I. What kind of steel is stainless steel?

Stainless steel is a type of steel. Steel refers to steel with a carbon (C) content of less than 2%, and steel with a carbon (C) content greater than 2% is iron. During the smelting process of steel, alloying elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added to improve the performance of steel, endowing it with corrosion resistance (i.e., no rusting), which is what we commonly refer to as stainless steel.

What exactly are "steel" and "iron", what are their properties, and what is the relationship between them? How do the terms 304, 304L, 316, and 316L that we usually talk about come about, and what are the differences among them?

Steel: A material with iron as the main element, a carbon content generally below 2%, and containing other elements. -- GB/T 13304-91 "Steel Classification";

Iron: A metallic element with atomic number 26. Iron materials have strong ferromagnetism and good plasticity and thermal conductivity.

Stainless steel: A type of steel that is resistant to weak corrosive media such as air, steam, and water, or has the property of not rusting. The commonly used steel grades are 304, 304L, 316 and 316L, which belong to the 300 series of austenitic stainless steel.

Common classifications

It is usually classified by metallographic structure as:

Generally, according to the metallographic structure, common stainless steels are classified into three types: austenitic stainless steel, ferritic stainless steel and martensitic stainless steel. Based on these three basic metallographic structures, for specific requirements and purposes, duplex steel, precipitation-hardening stainless steel and high alloy steel with an iron content of less than 50% have been derived.

Austenitic stainless steel.

The matrix is mainly an austenitic structure (CY phase) with a face-centered cubic crystal structure, which is non-magnetic and mainly strengthened through cold working (and may result in a certain degree of magnetism). The American Iron and Steel Institute marks it with numbers in the 200 and 300 series, such as 304.

2. Ferritic stainless steel.

The matrix is mainly ferrite structure (phase a) with a body-centered cubic crystal structure, which is magnetic. Generally, it cannot be hardened by heat treatment, but cold working can slightly strengthen it. The American Iron and Steel Institute marks it with 430 and 446.

3. Martensitic stainless steel.

Stainless steel with a martensitic matrix (body-centered cubic or cubic), magnetic properties, and its mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute marks it with the numbers 410, 420 and 440. Martensite has an austenitic structure at high temperatures. When cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (i.e., hardened).

4. Austenitic-ferritic (duplex) stainless steel.

The matrix of the stainless steel has a two-phase structure of austenite and ferrite, with the content of the less common phase matrix generally greater than 15%. It is magnetic and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has higher strength and significantly improved resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion.

5. Precipitation-hardening stainless steel.

Stainless steel with an austenitic or martensitic matrix that can be hardened through precipitation hardening treatment. The American Iron and Steel Institute marks it with 600 series numbers, such as 630, which stands for 17-4PH.

Generally speaking, except for alloys, austenitic stainless steel has relatively excellent corrosion resistance. In environments with low corrosiveness, ferritic stainless steel can be used. In environments with mild corrosiveness, if high strength or high hardness of the material is required, martensitic stainless steel and precipitation-hardening stainless steel can be adopted.

Characteristics and Uses

Surface treatment

Thickness distinction

1. Due to the slight deformation of the rolls caused by heating during the rolling process of the steel plant machinery, the thickness of the rolled plates varies, usually being thicker in the middle and thinner on both sides. When measuring the thickness of the board, the state stipulates that the middle part of the board head should be measured.

2. The causes of tolerances are generally classified into large tolerances and small tolerances based on market and customer demands, for instance

304 stainless steel is the most common type of steel. As a widely used steel, it features excellent corrosion resistance, heat resistance, low-temperature strength and mechanical properties. It has good hot workability such as stamping and bending, and no hardening phenomenon after heat treatment (non-magnetic, applicable temperature range: -196℃ to 800℃).

Household items (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)

Automobile parts (windshield wipers, mufflers, molded products)

Medical devices, building materials, chemicals, food industry, agriculture, ship components

 Performance Introduction

As a low-carbon 304 steel, under normal conditions, its corrosion resistance is similar to that of 304 steel. However, after welding or stress relief, its resistance to intergranular corrosion is excellent. It can maintain good corrosion resistance without heat treatment and can be used at temperatures ranging from -196℃ to 800℃.

