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What is Surface Hardening and Why Should I Use It?

Aug. 10, 2022

Surface hardening is a heat treatment process used to prevent rotating parts from becoming brittle and failing. Machines that use rotating parts such as ball bearings, gears and shafts are subject to "rolling contact fatigue" or "spalling". Rolling contact fatigue is caused by tiny cracks that occur just below the surface. The more components are used, the more these cracks creep up to the surface. Eventually, once the cracks reach the surface of the component, craters are created, which can lead to spalling. The more material that is pitting, the more the component will vibrate. Vibrating components are louder and usually lead to overall equipment failure.

Rolling contact fatigue can be prevented by heat treatment. However, heat treatment also has the potential to make components brittle, which can lead to another set of problems. Surface hardening machine is the process equipment used to prevent brittleness after heat treatment.


Surface hardening introduction

●Surface hardening produces a hard, wear-resistant outer layer while maintaining the ductile strength of the interior. Surface hardening of gearbox components creates a hard outer shell and a pliable inner layer. The hardened layer is called the outer shell. The thickness of the hardened layer is called the surface depth.

●There are several methods of surface hardening gears, including vacuum carburizing, atmosphere carburizing and induction hardening. Each process produces a different surface depth and a different degree of hardness. Whichever process you rely on depends on how much or how little you need to harden.


Carburizing Surface Hardening

●The two processes described below are carburizing processes, which means that carbon is introduced into the steel to increase its strength. Carburizing is suitable for steels with a carbon content between 0.15% and 0.25%. The surface carbon content of the steel should be increased to 0.8 to 1.0% after treatment.

●In the atmospheric carburizing process, the steel is heated in a furnace and mixed with carbon-containing gases such as methane, carbon monoxide and carbon dioxide. The steel absorbs the carbon from these gases and diffuses inward from the steel surface. Once they are mixed, the part is then cooled, usually using oil, but brine, molten salt, water or polymer solutions can also be used.

●The rapid cooling after quenching transforms the steel tissue from austenite to martensite, which increases its hardness. After quenching, the part is tempered in a furnace at 300 to 400 degrees Fahrenheit to prevent cracking and brittleness.

●Vacuum carburizing involves heating the steel in a vacuum chamber. Once the part reaches the necessary temperature, carbon gas is injected into the chamber. This method offers several advantages over atmosphere carburizing, including better control of carbon diffusion, which minimizes dimensional distortion. The steel part is then quenched in an inert gas (e.g., nitrogen) or in one of the liquids mentioned above. Finally, the part is tempered in a similar manner to atmosphere carburizing.


Induction Hardening

Induction surface hardening involves the use of electromagnetic induction to heat steel parts. The parts are then quickly cooled using a polymer solution or water spray. Unlike vacuum and atmosphere carburizing, induction hardening does not introduce new carbon into the steel. Therefore, induction hardening is only used for steels that already have a carbon content of about 0.4% to 0.6%. This method offers the advantage that it is fast and easy to automate.


Which one is best for you?

The surface hardening method you use depends on the needs of your company. It is useful to consult the technical requirements of the equipment depending on the application. The degree of fatigue will determine which process to use, as you need the hardened case to have a certain thickness to prevent fatigue. In general, carburizing processes produce thicker cases, so they are ideal for parts that are subjected to a lot of stress.


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