What are the various grading systems?

The grade of stainless steel pipe is governed by several established systems.

The Society of Automotive Engineers (SAE) employs a three-digit nomenclature that highlights the primary alloying elements and carbon content.

Alternatively, the American Society for Testing and Materials (ASTM) utilizes a six-digit system, which begins with 'A' to denote ferrous materials and incorporates additional specifications for various grades.

Understanding these grading systems is crucial for ensuring compliance and selecting appropriate stainless steel for specific applications.

The Society of Automotive Engineers 3-Digit System

Established by the Society of Automotive Engineers (SAE), the three-digit grading system classifies stainless steel based on its primary alloying elements and carbon content, facilitating standardized identification and specification across various applications. The SAE standards overview provides a clear framework for understanding the composition and performance characteristics of various  grape of stainless steel pipe , particularly relevant in the automotive industry usage.

Each grade number begins with a digit representing the primary alloying element—typically chromium or nickel—followed by two digits indicating the carbon percentage. This system allows for precise grade selection criteria, ensuring that engineers choose materials that meet specific corrosion resistance factors and mechanical performance requirements for their applications.

For instance, austenitic grade of stainless steel pipe like 304 and 316 are favored for their excellent corrosion resistance, making them suitable for parts exposed to harsh environments. Conversely, martensitic grade of stainless steel pipe may be selected for applications requiring higher strength and wear resistance.

Understanding the SAE three-digit system is essential for professionals in the field, enabling informed decisions about stainless steel applications across diverse sectors, including automotive, aerospace, and medical industries.

The American Society for Testing and Materials (ASTM) 6-Digit System

Utilizing a six-digit grading system developed by the American Society for Testing and Materials (ASTM) facilitates a comprehensive classification of stainless steel, beginning with an 'A' to denote ferrous materials and followed by specific numerical codes that indicate the stainless steel tube material's composition and properties.

This ASTM standards overview is vital for ensuring consistency across various applications in the industry.

The first two digits of the grade signify the primary alloying elements, while the subsequent digits detail additional elements and specific characteristics, aligning with robust stainless steel specifications.

Adhering to material compliance guidelines, stainles steel pipe manufacturers can ensure their products meet industry requirements, ultimately enhancing safety and performance.

Testing procedures explained in ASTM standards include mechanical, chemical, and corrosion resistance tests, which are critical for validating the integrity of stainless steel grades.

Furthermore, grade selection criteria are essential for engineers and designers to make informed decisions based on environmental conditions and mechanical demands.

This systematic approach fosters a sense of belonging within the engineering community, as professionals can rely on standardized classifications to communicate effectively and achieve optimal outcomes in their projects.

The American Society for Testing and Materials (ASTM) 6-Digit System

Adopting a six-digit grading system, the American Society for Testing and Materials (ASTM) provides a standardized method for classifying stainless steel, beginning with an 'A' to denote ferrous materials. This ASTM standards overview is crucial, as it defines compliance guidelines that ensure materials meet specific performance criteria.

The six-digit system enhances the grading system's importance by allowing for precise identification of alloy compositions and properties, essential for stainless steel applications across various industries, including construction, automotive, and medical.

Each grade within this system is formulated based on rigorous testing methodologies explained in ASTM documents, which outline how materials are evaluated for strength, corrosion resistance, and other critical properties. This systematic approach fosters consistency and reliability in material selection, ensuring that engineers and manufacturers can make informed choices aligned with industry standards.

Moreover, the six-digit classification aids in maintaining regulatory compliance, which is increasingly vital in a global market driven by quality assurance. Thus, the ASTM 6-digit system serves as a cornerstone for the effective utilization and application of stainless steel, reinforcing the need for standardized practices within the industry.

Austenitic Stainless Steel

Austenitic stainless steel exhibits non-magnetic properties and boasts exceptional resistance against corrosion. It excels in weldability, formability, fabricability, and ductility. This stainless steel variant upholds its corrosion resistance even in extreme temperatures, reaching as high as 1500°F. Its versatility makes it invaluable for applications requiring both low and high-temperature service. The following content are several type grade of stainless steel pipe and the chemical component and application of all kind of austentic stainless steel

Austenitic Grade of Stainless Steel Pipe

Type 301: Offers high ductility for shaped products and quick hardening during mechanical work, boasting good weldability. Exhibits superior wear resistance and fatigue strength compared to 304.

Type 302: Shares the corrosion resistance of 304 but holds slightly higher strength due to added carbon content.

Type 303: Recognized as an easier-to-machine version of 304 by incorporating sulfur and phosphorus. Referred to as "A1" as per ISO 3506 standards.

Type 304/304L :The most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with International Organization for Standardization ISO 3506.  

Type 309: Demonstrates improved temperature resistance compared to 304.

Type 316/316L: The second most prevalent grade, particularly suitable for food and surgical stainless steel applications. The addition of molybdenum prevents specific types of corrosion. Known as "marine grade" due to heightened resistance to chloride corrosion in contrast to type 304. Commonly used in constructing nuclear reprocessing plants and prevalent in many stainless steel watches.

