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What The Welding Inspector Should Know About Preheating And Postweld Heat Treatment
When welding some base materials and for some service conditions, preheating and/or postweld heat treatment may 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 heat treatment is costly since it demands extra equipment, extra time, and extra handling. For these reasons, heat treatment should only be undertaken after careful consideration of the advantages it may offer. In certain cases heat treatment will be mandatory, as with heavy sections of low alloy steels, whereas in other cases, it will be a justifiable precaution against early failure in service.
There are a number of reasons for the incorporation of these thermal treatments within the welding procedure, and we will consider some of the most common.
Preheat, as defined within the AWS Standard Welding Terms and Definition, is “the heat applied to the base metal or substrate to attain and maintain preheat temperature”. The preheat temperature is defined by the same document as “the temperature of the base metal in the volume surrounding the point of welding immediately before welding is started. In a multipass weld, it is also the temperature immediately before the second and subsequent passes are started” (Interpass Temperature).
Preheating may be performed by the use of gas burners, oxy-gas flames, electric blankets, induction heating, or by heating 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 weld joint should be heated evenly through the material thickness to the desired minimum temperature. To obtain a uniform temperature through the material thickness, it is desirable to apply the heating sources to one side of the material surface and to measure the material temperature on the opposite side. Whenever the heating and temperature measurement must be conducted from the same surface, the inspector must assure that more than just the surface of the material has been heated. It is important to ensure that the entire material thickness has been heated to a uniform temperature. In addition to establishing a preheat temperature, an interpass temperature limitation may need to be considered for some applications. This information should be shown in the welding procedure specification. When an interpass temperature is specified, the weld area must be inspected prior to depositing the next weld bead. Welding may not continue if the measured temperature exceeds the maximum interpass conditions specified in the welding procedure. The weldment must be permitted to cool down to the specified 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 welded component, preheat and interpass temperature may be evaluated for different reasons. For instance, a procedure for welding mild steel, which has a low carbon content, relatively low hardenability, and is used in an application with no special service requirements, may consider a minimum preheat and interpass temperature based on the material thickness. Welding procedures used for the heat-treatable low alloy steels and chromium-molybdenum steels with impact requirements will normally specify a minimum and maximum requirement for preheat and interpass temperatures. These low alloy materials can have high hardenability and are susceptible to hydrogen cracking. Allowing these materials to cool too quickly or overheating them can seriously affect their performance requirements. When welding the nickel alloys, we are concerned primarily with high heat input during the welding operation. The heat input of the welding process, and the preheat and interpass temperature can seriously affect these materials. High heat input can result in excessive constitutional liquation, carbide precipitation, and other harmful metallurgical phenomena. These metallurgical changes may promote cracking or loss of corrosion resistance. 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 interpass temperature 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, controllable heating systems are used, and thermocouples are attached to monitor the part being heated. These thermocouples provide a signal to the controlling unit that can regulate the power source required for heating. By using 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 material to a relatively low temperature, just above the boiling point of water. This will dry the plate surface and remove the undesirable contaminants that may otherwise cause porosity, hydrogen embrittlement, or cracking through the introduction of hydrogen during the welding process.
b) To lower the thermal gradient: All arc welding processes use a high temperature heat source. A steep temperature differential occurs between the localized heat source and the cool base material being welded. This temperature difference causes differential thermal expansion and contraction and high stresses around the welded area. Reducing the temperature differential by preheating the base material will minimize problems associated with distortion and excessive residual stress. If preheating is not carried out, a large differential in temperature can occur between the weld area and the parent material. This can cause rapid cooling, leading to the formation of martensite 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 remove any internal or residual stresses that may be present from the welding operation. Stress relief after welding may be necessary in order to reduce the risk of brittle fracture, to avoid subsequent distortion on machining, or to eradicate the risk of stress corrosion.
b) For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure. This treatment is generally performed after the weld has cooled, but under certain circumstances, it may be 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, may require normalizing after welding. This treatment will refine the coarse grain structure, reduce stresses after welding, 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 to undergo post-weld heat treatment to regain their original properties. In some cases, only an aging treatment is used, although a full solution heat treat and artificial aging treatment will provide better recovery of properties after welding.
When the welding operations involve preheating and/or post-weld heat treatment, it is important that the welding inspector understand these requirements in order to ensure that they are being conducted correctly and in terms of the relevant welding procedure specifications and/or code requirements.