Research on hot working process of large flange side shaft forgings

For the Large flange and large cross-sectional difference motor side shaft forgings, analyzed the disadvantages of the overall free forging process through the study of the forging forming mechanism and forging process, the use of tire die forging + free forging forming a combination of process solutions, so that the forgings have a better forging flow, improve the utilization of raw materials, to avoid the forging defects produced by free forging; reasonable choice of metal material thermal processing process control parameters, forging production inspection various indicators are excellent.

1. Forging main technical indicators

Metallurgical rolling mill equipment with a drive motor rotor for the three-segment shaft structure, two side shafts and hollow shaft through a large pin connection to transfer torque, motor speed of 45-100r/min, power of 6500kW, the rated speed of the working condition, the requirements of the side shaft can withstand 2.5 times the overload load. Side shaft forging materials generally use European standard C55E or Japanese standard SF590; the composition is equivalent to the national standard 55# steel and the main chemical composition: C is 0.52% -0.60%; Si ≤ 0.40%; Mn is 0.50% -0.80%; the requirements of the normalized performance: yield strength ≥ 300MPa, tensile strength ≥ 620MPa.
Side shaft forgings are shown in Figure 1; the maximum cross-section size of Φ2200mm × 315mm, the diameter of the small shaft is Φ750mm, the total length of 2160mm and the forgings belong to the large flange large cross-section difference T-shaped short shaft. Its main technical indexes:

• (1) Normalizing + tempering process after forging to ensure the mechanical properties required to detect the tangential and longitudinal mechanical properties at both ends;
• (2) Ultrasonic flaw detection requirements to meet the starting sensitivity Φ1.6mm do not allow the existence of equivalent diameter ≥ 3.0mm defects, while the bottom wave attenuation is not allowed ≥ 3dB;
• (3) Flange and shaft diameter transition R angle is a high-stress concentration area; Figure 2 shows the equivalent stress distribution under the maximum load numerical simulation. Reduce the large R angle position machining allowance as far as possible to maintain the continuity of the forging fiber flow line of the forging to extend the service life of the forging.

2. Forging process analysis

2.1 Free forging process analysis

Adopt traditional free forging process: upsetting → draw length, manufacturing difficulty and quality risk.

Figure.1 Side shaft forgings

Figure.2 Equivalent force distribution under the maximum load

(1) If the choice of small ingot production, the size of the length of the metal material is small, the quality risk of drawing a long small round, as shown in Figure 3; if the choice of large ingot, increase the size of the length of the metal material, the actual utilization of raw materials is low. Forging mass of about 2.1 × 104kg, free forging process if 3.2 × 104kg ingot, upsetting to diameter Φ2200mm, billet length of 780mm, according to Figure 3, the distribution of small round pulling out, the card table distribution length L is only 400mm, pulling long for the local upsetting, that is, pulling the length of the height-width ratio (2200/400) reached 5.5, according to the conditions of the pulling length of the L ≥ 0.3D accounting, the minimum distribution length should be ≥ 700mm. Minimum length of metal material should be ≥ 700mm. At this time, the heart of the billet cannot bulge out, resulting in shrinkage folding defects (see Figure 3); at the same time, the ingot water mouth deposition heap cannot be effectively extruded, resulting in forging dense defects in flaws equivalent to exceed the standard for scrapping, so the use of free forging process must be produced with a larger ingot, the raw material utilization rate is reduced to 40%-50%, the actual economic benefits of people experiencing poverty.

Figure.3 Free forging elongation defects
(2) In the transition corner of a large number of discharges, the actual size of forgings is beyond the process requirements on the deviation. Because of the flange and shaft diameter step difference, free forging process card table after the distribution of metal material, drawing long forging small round, step transition by tensile stress, a large number of billets piled up to the flange large transition angle position, and because of the deformation is not uniform, the flange inside the end of the uneven surface, affecting the size of the subsequent processing, the need to increase the length of the machining allowance, resulting in the forging blanks of the actual quality of the process beyond the requirements of the more than 20%, the poor economic efficiency.
(3) The shape of the forgings is difficult to control. Free forging draws long small circles; if the operation is not appropriate, small circles and flanges easy to produce serious eccentricity, resulting in subsequent processing needs to be many times in the direction of the diameter to find the right, and even scrapped due to failure to meet the size requirements.
(4) If the small circle cannot complete forging in heating, needs to return to the furnace heating, the diameter of the flange does not have a trim margin, repeated furnace heating caused by the flange part of the grain coarsening, ultrasonic flaw detection in the location of the grain boundaries to produce a large number of echoes reflections, cannot reach the beginning of the ultrasonic flaw detection sensitivity and the bottom of the wave to reduce the number of requirements due to the organization and the grain caused by the coarse echo interference leads to the inability to distinguish between defective waves.

