With the development of automation technology, robot welding has gradually replaced manual welding for excavator bucket. After MAG welding of the bucket arm of a certain type of excavator, cracks appeared in the circumferential weld of the bucket arm II shaft seat. The bucket arm is one of the key structural parts of the excavator, which bears the alternating load during the operation. In this paper, we will analyze whether the chemical elements are related to the occurrence of the circumferential weld crack of the shaft seat of the arm II shaft.
The weld crack can be divided into hot crack and cold crack, and the hot crack can be divided into crystal crack, high temperature liquefaction crack and polygonal crack. The cold crack can be divided into delay crack, hardening embrittlement crack and low plasticity embrittlement crack. The position of the circumferential weld crack on the II shaft seat of the stick is at the joint between the shaft seat and the plate of the stick. The II shaft seat is made of 35 forged steel and the plate is made of Q345B steel. The welding robot cracks immediately after welding, and it is preliminarily determined that it is one of the hot cracks.
Crystal crack is a crack that occurs when the residual liquid metal cannot meet the solidification and shrinkage requirements of the weld during the crystallization of the liquid metal of the weld. Crystal cracks mainly occur in the welds of carbon steel, low-alloy steel and single-phase austenitic steel containing more impurities, and are generally located in the center of the weld and crack longitudinally along the center of the weld. The location and time of the circumferential weld crack on the shaft seat of the bucket stick II are consistent with that of the crystal crack, which can prove that it is a crystal crack.
Through analysis, it is considered that the factors causing crystal cracks mainly include the following two aspects. First, the metal elements and impurities in metal materials increase the brittleness temperature region and the brittleness sensitive region of materials. Second, the defects in the welding process increase the weld stress and reduce the fluidity and replenishment of liquid metal.
Since the chemical elements such as C, Mn, Si, s and P in the weld have great influence on the crystal crack, the chemical composition and properties of the II shaft seat and the bucket rod are analyzed. When the content of S and P in the weld is too much, it will cause component segregation, produce low melting point eutectic and increase the tendency of crystal crack. When the content of C is too much, the crystallization temperature range will be increased, and the brittleness temperature range will be increased. Mn has desulfurization effect. When the content of C is less than 0.16%, the tendency of crystal crack decreases with the increase of manganese sulfur ratio (Mn / S). When the content of C is more than 0.16%, the effect of P on the formation of crystal crack is more than that of S. increasing the content of Mn has no obvious effect on the formation of crystal crack.
When the Si content is small, it is beneficial to prevent the occurrence of crystal cracks. However, when the Si content is greater than 0.42%, silicate with low melting point will be formed in the weld, thus increasing the tendency of crystal crack. According to the analysis results, the content of S and P elements in these two steels is within the standard range, while the content of Si in Q345B material is too high, which may increase the tendency of crystal crack.
