Introduction to some related issues of surfacing rolls
Rolls are the heart of a rolling mill. The use of rolls involves mill selection, roll preparation, maintenance and regular dressing during the rolling process, which are factors that affect mill productivity, product quality and production economics. Since the rolling process is a continuous process, operators need to constantly check the condition of the rolls so that the product can meet the user's requirements for tighter tolerances and surface quality. In order to maintain the competitiveness of the products, it is necessary to constantly ask the mill operators to reduce production costs, i.e. increase the rolling speed, increase the production flexibility of the rolling mill and reduce the consumption of the rolls. So what should we pay attention to when surfacing rolls?
(1) Before surfacing, it is necessary to remove the fatigue layer or defects on the surface of the roll body, especially cracks;
(2) During surfacing, in order to prevent cracks, the roll body needs to be preheated first, and the preheating temperature depends on the roll and surfacing materials;
(3) In order to obtain an ideal surfacing layer, some variable factors need to be comprehensively considered, such as welding voltage, welding speed, roll speed, roll insulation, welding current, welding materials, etc. For some roll cores with high carbon and alloy elements, in order to prevent cracks in the brittle zone, in addition to certain preheating measures, low carbon and low alloy transition layers are often used for pre-surfacing welding.
(4) Slow cooling turning after roll surfacing. In order to reduce cracks caused by volumetric stress caused by different cooling rates on the surface and inside, the cooling rate should be controlled. Then, the overlay roll is turned to restore the original size and smoothness.
And the performance of the roll after surfacing welding:
(1) The service life of the roll after surfacing is generally more than doubled;
(2) Greatly reduce the cost per ton of steel and improve the production effect;
(3) The surfacing roll has good crack resistance, wear resistance and thermal fatigue resistance. The main links in the manufacturing process of cold rolls are smelting, forging, heat treatment, processing and inspection.
The rolls are subjected to a lot of rolling stress during the working process. The welding seam, inclusions, edge cracks and other problems of the rolled piece are easy to cause instantaneous high temperature, which makes the work rolls subject to strong thermal shock, resulting in cracks, sticking, peeling and even scrapping. . Therefore, the roll should have the ability to resist cracking and spalling caused by bending stress, torsional stress and shear stress, and should also have high wear resistance, high contact fatigue strength, high fracture toughness and thermal shock strength, etc. Therefore, how to improve the service life of the roll has always been a major problem faced by the roll manufacturing industry.
Non-destructive testing of rolls is usually carried out with ultrasonic flaw detectors. In the cold roll flaw detection standard, the roll body is usually divided into a surface area, a central area and an intermediate area according to the force, and then the critical size of the allowable defects in each area is specified according to the principle of fracture mechanics. Flaw detection workers should have knowledge of roll manufacturing in order to judge the nature of defects, and should also have knowledge of roll use in order to estimate whether these defects will cause use damage under specific conditions of use.