The trial production process primarily involves smelting and pouring. Key steps include cleaning the solution pool and adding 0.90% of the material, followed by high-temperature decarburization at 1580°C during the oxidation period, which must be no less than 0.30% to ensure full degassing and removal of inclusions. Pre-deoxidation is carried out using Al (0.5 kg/Pt), with the C powder white slag method used for reduction. The composition before tapping is adjusted to a carbon content of C0.
Macroscopically coarse-grained 20Cr1Mo1VNbTiB forging material often exhibits a coarse grain structure, typically due to excessive heating cycles or overly high processing temperatures. To address this, a fired material is employed to eliminate the macroscopic coarsening. Cracking was observed in nine steel ingots after forming, with cracks reaching depths of up to 20 mm. Each ingot had only one crack. Metallographic analysis revealed that the crack tips were not expanded or passivated, with a decarburization depth of 0.11 mm. Additionally, some red ingots showed cracks on one side before rolling, suggesting that the cracking originated from longitudinal cracks in the ingot itself. These cracks are likely caused by improper annealing or excessive soaking time, requiring further investigation.
Tempering temperature significantly affects mechanical properties such as Rb, Rs, D5, and W. Within the range of 400–600°C, Rb and Rs increase with rising temperature, but drop sharply beyond 600°C. D5 remains relatively stable (14–15%) between 400–650°C, then increases significantly between 650–750°C. W shows a gradual rise with temperature. Below 400°C, tempering leads to a decrease in Rb and Rs, while D5 increases. At 300°C, the microstructure becomes more uniform, with strength loss mainly attributed to reduced internal stress.
For a tempering structure of 1020°C oil-cooled + 300°C for 6 hours air-cooled, Rb and Rs increase within the 400–600°C range, primarily due to secondary hardening of elements like Cr, Mo, and Nb. Secondary hardening occurs when alloy carbides form along dislocation lines in martensite at specific temperatures, precipitating in a dispersed manner and maintaining coherence with the parent phase, thereby enhancing hardness.
According to data <4>, molybdenum-containing steels show significant secondary hardening above 500°C, with vanadium exhibiting a pronounced effect, peaking after 5 hours of tempering at 600°C. This explains why Rb and Rs increase with temperature in the 400–600°C range, mainly due to secondary hardening of Mo, V, and Nb. After 550°C tempering for 6 hours, visible precipitation of alloy carbides is evident.
For a tempering structure of 1020°C oil-cooled + 550°C for 6 hours air-cooled, and another at 700°C for 6 hours, the granular carbides continue to aggregate and grow with increasing temperature. The needle-like ferrite morphology disappears, and carbides are distributed on a polygonal ferrite matrix, forming tempered sorbite, which offers excellent strength, ductility, elasticity, and toughness. After 700°C tempering for 6 hours, the needle-shaped ferrite is no longer visible, and carbides have grown significantly. In the annealed state, the microstructure is stable, with D5 remaining consistent within the acceptable technical range. Therefore, for comprehensive mechanical property requirements, a tempering temperature around 700°C is ideal.
In conclusion, the trial steel meets the technical specifications. The heat treatment process of 1020°C oil cooling + 700–720°C for 4–6 hours air cooling yields the best overall performance. A low Nb recovery rate is attributed to the high Nb content in NbFe, which raises its melting point too high to be fully melted. Effective degassing and deoxidation before adding boron (B) is crucial for B recycling.
Ultrasonic Level Gauge,Ultrasonic Tank Gauge,Ultrasonic Water Tank Level Meter,Ultrasonic Liquid Level Gauge
Jiangsu Pinpai Technology Co., Ltd. , https://www.jspingpa.com