Stainless steel expert analysis of all aspects of stainless steel

2020-07-29

A few big mistakes that stainless steel material recognizes

1, the magnet on the technetium must not be stainless steel.

This is the biggest mistake in Chinese people's mind! You go to any hardware store, so to speak, and the boss will tell you the same. If you believe it, you are wrong. Magnets don't have to be stainless steel. Stainless steel can be divided into many categories, in which the commonly used classification method has austenite, ferrite, biphasic, martensite, precipitation hardening five categories (don't ask me what these nouns mean, remember, these are all metallic nouns). Only austenitic stainless steel is non-magnetic, but this type of stainless steel occupies the vast majority of the stainless steel market (previously more than 70%, now estimated to be only 60%, to the future less than 50%), the most widely used. And other steel materials are magnetic, this is the Chinese domestic market with magnetic discrimination stainless steel origin. But all other kinds of stainless steel are magnetic. For example, a lot of times when you go to a fancy restaurant and try those shiny stainless steel knives and forks with magnets, they tend to be magnetic, in fact that is probably martensitic stainless steel, and car exhaust pipe stainless steel is also magnetic, that is ferrite.

Austenitic stainless steel often adds Ni, the world Ni price is very high, so a major development trend of stainless steel is how to save nickel, one of the good way is to develop excellent ferrite stainless steel and duplex phase. So in the future, there will definitely be more stainless steel that magnets can hold.

2, stainless steel must not rust?

The public also has a relatively big mistake, that is stainless steel will not rust. In fact, I inherited too much stainless steel corrosion. And I took over all these stainless steel corrosion accidents, a large part is because the user is not enough understanding of stainless steel, in the selection of materials without consideration of the characteristics of stainless steel, misuse of stainless steel, resulting in abnormal rust of stainless steel.

As is known to all, in general atmospheric environment, as long as it is qualified (in line with national standards) stainless steel, will not rust. This is because stainless steel contains an important alloy element Cr (stainless steel Cr content must be above 13%, otherwise it cannot guarantee its excellent corrosion resistance), Cr can form a layer of dense passivation film on the surface of stainless steel, protect stainless steel is not easy to be corroded.

But in many cases, stainless steel will be due to surface passivation film damage and local corrosion. There are many kinds of local corrosion, including point corrosion, crevice corrosion, intergranular corrosion, stress corrosion and galvanic corrosion (above called five local corrosion of stainless steel). One of the things that we see the most [in the home appliance, decoration and tableware industries] is point corrosion and crevice corrosion. The causes of these two kinds of corrosion are the same, and both are related to Cl-. To avoid, the first is to pay attention to avoid stainless steel in Cl- environment for a long time, the second is to pay attention to the use of stainless steel environment clean, to avoid dirt, in addition, also pay attention to avoid scratches on the surface of stainless steel. Stainless steel surface in scratches can often form a new passivation film, but the new formation of the passivation film scratches are generally more fragile, and scratches are easy to form a geometric gap, gap corrosion and rust.

Because there are many types of stainless steel, many types of corrosion, each type is not the same, so can not be generalized.

But if you have to give it somewhere easy to rust, I can give a practical but not necessarily scientific advice:

1) If this place refers to the area of space. In general, places containing more Cl- are more likely to rust, and humid places are more likely to rust. For example, Shanghai, close to the sea, has more Cl-, and is very humid, which is prone to rust.

2) If it refers to any part of the product. Generally speaking, the joint part of the product is easy to corrode because of the gap of the joint part; The deformation part is prone to corrosion due to the phase transition induced by residual stress and deformation. The welding part is easy to corrode because of the unreasonable welding process. The dirt hiding place is easy to corrode because of anoxia and the gap between dirt and stainless steel; Scratches are prone to corrosion; Often scour the impact of hard objects due to erosion corrosion easy to rust.

3, which stainless steel is the best stainless steel?

The reason why this question is put forward as a misunderstanding is that it is a misunderstanding to ask this question. There must be something in the best stainless steel that can't corrode it. In fact, this idea is wrong, at least, as far as I know, no such perfect stainless steel exists.