It is applied to outdoor machinery in the chemical, coal and petroleum industries with high requirements for resistance to intergranular corrosion, heat-resistant parts in building materials and parts that are difficult to heat-treat.

Performance Introduction

316 stainless steel, due to the addition of molybdenum, has particularly good corrosion resistance, atmospheric corrosion resistance and high-temperature strength, and can be used under harsh conditions. Excellent work hardening property (non-magnetic).

 Scope of application

Equipment used in seawater, chemical, dye, papermaking, oxalic acid, fertilizer and other production equipment; Photography, food industry, facilities in coastal areas, ropes, CD rods, bolts, nuts.

 Performance Introduction

As a low-carbon series of the 316 steel grade, in addition to having the same properties as 316 steel, it has excellent resistance to intergranular corrosion.

Products with special requirements for resistance to intergranular corrosion.

What kind of stainless steel is not prone to rust?

There are three main factors influencing the rusting of stainless steel:

The content of alloying elements.

Generally speaking, when the chromium content is 10.5%, steel is less likely to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, the nickel content of 304 material should be 8-10%, and the chromium content should reach 18-20%. Such stainless steel will not rust under normal circumstances.

2. The smelting process of the manufacturing enterprise also affects the corrosion resistance of stainless steel.

Large stainless steel factories with advanced smelting technology, advanced equipment and advanced processes can ensure the control of alloying elements, the removal of impurities and the control of billet cooling temperature. Therefore, the product quality is stable and reliable, with good internal quality and less prone to rust. On the contrary, some small steel mills have backward equipment and processes. During the smelting process, impurities cannot be removed, and the products they produce are bound to rust.

3. External environment: A dry and well-ventilated climate is less likely to cause rust.

Rust is prone to occur in areas with high air humidity, continuous rainy weather, or environments with high pH levels in the air. 304 stainless steel can also rust if the surrounding environment is too poor.

How should rust spots on stainless steel be dealt with?

1. Chemical method

Use pickling paste or spray to assist in re-passivating the rusted areas to form a chromium oxide film, thereby restoring its corrosion resistance. After pickling, it is very important to rinse it appropriately with clean water to remove all contaminants and acid residues. After all processing, re-polish with polishing equipment and seal with polishing wax. For areas with slight rust spots, a 1:1 mixture of gasoline and engine oil can be used to wipe off the rust spots with a clean cloth.

2. Mechanical methods

Sandblasting cleaning, shot blasting cleaning with glass or ceramic particles, immersion, brushing and polishing. It is possible to remove the contamination caused by previously removed materials, polished materials or submerged materials by mechanical means. All kinds of pollution, especially foreign iron particles, may become sources of corrosion, particularly in damp environments. Therefore, mechanical surface cleaning is best carried out under dry conditions for proper cleaning. Mechanical methods can only clean its surface and cannot change the material's inherent corrosion resistance. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax.

Why do some stainless steels also rust

Why does stainless steel also rust? When brown rust spots (or dots) appear on the surface of stainless steel pipes, people are greatly surprised, thinking that "stainless steel does not rust. If it rusts, it is no longer stainless steel and there might be a problem with the steel quality." In fact, this is a one-sided and wrong view due to a lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation - that is, its non-rusting property, and also has the ability to resist corrosion in media containing acids, alkalis and salts - that is, its corrosion resistance. However, the magnitude of its corrosion resistance varies with the chemical composition of the steel itself, the protective state, the usage conditions and the type of environmental medium. For instance, 304 steel pipes have absolutely excellent anti-rust ability in dry and clean air. However, when they are moved to coastal areas and exposed to sea fog containing a large amount of salt, they will rust very quickly. However, 316 steel pipes performed well. Therefore, not all types of stainless steel can resist corrosion and not rust in any environment.