The Ultimate Guide to Stainless Steel for Mechanical Designers & Product Developers

Stainless steel is the unsung hero in the world of materials. Imagine a robust, gleaming knight that resists the relentless advances of rust and decay—this is stainless steel in a nutshell. Its journey began in the early 20th century when industrial innovators sought materials that could endure the harshest conditions and stand the test of time. Stainless steel, born from necessity and innovation, quickly became indispensable in various applications. From the grandeur of towering skyscrapers and majestic bridges to the everyday utility of kitchen sinks and the precision of surgical scalpels, stainless steel has proven its worth. As a custom parts manufacturing expert, we understand the importance of choosing the right materials. We share this knowledge to help mechanical designers and product developers harness the power of stainless steel in their projects, ensuring durability, reliability, and superior performance.

factory stainless steel automotive cnc custom part

History of Stainless Steel

The story of stainless steel begins in 1798 when Louis Vauquelin introduced chromium to the French Academy. By the early 1800s, British scientists observed the oxidation resistance of chromium-iron alloys. In 1821, Pierre Berthier noted their corrosion resistance, suggesting their use in cutlery.

In the 1840s, Sheffield steelmakers in Britain and Krupp in Germany began producing chromium steel, with Krupp using it for cannons. By 1869, J. Baur of the Chrome Steel Works of Brooklyn was making chromium-containing steel in the US for bridge construction.

German chemist Hans Goldschmidt developed the carbon-free chromium production process in the late 1890s. By 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel (18/8 or AISI type 304).

Around the same time, in the US, Christian Dantsizen of General Electric and Frederick Becket at Union Carbide were developing ferritic stainless steel. This period marked the dawn of the versatile and durable stainless steel we use today.

Definition of Stainless Steel

What is Stainless Steel?

Stainless steel is an alloy primarily made of iron, with a minimum of 10.5% chromium content by mass. This chromium content allows stainless steel to form a protective layer of chromium oxide on its surface, which prevents rusting and corrosion. Additionally, stainless steel often includes other elements such as nickel, molybdenum, and nitrogen, which further enhance its strength, durability, and resistance to various forms of corrosion. Its unique properties make stainless steel a preferred material for applications ranging from kitchen utensils and medical instruments to construction materials and industrial equipment.

What is stainless steel is made of?

Stainless steel is primarily composed of iron, with at least 10.5% chromium by mass. This essential chromium content allows for the formation of a passive layer of chromium oxide on the surface, which prevents further corrosion. Alongside iron and chromium, stainless steel often includes other elements such as nickel, molybdenum, and nitrogen. These additional elements enhance its mechanical properties and corrosion resistance. For instance, nickel increases toughness and formability, while molybdenum boosts resistance to pitting and crevice corrosion. This unique combination of elements makes stainless steel incredibly versatile and durable, suitable for a wide range of applications.


Composition of Stainless Steel

Primary Components

Stainless steel is a complex alloy, but its primary components are iron, chromium, nickel, and molybdenum. Each of these elements plays a critical role in defining the alloy’s properties.

Role of Chromium

Chromium is the star of the show, providing the essential corrosion resistance that characterizes stainless steel. When exposed to oxygen, chromium forms a thin, inert layer of chromium oxide on the surface, shielding the underlying metal from further oxidation.

Importance of Nickel, Molybdenum, and Other Elements

Nickel enhances the alloy’s toughness and ductility, making it easier to work with. Molybdenum improves resistance to pitting and crevice corrosion, particularly in chloride environments. Other elements, such as manganese, nitrogen, and silicon, are added in smaller quantities to fine-tune the alloy’s properties for specific applications.

Alloying Elements and Their Effects

The addition of various alloying elements can significantly alter the properties of stainless steel. For instance, increasing the carbon content can improve hardness and strength, but may reduce corrosion resistance. Balancing these elements allows for the creation of different grades and types of stainless steel, each optimized for particular uses.

turning lathe CNC stainless steel 304 prototype

Types of Stainless Steel

Stainless steel can be classified into five main families, each differentiated by its crystalline structure and unique properties. These families are: Austenitic, Ferritic, Martensitic, Duplex, and Precipitation Hardening.

