Stainless steel | In detail

Ahmed Samy

5 Dec, 2023

Stainless steel, a remarkable alloy revered for its exceptional properties, has become an indispensable material in modern industries and everyday applications. Comprised mainly of iron, with a chromium content, stainless steel owes its name to its remarkable ability to resist stains, corrosion, and rust, making it an enduring choice for diverse environments and demanding conditions. This versatile metal's mechanical strength, hygienic nature, and aesthetic appeal have rendered it an ideal choice for a myriad of products, from cookware, medical instruments, and architecture to automotive components and aerospace equipment. Its enduring popularity stems from the seamless fusion of form and function, presenting a fascinating interplay of science and artistry within the realm of materials engineering.

What is stainless steel alloy?

Stainless steel or inox is a type of steel alloy that contains a minimum of 10.5% and maximum 30% chromium content by mass. It is an iron-based alloy, but the addition of chromium is what sets it apart from traditional carbon steel. The presence of chromium in stainless steel forms a protective passive oxide layer on its surface, known as the "chromium oxide layer" or "passive layer" that can protect the material and self-heal in the presence of oxygen. This layer is extremely thin but highly stable, and it prevents the underlying steel from being corroded or rusted by oxygen, moisture, or other environmental factors.

Apart from chromium, stainless steel may also contain other elements such as nickel, molybdenum, and sometimes other alloys to increase corrosion resistance to specific environments, improve oxidation resistance, and enhance specific characteristics. The varying composition of stainless steel alloys results in different grades, each with its unique characteristics suitable for particular applications.

Dose Stainless steel rust?

Under certain conditions, stainless steel can corrode or rust. Factors such as exposure to harsh environments, high temperatures, salt, or chemicals can compromise the protective oxide layer and lead to rusting. This is why it's essential to choose the right type of stainless steel for specific applications and to maintain it properly.

Stainless steel grades

Stainless steel grades(families) are specific categorizations of stainless steel alloys, each with its own composition and properties tailored for different applications. There are more than 100 grades of stainless steel. Stainless steels can be broadly categorized into five main groups, representing the majority of this versatile alloy family:

Austenitic Stainless Steel (e.g., 304, 316, 321): Known for their excellent corrosion resistance and formability, these grades are widely used in food processing, chemical, and architectural applications.
Ferritic Stainless Steel (e.g., 430, 446): These grades offer good resistance to corrosion, particularly in mildly aggressive environments, and are commonly used in automotive exhaust systems and decorative applications.
Martensitic Stainless Steel (e.g., 410, 420, 440): Known for their high strength and hardness, these grades are often used in cutlery, surgical instruments, and turbine blades.
Duplex Stainless Steel (e.g., 2205, 2507): Combining the qualities of austenitic and ferritic stainless steels, duplex grades offer enhanced strength and resistance to stress corrosion cracking, making them suitable for chemical processing and offshore applications.
Martensitic and Precipitation Hardening Stainless Steel (e.g., 17-4PH, 15-5PH): These grades can be heat-treated to increase strength and hardness, finding applications in aerospace components and high-performance machinery.

These are the major Stainless Steel grades and there are many other specialized stainless steel grades designed to meet specific requirements across a diverse range of industries and applications. The selection of the appropriate grade depends on factors like corrosion resistance, mechanical properties, temperature, and the intended use of the material.

About stainless steel grades

Stainless steel grades are classified using various systems, including three-digit identifiers established by the Society of Automotive Engineers (SAE) and six-character identifiers set by the American Society for Testing and Materials (ASTM), known as the Unified Number System (UNS) identifiers. Additionally, other countries and organizations have their grading systems, such as British Standards (BS), International Organization for Standardization (ISO), Japanese Industrial Standards (JIS), European Standard (EN), German Standard (DIN), and Chinese Standard (GB).

Navigating the numerous grading systems can be challenging, as the same stainless steel alloy may have different designations across various standards. For example, 304 stainless steel, classified under the SAE standard, can also be identified as EN number 1.4305, EN name X8CrNiN18-9, UNS S30400, DIN X5CrNi18-9, X5CrNi18-10, X5CrNi19-9, BS 304S 15, 304S 16, 304S 18, 304S 25, and En58E, as well as JIS SUS 304 and SUS 304-CSP.

To achieve a specific grade designation, the alloy must meet strict requirements specified by the relevant standard organization. For example, for 304 stainless steel to meet the common SAE standard, it must contain approximately 18% to 20% chromium, 8% to 10.5% nickel, 0.08% carbon, 2% manganese, 0.75% silicon, 0.045% phosphorus, 0.03% sulfur, and 0.1% nitrogen.

This complex landscape of stainless steel grading underscores the importance of understanding the appropriate standard for each application and ensures that the chosen alloy meets the required specifications for performance, composition, and suitability.

