Based on the current manufacturing and use status of CB2 steel castings, in order to grasp the quality differences between CB2 steel castings from different manufacturers, a comprehensive comparative analysis was made on the performance data of domestic and imported CB2 steel castings. The imported manufacturers involved include Voestalpine company in Austria, Sande company in Germany, JSW company in Japan, and the domestic manufacturers include China Erzhong Group, Ningxia shared Steel Casting Co., Ltd Dongfang turbine Co., Ltd. (casting Division) and Shanghai Honggang power plant equipment casting and Forging Co., Ltd.
1. Chemical composition comparison
The main alloy elements of CB2 steel castings are Cr, Mo, Co, Mn, Ni, Si, Nb, N, B. the chemical composition of domestic and imported CB2 steel castings is statistically analyzed, and the results are shown in Figure 1.
It can be seen from the figure that the control of main alloy elements Cr, Mo, CO and Nb in domestic and imported CB2 steel castings is at the same level; However, the distribution of Mn, Ni and n elements fluctuates greatly, and the content of individual elements does not meet the technical requirements of manufacturers’ procurement. Relevant research shows that the high content of Mn, Ni and Si in 9%~12%cr steel will promote the precipitation of harmful phases in the process of high-temperature creep, thus reducing the high-temperature creep strength of 9%~12%Cr steel. Therefore, it is suggested to control the content of Mn, Ni and Si in CB2 cast steel according to the lower limit to prevent exceeding the upper limit, which also puts forward stricter requirements for the steelmaking level of various manufacturers.
Because B and n elements are added to CB2 steel casting at the same time, when B and N in the steel are excessive, BN brittle inclusions will be easily formed (see Figure 2), and the formation of BN inclusions will reduce the effective solid solution B content in the steel, thus reducing the beneficial effect of B on the high temperature creep strength of the steel. Therefore, it is necessary to strictly control the n/b ratio and B content in CB2 steel casting, give full play to the role of interstitial solid solution strengthening of B element and stabilizing M23C6 carbides, and inhibit the formation of massive BN inclusions, so as to ensure the high temperature performance of CB2 steel casting.
2. Comparison of conventional mechanical properties
The room temperature tensile properties of domestic and imported CB2 steel castings are compared and analyzed. The results are shown in Figure 3. It can be seen from the figure that the tensile strength, yield strength and elongation at room temperature of domestic and imported CB2 steel castings are at the same level.
The room temperature impact properties and hardness of domestic and imported CB2 steel castings are compared and analyzed, as shown in Figure 4. It can be seen from the figure that the impact properties at room temperature are between 27~80j and the hardness is between 190~250hbw, and there is no obvious difference between the two. It can be seen that the conventional mechanical properties of domestic CB2 steel castings are at the same level as those of imported steel castings.
3. Microstructure comparison
The metallographic structure of CB2 steel castings from different domestic and imported manufacturers was statistically analyzed, and the metallographic photos are shown in Figure 5. It can be seen from the figure that the metallographic structure of all CB2 steel castings is tempered martensite, and there is a small amount in the microstructure of domestic steel castings δ- Ferrite is not found in imported steel castings δ- Ferrite. Relevant research shows that, δ- Ferrite is a phase rich in Mo, Cr, and V and poor in Mn. Its formation is related to the CR equivalent in the alloy, which will reduce the impact toughness and high temperature strength of steel. Therefore, the CR equivalent should be strictly controlled to avoid δ- Formation of ferrite.
The grain size of CB2 steel castings from different domestic and imported manufacturers is statistically analyzed. The grain size photos are shown in Figure 6 and the statistical distribution is shown in Figure 7. The macro grain size of all CB2 steel castings is m9.5~12.5, and the grain size of imported steel castings is significantly coarser than that of domestic steel castings. Because the grain boundary strength of the material at high temperature is lower than the intragranular strength, for high-temperature materials, larger grain size is beneficial to obtain better high-temperature creep strength. To sum up, there are still some differences between domestic CB2 castings and imported products in microstructure control and grain size control.
In addition, the contents of non-metallic inclusions in domestic and imported CB2 steel castings are compared and analyzed. The photos of non-metallic inclusions are shown in Figure 8. The results show that the forms of non-metallic inclusions in all CB2 steel castings are mainly divided into black gray spherical, dendritic and irregular. The inclusions are randomly distributed, and the particle size is about 50 μ Below m; The scanning electron microscope morphology and energy spectrum analysis results are shown in Figure 9, and its content distribution is shown in Figure 10. Through comparison, it is found that the types and forms of non-metallic inclusions in imported and domestic steel castings are the same, mainly oxides, sulfides, nitrides and their composite phases. The content of non-metallic inclusions in imported steel castings is small, and the particle size is small; The content of non-metallic inclusions in domestic steel castings is relatively large, and the particle size is coarse. It can be seen that there are great differences in the control of non-metallic inclusion content between domestic CB2 steel castings and imported products, which is related to the steelmaking level of the manufacturer and the purity control of molten steel.
Through the comparative analysis of the performance data of domestic and imported CB2 steel castings, it is found that the content of main alloy elements of both domestic and imported CB2 steel castings is similar, the conventional mechanical properties are at the same level, and the metallographic structure is tempered martensite. It is found that the quality difference between imported and domestic CB2 steel castings is mainly manifested in the content of non-metallic inclusions δ Ferrite content and grain size. Among them, the aggregation and distribution of non-metallic inclusions will destroy the continuity of the metal matrix. The metal is very easy to produce cracks at the inclusion aggregation due to external forces during service, resulting in the decline of strength and impact toughness, and affecting the service life of the material. and δ- A large number of alloy elements are dissolved in ferrite, which reduces the content of alloy elements in the matrix, resulting in the weakening of solid solution strengthening and precipitation strengthening of CB2 steel casting, which is unfavorable to the creep strength at high temperature; At the same time δ- The formation of ferrite results in uneven creep deformation around the matrix interface during high temperature endurance. Martensite and δ The interface between ferrites enhances the diffusion, promotes the coarsening of carbides and the recovery of martensite structure, thus reducing the durable strength of steel. Martensite and δ- Coarse carbides gather on the interface between ferrite, which weakens its combination with the original austenite grain boundary, so it is easy to crack initiation and propagation when subjected to external load, thus reducing the impact toughness of CB2 steel casting. Therefore, in order to ensure the overall performance of CB2 steel castings, it is suggested to strictly control the content of non-metallic inclusions and δ- Ferrite content.