Austenitic stainless steel, which makes up about two-thirds of all stainless steel production, possesses a face-centered cubic crystal structure. This structure, maintained at all temperatures due to alloying with nickel, manganese, and nitrogen, ensures that austenitic steels are not hardenable by heat treatment. The 200 and 300 series are the most common, with Type 304 (18/8 stainless) being the most widely used. Type 316, which includes molybdenum, provides enhanced resistance to corrosion from chloride ions, making it suitable for marine and chemical applications.

Ferritic stainless steel has a body-centered cubic structure similar to carbon steel, containing 10.5% to 27% chromium with little to no nickel. This type is known for its good ductility and corrosion resistance. Types 409 and 430 are commonly used in automotive exhausts and architectural applications. These steels are magnetic and not hardenable by heat treatment but can be strengthened through cold working.

Martensitic stainless steel offers a wide range of properties, including good hardness and strength. These steels can be heat treated and are used in applications requiring high wear resistance, such as cutlery, turbine blades, and surgical instruments. They are magnetic and less corrosion-resistant compared to austenitic and ferritic steels. The addition of elements like nickel improves their toughness, as seen in grades used for hydroelectric turbine components.

Duplex stainless steel features a mixed microstructure of austenite and ferrite, typically in a 50:50 ratio. This blend provides higher strength and improved resistance to stress corrosion cracking compared to austenitic and ferritic steels. Duplex stainless steels, such as lean duplex and super duplex, are cost-effective and used in industries like oil and gas, paper manufacturing, and structural applications.

Precipitation hardening stainless steels combine high strength and corrosion resistance. These steels can be heat treated to form precipitates that enhance their mechanical properties. Notable grades include 17-4 PH, used in aerospace, which undergoes solution treatment and aging to achieve exceptional yield strength and toughness. Semi-austenitic grades, such as 17-7 PH, and superalloy A286, used in high-temperature applications, highlight the versatility and advanced performance of precipitation hardening steels.

Grades of Stainless Steel

The grade of stainless steel indicates the specific chemical composition and mechanical properties of the alloy. Different grades are identified by numbers (e.g., 304, 316) and signify the varying proportions of elements like chromium, nickel, molybdenum, and others. These variations affect the steel’s characteristics, such as corrosion resistance, strength, temperature tolerance, and suitability for specific applications.

The stainless steel grading system attempts to differentiate alloys by toughness, corrosion resistance, magnetic properties, and composition. The first grading system was developed by the SAE (US Society of Automotive Engineers) using a 3-digit code, with grades belonging to 100, 200, 300, 400, 500, and 600 series classifications. This system was enhanced by ASTM (American Society for Testing and Materials) to a 6-digit code, known as UNS (Unified Number System) identifiers. In most cases, UNS identifiers share the same first three numbers as their AISI (American Iron and Steel Institute) equivalent.

For example, grade 304 is widely used for its excellent corrosion resistance and versatility, while grade 316 contains more molybdenum, offering superior resistance to chlorides and is often used in marine environments. Understanding the grade helps in selecting the right type of stainless steel for a particular use.

Table 1: Austenitic Stainless Steel

SAE UNS Equivalent
384 S38400
348 S34800
347 S34700
330 N08330
329 S32900
321 S32100
317L S31703
317 S31700
316N S31651
316F S31620
316L S31603
316 S31600
314 S31400
310S S31008
310 S31000
309S S30908
309 S30900
308 S30800
305 S30500
304N S30451
304Cu S30430
304L S30403
304 S30400
303Se S30323
303 S30300
302B S30215
302 S30200
301 S30100
254 S31254
205 S20500
202 S20200
201 S20100

Table 2: Ferritic Stainless Steel

SAE UNS Equivalent
446 S44600
442 S44200
436 S43600
434 S43400
430FSe S43023
430F S43020
430 S43000
429 S42900
409 S40900
405 S40500

Table 3: Martensitic Stainless Steel

SAE UNS Equivalent
440C S44004
440B S44003
440A S44002
431 S41623
422 S42200
420F S42020
420 S42000
416Se S41623
416 S41600
414 S41400
410 S41000
403 S40300

Table 4: Heat Resisting Stainless Steel

SAE UNS Equivalent
Super duplex Super duplex
Duplex Duplex
502 S50200
501 S50100
2507 S32750
2205

Properties of Stainless Steel

Corrosion Resistance Stainless steel stands out due to its exceptional resistance to corrosion. This property is primarily due to its chromium content, which forms a protective layer on the surface, shielding deeper layers from oxidation. Enhancing corrosion resistance further can be achieved by increasing chromium to over 11%, adding nickel to at least 8%, and incorporating molybdenum, which also improves resistance to pitting.

