How to correctly identify stainless steel materials？

How to correctly identify stainless steel and non-stainless steel materials?

It is believed that the magnet attracts stainless iron, but the stainless steel cannot attract. Good stainless steel has no magnet. In fact, this is a misunderstanding of people’s understanding of stainless steel.

In the eyes of many consumers, 304 stainless steel and 201 stainless steel are almost indistinguishable from each other with the naked eye. Here are some ways to distinguish 304 and 201.

Composition difference
AISI(304) ≤0.08 ≤1.00 ≤2.00 ≤0.045 ≤0.03 18-20 8-10
AISI(201) ≤0.15 ≤1.00 5.5-7.5 ≤0.05 ≤0.03 16-18 3.5-5.5
The composition of 201 is 17Cr-4.5Ni-6Mn-N, which is an alternative to Ni-saving steel and 301 steel. After cold processing, it has magnetism and is used in railway vehicles.
The composition of 304 is 18Cr-9Ni, which is the most widely used stainless steel and heat-resistant steel. Used in food production equipment, Xitong chemical equipment, nuclear energy, etc.

A: Can a magnet be used to distinguish stainless steel? ? ?
Q: No.

stainless steel is more and more popular with people because of its beautiful appearance, corrosion resistance and not easy to damage. stainless steel is used more and more in pots and pans, urban sculptures, buildings, and decorated rooms. When purchasing stainless steel kitchen utensils, some customers hold magnets to suck them around on the stainless steel utensils. In fact, this is wrong, and it is a misunderstanding of people’s understanding of stainless steel.

Iron and steel are distinguished by their carbon content. Iron-carbon alloys with carbon content below 2% are steel, and those with carbon content above 2% are called iron. Steel is widely used because of its toughness, elasticity and rigidity. All you come into contact with in life are steel, but people call it differently. For stainless steel, no matter whether the magnet is attracted or not, as long as it meets its quality standards, it is stainless steel.

Therefore, from a metallurgical point of view, there is no such thing as stainless iron. The main element of corrosion resistance of stainless steel is chromium. Steel with chromium content above 10.5% will not rust. The alloying elements added during smelting are different, and there is a difference between magnets that can attract and cannot attract.

stainless steel is generally divided into organizational structure, which can be divided into austenite, ferrite, martensite, etc. If different proportions of chromium-nickel are added to the molten steel, the austenitic steel produced is a stainless steel that cannot be attracted by magnets; if chromium and a small amount of nickel (or no nickel) are added to the molten steel, the steel produced is a magnet Absorbable stainless steel is also called ferritic stainless steel; the main alloying elements of martensitic stainless steel are chromium, iron and carbon.

There are more than 100 kinds of stainless steel due to the different alloy content. Except for austenitic stainless steel magnets that cannot be attracted, ferritic and martensitic stainless steels are magnetic. Nickel has very few reserves in the world, and the price is relatively expensive. Therefore, the price of stainless steel with high nickel content is also higher in the market, and the magnet cannot be attracted. In fact, there is another kind of stainless steel magnet that cannot be attracted. That is stainless steel with high manganese content and less nickel or no nickel. The market price of this kind of stainless steel is more than 1,000 yuan per ton lower than that of stainless steel with high nickel content. Some dealers use people’s misunderstanding that “good stainless steel magnets can’t attract” to deceive consumers and make their prices as expensive as high-nickel stainless steel.

There are more than 100 kinds of stainless steel, with different characteristics and functions. Generally, austenitic stainless steel is used for decoration, landscape and sculpture. Because of the low thermal conductivity of austenitic stainless steel, it is not appropriate to use it as a kettle, wok, and rice cooker. It will use a lot of energy and prolong the cooking time. Using ferritic stainless steel to make woks and rice pots not only has excellent corrosion resistance, but also its thermal conductivity is nearly half higher than that of austenitic stainless steel. Ferritic stainless steel should be used for the inner tub of washing machine, water heater, vegetable washing basin, etc., as long as the utensils in contact with water should be used. Some stainless steel knives are not fast because some stainless steels (such as austenitic stainless steel) cannot be hardened and are not suitable for knives and cutting tools. Tools made of martensitic stainless steel can change the hardness of stainless steel through heat treatment, such as quenching and tempering.

A: Why is stainless steel also magnetic?
Q: People often think that magnets absorb stainless steel to verify its quality and authenticity. It does not attract and has no magnets. It is considered to be good and genuine. If the magnet is magnetic, it is considered to be a counterfeit. In fact, this is an extremely one-sided and impractical method of identifying errors.

There are many types of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:
1. Austenite type: such as 304, 321, 316, 310, etc.;
2. Martensite or ferrite type: such as 430, 420, 410, etc.;

The austenitic type is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.
Most of the stainless steel commonly used for decorative tube sheets is austenitic 304, which is generally non-magnetic or weakly magnetic. However, due to chemical composition fluctuations or different processing conditions caused by smelting, magnetism may also appear, but this cannot be considered as What is the reason for counterfeit or unqualified?

As mentioned above, austenite is non-magnetic or weakly magnetic, while martensite or ferrite is magnetic. Due to component segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel will be caused. Body tissue. In this way, 304 stainless steel will have weak magnetic properties.

In addition, after 304 stainless steel is cold worked, the structure will also transform to martensite. The greater the degree of cold working deformation, the more martensite is transformed and the greater the magnetic properties of the steel. Like a batch of steel belts, Φ76 tubes are produced without obvious magnetic induction, and Φ9.5 tubes are produced. Because the bending deformation is larger, the magnetic induction is more obvious. The deformation of the square rectangular tube is larger than that of the round tube, especially the corner part, the deformation is more intense and the magnetism is more obvious.
In order to completely eliminate the magnetic properties of 304 steel caused by the above reasons, the austenite structure can be restored and stabilized by high-temperature solution treatment, thereby eliminating the magnetic properties.

In particular, the magnetism of 304 stainless steel caused by the above reasons is not at the same level as other materials of stainless steel, such as 430 and carbon steel. That is to say, the magnetism of 304 steel always shows weak magnetism.

This tells us that if the stainless steel is weakly magnetic or not at all, it should be judged as 304 or 316; if it is the same as carbon steel, it shows strong magnetism, because it is judged as not 304.