Introduction to stainless steel

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 holes. The surface of carbon steel can be protected by paint or oxidation-resistant metals (such as zinc, nickel and chromium) for electroplating, but, as people know, this protection is only a film. If the protective layer is damaged, the steel below will begin to rust. Steel that is resistant to weak corrosive media such as air, steam, and water, and chemically corrosive 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 called stainless steel, while steel that is resistant to chemical media is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical media, while the latter is generally rust-resistant. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Chromium is the basic element that makes stainless steel corrosion-resistant. When the chromium content in steel reaches about 12%, chromium reacts with oxygen in the corrosive medium to form a very thin oxide film (self-passivation film) on the steel surface, which can prevent further corrosion of the steel matrix. In addition to chromium, commonly used alloying elements include nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the requirements of stainless steel organization and performance for various uses.

Definition of stainless steel

A high-alloy steel that can resist corrosion in air or chemical corrosive media. Stainless steel has a beautiful surface and good corrosion resistance. It does not need to undergo surface treatment such as plating, but exerts the inherent surface properties of stainless steel. It is a kind of steel used in many aspects and is usually called stainless steel. Representative performance includes 13-chromium steel, 18-chromium-nickel steel and other high-alloy steels. From the perspective of metallography, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen that invades the steel and plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium.

Types of stainless steel

Stainless steel can be roughly classified according to its use, chemical composition and metallographic structure. Austenitic steel is basically composed of 18% chromium and 8% nickel. The amount of each element added varies, and steels for various purposes are developed.

1. Classification by chemical composition:

①CR series: ferrite series, martensite series; ②CR-NI series: austenite series, abnormal series, precipitation hardening series.

2. Classification by metallographic structure:

①Austenitic stainless steel; ②Ferritic stainless steel; ③Martensitic stainless steel; ④Duplex stainless steel; ⑤Precipitation hardening stainless steel.

3. Surface categories of stainless steel

The development of stainless steel has made the corrosion resistance, appearance, processability, strength and other characteristics of stainless steel far exceed other materials. Moreover, many surface treatment methods of stainless steel can achieve rich colors and shapes, which has made a great contribution to the development of stainless steel.