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Normalising
This unit of work addresses aspects of the following syllabus outcomes:
A student:
H2.1 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering
Extract from Engineering Studies Stage 6 Syllabus, © Board of Studies NSW 1999.
Introduction
Lifting devices in their various forms are constructed from a range of materials that have specific properties for the function they will perform. The inherent structure that provides these necessary properties may not be capable of meeting a range of “working demands”. It is then necessary to enhance these properties by some means.
Alloying, cold working processes and heat treatment are ways of altering the structure of a material to improve its properties. These structure-altering procedures are often employed in combination to achieve the desired results.
For example, iron alloyed with carbon produces steel that is further enhanced with other metals such as molybdenum, chromium and vanadium.
The forging process will again change the properties, as will heat treatment.
Normalising
The heat treatment process of normalising is an excellent example of how the structure/property relationship is controlled to produce a component that functions with optimum reliability over a range of working-environment conditions.
The normalising process is a simple one that follows the basics of process annealing (used for stress-relieving of welded structures and large castings) but the cooling process occurs in still air rather than in the furnace.
The component to be normalised is heated at temperatures around 40°C above the critical temperature for the particular metal alloy and soaked for several hours. Cooling in still air avoids large grain size occurring due to atomic diffusion. The resultant fine grain structure is stress free and the mechanical properties generally show a decrease in overall hardness, altered ductility and increased overall toughness, i.e. impact strength.
Cold rolling and casting are forming processes that produce grain structures that may require normalising before the component is put into service. Diagrams representing the microstructures of a cold rolled and a cast steel are shown in Fig. 1 and Fig. 2 respectively.
Figure 1 Microstructure of a cold rolled alloy showing elongated grains.
Figure 2 Microstructure of a cast alloy
Normalising at the correct temperature and for the right amount of time will modify these structures to produce a microstructure showing fine equiaxed grains as seen in Fig. 3
Figure 3 microstructure of a normalised alloy
Normalising can soften, harden or stress relieve a material depending on its initial state and chemical composition. For example, a normalised 0.4%C steel will have a tensile strength of 580 MPa compared to an annealed steel of 510 MPa. Its ductility will be 27% compared to 30%, and its hardness will be 165 BHN compared to 145 BHN. That is, the normalised steel will be stronger, and harder, but slightly less ductile than the annealed steel.
As a valuable heat treatment process, normalising will produce other mechanical and structural benefits.
Activity 1
Normalising is a type of heat treatment applicable to ferrous metals only and Normalised steels are harder and stronger than annealed steels. Access this web site to answer the following questions.
http://www.tpub.com/content/construction/14250/css/14250_30.htm 
- How does normalising differ from the annealing process?
- List some of the causes of internal stress, which normalising is designed to remove.
- How and why would you normalise low-carbon steels?
- What major property does normalised steel have and what type of loading will it best withstand?
Activity 2
Pollution, Greenhouse gases, Global
warming and sustainable energy are terms we often hear. Heat treatment processes will always pose a problem in these areas. A 'Fluidised Bed Furnace' is able to reduce these problems. Refer to the 'Quality Heat Technologies Q-Heat ' site to answer these questioned on 'fluidised bed furnaces':
- List some of the advantages of the fluidised bed furnace heating process.
- How are these furnaces controlled, heated and what is their temperature range.
- List some of the heat treatment operations, which this type of furnace can perform.
Activity 3
You will find a very good glossary of terms at AboutMetals.com
Explain the meaning of the terms:
- normalising
- non-destructive testing
- notched bar test.
Another useful resource:
http://www.efunda.com/processes
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