Stainless steel facts for kids

Stainless steel taps and sink

Stainless steel is used for industrial equipment when it is important that the equipment is durable and easy to clean.

Stainless steel is a special kind of iron-based metal mix, called an alloy. It contains chromium, which is what makes it super resistant to rust and corrosion. You might also hear it called inox (short for a French word meaning "non-rusting") or rustless steel.

The secret to stainless steel's rust-fighting power is its chromium content, which is usually 10.5% or more. This chromium creates a very thin, invisible protective layer on the surface. This layer acts like a shield and can even repair itself if it gets a scratch, as long as there's oxygen around.

Other elements like molybdenum, carbon, nickel, and nitrogen can be added to stainless steel. These additions give it even more special qualities for different uses. For example, some types are extra strong, while others are even better at resisting certain chemicals.

Stainless steel is used everywhere because it looks shiny and doesn't rust easily. It can be shaped into sheets, bars, wires, and tubes. You'll find it in cookware, cutlery, surgical instruments, and big major appliances like refrigerators. It's also used in cars, large buildings, and factories that make food or treat water. Some types can even be shaped by forging (hammering while hot) or casting (pouring into molds).

This metal is also very easy to keep clean, even better than copper or aluminum. This is why it's a top choice for places where cleanliness is super important, like hospitals and food processing plants.

Different kinds of stainless steel have special numbers, like a three-digit code from the AISI. These numbers help people know exactly what kind of stainless steel they are using.

Special Qualities of Stainless Steel

Stainless steel has many amazing qualities that make it useful.

Resisting Rust and Wear

Even though stainless steel is famous for not rusting, it can get a tiny bit of rust on its very outer layer. But don't worry! The chromium inside protects the deeper parts from rusting.

Adding elements like nitrogen and molybdenum makes stainless steel even better at fighting off tiny rust spots. Different types of stainless steel are made with various amounts of chromium and other elements. This helps them stand up to different environments, from kitchens to factories.

How Strong is it?

Stainless steel is very strong. A common type, called 304, is quite tough. It can be made even stronger by a process called "cold working," which means shaping it without heat.

Some super-strong types, like 17-4 PH, can be made incredibly tough with special heat treatments. These are used when extreme strength is needed.

Melting and Heat

Stainless steel needs a lot of heat to melt, usually between 1325 and 1530 degrees Celsius (2417 to 2786 degrees Fahrenheit). This is similar to regular steel and much hotter than aluminum or copper.

Electricity and Magnets

Stainless steel is not a great conductor of electricity, especially compared to copper. This is because of its protective layer. So, it's not usually used for electrical wires.

However, some types of stainless steel are magnetic, like ferritic and duplex stainless steels. Others, like austenitic stainless steel, are usually non-magnetic. If you bend or cut non-magnetic stainless steel, it can sometimes become slightly magnetic along the edges.

How Heavy is it?

The weight of stainless steel can vary a little. It usually has a density between 7.5 and 8.0 grams per cubic centimeter. This means it's quite dense, or heavy for its size.

The Story of Stainless Steel

An announcement about the development of stainless steel in Sheffield, England, from a 1915 newspaper.

The invention of stainless steel was a long journey with many scientific discoveries. It started way back in 1798 when a scientist named Louis Vauquelin first showed chromium to the French Academy.

In the early 1800s, other scientists noticed that mixes of chromium and iron could resist rusting. Pierre Berthier even suggested using these mixes for cutlery in 1821 because they didn't get damaged by some acids.

By the 1840s, steelmakers in Britain and Germany were making chromium steel. Germany's Krupp company even used it for cannons in the 1850s. In 1861, Robert Forester Mushet got a patent for chromium steel in Britain.

These early steps led to the first American company, Chrome Steel Works, making chromium-containing steel for bridges in the 1860s.

Who Discovered Stainless Steel?

A monument to Harry Brearley in Sheffield, England.

Many people helped develop stainless steel, but one key person was Harry Brearley. In 1913, he was working in Sheffield, England, trying to find a metal that wouldn't rust for gun barrels. He accidentally discovered a special stainless steel alloy.

His discovery was announced in The New York Times in January 1915. This new metal was later sold under the name "Staybrite" and was even used for the entrance canopy of the famous Savoy Hotel in London in 1929.

