How to Recognize Stainless Steel

In our daily life, we can always see stainless steel around us , such as stainless steel decorative hollow spheres, stainless steel tableware, etc. Its “non-rusting” performance may leave a great impression on people’s minds. Prevalent as it is, I believe that most people know very little about stainless steel. Don’t you believe? Then you can try to answer the following questions: How many types of stainless steel are there? What is the difference between the three most common grades of stainless steel, # 201 # 304 # 316? Does stainless steel really anti-rust? As a stainless steel hollow ball supplier, I often encounter people coming and asking me these questions. Therefore, I would like introduce to you how to recognize stainless steel from 6 aspects.

What is stainless steel?

Stainless steel is an alloy, which is formed by two or more separate elements. The most common ones we can see in our daily life is grade 304 stainless steel. Like all steels, the main alloying element of stainless steel is metallic iron. However, unlike other steels, stainless steel can only contain up to 1.2% carbon. In addition, the alloy also includes many different elements, such as nickel, silicon, manganese, titanium, molybdenum, nitrogen, niobium, etc. Adding metal chromium to stainless steel can make it have high rust resistance, which helps stainless steel strengthen its corrosion-resistant. It is fair to say that stainless steel is a combination of metal and other elements. It is the different combinations of these materials that lead to different properties of stainless steel. Here, Haixu Co., Ltd will introduce to you how to recognize stainless steel.

Stainless Steel

The type of stainless steel

Stainless steel is just a general term. In fact, it has many subdivisions. For example, you may have heard of 304 stainless steel, food grade stainless steel and the like. From a professional perspective, they can be divided into 5 categories. They are martensitic stainless steels, Ferritic stainless steels, Austenitic stainless steel, Duplex stainless steel, Precipitation hardening stainless steels.

Martensitic stainless steel

Martensitic stainless steel is a corrosion-resistant and hardenable stainless steel. All martensitic stainless steel grades are nickel-free chromium steel. All grades of martensite are magnetic. This type of stainless steel is mainly used in industries that require high hardness, strength and wear resistance. Common grades of martensitic stainless steel are # 410 # 414 # 416 # 420 # 431 # 440.

Martensitic stainless steel
Martensitic stainless steel

Ferritic stainless steels

Ferritic stainless steel is resistant to corrosion and oxidation. Although this type of steel is magnetic, it cannot be hardened by heat treatment. Although ferritic stainless steel has higher corrosion resistance than martensite, its corrosion resistance is still not as good as austenitic stainless steel. This type of stainless steel is commonly used in decorative strips, sinks, and certain automotive applications. Common grades of this type are: # 430 # 405 # 409 # 434 # 436 # 442 # 446.

Ferritic stainless steel
Ferritic stainless steel

Austenitic stainless steel

Austenitic stainless steel is the most common type of stainless steel that we see in our life. This type of stainless steel contains high chromium and nickel content, so it has excellent corrosion resistance and mechanical properties. Austenitic stainless steel cannot be hardened by heat treatment, but can be hardened by zero work. This type of stainless steel is basically non-magnetic. The most common austenitic stainless steel grades are # 304 # 316.

Austenitic stainless steel
Austenitic stainless steel

Duplex stainless steel

Duplex stainless steel is a combination of austenitic and ferritic materials. This type of stainless steel has better toughness and ductility than ferritic stainless steel, but it does not reach the level of austenite. # 2205 is the most commonly used material in duplex stainless steel.

Duplex stainless steel
Duplex stainless steel

Precipitation hardening stainless steels

The American Iron and Steel Institute uses 600 series numbers, such as 630, that is 17-4PH.

Generally speaking, in addition to alloys, the corrosion resistance of austenitic stainless steel is relatively excellent. In the environment with low corrosion, ferritic stainless steel can be used. In the mildly corrosive environment, if the material is required to have high For strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be used.

Precipitation hardening stainless steels
Precipitation hardening stainless steels
Martensitic stainless steel# 420 # 431Can be hardened by heat treatmentLow corrosion resistance compared with Austenitic
Ferritic stainless steels# 410 # 430Low cost, medium corrosion resistance and good formabilityCompared with austenite, corrosion resistance, moldability and high temperature resistance are poor
Austenitic stainless steel# 304 # 316The most widely used, relatively good corrosion resistance and low temperature toughness, excellent formability and weldabilityWork hardening limits formability and machinability
Duplex stainless steel# 2205Good mechanical strength in the annealed stateApplication temperature range is more restricted than austenitic stainless steel
Precipitation hardening stainless steels17/4PHCan be hardened by heat treatment, but has better corrosion resistance than martensiteCompared with austenite, corrosion resistance, formability and weldability are poor

Pros and cons of these 5 types of stainless steel

Which stainless steel is better, how to choose?

