Influence Of High Temperature Service Conditions On The Fracture Toughness Of Furnace Tube Material

Reforming furnace is a key equipment in the hydrogen production device in petrochemical industry,and the furnace tube is the core parts of hydrogen furnace.The service conditions of the furnace tubes are very harsh and the furnace tubes often suffer multiple damages such as high temperature creep,high temperature oxidation,high temperature corrosion.Once an accident happens,it will cause huge economic losses and lead to a bad social influence.So it is of great significance to study the damage and failure of the furnace tubes and ensure the long period safe operation of the reforming furnace and the whole hydrogen production device.Recent studies show that high temperature creep is the main cause of the damage of the tube material,and the high temperature is the direct cause of the creep.In order to obtained the influence of high temperature condition on the properties(especially the fracture toughness)of the reformer tube material,the related research was carried out in this paper.At first,The aging treatment of the furnace tube material at different temperatures was carried out by using the resistance furnace.Then the plane strain fracture toughness test and tensile test were carried out on the ten groups of tube materials after the different high temperature aging treatment at room temperature,and the macrofractography of the fracture toughness specimens was analyzed.Finally,the theory of fracture mechanics and finite element method were used to explore the reasons of toughness deterioration of the tube material caused by high temperature aging treatment.The main results and conclusions are as follows:(1)Through tensile testing at room temperature of ten groups of furnace tube specimens,the yield strength and tensile strength of the furnace tube material after high temperature aging treatment increased,but the extension rate and the section shrinkage rate of the plastic parameters of the material declined.(2)Through the plane strain fracture toughness test carried out on ten groups of the furnace tube specimens,it was found that the fracture toughness of the furnace tube material appeared deterioration after high temperature aging treatment.It was also found the aging time had little effect on the fracture toughness of the furnace tube material,and with the increase of the aging temperature,the fracture toughness of the furnace tube material decreased first and then rebounded slightly.(3)According to the theory of material damage,if the microstructure of material has damage,then dislocation-free zone between the crack tip and dislocation zone is easy to form micro crack.Crack propagation is essentially the result of the merger of the old and new cracks.By using the theory of fracture mechanics and finite element method to establish the three dimensions fracture finite element model of the three point bending specimen with different crack length and with micro cracks,without micro cracks at the crack tip,the maximum stress and the maximum stress intensity factor of the crack front were calculated.It was found that the maximum stress and maximum stress intensity factor of the crack front were much larger when there was a micro crack in the crack front and with the increase of crack length,the maximum stress and maximum stress intensity of the crack front would become larger.So if the material is damaged in the high temperature aging treatment,the ability to resist crack growth will become worse,in other words,the fracture toughness will be degraded.