Heat Treatment

To reduce the risk of cracking in the HAZ, preheat, controlled interpass temperatures and possibly post-heating may be applied to the weld areas.

Heat treatment is an expensive operation and is therefore only carried out when necessary, i.e. if there is a significant chance that adverse metallurgical structures and/or cracks could occur.

Preheat

Preheat is the application of heat to a joint prior to welding. Preheat is usually applied by a gas torch or induction system.

Preheating has many advantages:

• Preheat slows down the cooling rate of the weld and HAZ, which reduces the risk of hardening and also allows absorbed hydrogen a better opportunity of diffusing out, thereby reducing the chance of cracking. Basically speaking, the application of a preheat helps to counteract the adverse metallurgical effects produced by welding on the material.

• Preheat removes any moisture in the region of the preparation.

• Preheat improves the overall fusion characteristics during welding.

• Preheat ensures more uniform expansion and contraction and lowers the stress between the weld and parent material.

Preheat temperatures may be measured by the use of a touch pyrometer (thermocouple) or temperature indicating crayons (Tempil sticks). Temperature indicating crayons exist in two forms; the type that melt and the type that change colour. The method of temperature measurement to be used is sometimes stated in the specification for the work being carried out.

Preheat temperatures are measured at intervals along or around a joint to be welded. The number of measurements taken must allow the inspector to be confident that the required temperature has been reached over the full area to be welded. Specifications sometimes specify that the preheat temperature must be maintained over a specified distance from the joint faces, e.g. 50 – 100 mm.

The preheat temperature should be taken immediately prior to welding. If a gas heat source has been used, sufficient time must be allowed for the temperature to equalize throughout the thickness of the components to be welded, otherwise only the surface temperature will be measured. Time lapses vary depending on specification requirements, e.g. BS EN 1011 states 2 minutes for a 25 mm wall thickness.

The temperature of the joint during welding and between passes is known as the interpass temperature. It is often specified that the interpass temperature must not drop below the minimum preheat temperature.

Post-heat treatment

Post-heat treatment in this context is a process in which metal in the solid state is subjected to one or more controlled heating cycles after welding. The post heat treatment of welds (PWHT) is normally carried out for the purpose of stress relief, i.e. the reduction of localised residual stresses. Post-heat treatment may also be used to produce certain properties, such as:

• Softening after cold working.
• Hardening to produce improved strength and hardness, this may be very hard and brittle.
• Tempering to improve hardened structures giving ranges of strength with toughness.

Another PWHT process which may be used is for hydrogen release only.

The relevant variables for a PWHT process which must be carefully controlled are as follows:

• heating rate,
• temperature attained,
• time at the attained temperature,
• cooling rate – in certain circumstances

Stress relieving

Used to relax welding stresses without any significant affects on the component’s metallurgical structure because austenite is not produced.

Stress relief is achieved by heating to 550-650°C, holding for the required time, e.g. 1 hour per 25 mm thickness, and then cooling down in air. Local heating is carried out with gas flame or electric elements; whole components may be stress relieved in a furnace.

Annealing

Full anneal – is used to produce a very soft, low hardness material suitable for machining or extensive cold working. A full anneal is achieved by very slow cooling* after the steel has been heated to above 910°C and made fully austenitic. By the time the steel has been very slowly cooled down to 700°C, all the austenite changes to ferrite and pearlite with extensive grain growth. The component is cooled down in air from 680°C.

Sub-critical anneal – this process is also known as spheroidizing and is used to produce a soft, low hardness steel – cheaper than full anneal. Temperatures must not rise above 700°C. A sub-critical anneal is achieved by heating to 680-700°C, holding for sufficient time for full recrystallisation to occur, i.e. new ferrite grains to form; the component is then air cooled in most circumstances.

Normalising

Normalising is used to maintain and improve mechanical properties and to modify grain structures by making them more uniform giving a refined structure avoiding grain growth.

Normalising is achieved by heating the steel until it is fully austenitic – the same temperature as that used for full anneal – soaking for the minimum time necessary to achieve a uniform through thickness temperature and then air cooling.

Hardening/quenching

Hardening is achieved by very fast cooling from the austenite region.

The steel is first heated to produce austenite; it is then allowed to soak at this temperature to produce grain uniformity, and then fast cooled by quenching into oil or water (brine) to achieve the desired hardness.

After quenching, the steel is highly stressed, very hard and brittle, with a high tensile strength. Quenched steel is very prone to cracking and therefore requires tempering.

Precipitation hardening Precipitation hardening can be achieved by solution heat treatment. Solution heat treatment relies on locking in certain alloying constituents, sometimes referred to as precipitants, by heating the material to a pre- determined temperature and allowing these constituents to be uniformly distributed.

Cooling the material at a pre-determine rate hold these constituents in position which act as a barrier to dislocations (faults within the atomic lattice) restricting dislocation movement within the material causing hardening of the material. Hence precipitation hardening alloys.

Pure iron, which has no carbon content, cannot be hardened via heat treatment as there is no carbon to act as a precipitant. Mild steel cannot be heat treated very easily because there is little or no carbon available to act as a precipitant, but the hardness maybe increased via work hardening to a small degree, or by case hardening methods such as carbonising.

Carbonising is a heat treatment which takes place in a carbon rich atmosphere to harden the surface of the component.

Tempering

Tempering is used to produce a range of desired mechanical properties to meet specific requirements.

Tempering is achieved by slowly heating the hardened steel to a temperature between 200-650°C to produce the required tensile strength and toughness properties; the component may then be air cooled.

At 200°C, the quenching stresses are reduced and the steel will give maximum tensile and hardness with a reduced risk of cracking. Increasing the tempering temperature reduces the hardness and tensile strength whilst increasing the toughness and ductility. At 650°C, a full temper is produced, giving a very fine grained soft steel with a spheroidized structure.

Hydrogen release

Both normalising and annealing heat treatment processes will help to release hydrogen from a weld area. However, there may be a situation where only hydrogen release is required. This may be performed by heating the weld area to 150-200°C and soaking for approximately 10- 24 hours.