Steels - Work Hardening of Engineering Steels

Topics Covered

Introduction

Effect of Work Hardening on Mechanical Properties

Introduction

Resistance to continuing plastic flow as a metal is worked is termed 'work hardening'. When work is performed below hot working temperatures (i.e. below about 0.5Tm, where Tm is the melting point), and the crystal structure is forced to deform to accommodate the strain, microscopic shearing (or slip) occurs along definite crystalline planes. Discontinuities in the crystal structure, present in all metals and known as dislocations, increase in density during plastic flow and those moving on intersecting slip planes tangle and pile up. This means that an ever increasing shear stress is required for deformation, increasing the yield stress. Eventually the stress required to move dislocations is high enough for a crack to initiate and subsequently propagate, and the material breaks. Figure 1 demonstrates the effect of work hardening during a tensile test.

Most steels with appreciable alloy content possess a complex crystal structure resulting in numerous potential slip planes and intersection points, consequently most engineering steels are highly susceptible to work hardening.

Effect of Work Hardening on Mechanical Properties

Work hardening improves tensile strength, yield strength and hardness at the expense of reduced ductility (see Table 1). These effects can only be removed by annealing or normalising.

Table 1. The effect of heat treatment and work hardening on the properties of carbon steels.

Primary author: Justin Furness

Source: Materials Information Service

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