Stainless Steel Material Classification Basics
Jan 03, 2019

Since the invention of stainless steel in the early 2000s, stainless steel has combined the image of modern materials with the prestigious reputation of architectural applications, envying its competitors.

As long as the steel is selected correctly and properly maintained, the stainless steel will not corrode, pitting, rust or wear. Stainless steel is also one of the strongest materials in building metal materials. Because stainless steel has good corrosion resistance, it enables structural components to permanently maintain engineering integrity. Chromium-containing stainless steel also combines mechanical strength and high extensibility, making it easy to machine parts to meet the needs of architects and structural designers.

Stainless steel grade grouping

200 series - chrome-nickel-manganese austenitic stainless steel

300 series - chrome-nickel austenitic stainless steel

Model 301 - good ductility for molding products. It can also be hardened by mechanical processing. Good weldability. Wear resistance and fatigue strength are better than 304 stainless steel.

Model 302 - Corrosion resistance is the same as 304, because the carbon content is relatively high and the strength is better.

Model 303—It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.

Model 304 - universal model; ie 18/8 stainless steel. The GB grade is 0Cr18Ni9.

Model 309 - has better temperature resistance than 304.

Model 316—After 304, the second most widely used steel grade is used primarily in the food industry and surgical equipment, adding molybdenum to give it a special structure that resists corrosion. It is also used as "ship steel" because it has better resistance to chloride corrosion than 304. SS316 is commonly used in nuclear fuel recovery units. 18/10 grade stainless steel also generally meets this application level.

Model 321 - except for the addition of titanium to reduce the risk of weld corrosion of the material, the performance is similar to 304. Stainless steel decorative tube, 201 stainless steel tube, 304 stainless steel tube

400 Series - Ferritic and Martensitic Stainless Steel

Model 408 - good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.

Model 409 - the cheapest model (British and American), usually used as a car exhaust pipe, is a ferritic stainless steel (chrome steel).

Model 410—Martensite (high-strength chrome steel) with good wear resistance and poor corrosion resistance.

Model 416—Addition of sulfur improves the processing properties of the material.

Model 420—“cutting grade” martensitic steel, similar to the earliest stainless steel of Brinell high chromium steel. Also used for surgical knives, it can be done very brightly. Model 430 - ferritic stainless steel, for decorative purposes, for example for automotive accessories. Good formability, but poor temperature resistance and corrosion resistance.

Model 440—High-strength cutting tool steel with slightly higher carbon content. After proper heat treatment, it can obtain higher yield strength and hardness of 58HRC, which is among the hardest stainless steel. The most common application example is the "razor blade." There are three commonly used models: 440A, 440B, 440C, and 440F (easy processing type).

500 series - heat-resistant chrome alloy steel.

600 series - martensitic precipitation hardening stainless steel.

Model 630—The most commonly used type of precipitation hardened stainless steel, also commonly referred to as 17-4; 17% Cr, 4% Ni.

Why is stainless steel resistant to corrosion? Stainless steel decorative tube, 201 stainless steel tube, 304 stainless steel tube

All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing the rust to expand and eventually form pores. The carbon steel surface can be ensured by electroplating with paint or oxidation resistant metals (e.g., zinc, nickel, and chromium), but as is known, this protection is only a film. If the protective layer is destroyed, the underlying steel begins to rust.

First of all, let's first understand what is stainless steel. Generally speaking, steel that does not rust is called stainless steel, but it is academically resistant to weak corrosive media such as air, steam, water, and chemical etching of acids, alkalis, and salts. Medium corroded steel. Also known as stainless acid-resistant steel. In practical applications, steel that is resistant to weak corrosive media is often referred to as stainless steel, while steel that is resistant to chemical media is referred to as acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter generally has rust. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element for obtaining corrosion resistance of stainless steel. When the chromium content in steel reaches about 12%, chromium reacts with oxygen in the corrosive medium to form a thin oxide film (self-passivation film) on the steel surface. It can prevent further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of stainless steel structure and performance for various purposes.

Stainless steel is usually divided into matrix structure: 201 stainless steel tube, 304 stainless steel tube

1 ferritic stainless steel. Containing 12% to 30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is better than other types of stainless steel.

2 austenitic stainless steel. Containing more than 18% chromium, it also contains about 8% nickel and a small amount of elements such as molybdenum, titanium and nitrogen. Good overall performance, resistant to a variety of media corrosion.

3 austenitic-ferritic duplex stainless steel. It combines the advantages of austenitic and ferritic stainless steels and has superplasticity. 4 martensitic stainless steel. High strength, but poor plasticity and weldability.

The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection methods are different.

When the amount of chromium added reaches 10.5%, the atmospheric corrosion resistance of the steel is remarkably increased, but when the chromium content is higher, although the corrosion resistance is still improved, it is not obvious. The reason is that when chromium is used to alloy steel, the type of surface oxide is changed to a surface oxide similar to that formed on pure chromium metal. This tightly adhering chromium-rich oxide protects the surface from further oxidation. This oxide layer is extremely thin, through which you can see the natural luster of the steel surface, giving the stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface will react with the atmosphere to repair itself, and the "passivation film" is reformed to continue the protection.


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