Study On The Surface Treatment Process Of H13 Type Hot Work Steel And Its Subsequent Thermal Fatigue And Erosion Behaviors

The innovation of surface treatment to increase the associated performance of hot work dies is one of the most interesting subjects in laboratories and factories.A furnace with preheating equipment for water vapor is developed for surface oxidation treatment of hot work dies. This type furnace is successfully used in industry. The structural design including loop-pipe , stream chamber results in the sufficient preheating of water vapor and then steady feeding into oxidation room, which ensures the realization of oxidation process for die steel, and the reliability and security of the furnace. It is failed to combine together the two process of water vapor oxidation and gas nitriding in the same furnace. Consequently, the oxidation process is mainly carried in the special designed furnace and the nitriding process is carried in a commercial plasma nitriding furnace respectively.The oxidation behaviors of H13 steel are studied with both thermodynamical calculation and experimental information. Thermal-Calc software is performed to calculate the oxidation phases on H13 steel along with different partial oxygen pressures in the interesting temperature range from 500℃ to 700℃. In this range H13 steel samples are treated respectively in different atmospheres including flowing water vapor (0.2MPa), normal pressure air (0.1Ma) or low pressure air (0.001MPa). The microstructures and phase constitutions of the films formed in low pressure air are similar to those of the films formed in water vapor, and obviously different to those of the films formed in normal pressure air, which could be explained by result of calculation.Calculation with Thermal-Calc software indicates that higher partial oxygen pressure is favorable to the formation of Fe₂O₃ while lower partial oxygen pressure is favorable to the formation of Me₃O₄. Full compact Fe₂O₃ layer formed rapidly in high partial oxygen pressure would block the diffusion of species and then the formation of Me₃O₄. Therefore, in proper low pressure air the oxidation speed of H13 steel is obviously faster than that in normal pressure air. Besides the effect of partial oxygen pressure, highoxidation speed of HI3 steel in water vapor is more complicated and believed as the result of short-current diffusion of O ion. Calculation also shows that spinel phases of FeC^CU, FeV2O4, Fe2SiC>4 are also the main phases as well as phases Fe2C>3 and Fe3C>4 in oxides on HI3 steel.As to the combination of water vapor oxidation and plasma nitriding, it shows the pre-oxidation of HI 3 don't help the subsequent plasma nitriding course. The oxide film would be gradually reduced and no nitrogen could diffuse into the steel before vanish of the oxide. The activation of HI 3 surface caused by plasma bombardment and the subsequent higher oxygen diffusion rate into the steel seemed to be the reasons of higher oxidation speed. The nitride layer, generally formed on the steel surface under plasma nitriding process, could be substituted by oxide in subsequent oxidation process. Which could reduce the risk of heat cracking in hot work applications. Therefore, the process of plasma nitriding with post oxidation would be a proper choice for some hot work dies, which demands high hardness to avoid indentation as well as high toughness to avoid cracks. It is successfully applied on some heavy load forging dies.Self-restricted thermal fatigue tests are carried out on different surface treated HI3 steel specimens according to Uddeholm method with temperature range between room temperature and 700 °C . The processes include plasma nitriding, plasma sulfur-carbon-nitriding, boronizing and oxidizing. It shows that the thermally induced cyclic stress plays an important role in the generation and propagation of heat checking cracks. During thermal cycles, the residual compressive stresses on HI3 specimens without high hardness compound layer decreases gradually, and it is apt to form evenly distributed cracks. While the higher compressive stresses on HI3 specimens with high hardness compound layer drops quickly. The uneven distributed stresses, usually induced by a few cracks on compound layer, are the important factor for quickly developed main cracks. Optimized nitrogen diffusion layer in HI 3 steel shows positive effect on thermal fatigue due to the improved strength and limited decreased toughness. No obvious negative effect is found on thermal fatigue of surface oxidized HI 3 steel. In erosion test, HI3 hot work steel samples and surface water vapor oxidized HI 3...