After introducing the above five types of stainless steel, I believe you have a preliminary understanding of each type of stainless steel and its properties. Then someone will ask, which of the above stainless steel is better, how to choose?

In fact, no type of stainless steel is good or bad. Whether it is good or not depends on whether it is suitable for your usage or not. Stainless steel is an iron alloy, in which the chromium content is at least 10.5%, while other alloying elements are used to increase its performance. But no matter what, basically speaking, stainless steel is a kind of steel with corrosion resistance.

From an economic perspective, stainless steel can compete with better-cost engineering metals and nickel- or titanium-based alloys while providing corrosion resistance. Their strength is higher than most polymer products (GRP).

When choosing stainless steel, the most important thing is to consider the following points:

1. Mechanical and physical properties

2. Corrosion resistance

3. Manufacturing process requirement for formability, strength and high temperature resistance 

4. Environmental and material costs

For example, if we want to make stainless steel railings and stainless steel balls, we can choose grade 304 stainless steel; if we want to make stainless steel bowls, stainless steel cups and other food grade products, we should use food grade 304 stainless steel.

What is the difference between the most common stainless steel grade201, 304 and 316?

You may not understand the relationship between 201, 204 and 316 stainless steel. Haixu will teach you how to distinguish them. Before explaining the difference between these three, let’s take a look at the difference in their chemical composition.

Chemical Composition

%C0.150 Max0.080 Max0.080 Max
%Si1.00 Max0.75 Max1.00 Max
%Mn5.50-7.502.00 Max2.00 Max
%P0.060 Max0.045 Max0.045 Max
%S0.030Max0.030 Max0.030 Max
%N0.250 Max0.100 Max

Grade 201 stainless steel

# 201 stainless steel existed because of the soaring nickel price. In order to avoid the excessive rise in nickel prices and lead to rising raw material products, 201 was invented. From this perspective, this also means that grade 201 stainless steel is relatively low in nickel content and relatively low in price. However, through the above analysis, we also know that the higher the nickel content, the stronger the corrosion resistance, and vice versa. As I said above, stainless steel is not good or bad, the point is on its use. # 201 stainless steel decorative ball is suitable for people who look for more durability at a lower cost and do not worry about being exposed to corrosive elements, such as certain projects or home decoration.

Stainless Steel Hollow Sphere
# 201 Stainless Steel Hollow Hemisphere

Grade 304 stainless steel

# 304 stainless steel is the most common type of stainless steel. The nickel content of 304 stainless steel is higher than other general types of stainless steel, such as 201 stainless steel. Due to the increase in nickel prices, the price of 304 stainless steel will be slightly more expensive than # 201. However, due to the high nickel content, the corrosion resistance of 304 stainless steel is also stronger than that of 201. This is why Haixu’s stainless steel balls are usually made of 304 stainless steel.

# 304 Stainless Steel Gazing Ball

Grade 316 stainless steel

As what we can see from the table above, the lowest nickel content is # 201 stainless steel, followed by # 304, and the highest is # 316. Therefore, it is obvious that the corrosion resistance of 316 is the strongest among # 201 and # 304, and the price is also the highest. However, for some situations, the high cost is worth it. In environments with more chloride, such as coastal areas, # 316 stainless steel is the most ideal choice. Although 304 stainless steel seems to be a more cost-effective option, its resistance to salt and chloride is far less good than # 316. From a long-term perspective, the chloride environment accelerates the decomposition rate of # 304. If you need to replace it, you will cost more. The choice of stainless steel type depends on its use. In some cases, it is not cost-effective to buy # 316 stainless steel balls. In other cases, in the long run, high-priced # 316 stainless steel balls can save you more money.

Stainless Steel Pattern Balls
# 316 Stainles Steel Sphere Water Feature

If you have the idea of putting decoration to your indoors or outdoors, you might consider stainless steel decoration balls. It is famous for its practical, durable and beautiful features. In the choice of steel, if you have low budget and there are no corrosive elements in the location, then 201 stainless steel hollow balls can meet your needs. And if you are not sure whether the environment you place contains corrosive elements, then we can choose 304 stainless steel balls, which will be more durable and more corrosion resistant than 201 stainless steel balls. This is also a steel choice we recommend. If the environment you place needs more corrosion-resistant metal balls, then 316 stainless steel decorative balls will be your ideal choice.

