The temperature cycles occurring during welding (temperature-time curve) have a decisive influence on the mechanical properties in the weld metal and in the heat-affected zone. The temperature cycles in turn depend on the welding conditions. Welding conditions are understood to be a multitude of influencing variables such as arc voltage, welding current, welding speed, working temperature, sheet thickness, welding process and seam shape [1]. The welding parameters arc voltage, welding current and welding speed can be summarized as line energy.
According to [2] the energy per unit length is calculated as:
Formula:
E = (U * I) / v
with
- U: arc voltage
- I: arc current
- v: travel speed
The energy per unit length is thus a measure of the energy supplied to the welding process.
However, it must be taken into account that not all of the electrical energy taken from the power source can be fed into the weld pool, but only a certain part, depending on the welding process and welding conditions. However, only the energy actually introduced into the weld seam area has an influence on the solidification process in the weld metal and the thermally induced structural changes in the heat-affected zone. For this reason, it is necessary to consider the energy losses in a differentiated way [3].
This can be done by extending the energy per unit length E by a factor eta, which is the ratio of the energy introduced into the weld area to the energy supplied to the welding process. The heat input Q thus defined is thus calculated as [2]:
Q = eta * E = eta * (U * I) / v
with
- Q: heat input
- E: energy
- eta: thermal efficiency
- U: arc voltage
- I: arc current
- v: travel speed
Unless otherwise specified, the thermal efficiency of welding processes (eta) shall be based on the values given in the following table [5].
| Thermal efficiency of welding processes | |
| Process | Factor eta |
| Submerged arc welding | 1,0 |
| Manual metal arc weldingwith stick electrode | 0,8 |
| Gas metal arc welding (MAG) | 0,8 |
| Inert gas metal arc welding (MIG) | 0,8 |
| Gas tungsten arc welding (WIG) | 0,6 |
Unless otherwise specified, the thermal efficiency of welding processes (eta) shall be based on the values given in the table above [2].
However, in the case of a planned welding task which is specified by welding process, sheet thickness and seam shape, it is often necessary to obtain a very specific microstructure in the heat-affected zone which is determined by a specified cooling time t8/5. By transforming the general formulas for calculating the cooling time t8/5, the maximum permissible heat input and, from this, the maximum energy per unit length can be calculated [2]. This makes it possible to determine suitable welding parameter combinations (arc voltage, welding current, welding speed) for the planned welding task.
When calculating the energy per unit length for a given welding task, however, a distinction must be made between three- and two-dimensional heat dissipation. When welding relatively thick workpieces, the heat dissipation is three-dimensional. The heat introduced via the arc can be dissipated in the workpiece plane and additionally in the direction of the workpiece thickness. This therefore has no effect on the cooling time. With two-dimensional heat dissipation, however, the heat flow occurs exclusively in the plane of the workpiece. In this case, the workpiece thickness is decisive for the cross-sectional area available for heat dissipation and thus has a pronounced influence on the maximum permissible energy per unit length [4].
When welding relatively thick plates (three-dimensional heat dissipation), the energy per unit length is calculated according to the following equation:
Formula (three-dimensional heat dissipation):
E = t8/5 / [(6700 - 5 * T0) * eta * ((1 / (500 - T0)) - (1 / (800 - T0))) * F3]
with
- t8/5: cooling rate t8/5
- T0: Preheat temperature
- eta: Thermal efficiency
- F3: Seam factor for three-dimensional heat dissipation
Two-dimensional heat dissipation is present when welding products of relatively small thickness. The energy per unit length is calculated according to the following equation:
Formula (two-dimensional heat dissipation):
E = (t8/5 * d2 / [(4300 - 4.3 * T0) * 105 * eta2 * ((1 / (500 - T0))2 - (1 / (800 - T0))2) * F2])0,5
with
- t8/5: Cooling time t8/5
- d: Plate thickness
- T0: Preheat temperature
- eta: Thermal efficiency
- F2: Seam factor for two-dimensional heat dissipation
The number of conceivable seam types is so large that a quantitative clarification of the influence of all of them on the maximum energy would require an extremely high effort. For this reason, the table below summarizes only the seam factors for the most common seam types with three-dimensional heat dissipation (F3) and two-dimensional heat dissipation (F2) [5].
| Seam factors | ||
| Seam type | F3 | F2 |
| Deposit weld | 1,0 | 1,0 |
| 1. and 2. fillet weld on T- or cross joint | 0,67 | 0,45 to 0,67 |
| 3. and 4. fillet weld on T- or cross joint | 0,67 | 0,3 to 0,67 |
| Fillet weld at corner joint | 0,67 | 0,9 |
| Fillet weld at lap joint | 0,67 | 0,7 |
| Root position of V-seams (opening angle 60°, weld face3 mm) | 1,0 to 1,2 | rd. 1,0 |
| Root position of double V seams (opening angle 50°, weld face 3 mm) | 0,7 | rd. 1,0 |
| Center layers of V and double V seams | 0,8 to 1,0 | rd. 1,0 |
| Cover layers of V and double V seams | 0,9 to 1,0 | 1,0 |
| I-seam, 'position-counter position welding' | - | 1,0 |
If the respective workpiece thickness is close to the transition sheet thickness (see below), the value of the seam factor F2 corresponds to that of F3. The smaller the workpiece thickness compared to the transition sheet thickness, the more clearly F2 and F3 differ [4].
The sheet thickness at the transition from three- to two-dimensional heat dissipation is called transition sheet thickness dü. It is obtained by equating the formulas for calculating the cooling time t8/5 for three- and two-dimensional heat dissipation:
dü = [((4300 - 4,3 * T0) / (6700 - 5 * T0)) * 105 * Q * (( 1 / (500 - T0)) + (1 / (800 - T0)))]0,5
with
- Q: heat input
- T0: preheat temperature
Literature:
[1] Degenkolbe, J., Uwer, D., und Wegmann, H. G.:
Kennzeichnung von Schweißtemperaturzyklen hinsichtlich ihrer Auswirkung auf die mechanischen Eigenschaften von Schweißverbindungen durch die Abkühlzeit t8/5 und deren Ermittlung. Thyssen Technische Berichte, Heft 1/85, S. 57 - 73
[2] Stahl-Eisen-Werkstoffblatt 088 Beiblatt 2:
Schweißgeeignete Feinkornbaustähle - Richtlinien für die Verarbeitung, besonders für das Schmelzschweißen; Ermittlung der Abkühlzeit t8/5 zur Kennzeichnung von Schweißtemperaturzyklen. 4. Ausgabe, Oktober 1993, Verlag Stahleisen, Düsseldorf
[3] Uwer, D. und Wegmann, H. G.:
Temperaturzyklen beim Lichtbogenschweißen - Einfluß von Schweißverfahren und Nahtart auf die Abkühlzeit. Schweißen und Schneiden, Jahrgang 28 (1976), Heft 4, S. 132 - 136
[4] Uwer, D.:
Rechnerisches und grafisches Ermitteln von Abkühlzeiten beim Lichtbogenschweißen. Schweißen und Schneiden, Jahrgang 30 (1978), Heft 7, S. 243 - 248
[5] Uwer, D. und Degenkolbe, J.: Kennzeichnung von Schweißtemperaturzyklen hinsichtlich ihrer Auswirkung auf die mechanischen Eigenschaften von Schweißverbindungen. Stahl und Eisen 97 (1977), Nr. 24, S. 1201 - 1207