2.2 Tire die forging process analysis

Shaft parts free forging process is the forging process of the outer circle step by big to small section by section; the flange part of the forging flow is axial-based, machining, the inside of the flange machining excision is larger and the forging flow is cut off, forging the continuity of the fiber is destroyed. The use of the die forging process forms the flange part not only in the billet drawing process produced by the axial flow line but also in the subsequent die forging process produced by the circumferential flow line. Therefore, the flange part of the performance and organization of the isotropy is significantly better than the free forging forming process; furthermore, the flange inside of the amount of excision is less than the free forging, so the processing can be retained in the flange part of the continuity of the forging fibers, compared with the free forging fiber flow line forming More complete.

3. Optimization of process solutions

Through the above analysis, forgings to be used “free forging + tire die forging” composite forging forming technology can effectively improve the surface quality of the forging blanks and obtain a complete forging fiber flow to avoid the defects that occur in the process of free forging draw length, the overall thermal processing technology program: risers clamp handles → ingot upsetting → wide anvil draw length of the main deformation → rounding Drawing long, billet making, removing risers → head upsetting and rounding → trimming small round, out of the finished product → after forging (performance) heat treatment, as shown in Table 1. The key process control points of heat processing are as follows.

• (1) Ingot smelting. 55# steel chemical composition to add 0.03% of trace elements Nb, reduce the sensitivity of the steel overheating, increase the forging temperature, the generation of highly dispersed carbide NbC, prevent grain growth, and improve the forging after forging heat treatment furnace temperature. Ingot smelting process: EBT eccentric bottom out steel electric arc furnace primary steel refining → LF furnace refining → VD vacuum treatment degassing, improve the purity of steel → VC vacuum casting.
• (2) Ingot upsetting, wide anvil draw length. The forging process of large forgings includes denaturation and deformation of two parts, of which the first fire upsetting and pulling long main deformation belongs to the process of denaturation, is the key control process of forging, upsetting forging ratio should be ≥ 2, the purpose is to break the ingot in the casting state of the organization; upsetting after the use of a wide anvil pulling a long strong compression method, the requirements of pulling a long forging ratio of > 2.0, process control pulling a long anvil width ratio of 0.5-0.8, the amount of pressure down for the pressure of the former height of 20%, to ensure that the billet heart is always in the heart of the billet, the wide anvil pulling a long time. Ensure that the heart of the billet is always in a three-way compressive stress state. To achieve the purpose of forging through compaction, the drawing process requires 2 consecutive hammer pressures between the anvil width of 10% of the lap to prevent deformation of the dead center and avoid leakage of pressure.
• (3) Rounding and drawing blank making. Billet making process is to ensure that the forgings get the ideal shape of the key link; the flange part of the billet should ensure that the flange size is suitable after die forging; the small axis should be avoided in the making of billet on the drawing length to the required size, due to upsetting is carried out at high temperature, the small axis should be left with a certain amount of deformation, the use of residual forging ratio and forging dynamic recrystallization of the principle of the last trimming the small axis size to the requirements of the process, you can get the fine after forging grain organization.
• (4) Head upsetting and rounding. Upsetting should be completed at a high temperature, should pay attention to controlling the upsetting rate, reduce the crack produced by the outer circle tensile stress, upsetting half height, take the local upsetting, spinning method, around the first spinning a circle and then upsetting the center position, the last upsetting, due to the constraint of the outer circle, so that the center of the billet has high hydrostatic pressure, the heart of the compacting effect is good. During the upsetting process, pay attention to observing whether the forging is always in the center position of the mold to avoid upsetting bias.
• (5) Trimming a small round out of the finished product. After the forging is released from the mold, due to the small circle wrapped in the mold still maintaining a high forging temperature, clamping the flange and trimming the small circle to the required size of the forging under high-temperature conditions.
• (6) Final forging temperature control. 55# steel phase transition temperature point A_c1 about 720 °C, A_c3 about 760 °C, the final forging temperature selection of 750 °C, reduce the final forging temperature is conducive to obtaining more fine ferrite grains and organization, forging and other accelerated cooling measures, so that forging to a faster cooling rate through the A₁-A₃ zone, the outer circle cooled to 400-450 °C, the heart to obtain a uniform pearlitic + ferrite organization. Forging size, heart diameter Φ300mm range of slow cooling speed, if a long time in the high temperature stay, the grain boundaries of a large number of precipitation block ferrite, the formation of coarse ferrite mesh organization, resulting in ultrasonic flaw detection of the bottom wave attenuation, cannot meet the requirements of the starting sensitivity.
• (7) Forging heat treatment process. The austenitizing heating temperature should ensure mechanical properties and grain refinement, process selection heating temperature of 790-830 °C, normalizing after taking blast cooling to room temperature, accelerate the cooling rate to reduce ferrite precipitation, grow up, increase the pearlite content and improve the mechanical properties of the strength index, tempering heating temperature of 540-570 °C, to obtain the matching strength and toughness index.

