By adopting a more refined tempering process and improving the furnace charging, layering, and increasing the auxiliary support points, we can meet the requirements of high hardness and high uniformity of large ring forgings and effectively reduce the deformation.
Our company has often undertaken the quenching and heat treatment of large ring forgings. This ring type is a key component of a large mining crusher used for crushing ores in mines. The main structure of the machine has a ring-shaped track and a ring-shaped track with a ring-type grinding plate, and the grinding plate is composed of several grinding rollers. This type of ring is a ring-shaped track in the machine, which requires highly comprehensive mechanical properties and accurate dimensional accuracy. These large ring parts’ large diameter and thin thickness are easily deformed during processing and cannot be corrected after deformation. The hardness requirement is high, and the uniformity is challenging to control. To solve these problems, this paper will improve the process and production operation and strictly control it to improve the overall quality of the workpiece.
Introduction of the workpiece
This kind of ring forgings are divided into two kinds, the first for the ring rail, material 40CrNi2Mo, maximum size Ø4572/4252mm × 100mm, single weight 3900kg; the second for the tooth ring, material 34CrNi3Mo, maximum size Ø5232/4660mm × 140mm, single weight 6000kg.
- Technical requirements: hardness of 320-360HBW, 340-380HBW respectively, uniformity not more than 30HBW.
- Deformation requirements: after tempering, the ring rail does not exceed 30mm (single side margin of 20mm), and the tooth ring does not exceed 20mm (single side margin of 15mm).
Surface quality: no cracks and other defects. The actual chemical composition of the workpiece is shown in Table 1.
2. Process development
According to the material’s solid phase change point Ac1 and Ac3 and the performance requirements of the material, the selected heat treatment temperature of the material is T1, and the tempering temperature is T2. The heating process considers the characteristics of the workpiece and controls the speed of heating to have into the furnace insulation and intermediate insulation process to reduce the stress of the quenching process and minimize the generation of deformation. Quenching and holding time is selected, according to the effective cross-section of each millimeter insulation 1 – 1.3min, the appropriate extension of the average temperature, and the total quenching and holding time is controlled within 4h. The specific process is shown in Figure 1.
Table.1 Chemical composition (mass fraction, %)
3. Quenching treatment
3.1 Production operation
First, inspect the surface of the incoming parts for obvious defects such as sand holes and cracks, and check whether there are sharp corners and whether the angles need to be rounded. Accurately measure the size of each piece and check the drawing to calculate the size of the margin.
The maximum diameter of the ring rail is 4572mm, the effective thickness is 100mm, the maximum diameter of the tooth ring is 5232mm, and the effective thickness is 140mm, both of which are large-size thin plate workpieces. It is not easy to quench the high hardness of this shape in the same material, and the effective thickness needs to be thicker and easier to deform.
The company has a quenching furnace with a maximum heating zone of 5000mm, and the workpiece diameter is so large that the upper limit of the furnace body is reached, and the pallet cannot be used. According to past experience, the company made a batch of new pads suitable for easy lifting to achieve better flatness. The pads are placed parallel, and the spacing refers to the distance between the ring pieces after four equal divisions.
Figure.1 Process curve
I-beam and 4 horseshoe chains are used for the spreader, and the horseshoe-shaped buckle at the end is made according to the size and shape of the mat end. The pieces need to be spaced out evenly with small pads, 8 points per piece; 80 pads are prepared in advance, with the same specifications and height. The key parts are easily deformed during furnace loading to increase the auxiliary support points.
3.2 Test results
The hardness test result is shown in Table 2, and the deformation test result is shown in Table 3; from the test result, the hardness is qualified and uniform and meets the customer’s requirement. The deformation of the 100mm thick section is 22mm, and the 140mm section is 18mm; However, within the margin, the deformation is large, which causes inconvenience to the customer’s subsequent processing. After a comprehensive study, the furnace loading method was improved to reduce deformation further.
Table.2 Hardness test results
| Material | HBW (detection at 90 ° intervals) | ||
| 40CrNi2 Mo |
340,345,330,320
330,345,340,325
350,355,345,330
|
||
| 34CrNi3 Mo |
350,355,365,340
370,360,365,345
350,355,360,375
|
||
Table.3 Deformation test results (mm)
| Material | Incoming parts | After tempering | Diameter direction | Vertical direction |
| 40CrNi2 Mo |
4572,4572,4572, 5230,5230,5230 |
4590,4584,4381 5240,5241,5242 |
+18,+12,+9 +10,+11,+12 |
+ 16,+20,+22 + 18,+11,+12 |
| 34CrNi3 Mo |
Table.4 Deformation testing results after improving the production method (mm)
| Incoming parts | After tempering | Diameter direction | Vertical direction |
| 4644,4644,4628,4628 | 4640,4649,4625,4635 | +4,+5,+3,+7 | +6,+10,+ 12,+9 |
4. Improvement of production method
Summarizing the production experience, we found that the furnace loading method has a greater impact on the deformation, mainly due to the uneven force. The sinking phenomenon will occur easily if the distance between the two pads is large. Increase the force support point with the bottom plate of the furnace so that the force is more evenly dispersed. So double pads loaded furnace, support points from the original 8 increased to 12. At the same time, increase the distance between the bottom plate and the furnace, effectively avoid the location of the burner, increase the temperature uniformity, and effectively avoid the risk of local temperature overheating.
Cooling oil temperature should be controlled below 50 ℃; the highest quenching oil temperature should not exceed 80 ℃. Cooling oil tank specifications for 15m × 6.5m × 6m, with oil temperature monitoring circulating cooling system. Pay attention to smooth lifting, rapid operation, smooth oil in the shortest possible time, and a crane with tandem cooling to improve the uniformity of hardness. Control the oil cooling time, master the final cooling temperature, and keep warm after the oil is released as soon as possible. Increase the intermediate inspection to detect the hardness after quenching to provide an accurate reference for tempering. Deformation test results are shown in Table 4.
Compared with the single-layer charging method, the deformation is significantly reduced. The improvement effect is good for the maximum deformation of the diameter direction from 18mm to 7mm, and the maximum vertical direction from 22mm to 12mm.
5. Conclusion
- (1) Large ring forgings are easily deformed in the process, and the thinner the effective thickness, the greater the deformation. Slow heating and holding in stages can reduce thermal stress and increase temperature uniformity.
- (2) The furnace loading and operation process control is also very important to the deformation of ring parts and the final quality. Double pads are placed, and multiple points are evenly padded and stabilized; the effective support points between the bottom plate and the furnace are increased, and the distance between the ring and the burner is raised; the lifting process is smooth and rapid, which can effectively control the deformation of large ring forgings.
Author: Li Xiaohong, Yang Di, Huang Shouji