Type 321: Shares similarities with 304 but lowers the risk of weld decay by including titanium. Also, consider type 347, which incorporates niobium for desensitization during welding.

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Alloy Component  /Characteristics /  Application

What is the difference between 316 and 304?

Cost

The cost of stainless steel largely depends on the alloys present in its composition. All stainless steels require a minimum of 10.5% chromium and are primarily alloyed with iron, which constitutes a significant portion of their final makeup. However, the other elements present affect the properties and characteristics, ultimately influencing the cost.

316 stainless steel contains at least 2.0% molybdenum, making it much more corrosion-resistant than 304. Due to molybdenum being a more expensive element, 316 is generally a more expensive grade of metal.

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Chemical composition

Magnetism

Both grades are austenitic, meaning they do not possess magnetic properties. However, cold working can affect this. When stainless steel is formed or cut, there is a slight increase in its magnetic properties, although this increase is minor.

Corrosion Resistance

Stainless steel, as a family of metals, is highly corrosion-resistant. The addition of molybdenum in 316 increases its ability to withstand harsh environments, often earning it the designation of marine grade. 316 is suitable for use in more aggressive environments than ambient ones, though regular cleaning is still recommended to prolong its service life.

Applications for 304 Stainless Steel

The high concentration of chromium in stainless steel 304 gives it excellent corrosion resistance, making it suitable for various applications, including:

• Kitchen appliances like refrigerators and dishwashers
• Heat exchangers
• Commercial food processing equipment and kitchen fittings like sinks and splashbacks
• Saucepans, cutlery, and flatware
• Architectural paneling
• Nuts, bolts, screws, and nuts
• Brewery, food, and pharmaceutical production equipment

Generally, stainless steel is used in applications where standard carbon steel would corrode.

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Applications for 316 Stainless Steel

Originally developed for use in paper mills, stainless steel 316 is now commonly used in various applications, including:

• Food, chemical, and petrol production and processing equipment
• Laboratory benches and equipment
• Architectural paneling in coastal regions
• Boat fittings
• Mining screens
• Kitchenware, sanitaryware, and troughs
• Tubing
• Medical implants

316 stainless steel offers better corrosion resistance than 304 and often exhibits greater strength at high temperatures.

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Ferritic Stainless Steel

Ferritic stainless steel, being magnetic and high in carbon content, tends to be brittle and exhibits relatively lower corrosion resistance when compared to austenitic stainless steel. It remains non-hardenable through heat treatment but showcases robust resistance against chloride stress corrosion cracking.

Ferritic Grade of Stainless Steel Pipe

Type 408:

Heat-resistant but exhibits poor corrosion resistance with 11% Chromium and 8% Nickel content.

Type 409:

Economical variant primarily used in automobile exhausts; ferritic in nature, composed of iron and chromium.

Type 416:

Notable for its ease of machining due to additional sulfur content.

Type 430:

Mainly used for decorative purposes such as automotive trim; a ferritic stainless steel featuring good formability but exhibiting reduced temperature and corrosion resistance.

Alloy Component  /Characteristics /  Application

Type 309- Better temperature resistance than 304

Type 309 - Exhibits better temperature resistance compared to Type 304.

Type 316 - This grade  of stainless steel pipe ranks as the second most common after Type 304. It is widely used in food processing  and surgical stainless steel applications. The addition of molybdenum enhances its resistance to specific types of corrosion, making it ideal for marine environments and earning it the nickname "marine grade stainless steel." Type 316 is also commonly used in the construction of nuclear reprocessing plants. Most stainless steel watches are made with this grade, except for Rolex, which uses a different quality.

Type 904L - Known as 18/10 stainless steel, this grade is also referred to as “A4” according to the International Organization for Standardization ISO 3506.

Type 321 - Similar to Type 304, but with a lower risk of weld decay due to the addition of titanium.

Duplex Stainless Steel

Duplex stainless steels are a combination of austenitic and ferritic stainless steels, and are composed of chromium, molybdenum, nickel and other elements. Typical grades of duplex stainless steel pipe include 1Cr18Mn10Ni5Mo3N, 2101 and 2205.

The duplex grade of stainless steel pipe

UNS S31803 (SAF 2205):

This is the most widely used duplex stainless steel grade. It contains approximately 22% chromium, 5-6% nickel, and 3% molybdenum. UNS S31803 offers excellent corrosion resistance, high strength, and good weldability.

UNS S32205 (SAF 2205):

 Similar to UNS S31803, UNS S32205 offers the same composition and properties. It provides high resistance to stress corrosion cracking, making it suitable for applications involving harsh environments and chloride-containing solutions.

UNS S32750 (SAF 2507): 

This grade, also known as super duplex stainless steel, contains approximately 25% chromium, 7% nickel, and 4% molybdenum. UNS S32750 offers exceptional corrosion resistance, high strength, and excellent resistance to pitting and crevice corrosion. It is commonly used in offshore and marine environments, chemical processing plants, and desalination systems.

