A Quick Look at Heat Treatment of Steel
Heat treatment is the process of heating and cooling metals to alter the physical, sometimes chemical and properties of steel. In fastener manufacture industry, steel undergo heat treatment are to improve its hardness, strength, toughness, ductility, and corrosion resistance, that can improve performance when molding, welding or connecting. Common technical methods for heat treatment include:
Flame hardening is a surface hardening method used on medium carbon steel which include 0.4 - 0.5% carbon or alloy steels, following by quenching to increase core hardness and case surface hardness on fasteners. Usually the surface hardness range is about 50 - 60 HRC in medium carbon steel, hardened depth depends on strength of flame, heating time, temperature and moving speed. In the same condition, longer heating time with slower moving speed, more hardened depth.
Flame hardening is a rapid, economical, cheaper method that also suitable for mass quantity. But this process is applied to selected metal surfaces of carbon steel, alloy steel even some stainless steel, the disadvantage of flame hardening method is material surface couldn't heat evenly and sometimes deform the shape.
High Frequency Hardening Method
The principle of high frequency hardening is similar to flame hardening, but by using electromagnetic induction to heat the parts. Using the current of electricity induced through electromagnetic induction, coil is replaced with material into which the current of electricity will flows into the part therefore the parts heat up. The higher the frequency of the power source, the more induced current of electricity will concentrate on the surface of the part with higher hardness.
The temperature can be controlled through changing frequency of the power source and it not only heat up evenly but also support to specify area according to customer's request. But it's high capital investment and only certain steels can be induction hardened. This method is also restricted to fasteners having a shape that is suitable for induction hardening and mostly hardening one piece at one time, the production is less and price is higher.
Carburizing Heat Treatment Process
Carburizing is a heat treatment process that steel absorbs carbon while the metal is heated in the presence of gas or liquid carburizing compound, the results in a change in the carbon concentration between the carbon rich surface and the interior core of steel, it's commonly used in flame hardening method. The depth of carburization also affected by different carburizing compounds, steel, temperature and time.
Vacuum (gas) nitriding is a heat treatment process that diffuses nitrogen into metal at certain chemical mediator usually ammonia (NH3), after heat it dissociates into nitrogen and hydrogen and to create a case-hardened nitride layer. These processes most commonly used on low-carbon alloy steels.
The advantage of this process is components with excellent wear resistance, corrosion resistance and high temperature properties, the hardness result and also better than carburizing heat treatment process.