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Preheating is often considered quite ordinary in itself. It refers to heating the workpieces to be welded above room temperature before or during welding. Current standards generally require several preheating temperature grades based on the application standards of the material. The necessity, benefits of preheating, as well as the consequences of improper preheating, will all be illustrated with examples. 01 Process Preheating refers to heating the workpieces to be welded above room temperature before or during welding. Preheating is required by both the pre-welding and post-welding specifications. However, under certain conditions, other methods of preheating can also be used. Whether preheating is required or not, it offers the following advantages: • It reduces the shrinkage stress at the weld joint and the adjacent base material, which is particularly useful for welds with high stress levels. •It slows down the cooling rate in the critical temperature ranges during weld cooling, preventing excessive hardening and reducing the ductility of the weld and the heat-affected zone (HAZ). •In the 400°F temperature range, slowing down the cooling rate gives hydrogen more time to escape from the weld and the adjacent base metal, thereby preventing hydrogen-induced cracks. •Remove pollutants. •The amount of preheating is not determined by specified minimum standards, but rather by one or more of the following methods: • Calculation tables • Carbon equivalent estimation • Crack parameter estimation • Spark test estimation • Rules of thumb. The preheating temperature range usually corresponds to various weld groove sizes and constraint conditions. Although minimum preheating temperatures are specified in many standards, lower preheating temperatures are still used in some cases, while higher preheating temperatures are used in others. 02 Calculation Tables Various “preheating calculation tables” have been available over the years. Many “preheating calculation tables” take the form of linear or circular calculators, which predict the preheating temperature by identifying the material of the base metal and its thickness. 03 Carbon Equivalent The Carbon Equivalent (CE) is a way to determine whether preheating is necessary and to what extent. If CE ≤ 0.45%, preheating can be chosen at discretion. If 0.45 ≤ CE ≤ 0.60%, the preheating temperature range should be 400° to 700°F (200°–350°C). When CE > 0.5, it is necessary to delay the final non-destructive evaluation (NDE) by at least 24 hours, in order to determine whether any delayed cracking occurs. 04 Crack parameters: When the carbon equivalent is equal to or less than 0.17 wt-%, or when high-strength steel is used, the Ito & Bessyo parameter crack detection (Pcm) can be employed. This method can accurately predict when to perform preheating, when to carry out forced preheating, and to what temperature the preheating should reach. Specifically, if Pcm ≤ 0.15%, preheating can be chosen arbitrarily; if it is between 0.15% and 0.26–0.28%, preheating should be done to 400°–700°F (200°–350°C). 05 Spark testing: Spark testing has been in use for decades; it is a method for estimating the carbon content in carbon steel. The higher the carbon content, the better the sparks, and the more preheating is required. This method is not very accurate, but it is simple. It is possible to determine the relative level of the preheating temperature. 06 Rule of thumb: Another method for selecting the preheating temperature, though less precise, is to calculate it by increasing the preheating temperature by 100°F (50°C) for every 0.10 wt-% increase in carbon content per 10 points. For example, if the carbon content is 0.25 wt-%, then the preheating temperature is 250°F (125°C), or preheating should start at least at 250°F (125°C). If there is a coating or other components near the weld, then the preheating temperature specified in the original production specifications is not appropriate. However, if the welding heat input is near the maximum limit allowed by the standard process, the heat transferred to the welded joint may be balanced by the welding heat input, causing the affected metal to be heated to or above the minimum value required for preheating; therefore, a more relaxed preheating can be achieved using external methods. It should be noted that range and imprecise conversions (°F to °C) are used here. It was done on purpose. Preheating is not a highly precise science. In many cases, it is also normal to continuously increase the preheating temperature until the problem is resolved (such as the cracks disappearing). On the contrary, in certain specific situations, it is possible to achieve the desired result even by using a preheating temperature that is lower than the recommended value or the value specified by the standards. 07 Practical Application: Special attention must also be paid to the techniques used in actual operations, in order to avoid the problem of material softening caused by preheating. Choose welding processes and electrodes that introduce very little hydrogen. Certain techniques can reduce or decrease residual stress. Monitor carefully to ensure that the preheating method is used correctly. Some of the following descriptions are very important for the successful application of these techniques. 