Little known Faq Of Stainless Steel Material

1. It must not be stainless steel that the magnet can attract.

This is the biggest misunderstanding in people's minds! It can be said that if you go to any hardware seller, the boss will tell you so. If you believe it, then you are actually wrong. What magnets can attract is not necessarily stainless steel. (Attention, there are three not words in this sentence) Stainless steel can be divided into many categories, among which the commonly used classification methods are austenite, ferrite, duplex, martensite, precipitation hardening and other five categories. Among them, only austenitic stainless steel is non-magnetic, but this kind of stainless steel accounts for most of the stainless steel market (previously, it was more than 70%, now it is only estimated to be 60%, and it will be below 50% in the future), and it is the most widely used. Other steel materials are magnetic, which is why the Chinese domestic market uses magnetism to distinguish stainless steel. But other types of stainless steel are magnetic. For example, many times you go to high-end restaurants to try those shiny stainless steel cutlery with magnets. They are often magnetic. In fact, they are probably martensitic stainless steel, and the stainless steel of automobile exhaust pipes is also magnetic. It's ferritic.

Ni is often added to austenitic stainless steel, and the current world price of Ni is very high. Therefore, a major development trend of stainless steel is how to save nickel. One of the good ways is to develop excellent ferritic stainless steel and duplex. Therefore, in the future, there will be more stainless steel that can be absorbed by magnets.

2. Stainless steel will not rust?

The public still has a big misunderstanding, that is, stainless steel will not rust. In fact, I have taken over too many things about stainless steel corrosion. And a large part of all these stainless steel corrosion accidents that I took over was because users did not know enough about stainless steel. They did not consider the characteristics of stainless steel materials when selecting materials. The abuse of stainless steel caused abnormal corrosion of stainless steel.

As we all know, in the general atmospheric environment, as long as it is qualified (conforming to the national standard) stainless steel, it will not rust. This is because stainless steel contains an important alloying element Cr (the content of Cr in stainless steel must be more than 13%, otherwise its excellent corrosion resistance cannot be guaranteed). Cr can form a dense passivation film on the surface of stainless steel to protect Stainless steel is not easily corroded.

However, in many cases, stainless steel still undergoes local corrosion due to damage to the passive film on the surface. There are many types of local corrosion, including pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion and galvanic corrosion (the above are referred to as the five local corrosion of stainless steel). Among them, the most encountered (household appliances, decoration and tableware industries) are pitting corrosion and crevice corrosion. The causes of these two types of corrosion are the same, and both are inseparable from Cl-. To avoid it, firstly try to avoid stainless steel in the Cl- environment for a long time, secondly, pay attention to the cleanliness of the stainless steel use environment to avoid dirt, and also pay attention to avoid scratches on the stainless steel surface. A new passivation film can often be formed on the surface of stainless steel after scratches, but the newly formed passivation film from scratches is generally fragile, and the scratches are easy to form geometric gaps, which are prone to crevice corrosion and rust.

Because there are many types of stainless steel and many types of corrosion, each type is not the same, so it is not possible to generalize. However, if you must give a place that is prone to rust, I can give a more practical but not necessarily scientific and rigorous suggestion:

• 1) If this place refers to a spatial area. Generally, places with more Cl- are more prone to rust, and damp places are more prone to rust. For example, Shanghai, near the sea, has a lot of Cl-, and it is very humid, prone to rust.
• 2) If it refers to any part of the product. Generally speaking, the connection part of the product is prone to corrosion due to the crevice of the connection part; the deformed part is prone to corrosion due to the phase change induced by residual stress and deformation; the welded part is prone to corrosion due to the unreasonable welding process; the place where dirt and dirt are easily stored is easy to corrode. Corrosion occurs in the gap between the lack of oxygen and dirt and stainless steel; the scratched area is prone to corrosion; the area that is often scoured and hit by hard objects is prone to rust due to erosion and corrosion.

