26th June, 2017

Processes - Introduction

The heat treatment or surface engineering of engineering materials consists of a number of competing methods such as carburising, carbonitriding, nitriding, induction hardening, laser hardening etc. The choice of a particular method for an engineering component whether it be for example, carburising or induction hardening is based on many factors including cost, subsequent service performance requirements, wear resistance, strength, endurance and toughness.

The methods used to heat treat steel can be divided between (A) Simple thermal treatments such as induction hardening, and through hardening where only thermal energy is applied. These methods require the use of a medium carbon steel. (B) Thermochemical methods where both a chemical element (usually carbon, nitrogen or boron) is added, combined with thermal energy. Typical processes would be carburising, carbonitriding and nitriding.

A further feature that can be used to differentiate the hardness of methods of steel is the existence of the 'allotropic' transformation that exists with steel alloys. This unique feature means that the atomic arrangement (or crystal lattice) can change for BCC to FCC at around 900°C. The BCC crystal structure in ferrous alloy can only take into solution 0.02% carbon. However, the FCC crystal structure can take into solution over 0.8% carbon. This results in a 'hardness mechanism' called "transformation hardening". The method involves a steel with say 0.3 to 0.6% carbon taken to a temperature of around 900°C where all the carbon is dissolved into an FCC crystal structure. Rapid cooling from this temperature (called quenching) traps the carbon in the lattice resulting in hard, strong structures called martensite.

Unfortunately the transformation method of hardening can result in cracking and distortion. Despite these disadvantages transformation hardening the most frequent method and is exploited in through hardening, carburising and induction hardening etc. These are all processes where temperatures above 900°C have to be achieved.

Alternative low distortion methods of heat treatments involve processes that keep the ferrous alloys in the BCC crystal structure by processing at 500 to 600°C. These processes are nitriding and nitrocarburising. They involve the diffusion of nitrogen into the surface of the component and the formation of a surface 'compound layer' and a 'diffusion zone'.

This introduction is a simplification of the complex nature of hardening of engineering materials. However, it is easy to appreciate that to achieve a 'fitness for purpose' component, it involves material selection, method of manufacture and finally the correct choice of heat treatment.

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