Development of rare earth heat-resistant cast steel

Equipment boilers, turbines, aviation, petroleum and chemical industries often used in the components working at high temperatures, in addition to requiring high oxidation resistance, toughness and adequate stability in certain tissues, also requires more High high temperature strength. Steel added rare earth not only play deoxidation, desulfurization, to change the shape of inclusions, such as purification and metamorphism, but also in certain steel micro alloyed effect. A large number of research results show that rare earth elements can significantly improve the oxidation resistance, high temperature strength and plasticity, fatigue life, corrosion resistance and crack resistance of heat resistant steel and electrothermal alloy steel. In order to improve the heat resistance of cast steel materials and expand the application range and quantity of rare earths, the effects of heat-resistant cast steel on the thermal properties of rare earth elements Ce or La were studied experimentally, and the test results were used for Fe-Cr- The Ni-AI heat-resistant alloy steel was developed. The results show that the alloying method is used to optimize the screening of a component alloy by chemical casting. The heat resistance temperature of the alloy is increased to 1250-1300 °C, and excellent temperature is obtained at room temperature. Castability, weldability, good mechanical properties and high temperature oxidation resistance at high temperatures.

I. Test materials and methods

The test material is made of Cr24Ni7N steel and Cr25Ni20 cast steel. After adding 0.01% to 0.10% of the metal element La or Ce in the cast steel, it is smelted in an intermediate frequency furnace. Each steel uses the same furnace steel, half without rare earth and the other half with rare earth. The durability and creep tests were carried out on BII-2 and RD-23 test machines, respectively. The cast sample was subjected to an aging treatment at a test temperature for 24 hours, and then processed into a sample of M12 mm × 66 mm. Depth analysis of rare earth on the fracture surface (ie, crystal interface) was carried out using an ELC-3133A ion probe. Fe-Cr-Ni-AI alloy heat developed by conventional casting method, the charge used for industrial pure iron, carbon ferrochrome, nickel, iron, aluminum wire, commercially pure silicon, rare earth ferrosilicon alloy No. 1. The alloy is smelted by an intermediate frequency induction furnace; the slag material is lime and fluorite ; the molten steel is discharged at a temperature of 1550 to 1570 ° C; the blank test bar is cast by a split graphite type.

Second, the test results analysis

(1) Effect of rare earth on the properties of heat-resistant cast steel (Table 1)

Table 1 Effect of rare earth on properties of heat-resistant cast steel

Under different strain rate conditions, the tensile strength of the alloys with rare earth elements La and Ce increased significantly compared with the tensile strength of alloys without rare earth elements. The long-term strength of steel mainly depends on the structural characteristics and purity of steel, but steel with better oxidation resistance also has a good effect due to less surface ablation. In order to eliminate the influence of oxidation, we performed a 1000 °C vacuum durability test on Cr24Ni7N (RE) steel castings. The results show that the addition of rare earth in the cast steel extends the fracture time under the same stress by more than 2 times.

(II) Effect of rare earth on creep properties

Figure 1 shows the test results of 870 °C high temperature creep of cast samples of Cr24Ni7N (RE) steel. When the metal La is added to the steel, the creep rate is lowered from 1.14×10 ^-3 %/h to 3.6×10 ^-4 %/h, and the fracture time is prolonged. This is because the atomic radii of the rare earth elements La and Ce are much larger than that of Fe, and they dissolve in iron to produce a large lattice distortion energy. According to the theory of equilibrium equilibrium of solute atoms, they will be segregated on the grain boundaries, which is confirmed by our experimental results. At home and abroad, it is also found that rare earth elements are segregated on the grain boundaries of steel, and the rare earths segregated at the grain boundaries tend to occupy the vacancies and distortion regions in the grain boundaries, which may reduce the grain boundary diffusion rate of the matrix atoms. The grain boundary sliding controlled by diffusion is hindered, and grain boundary cracks are not easily formed, and grain boundaries are strengthened. The rare earth purifies the grain boundary, reduces the impurity elements of the grain boundary, improves the thermoplasticity of the steel, and causes the stress concentration at the crack tip of the grain boundary to be easily loosened by deformation, and the crack is difficult to expand, thereby prolonging the fracture life.