4. Tire mold design

According to the shape of the forging, the tire mold forming die shown in Fig. 4(a) was designed, and Forge software was used to simulate and analyze the forging of the flange. The die forging pressure curve is shown in Fig. 5. When forging is finished, the billet is not filled with the cavity, there is a void in the ellipse area shown in Fig. 6; the analysis may be due to the temperature of the billet and the mold parts contact surface decreases fast, the local temperature is lower than 750 ℃, and the lowest temperature reaches 475 ℃, resulting in these areas of reduced plasticity, the metal flow is difficult, and at the same time, excess billet is squeezed between the mold parts and the upsetting cover. The peripheral constraints on the mold parts also lead to a linear increase in forging force. Figure 4 (a) shows forming die on the flange size limit is too strict due to the forging billet using manual scribing, chopping knife cutting or acetylene gas cutting under the material; under the material accuracy is poor, under the material size is small cannot be full of billet mold, under the material size is large will be the excess material will be extruded to the outer edge of the tire mold appeared 2 steps, in addition, in the process of forming the flange mold there is a large expansion of the shape of the force, easy to cause cracking and scrapping. Due to the outer circle step binding, forgings are not easy to come off the mold. The forging is not easy to be demolded due to the binding of the outer round step. The optimized die structure shown in Fig. 4(b) is simple in design, with a good flange forming effect, small expansion force and large cone angle for easy demolding during forming and long service life. Figure 7 shows the forging flow line calculated by simulated forging.

5. Production practice

According to the above technology program for the production of forgings, shown in Figure 8 for the side shaft forging blanks in kind, forged flange end face regular, flange and small circle without eccentricity, flange large beveled transition has a large profiling angle. Inside the flange part of the transverse and longitudinal mechanical properties of the sample, performance measured values shown in Table 2, transverse and longitudinal performance without significant fluctuations. The ultrasonic flaw detection of the forging meets the starting sensitivity Φ1.6mm, and no coarse crystal wave is found.
Table.1 Design of forging process of side shaft forging piece

Heating frequency	Process	Illustration	Forging ratio
1	Riser clamp handle		–
2	Steel ingot upsetting		Upsetting ratio 2.0
	Wide anvil elongation		Ratio 2.9
3	Round drawing, billet making, and riser removal		–
4	Head sensitivity and roundness		Upsetting ratio 2.1
	Trim small circles and produce finished products		–

Fig.4 Optimization scheme of tire die design

Fig.5 Pressure curve of die forging

Fig.6 Forging simulation

Fig.7 Forging flow line calculated by simulation

Figure.8 Forging blanks in kind

Table.2 Measured mechanical properties of forging parts

Performance		Rp0.2/MPa	Rm/MPa	A/%	Z/%	Kv2/J
Flange	Horizontal	360	650	28	51	23.8\23.2\23.2
		357	647	25.5	47
	Vertical	368	648	26	50	–
		366	658	22	49	–
		354	654	25.5	46	–
		353	654	26.5	48	–

6. Conclusion

In view of the shape characteristics of the large flange side shaft forgings, proposed a combination of free forging + die forging process plan, the use of computer simulation software to simulate the flange forming process, the reasonable optimization of the die structure to obtain the ideal forging flow line. The new forging process plan can effectively ensure the size, appearance and internal quality of the forgings while reducing the gross to net ratio and improving the utilization rate of raw materials for the forgings, which has achieved better economic benefits.
Author: Qin Hongfu