08 Welding groove dimensions and techniques: The techniques used during welding have a significant impact on factors such as the shrinkage of the workpiece after welding, residual stress levels, heat input control, and the prevention of crack formation. Short welds exhibit less longitudinal contraction compared to long welds. Backhand welding or a special welding sequence can also be used to reduce residual stress. Control or reduce heat input. Linear welds with small oscillations can be used instead of welds with large swings. 09 Reducing cracks: Arc pits and weld cracks can be reduced or eliminated by using appropriate manufacturing processes. 1) Welds with a circular cross-section produce the fewest cracks during welding compared to those with a thin, wide cross-section. 2) Avoid sudden start or stop of welding. Welding operations and weld shape are controlled using up/down bevel welding techniques or electrical methods via the welding power supply. 3) There must be enough filler material to prevent cracks caused by welding shrinkage or normal welding effects. A practical approach to avoiding cracks caused by insufficient weld deposit material – a requirement specified in many manufacturing standards – is to ensure that the amount of deposited metal is at least 3⁄8 inch (10 mm), or 25% of the thickness of the weld groove. 10 Preheating methods: In workshops or in the field, preheating can be achieved using methods such as flame heating (air-fuel or acetylene fuel), resistance heating, and electron induction heating. Regardless of the method used, preheating must be uniform; unless there are special requirements, the preheating should penetrate the entire thickness of the weldment. Figure 1 shows the equipment using resistance (without insulation, to be used later) and induction heating. 11 Preheating monitoring: Many devices can be used to measure and monitor temperature. The components or weldments to be welded should first be preheated to a temperature at which heat can fully penetrate the material. If possible, the degree of hot penetration should be measured or evaluated. Generally, for most welding applications, it is sufficient to monitor the temperature at a certain distance from the weld edge. It is essential to prevent operations such as monitoring or reading temperature values from contaminating the welding groove. 12 Temperature indicator pens: These pens, or pencils-like tools, melt at a certain temperature value; they can be used to determine in a simple and cost-effective manner the minimum temperature reached during preheating – that is, the temperature at which the indicator pen melts. The downside is that it will not work if the temperature of the workpiece is higher than the melting point of the indicator pen. When the workpiece temperature is too high, more indicator pens with different melting points are required. 13 Electron temperature monitoring: For preheating and welding operations, direct measurement devices such as contact thermometers or directly reading thermocouples (with analog or digital readings) can also be used. All measuring devices must be calibrated, or their ability to measure temperature ranges must be verified in some way. Since thermocouples can continuously monitor and store data, they can be used with curve recorders or data acquisition systems for preheating or PWHT operations. AWS D10.10 provides a variety of solutions and examples for thermocouple placement. 14 “Homegrown” methods of monitoring: Many “homegrown” methods for determining whether the preheating temperature is sufficient have also been in use for decades. Of course, one method is to directly spray water or smoke liquid onto the workpiece. The volume of the \"sound\" produced when saliva is sprayed onto it serves as the temperature indicator. Although not very accurate, many ‘veterans’ use it. Another more accurate method for determining the preheating temperature is to use an acetylene torch. The flame is adjusted to achieve high carbonization, forming a layer of smoke-gray in the areas that require preheating. Then, adjust the welding torch to medium heat and heat the smoke-gray area. When the smoke gray color disappears, the surface temperature can reach over 400°F (200°C). It is necessary to ensure that the preheating temperature is reached across the entire thickness of the workpiece and the welded area. Most inspections focus only on the outer surface of the workpiece. The recommended practice for AWS D10.10 is to provide useful guidance on the heat distribution area, requiring that during pipe-to-pipe welding, the entire thickness of the workpiece be heated. Careful observation is necessary during preheating to avoid overheating of the base material, especially when using resistance heating or induction heating methods. Now many cargo owners require thermocouples to be placed under each resistance heating plate or induction coil assembly in order to monitor and prevent overheating. 15 Summary: Whether preheating is required or not, and regardless of the method used for preheating, it brings the following benefits: it reduces the shrinkage stresses in the weld and the adjacent base material, which is particularly advantageous for highly constrained welded joints; it slows down the cooling rate of the workpiece in critical temperature ranges, preventing excessive hardening of the workpiece and reducing softening in the weld and HAZ; it slows down the cooling rate as the workpiece passes through the 400°F (200°C) temperature range, giving hydrogen more time to diffuse out of the weld and adjacent base material and thus preventing hydrogen-induced cracks; it helps remove contaminants. It is best to ensure that the entire thickness of the workpiece is heated evenly at the specified preheating temperature during preheating. Excessive local heating may cause material damage, so try to avoid it.