Research on the Manufacturing Process and Various Properties of Red Ring Forgings

Research on the Manufacturing Process and Various Properties of Red Ring Forgings

This article elaborates on the red sleeve ring forgings manufacturing process for the high-pressure inner cylinder of ultra-supercritical steam turbines. The tensile properties, impact properties, grain size, inclusions, and other properties of the forgings in different parts were measured through anatomical tests. The results show that the manufacturing process of the forging is complete and reliable, and the various properties of the forging are excellent, especially the uniformity is very good, achieving the goal of localization.

0. Introduction

When the main steam pressure of the ultra supercritical steam turbine reaches over 28MPa, the traditional flange structure can no longer meet the sealing load requirements for the split surface of the high-pressure inner cylinder. ABB in Switzerland and Alstom in France have produced steam turbines with a red ring cylindrical high-pressure inner cylinder structure since 1988 and 2000, respectively. After adopting a symmetrical cylindrical structure, the red ring high-pressure inner cylinder has a uniform temperature field distribution and low thermal stress, making it easy to start and change working conditions quickly. Since 2013, main domestic engine manufacturers have gradually adopted a red collar structure for ultra supercritical units with a main steam pressure 28MPa. The steam turbines produced by ABB, Alstom, and Beizhong Alstom (Beijing) Electric Equipment Co., Ltd., which use the high-pressure cylinder with a red ring, have been put into operation in Huaneng Shidongkou No.2 Power Plant, Hubei Dabie Mountains Power Plant, Guodian Baoji Power Generation Co., Ltd., and other power plants. The 660MW ultra-supercritical steam turbine of Harbin Turbine Works Co., Ltd. adopts a red ring structure and has also been implemented in power plants such as Zouping.
The operating conditions of the red collar determine that its materials must have excellent high-temperature performance, especially with high requirements for the uniformity of material structure and performance. Therefore, the red collar is generally made of 9% -12% Cr ferrite heat-resistant steel material with excellent high-temperature performance, and the product form is rolled ring forgings. Ring rolling is a plastic processing process that utilizes a ring rolling machine to produce continuous local plastic deformation of the ring, thereby achieving wall thickness reduction, diameter expansion, and cross-sectional contour forming. Its biggest advantage is the excellent microstructure and properties of forgings.
This article discusses the manufacturing process of red ring forgings produced by a domestic forging manufacturer, including smelting, forging, ring rolling, heat treatment, etc. Through anatomical experiments, various indicators such as room temperature and high temperature tensile properties, impact properties, grain size, and inclusions were measured at different positions of the forging. The forging has excellent performance and uniformity, fully meeting the usage requirements and achieving the goal of localization of the forging.

1. Manufacturing process of Ring Forgings

1.1 Raw materials

The material used for the red collar forging is 23Cr12Mo1V. This steel belongs to 9% -12% Cr ferrite heat-resistant steel. It is widely used in high-temperature components such as steam turbine red collars, fasteners, and blades, with excellent high-temperature performance.
The smelting process of materials is electric furnace + ladle refining + Electro-slag remelting.

1.2 Manufacturing process flow of ring forgings

The production process of red collar forgings is as follows:

Raw material re-inspection → cutting and cutting → billet making → ring rolling → annealing → positive tempering → rough turning → ultrasonic testing → quenching and tempering → mechanical properties and metallographic structure testing → rough turning → stress relief treatment → ultrasonic testing.

1.3 Key Process Processes

1.3.1 Forging (blooming)

Adopting a natural gas box type heating furnace, the heating temperature of the billet is 1100 ℃, and the insulation time is not less than 1.5 hours; The press adopts a 2500t hydraulic press, and the final forging temperature is not less than 850 ℃. Strictly control the temperature of the billet during the forging process, and take certain insulation measures to minimize the heating frequency as much as possible.

1.3.2 Grinding ring

Adopting a natural gas box type heating furnace, the heating temperature of the billet is 1100 ℃, and the insulation time is not less than 1.5 hours; Using a 2500mm ring rolling machine, the final forging temperature shall not be lower than 950 ℃. The forging ratio is greater than 4:1. After the rolling is completed, it is slowly cooled, followed by annealing treatment at 730 ℃ -780 ℃, and then cooled in the furnace.

1.3.3 Heat treatment

Heat treatment is mainly divided into two parts:

• (1) Positive tempering: The main purpose is to homogenize the microstructure of the forging and prepare for subsequent testing and tempering;
• (2) Quenching and tempering: Quenching+2 tempering cycles aim to achieve the required mechanical properties of the forging.

