Author: haroonkhan

Steel parts may be surface hardened by heating in contact with a cyanide salt, followed by quenching. Only a thin case is obtained by this method. Cyaniding is, however, a rapid and economical method of case hardening and may be used in some instances for relatively unimportant parts. The work to be hardened is immersed…

Carburizing is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon-rich atmosphere, such as charcoal or carbon monoxide, with the intent of making the metal harder. Depending on the amount of time and temperature, the affected area can vary in carbon…

Martempering or marquenching permits the transformation of austenite to martensite to take place at the same time throughout the structure of the metal part. By using interrupted quench, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the centre to cool to the same temperature as the…

This is the method that can be used to overcome the restrictions of conventional quench and tempering. The quench is interrupted at a higher temperature than for martempering to allow the metal at the centre of the part to reach the same temperature as the surface. By maintaining that temperature, both the centre and the…

Tempering is a process of reheating of hardened steel below critical range and cooled at the decreased rate (approximately 4–5 min for each mm of the section). There is partial transformation of martensite to secondary constituent troostite and sorbite. The purposes of tempering are: (a) to reduce some amount of hardness produced during hardening and…

The purposes of hardening are: (a) to harden the steel to resist wear, and (b) to enable it to cut other metal. The metal is heated 30–50°C above the upper critical temperature for hypoeutectoid steel and above the same amount above the lower critical temperature for hypereutectoid steel. It is left for soaking for considered…

Spheroidizing is used to improve the machinability of steel. The workpiece is heated to 730–770°C, slightly above the lower critical temperature, and cooled 25–30°C per hour.

The purposes of annealing are: (a) to soften the metal for easy machining, (b) to remove internal stress caused by working, (c) to increase ductility, (d) to refine grain size, and (e) to modify electrical and magnetic properties. Normalized steel is less ductile and have more yield point and tensile strength than the annealed steel.…

Normalizing is a process of heating about 30–50°C above higher critical point for the time duration of 15 min and cooling in still air. The purposes of the process normalizing are: (a) to reduce grain size of steel, (b) to remove internal stress caused by working, and (c) to improve some of the mechanical properties.…

The time–temperature–transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite (Figure 24.2). Above 550°C, austenite transforms completely to pearlite. Below 550°C, both pearlite and bainite are formed and below 450°C, only bainite is formed. The horizontal dotted line that runs between the two…