Fig.1 Effect of Cr24Ni7N steel on creep properties after adding RE

Third, the analysis of factors in improving the thermal strength of rare earth

High temperature fracture, especially high temperature and long-lasting fracture, is generally along the crystal fracture (in the as-cast state or along the dendrite fracture), so for heat-resistant steel, the key to the thermal strength is the grain boundary strength. We conducted a depth analysis of the fracture surface (ie, crystal interface) rare earth on the fracture surface of the Cr24Ni7NLa steel at 1000 °C using the ELC-3133A ion probe method. The results are shown in Fig. 2. As the sputtering time increases, the rare earth content away from the fracture surface (crystal interface) is significantly reduced, indicating that the rare earth is enriched in the grain boundary.

Fig. 2 Depth distribution results of Cr24Ni7NLa steel at 1000 °C vacuum permanent fracture

Development of Fe-Cr-Ni-Al heat resistant alloy steel

Fe-Cr-Ni-Al heat-resistant alloy steel is prepared by ordinary casting method through the ratio of each element alloy. Test alloy composition (%): c0.06, Cr 24, Ni 10, Al 3, Si 1.5, Re ≤ 0.5, S ≤ 0.03, P ≤ 0.03. Metallographic analysis of the alloy by electron microscopy showed that the as-cast matrix structure of the alloy consisted of two phases (ferritic ten austenite). In Figure 3, white ferrite and black austenite were observed. The two phases are distributed between the two phases, and two comparative examples and microstructure characteristics can be observed. These factors determine the high temperature mechanical properties and high temperature oxidation resistance of the alloy.

Figure 3 Alloy as-cast microstructure

Figure 4 High temperature mechanical properties of alloys with temperature

The temperature curve of the test alloy short-term tensile strength value and the section shrinkage rate is shown in Fig. 4. Each point in the figure is the average value of three samples. It can be seen from Fig. 4 that in the high temperature range of 1050~1300 °C, the short-term tensile strength value decreases linearly with the increase of temperature. At 1250 ° C, the short-term tensile strength of the alloy still reaches 40 MPa. The shrinkage ratio of the alloy section is known from the figure, peaking at 1150 ° C, and still reaching about 30% at 1250 ° C. According to the thermoplastic curve of steel, the alloy is in the austenitizing zone at 1100-1200 °C, at which time the alloy has the best plasticity and toughness; when the temperature exceeds 1250 °C, the grain grows sharply, the matrix structure deteriorates, and the grain boundary strength Lowering, most alloys exhibit a brittle fracture tendency at 1300 ° C; at less than 1100 ° C, the alloy is in a low plastic zone.

The alloy was subjected to a high temperature oxidation test at 1250 °C according to GB/T13303-91. The results are shown in Fig. 5. The oxidation weight gain rate curve of the test alloy at the high temperature has a large slope at the beginning of 200h, and the slope increases with time after the increase of 300h, and the curve becomes smooth, indicating that the alloy oxide film has high stability at high temperature, and the alloy interior is high. The matrix structure plays a good protective role. From the test data in the figure, it can be calculated that the oxidation weight growth rate of the test alloy at 1250 ° C is stable at about 0.2 g / (m ² · h), and has excellent high temperature oxidation resistance.

Figure 5 Alloy oxidation kinetics curve

V. Conclusion

Adding an appropriate amount of metal Ce or La to the steel can significantly improve the permanent strength of the heat-resistant cast steel and reduce its creep rate. When the Fe-Cr-Ni-AI heat-resistant alloy produced by ordinary cast graphite type molding method is used at 1250 °C, the short-term tensile strength of the alloy reaches 40 MPa or more, the area shrinkage rate is about 30%, and the oxidation weight-increasing rate is stable at 0.2 g/ (m ² ·h> left and right.

Molybdenum Rod

Molybdenum can increase steel`s corrosion resistance in the acid-based solutions and liquid metals, thus improve its wear resistance, hardenability, weldability, and heat resistance. As an element which easy to form carbides, molybdenum can effectively avoid oxidation during steel making process, and it can be used alone or mix with other alloy element to achieve the best performance.