Understanding Preheating and Post-Weld Heat Treatment for Welding Inspection

In the realm of welding, certain base materials and service conditions necessitate the application of preheating and/or post-weld heat treatment. These thermal procedures play a pivotal role in ensuring the integrity of welds and mitigating undesirable characteristics that may arise during the welding process. It is worth noting, however, that heat treatment carries a financial cost, demands additional equipment, consumes time, and requires meticulous handling. Consequently, the decision to implement heat treatment should be made judiciously, taking into account its inherent advantages. While heat treatment becomes obligatory in specific scenarios, such as welding heavy sections of low alloy steel, it serves as a prudent precaution to prevent premature failure in other cases.

Numerous factors underpin the incorporation of these thermal treatments within welding procedures, and we shall delve into some of the most prevalent considerations.

Preheating The American Welding Society's Standard Welding Terms and Definitions defines preheat as "the application of heat to the base metal or substrate to attain and sustain the preheat temperature." This standard also stipulates that preheat temperature refers to "the temperature of the base metal in the vicinity of the welding point immediately prior to commencement. In the context of a multipass weld, it is also the temperature immediately preceding the initiation of subsequent passes" (Interpass Temperature).

Preheating can be accomplished through various methods, including gas burners, oxy-gas flames, electric blankets, induction heating, or furnace heating. It is imperative that the heating process achieves uniformity across the joint area. Intense, non-uniform heating is counterproductive, potentially leading to heightened residual stresses, distortion, or undesirable metallurgical alterations in the base material. To ensure uniformity, the entirety of the weld joint should be uniformly heated through the material thickness to the prescribed minimum preheat temperature. Achieving uniform temperature distribution throughout the material thickness is facilitated by applying heating sources to one side of the material surface and measuring the temperature on the opposite side. When heating and temperature measurement must be conducted from the same surface, the inspector must ascertain that the entire material thickness attains uniform temperature.

Additionally, some applications may necessitate adherence to an interpass temperature limit, a detail specified in the welding procedure specification. In instances where an interpass temperature is mandated, the weld area must undergo inspection before proceeding to deposit the next weld bead. Welding cannot continue if the measured temperature surpasses the maximum interpass conditions outlined in the welding procedure. The weldment must cool down to the specified upper limit of the interpass temperature before welding can resume.

The selection of preheat and interpass temperatures is contingent upon the metallurgical properties of the material and the desired mechanical characteristics of the welded component. For example, when welding mild steel, characterized by low carbon content and modest hardenability, with no specific service requirements, the minimum preheat and interpass temperature may be determined based on material thickness. Conversely, welding procedures for heat-treatable low alloy steels and chromium-molybdenum steels, which have impact-related prerequisites, typically mandate a range for preheating and interpass temperatures, encompassing both minimum and maximum values. These low alloy materials exhibit high hardenability and susceptibility to hydrogen-induced cracking. Inadequate cooling or excessive heating can significantly compromise their performance requirements. Welding of nickel alloys places considerable emphasis on managing heat input during the welding process, as excessive heat input can result in detrimental metallurgical changes and potential issues such as cracking and corrosion resistance loss. Welding aluminum alloys of the heat-treatable 2xxx, 6xxx, and 7xxx series often necessitates minimizing overall heat input to mitigate adverse effects on the heat-affected zone (HAZ) and the associated tensile strength loss.

In applications of critical significance, precise control of preheat temperature is imperative. In such instances, controllable heating systems are employed, complemented by thermocouples for temperature monitoring. These thermocouples transmit signals to the control unit, enabling regulation of the requisite heating power source. This equipment allows for meticulous control of the heated component, maintaining exceedingly close tolerances.

Preheating serves several key purposes:

a) Moisture Evaporation: Preheating effectively removes moisture from the weld area by heating the material's surface to a temperature slightly above the boiling point of water. This moisture removal mitigates the introduction of hydrogen during welding, which can otherwise lead to porosity, hydrogen embrittlement, or cracking.

b) Thermal Gradient Reduction: Arc welding processes entail the use of high-temperature heat sources, resulting in a pronounced temperature differential between the localized heat source and the cooler base material. This disparity triggers differential thermal expansion, contraction, and heightened stresses around the welded region. Preheating reduces this temperature differential, minimizing distortion and excessive residual stress. Failure to preheat may cause a substantial temperature differential between the weld area and the parent material, potentially leading to rapid cooling, martensite formation, and potential cracking in materials with high hardenability.

Post-Weld Heat Treatment Several types of post-weld heat treatments are applied for various purposes:

a) Stress Relief: Post-weld heat treatment is primarily employed for stress relief, aiming to eliminate internal or residual stresses resulting from the welding process. Stress relief may be necessary to mitigate the risk of brittle fracture, prevent subsequent machining-related distortions, or eliminate the potential for stress corrosion.

b) Metallurgical Structure Modification: Some alloy steels may require thermal tempering treatment post-welding to achieve the desired metallurgical structure. Typically, this treatment occurs after the weld has cooled, although certain circumstances may dictate its execution prior to cooling to prevent cracking.

c) Coarse Weld Structure Refinement: Extremely coarse weld structures, as encountered in steel welding through electro-slag welding, may necessitate normalization after welding. This treatment refines the coarse grain structure, reduces post-weld stresses, and eliminates hard zones within the heat-affected zone.

d) Regaining Material Properties: Precipitation-hardening alloys, such as heat-treatable aluminum alloys, may require post-weld heat treatment to restore their original properties. Depending on the case, an aging treatment or a comprehensive solution of heat treatment and artificial aging treatment may be employed to restore properties after welding.

In scenarios where welding operations involve preheating and/or post-weld heat treatment, it is imperative that the welding inspector possesses a comprehensive understanding of these requirements. This knowledge ensures that the procedures are executed correctly, adhering to the relevant welding procedure specifications and code requirements.