What Is Stainless Steel and What Are The Common Stainless Steel

Types
Stainless steel is a very common metal material in our lives and plays an important role in many fields. Despite that, many of us are still not very familiar with what stainless steel is when it comes to things such as its nature, pros and cons, as well as categorization. So we are going to have an in-depth introduction on what stainless steel is exactly and what the common stainless steel types are for you!

What Is Stainless Steel

Stainless steel is the abbreviation of stainless steel and acid-resistant steel. In the fields of metallurgy and materials science, based on the main performance characteristics of steel, a series of iron-based alloys that contain more than 10.5% chromium and have corrosion resistance and stainlessness as their main properties are called stainless steel. Usually, steel types that are stainless and corrosion-resistant in less corrosive media such as atmosphere, water vapor, and fresh water are called stainless steel; steel types that are corrosion-resistant in highly corrosive environments such as acids, alkalis, and salts are called acid-resistant steel. The two types of steel have different corrosion resistance due to differences in composition. The former has a low degree of alloying and is generally not acid-resistant; the latter has a high degree of alloying and is both acid-resistant and stainless.

Stainless Steel Properties

The most basic characteristics of stainless steel are its rust resistance under atmospheric conditions and corrosion resistance in various liquid media. This characteristic is directly related to the chromium content in the steel. The presence of chromium causes an invisible film to be formed on the surface of the steel, which can prevent oxidation and make the material "passivated" or resistant to corrosion. Adding other elements such as nickel or molybdenum can improve corrosion resistance, strength and temperature resistance.

Corrosion resistance increases with increasing chromium content. When the chromium content reaches more than 10.5%, this characteristic of steel changes suddenly, from easy to rust to stainless, and from non-corrosion to corrosion-resistant. Moreover, as the chromium content continues to increase from 10.5%, its rust resistance and corrosion resistance are also continuously improved. Generally, the maximum chromium content of stainless steel is 26%, and higher chromium content is no longer necessary.

Common Types of Stainless Steel

At present, commonly used classification methods of stainless steel are to classify it according to the structural characteristics of the steel, the chemical composition characteristics of the steel, or a combination of the two. The most common way to categorize stainless steel is to have it divided into five categories: martensitic steel, ferritic steel, austenitic steel, duplex steel and precipitation hardened steel.

1. Austenitic Stainless Steel

Austenitic stainless steel is a stainless steel with an austenite structure (mainly γ phase) with a face-centered cubic crystal structure. It is non-magnetic and is mainly strengthened by cold working (and may lead to certain magnetism). 18-8 stainless steel is a typical austenitic stainless steel type containing approximately 18% chromium and 8% nickel. 18-8 stainless steel includes 302, 303, 304 and other grades.

Under normal heat treatment conditions, the matrix structure of steel is austenite. However, a small amount of carbides and ferrite structures may exist in the austenite matrix under improper heat treatment or different heating conditions. Besides, austenitic stainless steel cannot change its mechanical properties through heat treatment and can only be strengthened by cold deformation. Different steel types suitable for various usage conditions can be obtained by adding alloying elements such as molybdenum, copper, and silicon.

Austenitic stainless steel is generally non-magnetic, but some parts may become slightly magnetic after cold working. The important characteristics of this type of steel are good low-temperature properties, easy formability and weldability. And it also has high corrosion resistance. 

2. Ferritic Stainless Steel

Ferritic stainless steel is a common type of stainless steel that is dominated by a ferrite structure (α phase) with a body-centered cubic crystal structure. It is magnetic and generally cannot be hardened by heat treatment, but it can be slightly strengthened by cold working. Ferritic stainless steel contains 12% to 18% chromium, but the carbon content is less than 0.2%. The common steel types are 409 and 430, and their corrosion resistance is not as good as austenitic stainless steel. 

Due to its high chromium content, ferritic stainless steel has relatively good corrosion resistance and oxidation resistance, but poor mechanical and process properties. It is mostly used in acid-resistant structures with little stress and as anti-oxidation steel. This type of steel can resist corrosion from the atmosphere, nitric acid and salt water solutions, and has good high-temperature oxidation resistance and small thermal expansion coefficient. It is typically used in nitric acid and food factory equipment, and can also be used to make parts that work at high temperatures, such as gas turbine parts.

3. Martensitic Stainless Steel

The matrix of martensitic stainless steel is martensite structure, which is magnetic and its mechanical properties can be adjusted through heat treatment. Martensitic stainless steel contains 12~18% chromium. When chromium exceeds 15%, it is often necessary to add a certain amount of nickel or appropriately increase the carbon content to balance the structure. Common stainless steel types include 410 and 420.

Martensitic stainless steel has strong magnetism at room temperature. Generally speaking, its corrosion resistance is not outstanding, but has high strength, and it is used in high-strength structural steel. Martensitic stainless steel has a stable austenite structure at high temperatures, which can transform into the martensite phase under air cooling or oil cooling, and has a complete martensite structure at room temperature. Therefore, the mechanical properties can be improved and changed through heat treatment methods. Martensitic steel has high tensile strength, and its maximum strength can reach 1600MPa. It needs to be tempered to improve its plasticity so that it still has sufficient formability under such high strength. It is a commercial high-strength steel with the highest strength among steel plates.

4. Duplex Stainless Steel

Duplex stainless steel has the characteristics of austenitic stainless steel and ferrite stainless steel because it has a dual-phase structure of austenite plus ferrite, and the contents of the two phase structures are basically equal. The yield strength can reach 400Mpa ~ 550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has high toughness, low brittle transition temperature, and significantly improved intergranular corrosion resistance and welding performance. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel. Common stainless steel types include 2304, 2205, and 2507.

Duplex stainless steel has good welding performance. Compared with ferritic stainless steel, it does not have the welding heat-affected zone that greatly reduce the plasticity and toughness due to severe grain coarsening. On the other hand, it is not as sensitive to welding hot cracks as austenitic stainless steel. Due to its special advantages, duplex stainless steel is widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, papermaking machinery and other industrial fields. 

5. Precipitation Hardening Stainless Steel

Precipitation hardening stainless steel is a stainless steel whose matrix is austenite or martensite and can be hardened through precipitation hardening treatment. Precipitation hardening stainless steel can be divided into martensitic precipitation hardening stainless steel, semi-austenitic precipitation hardening stainless steel and austenitic plus ferrite precipitation hardening stainless steel according to its structure. This type of material uses aging to precipitate metal compounds such as Cu, Al, Ti, Nb, etc. after heat treatment to improve the strength of the material. 

The chromium content of precipitation hardening stainless steel is about 17%, and it also contains elements such as nickel and molybdenum. Therefore, in addition to having sufficient stainlessness, its corrosion resistance is close to that of 18-8 austenitic stainless steel.

Summary

The above sums up the 5 common types of stainless steel categorized according to the structural characteristics. It is vital to have a deep understanding of your own needs or your clients’ specific needs and expectations when deciding on which type of stainless steel to purchase. Each common type of stainless steel consists of quite a number of specific grades that have their own characteristics and settings of use.

As a leading manufacturer and supplier of steel products with more than 20 years professional experience in this field, Jiuzhou Metal is committed to manufacturing and offering a wide range of common types of stainless steel. The company has passed ISO9001 quality management system certification and CE certification. All the products can be executed according to ASTM, JIS, DIN, BS and other common international standards. At Jiuzhou Metal, we always give great importance to optimizing our products and services, and prioritize the needs of our clients. Nowadays, more and more fellows worldwide seek assistance from Jiuzhou Metal in meeting their future demands for steel, and thus fueling their business development!