Hardening Process
Purpose
1. To increase the hardness of steel to maximum extent by austenite to martensite transformation.
2. To increase the wear resistance and cutting ability of tool steels.
Principle
Hypoeutectoid steel are heated to above A3 and hyper eutectoid steels are heated to above A1 by 30 to 50°C followed by sudden cooling to room temperature or below room temperature in a quenching medium, after uniform austenization. To prevent oxidation and decarburisation effect we are not going up to Acm.
- Martensite is defined as a super saturated solid solution of carbon in BCC form of α-iron which exist in BCT configuration.
Characteristics of Austenite to Martensite Transformation
• Austenite to martensite transformation take place at the speed of sound.
• Austenite to martensite transformation is independent of time.
• Austenite to martensite transformation is a “diffusionless” process.
• Austenite to martensite transformation takes place by process of shear.
• Austenite to martensite transformation never goes to 100% completion at room temperature.
Problems with sudden cooling
• Residual stresses and due to this corrosion resistance decreases.
• Distortion of shape take place.
• Quench crack formation take place when component is not capable of withstanding rapid density changes.
• Retained austenite formation take place. Retained austenite brings out the dimensional changes in the previously quenched components from season to season since this is not acceptable in precision engineering application retained ‘γ’ should be eliminated.
