Rollers are the core components of a rolling mill. Roller usage involves mill selection, roller preparation, maintenance, and regular adjustments during the rolling process; these all affect the mill's productivity, product quality, and economic efficiency. Because the rolling process is continuous, operators need to constantly check the roller condition to ensure the product meets customer requirements for tighter tolerances and surface quality. To maintain product competitiveness, it's necessary to constantly require mill operators to reduce production costs, i.e., increase rolling speed, increase mill production flexibility, and reduce roller consumption. So what should be noted when hardfacing rollers?
(1) Before hardfacing, the fatigue layer or defects on the roller surface, especially cracks, need to be removed;
(2) During hardfacing, to prevent cracking, preheating of the roller body is required. The preheating temperature depends on the roller and hardfacing material;
(3) To obtain an ideal hardfacing layer, several variable factors need to be considered comprehensively, such as welding voltage, welding speed, roller speed, roller insulation, welding current, and welding material. For some high-carbon and alloy element cores, to prevent cracks from forming in the brittle zone, in addition to certain preheating measures, a low-carbon and low-alloy transition layer is often used for pre-hardfacing.
(4) Slow cooling and machining after roller hardfacing. To reduce cracks caused by volume stress from different cooling rates on the surface and inside, the cooling rate should be controlled. Then, rotate the hardfaced roller to restore the original size and smoothness.
And the performance of rollers after hardfacing:
(1) The service life of hardfaced rollers is generally doubled or more;
(2) Significantly reduces the cost per ton of steel and improves production efficiency;
(3) Hardfaced rollers have good crack resistance, wear resistance, and resistance to cold and hot fatigue. The main stages in the cold rolling mill manufacturing process are smelting, forging, heat treatment, processing, and inspection.
Rollers bear significant rolling stress during operation. Welding seams, inclusions, and edge cracks in the workpiece can easily lead to instantaneous high temperatures, causing the working rollers to experience strong thermal shock, resulting in cracks, sticking, scaling, or even scrapping. Therefore, rollers should have the ability to resist cracking and spalling caused by bending, torsional, and shear stresses, and should also have high wear resistance, high contact fatigue strength, high fracture toughness, and thermal shock resistance. Therefore, how to improve the service life of rollers has always been a major challenge facing the roller manufacturing industry.
Non-destructive testing of rollers usually uses ultrasonic flaw detectors. In cold rolling mill flaw detection standards, the roller body is usually divided into surface, center, and intermediate zones according to the stress conditions, and then the critical size of allowable defects in each zone is specified based on fracture mechanics principles. Flaw detection workers should have knowledge of roller manufacturing to judge the nature of defects, and should also have knowledge of roller use to estimate whether these defects will cause damage under specific usage conditions.