Study On Differential Heating Heat Treatment Of High Chromium Cast Iron Rolls

High chromium cast iron rolls have the significant advantages of simple manufacturing technology,low cost,good processing performance,wear resistance and excellent fatigue resistance.However,due to the oversized specifications of such workpieces,conventional heat treatment process have a very long heating time and high energy consumption to result in such problems of serious oxidation,great deformation,easy cracking,low yield and so on.How to solve the practical problems of the heat treatment process and reduce production costs are important issues faced by the company’s production.The reform and innovation of the heat treatment process is the key.Differential heat treatment is an innovative heat treatment process,which can effectively save energy and reduce consumption,and improve the product quality and yield.In this investigation,conventional heat treatment processes and practical problems of the current Cr18 series high chromium cast iron rolls were studied firstly.JMatpro software was used to simulate and calculate for Cr18 high chromium cast iron rolls.Based on the simulation results,differential heat treatment processes,including preheating,quenching and tempering of Cr18 series high chromium cast iron rolls were designed.By using 15 t class?800mmCr18 series large high chromium cast iron rolls,online research was carried out to study the temperature changes of different sections from the surface to the core of the trail roll,and the structure and mechanical properties after final treatment during differential heat treatment process.This topic through the optimization of heat treatment process,further energy saving and consumption reduction are realized.The main conclusions are as follows:1)The main problems of two conventional heat treatment processes for high chromium cast iron rolls were analyzed.For annealed heat-treated rolls,there are severe structure segregation,low hardness on the working layer,poor wear resistant and easy to crack during a roll is used.For overall quenching heating rolls,long heating cycles bring about high energy consumption,high production cost and severe oxidation as well as great deformations.There were also large cracking tendency during quenching and poor hardness uniformity on the working layer.2)The simulation calculation results of JMatpro software showed as follow.The elastic-plastic transition temperature of the core nodular cast iron and the outer high chromium cast iron is calculated.The creep temperature is less than 600℃.The CCT curves,transformation temperatures of Acm,A₁ and Ms of Cr18 high chromium cast iron working layers at different austenitizing temperatures from 960℃¹050℃are obtained.The CCT curve analysis shows that the martensite mainly occurs in the working layer at a very low cooling rate（0.02℃/s）,and the hardness can reach above 51HRC.The CCT curve of austenitizing of nodular cast iron at 1000℃is simulated,A1=832.7℃.Under the condition of cooling rate less than 10℃/s,only pearlite transformation occurs,and the hardness is between 17²3 HRC.Ensure that the overall toughness and tensile strength of the roll meet the performance requirements.3)The differential heat treatment process about Cr18 series high-chromium cast iron rolls was designed with three parts of preheating,quenching and tempering.The online research shows that the differential temperature preheating process could make the working layer and core temperature reaching the set temperature of 650°C in a shorter time.Under the designed heating temperature of 1050℃¹100°C at differential temperature quenching process,The results show that the austenitizing temperature and cooling rate are reached in the working layer and core of different parts,so that corresponding martensite transformation and pearlite transformation of the working layer and the core would be guaranteed during the quenching process.4)After the final heat treatment of 500℃⁵20℃three-stage differential tempering,the metallographic structure and property analysis show that the structure of the working layer is a mixture of martensite+primary carbide+secondary carbide+a small amount of retained austenite.The hardness of the working layer is 56-58 HRC and uniform.The microstructure of roll core is nodular cast iron with fine pearlitic matrix and the hardness is 16-25hrc.The on-line research shows that the designed differential temperature heat treatment process can meet the requirements of obtaining high hardness and high toughness core layer.5)Comparing to the energy consumption,Cr18 series high chromium cast iron rolls with differential temperature heat treatment is about 325kwh/t lower than the overall heating and quenching.And it has remarkable advantages of less oxidation and deformation,small stress during heat treatment,low rejection rate,and low uniformity of the structure.The hardness of working layer is about 3HSD higher than conventional heat treatment.After process optimization,the energy consumption of the differential heat treatment process is reduced by 32%compared with the conventional overall quenching,and energy saving and consumption reduction are significant.