The manufacturing process of motor guard ring forgings

The manufacturing process of motor guard ring forgings

Studied the hot forging process, solid solution treatment process, and cold deformation hydraulic bulging strengthening process of the 1Mn18Cr18N motor retaining ring, determined the technical process parameters, and ultimately produced the required retaining ring forgings.

The protective ring of a 20MW high-strength electric motor with ultra-high speed is a key component of the driving motor on the booster pump of the national key equipment localization project – the West-East Gas Pipeline Project. The key to solving the localization process of this electric motor is whether it can be mass-produced domestically. Due to the high technical requirements and manufacturing difficulty of the protective ring in this specification, imported forgings are used for installation, resulting in high prices and a heavy cost burden on the entire machine, which has restricted the localization process of this specification electric motor. To replace imports and eliminate technical barriers, our company has developed this product and succeeded, forming a batch manufacturing capability.

As the rotor rotates at high speed (5200r/min), the retaining ring needs to withstand a large centrifugal force at the end of the rotor and prevent damage to the end of the rotor coil. The retaining ring is required to have good mechanical properties (R_p0.2>1070MPa). Using 1Mn18Cr18N as a material to produce the protective ring cannot only avoid heating the protective motor ring during operation due to electromagnetic induction but also effectively improve the working efficiency and safety of the motor during operation. Ordinary heat treatment methods cannot improve the mechanical properties of austenitic materials with no phase change at room temperature. Only hydraulic cold deformation strengthening can be used to enhance the mechanical properties of this material.

1. Technical specifications of the protective ring for the 20MW1Mn18Cr18N motor

When delivering the finished protective ring, it is necessary to meet the customer’s requirements for its performance indicators and internal quality. The customer requires that the chemical composition of the finished product of the retaining ring forging meet Table 1, the mechanical properties meet Table 2, and the magnetic permeability should not exceed 13.2 when the magnetic field strength is 8000A/m × 10^-7H/m, residual stress should be less than 10% of yield strength. A finished protective ring according to Ф 800mm ×Ф 670mm × 585mm size is delivered, and the interior is inspected according to the non-destructive testing requirements in JB/T7030-2002 to ensure that there are no excessive defects.

Table.1 Chemical Composition Requirements for 1Mn18Cr18N (Mass Fraction, %)

C	Si	Mn	P	S	Cr	N
≤0.12	≤0.80	17.5-20.0	≤0.05	≤0.015	17.5-20.0	≥0.47

Table.2 Mechanical Property Requirements

Project	Rp0.2/MPa	Rm/MPa	A4（ %）	Z（ %）	KV/J
Tangential Direction	1070 – 1210	≥1070	≥15	≥52	≥82
Test Temperature/T	95 – 105				20 – 27

2. Development of hot forging process for protective rings

Based on the mechanical performance requirements of the customer for the test ring of the protective ring workpiece, combined with years of production and manufacturing experience and the performance characteristics of the material itself, relevant data is obtained to provide the rough turning dimensions and forging process dimensions of the protective ring forging. This is the biggest difference from the conventional forging process settings.

The production process of the protective ring is as follows: smelting → ingot casting → heating → forging → rough turning → solid solution treatment → precision processing before strengthening → cold deformation hydraulic bulging strengthening → residual stress relief heat treatment → physical and chemical testing → non-destructive testing after precision processing → packaging and delivery. The dimensions of the blank forging of the retaining ring are shown in Figure 1, and the hot forging process steps are shown in Table 3, which are forged using electro slag remelting steel ingots.

Figure.1 Dimension of Retaining Ring Blank Forgings

Table.3 Hot Forging Process Plan

Fire frequency	Temperature/˚C	Explanation of deformation process
1st fire	1 000-1200	Upsetting to a height of 400 mm
2nd fire	1 000-1200	The punching diameter is Ф 300 mm
3rd fire	1 000-1200	The diameter of the expansion hole is Ф 470 mm
4th fire	1 000-1200	Extend the core rod to 600 mm
5th fire	950-1200	Water cooling after elongating and leveling the end face to meet the process requirements

Figure.2 Solid Solution Treatment Process Curve
In the process of hot forging retaining ring forgings, because 1Mn18Cr18N retaining ring steel belongs to high non-magnetic manganese austenitic stainless steel, with manganese and chromium content of 18% and nitrogen content of over 0.5%, surface cracks are easily generated during the forging process. To prevent the occurrence of surface cracks, it is necessary to strike during forging lightly, reasonably use a small reduction amount, and control the feeding amount.

