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The reasons for work hardening of metal materials

The fundamental reason for work hardening of metal materials, also known as strain hardening, is that the density of internal defects (mainly dislocations) in the metal increases significantly during plastic deformation, as well as the complex interactions between these defects.

1. Core mechanism: proliferation and interaction of dislocations

The plastic deformation of metals is not achieved through the sliding of the atomic layer as a whole, but through the movement of a type of line defect called a "dislocation". You can imagine a dislocation as a wrinkle on a carpet - moving this wrinkle is much more effortless than dragging the entire carpet.

In annealed (softened) metals, the dislocation density is low and dislocations can move relatively freely. When we apply external force to deform metal, the following situations occur:

·Dislocation proliferation: During the deformation process, dislocations proliferate in large quantities through various mechanisms (such as the Frank Reed source), leading to a sharp increase in their density (which can increase from 10 ⁶/cm ² to over 10 ¹ ²/cm ²).

·Dislocation interaction and entanglement:

·Dislocation Encounter: Moving dislocations will encounter other dislocations. They may cut each other, creating a 'cutting step' that hinders further movement of dislocations.

·Obstacle formation: A large number of dislocations will entangle together, forming complex "dislocation tangles" and "dislocation cell structures". These structures are like countless roadblocks set up on the road.

·Obstacle to movement: These proliferating and entangled dislocation networks become a huge obstacle to subsequent dislocation movement. To continue the deformation, greater external force must be applied to "push away" these obstacles, or to make way for them and cut them. It's like a person struggling to navigate through a crowded crowd.

·Annealing state: Like a sparse crowd, you can easily pass through.

·Work hardening state: Like a dense crowd of people squeezing and pushing each other, if you want to continue moving forward, you must spend much more effort pushing away the people around you. This' crowd 'is high-density, entangled dislocations.

2. Macroscopic performance: increased strength and hardness, decreased plasticity

This microscopic change directly leads to the phenomena we observe at the macroscopic level:

·Strength and hardness increase: Because moving dislocations requires greater force, the metal's ability to resist further plastic deformation is enhanced, resulting in an increase in yield strength and tensile strength.

·Decreased plasticity and toughness: The ability of metals to continue to deform decreases, becoming more "brittle". In the tensile test, it is manifested as a decrease in elongation.

·Stress strain curve: On the true stress-strain curve, work hardening is manifested as the curve continuously rising with increasing strain.

So, the reasons for work hardening can be summarized as:

During the cold working (plastic deformation) process of metals, the internal dislocation density increases sharply, and these dislocations interact, entangle, and pin together, forming a barrier that hinders subsequent dislocation movement. In order to overcome these barriers and continue deformation, external forces must be continuously increased, which is manifested macroscopically as the strength and hardness of the material continue to improve.

·The favorable side:

·Important strengthening methods: For metals that cannot be strengthened by heat treatment (such as pure metals, austenitic stainless steel, and certain aluminum alloys), work hardening is the only effective strengthening method.

·Ensure uniform deformation: During the stretching process, if a local area undergoes necking (thinning), the amount of deformation in that area will increase, rapidly hardening and preventing it from further thinning, transferring the deformation to adjacent weaker areas and making the deformation more uniform.

Some customers require low carbon steel to meet both welding requirements and certain mechanical strength requirements. Therefore, cold working plastic deformation is used to obtain higher mechanical performance strength, which is simpler and easier to control in terms of size and surface compared to processes such as heat treatment. Especially for accessories of construction machinery products, such as pins, shaft sleeves, etc,

·Disadvantage side:

·Subsequent processing difficulties: In processes such as cold rolling, cold drawing, and stamping, the material will become harder and more brittle, which poses difficulties for subsequent forming and may even lead to cracking.

·Intermediate annealing is required: In order to restore the plasticity of the material for further processing, a "recrystallization annealing" treatment must be carried out between processes. Heating is used to re nucleate and grow deformed grains, eliminate defects such as dislocations, and soften the material.

Another unfavorable factor is that cold processing cannot produce large-sized products, and the required processing capacity and equipment are limited to a certain extent. And generally there is a minimum order quantity requirement. Compared to foreign countries, China's well-established supply chain configuration also has some small and micro enterprises willing to meet customers' various quantity requirements, but the price will be slightly higher than ordinary processing.