If designing high-temperature stress components, such as water-free cooling furnace roller, the designer must consider the specific performance characteristics, because the components need to operate reliably for a long time under the condition of ultra-high temperature and high load. The experienced furnace designer is very clear that the material characteristics will not change with time under the condition of room temperature (or only moderate temperature rise), but also when designing furnace roller Considering the creep over time, because with the increase of load time, the material of furnace roller still has tension, and the mechanical properties such as tensile strength and yield strength are the main considerations in manufacturing design. In this case, the designed furnace roller is only suitable for short-term use or operation in low temperature environment (such as ≤ 500-600 ℃, depending on the material used).
For all heat-resistant materials used in the case of temperature rise, in addition to the mechanical properties, the equipment manufacturing company shall provide the stress and creep fracture value of the furnace roller materials in the whole temperature range, so that the furnace roller engineer can design the furnace roller meeting the reliability requirements, which can bear the requirements of high load, high temperature and long-term operation. Safety measures should also be taken into consideration when designing water free cooling furnace rolls in high temperature environment.
The furnace roll usually runs continuously under the temperature rising environment, which accelerates the degradation due to the corrosion and affects the service life of the furnace roll. It is very important to ensure the mechanical properties under stable working conditions for the integrity of the material structure, and the anti creep performance is also a necessary feature, which is the most important for material selection. Creep resistance determines the ability of materials to withstand deformation under high stress, high temperature and long-term working conditions. Under the given stress and temperature, creep test and stress fracture test are carried out to quantify the long-term operating conditions of materials. Creep test is related to time. Under constant stress or load lower than yield strength, measure the size change and elongation of the sample at a certain temperature, and use the measured value to determine the way and speed of deformation of the component during operation. Creep test is very significant to determine the deformation rate of components or parts under given load and temperature. When designing the furnace roll, it is necessary to refer to the creep test data. In the industry, materials are usually ranked according to the stress required to produce 1% elongation per 10000 hours at a specific temperature. When the material is subjected to constant load at constant temperature, the time required for material failure is determined by stress fracture test, and the material is selected purposefully according to these parameters. Although the dimensional tolerance is not very critical, no cracking is allowed. Stress cracking test is similar to creep test, but the stress is large. Stress cracking test is usually a short-term test, while creep test takes a long time. The Larson Miller parameter (LMP) clearly explains the temperature time equivalence of Superalloy in the process of creep stress fracture. According to these parameters, the curves of stress cracking and fracture time and temperature are drawn, and the equivalent time required for stress cracking can be calculated at different temperatures. Each alloy has its own LMP curve, which can be used for material selection. It is very time-consuming and costly to determine the parameters of various materials. A foundry company with R & D ability and focusing on scientific and technological development can be entrusted to provide effective parameters (including furnace number and test variables) of specific alloys, so as to design and select materials for furnace rolls of high-temperature heating furnace.
Another very important material characteristic is the oxidation resistance of the metal material used to make the furnace roll. The oxidation resistance is based on the possibility of forming the oxide rust layer between the metal and the gas. The oxide rust layer separates the metal and the gas and has the resistance to the bottom metal corrosion. In fact, the oxide rust layer has the characteristics of stable chemical properties, dense structure and slow dynamic growth. Oxides such as FeO and NiO are generated faster, while SiO2, Cr2O3 and α – Al2O3 are generated slower. Therefore, in engineering design, the oxidation protection usually refers to Cr2O3, SiO2 and α – Al2O3.
High temperature tensile test data is of great significance to understand the mechanical properties of materials, especially the ductility and strength of materials in high temperature environment. However, the test data at room temperature can only obtain the performance index at room temperature, which is not significant for the materials in service at high temperature, because it is necessary to maintain the function of materials at high temperature and control their quality stability. Physical properties, such as thermal conductivity and thermal expansion coefficient, play an important role in material selection.