3. Solid solution treatment

Solid solution treatment refers to heating an alloy to a high-temperature single-phase zone and maintaining it at a constant temperature, allowing carbides to dissolve into the austenitic matrix fully and then rapidly cool down in order to obtain a uniform and stable austenite structure, improve toughness, and corrosion resistance, and enhance the cold deformation strengthening the performance of the material. The solid solution treatment curve of the billet after rough machining of the 1Mn18Cr18N retaining ring is shown in Figure 2.

4. Cold deformation strengthening

• (1) To meet the customer’s requirements for the mechanical properties and dimensions of the retaining ring forgings, it is necessary to strengthen them through hydraulic bulging and cold deformation.
• (2) Use a high-pressure pump to inject water into the enclosed space formed by the protective ring and the mold, causing plastic deformation of the protective ring after rough turning. This can facilitate the control of the protective ring’s external dimensions and deformation speed.
• (3) This protective ring is slender compared to conventional protective rings. It has a large aspect ratio before bulging, which can easily cause a concave busbar (commonly known as bell mouth) during the hydraulic bulging. We had manufactured positioning and limiting devices before expanding the protective ring. We determined the instantaneous low pressure parameter values that ensure that the protective ring’s hydraulic expansion process matches the ultra-high pressure liquid required for expansion. After careful control and continuous improvement of control parameters, the protective ring has been deformed from a size of 660mm before bulging to 810mm, with a tolerance of ± 2mm.

5. Stress relief heat treatment

After strengthening, the protective ring forging must undergo heat treatment to stabilize the organization and reduce residual stress. The stress relief curve of the retaining ring is shown in Figure 3.

6. Test results

The manufacturer takes the middle ring at both ends of the retaining ring and cuts the test ring for mechanical performance testing. The number and location of tensile and impact specimens for each test ring are shown in Figure 4. Taking furnace T1205271 as an example, the mechanical performance test results are shown in Table 4, and the ultrasonic test results are shown in Table 5. The metallographic examination results show a radial grain size of 4 levels and a longitudinal grain size of 4 levels. The metallographic structure is composed of austenite and trace point like carbides. The magnetic permeability test was conducted by the provisions of the “Technical Agreement for Forging and Manufacturing of Motor Retaining Rings for the West-East Gas Pipeline,” The test results were one order of magnitude lower than the user’s requirements.

Figure.3 Stress Relief Curve

Figure.4 Sampling Location and Quantity

Table.4 Actual Test Results of Mechanical Properties

Rp0.2/MPa	Rm/MPa	A（%）	Z（%）	kv2/J
1120	1124	23.5	60	115
1083	1099	24.5	62	129
1103	1117	24.5	64	133
1085	1098	24	62	130.5
1130	1143	22.5	62	144
1130	1145	23	63	137.5
1103	1113	22.5	66	145
1136	1147	24	62	156

Table.5 Ultrasonic Testing Results

PXUT350 + parameters	Probe frequency/MHz	Sensitivity/ mm	Couplant	Surface roughness Ra/ μ m
	2. 5	Ф2	Engine oil	3.2
Acceptance criteria	JB/T 7030—2002
Inspection results	No exceeding standard defects were found, and the internal quality of the workpiece meets the standard requirements

7. Conclusion

The manufacturing process of motor retaining rings includes hot forging, solid solution treatment, cold deformation hydraulic bulging strengthening, etc. The determined process technical parameters ensure quality control of the actual manufacturing process, meet the user’s requirements for various technical indicators of this specification of retaining ring forgings, and fill the gap in the domestic manufacturing of 1Mn18Cr18N retaining rings with ultra-high speed.

Author: Zhang Xudong

Source: China Ring Forgings Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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