Preheating in steel fabrication involves warming the base metal to a specified minimum temperature before welding begins. This controlled application of heat is fundamental for ensuring the integrity and longevity of the finished weldment. By raising the temperature of the workpiece, fabricators manage the cooling rate of the weld area, which prevents the formation of brittle microstructures that could lead to cracking. The necessity for preheat is determined by the steel’s chemical composition, its thickness, and the degree of joint restraint. Achieving and maintaining this specific thermal condition guarantees the mechanical properties of the welded joint.
The General Temperature Range for Common Steels
The most common preheating temperature range for structural steels, particularly low-carbon and mild steels, is 93°C to 204°C (200°F to 400°F). For routine applications involving materials like ASTM A36 or A572 steel of moderate thickness, a minimum temperature of 50°C to 150°C (122°F to 302°F) is often sufficient. This range applies to materials with low carbon equivalents and thicknesses under about 25 millimeters. The purpose of this minimum heat is to counteract the rapid heat dissipation that occurs as the surrounding material acts as a heat sink.
This standard range is a minimum guideline, not a universal rule for all steels or thicknesses. Codes, such as the American Welding Society (AWS) D1.1, specify minimum preheat temperatures that increase with material thickness. While 93°C to 204°C is frequently cited for common structural work, the precise requirement must be confirmed by the specific material composition and component dimensions. Furthermore, the minimum preheat temperature must be maintained as the interpass temperature during multi-pass welding.
The Metallurgical Purpose of Preheating
Preheating serves a dual metallurgical function by controlling the cooling rate of the weld metal and the adjacent heat-affected zone (HAZ). Raising the base metal temperature significantly slows the cooling rate, which prevents the formation of brittle phases like martensite. Martensite is a hard microstructure that forms when certain steels cool too quickly, making the joint susceptible to cracking.
The second major purpose is to facilitate the diffusion of hydrogen out of the weld area, preventing Hydrogen-Induced Cracking (HIC). Hydrogen atoms, introduced from moisture or consumables, become trapped during rapid cooling. Preheating keeps the weld area elevated, often above 100°C, allowing hydrogen atoms time to escape before the structure solidifies. This extended diffusion time is crucial for preventing cold cracking. Preheating also minimizes shrinkage stresses and the risk of warping by reducing the thermal stress differential between the weld metal and the surrounding plate.
Key Factors Influencing Temperature Adjustments
The minimum preheat temperature is adjusted upward based on three main variables that increase the risk of cracking: chemical composition, material thickness, and joint restraint. Chemical composition is quantified by the Carbon Equivalent (CE) value, which summarizes the hardening potential of all alloying elements. Steels with a higher CE value have a greater tendency to form brittle martensite, necessitating a higher preheat temperature to slow the cooling process.
Material thickness is a major determinant because thicker sections act as more effective heat sinks, pulling heat away from the weld rapidly. Increasing plate thickness requires a higher preheat temperature to counteract this faster heat dissipation and maintain the desired slow cooling rate. Sections exceeding 40 millimeters often require significantly higher preheat, sometimes reaching 200°C or more, especially if they have a higher CE.
The third variable is the degree of joint restraint, which relates to how much the surrounding structure prevents the weld metal from shrinking freely as it cools. Highly restrained joints, such as those in heavy structural frames, accumulate greater residual stresses. To manage these elevated stresses and the associated risk of cracking, a higher preheat is specified to ensure the material retains maximum ductility during cooling.
Methods for Measuring and Maintaining Preheat
Accurately measuring the preheat temperature ensures the minimum required heat is achieved and maintained. Common monitoring tools include temperature-indicating crayons, which melt instantly when a specific temperature is reached. Digital pyrometers and thermocouples provide more precise, real-time readings, often used in complex or code-governed applications.
Heat can be applied using several methods, depending on the size and geometry of the workpiece. Portable flame torches suit spot heating small areas, while electrical resistance blankets or induction heating systems provide uniform heating for larger components. For multi-pass welds, the interpass temperature must be monitored and maintained above the specified preheat temperature before the next weld bead is laid.