What is stainless steel? A detailed explanation of its characteristics, types, and why it is resistant to rust
- Materials
Stainless steel has a wide range of uses, and is used to make a wide variety of products, from everyday items such as kitchen sinks, tableware, and cutlery, to building exteriors, computers, semiconductor manufacturing equipment, automobile parts, and aircraft structural materials.
The biggest feature of stainless steel is its resistance to rust, but there are many types, with over 200 types specified by the JIS alone. There is a wide range of chemical properties, mechanical properties, and prices, so it is important to choose the right type for your application.
In this article, we will first discuss the characteristics and types of stainless steel, then carefully explain the mechanism behind its rust resistance, and finally, the circumstances under which stainless steel can rust.
What is stainless steel?
Simply put, stainless steel is steel (an alloy of iron with a few percent carbon) mixed with more than 12% chromium.Its biggest feature is that it is resistant to rust, so the name comes from the English word "stainless," which means that it does not rust. Although it is technically called stainless steel, it is commonly called stainless steel.
Stainless steel whose main components are only iron and chromium is called chromium-based stainless steel, and stainless steel whose main components are nickel is called chromium-nickel-based stainless steel.
A few percent of other elements are added to improve the properties of stainless steel, but remember that the main elements are iron, chromium, and nickel, and may or may not be present.
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Characteristics of Stainless Steel
There are many different types of stainless steel, but we will introduce some common characteristics of most stainless steels.
- Hard to rust
- Low thermal conductivity
- Hard to cut
Hard to rust
The greatest feature of stainless steel is its resistance to rust.
First of all, what exactly is rust? If an iron nail is left outdoors, it will change from its shiny silver color to a rough, reddish-brown surface. This is called rust, and in the case of iron, it is called red rust because it is red.
Rust is the reaction of metal surfaces with oxygen and moisture, resulting in the formation of an oxide layer.Most metal elements are more stable when ionized as oxides than when they remain in their metallic state. For example, iron is normally covered with red rust.
Since metal is in a corroded state, rust is also called "corrosion" and the resistance to rust is also called "corrosion resistance."
However, because iron is difficult for humans to use when it is rusty, a rust-resistant iron was invented. This is stainless steel. The reason why stainless steel is rust-resistant is explained in "Why stainless steel is rust-resistant".
Low thermal conductivity
Stainless steel has low thermal conductivity,It is about half to one-third the weight of steel, and about one-eighth to one-thirteenth the weight of aluminum.
Therefore, stainless steel pots have the disadvantage that they do not conduct heat well and are prone to burning due to localized heating. However, they are still used because of their good looks and rust resistance.
One application where the low thermal conductivity of wood can be utilized to its advantage is the inner walls of water bottles and pots. Because wood does not easily transfer heat, it has excellent heat retention properties.
Hard to cut
Stainless steel is characterized by being difficult to cut (poor machinability).There are two reasons for this. The first is the high degree of work hardening. Work hardening occurs in ordinary steel (carbon steel) that is not stainless steel, but the degree of work hardening in stainless steel is said to be twice that of carbon steel.
Another factor is its low thermal conductivity. Although low thermal conductivity is an advantage in terms of heat retention, it has the disadvantage in cutting processes, as heat generated during processing accumulates and makes tools more susceptible to damage.
Five types of stainless steel
In "What is Stainless Steel?" we explained that there are chromium-based and chromium-nickel-based stainless steels.
Furthermore, by classifying them by the type of metal structure,There are two types of chromium-based alloys and three types of chromium-nickel-based alloys.
Martensite, ferrite, and austenite, which will be explained later, are names for the metallic structure of iron, and each has a different crystal structure.
- Martensitic stainless steel
- Ferritic stainless steel
- Austenitic stainless steel
- Duplex stainless steel
- Precipitation hardening stainless steel
1. Martensitic stainless steel
It is a type of chromium-based stainless steel and is relatively inexpensive, so it is widely used as a general-purpose stainless steel.
Typical materials include SUS13 and SUS403, which contain about 410% chromium.
Stainless steel is characterized by its hardness, which comes from the large amount of strain that remains within the crystals (a process called strain hardening).
However, it has the lowest corrosion resistance of all stainless steels and will rust in harsh environments.
Applications include structural components, shafts for office equipment, knives, etc.
2. Ferritic stainless steel
This is also a chromium-based stainless steel and is a relatively inexpensive material that does not contain expensive nickel.
A typical material is SUS18, which contains about 430% chromium.
It has excellent corrosion resistance among stainless steels. However, its ferrite crystal structure is brittle and prone to cracking, so it cannot be used in places where it will be subjected to shock loads.