Each stainless steel has its own specific application and design purpose. When researching, developing and designing materials, it is impossible to take all into account. Cost, corrosion resistance, forming and welding are often difficult to take into account. Even if only looking at corrosion resistance, product development composition design can not take all corrosion types into account. However, it can only be selected and emphasized. For example, Mo should be increased to improve pitting resistance, Ti and Nb should be increased to improve intergranular corrosion resistance. In fact, according to the composition of these different, many grades of stainless steel, up to hundreds of. Among them, the most we encountered are the following:

We have encountered the most classic stainless steel of 304, with 17% -- 19%Cr and 8-11% Ni. The current market price is about 3-4W/t.

316 is resistant to seawater corrosion, with an increase of Mo over 304, and the price is more than 6W.

301 and 302 have higher C content than 304, and the corrosion resistance is also slightly lower.

321 plus Ti, Nb increase weldability, in the past more popular in China, now gradually fade out.

Estimated that you encounter more than these kinds of.

4, how to identify different types of stainless steel

Is this a misunderstanding? That's right. Because by the naked eye, you can hardly tell the difference between different types of stainless steel. It's almost impossible. In addition to the magnetic can simply distinguish between austenitic stainless steel or other types of stainless steel, even stainless steel experts, even if it is to do a lifetime of old experts, can not be distinguished. Distinguish only by special instruments. Of course, the person inside line can estimate according to price, without magnetism, producing area to wait roughly, but absolutely cannot give you very clear very guarantee answer.

What kind of stainless steel should not buy as far as possible not to buy?

1. Too cheap stainless steel. Although this is a bit excessive, but there is definitely a certain truth. To buy stainless steel sink for example, if a carry in the hand more than 10 kilograms (have? Not clear, take bibcock weight) the stainless steel sink of left and right sides, if sell price in 300 below, and the boss still tells you to do not have magnetism, it is stainless steel certainly. So, tx, you don't want to buy it. Why is that? Didn't I tell you the current market price of 304? (Although the price is only in the second half of last year, but I believe the price this year should only be higher than this price), 3W per ton, that is only the factory price oh, you can not get this price without buying hundreds of tons. That's 30 pieces per kg (that's expensive, you know why the stainless steel hardware is so expensive?). So, the cost of materials alone, a 10 kg most water tank, to 300 yuan, including wear and loss, processing costs, transportation, taxes, the boss to earn profit, management fees and so on, less than 400 yuan you can buy it? So there's only one possibility, adulteration.

2, 200 stainless steel do not buy. So far as the question of adulteration is concerned, let us proceed. In fact, why want to post here, is because considering a lot of places in the decoration to use stainless steel, and a lot of TX, including sales, may be kept in the dark. Post, the most important is to tell you do not blindly believe in magnetic discrimination, do not be intentionally or unintentionally cheated by the sales.

Seems to have said a lot of nonsense, next to get down to business. Since 2000, due to the magnetic psychology of Chinese users, there have been a lot of inferior low-NI stainless steels in China, they are called 200 (actually they should be called 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 steels will be easy to become ferrite (ferritic stainless steels contain little or no Ni), which will not make use of everyone's magnetic psychology for publicity and promotion. As a result, manufacturers have to take the next step -- reduce Cr and increase C and N content, because then they can get austenite again. (Very professional, if you don't understand why this is happening, I can explain this, but I don't think it is necessary). However, this has created a very serious problem, read the previous post tX know, stainless steel corrosion resistance (may not be very scientific, but because it can be said so) and Cr content has a lot to do with Cr content, Cr content is higher corrosion resistance is better. In addition, high C% will also have a great negative impact on the corrosion resistance of stainless steel, while the N% content is very difficult to be high (high will become nitrogen overflow, of course, high N is an important development direction of stainless steel), and high formability is not good. As a result, the corrosion resistance of this stainless steel is low (it will be much better than ordinary steel, but much worse than the previous 304, I am doing this research, research on the difference between the two corrosion resistance).