Research on the Production Process of Large Shaft Forgings

Large forgings are the core components of heavy equipment, the quality of which directly affects the stability of equipment operation. The manufacturing technology of large forgings is an important indicator to measure the development level of the national heavy equipment manufacturing industry. In this paper, we mainly design the forging process of large shaft forgings and develop a reasonable plan to ensure the success of product development.

Figure 1 shows a large shaft forging, the quality of which is more than 100t. Due to its large size and mass, complex flow field inside the die, and long solidification time, the forging has defects such as composition segregation and uneven grain size, as well as non-metallic inclusions and microscopic voids; at the same time, due to the tissue heredity, the banded tissue cannot be eliminated by heat treatment; forging requires repeated and long time heating and multiple deformations, which makes the forging defects and tissue evolution law more complicated. In this study, the forging and heat treatment processes are designed to ensure that the performance of the shaft forgings meets the requirements of use.

Figure.1 Forging diagram (mm)

1. Experimental scheme

The production process of the large shaft forgings is smelting → forging → post-forging heat treatment → sampling, and testing. A reasonable heat treatment process plan was developed by reviewing the literature and combining it with the actual production experience. A tempering temperature of more than 500°C and axial samples was taken 75mm from the surface. The steel belongs to the essence of coarse crystalline steel; the forging diameter is large and needs to refine the grain through heat treatment.

1.1 Chemical composition control

Because the ingot is relatively large, to reduce the composition of segregation, to ensure the consistency of the chemical composition of the bottom and riser end, to control non-metallic inclusions, the use of double vacuum pouring, two packages together when pouring C, Mn elements for anti-bias control. The composition is controlled according to the requirements of Table 1. SPECTROLabM12 direct reading spectrometer was used to analyze the specimens’ chemical composition. The content of O, N, and H elements was analyzed by NAK Oxygen, Nitrogen, and Hydrogen analyzer, and the results are shown in Table 1.

Table.1 Chemical composition analysis results of the forgings (mass fraction, %)

Element	C	Si	Mn	P	S	Cr	Ni
Target Value	0.42-0.44	1.30-1.50	0.25-0.35	≤0.012	≤0.010	≤0.20	0.35-0.45
Measured Value	0.428	1.46	0.3	0.01	0.0008	0.18	0.38
Element	Mo	Cu	Al	V	N	O	H
Target Value	≤0.15	≤0.10	0.015-0.025	0.02-0.05	≤0.0070	≤0.0040	≤0.0002
Measured Value	0.13	0.08	0.018	0.034	0.0048	0.0032	0.00012

1.2 Forging and heat treatment process

Due to the large size and weight of the large shaft forgings, the forging process route is upper jaw (back to the furnace) → repeated upsetting and drawing (back to the furnace) → forging out the finished product, forging temperature is 1250-750 ℃, forging ratio is 6.8, the upsetting process does not allow the leakage of pressure area.

To cut off and stop the tissue genetic effect and reduce the grain coarseness and the chance of mixed crystal, we need to develop a suitable heating rate, forging holding time, and cooling system; the specific heat treatment process curve is shown in Figure 2.

Figure.2 Test heat treatment process

2. Test results analysis

2.1 Composition control

To ensure that the chemical composition of the steel is in line with the smelting requirements is the main content of large forgings with ingot smelting, the need to minimize the phosphorus, sulfur, non-metallic inclusions, and gas content. Ingot internal defects are mainly inclusions, porosity, segregation, shrinkage and loosening, and the distribution of various defects in different locations. With the increase of ingot size, the uneven distribution of chemical composition, organization, and inclusions inside the ingot becomes increasingly serious, which becomes the main reason for a scrap of large forgings. In this study, the ingot is smelted by double vacuum pouring, and the C and Mn elements are controlled by anti-bias when the two packages are combined.

2.2 Mechanical properties analysis

From the forging end of the mechanical properties of the specimen, using the INSTRON 300kN universal material testing machine and Brinell hardness tester were mechanical properties of the test; the results are shown in Table 2. It can be seen that the mechanical properties of specimens’ tensile strength, yield strength elongation after the break, sectional shrinkage, and hardness are within the target range.

Table.2 Mechanical properties testing results of forgings

Name	Yield strength ReH/MPa	Tensile strength Rm/Mpa	Elongation after fracture A(%)	Reduction of area Z (%)	Hardness (HB)
Target	≥350	550-700	≥21	≥45	≥200
Measured Value	376	658	27	49	213

2.3 Microstructure analysis

For large forgings, tissue inheritance is the main cause of coarse grains and the use of a ZEISS metallurgical microscope for metallographic analysis of forging specimens before and after the process improvement. Figure 3 shows the results of the metallurgical analysis of forgings before the process improvement. Figure 3 (a) is the normal ferrite + pearlite organization, with an average grain size of 7, and grain size is more uniform; Figure 3 (b) is the microstructure of the mixed crystal area; it can be seen that the mixed crystal area grain size is different, part of the grain is coarse, the average grain size is about 4, and the average grain size of smaller grains is about 7. Figure 4 shows the results of the metallographic analysis of the forgings after a process improvement. The forging is normalized and tempered, and the organization is ferrite + pearlite. The grain size of the forging is evenly distributed in different positions, and the grain size is 7-8 grades, which can meet the use requirements.

Figure.3 Metallographic organization of forgings before process improvement

Figure.4 Metallographic organization of forgings in different positions after process improvement

The forging process of large forgings usually has a strong tendency of tissue inheritance; that is, when the coarse original austenite grains are austenitized again after cooling, the new austenite will inherit the original grain size and orientation, which leads to the coarse and inhomogeneous grains in large forgings are difficult to refine and homogenize. This study aims to refine and homogenize the coarse grains and mixed grains caused by forging, eliminate the residual stresses after forging, improve the uniformity of the internal composition and organization, and ensure the excellent mechanical properties of the forgings.

Normalizing temperature ≥ 880 ℃, the heating temperature is located in the single-phase austenite region, the organization to achieve complete austenitization, high-temperature normalizing makes austenite recrystallization, α → γ phase transition in the steel volume expansion caused by large internal stresses, and the heating process of the combined effect of thermal stress so that the steel “internal hardening,” to provide the driving force for the recovery and recrystallization This provides the driving force for recovery and recrystallization. The recrystallized grains have a new orientation and no fixed orientation relationship with the original coarse grains, so the growth process does not merge, which can effectively cut off the tissue inheritance. As shown in Figure 4, no needle-like pearlite or massive ferrite has not been recrystallized in the tissue.

In the austenitization process, lamellar austenite inherited the original austenite orientation along the lath boundary residual austenite formation. The α-phase with which the co-lattice relationship becomes a wide space for the rapid growth of lamellar austenite, so that lamellar austenite in just over the Ac1 temperature quickly generated and grown, and when they meet each other into the original austenite grain size, shape and orientation of the same coarse austenite grains, the occurrence of the tissue inheritance occurred. And through high-temperature tempering treatment can break down the residual austenite organization to achieve the purpose of grain refinement. Figure 4 shows that after normalizing + tempering heat treatment of the shaft forgings, the forging edge and heart organization is ferrite + pearlite organization, the grain size is uniform, and the grain size is about 7.

3. Conclusion

• (1) The size of the shaft forgings is large, and the material used is carbon manganese, essential coarse crystalline steel; through the composition control and design of a good heat treatment process, the chemical composition and mechanical properties of the products meet the requirements.
• (2) After normalizing and tempering treatment, the forgings are ferrite + pearlite, the average grain size is 7-8, and the grain size is more uniform in different positions, which solves the problem of coarse grain size.

Author: Gao Dongsheng