical-grade stainless steel is essential for ensuring safety, durability, and performance in healthcare environments. As the industry evolves, advanced technologies are shaping the future of medical stainless steel.

At SteelPRO Group, our stainless steel meets SAE, AISI, and ASTM standards recognized in the medical field. We can tailor our products to fit your exact requirements. For more information, visit our blog page and contact our experts for the best quote!