It is well known that there are three main methods for exhibiting grain size: oxidation method, grain boundary corrosion method, and carburization method. Oxidation method is applicable to all steel types, especially for stainless steel and heat-resistant steel. It is difficult to master for structural steel; grain boundary corrosion method is suitable for most steel types, especially for displaying martensite The bainite microstructure steel has the best effect on the prior austenite grain; the carburizing method is mainly used for carburizing steel, but in actual production, the carburizing method shows that the grain length is long and the process is complicated, and the carburized steel is also often used. The grain size is shown by grain boundary etching.
1 Experimental steel and experimental method The 5 furnace 18CrMnB steel used in this experiment is smelted under the protection of Ar gas by vacuum induction furnace, the weight of the ingot is 28.5kg, the chemical composition and its content.
The steel ingot was heated to 1250 ° C for 1 h in an electric furnace, then forged into a round material of Φ 12 mm, and subjected to two kinds of pre-heat treatment.
Two kinds of preheated round materials were finished into a sample of Φ10×18 mm. Then carry out the following experiments: heat preservation at 9001060 °C for 1 h, water quenching (grain boundary corrosion method); solid carburizing at 9001060 °C for 8 h, furnace cooling; solid carburizing at 9001060 °C for 8 h, Water quenching. The water quenched samples were all mixed with supersaturated bitter acid aqueous solution and Haiou brand detergent (10:1) and heated to 5080 °C for thermal erosion; the furnace cooled samples were etched with 4 nitric acid alcohol solution. After the austenite grain boundary is shown, the average chord length of the austenite grain is measured under a microscope by the string cutting method, and the temperature corresponding to the chord cut value reaches the austenite grain coarsening temperature.
2Experimental results 2.1 Comparison of grain coarsening temperature of carburized layer measured by different ways of solid carburizing of 18CrMnB steel in pre-heat treated state. The next page shows two kinds of pre-heat treated 18CrMnB steel through 8h solid carburizing. Thereafter, a binary correlation diagram of the grain coarsening temperature of the carburized layer measured by the furnace cooling method and the water quenching method is employed.
After solid carburizing of 18CrMnB steel in two preheated states, the difference in grain coarsening temperature measured by different methods (furnace cooling, water quenching) is small.
2.2 Comparison of grain coarsening temperature measured by different grain size display methods for 18CrMnB steels with different preheating conditions. Binary correlation diagram of grain coarsening temperature measured by carburizing method and grain boundary corrosion method for 18CrMnB steel.
The roughening temperature of 18CrMnB steel measured by different grain size display methods is very different, and the roughening temperature measured by carburizing method is higher.
3 As discussed before, the austenite grain coarsening temperature measured by carburizing method and grain boundary corrosion method of 18CrMnB steel is very different, and the correlation between them is very poor. The reason is that the carburization method and the grain boundary corrosion method have significant differences in the method of forming the crystal grains: the carburizing method has a slower heating rate than the grain boundary corrosion method, has a long holding time, and has a slow cooling rate; When the method shows grain size, the carburizing process also changes the surface composition of the steel.
3.1 Effect of heating rate on grain size Heating rate is essentially a problem of superheat. The higher the heating rate, the greater the degree of superheat, that is, the higher the actual formation temperature of austenite. Since the nucleation rate of austenite at a high temperature is large, a fine initial grain size is obtained. However, since the initial grain size is small, the grain boundary energy is high, and the austenite grain growth tends to be large, and if it is kept for a long time, the crystal grains will grow sharply.
3.2 The influence of heating temperature and holding time on grain size The growth of grain is mainly caused by the movement of grain boundary. It is essentially the diffusion of atoms near the grain boundary, so the higher the heating temperature, the longer the corresponding holding time. Atomic diffusion is easier and more complete, and the austenite grains will become coarser.
The higher the heating temperature, the faster the austenite grain growth process proceeds; in addition, the grain growth process has an acceleration period at each heating temperature. When the grain reaches a certain size, the growth process will Weaken and stop; when the temperature is low, the holding time has little effect on the grain, and at the high temperature (above 900 °C), the effect of holding time (within 30 min) is larger and then weakened.
3.3 The influence of the carbon content of steel on the grain size, austenite grains increase with the increase of carbon content within a certain carbon content range. The limits of carbon mass fraction are: 1.2 at 900 °C, 1.4 at 1000 °C, 1.6 at 1100 °C, 1.2 at 1200 °C, 1.2 at 1300 °C. When this limit is exceeded, the carbon content is further increased and the austenite grains are reduced. . In the carburizing process, as the carbon content increases, the diffusion rate of carbon in austenite and the self-diffusion rate of iron increase, which promotes the austenite grain growth. However, when the carbon content exceeds the above limit, the secondary cementite is formed to prevent grain boundary migration, and the austenite grain growth tends to decrease.
It is the combination of these influencing factors that leads to a large difference in the austenite grain coarsening temperature of 18CrMnB steel measured by carburizing method and grain boundary corrosion method, and the correlation between them is very poor.
4 Conclusion After solid carburizing of carburized boron steel 18CrMnB steel, the austenite grain coarsening temperature measured by different cooling methods is basically the same. The grain coarsening temperature of 18CrMnB steel measured by carburizing method and grain boundary corrosion method is very different. Since the carburizing method is used to determine that the crystal grains are relatively close to the actual heat treatment conditions of the carburized steel, it is preferable to measure the grain size by carburizing the carburized steel.
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