Heat Treatment & Mechanical Properties of ASTM A350 LF3 Flanges (Part Two)

3. Process tests
The flange forging deformation must be sufficient; the total forging ratio should be greater than and equal to 6.0, and the final forging temperature should not be lower than 850℃. Rough machining should be performed after forging. According to literature reports, ASTM A350 LF3 steel forgings can obtain mechanical properties that meet standard requirements by normalizing and tempering. According to the chemical composition of ASTM A350 LF3 steel, in order to obtain stable low-temperature impact properties of the flange, it is more reasonable to adopt the process of normalizing after forging, rough turning, quenching and tempering. The preliminarily determined heat treatment process of ASTM A350 LF3 steel flange forgings is shown in Figure 2.
  
To verify the effectiveness of the above heat treatment process, test blocks I and II were made separately, with a cross-sectional size of 80 mm×80 mm. After forging, they were air-cooled, then normalized, and then quenched and tempered (water quenched) according to the process shown in Figure 2. The mechanical properties are listed in Table 3, and the microstructure is shown in Figure 3.
 
 
Figure 2 Heat treatment of ASTM A350 LF3 steel flange forgings determined for the first time
 
From Table 3, it can be seen that the tensile strength and yield strength of the test block are much higher than the standard requirements. Although the elongation after fracture and low-temperature impact toughness meet the standard requirements, the elongation after fracture is just qualified. Therefore, the quenching and tempering process of test block II was adjusted, as shown in Figure 4. The properties after quenching and tempering are listed in Table 4, and the microstructure is shown in Figure 5. From Table 4, it can be seen that the mechanical properties of test block II treated by the adjusted process meet the requirements; the elongation after fracture is significantly improved, and the microstructure is uniform tempered troostite.
 

Figure 3 Microstructure of ASTM A350 LF3 steel specimen I after quenching at 880℃ and tempering at 650℃
  
Table 3 Mechanical properties of ASTM A350 LF3 steel specimen I after quenching at 880℃ and tempering at 650℃

 
Table 4 Mechanical properties of ASTM A350 LF3 steel specimen II after quenching at 860℃ and tempering at 670℃

 
 
Figure 4 Quenching and tempering for the ASTM A350LF3 steel test block Ⅱ
 
4. Heat treatment of flanges
According to the test results of the heat treatment of the test block, the two flanges were quenched and tempered according to the adjusted process. The blind flange has smaller sizes and is stacked in multiple layers with pads between layers to ensure uniform heating of the workpiece. The butt-weld flange is loaded in a furnace in a multi-layer staggered arrangement with pads between each layer. The mechanical properties of the flange after quenching and tempering are shown in Table 5. It can be seen from Table 5 that the mechanical properties of the flange meet the technical requirements, indicating that the adjusted heat treatment is feasible. Table 5 shows that the low-temperature impact performance of the blind flange is better than that of the butt-weld flange, which may be related to the different forging processes. The blind flange is free forged, while the butt-weld flange is die forged, with a greater forging ratio and more complete forging.
 
 
Figure 5 Microstructure of ASTM A350 LF3 steel specimen II quenched at 860℃ and tempered at 670℃
 
Table 5 Mechanical properties of ASTM A350 LF3 steel flange quenched at 860℃ and tempered at 670℃

 

5. Conclusion

(1) The mechanical properties of ASTM A350 LF3 steel flange quenched at 900℃, water quenched at 860℃ and tempered at 670℃ were significantly improved. The tensile properties and -101℃ impact properties of LF3 steel blind flanges and butt-welded flanges meet the technical requirements.
(2) For parts made from the same batch of materials, the forging ratio has better low-temperature impact properties after being quenched and tempered by the same process.