Stainless Steel : Difference of 4 Types You Should Know

What Is Stainless Steel?

The so-called stainless steel is a high-alloy steel smelted with iron as the basic material and adding alloying elements.
 

The Raw Materials of Stainless Steel

Its chemical composition is mainly iron, carbon, and chromium, plus other alloying elements, such as nickel, Molybdenum, copper, titanium, niobium, etc. Chromium is the first major alloying element, and the chromium content must be at least 11.5% by weight to be called stainless steel, while nickel is the second major alloying element. The reason why the surface of steel is easy to rust is that the iron is oxidized by oxygen in the air to form a porous iron oxide rust layer.

Due to the air and moisture in the atmosphere, the rust layer can still penetrate into the interior through the pores. Cause the corrosion phenomenon to continue to occur. However, stainless steel contains chromium. Because chromium is more active than iron, a layer of chromium oxide film with good adhesion, dense and airtight will be formed on its surface. This film can prevent corrosive gas or liquid from penetrating into the interior, so it will not Corrosion and rust continue to occur, hence the name. In particular, even if this protective film is damaged by dents, scratches or abrasion, it can be re-formed as long as there is air (oxygen), so it is not easy to corrode and rust.
 

4 Main Types of Stainless Steel

Classification of stainless steel: There are many types of stainless steel. Generally speaking, they can be divided into four categories according to their metallographic structure:

• Ferritic Stainless Steel
• Martensitic Stainless Steel
• Austenitic Stainless Steel
• Precipitation-hardening Stainless Steel

Take the specifications of American Iron and Steel Institute (AISI) and American Society for Testing and Materials (ASTM) as examples, which classifies Wrought Stainless Steels with "three digits." Characters to indicate various standard grades of stainless steel series. Each series has different metallographic structure, magnetic properties and uses. They are briefly described as follows:
 

1. Ferritic Stainless Steels

This is the chromium series of AISI number 400 series Stainless steel, such as 405, 430, 446, etc. This series has a high chromium content (about 11.5--27% chromium), no nickel, and low carbon content (≤0.2% carbon). The commonly used basic type is 430 (16-18% chromium, ≤0.12% carbon). Because it is maintained as a fat granule structure at various temperatures, it cannot be hardened by heat treatment.

Generally, cold working can only be used to increase the hardness. In terms of physical properties, like iron (Fe, body-centered cubic structure), it has strong paramagnetism; its main uses include construction and automobile decorations.
 

2. Martensitic Stainless Steels

Martensitic Stainless Steels AISI code is also 400 series of chromium stainless steels, such as 403, 410, 420, etc. The chromium content of this series is lower than that of fertilizer grain stainless steel (about 11.5-15% chromium), and has a higher carbon content (0.08-1.2% carbon). The basic type is 410 (11.5-13.5% chromium, 0.15% carbon) . As the temperature changes, the Matian bulk phase transformation will occur, so the hardness and strength can be increased by heat treatment.

The structure of Matian loose body is body-centered cubic or body-centered square crystal, so it is also magnetic in terms of physical properties; the main uses are knives, surgical appliances, and engineering components that are resistant to high temperature and pressure. In addition, although the 500 series also belong to Matian bulk stainless steel, its chromium content is only 4-6%, so it is classified as a branch series with extremely low chromium content. Due to its poor corrosion resistance, it is generally only used as heat-resistant alloy steel.
 

3. Austenitic Stainless Steels

It has two stainless steels of 300 and 200 series in AISI number. The 300 series is chromium-nickel stainless steel, such as 302, 304, 316, etc. The commonly used type is 304. Its standard chemical composition contains 18% chromium and 8% nickel, which is generally called 18-8 stainless steel. Because it is maintained as austenitic tissue at various temperatures, it cannot be hardened by heat treatment, and generally only cold working can be used to harden the material.

This series of stainless steel has the best corrosion resistance, of which 304 is the most versatile, such as tableware, lunch boxes, medical and sanitary appliances, food processing equipment, construction and decorations. Because the austenitic body structure is a face-centered cubic crystal, it is non-magnetic; however, if it is subjected to cold working (such as rolling, stretching, polishing, etc.), the surface may be affected by stress and induce a small amount of hepatic loose body deformation. Make it weakly magnetic.

In addition, the magnetic properties of cast stainless steels and forged stainless steels are also different. For example, AISI 304 forged stainless steel corresponds to the cast stainless steel ACI (Alloy Casting Institute) whose code is CF-8. For fluidity, a large amount of silicon (2%) needs to be added. However, silicon is a stabilizing element of fat granules, which will promote the formation of a small amount of island-shaped fat granules in the austenitic base, so it also has weak magnetic properties.

Looking at the above, if 304 stainless steel is judged only by whether it has magnetism, there may be doubts. The 200 series is a branch of the 300 series, such as 201 and 202. Its chemical composition is to replace part of nickel with cheaper manganese to become chromium-nickel-manganese stainless steel. This is because manganese and nickel are both austenitic. Stabilizing element, usually nickel (Mn/Ni≅2) which replaces manganese with about twice the amount, is a relatively inexpensive austenitic stainless steel, which is the same as the 300 series and is also non-magnetic. It is generally used in automobile wheel covers and kitchen equipment.
 

4.Precipitation-hardening Stainless Steels

Precipitation-hardening Stainless Steels AISI numbered 600 series stainless steels. This type of stainless steel was developed in response to the needs of the aviation industry during the Second World War. It is mainly made of different chromium and nickel The main content is ferroalloy, and alloying elements such as copper and aluminum are added.

This series of stainless steels can be divided into three types according to their different metallographic structures:

(a) Mattian bulk precipitation hardening stainless steel, which is magnetic, such as 630 (brand name 17-4 PH)
(b) semi-austite precipitation hardening stainless steel , Magnetic, such as 631 (brand name 17-7 PH)
(c) Austenitic precipitation hardening stainless steel, non-magnetic, such as 660 (brand name A286)

this type of stainless steel, although the hardness and strength are high, but It is generally used in the aviation and rocket industry, but it seems that it is not used much in the engineering field. It is customary to use brand names.

References 1. J.G. Parr A. Hanson(1993). Stainless Steel, ASM, Metals Park.
2. Smith, W. F. (1993). Structure and Properties of Engineering Alloys. McGraw-Hill Inc.
3. D. Peckner, I.M. Bernstein(1977). Handbook of Stainless Steel. McGraw-Hill Inc.