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What The Welding Inspector Should Know About Preheating AndPostweld Heat Treatment

When welding some base materials and for some service conditions, preheating and/or postweld heat treatmentmay be a requirement. These types of thermal treatments are generally required in order to ensure suitable weld

integrity and will typically prevent or remove undesirable characteristics in the completed weld. Any form of heattreatment is costly since it demands extra equipment, extra time, and extra handling. For these reasons, heattreatment should only be undertaken after careful consideration of the advantages it may offer. In certain casesheat treatment will be mandatory, as with heavy sections of low alloy steels, whereas in other cases, it will be ajustifiable precaution against early failure in service.

There are a number of reasons for the incorporation of these thermal treatments within the welding procedure, andwe will consider some of the most common.

Preheating

Preheat, as defined within the AWS Standard Welding Terms and Definition, is the heat applied to the base metalor substrate to attain and maintain preheat temperature. The preheat temperature is defined by the samedocument as the temperature of the base metal in the volume surrounding the point of welding immediatelybefore welding is started. In a multipass weld, it is also the temperature immediately before the second andsubsequent passes are started(Interpass Temperature).

Preheating may be performed by the use of gas burners, oxy-gas flames, electric blankets, induction heating, or byheating in a furnace. For good results, it is essential for the heating to be uniform around the joint area. Intense,non-uniform heating is of little use in retarding cooling and may be detrimental in causing higher residual stresses,distortion, or undesirable metallurgical changes in the base material. When preheating is specified, the entire weldjoint should be heated evenly through the material thickness to the desired minimum temperature. To obtain auniform temperature through the material thickness, it is desirable to apply the heating sources to one side of thematerial surface and to measure the material temperature on the opposite side. Whenever the heating andtemperature measurement must be conducted from the same surface, the inspector must assure that more thanjust the surface of the material has been heated. It is important to ensure that the entire material thickness hasbeen heated to a uniform temperature. In addition to establishing a preheat temperature, an interpass temperaturelimitation may need to be considered for some applications. This information should be shown in the weldingprocedure specification. When an interpass temperature is specified, the weld area must be inspected prior todepositing the next weld bead. Welding may not continue if the measured temperature exceeds the maximuminterpass conditions specified in the welding procedure. The weldment must be permitted to cool down to thespecified upper limit of the interpass temperature before continuing with the weld.

Dependent on the metallurgical properties of the material, and/or the desired mechanical properties of the weldedcomponent, preheat and interpass temperature may be evaluated for different reasons. For instance, a procedurefor welding mild steel, which has a low carbon content, relatively low hardenability, and is used in an applicationwith no special service requirements, may consider a minimum preheat and interpass temperature based on thematerial thickness. Welding procedures used for the heat-treatable low alloy steels and chromium-molybdenumsteels with impact requirements will normally specify a minimum and maximum requirement for preheat andinterpass temperatures. These low alloy materials can have high hardenability and are susceptible to hydrogencracking. Allowing these materials to cool too quickly or overheating them can seriously affect their performancerequirements. When welding the nickel alloys, we are concerned primarily with high heat input during the weldingoperation. The heat input of the welding process, and the preheat and interpass temperature can seriously affectthese materials. High heat input can result in excessive constitutional liquation, carbide precipitation, and otherharmful metallurgical phenomena. These metallurgical changes may promote cracking or loss of corrosionresistance. Procedures for welding some aluminum alloys such as the heat-treatable, 2xxx, 6xxx, and 7xxx series,are often concerned with overall heat input reduction. With these materials, the maximum preheat and interpasstemperature is controlled in order to minimize its annealing and over-aging influence on the heat-affected zone(HAZ) and consequent loss in tensile strength.

On critical applications, the preheat temperature must be precisely controlled. In these situations, controllableheating systems are used, and thermocouples are attached to monitor the part being heated. Thesethermocouples provide a signal to the controlling unit that can regulate the power source required for heating. Byusing this type of equipment, the part being heated can be controlled to extremely close tolerances.

Some of the reasons for preheating are:

a) To drive away moisture from the weld area: Typically, this is performed by heating the surface of the materialto a relatively low temperature, just above the boiling point of water. This will dry the plate surface and remove theundesirable contaminants that may otherwise cause porosity, hydrogen embrittlement, or cracking through theintroduction of hydrogen during the welding process.

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b) To lower the thermal gradient: All arc welding processes use a high temperature heat source. A steeptemperature differential occurs between the localized heat source and the cool base material being welded. Thistemperature difference causes differential thermal expansion and contraction and high stresses around the weldedarea. Reducing the temperature differential by preheating the base material will minimize problems associatedwith distortion and excessive residual stress. If preheating is not carried out, a large differential in temperature canoccur between the weld area and the parent material. This can cause rapid cooling, leading to the formation ofmartensite and probable cracking when welding some materials with high hardenability.

Postweld Heat Treatment

A number of different types of post-weld heat treatments are used for different reasons and for different materials.

a) Post-weld heat treatment is most generally used for stress relief. The purpose of stress relieving is to removeany internal or residual stresses that may be present from the welding operation. Stress relief after welding maybe necessary in order to reduce the risk of brittle fracture, to avoid subsequent distortion on machining, or toeradicate the risk of stress corrosion.

b) For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgicalstructure. This treatment is generally performed after the weld has cooled, but under certain circumstances, it maybe necessary to perform this treatment before it has cooled to prevent cracking.

c) Extremely coarse weld structures in steel, such as those obtained with the electro-slag welding process, mayrequire normalizing after welding. This treatment will refine the coarse grain structure, reduce stresses afterwelding, and remove any hard zones in the heat-affected zone.

d) The precipitation hardening alloys, such as the heat treatable aluminum alloys, are sometimes required toundergo post-weld heat treatment to regain their original properties. In some cases, only an aging treatment isused, although a full solution heat treat and artificial aging treatment will provide better recovery of properties afterwelding.

When the welding operations involve preheating and/or post-weld heat treatment, it is important that the weldinginspector understand these requirements in order to ensure that they are being conducted correctly and in terms ofthe relevant welding procedure specifications and/or code requirements.

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