Strength The most common stainless steel type, 304, boasts a tensile yield strength of around 210 MPa (30,000 psi) in its annealed state. Cold working can significantly increase its strength to about 1,050 MPa (153,000 psi). The strongest stainless steels are the precipitation hardening types like 17-4 PH, which can reach tensile yield strengths up to 1,730 MPa (251,000 psi) after heat treatment.

Melting Point Stainless steel has a high melting point, typical of steel, ranging between 1,400 and 1,530 °C (2,550 to 2,790 °F), depending on the alloy composition. This range is higher than that of metals like aluminum and copper.

Conductivity Stainless steels are poor conductors of electricity compared to metals like copper. The electrical contact resistance (ECR) is relatively high due to the dense protective oxide layer. Although copper alloys and nickel-coated connectors are preferred for applications requiring low ECR, stainless steel is used where corrosion resistance and high-temperature performance are crucial.

Magnetism Magnetism in stainless steel varies by type: martensitic, duplex, and ferritic stainless steels are magnetic, whereas austenitic stainless steel is typically non-magnetic. Ferritic steel’s magnetism comes from its body-centered cubic structure. Austenitic steels can become slightly magnetic through work hardening, especially along edges after bending or cutting.

Wear Galling, a severe form of adhesive wear, can occur in stainless steel fasteners under heavy pressure and relative motion. This issue can be mitigated by using dissimilar materials or lubricating threaded joints. Nitronic 60, an alloy with manganese, silicon, and nitrogen, shows reduced galling tendencies.

Density The density of stainless steel varies between 7.5 to 8.0 g/cm³, depending on the specific alloy composition.


Chemical Properties of Stainless Steel

Property Notes
Corrosion Resistance Ranges from excellent in non-salt environments to good in salt environments, and varies in hot/salt/acidic environments based on grade.
Chemical Resistance Generally high to very high; specific grades offer resilience in challenging environments, such as acidic or alkaline conditions.
Heat Resistance High to very high, withstanding temperatures up to 2,000 °C for certain grades like 330.
Cold Resistance Excellent, with non-stabilized 304-grade performing well down to -200 °C.
Sensitization Corrosion Severe in 304 and 316 grades at welds; mitigated by using low heat and low-carbon fillers.
Oxidation Resistance Maintains a protective oxide layer in high-temperature environments, enhancing longevity.
Chloride Stress Corrosion Cracking Prone to cracking in chloride environments, especially in high-stress applications; duplex grades offer better resistance.
Acid Resistance Superior in grades like 316L, which contains molybdenum to resist sulfuric, hydrochloric, acetic, and tartaric acids.
Alkaline Resistance Performs well in mildly alkaline environments; higher grades withstand stronger alkaline conditions.
Pitting Resistance Enhanced in grades with higher chromium, molybdenum, and nitrogen contents, such as 2205 duplex stainless steel.
Crevice Corrosion Grades like 316L and super duplex stainless steels are designed to resist crevice corrosion in tight spaces.
Galvanic Corrosion Occurs when in contact with more noble metals; requires careful material selection and isolation techniques.
Passivation Forms a self-healing oxide layer that protects against environmental factors, maintaining stainless properties.


