welding technology-Preheating and interlayer temperature control-GB50661

2026-04-10

1. Principles for Determining Minimum Preheating Temperature

The preheating temperature and the temperature difference between layers should be determined based on factors such as the chemical composition of the steel, the restraint state of the joint, the size of the heat input, the hydrogen content level of the deposited metal, and the welding method used. Alternatively, welding tests should be conducted to determine the minimum preheating temperature for actual engineering structure welding.

When using medium heat input for welding common structural steel, the minimum preheating temperature should comply with the provisions in Table 2.

Table 2 Minimum Preheating Temperature Requirements for Common Structural Steels.

常用钢材牌号	接头最厚部件的板厚t（mm） t＜20	20≤t≤40	40＜t≤60	60＜t≤80	t＞80
Q235、Q295	/	/	40	50	80
Q345	/	40	60	80	100
Q390、Q420	20	60	80	100	120
Q460	20	80	100	120	150

Notice:

"/" indicates that no preheating is required.
When using non-low-hydrogen welding materials or welding methods, the preheating temperature should be increased by 20℃ compared to the temperature specified in this table.
When the temperature at the welding site of the base material is lower than 0℃, the preheating temperature of the base material in the table should be increased by 20℃, and this minimum inter-pass temperature should be maintained throughout the welding process.
Medium heat input refers to a welding heat input of approximately 15 to 25 kJ/cm. For every 5 kJ/cm increase in heat input, the preheating temperature can be reduced by 20 degrees Celsius.
When the thickness of the welded joint varies, the minimum preheating temperature and inter-pass temperature should be selected based on the thicker plate thickness of the joint.
When the materials of the welded joints are different, the lowest preheating temperature should be selected based on the steel with the higher strength and higher carbon content in the joint.
The values in this table are not applicable to steel with a supply condition of quenching and tempering treatment; the minimum preheating temperature that can be reduced for controlled rolling and controlled cooling (hot mechanical rolling) steel is determined by experiments.

2.Preheating Requirements for Special Welding Processes

When performing electroslag welding and gas tungsten arc vertical welding under an ambient temperature above 0℃, no preheating is required. However, when the plate thickness is greater than 60mm, it is advisable to preheat the base metal in the arc initiation area and the temperature should not be lower than 50℃.

3.Interpass Temperature Requirements

During the welding process, the minimum inter-pass temperature should not be lower than the preheating temperature; for static load structures, the maximum inter-pass temperature should not exceed 250℃; for cyclic load structures and quenched and tempered steels, the maximum inter-pass temperature should not exceed 230℃.

4.Requirements for Control of Preheating and Interpass Temperature

Preheating and inter-pass temperature control should comply with the following regulations:

Preheating and maintaining the inter-pass temperature should be carried out using heating methods such as electric heating, flame heating, and infrared heating, and should be measured using dedicated temperature measurement instruments;

The heating area for preheating should be on both sides of the weld bevel, with a width of more than 1.5 times the plate thickness at the welding site of the weldment, and not less than 100mm; The preheating temperature should be measured on the back side of the heated surface of the weldment, and the measurement points should be at least 75mm away from the welding point before the arc passes; When using a flame heater for preheating, the forward temperature measurement should be conducted after the heating stops.

5.Requirements for Special Steels

For steel types Ⅲ and Ⅳ as well as quenched and tempered steel, the preheating temperature and inter-pass temperature should comply with the guiding parameters provided by the steel mill.

6.Explanation

The principle of the minimum preheating temperature and inter-pass temperature requirements from 1 to 7 is to control the cooling rate of the weld metal and the adjacent base metal. Higher temperatures can facilitate the diffusion of hydrogen and reduce the tendency for cold cracking. Under given conditions, the cooling rate of the unwelded welding joint without preheating will be higher than that of the preheated welding joint. The higher the preheating temperature, the lower the cooling rate. When the cooling rate is sufficiently slow, it will effectively reduce hardening and crack tendency.

For quenched and tempered steel, a slower cooling rate is not desired. And steel mills do not recommend such a practice either.

Although the minimum preheating temperature and inter-pass temperature are specified in the code, these values are only based on the actual chemical composition levels of common steels, moderate structural restraint, commonly used low-hydrogen welding methods and welding materials, and moderate welding line energy, etc., to provide the lowest temperature that can avoid the formation of hardening or cracks in the welding joint. Practice and experiments have also proved that choosing the minimum preheating temperature and inter-pass temperature specified in the code can prevent cracks from occurring in joints with general restraint during welding. However, during actual welding, to produce weld joints with no cracks and good plasticity, the preheating and inter-pass temperatures should be higher than the minimum values specified in the code. At the same time, an upper limit for the inter-pass temperature is specified to avoid overheating of the base material and cause the joint to become brittle and reduce the performance of the joint.

During actual welding, the minimum preheating temperature for the joint should be determined based on factors such as the chemical composition of the steel, the strength grade of the base material, the carbon equivalent level of the base material and welding material, the restraint state of the joint, the size of the welding line energy, the hydrogen content level of the weld metal, and the welding method used. If there is sufficient experimental evidence to prove that the selected preheating temperature and inter-pass temperature are sufficient to prevent cracks from occurring during joint welding, a preheating and inter-pass temperature lower than the minimum values specified in Table GB50661-7.6.2 can be selected.

The code also stipulates the heating range for preheating, to ensure that the preheating temperature of the joint is uniform, and during cooling, there is a smooth cooling gradient, avoiding a rapid cooling rate.

For electroslag welding, gas-electric vertical welding, and stud welding, the welding line energy is large, and the welding speed is generally slow. Generally, there are no requirements for welding preheating.