SteelsĀ having a carbonĀ content between 0.3% and 0.8% are capable of being through hardened. As the carbon content increases, so does the degree ofĀ hardnessĀ achievable. The depth to which a steel grade will fully harden depends upon the rate of quenching, with faster quenches in brineĀ or water producing a deeper hardeningĀ effect than with oil, air or inertĀ gas. The addition of alloying elements, such as manganese, nickel, chromiumĀ and molybdenum, increase the achievable hardening depth, i.e. the hardenabilityĀ of the steel is thereby increased.
For every steel composition there is a limiting ruling section at which the specified combination of properties can be achieved. In parallel with hardening, the brittleness of the steel increases. This is the reason for the secondary treatment which follows hardening, which is termed tempering. The instability of steel in the as-hardened condition, due to the high level of internal stresses present, is prone to induce cracking. The cracking tendency increases with increasing hardenability and with the severity of the quenchantĀ used in hardening. In order to relieve the internal stressĀ produced in the microstructural change causing hardening (the formation of martensite), it is necessary to re-heat the quenched steel to a temperature below the martensite transformationĀ finish temperature, suitable for the particular steel in question.
The cracking propensity increases with increasing hardness i.e. with increasing carbon and alloyĀ content. Thus, tempering has to be carried out with as short a time delay as possible following hardening, particularly for tool steels. During tempering, in addition to stress relief, many steels undergo a further sub-microscopic structural change consisting of the precipitationĀ of carbide particles from the martensite. Tempering produces a reduction in hardness and a corresponding improvement in ductility. The effect is both time and temperature dependant, with higher temperatures and longer soak times producing maximum reduction in hardness and increase in ductility. Ultimately with some steels, over-tempering can produce a breakdown of the martensite structure and the formation of a spheroidised carbide structure.
Low alloy steels are usually tempered in the range 450-650°C for the most useful combination of mechanical properties. Some high alloy tool steels exhibit secondary hardening during the tempering treatment, due to the precipitation of hard alloy carbides.
SteelsĀ having a carbonĀ content between 0.3% and 0.8% are capable of being through hardened. As the carbon content increases, so does the degree ofĀ hardnessĀ achievable. The depth to which a steel grade will fully harden depends upon the rate of quenching, with faster quenches in brineĀ or water producing a deeper hardeningĀ effect than with oil, air or inertĀ gas. The addition of alloying elements, such as manganese, nickel, chromiumĀ and molybdenum, increase the achievable hardening depth, i.e. the hardenabilityĀ of the steel is thereby increased.
For every steel composition there is a limiting ruling section at which the specified combination of properties can be achieved. In parallel with hardening, the brittleness of the steel increases. This is the reason for the secondary treatment which follows hardening, which is termed tempering. The instability of steel in the as-hardened condition, due to the high level of internal stresses present, is prone to induce cracking. The cracking tendency increases with increasing hardenability and with the severity of the quenchantĀ used in hardening. In order to relieve the internal stressĀ produced in the microstructural change causing hardening (the formation of martensite), it is necessary to re-heat the quenched steel to a temperature below the martensite transformationĀ finish temperature, suitable for the particular steel in question.
The cracking propensity increases with increasing hardness i.e. with increasing carbon and alloyĀ content. Thus, tempering has to be carried out with as short a time delay as possible following hardening, particularly for tool steels. During tempering, in addition to stress relief, many steels undergo a further sub-microscopic structural change consisting of the precipitationĀ of carbide particles from the martensite. Tempering produces a reduction in hardness and a corresponding improvement in ductility. The effect is both time and temperature dependant, with higher temperatures and longer soak times producing maximum reduction in hardness and increase in ductility. Ultimately with some steels, over-tempering can produce a breakdown of the martensite structure and the formation of a spheroidised carbide structure.
Low alloy steels are usually tempered in the range 450-650°C for the most useful combination of mechanical properties. Some high alloy tool steels exhibit secondary hardening during the tempering treatment, due to the precipitation of hard alloy carbides.
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