In 2000, Ethylene Industry Corporation and our company ordered a large main reducer for the ZZM840 mixer. The total weight of the main reducer is 1.8 tons; the length of the reducer is 2360mm long, 2280mm wide, 1480mm high and weighs 1.4 tons. The material is HT250 gray cast iron. When the large main reducer is installed at the site, due to unstable lifting, the corner of the box base is broken at the corner when the reducer is dropped. The broken part passes through the anchor bolt hole, and the speed reducer cannot be installed or tested. See details below.
2 Weldability analysis 2.1 Base breaks off the corner Case base thickness 70mm, the corner dropped from the box is 200mm long, 150mm high, 250mm bevel edge, about 45°, with Y50 anchor bolt hole in the middle, see.
2.2 Fracture microstructure analysis of gray cast iron. The fracture is dark gray, and the microstructure is mainly pearlite matrix. A large amount of flake graphite is basically distributed. Flake graphite is a free form of carbon, which is very loose and lubricated. The presence of flake graphite gives cast iron excellent properties such as wear resistance, shock absorption, and excellent machinability. However, graphite has low strength and plasticity is almost zero. Therefore, flake graphite is equivalent to small cracks in cast iron. The split matrix makes the strength of gray cast iron low, and the plasticity is almost zero.
2.3 Solderability of gray cast iron When repairing grey cast iron, the difficulty often encountered is the generation of white tissue and cracks.
When welding, because the heat transfer rate of cast iron is much faster than that of sand, the welding pool is very small, and the cooling rate during welding is many times faster than that during casting. The main reason for the white-colored structure is that the cooling rate is fast. Therefore, it is easy to repair the cast iron when welding. White mouth tissue. The production of white tissue will make the weld and fusion zone very hard and difficult to machine. For gray cast iron, preheating 600~700°C according to the different wall thickness can prevent white tissue. Because the box is too large, preheating welding is difficult, and only non-cast iron welding materials can be considered to solve the problem of white mouth organization.
Due to the large amount of graphite flakes distributed within the cast iron, it is as if the matrix of the steel is cracked by a large number of small cracks. Therefore, the cast iron has low strength and poor plasticity, and does not easily produce plastic deformation such as elongation and bending under the action of force like low carbon steel. Therefore, when the welding stress exceeds the strength of cast iron, the manufacturing process of the fan technology may suddenly break along the weak point of the weld repair zone, or it may crack along the fusion zone or the heat affected zone to cause the weld seam to peel off and form a thermal stress crack.
The angle at which the gearbox breaks is at the corner of the end of the box. The cast iron repair area is at the corner of the end. Thermal expansion is less hindered when heated and the resistance to cooling shrinkage is smaller, that is, it can It is more flexible. Therefore, the thermal stress during the welding process will not be so great that it will not generate thermal stress cracks.
Nickel-based cast iron 08 welding, welding is face-centered cubic lattice nickel-base alloy, no organizational transformation when cooling, will not be hardened due to rapid cooling, the weld is soft, you can eliminate the stress by hammering, crack resistance better. Nickel is a graphitized element, which helps to reduce the white area in the semi-melting zone. When the process parameters are correct, the semi-melting zone is thinner and sequenced, and the processing performance is better.
3 REPAIR PROCESS 3.1 Preparation before repair welding (1) Shave the sloping side of the broken edge. The form of the groove is as shown.
The falling corner of the digging groove burns the base and breaks it until it can remove oil and dirt. The firearm swings during grilling to prevent the deformation of the base caused by the grill, the heating temperature does not exceed the 400-A view, and the welding groove on the side of the base is sharpened. When the falling corner is combined with the two bases of the base, the X groove is formed. The gap is 0~3mm. Before the welding, use alcohol and acetone to clean the oil on both sides of the groove and 30~50mm from the groove surface.
3.2 Cold welding required current / = 150 ~ 170A; using a small current, the current will increase the depth of fusion and fusion zone white layer thickness.
~50mm, thoroughly cleaned after welding, tapping the weld with a hand hammer, 80 to 100 points per square centimeter.
Each bead and interlayer temperature must be controlled at 50 ~ 60 °C, and then continue welding next. Each solder layer should not be too thick.
When welding, the welding is performed at a faster speed as much as possible. The welding rod does not swing laterally, and the arc pit must be filled when the welding tail is received.
Solder half of the groove and then weld the other side. When the other half is welded to half, drill a hole with a diameter of Y20mm at the center of the thickness of the base. Inserting two pins is equivalent to adding two steel bars to strengthen the anchor screw. Solid strength, according to the above requirements will be welded on both sides of the groove, the base of the outer wall of the two pin holes welded. As shown.
The method of using arc cold welding gray cast iron, as long as strictly in accordance with the requirements of the process, the white mouth organization and welding cracks can be controlled, after welding by the coloring test found that the surface without any cracks and defects, in full compliance with the design requirements. By means of welding and pinning, both the welding quality and the fastening strength of the anchor bolts during installation are ensured. In January 2004, the user was installed and the test run was successful. It is still stable and no noise.