Heat Treatment: Critical Points of Steel

When a piece of steel is heated at a uniform rate, the temperature of the steel will, at first, rise steadily.  When the temperature reaches approx.700°C (a dull red colour) it will remain there for certain time then continue to rise again at a uniform rate.  If the heating is continued further then occurs a second arrest in the rise in temperature.  After this, if the heating is continued, the temperature will continue to rise at approximately the initial rate.

During these periods of arrest, the metal absorbs heat, but instead of raising the temperature, the heat brings about a structural change in the steel.  The temperatures at which these periods occur are called ‘critical’ or ‘arrest’ points.

If the steel is heated to 900°C (a bright reddish yellow colour) then removed from the furnace and observed in a darkened room, it will be seen that, as it cools, it will lose its brilliance.  At the points where it received its checks in heating the metal is seen to glow more brightly and it will seem that the cooling has stopped.  In fact the steel will be seen to take on an extra glow as though it was reheated.  After this the rate of cooling will be normal until the metal is cold to room temperature or becomes equal to environmental temperature. 

The temperature at which the changes start (lower critical point) is the same for all steels and is about 700°C.  At this temperature, the internal structural change is as that the pearlite disappears and the laminae of ferrite and cementite of which it is composed, dissolves and forms the solid solution Austenite.  The finishing point of the transformation is known as the upper critical point this point varies according to the steel carbon content.  

Aircraft Structural Health Monitoring (SHM)

It is extremely important that the level of fatigue, imposed on an aircraft structure and associated components can be monitored and recorded so that the respective fatigue lives are not exceeded. Several methods have been developed to assist in the vital tasks involved with SHM

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Fatigue Meters

Fatigue meters are used to check overall stress levels on aircraft and to monitor the fatigue history of the aircraft. Fatigue meters also allow a check to be made on the moment in time when the aircraft exceeds the design limits imposed on it.

Strain Gauges

Strain gauges may be used to monitor stress levels on specific aircraft structures. Strain gauges are thin-foil electrical resistor elements that have been bonded to the aircraft structure during manufacture. Their resistance varies proportional to the applied stress loading.

Fatigue Fuses 

Fatigue fuses are metallic fuses, which are bonded to the structure and which fail at different fatigue stresses. The electrical current flowing through the fuse, will vary and so provide an indication of the stress level.

Intelligent Skins Development

Modern developments in aircraft structures will allow the structures to be designed and built with a variety of sensors and systems embedded into the structure and skin. This would mainly be restricted to structures manufactured from composite materials. One major benefit of this is to allow the structure to monitor it's own loads and fatigue life.

Aluminium Alloy Identification

Aluminium and its alloys are internationally classified into eight groups identified by a four figure series number.  

 The first digit indicates the principal alloying element.  For example any alloy in the 2000 series such as 2117 or 2024 has copper as its main alloying element.  7075 has zinc as its main alloy.

Aluminium Alloys grouped by main alloying elements.	99.0% minimum	1000
	Copper	2000
	Manganese	3000
	Silicon	4000
	Magnesium	5000
	Magnesium and Silicon	6000
	Zinc	7000
	Lithium and others	8000

 

The second digit identifies the alloy modification state, 0 indicates that the alloy is original.  1 indicates that the alloy has been modified once.  The third and fourth digits identify the specific aluminium alloy.  In the case of 2024, the alloy consists of 4.5% copper, 1.5% magnesium, 0.1% chromium and the remainder aluminium.

There are two basic divisions of aluminium alloys: 

• NON-HEAT TREATABLE – those that can be softened but not hardened by heat treatment.  These alloys are strengthened by controlled working, eg cold rolling. 

• HEAT TREATABLE – those that can be softened and hardened by heat treatment.

If non-heat treatable alloys are heated to their annealing temperature and allowed to cool slowly they will be softened to their annealed, or –0 condition.

The 2000, 4000, 6000, 7000 and 8000 series alloys are strengthened by solution treatment followed by age hardening.

The 1000, 3000 and 5000 series alloys are cold worked to increase their strength.

IDENTIFYING STRAIN HARDENING PROCESSES

Non-heat treatable aluminium alloys can be hardened by strain hardening.  This is usually done by rolling the sheets of metal.

The cold worked temper of wrought alloys is indicated by the letter –H followed by a number.  Tempers in the –H series are not applicable to castings.

The first digit following –H indicates the form of the strain hardening.

A second digit 2, 4, 6 or 8 indicates the final degree of hardness.  The fully hard condition is 8.4 indicates material having a strength midway between fully annealed –0 and full hard i.e. half hard.  -0  Indicates the soft, annealed condition.  Applies only to the wrought alloys.

   5052 – H24----------->Strain hardened +  Half hard Then partially annealed

IDENTIFYING HEAT TREATMENT PROCESSES

The heat treatable alloys can be hardened and strengthened by solution heat treatment or by precipitation heat treatment (artificial ageing).

The heat treated temper of aluminium alloys is indicated by the letter –T followed by one or more numbers.  The number following the letter T shows the type of heat treatment.  Any variations are indicated by a second number.

-T3      Solution heat treated and strain hardened (cold rolled).

-T36    Solution heat treated and cold rolled to reduce the thickness of the sheet by 6 per cent.

-T4      Solution heat treated then naturally aged.

-T42    Solution heat treated by the user regardless of the previous temper.

2024   –  T4--------------> Heat treated alloy + Solution treatand +naturally aged