Applications of Stainless Steel

  1. Marine Industry Stainless steel is crucial in the marine industry due to its excellent resistance to rust and corrosion, even in saltwater environments. It’s used in boat fittings, propellers, and marine fasteners. The material’s durability and resistance to harsh marine conditions make it indispensable for ships, submarines, and other sea-faring vessels.
  2. Jewelry Stainless steel is increasingly popular in jewelry making due to its strength, durability, and hypoallergenic properties. Stainless steel jewelry can withstand daily wear and tear while maintaining its luster. It’s often used in watches, bracelets, necklaces, and rings. Its resistance to tarnishing and rust makes it an excellent choice for accessories meant to last a lifetime.
  3. Industrial Machinery Stainless steel is essential in industrial settings due to its strength and resistance to wear and tear. It’s used in the production of automotive parts, industrial machinery, and chemical plant construction, where exposure to corrosive materials is common. Its durability ensures long-lasting machinery and equipment.
  4. Construction Sector In the construction industry, stainless steel is used for building structures like skyscrapers and bridges. It provides structural integrity and a sleek, modern appearance. Stainless steel’s resistance to corrosion makes it ideal for outdoor structures exposed to varying weather conditions.
  5. Heavy Industries Heavy industries, such as mining and oil and gas, rely on stainless steel for equipment that must withstand harsh environments and extreme temperatures. Drill rigs, pipelines, and mining equipment are often made from stainless steel to ensure reliability and safety under demanding conditions.
  6. Household Products and Appliances Stainless steel is a staple in kitchens, where it’s used for sinks, countertops, and a wide range of cookware due to its durability and ease of maintenance. Stainless steel cutlery and utensils are also common in households, valued for their longevity and resistance to staining and rust. Home appliances like refrigerators, ovens, and dishwashers often feature stainless steel exteriors, providing a modern and clean look.
  7. Medical Equipment In the medical industry, stainless steel is used to manufacture surgical instruments, medical devices, and implants. Its non-porous surface prevents bacteria from adhering, making it easy to sterilize and maintain. Stainless steel’s biocompatibility ensures it does not react adversely with bodily tissues, making it ideal for surgical implants and medical tools.
  8. Food Processing In the food industry, stainless steel is extensively used in the production and processing of food. Equipment like mixers, tanks, and conveyors are made from food-grade stainless steel, ensuring the food remains uncontaminated. Its resistance to corrosion from food acids and non-reactive nature help maintain food safety standards. Stainless steel’s ease of cleaning and sterilization makes it perfect for maintaining hygiene required in food processing environments.

Examples of Industrial Applications:

  • Manufacturing Machinery: Used in machines that manufacture automotive parts, electronics, and industrial components.
  • Construction: Utilized in the framework of skyscrapers, bridges, and architectural facades.
  • Heavy Industries: Essential for constructing pipelines, drill rigs, and mining equipment.

Examples of Consumer Applications:

  • Kitchen: Sinks, countertops, cookware, cutlery, and utensils.
  • Home Appliances: Refrigerators, ovens, dishwashers, and microwaves.

Examples of Medical and Food Industry Applications:

  • Medical: Surgical instruments, medical devices, implants, and hospital equipment.
  • Food Industry: Mixers, tanks, conveyors, and other food processing equipment.

Examples of Marine Industry Applications:

  • Marine Equipment: Boat fittings, propellers, and fasteners used in ships and submarines.

Examples of Jewelry Applications:

  • Accessories: Watches, bracelets, necklaces, and rings known for their durability and lasting shine.

Stainless steel’s versatility and exceptional properties make it a cornerstone material in industrial, consumer, medical, jewelry, and marine applications, proving its worth across a wide spectrum of uses.




Advantages of Using Stainless Steel

Advantages of Using Stainless Steel

Durability and Longevity Stainless steel’s long lifespan makes it a superior choice for projects where longevity is crucial. Its resistance to corrosion, even in harsh environments, significantly reduces the need for frequent replacements. For example, stainless steel railings on a coastal property will withstand saltwater spray without rusting, maintaining both functionality and appearance for decades.

Why Stainless Steel Outlasts Other Materials The unique combination of corrosion resistance and mechanical strength ensures that stainless steel components stay robust and attractive. Unlike iron that rusts or plastic that degrades, stainless steel remains steadfast. Imagine a stainless steel kitchen sink: it endures constant exposure to water and cleaning chemicals while retaining its integrity and shine, far outlasting other materials.

Aesthetic Appeal Stainless steel is renowned for its sleek, modern look, making it a popular choice in design and architecture. Its visual appeal is unmatched, bringing a sense of sophistication and cleanliness to any application. For instance, stainless steel appliances in a kitchen not only perform reliably but also elevate the overall aesthetic with their polished surfaces.

