How is the Hardness of High Speed Steel (HSS) Maintained?

High speed steel is a subset of tool steel which is basically used as cutting tool material. It is commonly used in power-saw sharp bores and edges. It is much stronger than carbon tool steel and has in fact replaced the latter’s demand in the market. High speed steel is easily available, has improved strength, machinability, cutting strength and better affordability. Besides, it can easily endure high temperatures without having any effect on their strength and durability. Hence, HSS operates at a much faster pace compared to high speed steel. Due to all these exceptional properties, high speed steel has been widely in use across various industries. Traditionally, high speed steel is manufactured using traditional steel melting procedure. It includes ingots solidification, rolling or forging, heat treatment and finally, the quality inspection of the final product. 

Hardness of high speed steel – High speed steel has extremely high resistance against wears and abrasions due to the carbides in the steel. It has a structural ability up to 560 degree Celsius, which makes it resistant against softening at high temperatures. Furthermore, HSS also exhibits extremely high resistance against deformation because of high hardness. It has a Rockwell hardness 60 which allows it to be used in various ways.

Types of high speed steel determining hardenability – There can be various grades of high speed steel and each determines where that particular grade can be used. Besides, each grade has its specific alloy which strengthens a certain feature of the steel. High speed steel with molybdenum ensure higher levels of toughness. Tungsten exhibits properties of hot hardness. This hardness is acquired at very high temperatures. Vanadium in HSS improves resistance against wears, abrasions and increases hot hardness due to the formation of stable vanadium carbides. Chromium is often used for maintaining hardenability. It also reduces any chance of oxidation damage when HSS undergoes heat treatment. Due to cobalt content in high speed steel, hot hardness of the metal is increased. However, along with that the brittleness increases as well. Traces of Sulphur or silicon are also found in the steel which neither contribute nor reduce any of its existing properties.

Salt bath or air cooled – High speed steel already has excellent hardenability. Tools made from these kind of steel are either air cooled or quenched in a salt bath. After quenching in a normal hardening temperature, HSS contains approximately 20-30% retained austenite. While cooling down from a high temperature of approximately 575 degree Celsius, austenite is changed into martensite. At the same time, the newly formed martensite always gets tempered. This is followed by a second tempering or heat treatment where the martensite is heated up to exhibit best properties. Retained austenite can be transformed also by using subzero treatment. Tempering temperature above 550 degree Celsius produces the desired hardness which varies depending upon the holding time. The toughness needs to be pre-decided to later decide on the temperature. The impact strength becomes standard when the steel is heated between 250 and 450 degree Celsius. Tools which are subjected to extreme pressure give excellent results if they are tempered at 600 degree Celsius. 

To increase the hardness of high speed steelHigh speed steel are mostly used for machining or cutting other tools. This is why, the hardness cannot be temporary in these tool steel. The quality of hardness must be persistent and balanced to reduce manufacturing cost. T1 high speed steel is austenitized at 1200 degree Celsius, whereas M2 is austenitized at 1220 degree Celsius. The hot hardness increases along with the hardening temperature. However, another determining factor is the composition of steel. As discussed earlier, the alloy in the steel will contribute its properties during the heat treatment. These alloys will also ensure that the steel will retain its hardness even when used repeatedly. Brittleness cannot be expected from HSS. 

High speed steel is one of the best available cutting tool steel in the market. Its hardness and other properties increase its demand amongst suppliers.

Comments are closed