Work Hardening Metal: Processes, Benefits, and Applications

Work hardening is one of the most important phenomenon in metal working processes. Be it manufacturing of beverage cans through deep drawing process or the shaping of common bicycle frame, work hardening plays a basic yet pivotal role. Our curious readers should stay tuned to get a deep insight to this topic.

What Is Work Hardening Metal?

Metals get work hardened sometimes desirably or sometimes undesirably, depending on the process. It increases their load bearing ability or yield strength. This article will give a detailed overview of the work hardening basics, work hardening calculations, strain hardening, and almost everything related to it in a technical yet a reader friendly language.

Definition and Importance in Metallurgy

Work hardening in metals is a process by which their mechanical properties change as a result of increased loading beyond a particular point, through plastic deformation. The most profound effect is on the yield strength which increases. Ductility decreases! Manufacturers, specially in the field of metal products, give due diligence to the usefulness of this phenomenon. Take manufacturing of aluminum beverage cans as an example: series 3 aluminum alloy is usually relatively soft and ductile in an annealed condition, but as work is done on it by deep drawing, its strength improves, such that it can be used to contain and transport beverage without the fear of leaking due to deformation.

This process can be used to our advantage in other process also, like, rolling, bending, shearing, etc.

What Are the Three Stages of Work Hardening?

Three stages of work hardening are discussed below:

1. Yeilding: At the onset of yield point the material start to deform plastically. The required shear stress is almost constant for plastic deformation
2. Strain hardening: To continue plastic deformation, the magnitude of required shear stress continues to increase as strain increases.
3. Necking: Due to activation of cross slip planes, magnitude of required stress decrease.

Watch this video on Strengthening by Strain Hardening: Strengthening by Strain Hardening

The Science Behind Work Hardening

Work hardening in metals relates to the scientific concepts like storage of energy, distortion of crystal lattice, movement, pinning and pile up of dislocations at the grain boundaries. More of it is being discussed here.

Elastic vs. Plastic Deformation

Below table highlights some of the key differences between elastic deformation and plastic deformation in the context of work hardening:

Dislocations and Lattice Strain Fields

Dislocations play a vital role in the work hardening process. As the material is subjected to a stress level beyond its yield point, the movement of dislocations intensify. They move and interact with each other creating distorted lattice strain fields. The more the dislocations dislocations a material has the more is the hinderance to the movement of dislocations and hence the more is the work hardening.

How Dislocations Accumulate During Work Hardening

As applied stress continues to increase, the dislocations motion increases but they find grain boundary as an unsurmountable barrier. Hence they pile up at the grain boundary creating a great deal of hindrance to their further movement.

Impact of Temperature on Work Hardening Dynamics

At high temperatures, dislocations movement is facilitated by the increase in overall energy mainly in the form of heat. Hence less amount of work is required to be done to deform the material. Work hardening become difficult. Dislocations can climb easily.

Intentional Work Hardening: Leveraging Its Benefits

In many metal working applications, work hardening is sought for to make a material suitable for a particular application. Its benefits are leveraged in processes like deep drawing, bending, shearing, rolling etc. Take the example of aluminum beverage can, for instance (as discussed earlier).

Strengthening Metals Through Work Hardening

As metals are work hardened, their strength is increased at the expense of their ductility, which is many a times, the desired goal. For example, before work hardening, the ductility of aluminum helps the blank of beverage cans to attain shape and as it gets work hardened, the increased strength helps in preserving the shape.

Common Techniques: Rolling, Hammering, and Drawing

Applications in Tool and Die Making

Tool and die materials should be relatively high in strength to endure a frequent and high magnitude of stress that is applied on them in their lifetime.

Applications in Sheet Metal Forming

Cold rolled and unannealed sheet of metal has an improved level of strength comparatively with the hot rolled metals. Cold rolling imparts work hardening.

What Is the Problem with Work Hardening?

While there is an ample usefulness of work hardening in manufacturing processes, sometimes it happens unintentionally and undesirably. A common problem due to work hardening occurs in the CNC and machining industry. More of it will be explored!

Causes of Undesirable Work Hardening

Some of the common causes of undesirable work hardening are listed below:

1. Improper tool alignment in the machining process causes top surface to work harden.
2. Lack of lubrication increases friction which aggravates work hardening
3. High strain rate causes stress concenteration
4. Lack of annealing where required

Heat Generation and Surface Hardening

During manufacturing processes, lack of lubrication causes local stress concenteration and heat build up which undesirable work harden the workpiece

Impact on CNC Machining and Formability

In the CNC machining industry, the surface of the workpiece might get work hardened due to improper setting of the tool in the initial passes. This work hardened surface causes problems later on in the machining process. The work piece surface might become rough and the tool’s lifespan also decreases due to excessive wear. Also, it impacts precision.

Quantifying Work Hardening

In mechanics of materials, work hardening calculations are carried out by introducing a coefficient (n) of work hardening. It basically is like a power law and the coefficient acts like an exponent. Commonly, Holloman relationship or Ludwik equation are used for these calculations. But still there are other empirical relations as well.