Visual Benefits and Finishes Stainless steel offers a range of finishes to suit various design needs. It can be polished to a mirror-like shine, adding a touch of luxury to public sculptures or high-end kitchenware. Alternatively, it can be brushed to create a matte finish that provides a subtle, modern look, perfect for contemporary architecture and interior design.

Environmental Impact In today’s eco-conscious world, the environmental impact of materials is a significant consideration. Stainless steel stands out due to its sustainability credentials.

Recyclability and Sustainability Stainless steel is 100% recyclable, making it an environmentally friendly choice. Its production has a lower environmental impact compared to many other materials. Using stainless steel supports sustainable practices, reducing waste and conserving resources. For example, old stainless steel products, like pipes or appliances, can be melted down and repurposed without loss of quality, contributing to a circular economy.




Challenges and Limitations

Cost Considerations One of the primary challenges with using stainless steel is its initial cost. Stainless steel can be more expensive upfront compared to other materials like mild steel or aluminum.

Initial Investment vs. Long-Term Benefits Despite the higher initial cost, the long-term benefits often outweigh the initial expenses. For example, consider a company deciding between stainless steel and a cheaper alternative for constructing a chemical storage tank. The stainless steel tank will resist corrosion and require fewer repairs and replacements over its lifespan, ultimately saving money and downtime. In contrast, a cheaper material might corrode and degrade faster, necessitating frequent repairs and replacements that add up over time.

Fabrication and Welding Processing stainless steel isn’t straightforward. Its thermal properties and the need for precision can make fabrication and welding particularly challenging.

Difficulties and Solutions in Processing Stainless Steel Welding stainless steel can be tricky because of its tendency to warp and distort under heat. For instance, constructing a stainless steel pipeline for a food processing plant requires precise welding to ensure no contamination or leakage. This process can be challenging due to the metal’s expansion and contraction properties. However, advanced techniques and skilled labor can address these issues. Methods such as TIG (Tungsten Inert Gas) welding offer the precision needed for such applications, ensuring strong, clean welds that maintain the integrity of the material. Additionally, using proper cooling techniques and fixturing can help manage thermal distortion during fabrication, ensuring that the final product meets the necessary specifications.



FAQs

Which is better, 304 or 316 stainless steel?

316 stainless steel is better for corrosion resistance, especially in environments exposed to saltwater or chemicals. However, 304 stainless steel is more affordable and suitable for general-purpose use.

What is the stainless steel grade?

Stainless steel grade refers to the classification based on its composition and properties. Common grades include 304, 316, 409, and 420, each with specific characteristics suited for different applications.

Is 304 or 409 stainless steel better?

304 stainless steel is generally better for corrosion resistance and overall durability. 409 stainless steel is more affordable and used mainly in automotive exhaust systems due to its adequate resistance to high temperatures and lower cost.

Which is better, 304 or 420 stainless steel?

304 stainless steel is better for corrosion resistance and general applications, while 420 stainless steel is harder and more suitable for cutlery and tools due to its higher carbon content and wear resistance.

How do I know if my SS is 304 or 316?

You can identify if your stainless steel is 304 or 316 by performing a chemical analysis or using a magnet. 316 stainless steel contains molybdenum, which improves corrosion resistance but isn’t present in 304. 316 is also less magnetic than 304.

Is 304 or 316 better for kitchen?

316 stainless steel is better for kitchen environments where high corrosion resistance is needed, such as in professional kitchens and coastal areas. For most household kitchen applications, 304 stainless steel is sufficient and more cost-effective.

What is the cheapest grade of stainless steel?

The cheapest grade of stainless steel is typically 409, which is often used in automotive exhaust systems and other applications where high corrosion resistance is not critical.

Conclusion

Stainless steel stands out as a versatile, durable, and aesthetically appealing material that plays a crucial role in various industries. From its unique composition and diverse grades to its exceptional properties and wide-ranging applications, stainless steel continues to be a material of choice for many demanding environments.

At Modo Rapid, we pride ourselves on being an ISO9001 and IATF16949 certified factory, specializing in on-demand manufacturing. If you have any questions related to production feasibility or need expert advice on stainless steel applications, please contact our team. We’re here to help bring your projects to life with precision and quality.

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