In the manufacturing process of the crane, the steel rail laid on the box-shaped main beam is often welded into a seamless long steel rail in the form of butt joints. In the installation of cart rails , there are also many types of joints that use welding. The case of rail butt welding process in actual work is summarized as follows.
There are three types of crane trolley tracks :
(1) Crane rails such as QU70, QU80, etc.
(2) P-type rails such as P24, P38, P43, etc.
(3) Square steel such as: 30mm×40mm, 40mm×40mm, etc.
The top of the first two rails is made convex, and the bottom is a flat plate with a certain width, which can increase the contact surface with the foundation. The cross section of the rail is I-shaped, with good bending strength, and its carbon content and manganese content are relatively high, wC=0.5%～0.8%, wMn=0.6%～1.5%. The material of the square steel is Q275, and the top is flat and wears a lot to the wheels.
The butt weld of the rail requires no treatment to achieve the surface hardness of the rail. Below the head of the track, use E5016 electrode; at the head of the track, use surfacing electrode D322 (chromium tungsten molybdenum vanadium cold die electrode). This is economical and practical, not only can ensure the quality and strength of the butt weld, but also can make the hardness of the surfacing layer (air cooling after welding) ≥55HRC.
The above two kinds of welding rods are both AC and DC, with a diameter of 5mm, a welding current of 180-240A, and an arc voltage of 36-24V.
2. Butt welding process
1. Tools, materials and welding preparation
1 to 2 electric welding machines, 2 to 3 welding torches, a thermometer at 0 to 300°C, oxygen and acetylene gas. Before welding, put the welding rod in an oven at 350～400℃ and bake it for 1h, then put the butt-joined rail on the concrete floor and support it with a gap of 20mm between butt welds. Sand and scrub clean.
Due to the poor welding performance of the steel rail, the welding process is more complicated. A thermometer of 0～300℃ should be fixed on the steel rail. At a position 100mm long from both sides of the welding seam, use 2～3 welding torches to preheat the rail at the same time. When the rail temperature reaches 230～250℃, first use E5016 welding rod from the bottom of the rail while surfacing welding. When surfacing to the track head, use D322 welding electrode while heating while surfacing. Welding should be carried out intermittently, minimize the heat of the welding part, and keep the track temperature of 230～250℃ during the welding process. After all welding is completed, continue to heat to 250°C, and then slowly cool the rail to outdoor temperature (about 30°C) in the air for ≥0.5h to prevent cracks. After welding, check whether there are obvious traces and the hardness after welding at the weld and the joint with the rail.
After welding, use pneumatic grinding wheel to grind to keep the surfacing seam of the rail head on the same level as the original rail surface, with the same surface roughness.
Steel has different internal organizational structures at different temperatures, and changes in internal organizational structure will inevitably change the mechanical properties of steel. The wC of the rail is 0.5%～0.8%, which belongs to hypoeutectoid steel. Below the AC1 line (723℃), the internal structure of the steel is ferrite and pearlite. The surface of this steel forms a martensite structure after quenching. Very high strength and hardness.
Before welding, the steel rail is preheated to 230～250℃, and a low temperature tempering treatment is performed to prevent the metal at the welded joint from cracking after cooling. At this time, the structure of the hardened layer changes from martensite to tempered martensite. The internal stress of the steel is partially eliminated, the toughness is increased, and the hardness is almost not reduced, and it can also ensure that the rail is not deformed at the preheating temperature. If the preheating temperature is too high, the martensite structure of the original surface and hardened layer will be transformed into other structures, such as sorbite, which will reduce the hardness of the rail.
During rail butt welding, as the welding temperature increases, the tempered martensite at the welded part begins to transform to other structures. Welding should be carried out intermittently to minimize the heat of the welding part, so that the temperature of 230-250℃ is always maintained during the rail welding process. When the welding of this part is finished, if it is rapidly cooled to 230～250℃ in air, temper brittleness is a frequently encountered problem. For general carbon steel, the starting temperature of martensite formation is 200-350℃, while the rail is an alloy steel containing manganese elements, and the martensite formation temperature of the rail should be 100-200℃. This requires that after the rails are all butt welded, they should be heated to 230～250℃ and then slowly cooled in the air. After ≥0.5h, the temperature will be slowly cooled from 230～250℃ to the outdoor temperature, so that the weld metal can be gradually transformed into Martensitic organization. Since the alloy at the welding seam and the track as a whole are slowly cooled to the outdoor temperature, the internal stress at the welding seam is reduced and no cracks are produced on the surface. In addition, through the hardness tester, the weld on the top of the track has reached the original hardness of the top surface of the track, indicating that the surface hardness has also reached the requirement.
The above-mentioned butt welding process is used to successfully use the steel rails with poor welding performance in the production of trolley rails for crane products.
The steel rail laid on the box-shaped main beam of the crane is welded to form a seamless whole rail, which can greatly increase the rigidity of the box-shaped beam of the crane, which not only reduces the bowing of the main beam, but also reduces the crane The noise of the trolley running makes the trolley run smoothly and the wheel pressure is even. In addition, it reduces the wear of the wheels of the trolley and prolongs the life of the wheels, thereby prolonging the service life of the crane, improving the reliability, reducing the maintenance workload, saving the user's cost in use, and being praised by the users. This welding process is particularly important for cranes with large tonnage and large span.
We believe that with the improvement of the design and manufacturing technology of steel structure workshops year by year, and the span of the workshops is getting larger and larger, the economical and practical technology of rail butt welding is becoming more and more important.
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