Analysis of Stainless Steel in Machining Industry

Analysis of stainless steel

1. What is stainless steel?

Stainless steel is a type of steel. Steel refers to steel containing less than 2% carbon (C), and more than 2% is iron. Adding chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo) and other alloying elements in the steel smelting process improves the performance of the steel and makes the steel have corrosion resistance (that is, no rust) is what we often call stainless steel.

2. Why do stainless steel have different grades?

During the smelting process of stainless steel, due to the different types of alloying elements added, the amount of different types added is different. Its characteristics are also different. In order to distinguish them, different steel grades are crowned. The following is a table of "alloy element" content of different steel grades of common decorative stainless steel for reference only:

Steel grade carbon (C) silicon (Si) manganese (Mn) phosphorus (P) sulfur (S) chromium (Cr) nickel (Ni) molybdenum (Mo) copper (Cu)

304 ≤0.08 ≤1.00 ≤2.00 ≤0.045 ≤0.03 18-20 8-10

301 ≤0.15 ≤1.00 ≤2.00 ≤0.045 ≤0.03 16-18 6-8

202 ≤0.15 ≤1.00 7.5-10 ≤0.05 ≤0.03 17-19 4-6

201 ≤0.15 ≤1.00 5.5-7.5 ≤0.05 ≤0.03 16-18 3.5-5.5

3. What kind of stainless steel is not easy to rust?

There are three main factors affecting the corrosion of stainless steel:

One: the content of alloying elements, generally speaking, steel with a chromium content of 10.5% is not easy to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, the content of nickel in 304 material should be 8-10%, and the content of chromium should reach 18-20%. Such stainless steel will not rust under normal circumstances.

Second: The smelting process of the production enterprise will also affect the corrosion resistance of stainless steel. A large stainless steel factory with good smelting technology, advanced equipment and advanced technology can guarantee the control of alloy elements, the removal of impurities, and the control of billet cooling temperature. Therefore, the product quality is stable and reliable, and the internal quality is good. , not easy to rust. On the contrary, some small steel mills have backward equipment and backward technology. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.

Third: the external environment, dry and well-ventilated environment is not easy to rust. The air humidity is high, the continuous rainy weather, or the environmental area with high pH in the air is easy to rust. 304 stainless steel, if the surrounding environment is too bad, it will rust.

4. Stainless steel is without magnetism. Is it good stainless steel without magnetism? If the microstrip is magnetic, isn't it 304?

Many customers go to the market to buy stainless steel, and they bring a small magnet with them. When they look at the goods, they think that the ones that cannot be sucked are good stainless steel. There will be no rust without magnetism. In fact, this is a wrong understanding. Whether the stainless steel belt is magnetic or not is determined by the organizational structure. During the solidification process, due to the different solidification temperatures, stainless steel with different organizational structures such as "ferrite", "austenite" and "martensite" will be formed. Among them, "iron Element body" and "martensitic" stainless steel are magnetic. The "austenitic" stainless steel has good comprehensive mechanical properties, process performance and weldability, but only in terms of corrosion resistance, the magnetic "ferritic" stainless steel is stronger than the "austenitic" stainless steel. The so-called 200 series and 300 series stainless steels currently in the market with high manganese content and less nickel content are not magnetic, but their performance is far from that of 304 with high nickel content. On the contrary, 304 has been stretched and annealed , Polishing, casting and other processes will also be slightly magnetic, so it is a misunderstanding and unscientific to judge the quality of stainless steel with or without magnetism.

5. Why does stainless steel rust?

When brown rust spots (spots) appear on the surface of stainless steel, people are greatly surprised: they think that "stainless steel will not rust, and if it gets rusty, it is not stainless steel. It may be that there is a problem with the quality of the steel." In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel also can get rusty under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation, that is, stainless, and also has the ability to resist corrosion in media containing acids, alkalis, and salts, that is, corrosion resistance. But the size of its resistance to corrosion with the chemical composition of its steel itself, mutual state, working conditions and surrounding medium type and change. Like 304 steel pipes, in the atmosphere of dry cleansing, absolute good resistant to corrosion ability is arranged, but it is moved on to riviera, in containing the sea fog of a large amount of salts, and will soon get rusty. Therefore, not any kind of stainless steel can resist corrosion and rust at any time.

Stainless steel obtains the ability to resist corrosion by means of an extremely thin, firm, dense and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to infiltrate and oxidize. Once there is some reason, this film is continuously damaged, and the oxygen atoms in the air or liquid will continuously infiltrate or the iron atoms in the metal will continuously separate out to form loose iron oxide, and the metal surface will be continuously damaged. rust. There are many forms of damage to this surface film, and the most common ones in daily life are as follows:

1. The surface of stainless steel accumulates dust containing other metal elements or attachments of heterogeneous metal particles.

In the air, the condensed water between the attachment and the stainless steel connects the two into a micro-battery, which triggers an electrochemical reaction and damages the protective film, which is called electrochemical corrosion.

2. Organic juices (such as vegetables, noodle soup, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, organic acids will be formed. For a long time, organic acids will form organic acids that will corrode the metal surface.

3. The surface of stainless steel adheres to substances containing acids, alkalis, and salts (such as alkaline water and lime water sprayed on walls for decoration) to cause local corrosion.

4. In polluted air (atmosphere containing a large amount of sulfides, oxides, and hydrogen oxides), when it encounters condensed water, it will form sulfuric acid, nitric acid, and acetic acid liquid points, causing chemical corrosion.

The above conditions can cause damage to the protective film on the stainless steel surface and cause corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not rusted, we recommend:

① The surface of the decorative stainless steel must be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause corrosion.

② There is a kind of 201 and 202 stainless steel on the market that is easy to rust in coastal areas, and is suitable for use in environments without industrial pollution and air corrosion.

③ Seaside areas should use 304 stainless steel, which can resist seawater corrosion.

6. How to deal with rust spots on stainless steel?

a) Chemical method:

Use pickling cream or spray to assist the re-passivation of the rusted parts to form a chromium oxide film to restore the corrosion resistance. After pickling, in order to remove all pollutants and acid residues, it is very important to rinse properly with clean water. After all treatment, re-polish with polishing equipment and seal with polishing wax. For those with slight rust spots, you can also use a 1:1 mixture of gasoline and engine oil to wipe off the rust spots with a clean rag.

b) Mechanical method:

Sand blasting, shot blasting with glass or ceramic particles, obliteration, brushing and polishing. It is possible to mechanically wipe away contamination from previously removed material, polishing material or obliterating material. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should ideally be properly cleaned under dry conditions. The use of mechanical methods can only clean the surface, and cannot change the corrosion resistance of the material itself. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax.