Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M

Year: May 2014

The hardenability of steel is defined as that property which determines the depth and distribution of hardness induced by quenching from the austenitic condition. The dependence of hardness upon quenching rate can be understood from the time-temperature-transformation characteristics of steel, and, for particular steel, can be estimated from the T-T-T diagram.

A part may be hardened by quenching into water, oil, or other suitable medium. The surfaceof the part is cooled rapidly, resulting in high hardness, whereas the interior cools more slowly and is not hardened. Because of the nature of the T-T-T diagram, the hardness does not vary linearly from the outside to the center. Hardenability refers to capacity of hardening (depth) rather than to maximum attainable hardness.

The hardenability of steel depends on

(1) Composition of the steel

(2) Austenitic grain size

(3) Structure of the steel before quenching.

In general hardenability increases with carbon and alloy content. The most important factor influencing the maximum hardness that can be obtained is mass of the metal being quenched

Procedure

1. Sample of medium carbon steel machined to the shape shown in Fig. It is a cylindrical bar with a 25 mm diameter and 100 mm length. The specimen is placed in the furnace at 900°C
2. For about 1/2 hour. The water flow rate is adjusted so that the water column is approximately at the distance 50 mm above the end of the pipe, when water is flowing freely.
3. After the sample has been austenitized it is removed from the furnace and placed directly into the quenching apparatus. A jet of water is quickly splashed at one end of the specimen.
4. Afterthe entire sample has cooled to room temperature the scale oxidation is removed. Twoopposite and flat parallel surfaces are ground along the length of the bar. Rockwell C hardnessmeasurements are then made every 2 mm and these readings are recorded.