The Science Behind Temperature Changes in Welding Zone: An Analysis

1. Welding heat source

Heating is a crucial requirement for fusion welding. The welding thermal process encompasses the entire welding process and governs 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 the heat of welding heat source.

It includes heat for melting the 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 the 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.

be careful:

1. The investigation object is the temperature distribution in a certain range of space.
2. 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.

temperature gradient

Stable temperature field, unstable temperature field, quasi-stable temperature field

2) Factors affecting the welding temperature field

1. Properties of heat source
2. Welding parameters (welding line energy)
3. Thermophysical properties of the metal to be welded

• Thermal conductivity
• Specific heat capacity
• Thermal diffusivity
• Surface heat transfer coefficient
• Enthalpy

4. Geometric dimensions of the metal to be welded

3. Welding thermal cycle

1) Basic concepts of welding thermal cycle

Under the action of the heat source, the temperature of a certain point on the weldment changes with time, which is called the welding thermal cycle.

2) Basic parameters of welding thermal cycle

• Heating speed
• Maximum heating temperature
• Residence time above the phase transition temperature
• The cooling rate at a 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 the welding thermal cycle

• Welding line energy and preheating temperature
• Welding method
• Weldment size
• Joint form

② Method for adjusting welding thermal cycle

The appropriate welding method should be selected based on the composition and characteristics of the metal being welded.

It is important to make a reasonable selection of welding parameters.

Preheating, post-welding insulation, or slow cooling should be implemented to decrease the cooling rate.

When performing multilayer welding, it is necessary to regulate the number of layers or the length of the weld bead, and to control the interlayer temperature.

4. Summary

Welding Heat Source and Its Main Characteristics, and the Main Characteristics of an Ideal Heat Source

This section will discuss the heat source used in welding and its primary characteristics, as well as the main characteristics of an ideal heat source.

Effective Thermal Power of Arc and Melting Efficiency of Heat Source

The effective thermal power of the arc and the melting efficiency of the heat source will be discussed in this section.

Concept of Welding Temperature Field

This section will introduce the concept of the welding temperature field.

Heat Transfer Mode

The heat transfer mode used in welding will be discussed in this section.

Temperature Gradient

This section will discuss the temperature gradient in welding.

Characteristics of Actual Welding Temperature Field

The characteristics of the actual welding temperature field will be discussed in this section.

Stable, Quasi-Stable and Unstable Temperature Field

The stable, quasi-stable, and unstable temperature fields in welding will be discussed in this section.

Factors Affecting Welding Temperature Field

This section will discuss the factors that affect the welding temperature field.

Concept of Welding Thermal Cycle and Its Basic Parameters

The concept of the welding thermal cycle and its basic parameters will be introduced in this section.

Method of Adjusting Welding Thermal Cycle

This section will discuss the method of adjusting the welding thermal cycle.