Applications include kitchen utensils and automotive parts (excluding those used in high temperature environments).
3. Austenitic stainless steel
It is a type of chromium-nickel stainless steel and is mass-produced due to its excellent characteristics.
A typical material is 18-8 stainless steel, which contains 18% chromium and 8% nickel.Among 18-8 stainless steels, SUS304, which has a low carbon content, is well-known.
Due to its austenite crystal structure, it has good workability and excellent corrosion resistance.
Its wide range of applications include kitchen utensils, the food and chemical industries, automobile parts, and scientific equipment.
4. Duplex stainless steel
It is a type of chromium-nickel stainless steel and is one of the more expensive stainless steels. Since it has two metal structures, ferrite and austenite, its physical properties are roughly intermediate between the two.
A typical material is SUS25J5, which contains 2% chromium, 329% nickel, and 1% molybdenum.
It is resistant to stress corrosion cracking and has excellent seawater resistance.
Applications include seawater condensers, seawater desalination plants, and salt production plants.
5. Precipitation hardening stainless steel
It is a type of chromium-nickel stainless steel, and is also expensive. It is made by adding copper, aluminum, titanium, etc. to the main elements of stainless steel, iron, chromium, and nickel, and precipitation hardening it at high temperatures.
A typical material is SUS17, which contains 4% chromium, 4% nickel, and 630% copper.
It is characterized by both high strength and good corrosion resistance. The martensitic type mentioned above also has high strength, but its corrosion resistance is inferior.
Applications include shafts, engine parts, and structural materials for aircraft and rockets.
Why stainless steel is resistant to rust
Here, we will explain why stainless steel is resistant to rust, and the environments in which even stainless steel can rust.
- Rust resistance mechanism
- Is the stainless steel really rusted? Possibility of contagious rust
- Environments in which stainless steel is prone to rust
Rust resistance mechanism
As mentioned in What is stainless steel?, stainless steel is a metal alloy containing about 12% chromium in iron. This chromium forms a hydroxide film on the surface of the stainless steel,Because it blocks oxygen and moisture from the surrounding area, iron oxide is less likely to form (rust is less likely to occur).
This film that covers the surface of metals and prevents corrosion (rust) is called a "passive film." It is an extremely thin film, only about one millionth of a millimeter (a few nanometers), and cannot be seen by the human eye.
In other words, before the iron reacts with oxygen and air to become an oxide (rust), the chromium forms a film that covers the surface, making stainless steel less likely to rust.
Is the stainless steel really rusted? Possibility of contagious rust
Even stainless steel, which is known for its rust resistance, sometimes receives complaints about rust.However, in reality, it is often the case that another iron part has rusted and that rust has simply adhered to the stainless steel.This phenomenon is called "rust transfer."
In addition to cases where rust adheres to the surface like this, there are also cases where iron powder flies in and adheres to the stainless steel, causing rust. Depending on the work environment, there may be iron powder flying around, so it is necessary to check the surrounding environment.
Elemental analysis can determine whether the stainless steel is rusted or has rusted from other materials.Stainless steel should contain about 12% chromium, so if no chromium is detected in the rust, it is likely to be contaminated rust.
Environments in which stainless steel is prone to rust
Although stainless steel is resistant to rust, it is not an all-purpose material. Here are two cases where it can rust.
One is an environment that contains halogen ions, such as chloride ions.Chloride ions can locally destroy the chromium film, causing holes. Seawater contains salt, which means chloride ions, so seawater and the vicinity of the sea are harsh environments for stainless steel.
The other is in an environment containing hydrochloric acid, sulfuric acid, phosphoric acid, organic acids, etc.In this case, the chromium passive film is not formed and the surface will rust uniformly.
Summary of Stainless Steel
In this article, we have explained in detail the characteristics and types of stainless steel, as well as the mechanism behind its rust resistance. Stainless steel, which has good performance and good looks, is used in a wide range of applications from everyday items to industrial products, and new types of stainless steel have been developed to improve its performance, further expanding its uses.
Sanwa Needle Bearing is a company with strengths in ultra-precision processing, with grinding at its core.To achieve high-precision processing at the ±0.0001mm level, we select the optimal heat treatment depending on the application and purpose, and provide stable quality.
Although it is said that "processing stainless steel is difficult," our company is able to provide stable processing thanks to our many years of technology and know-how.Our products are compatible with a wide range of materials, mainly stainless steel, but also hard-to-cut materials such as bearing steel, titanium, and ceramics, and are used in a variety of industries and applications.
We can flexibly respond to any material or application, so if you have any metal processing needs, please feel free to contact Sanwa Needle Bearing.