Work Hardening of Metals ( also known as strain hardening or cold working)

Work hardening, also known as strain hardening or cold working, is a process in metallurgy where a metal undergoes plastic deformation at temperatures below its recrystallization point. This plastic deformation leads to an increase in the hardness and strength of the metal. The key characteristic of work hardening is that it occurs through the application of mechanical stress or strain.
Dislocation Density Increase: Cold working increases the density of dislocations within the metal structure. This increased density makes it more difficult for dislocations to move through the crystal lattice, leading to enhanced strength.
Grain Boundaries: The movement of dislocations is impeded by grain boundaries. As dislocation density increases, the chances of dislocations encountering grain boundaries also rise, contributing to the hardening effect.

1. Work Hardening of Metals Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.) Assistant Professor, Department of Mechanical Engineering, Mookambigai College of Engineering, Pudukkottai-622502, Tamil Nadu, India. Email:[email protected] Dr. R.Narayanasamy , B.E., M.Tech ., M.Engg ., Ph.D., (D.Sc.) Retired Professor (HAG), Department of Production Engineering, National Institute of Technology, Tiruchirappalli-620015, Tamil Nadu, India. Email: [email protected]

2. Work hardening, in metallurgy, increase in hardness of a metal induced, deliberately or accidentally, by hammering, rolling, drawing, or other physical processes. Although the first few deformations imposed on metal by such treatment weaken it, its strength is increased by continued deformations. Definition

3. True Stress – True Strain Diagram for Ductile Metal As true stress increases with respect to strain, we can say that metal undergoes work (or) strain hardening. Beyond the stress level ( σ ) the plastic deformation takes place.

4. Engineering Stress Vs Engineering Strain As the engineering stress increases with increasing amount of strain it is called work hardening. Beyond the yield stress ( σ 1 ), stress increases till the maximum stress is reached with increasing strain. Beyond the maximum stress, the engineering stress decreases till fracture (or) failure occurs. Beyond σ 1 , the metal undergoes plastic deformation.

5. Stress Strain Stress – Strain Diagram When there is no cold stretching, the amount of strain before fracture is high. When the metal is cold stretched, the total amount of strain before fracture is less. However, in the case of cold stretched the stress level is high compare to un-stretched.

6. Cold Rolling of Metal The metal is rolled in between two similar rolls. But, rolls are rotated in opposite direction. During cold rolling, the metallurgical grains are getting elongated in the direction of the rolling. After cold rolling Before cold rolling

7. Work Hardening Tensile Force is applied. After unloading opposite forces will setup.

8. Crystallographic Structure of Metals

9. Contd. Most of the metals are made up of unit cell crystal structure as shown in figure 1.1. The metal structure is made up of an individual crystalline area called grain (figure 1.2).

10. Contd.

11. The linear defect occurs when the group of atoms in the structure is in an irregular position. The movement of these dislocations involves breaking of atomic bonds in the crystal. This movement makes atoms in the crystal planes slip over one another. The slip occurs along the parallel plane within the grain. Any defects in the regular crystal affect the motion of dislocation which makes the movement of dislocation difficult. So the strength of the metal can be raised by increasing the number of dislocations.  In plastic deformation, the dislocation movement produces additional dislocations, the sliding of which often hinders the movement. This increases the force needed to move the dislocation and strengthen the metal.  Contd.

12. Figure 1.4 depicts the uniaxial stress-strain behavior of a typical metal.

13. When metal is loaded (standard tensile test) it deforms elastically till the yield point (A) and then hardens after that point. In hardening, the stress value rises with an increase in strain. Perfectly plastic behavior is also shown in the figure. Here , after the yield point, the stress value remains constant and the material only strains under constant stress without any hardening. If the load is removed at point B after some hardening, there is elastic unloading. The elastic strain ( εe ) is recovered and there is an only permanent plastic strain ( εp ) left in the metal. This phenomenon is described by  elasto -plastic  material models. Contd.

14. 4.13. (a) Schematic representation of dislocation formation by the Frank–Read source. (b) High magnification photograph showing the formation of dislocations. (from M. F. Ashby and D. R. H. Jones,  Engineering materials 1 , Butterworth–Heinemann, 1996) Dislocation Formation by t he Frank–read Source

15. Work-hardening ∆ σ ( a) and work-hardening rate d∆ σ/ d ε p (b) versus plastic strain in tensile stressed interstitial-free (IF) low-carbon steels (adapted from [20]), ultra-low-carbon steels (adapted from [21]), and extra-pure large-grained Fe samples (adapted from [22]). 

16. From: Ashby and Jones, "Engineering Materials I," Pergamon (1980) Work Hardening (Or) Strain Hardening

17. Work hardening (strain hardening) manifests as the increase in stress that is required to cause in increase in strain as a material is plastically deformed. On the diagram, the red curve is for a material that does not work harden - an ideal plastic material. Plastic deformation begins when the yield stress is reached and this material deforms to fracture at the same stress value. The black curve shows the true resolved shear stress/shear strain response of a material that work hardens. Yield again starts at the yield stress, but as the strain increases an increase in stress is required to maintain the same strain rate. The difference between the two curves measures the degree of work hardening. Contd.

18. The insert on the diagram shows a mechanism for work hardening. Dislocations on intersecting slip planes permit both elastic interactions and dislocation reactions to contribute to work hardening. Contd.

19. Improvement of Mechanical Properties and Work-Hardening Behavior of Intercritically Annealed Dual Phase Steel

20. Property Changes due t o Cold Working

21. As the internal energy of cold worked state is high, the chemical reactivity of the material increases i.e. the corrosion resistance decreases, and may cause stress corrosion cracking in certain alloys. The rate of strain hardening (slope of flow curve) is generally lower in HCP metals than cubic metals. High temperatures of deformation also lower the rate of strain-hardening. Contd.

22. Loading/Unloading Cycle in Material Work Hardening Curve Example: Mn steel

23. Fig. Plastic deformation.

24. Work hardening. The meaning of work hardening in simple tension is just that stress is a monotonically increasing function of strain, Fig. 1. The plastic deformation is then .said to be stable . For more general states of stress and paths of loading no such simple picture can be drawn. The concept of work hardening, or stability in a restricted sense , can be expressed in terms of the work done by an external agency which slowly applies an additional set of stresses and then slowly removes them. The original configuration , or state of strain, may or may not be restored. This external agency is to be understood as entirely separate and distinct from the agency which causes the existing state of stress and which has produced the existing state of strain. Work hardening implies that for all such added sets of stresses the material will remain in equilibrium and further that a ) positive work is done by the external agency during the application of the set of stresses, and b ) the net work performed by it over the cycle of application and removal is zero or positive. Contd.

25. References: Authors of Technical articles and Scopus Journals are Acknowledged.

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