In order to reduce the dead weight of the structure and improve the bearing capacity, the application of low-alloy high-strength steel in industrial and mining machinery has been paid more and more attention. In recent years, ultra-high strength steel with yield strength>800MPa has been widely used in domestic construction machinery to meet the demand of large-scale, lightweight and high-efficient development of construction machinery. Due to the complex alloy system and high hardenability of ultra-high strength steel, cold cracks are easy to occur during welding; In addition, because of the high strength grade of ultra-high strength steel, it is easy to cause embrittlement of the welded joint including the welding heat affected zone during the welding process. Therefore, it is the key of welding technology for this series of steel to prevent the generation of welding cold cracks and ensure that the welded joints have excellent mechanical properties.

Selection and matching of welding materials

As the strength of ultra-high strength steel increases, the plasticity and toughness of steel decrease correspondingly. If the equal-strength principle is still adopted and the welded joint with high assembly is selected, the toughness of the weld is not easily guaranteed, which may lead to low stress brittle failure due to insufficient toughness of the weld metal. Therefore, the principle of equal toughness should be adopted for the welding of high-strength steel, and it is reasonable to select the low-assembly welded joint with weld toughness not lower than the base metal. The use of low strength weld metal does not always mean that the strength of the welded joint must be lower than the base metal. According to the mechanical property test experience of welded joints for many years, as long as the strength of the weld metal is not less than 87% of the base metal, the strength of the joint and the base metal can still be guaranteed.

When welding thicker ultra-high strength steel plate, welding materials of different strength levels shall be matched at different parts of the weld. That is, the root pass is made of low strength welding material, and the filler and cover pass are made of high strength welding material; For fillet welding, low strength welding materials are usually used. The advantages of selecting low strength welding materials over high strength welding materials are that the plastic toughness reserve of weld metal is large, and the ductility of welded joints is good, which reduces the possibility of cracks in joints.

Ultra-low hydrogen welding materials shall be selected for welding ultra-high strength steel. The hydrogen content of the deposited metal shall not exceed 5 ml/100 g (mercury method) to minimize the hydrogen content brought into the welding joint by the welding materials during the welding process. At the same time, in order to avoid moisture absorption, welding materials shall be stored according to regulations and re-baked as required before use.

Determination of preheating temperature

In the actual welding process, special attention should be paid to the preheating of the butt weld and root pass of ultra-high strength steel. The thicker the steel plate, the greater the need for preheating. The preheating temperature is related to the equivalent plate thickness of the steel plate. In addition, the preheating temperature should be adjusted according to the actual situation:

(1) If the ambient humidity is high or the temperature is lower than 5 ℃, the preheating temperature should be increased by 25 ℃; If the workpiece is rigidly fixed, the preheating temperature should also increase accordingly;

(2) When the equivalent plate thickness is less than the limit value, the workpiece temperature is lower than 5 ℃ or the air humidity is greater than 65%, the workpiece shall be preheated to 50~80 ℃.

Welding heat input control

The change of welding heat input will change the welding cooling rate, thus affecting the microstructure of weld metal and heat affected zone, and ultimately affecting the mechanical properties and crack resistance of welded joints. In order to avoid welding cold cracks and the reduction of toughness of weld heat affected zone during welding of ultra-high strength steel, it is necessary to strictly control the welding heat input and control the welding cooling rate to obtain the ideal microstructure of weld and weld heat affected zone. The cooling time t8/5 is an important parameter that determines the properties of post-weld super strength steel and welded joints. The cooling time mainly depends on the welding heat input, preheating or interpass temperature. In order to ensure that the performance of super strength steel will not be weakened by excessive heat input during welding, the upper limit of cooling time must be set. On the other hand, if the cooling is too fast, it will cause brittle and hard structure in the heat affected zone, and prevent the removal of hydrogen, resulting in the generation of cold cracks. Therefore, the lower limit of cooling time must also be set.

According to the different characteristics of various super-strength steels, determine a reasonable cooling time, which can effectively prevent the appearance of cold cracks while ensuring the performance of the welding heat affected zone, so as to ensure the quality of the welded joints.

Other process measures

(1) Ensure that the welding surface is clean and dry. The main reason for cold cracking is the presence of hydrogen in the weld metal with stress. Before assembly, the weldment shall be completely cleaned of oxide scale, rust, oil stain, moisture, etc. on the groove surface and the adjacent base metal until the metal luster is exposed and no cracks, interlayer and other defects are guaranteed within the cleaning range.

(2) Reduce the internal stress of the component. Adopt reasonable welding sequence for assembly and welding to avoid strong assembly to reduce residual stress of components. During welding and assembly, the workpiece shall be pressed or cushioned firmly to prevent additional stress and deformation caused by welding heat.

(3) Post-weld hydrogen removal treatment. After welding, immediately heat the weldment to 150~250 ℃, and conduct constant temperature treatment for at least 5 min per millimeter of plate thickness and slow cooling (and the total constant temperature time shall not be less than 1 h) to ensure the diffusion and escape of residual hydrogen energy in the welded joint and reduce the generation of delayed cold cracks.

(4) Post weld heat treatment. The purpose of post-weld heat treatment is to reduce welding residual stress. Generally, high-strength steel is not subject to post-weld heat treatment after welding. Heat treatment will reduce some mechanical properties of the joint, such as impact toughness. Post-weld heat treatment shall be carried out only when there are special instructions in the design rules. However, it should be noted that the post-weld heat treatment temperature should not exceed its tempering temperature.