How to identify stainless steel grade 201, 304 and 316?

Identifying stainless steel types require a certain amount of professional knowledge. Haixu, as a professional stainless steel ball/hemisphere/sculpture supplier, will share with you a simple way to distinguish #201 #304 #316 stainless steel.

The simplest and more accurate detection method is to use stainless steel pickling paste. It can approximate the content of nickel and molybdenum in stainless steel by the chemical compound in the liquid medicine and the nickel and molybdenum in stainless steel with a specific color complex. For example: apply the pickling paste to these two types of stainless steel separately, and then after waiting for 2 minutes, observe the color change of the stainless steel. If it appears dark color, it means it is # 201. if white or colorless, then it is #304.

However, the stainless steel test solution cannot detect the content of other chemical components in stainless steel, such as chromium. Therefore, if you want to know the precise data of each chemical composition in stainless steel, you must send it to a professional tester. Therefore, if possible, try to choose a reliable supplier. Haixu is a  stainless steel ball/hemisphere/sculpture manufacturer and supplier in China. We have been on this industry for more than 20 years and are known for high-quality products and excellent service.

Does stainless steel really anti-rust?

Stainless steel will also corrode under certain conditions. First, let’s take a look at the principle of corrosion resistance.

Stainless steel forms an oxide on the surface. At present, the stainless mechanism of all stainless steels on the market is due to the presence of Cr element. The fundamental reason (mechanism) of stainless steel corrosion resistance is the passivation film theory. The so-called passivation film is a thin film mainly made of Cr2O3 on the surface of stainless steel. Because of the existence of this film, the corrosion of the stainless steel substrate in various media is hindered. This phenomenon is called passivation.

There are two situations for the formation of this passivation film. One is that stainless steel itself has the ability to self-passivate. This self-passivation ability accelerates with the increase of chromium content, so it has rust resistance; the other one is that stainless steel forms a passivation film in the process of being corroded in various aqueous solutions (electrolytes), which prevents corrosion. When the passivation film is damaged, a new passivation film can be formed immediately.

The reason why the stainless steel passivation film has the ability to resist corrosion is for its three characteristics. one is that the thickness of the passivation film is extremely thin, and under the condition of chromium content> 10.5%, it is generally only a few microns. The second is the specific gravity of the passivation film greater than the specific gravity of the substrate; these two characteristics indicate that the passivation film is both thin and dense. Therefore, the passivation film is difficult to be penetrated by the corrosive medium to quickly corrode the substrate. The third feature is the chromium concentration ratio of the passivation film. The substrate is more than three times higher; therefore, the passivation film has high corrosion resistance.

In short, , a very stable film will be formed on the surface of stainless steel, generally at the nanometer level. This layer of film is called a passivation film, which is not easy to chemical and electrochemical reactions in the environment so to protect the material from rust and have a strong self-healing ability, and it will be generated soon after it is broken.

Why is stainless steel corroded and under what circumstances it will be corroded?

The corrosion resistance of stainless steel is conditional. A grade of stainless steel is resistant to corrosion in one medium, but may be damaged in another medium. At the same time, the corrosion resistance of stainless steel is also relative. So far, no stainless steel has been absolutely non-corrosive in all environments. And stainless steel is not necessarily not rusty, but the corrosion rate is much lower than other steels in the same environment, and sometimes it can even be ignored.

Conditions that cause corrosion include:

  1. Characteristics of materials: physical and chemical properties of materials
  2. Environmental conditions: temperature, humidity, fluid velocity, oxygen content and PH
  3. Media causing corrosion: acids, alkalis, salts, oxides, organic matter and microorganisms

Take the 304 stainless steel decorative ball as an example. Under normal circumstances, the 304 stainless steel ball is not easy to corrode, but when it is in an environment containing salts and chlorides, some chemical substances in the 304 stainless steel ball will chemically react with it to cause rust. But if 316 stainless steel balls are used in this environment, the situation will be much better. The different chemical composition of 316 stainless steel ball makes it less susceptible to corrosion in this environment.

You may want to know how to get rust spot off stainless steel sphere.