What is Case Hardening? An Introduction to Surface Hardening

Case hardening is also called carburization. It makes the outside of metals hard. The inside stays tough and bendable. You get a strong outside that does not wear out easily. Many industries use case hardening to make parts last longer. These industries include construction, fencing, machining, grating, and metalworking.

What is Case Hardening

Surface Hardening Explained

Case hardening makes metal parts tough on the outside but soft inside. Only the surface changes, not the whole part. The outside gets a thin, hard layer. This layer protects against damage and wear. The inside stays strong and flexible.

Tip: Surface hardening helps parts that rub or scratch, like gears and camshafts.

Here is what happens during surface hardening:

• You heat the metal until it is very hot.

• The surface takes in new elements like carbon or nitrogen.

• The outside gets hard, but the inside stays bendable.

Surface hardening and through hardening are different. Through hardening makes the whole part hard. This can make the part break more easily. Surface hardening only changes the outside, so the inside stays tough.

How Case Hardening Works

Case hardening uses a few main steps. First, you heat the metal part to a high temperature. This gets the surface ready to take in new elements. Next, you put the metal in a place with lots of carbon or nitrogen. The surface soaks up these elements and gets harder.

Here are the steps:

1. Heating: Heat the metal so the surface can take in carbon or nitrogen.

2. Diffusion: The surface absorbs carbon or nitrogen from the air.

3. Cooling (Quenching): Cool the metal quickly to keep the hard surface.

The science behind case hardening is interesting. Carburization adds carbon to the surface, raising it to about 0.8%. When you quench the metal, the structure changes from austenite to martensite. This makes the surface very hard. Compressive stresses also help make it harder.

If you look at the microstructure, you see big changes between case-hardened steel and untreated steel:

1. Case-hardened steel has a hard, wear-resistant outside.

2. The inside stays softer and bendable.

3. Untreated steel does not have both hardness and toughness.

Different case hardening methods control how deep the hard layer goes. Carburizing gives a slow change because carbon moves slowly. Induction hardening makes a sharp change because it heats and cools fast. The hard layer is usually about two-thirds as deep as the total hardened area.

Note: Most case hardening happens after shaping or machining the part. This is because the hard surface makes more work difficult.

Case hardening helps metal parts last longer and work better. You get a tough inside and a hard, wear-resistant outside.

Case Hardening Methods

There are a few ways to make metal surfaces harder. Each way changes the surface in its own way. The most used ways are carburizing, nitriding, and heating with quenching. These ways add carbon or nitrogen to the surface. This makes the metal strong and helps it last longer.

Carburizing

Carburizing is the most common way to harden steel. You heat the metal where there is lots of carbon. Carbon moves into the top layer of the metal. This is done at very high heat, between 1650°F and 1750°F (898°C to 954°C). You can use gas, pack, or liquid for carburizing. The surface gets very hard, between 58 and 64 HRC. Carburizing makes the outside tough, but the inside stays soft.

Carburizing is good for gears and shafts that need a hard surface.

Nitriding

Nitriding puts nitrogen into the surface of steel. You heat the metal in a gas with lots of nitrogen or use plasma. This happens at lower heat, around 930°F to 1020°F (500°C to 550°C). Nitrogen makes hard nitrides in the surface layer. Nitriding gives a very hard surface, up to 1200 HV, and better wear resistance than carburizing. It also causes less change in shape and lets you control the final size better.

• Nitriding makes the metal harder and helps stop rust.

• The treated layer has iron nitrides and compressive stresses, which help the part last longer.

Heating and Quenching

Heating and quenching is an important step in case hardening. You heat the metal and then cool it very fast. Fast cooling changes the metal to martensite, which is very hard. Quick cooling makes the metal harder but also more brittle. Slow cooling keeps the metal tough but not as hard.

Pick the best way based on how hard and tough you want your part to be.

Materials and Applications

Suitable Metals

Case Hardening works on many metals. Low carbon steel is used the most. It gets a hard outside but stays tough inside. Alloy steel is also good if you need more strength. Cast iron and stainless steel can be case hardened too. Each metal changes in its own way. The table below shows which metals work best with each method:

Steel alloys are picked a lot. They make strong and long-lasting parts. People use these metals to get better wear resistance and surface hardness. They also help parts handle stress better.

Benefits of Case Hardening

Case Hardening makes the outside much harder. This helps stop damage and wear. It also makes parts last longer under stress. Here are some main benefits:

• The hard surface stops scratches and dents.

• The tough inside takes hits and keeps the part safe.

• Parts wear out slower, so they last longer.

• Parts can handle stress better, which helps gears and moving parts.

• You save money and time because parts do not break as fast.

Car makers now give longer warranties for powertrain parts. This shows that parts are stronger and last longer.

Common Uses

Many industries use case-hardened parts. These parts are best where strength and wear resistance are needed. The table below shows some common uses:

You also see case-hardened parts in transport, energy, and big machines. These parts help machines work better and last longer, even when things get tough.

Case hardening makes the outside hard and the inside tough. This helps parts last longer and work better. The hard surface stops wear and damage. The tough core lets the part handle heavy loads. Parts can be used again and again without breaking. You can also harden only the areas that need it.

You can find more information in these resources:

• The Essential Guide to Case Hardening in Metallurgy

• Steel Case Hardening course

FAQ

What metals can you case harden?

You can case harden low-carbon steel, alloy steel, cast iron, and some stainless steels. Each metal responds differently to the process.

How does case hardening help your parts?

Case hardening gives your parts a tough surface. This helps stop wear, scratches, and dents. Your parts last longer and work better under stress.

Can you case harden at home?

You need special equipment and high heat for case hardening. Most people cannot do it safely at home. You should use a professional service.