Hardness Tests and Their Role

To quantify the actual work hardening, Brinell hardness test or Vickers hardness test can be carried out. Brinell test usually gives hardness over a large area while Vicker’s test can be used for a more precise and micro level hardness test.

Stress-Strain Curves and Their Significance

Work hardening can also be estimated from a stress-strain curve. Before the yield point and elastic limit, it follow a linear pattern, i.e, the amount of strain increases proportionally with the amount of stress. But beyond the yield point, the curve become more inclined towards the x-axis, which represents stress. Hence, it becomes more difficult to strain with the same proportion of increase in the stress value.

With regards to dislocations, stress normally follows a half root relationship with the dislocation density:

Where is the stress and is the dislocations density.

Empirical Relations in Work Hardening

Common empirical relations like Holloman equation or Ludwik equation quantifies the impact of work hardening on the stress required to deform a work hardened material. Ludwik equation is given below:

Where is the yield strength, K is the strength coefficient, is the plastic strain and n is the work hardening coefficient.

What Metals Can Be Work Hardened?

Most of the common metals can be work hardened easily due to the presence and moveability of dislocations. But some alloys which are hard an d brittle, like some cast irons, are too difficult to work harden, so they are shaped via metal casting instead of metal working.

Steel

It is one of the most common and widely applicable examples of “strengthening via work hardening”. Bending, squeezing, pressing etc are the common processes of it.

Copper

Copper and brass can be work hardened easily. Some controlled hardening processes like thermo-mechanical treatment or partial annealing enhances both: strength and ductility.

Aluminum

Aluminum is quiet soft in an annealed condition, work hardening in processes like cold rolling or deep drawing enhances the strength.

Gold and Precious Metals

Goldsmiths leverage the benefits of both alloying and work hardening to give the desired strength to precious jewellery, without which it’ll be prone to damage.

Platinum and silver can also be strengthened for use in electronics applications

Metals Work Hardened Charts

The below table summarizes the work hardening characteristics, advantages and limitations of the commonly used metals:

How to Avoid Work Hardening During Machining

Undesirable work hardening which can occur during machining can lead to higher operating costs, process inefficiency, material wastage, rapid tool wear and lack of precision. It must be avoided to optimize the process.

Optimizing Speeds and Feeds

A balanced and optimized speed of the cutting tool can minimize the friction and heat buildup which results in work hardening. Feed rate should be high while cuts should be deep to avoid unnecessary rubbing of the tool with the work piece.

Using Climb Milling and Advanced Tool Coatings

Climb milling significantly hampers the unneccesary rubbing and excessive friction between the tool and the work piece.

Advanced tools that are sharp and much harder like ceramics or coated tools like DLC (diamond like coating) or TiAlN or TiN coated tools have a low coefficient of friction. Also they can dissipate heat well and they remain sharp even after many passes.

Effective Coolant Application Strategies

Frequent application of coolant helps in reducing friction and heat buildup, thus it make the material less prone to work hardening

What is an Example of Work Hardening?

There are numerous applications of work hardening in various industries like building and construction, automobiles, locomotives and aerospace.

Work Hardening in Bolt Manufacturing

Bolts made from metals like steel passes through various steps in their manufacturing like cold drawing, head forging, and thread rolling. Due to an enormous amount of deformation, they attain work hardening. They tend to exhibit an increase in strength and wear resistance.

Applications in Manufacturing and Construction

Common bending and shearing operations on sheet metal in the manufacturing of trusses, columns and purlins impart work hardening in the metal. As a results their strength enhances.

Case Studies in Aerospace and Automotive Sectors

Aerospace and automotive components that go through presses, squeezing and cutting operations improve their strength due to work hardening.

Advantages of Work Hardening Materials in CNC Machining

In CNC machining, materials undergo work hardening during certain processes, like squeezing or forming, which can enhance their strength. This process can improve the material’s hardness, wear resistance, and fatigue strength, making it ideal for applications requiring high durability. However, work hardening can also be undesirable in cutting operations, where it may lead to increased tool wear and difficulty in achieving high-quality finishes.

Work Hardening vs. Heat Treatment

Work hardening and heat treatment are two different processes. In work hardening only mechanical forces play a role, but in heat treatment, both mechanical forces and application of heating and cooling plays a role, while the latter is more important.

How Work Hardening Differs from Annealing

Work hardening and annealing differs in many aspects. While the main goal in work hardening is an increase in strength, annealing does quite the opposite: it increases ductility.

Comparing Durability: Work Hardened vs. Heat-Treated Metals

Heat treatment has a diverse number of uses and comparing the durability of both is subjective. In some aspects heat treatment is more durable than work hardening and vice versa.

FAQ

How does work hardening happen?

Work hardening primarily happens due to movement and interaction of dislocations. Distortions in the lattice strain fields and accumulation of dislocations at the grain boundaries causes work hardening.

Why Can’t Some Metals Be Work Hardened?

Due to lack of ductility in some metals like cast irons they cannot be metal worked

Does 316 stainless steel work harden?

Yes, 316 stainless steel can be work hardened.

Can work hardening improve machinability?

For soft metals, work hardening can improve machinability.