Due to the high content of Cr and Ni in austenitic heat-resistant cast steel, the cost remains high. In order to save rare metal elements and ensure safe and reliable operation, high cost-effective nickel ferrite heat-resistant cast steel has received special attention.
GX40CrSi17 ferritic heat-resistant cast steel has the outstanding characteristics of low cost and low thermal expansion rate, as well as good creep resistance, corrosion resistance, and oxidation resistance. It is widely used in long-term high-temperature and low load working environments, such as components of industrial heating furnaces, carburization boxes, automotive exhaust system components, etc. Due to long-term high-temperature effects, atomic diffusion is promoted, leading to changes in grain size and microstructure. Changes in alloy chemical composition also affect the type and content of precipitates at high temperatures, thereby affecting the structural stability and high-temperature performance of materials. Long term service in high-temperature and high-pressure environments directly affects the service life and operational safety of parts due to the stability of the structure. Therefore, it is important to study the microstructural changes of heat-resistant cast steel in actual working conditions.
Due to the relatively limited research on GX40CrSi17 heat-resistant cast steel, ZHY Casting uses thermodynamic software to simulate the phase composition at different temperatures and its variation with temperature based on the chemical composition of the cast steel used in the study. The main focus is on the microstructure after aging at different temperatures, with a focus on the influence of aging temperature on microstructure and hardness. The characterization and analysis of high-temperature precipitates were carried out using methods such as OM, SEM, EDS, etc. The relationship between composition simulation and precipitates after high-temperature aging treatment was established, laying an experimental foundation for the study of high-temperature mechanism and safety service evaluation of this heat-resistant cast steel.
The chemical composition of the cast GX40CrSi17 ferrite heat-resistant cast steel used in the experiment is shown in the table. Compared with the German DIN standard grade 1.4740, the elemental content of the cast steel used in the experiment meets the standard requirements.
| Elements | C | Si | Mn | P | S | Cr | Ni | Fe |
| Actual measured value | 0.43 | 1.16 | 0.58 | 0.03 | 0.01 | 16.66 | 0.26 | Bal. |
| Standard value | 0． 3 ～ 0． 5 | 1． 0 ～ 2． 5 | 0． 5 ～ 1． 0 | ≤ 0． 045 | ≤ 0． 030 | 16． 0 ～ 18． 0 | — | Bal． |
Based on the chemical composition of the experimental materials, JMatPro software based on the CALPHAD algorithm was used to simulate the thermodynamic equilibrium phase diagram of the alloy at 600-1500 ℃, and the changes in precipitated phases at different temperatures were analyzed. The samples were subjected to high-temperature aging treatment using the KSL-1400X high-temperature box type resistance furnace, with aging temperatures of 850, 950, and 1050 ℃, and insulation time of 100 hours. Then, they were cooled to room temperature by water. The as cast and effective samples were polished and polished, and then immersed at room temperature with Villa reagent (1 g of picric acid, 5 mL of hydrochloric acid, and 100 mL of anhydrous ethanol) for 1-3 minutes. The microstructure was observed using an AERXIOVT.A1 optical microscope, and the microstructure was observed using a VEGA3 scanning electron microscope. Using HB-3000 Brinell hardness tester to measure the hardness of specimens in different states, the diameter of the hard alloy indenter is φ 2.5 mm, with a load of 1837.5 N and a holding time of 15 seconds, measure 5 points on each sample and take the average value.
1) According to the simulation results of JMatPro software, GX40CrSi17 cast steel is mainly composed of ferrite and M23 C6 at 850-1050 ℃; The content of M23 C6 type carbides decreases with increasing aging temperature, reaching its highest at 850 ℃, about 7.70wt%; As the aging temperature increases, the content of M23C6 phase decreases. At 1050 ℃, the content drops to about 4.42wt%, and the ferrite content also significantly decreases, while the amount of austenite phase significantly increases.
2) High temperature aging at 850 ℃, the microstructure is a ferrite matrix with a large number of small and dispersed M23 C6 carbides; As the temperature rises to 950 ℃, the amount of M23C6 phase on the ferrite matrix significantly decreases due to partial dissolution, and the grain size tends to grow, with grain boundaries tending to flatten and develop; Aged at 1050 ℃, the grain boundaries are flatter, and the matrix structure is composed of ferrite and austenite phases; As the temperature increases at any time, the hardness change in the aged state shows a continuous decrease, which is lower than the as cast hardness. The change in hardness is related to the dissolution of M23C6 carbides and structural changes.
3) The recommended working temperature for GX40CrSi17 ferritic heat-resistant cast steel is between 850-900 ℃, which is not suitable for long-term use at temperatures above 950 ℃.