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Basic Knowledge of Stainless Steel Classification

Jul. 03, 2019

As a Tube Nickel 200 Factory, let's share some of the basics of stainless steel classification.

First, let's first understand what is stainless steel. Popularly speaking, steel that does not rust is called stainless steel. However, it is academically resistant to corrosion of corrosive media such as air, steam, water, and chemically etched media such as acids, alkalis, and salts. 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. 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 the base 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 number of elements such as molybdenum, titanium, and nitrogen. Such as Austenitic Stainless Steel Pipe, its comprehensive performance is good, can withstand a variety of media corrosion.


Austenitic Stainless Steel Pipe


3. Austenitic-ferritic duplex stainless steel. Duplex Stainless Steel Pipe combines the advantages of austenitic and ferritic stainless steels with 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.

For more information on China Welded Stainless Steel Tubing, please visit our website, https://www.baowu-specialsteel.com/



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