The volume fraction (3.3% – 4.6%) of M7C3 carbide calculated from two-step thermokinetics corresponds to the result (3.6% – 4.8%) determined by optical micrograph, as shown in Table 2. On the other hand, it seems that the volume fraction of ferrite in the phase diagram is overestimated. As vanadium is a kind of ferrite forming element, the volume fraction of ferrite in n16v1 sample is about 8% in the phase diagram, but only 1.8% in the actual microstructure. Although the calculated volume fraction of ferrite is close to 5%, ferrite does not exist in other samples. The results show that there is a certain difference between the calculated value and the measured value of ferrite because the casting process does not reach the equilibrium state. The existence of ferrite will affect the tensile properties. At room temperature, because the properties of ferrite are similar to that of austenite, the yield strength and tensile strength increase with the increase of vanadium content, but they have nothing to do with the existence of ferrite, but the elongation will decrease, as shown in Table 3.
However, at 900 ℃, because the properties of ferrite are not as good as that of austenite, the strength of ferrite will decrease. According to the phase diagram of EBSD imaging quality (IQ), the volume fraction of ferrite is 1.8% and 1.0% respectively in n16v1 sample quenched at 900 ℃ for 1H. After quenching at 900 ℃ and 1 h, the volume fraction of ferrite decreases, so the volume fraction at 900 ℃ is smaller than that at room temperature. The existence of ferrite will have an adverse effect on the high-temperature tensile properties, because it will be used as the starting point of holes or cracks, thus reducing the yield strength and tensile strength. Therefore, these performance indexes of n16v1 sample are lower than those of n16 or n16v0.5 sample, as shown in Table 3. However, due to the coexistence of ferrite and carbide and the small size, it is difficult to find the lower cross section of the fracture surface, as shown in Figure 4. Since the hardness of M7C3 carbide is 2000 vhn, which is much higher than that of austenite matrix from 175-216vhn, it can be predicted that the strength is directly proportional to the volume fraction of carbide at room temperature and high temperature. However, since the strength increases with the increase of vanadium or copper content, the strength shows the same trend after the addition of alloy elements such as vanadium or copper, as shown in Table 3. Considering that the volume fraction of M7C3 carbide is only 3.6% – 4.8%, and the change is not big, therefore, it is not the volume fraction of M7C3 carbide but the hardness of matrix that has more influence on the strength. The addition of vanadium or copper can improve the hardness of the matrix, which is mainly due to the solution strengthening effect. In austenite matrix, copper is the most important element that affects the effect of solution strengthening. Copper contained in austenite matrix is an austenite stabilizer. When the copper content is very high, for example, in n16cu4 sample with 4wt% copper content, the distribution of copper in austenite matrix is not necessarily uniform. It also shows that, because the melting point of copper is lower than that of other elements, there will be serious dendrite segregation along the solidification cell boundary, which is the end of solidification area.
The tensile properties of copper dendrite segregation will decrease, as shown in Table 3, because its strength is about 85vhn higher than that of the base. Especially at 900 ℃, copper dendrite segregation does not dissolve, and it can also restrain the deformation of the matrix. Then, the cracks appeared and propagated rapidly along the copper dendrite segregation region. Therefore, although the high-temperature strength is the highest of the five cast steel samples, the thermal ductility of n16cu4 sample is significantly reduced to 3.8%. The effect of adding vanadium or copper on the high temperature properties explains the idea that the strength of the cast steel can be improved by further understanding the austenite and carbide structure. Compared with n16, because of the addition of vanadium or copper, the yield strength and tensile strength of the sample at high temperature have been improved, which shows that the addition of vanadium or copper has a positive impact on the strength. This is because the addition of vanadium or copper not only increases the volume fraction of M7C3 carbide, but also increases the hardness of austenite matrix. Therefore, the sample with vanadium or copper is very suitable for high performance turbocharger shell and exhaust emission system. However, if the amount of vanadium or copper is too high, such as 1wt% vanadium or 4wt% copper, the formation of ferrite or copper dendrite segregation will lead to the degradation of high temperature performance. Especially, the hardness of austenite matrix of n16cu4 is the highest, and the volume fraction of M7C3 carbide is the highest. The copper dendrite segregation will lead to the generation of cracks, and expand along the copper dendrite segregation area, thus reducing the thermal ductility. Therefore, in order to improve the high temperature properties, it is better not to form ferrite or copper dendrite segregation, and to improve the hardness of the matrix mainly by means of alloy design and microstructure control.
According to the daily price of London Metal Exchange in January 2018 and the standard alloy price of ferroalloy, compared with the traditional HK40 steel, the alloy element price of n16v0.5 and n16cu2 samples can be saved by 8% and 10%, respectively. Considering the alloy price and high temperature performance, n16v0.5 and n16cu2 are the high performance materials which can be used as turbocharger shell.