In order to reduce the weight of the structure and improve the bearing capacity, the application of low alloy high strength steel in industrial and mining machinery is 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 needs of large-scale, lightweight and high-efficiency 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, the high strength level of ultra-high strength steel is easy to cause embrittlement of welded joints including welding heat affected zone during welding. Therefore, it is the key to prevent the welding cold crack and ensure the excellent mechanical properties of the welded joint.

Selection and matching of welding materials

Ultra-high strength steel due to increased strength, steel plasticity, toughness decreased accordingly. If the principle of equal strength is still adopted and the welding joint with high assembly is selected, the toughness of the weld is not easy to guarantee, which may lead to low stress brittle failure caused by insufficient toughness of the weld metal. Therefore, the principle of equal toughness should be adopted for high-strength steel welding, and it is reasonable to select low-assembly welded joints with weld toughness not lower than that of 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 many years of experience in the mechanical properties of welded joints, 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 plates, different parts of the weld should be matched with welding materials of different strength levels. That is, the root weld bead is primed with low-strength welding material, and the filling and cover weld bead is filled with high-strength welding material; for fillet welding, low-strength welding material is usually used. The advantages of selecting low-strength welding materials than selecting high-strength welding materials are that the weld metal has a large reserve of plasticity and toughness, and the weld joint has good extension performance, which reduces the possibility of cracks in the joint.

Ultra-high strength steel welding should use ultra-low hydrogen welding materials, the hydrogen content of the deposited metal should not exceed 5 ml/100g (mercury method), in order to minimize the welding process from the welding materials into the welding joint hydrogen content. At the same time, in order to avoid moisture absorption, welding materials should 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 bead of ultra-high strength steel. The thicker the steel sheet, the greater the need for preheating. The preheating temperature is related to the equivalent 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 be increased 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 should be preheated to 50~80 ℃.

Welding heat input control

The change of welding heat input will change the welding cooling rate, which will affect the microstructure of weld metal and heat affected zone, and finally affect the mechanical properties and crack resistance of welded joints. In order to avoid welding cold cracks and the reduction of the toughness of the 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 weld and welding heat-affected zone metallographic organization. The cooling time t8 /5 is an important parameter that determines the performance of the ultra-strong steel after welding and the performance of the welded joint. The cooling time mainly depends on the welding heat input, preheating or interlayer temperature. In order to ensure that the performance of the super-strong steel will not be weakened due to excessive heat input during the welding process, an upper limit must be set for the cooling time. On the other hand, if the cooling is too fast, it will cause brittle and hard structures in the heat-affected zone and prevent the elimination of hydrogen, resulting in the generation of cold cracks, so the lower limit of the cooling time must also be set.

According to the different characteristics of various super steel, determine the reasonable cooling time, so as to ensure the performance of welding heat affected zone at the same time can effectively prevent the occurrence of cold cracks, so as to ensure the quality of welded joints.

Other process measures

(1) Ensure that the welding surface is clean and dry. The main cause of cold cracking is the presence of hydrogen in the weld metal where there is stress. Before assembling the weldment, the scale, rust, oil stain, moisture, etc. on the groove surface and nearby base metal shall be thoroughly removed until the metallic luster is exposed and the cleaning range shall be free of defects such as cracks and interlayers.

(2) Reduce the internal stress of the component. Adopt reasonable welding sequence for assembly and welding, avoid strong assembly to reduce the residual stress of components, and press or pad the workpiece firmly during welding assembly to prevent additional stress and deformation due to welding heat.

(3) hydrogen elimination treatment after welding. After the welding is completed, immediately heat the weldment to 150~250 ℃, and carry out constant temperature treatment for not less than 5 min per mm of plate thickness before slow cooling (and the total constant temperature time shall not be less than 1 h), so as 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. Post-weld heat treatment is carried out in order to reduce welding residual stress, high-strength steel after welding generally do not carry out post-weld heat treatment, heat treatment will reduce some of the mechanical properties of the joint, such as: impact toughness. Post-weld heat treatment shall be carried out only if there are special instructions in the design rules. However, it should be noted that the post-weld heat treatment temperature cannot exceed its quenching and tempering temperature.