UNS S32760 (Zeron 100): 

Another super duplex stainless steel grade, UNS S32760 contains higher amounts of chromium, nickel, and molybdenum compared to UNS S32750. It provides excellent corrosion resistance, high strength, and improved resistance to corrosion fatigue and erosion. UNS S32760 is often used in applications where superior corrosion resistance and mechanical properties are required.

Alloy Component  /Characteristics /  Application

Martensitic Stainless Steel

Martensitic stainless steel pipe is a type of stainless steel pipe which contains a high carbon content and is typically used for applications requiring high strength and hardness.

The martensitic grade of stainless steel pipe

How to choose the right grade of stainless steel pipe?

Opt for the Right Metal in Your Working Environment

To determine the most suitable  grade of stainless steel pipe  for a specific environment, assess the conditions your final product will endure. Factors like extremely low pH, high stress, elevated temperatures, and crevice corrosion can detrimentally affect the performance of stainless steel. Austenitic  series steels, such as the commonly used 316 and 304 alloys, maintain their strength, toughness, and corrosion resistance across a wide temperature spectrum.

The primary consideration for selecting austenitic grade pf stainless steel pipe  is their corrosion resistance. Type 316, fortified with molybdenum, even combats chloride ions commonly found in marine and chemical processing settings. Irrespective of the grade of stainless steel pipe , a well-designed structure is the best defense against corrosion.

Emphasize Strength, Ductility, and Toughness

• Strength: A metal's ability to endure stress before deforming or fracturing.
•  
• Ductility: The material's capacity to change shape, like being drawn into a wire, without losing strength or breaking.
•  
• Toughness: The metal's ability to deform and absorb energy before fracturing.

Stainless steel, containing 10–30% chromium as an alloying element, resists corrosion. The inclusion of nickel in austenitic grades provides superior toughness and ductility among stainless grades. Alloys rich in chromium, molybdenum, and nickel exhibit the highest corrosion resistance.

Apart from alloy content, consider the material's processing, which impacts its mechanical properties. Cooling duration at various temperatures and the overall cooling rate during steel production affect its overall quality.

While heat treatment increases the hardness of carbon steels, cold working operations like rolling, bending, or drawing at lower temperatures harden austenitic stainless steel. However, note that increased hardness due to cold working may decrease properties like elongation and impact resistance.

Account for Form and Process

Austenitic stainless steel is available in various forms such as bar, wire, tube, pipe, sheet, and plate. Most products require additional shaping or machining for specific applications.

For instance, stainless steel tubing might need bending, coiling, machining, welding, or end forming. When machining processes like CNC machining are involved, select a machining rate that mitigates work hardening or opt for a "free-machining" grade containing sulfur.

During welding, embrittlement in the weld area is a concern. Opt for lower carbon grades like 304, 304L, or 316L to minimize carbide formation.

Consider Customer Preferences

Designers often select stainless steel for its aesthetic appeal—be it a shiny, electropolished "bright" finish, a dull "pickled" finish, a specific RMS polished matte surface, or a light-absorbing black oxide coating. Austenitic stainless steel grades can adopt these finishes and commonly undergo passivation.

Customers might also require certification for application-specific specifications, such as ASTM standards for different industrial uses. These standards ensure compliance with technical specifications for chemical composition, heat treatment, and other attributes.

Manage Material Costs and Availability

While high-performing austenitic stainless steels are initially more expensive, they prove worthwhile due to their corrosion resistance. Choosing a corrosion-resistant material suited to the application minimizes maintenance, downtime, and replacement expenses. Life-cycle costing methods enable a comprehensive comparison of different materials in terms of current and future costs.

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American (ASTM) standards for stainless steels for general applications

The table displays only the most up-to-date standards, as ASTM standards undergo annual review and status updates, regardless of whether changes have been made. Therefore, the original and current years of issue are not included in the table. To locate a specific standard, use the ‘Ctrl’ and ‘F’ keys.

Table

The above table originating from :

https://bssa.org.uk/bssa_articles/american-astm-standards-for-stainless-steels-for-general-applications/

Conclusion

In essence, grade of stainless steel pipes epitomize the marriage of innovation and functionality. From the corrosion-resistant Austenitic to the robust Martensitic grades, each category offers a distinct set of attributes catering to diverse industries. The Duplex variety bridges the gap with a harmonious blend of strengths, while the Ferritic types excel in reliability.

What’s more, grade of stainless steel pipe is an important factor to consider when selecting a pipe for a particular application. The grade of stainless steel pipe should be chosen based on the chemical composition of the metal, the desired properties of the pipe, and the requirements of the application.

Contact the stainless steel pipe expert 

It is of utmost importance to seek the guidance of a professional expert when considering the selection of the appropriate grade of stainless steel pipe, particularly for new entrants in the stainless steel market. Foshan Vinmay Stainless Steel Co., Ltd. is your reliable source for expert advice and solutions regarding stainless steel pipes. Contact us today for your planing.