Stainless Steel properties

Density:

Stainless steel has a relatively high density, typically ranging from 7.7 to 8.0 grams per cubic centimeter (g/cm³), depending on the specific grade and composition.

Melting Point:

The melting point of stainless steel can vary with its composition but generally falls within the range of 1400 to 1510 degrees Celsius (2552 to 2750 degrees Fahrenheit).

Electrical Conductivity:

Stainless steel is a relatively poor conductor of electricity compared to metals like copper or aluminum. Its electrical conductivity is typically lower, but certain grades with higher nickel content may exhibit better conductivity.

Thermal Conductivity:

Stainless steel's thermal conductivity is lower than that of many other metals. It tends to conduct heat more slowly, which can be both advantageous and disadvantageous in different applications.

Magnetic Properties:

The vast majority of stainless steel grades are non-magnetic in their annealed (softened) state. However, some specific grades may exhibit magnetic properties depending on their microstructure and alloying elements.

Strength and Toughness:

Stainless steel is known for its excellent strength and toughness, which contribute to its durability and ability to withstand mechanical stresses.

Expansion and Contraction:

Like most materials, stainless steel expands and contracts with changes in temperature. However, its coefficient of thermal expansion is relatively low compared to other materials, which can be beneficial in applications with temperature variations.

Appearance and Finish:

Stainless steel's smooth and shiny surface, along with its ability to take on various finishes (e.g., brushed, polished, matte), makes it a popular choice for architectural and decorative applications.

Ferromagnetism:

Some stainless steel grades can be made ferromagnetic (magnetic) through specific treatments like cold working or adding certain elements like manganese.

Chromium Content:

Stainless steel contains a minimum of 10.5% chromium by mass. The presence of chromium is crucial as it forms a passive oxide layer on the surface of the steel, which provides exceptional corrosion resistance.

Corrosion Resistance:

The passive oxide layer formed by chromium prevents the underlying steel from being corroded or rusted by environmental factors like oxygen, moisture, and certain chemicals. This property makes stainless steel highly resistant to rust and corrosion.

Alloying Elements:

In addition to chromium, stainless steel may contain other alloying elements such as nickel, molybdenum, titanium, and others. These elements contribute to specific properties like increased strength, improved weldability, and enhanced resistance to certain types of corrosion.

Temperature Resistance:

Certain stainless steel grades maintain their strength and corrosion resistance even at high temperatures, making them suitable for applications in elevated temperature environments.

Acidity and Alkalinity Resistance:

Stainless steel exhibits excellent resistance to both acidic and alkaline environments, making it suitable for applications in chemical processing and other industries where exposure to corrosive substances is common.

Hygienic Properties:

Stainless steel has a smooth surface that is easy to clean, making it a preferred material in food processing, medical instruments, and sanitary applications.

Magnetism:

Most stainless steels are non-magnetic in the annealed condition due to their microstructure. However, some stainless steel grades can be made magnetic through specific processing.

Weldability:

Depending on the grade, stainless steel can have good weldability, making it easier to join components and fabricate complex structures.

Stainless steel uses

Stainless steel is widely used in the production of pots, pans, cutlery, and other kitchen utensils due to its hygienic properties and corrosion resistance.

Stainless steel is used in building facades, handrails, structural components, and roofing due to its aesthetic appeal, durability, and resistance to weathering and corrosion.

Stainless steel is employed in surgical instruments, medical equipment, and implants due to its biocompatibility, corrosion resistance, and ease of sterilization.

Stainless steel is used in various automotive components, including exhaust systems, trim, and grilles, due to its corrosion resistance and high-temperature performance.

Stainless steel finds applications in aerospace components, such as aircraft structural parts and jet engine components, owing to its strength, corrosion resistance, and ability to withstand extreme conditions.

Stainless steel is used in pipelines, storage tanks, and other equipment in the oil and gas sector due to its corrosion resistance and suitability for harsh environments.

Stainless steel is employed in chemical processing plants for reactors, storage vessels, and piping systems due to its resistance to various corrosive chemicals.

Stainless steel is used in food processing equipment, brewing tanks, and storage tanks due to its hygienic properties and ease of cleaning.

Stainless steel is utilized in water treatment plants for pipes, pumps, and filtration systems due to its corrosion resistance and suitability for handling different water conditions.

Stainless steel is employed in marine environments for boat fittings, hull components, and offshore structures due to its corrosion resistance in seawater.

Stainless steel is used in the production of household appliances like refrigerators, dishwashers, and washing machines due to its durability and modern appearance.

Stainless steel price

The cost of stainless steel can vary significantly based on the grade or type being acquired. Lower grades generally begin around $800 per ton, whereas higher grades can soar to $2,500 per ton or even higher.