In order to ensure the performance of turbocharger shell at 1050 ℃, a high nickel (20wt%) austenitic steel ASTM HK40 has been adopted in the industry. In this study, by adding alloy elements such as vanadium or copper, a better solution strengthening effect is produced, which is an effective process. Five kinds of cast steels were prepared by adding vanadium or copper to the improved HK40 steel (n16 steel; 4% nickel was replaced by 4.6% manganese). At the same time, the improvement of high temperature properties was explained by the detailed microstructure evolution mechanism combined with the thermodynamic calculation phase diagram. Because the addition of vanadium or copper can improve the hardness of austenite matrix and the volume fraction of M7C3 carbide, the modified steel with vanadium or copper is better than n16 steel in high temperature performance. Since the volume fraction of M7C3 carbide in five kinds of austenitic cast steel is only 3.6% – 4.8%, the strength of steel sample is more affected by matrix hardness. However, when the content of vanadium or copper is very high, ferrite or copper dendrite segregation (Cu coring) will be formed, which will lead to serious degradation of high temperature performance. Therefore, if the amount of vanadium or copper is in the appropriate range, steel is very suitable for automotive turbocharger shell which requires high temperature tensile properties.
Because of its high strength and stable thermal fatigue resistance when the temperature of exhaust gas is higher than 950 ℃, austenitic cast steel has been used as the material of heat-resistant turbocharger shell. In order to stabilize austenite, form all kinds of hard carbides and improve hardness and corrosion resistance, this kind of steel usually contains a lot of alloy elements such as nickel, chromium, tungsten and niobium. In order to use this kind of steel when the temperature of tail gas is higher than 1050 ℃, more excellent high temperature strength is urgently needed. In the petrochemical industry, a high nickel austenitic steel, such as ASTM HK40 steel, is usually used in steam pipe or reformer, with nominal composition of 0.4C – 1.0mn – 1.2si – 25cr – 20ni (wt%), which has been proposed as an excellent alternative material for turbocharger shell. In order to form high stable austenite, the HK40 steel contains a large amount of nickel, up to 20wt%. In order to control the carbide at the lowest level, there will be no carbide forming elements such as tungsten and niobium. Recently, Jung et al. Used manganese (6.9wt%, another cheap austenite stabilizer) instead of nickel (6wt%), while ensuring a certain high temperature strength, which can save about 10% of the alloy cost.
In order to improve the high temperature properties of HK40 steel, a new type of cast steel was designed in this study. The solution strengthening effect can be produced by adding vanadium or copper, which is a very promising method. As an austenite stabilizer, copper can stabilize the austenite matrix and contribute to the solution strengthening effect of austenite steel. Due to the addition effect of vanadium or copper in austenitic cast steel has not been fully studied. In order to develop steel grades and improve properties, the effect of the content of added alloy on carbide formation and matrix strengthening should also be clarified. In the modified HK40 steel (4% nickel is replaced by 4.6% manganese), five kinds of cast steels are prepared by controlling the content of vanadium or copper. Their properties were improved by optimizing the formation of carbides and strengthening of austenite matrix. In order to study the evolution of microstructure, the high temperature equilibrium phase was calculated by thermal dynamic force and compared with the measured microstructure data. The complex microstructure is composed of carbide and austenite matrix. In order to connect the evolution of microstructure with high temperature tensile properties, the strengthening mechanism inside the microstructure was verified.
Five kinds of austenitic cast steel were prepared by adding vanadium or copper to the improved ASTM HK40 steel (n16 steel). The mechanism of carbide formation and austenitic matrix strengthening was used to explain how to improve the high temperature properties of the steel.
1) The volume fraction of austenite, ferrite and carbide was calculated by two-step method. In n16v1 sample, vanadium is a ferrite forming element, so although there is 8% ferrite in the phase diagram, the actual microstructure is only 1.8%. Even though the calculated volume fraction of ferrite is close to 5%, ferrite is not found in other samples. The strength of n16v1 sample is smaller than n16 or n16v0.5 sample due to the degradation of high temperature performance caused by ferrite.
2) The formation of M7C3 carbide is inferred from the equilibrium phase diagram. The measured value of M7C3 carbide volume fraction has a good correspondence with the calculated value. Since the volume fraction of M7C3 carbide is only 3.6% – 4.8%, and the change is not large, the strength is not affected by the hardness or the volume fraction of M7C3 carbide, but more by the matrix hardness. The hardness of matrix can be increased by adding alloy elements such as vanadium or copper, which is mainly due to the effect of solution strengthening.
3) When the copper content is very high, such as the n16cu4 sample with the copper content of 4wt%, the distribution of copper in the austenite matrix is uneven. This indicates that copper will have serious dendrite segregation along the solidification cell boundary, and the solidification will end in this region. Although the high-temperature strength is the highest among the five cast steel samples, the high-temperature ductility of n16cu4 sample will be significantly reduced to 3.8% due to the occurrence of cracks and the propagation along the copper dendrite segregation region,
4) The yield strength and tensile strength of the samples with vanadium or copper are better than those of n16, which shows that the addition of vanadium or copper is beneficial to the improvement of the strength. This is because the addition of vanadium or copper can improve the hardness of austenite matrix and the volume fraction of M7C3 carbide at the same time. In addition, compared with the traditional HK40 steel, the alloy element prices of n16v0.5 and n16cu2 samples can be saved by 8% and 10%, respectively.