TPT May 2022

AR T I C L E

Thermatool Corp

Power beam processes Power beam processes were relatively easy to relate to the traditional heat input calculations, as the beam power (P) had to be divided by line speed, resulting in similar energy per unit length units (kJ/in or kJ/mm) for heat input in laser beam welding (LBW) and electron beam welding (EBW). However, the efficiency factor of the process may vary with the power intensity of the beam, and joint configuration. In LBW, the efficiency factor is not included in any of the heat input calculations. Due to differences in weld shape, the effect of heat input on cooling rate is not the same for power beam processes as it is for arc welding processes [2] .

Process validation is performed using standard inspection methods including both non-destructive (NDT) and destructive testing. Once the process h s be n validated, a high level of statistical reproducibility is achieved by maintaining precise weld heat input and process control during every run. The new version of HCT software gives the operator the ability to create upper and lower control limits (UCL and LCL) according to validated process control limits. The result – Six Sigma weld quality levels are now possible based on real world experience and data. How did we get here? By precisely controlling HF weld heat input. So what is exactly weld heat input? Simply put, heat input is the amount of heat generated by the welding arc per unit length of the weld. Welding engineers soon realised that heat input could be used to characterise the strength and performance of the weld HAZ, as well as other characteristics such as width and height of the weld bead, hardness, distortion, toughness, residual stresses and so on. Today, weld heat input is used as the main control parameter in industries where weld quality is critical. There are several ways of calculating weld heat input. In this section we will discuss two of the most common methods of calculating the heat input for fusion welding. The American system (given in ASME BPVC Section IX – QW 409.1 (a) and various AWS standards provides this equation for calculating heat input:

Shandong Province Sifang Technical Development Group

Friction stir welding Per recent study [3] , heat input in friction stir welding (FSW) can be calculated using a similar approach as fusion welding methods. The assumption is that the source of heat generated during FSW is mainly from the friction between the tool and the stirred material and therefore, the heat input during FSW can be calculated using the following equation:

Where heat input is expressed in J/in or J/mm, voltage is expressed in volts, travel speed is expressed in in/min or mm/min. The unit for heat input obtained by this formula shall be either in J/In or J/mm. An additional parameter of thermal efficiency (also referred to as process efficiency or arc efficiency) is used while calculating heat input as per European standards (EN ISO 1011-1 and PD ISO/TR 18491).

Where T is the torque (N·m), ω is the rotational speed (rpm), v is the linear speed (mm/min) and η is the efficiency of heat transfer, (η = 0.9). Why heat input is so impor tant in welding? Heat input directly affects the resulting microstructure of the heat affected zone (HAZ), which in return affects mechanical properties of the welded joint, especially toughness. Weld toughness is directly related to cold weather service performance and Charpy impact results. With high weld heat inputs, slower cooling rates are typically experienced resulting in excessive grain growth. Excessive grain growth in the weld HAZ results in changes of mechanical properties, mainly a decrease in the material’s cold weather toughness (Charpy impact). Therefore, it is very important to precisely control weld heat input in order to achieve an optimal microstructure and good weld quality. Precise heat input during welding also avoids weld flaws and costly scrap.

Each arc welding process has a different efficiency rating. To simplify the rating system, all efficiency factors relate to the effi ciency of submerged arc welding and can be found in table 1.

Efficiency factor

Welding Process

Submerged arc welding (SAW)

1.0 0.8 0.8 0.8

Manual metal arc (MMA) / Shielded metal arc welding (SMAW) Cored wire welding / Flux cored arc welding (FCAW)

Metal active gas / Metal inert gas (MAG/MIG) / Gas metal arc welding (GMAW)

Tungsten inert gas (TIG) / Gas tungsten arc welding (GTAW)

0.6

Plasma arc welding 0.6 Table 1:Thermal efficiency for different arc welding processes

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MAY 2022