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.

Figure 1. Loading and unloading cycles in a tensile test demonstrating work hardening.

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.

BS 970
Grade

Heat
Treat

Cond.

Limiting
Section (mm)

Tensile
Str. (MPa)

Yield
Str. (MPa)

Elong
(%)

070M20

N

TN

≥6≤150

430

215

21

AISI 1020

HR

CD

≥6≤13

560

440

10

080M30

N

TN

≥6≤150

490

245

20

AISI 1030

HR

CD

≥6≤13

620

480

9

080M40

N

TN

≥6≤150

550

280

16

AISI M40

HR

CD

≥6≤13

660

530

7

070M55

N

TN

≥6≤63

700

355

12

AISI 1053

N

CD

≥6≤13

760

610

6

 

Key

N

Normalised – ie heated above the transformation temperature to allow the carbon to go into solution and air cooled below that temperature.

 

HR

Hot rolled

 

TN

Turned

 

CD

Cold drawn - ie cold worked

 

Primary author: Justin Furness

Source: Materials Information Service

 

For more information on Materials Information Service please visit The Institute of Materials.

 

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