3. Which stainless steel is the best stainless steel?

The reason why this question is also raised as a misunderstanding is because in fact, asking this question itself has a misunderstanding. That is, there must be the best stainless steel that cannot be corroded by anything. In fact, this idea is wrong, at least, as far as I know, there is no such perfect stainless steel. Each type of stainless steel has its specific application area and its design purpose. When materials are developed and designed, it is impossible to balance everything. Cost, corrosion resistance, forming, and welding are often difficult to balance. Even if we look at corrosion resistance alone, it is impossible to take into account all types of corrosion in the design of product development components. But it can only choose to focus, such as increasing Mo in order to improve the resistance to pitting corrosion, and increasing Ti and Nb in order to improve the resistance to intergranular corrosion. In fact, according to the difference of these components, there are many grades of stainless steel, as many as hundreds. Among them, the ones we have encountered most are the following:

• 304 is the most classic stainless steel, we have encountered the most, 17%-19% Cr, 8-11% Ni, the current market price is about 3-4W/t.
• 316 is resistant to seawater corrosion. Compared with 304, Mo is increased, and the price is more than 6W. 301 and 302 have higher C content than 304, and correspondingly lower corrosion resistance.
• 321 Adding Ti and Nb to increase weldability, which was popular in China before, has gradually faded out. It is estimated that these are the ones that you have encountered more.

4. How to identify different types of stainless steel

This question is also misunderstood? correct. Because it is difficult for you to distinguish between different types of stainless steel with the naked eye. It's almost impossible. In addition to being able to simply distinguish whether it belongs to austenitic stainless steel or other types based on its magnetic properties, even an expert in stainless steel, even an veteran expert who has been working for a lifetime, cannot distinguish it. It can only be distinguished by special equipment. Of course, people in the industry can get a rough idea based on the price, whether there is magnetism, the place of production, etc., but they can never give you a clear and guaranteed answer.

What kinds of stainless steel should be avoided if possible?

• Too cheap stainless steel. Although this statement is a bit excessive, it definitely makes sense. Take buying a stainless steel sink as an example. If a stainless steel sink with more than 10 kilograms in your hand (does it? I don’t know the weight with a faucet), if the selling price is less than 300 yuan, and the boss told you that it is not magnetic, sure It is stainless steel. Then, you must never buy. why? Didn’t I tell you the current market price of 304? (Although it is only the price in the second half of last year, but I believe this year's price should only be higher than this price), 3w per ton, that is only the ex-factory price, you can't get this price if you don't buy hundreds of tons. It is 30 yuan per kilogram (so expensive, do you know why stainless steel hardware is so expensive?), then, the cost of light materials, a 10 kilograms of the most common sink, will cost 300 yuan, plus the loss, processing cost, transportation, Taxes, profits to be earned by the boss, management fees, etc., are you likely to buy it for less than 400 yuan? Therefore, there is only one possibility, adulteration.
• Do not buy 200 stainless steel. Since 2000, because of the magnetic psychology of catching users, there have been many inferior low-Ni stainless steels in China. They are called 200 (in fact, they should be called the new 200 series). These stainless steels contain very low Ni. If the Ni content is low, there will be a problem. If the Ni content is low, the stainless steel will easily become ferrite (ferritic stainless steels contain little or no Ni), so you cannot use your magnetism. Psychology has been promoted. As a result, the manufacturers have to take another step-reduce the content of Cr and increase the content of C and N, because in this way, the austenite structure can be obtained again (good professional, if you don’t understand it, you don’t have to investigate why this is the case. , I can explain this clearly, but I don't think it is necessary). However, this has produced a very serious problem. The corrosion resistance of stainless steel (which may not be very scientific, but because it can be said) has a lot to do with the Cr content. The higher the Cr content, the better the corrosion resistance. In addition, high C% will also have a great negative impact on the corrosion resistance of stainless steel, while the N% content is difficult to high (higher will become nitrogen overflow, of course, high N is an important development direction of stainless steel), and high Formability is not good. In this way, the corrosion resistance of this kind of stainless steel is low (it will be much better than ordinary steel, but much worse than the previous 304), but due to the decrease of Ni content (most of this kind of stainless steel has Ni content almost below 1%) Therefore, the cost of this type of stainless steel is very low, and the ex-factory price can be only about 1.5w.

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By Minghe Die Casting Manufacturer |Categories: Helpful Articles |Material Tags: Aluminum Casting, Zinc Casting, Magnesium Casting, Titanium Casting, Stainless Steel Casting, Brass Casting,Bronze Casting,Casting Video,Company History,Aluminum Die Casting |Comments Off