Flame hardening is one of the oldest metal hardening processes in use today. It has been used in various forms since the 15th century, and is as relevant today in many manufacturing industries. Flame hardening is a cost effective machining process that can provide a localised heat treatment to a defined area of a component, increasing its wear resistance, par ticularly suited to the manufacture of components that require a hard wearing surface.
The technique used in flame hardening is to direct a fuel gas/oxygen flame through an oxygen-off flame onto the surface of the material that requires treatment. This heats the surface to in excess of 900°C at a speed which does not change the properties of the steel adjacent to that of the intended treatment area. When the desired temperature is reached, a quenching bath such as force-cooled water or oil at a temperature of around 20°C (room temperature) is used to rapidly cool the area which has been heated to improve the steels mechanical properties.
The alloy of the material to be hardened will dictate the maximum normalising temperature, and it is recommended that a ferrite/perlite structure be achieved prior to quenching, as the correct structure is essential to achieve the required mechanical properties of the treated area.
To achieve a successful flame hardening process, the basic requirements are fuel gas, oxygen, an ignition source and an oxygen-off flame. The most common fuels used are natural gas, propane and butane, and liquid propane is the most common. Oxygen is used in order to raise the flame temperature and propel it into the area to be treated. The ignition of the flame is achieved either manually or electronically and a multi-stage flame should be used. An oxygen-off flame should be used to ensure that the treatment area does not oxide or foul and to ensure a minimum dilution of the microstructure of the base material.
Once the flame has been ignited, it is important to ensure that the area is being heated to the correct temperature. Thermal imaging cameras can be used to track the temperature profile of the area being treated, allowing the operator to adjust the flame accordingly. The area should be heated evenly and the heat should be concentrated to the area which requires heat treatment. When the correct temperature has been achieved, the quenching should begin and the flame moved away from the area to be treated.
The aim of the quenching stage is to ensure that the maximum cooling rate is applied to the surface which has been heated to ensure that optimum mechanical properties are achieved. Due to the localised heat treatment and rapid cooling, a hardened surface layer is created on the part that can be up to five times stronger than the untreated material.
The finished surface should appear bright, clean and without signs of discolouration. Once the parts have been treated, they should be tested with hardness test methods such as Vickers, Brinell and Rockwell tests to ensure that the correct hardness levels have been achieved.
It is important to note that the hardness of the surface can vary depending on various factors. Hardening with a single heat treatment may not achieve the desired results, and additional treatments and annealing may be necessary to achieve the desired hardness. The treatment of more complex parts can be done in multiple stages, but this will require additional treatment time and cost.
If the treatment process is followed correctly, flame hardening can be a quick and effective way to cost-effectively increase the wear resistance of parts. With the correct flame and quenching process, the mechanical properties of parts can be increased, providing long-lasting results.