Early Names for Stainless Steel

Harry Brearley first called his new metal "rustless steel." In the United States, it was sold under names like "Allegheny metal."

A local cutlery maker in Sheffield gave it the name "stainless steel," which eventually became the most popular name worldwide. Even today, in Japan, people often just say "stainless" when talking about cutlery.

By 1929, before the Great Depression, over 25,000 tons of stainless steel were made and sold in the US each year!

Different Kinds of Stainless Steel

Stainless steel comes in five main "families," each with its own special features. These families are mostly named after their internal structure.

Austenitic Stainless Steel

This is the most common type, making up about two-thirds of all stainless steel produced. It has a special crystal structure that stays the same at almost all temperatures.

200 series: These types use manganese and nitrogen to save on nickel. They are about 50% stronger than the 300 series. Examples include Type 201 and Type 202.
300 series: These are the most widely used. They get their special structure mainly from nickel. Type 304 and Type 316 are well-known examples.

Ferritic Stainless Steel

Ferritic stainless steels are magnetic and can be made harder by shaping them when cold. They contain a lot of chromium (10.5% to 27%) but very little or no nickel, making them less expensive than austenitic types.

You can find ferritic stainless steels in:

Car exhaust pipes
Parts of buildings
Power plates in fuel cells

Martensitic Stainless Steel

Martensitic stainless steels are also magnetic and can be made harder with heat treatments or cold shaping. They are used for things like engineering parts and tools. They are not as rust-resistant as ferritic or austenitic types because they have less chromium.

These steels can be made stronger by heating them up, then quickly cooling them (quenching), and then reheating them to a lower temperature (tempering).

Duplex Stainless Steel

Duplex stainless steels are a mix of austenitic and ferritic types. They have more chromium (19–32%) and molybdenum (up to 5%) but less nickel than austenitic steels.

They are about twice as strong as austenitic stainless steel and are very good at resisting certain types of rust, especially in salty environments. They are often used in the oil and gas industry and for building structures near the coast.

Keeping Stainless Steel Safe from Rust

Stainless steel (bottom row) resists salt-water corrosion better than aluminium-bronze (top row) or copper-nickel alloys (middle row).

Unlike regular steel, stainless steel doesn't rust evenly when it gets wet. Regular steel rusts easily in air and moisture, and its rust layer flakes off, exposing more steel.

Stainless steel, however, forms a thin, invisible layer of chromium oxide. This layer blocks oxygen from reaching the metal, stopping rust from spreading. This protective film can even repair itself!

The ability of this film to resist rust depends on the stainless steel's chemical makeup, especially how much chromium it has.

General Rusting

General rusting happens in very harsh environments, like places that make or use strong chemicals. The entire surface of the steel is attacked.

This type of rust is usually easy to avoid by choosing the right kind of stainless steel for the specific chemicals and temperatures. For example, hydrochloric acid can damage any stainless steel and should be avoided.

Stainless steel is not completely immune to corrosion, as shown in this desalination equipment.

Some acids, like phosphoric acid and nitric acid, don't affect stainless steel at room temperature. Strong bases, like sodium hydroxide, can cause some damage at high temperatures.

Tiny Spot Rusting

Tiny spot rusting, also called pitting or crevice corrosion, can happen in places with chloride ions, like saltwater. Higher levels of chloride need stronger types of stainless steel.

This kind of rust is harder to predict. It depends on things like:

How much chloride is present.
The temperature.
How acidic the environment is.
If water or chemicals are sitting still on the surface.

The more chromium, molybdenum, and nitrogen a stainless steel has, the better it resists these tiny rust spots. Good design and choosing the right alloy can prevent this.

Cracks from Stress

Sometimes, a combination of pulling forces (stress) and a harsh environment can cause stainless steel to crack unexpectedly. This is called stress corrosion cracking.

It happens when:

The metal is under stress.
It's in a corrosive environment.
The specific type of stainless steel is prone to this.

You can prevent this by removing one of these conditions.

Mixing Metals

The nut on the left is not stainless steel and is rusty, unlike the nut on the right.

When two different metals touch in a wet environment, one can rust faster than it normally would. This is called galvanic corrosion. Stainless steel usually acts as the "stronger" metal, making the other metal rust faster.

