The bucket body material of WY25 excavator is Q345, and its weldability is good. The bucket tooth material is ZGMn13 (high manganese steel), which is single-phase austenitic structure at high temperature and has good toughness. It has high wear resistance due to the surface hardening under impact load. However, the weldability of this steel is poor: first, carbide precipitated in the welding heat affected zone causes material embrittlement; The second is that the weld produces hot cracks, especially liquefaction cracks near the weld.
(1) Embrittlement caused by carbide precipitation in heat affected zone
When ZGMn13 high manganese steel is reheated at more than 250 ℃, carbides may precipitate along the grain boundary, which greatly reduces the toughness of the material and seriously damages the excellent properties of high manganese steel. According to the analysis, when the high manganese steel is reheated and cooled rapidly, the carbides first precipitate at the grain boundary. With the extension of the residence time, the grain boundary carbides will change from discontinuous particle state to network distribution, and their brittleness will increase significantly. Therefore, when the high manganese steel is reheated after welding repair or welding, carbides will be precipitated in different degrees in a section of the welding heat affected zone and may be transformed into martensite, which not only makes the material brittle, but also reduces its wear resistance and impact toughness. Moreover, the longer the residence time in the temperature range (about 650 ℃) where carbides are easy to precipitate in the heat-affected zone, the more carbides precipitate.
In order to reduce the precipitation of carbide and prevent the material from losing its toughness and becoming brittle, measures should be taken to accelerate its cooling rate, that is, to shorten the residence time at high temperature. For this reason, short section welding, intermittent welding and bubble welding shall be used during welding.
(2) Welding hot crack
The way to prevent hot cracks is to reduce the content of S and P in the base metal or welding material; Measures can also be taken from the welding process to reduce the welding stress as much as possible, such as short welding, intermittent welding, scattered welding and post-welding hammering. When overlaying high manganese steel on the bucket body, a layer of Cr-Ni, Cr-Ni-Mn or Cr-Mn austenitic steel can be welded first as an isolation weld bead to prevent cracks.
2. Welding process of bucket body and bucket tooth
(1) Preparation before welding
First cut off the worn bucket teeth from the bucket body, and then use an angle grinder to polish the installation bucket teeth clean, free of dirt and rust, and carefully check for cracks and other defects; Use carbon arc air gouging to make a groove at the position to be welded on the bucket tooth, and clean it with an angle grinder.
(2) Welding
① First, the bucket body (the joint with the bucket tooth) shall be overlayed with GBE309-15 welding rod. The welding rod shall be dried at 350 ℃ for 1.5 h before welding. The welding current shall be too high and the welding speed shall be slightly slow to ensure the nickel content in the fusion zone is 5%~6%, and to prevent the formation of martensite sensitive to cracks.
② Conduct tack welding. After the bucket teeth are assembled in place, use D266 welding rod with a diameter of 3.2mm to conduct symmetrical tack welding on both sides, and the weld length shall not exceed 30mm. Water cooling and hammering immediately after welding.
③ Backing welding. Use D266 electrode with a diameter of 3.2mm for backing welding. Use small current, DC reverse polarity, intermittent welding and short welding. Water cooling shall be carried out in time when welding is stopped each time, and the interlayer temperature shall be controlled below 50 ℃.
④ Perform filler welding. After the bottom welding is completed, the D266 electrode with a diameter of 4mm is used for intermittent welding. The welding current is 160A. One electrode is welded in 3 to 4 times. When the welding is stopped, the weld bead must still be cooled to below 50 ℃ with water in time and hammered to remove stress and prevent carbide precipitation; After each two layers are welded, the left and right welds shall be rotated to ensure the symmetry of welding until the weld leg reaches 16mm. Use a magnifying glass to check the cracks after each welding. If there are cracks, use carbon arc gouging * before welding.
After being repaired by this method, the bucket teeth have been used for more than one year, and the welds are still good. This method has high efficiency, low cost and good effect in repairing bucket teeth, and has certain practical value.