1. Welding heat source
Heating is a necessary condition for fusion welding.
The welding thermal process runs through the whole welding process and determines the welding stress, strain, metallurgical reaction, crystallization and phase transformation.
1) Common welding heat sources
① Arc heat
② Chemical heat
③ Resistance heat
④ Friction heat
⑤ Plasma arc
⑥ Electron beam
⑦ Laser beam
⑧ High frequency induction heat
2) Main characteristics of welding heat source
Characteristics of ideal heat source:
- Small heating area;
- High power density;
- High heating temperature
3) Thermal efficiency of the welding process
Definition of thermal efficiency: utilization rate of heat of welding heat source. It includes heat for melting base metal, welding material and conduction through base metal.
Effective thermal power of arc
Arc output power Po
Melting efficiency of heat source: the percentage of heat used to melt metal in the power of heat source P.
2. Welding temperature field
1) Concept of welding temperature field
Three ways of heat transfer
Main heat transfer modes from heat source to weldment: radiation and convection
Main internal heat transfer mode of weldment: conduction
Concept of field
Welding temperature field: the temperature distribution on a transient weldment.
- The investigation object is the temperature distribution in a certain range of space.
- The temperature field is an instantaneous temperature field.
Representation of temperature field: formula, table and image
Simple temperature field
Actual welding temperature field: the isotherm is dense in front and sparse in back, irregular ellipse.
Stable temperature field, unstable temperature field, quasi stable temperature field
2) Factors affecting welding temperature field
- Properties of heat source
- Welding parameters (welding line energy)
- Thermophysical properties of the metal to be welded
- Thermal conductivity
- Specific heat capacity
- Thermal diffusivity
- Surface heat transfer coefficient
- Geometric dimensions of the metal to be welded
3. Welding thermal cycle
1) Basic concepts of welding thermal cycle
Under the action of heat source, the temperature of a certain point on the weldment changes with time, which is called welding thermal cycle.
2) Basic parameters of welding thermal cycle
- Heating speed
- Maximum heating temperature
- Residence time above phase transition temperature
- Cooling rate at specified temperature
3) Welding thermal cycle of multi-layer welding
- Welding thermal cycle of long multi-layer welding
- Short section multilayer welding
4) Basic factors affecting welding thermal cycle and adjustment methods
① Factors affecting welding thermal cycle
- Welding line energy and preheating temperature
- Welding method
- Weldment size
- Joint form
② Method for adjusting welding thermal cycle
- The applicable welding method shall be selected according to the composition and properties of the welded metal.
- Reasonable selection of welding parameters
- Preheating, post welding insulation or slow cooling shall be adopted to reduce the cooling rate.
- Adjust the number of layers or weld bead length of multilayer welding and control the interlayer temperature
- Welding heat source and its main characteristics, and the main characteristics of ideal heat source;
- Effective thermal power of arc and melting efficiency of heat source;
- Concept of welding temperature field;
- Heat transfer mode;
- Temperature gradient;
- Characteristics of actual welding temperature field;
- Stable, quasi stable and unstable temperature field;
- Factors affecting welding temperature field;
- Concept of welding thermal cycle and its basic parameters;
- Method of adjusting welding thermal cycle.