The tempering process is as follows:

• Quenching: 1020 ℃ -1070 ℃; 1st tempering: 660-760 ℃; Second tempering: 650 ℃ -730 ℃.
• Especially for 9% -12% Cr heat-resistant steel, the higher the tempering temperature, the more stable the structure. Therefore, while ensuring the mechanical properties of the forging, the tempering temperature should be increased as much as possible.

2. Anatomical performance test

2.1 Performance requirements for forgings

The performance requirements for forgings are shown in Table 1.

2.2 Sampling location and testing content

The sampling location and testing content are shown in Table 2.

2.3 Room temperature tensile properties

Figure 1 shows the tensile properties of different orientation positions, which can be seen as follows:
(1) The yield strength is relatively uniform and has a high margin;
Table.1 Exporting Forging Performance Requirements

Sampling Direction	Rp0.2 N/mm2	Rm N/mm2	A/%	Z/%	KV2/J
Tangential Direction	≥700	900-1050	≥13	Actual measurement	≥20
Portrait	≥700	900-1050	≥14	Actual measurement	≥27
Radial	≥700	900-1050	≥14	Actual measurement	≥27

Table.2 Sampling Location and Testing Content

Sampling Direction	Sampling Location	Test content (coding)
Tangential Direction	0°	Room temperature stretching (T)
Portrait	90°	High temperature 500 ℃ tensile test block (T1H)
Radial	180°	High temperature 600 ℃ stretching (T2H)
	270°	Impact (KV)
		Grain size (GS)
		Non metallic inclusions (MI)

(2) The tensile strength meets the standard requirements but generally approaches the lower limit.
Figure 2 shows the elongation at different orientation positions, and it can be seen that the elongation at all orientation positions is high and relatively uniform.
Figure 3 shows the impact performance of different orientation positions. It can be seen that the impact performance of all orientation positions has good richness and is relatively uniform.

Figure.1 Tensile property at different orientation positions

Figure.2 Elongation at Different Orientation Positions

Figure.3 Impact performance of different orientation positions

2.4 High temperature tensile properties

The high-temperature tensile performance reflects heat-resistant steel’s most basic thermal strength and is the basis for studying high-temperature creep and endurance properties. The high-temperature instantaneous tensile performance is a common basic indicator in the design of high-temperature load-bearing components. It is an important basis for determining the service temperature of high-temperature materials.
Figure 4 shows the high-temperature tensile properties of different sampling positions, indicating that the high-temperature tensile strength of the forging is very uniform.

Figure.4 High-temperature tensile properties at different sampling positions

2.5 High and low magnification organization

2.5.1 Grain size

Organization determines performance, and uniform organization ensures uniform performance. The grain size inspection results at different positions of the red ring forging are shown in Figure 5. It can be seen that the grain size at different positions meets the standard’s requirements and is very small and uniform.

Figure.5 Grain size at different sampling positions

2.5.2 Non-metallic inclusions

As the steel used in forging is smelted by ladle refining and Electro-slag remelting process, the non-metallic inclusions in the steel are very few. It can be seen from Table 3 and Figure 6 that there are no other types of inclusions except a very small amount of oxide non-metallic inclusions.

2.5.3 Macrostructure

Similarly, since the steel for forgings is smelted by ladle refining and Electro-slag remelting process, the degree of steel segregation is very small. As seen from Table 4 and Figure 7, except for a small amount of general porosity, there is no other type of porosity.
Table.3 Detection of non-metallic inclusions at different positions

Sample	Type A		Type B		Type C		Type D		DS
	Fine series	Coarse series	Fine series	Coarse series	Fine series	Coarse series	Fine series	Coarse series
MI11	0	0	0	0	0	0	1	1	0
MI21	0	0	0.5	0	0	0	1	1	0
MI31	0	0	0	0	0	0	1	1	0
MI41	0	0	0	0	0	0	1	1	0

Figure.6 Non-metallic inclusions at different sampling positions
Table.4 Export of Macroscopic Tissue Detection

Sample	General looseness	Center porosity	General point segregation	Ingot segregation
MAC11	1	0	0	0
MAC21	1	0	0	0
MAC31	1	0	0	0
MAC41	1	0	0	0

Figure.7 Macrostructure at Different Positions of Forgings

3. Summary

This article elaborates on the red sleeve ring forgings manufacturing process for the high-pressure inner cylinder of ultra supercritical steam turbines. It identifies the key points of forging, ring rolling, heat treatment, and other processes. The room temperature and high temperature tensile properties, grain size, non-metallic inclusions, and macrostructure of forgings were measured through anatomical experiments. The results indicate that the manufacturing process of the forging is complete and reliable, and the various application performance of the forging is excellent, meeting the localization requirements of the forging.
Author: Ju Hongxia