Annealing

[edit] Introduction

Annealing is a much-used process of heat treatment which involves heating a material for a given time then allowing it to cool in a slow, controlled manner. The aim is to increase ductility and strength and to remove internal stresses. It is usually applied to glass and metals.

[edit] Glass

The annealing of float glass immediately after it has been formed helps dissipate any internal stresses which may have been induced during the manufacturing process. The result is a glass that is harder to break than ordinary glass.

Manufacturing smaller glass objects may involve annealing as an incidental process – i.e cooling takes place over time during manufacture without a specific annealing process. But with larger or more complex objects, such as long ribbons of float glass, a specific annealing process can be introduced using a temperature-controlled kiln called a lehr. NB There can be safety concerns using annealed glass as it can break into large, jagged shards.

A typical annealing process that may form part of the manufacture of float glass

• After the glass has been poured onto the molten tin in ribbons at an initial temperature of around 1,200°C, it travels under gravity or by top roller gears propelling it forward.
• Having cooled slightly to around 1,100°C, the glass enters a lehr oven to be annealed i.e allowed to cool very gradually over a specific time period until it reaches the required temperature. The time required to anneal glass will depend on the glass type and the thickness.
• When it finally emerges from the oven, the glass may be further cooled by jets of air after which scanners seek any imperfections which, if found, will result in the glass being discarded or recycled.

If glass is not properly annealed, it may retain thermal stresses which can significantly decrease its strength and make for a less reliable product; it may therefore crack or shatter even under small temperature changes, mechanical shock or stress. It can even suffer sudden failure.

[edit] Metal

The annealing of metals – such as alloys, stainless steel, copper, silver and brass – also involves a heat treatment process that alters their physical characteristics, reducing hardness and increasing ductility and malleability. Therefore, metals can be softened in preparation for further processing such as forming, shaping and stamping.

The annealing process for metals is essentially similar to that for glass: the metal is heated to above the temperature required for recrystallisation – in the case of steel, copper, silver and brass this will be until they glow. The temperature is maintained for the required time and then cooled in a slow, controlled process, possibly down to room temperature. As the metal cools, recrystallisation takes place within the matrix until the required temperature is reached. However, further heat treatments may be needed, depending on the degree of malleability and hardness required.

Steel requires slow cooing in air to anneal, but copper, silver and brass can also be cooled by quenching (quench annealing) whereby they are cooled quickly by immersion in water.

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