Question

Draw a typical Time vs. Temperature plot for Quench Hardening heat treatment process. Label the axes properly.

Answer 1

Conventional heat treatment procedures to produce martensite steels involve continous and rapid cooling of austinite specimen in material medium which could be air, water or oil depending on desired properties of final product.   

During quenching it is not possible to have uniform cooling throughout the specimen, surface will cool faster than interior of specimen. Therfore, austenite will transform over a range of temperatures, which will give diversity in microstructure of resulting specimen throughout.

To produce predominatly martenistic composition throughout depends on following:

1. Composition of alloy

2. Quenching medium

3. Size and shape of specimen

Sometimes, it is convenient to relate hardness to cooling rate the are drawn as shown below:

Depending on cooling we will have different resulting microstructure

A: Pearlite (Holding at a temperature then quenching, coarser relative to B)

B: Fine Pearlite (Holding for some time at high temperature and then quench)

C: Martensite and Pearlite ( Since cooling passes from transformation region some pearlite is formed)

D: Martensite (Rapid cooling so that cooling curve does not pass transforamtion region we will have complete martensite microstructure at the end of quenching)