For example, if aluminum rivets are used to fasten stainless steel sheets in water, the aluminum will rust quickly. It's usually okay to use stainless steel screws with regular steel plates, but not the other way around.

Hot Temperatures

At very high temperatures, all metals react with hot gases, especially oxygen in the air. Regular steel can only handle about 480 degrees Celsius (896 degrees Fahrenheit) before it starts to oxidize.

Stainless steel resists high temperatures much better. Adding chromium, silicon, and aluminum helps. The minimum 10.5% chromium in stainless steel protects it up to about 700 degrees Celsius (1292 degrees Fahrenheit). Types with 18% chromium, like Type 304, can resist up to about 870 degrees Celsius (1598 degrees Fahrenheit).

How Stainless Steel Looks

316L stainless steel, with an unpolished, mill finish

Stainless steel can have many different looks, called "finishes." These finishes are made by rolling the steel and then sometimes polishing or brushing it.

After the steel is shaped and heated, any rust that forms on the surface is removed. Then, a protective layer is created. Finally, a special finish is applied to give it the desired look.

Some common finishes include:

No. 1: Hot-rolled, annealed, and passivated (a basic, dull finish).
No. 2B: Cold rolled and polished for a smooth, slightly reflective look.
No. 4: A brushed finish, often seen on kitchen appliances.
No. 8: A mirror-like, highly reflective finish.

Putting Stainless Steel Together

There are many ways to join pieces of stainless steel, but welding is the most common.

Some types of stainless steel, like austenitic, are quite easy to weld. Others, like martensitic, need special care and heating before and after welding to prevent cracks.

Common welding methods include:

MIG/MAG welding: Uses a wire that feeds automatically.
TIG welding: Uses a non-melting tungsten electrode.
Shielded metal arc welding: Uses a coated electrode.

Stainless steel can also be glued together using strong adhesives like silicone or epoxies.

Making Stainless Steel

Most stainless steel around the world is made using these steps:

Melting: Stainless steel scrap, other metal scraps, and alloys are melted together in a huge Electric arc furnace.
Refining: The melted metal then goes into an AOD process. Here, carbon is removed, and other elements are adjusted to get the exact mix needed.
Shaping: The liquid metal is poured into molds to solidify into slabs (for flat products) or blooms (for bars and wires).
Hot Rolling: These slabs and blooms are reheated and rolled while hot. This makes them thinner, creating coils, bars, or wire rods.
Cold Finishing: The hot-rolled products are then cleaned with acid to remove surface rust. They are then cold-rolled and heated in a special atmosphere to get the final thickness and surface look.

Major producers of stainless steel include China, Japan, South Korea, Taiwan, India, the US, and countries in the European Union.

In 2017, the most common types produced were:

Austenitic stainless steels (300-series): 54%
Austenitic stainless steels (200-series): 21%
Ferritic and martensitic stainless steels (400-series): 23%

Where We Use Stainless Steel

Stainless steel is used in countless places! You can find it in:

Architecture: For parts of buildings and decorations.
Art: Sculptures and other artworks.
Food and Drink: Kitchen appliances, cookware, and equipment for making food and beverages.
Vehicles: Cars, trains, and even some airplanes.
Medicine: Surgical instruments and hospital equipment.
Energy: In power plants and other energy systems.
Firearms: For certain parts of guns.

Caring for Our Planet with Stainless Steel

Stainless steel is a very environmentally friendly material because it is 100% recyclable. This means it can be melted down and used again and again without losing its quality.

On average, a stainless steel item is made from about 60% recycled material. About 40% of this comes from old products that have reached the end of their life, like old kitchenware. The rest comes from leftover pieces during manufacturing.

The process of recycling stainless steel helps save energy and reduces the need to mine new raw materials. It's a great example of how we can reuse materials to protect our planet.

Stainless Steel and Your Health

Stainless steel is generally considered safe and doesn't react much with our bodies.

When people weld stainless steel, fumes are created. These fumes can contain tiny particles that are not good to breathe in. Because of this, welders always wear special masks and safety gear to protect themselves.

While very tiny amounts of metals can sometimes transfer from new stainless steel cookware to highly acidic foods, this is generally not a health concern.