Forming technology of Φ7.3m extra large 09MnNiD low temperature steel flange forging

For the first forging production of Φ7.3m extra large 09MnNiD low temperature steel flange forgings, through analysis and comparison of three forming solutions, including segmental bending and then welding forming, ring rolling forming and free forging and flaring forming, the free forging and flaring forming solution was determined. In order to eliminate the influence of flaring width and cold shrinkage rate of hot forgings on the finished flange dimensions during forging and forming, the analysis of flaring width and cold shrinkage rate of hot forgings during forging and forming of extra-large rings determines the parameter setting basis of flaring width during forging process design, and the reference data of cold shrinkage rate of hot forgings are statistically analyzed. For 09MnNiD low-temperature steel flange forgings with a wall thickness of more than 400mm, resulting in the risk of poor heat treatment hardening effect and unqualified performance, the heat treatment process test was carried out systematically. According to the results of the heat treatment process test, the best heat treatment process plan of 09MnNiD low temperature steel was determined. Through actual production, the reasonableness of the design of forging and forming process of 09MnNiD low-temperature steel for extra-large flanges in terms of reaming spreading amount, cold shrinkage rate of hot forgings and heat treatment process parameters was verified.

Cryogenic steels are mainly used for the manufacture of various liquefied gases such as liquefied petroleum gas, liquid ammonia, liquid nitrogen and other liquefied gas production and storage equipment [1]. With the in-depth research on cryogenic steel, its application is becoming more and more widespread. At present, cryogenic steel has been promoted to the equipment testing industry for the manufacture of large cryogenic testing devices to simulate and test the operation of electrical, instrumentation and mechanical equipment in extremely cold regions. And, with the level of large-scale equipment and device reliability requirements continue to improve, the specifications of low-temperature steel forgings are getting bigger and bigger, on the one hand, for the development of low-temperature steel forgings brought opportunities; and on the other hand, also for the manufacture of low-temperature steel forgings brought a great challenge. 09MnNiD for -70 ° C level low-temperature steel, is China’s 0.5% Ni low-temperature steel system in the Germany and France on the basis of the corresponding steel grades to adjust the chemical composition of the steel grade [2]. At present, 09MnNiD cryogenic steel has become one of the main steel grades for cryogenic pressure vessels in China and is included in the NB/T47009-2017 [3] standard for alloy steel forgings for cryogenic pressure-bearing equipment. The Φ7.3m extra-large 09MnNiD cryogenic steel flange forgings made by Yaang Pipe Industry Co., Limited. are used in a large cryogenic testing device, which is the largest cryogenic steel ring-type forgings in terms of size and unit weight made by the company. It is understood that the specification of the cryogenic steel forgings in the domestic is the first case, and the forging wall thickness has exceeded the standard range of NB/T47009-2017 [3], its heat treatment quenching effect is poor, manufacturing is extremely difficult. This paper mainly focuses on the key parameters of forging and forming process and heat treatment process of this flange forging. 

1. Φ7.3m very large flange parameters forging

Flange shape size specifications as shown in Figure 1, forging outer diameter of Φ7350mm, height of 1520mm, mass of 123700kg. According to the production rate of large ingot forging process, it is necessary to use about 200000kg level of very large ingot forging.

Figure.1 Schematic diagram of the shape and size of the extra-large flange forgings
The material of the flange forgings is 09MnNiD low temperature steel, and its melting chemical composition should meet the requirements of Table 1.
Table.1 Chemical compositions of 09MnNiD low temperature steel (%, massfraction)

Forging heat treatment state Q + T, tempering temperature of not less than 620 ° C; and technical conditions require the selection of tangential specimens, at the end of the forging, from the wall thickness of 1/2 sample. The mechanical properties of the forgings are required as shown in Table 2.
Table.2 Mechanical properties of 09MnNiD low-temperature steel

Parameter	Tensile strength Rm / MPa	Yield strength Rel / MPa	Elongation A /%	Impact absorbed energy Kv2 / J (- 70 ° C)
Numerical value	430～580	≥270	≥23	The average value of three samples is ≥ 60, and the impact value of one sample is allowed to be < 60, but it must be ≥ 42

The grain size of the forgings was determined according to the standard GB/T6394-2017 [4], requiring ≥5 grade.

2. Selection of forming solution

For this kind of extra-large ring forgings, some scholars have studied the forming process using segmental bending and forming, and then sputtering and welding for the whole ring [5], but the presence of welding seams will affect the integrity of the parts, and bending and forming need to make special tooling and dies, and the manufacturing cost is high. Therefore, a reasonable forming process needs to be explored to ensure the external dimensions and internal quality of the product. Rolling ring forming has the features of high accuracy of forging blank, small machining allowance and high material utilization rate [6]; however, the single weight of the flange is large, and extra large heavy ring rolling machine is required for rolling. The wall thickness of the flange forging blank is nearly 500 mm, and the wall thickness of the prefabricated hollow billet before ring rolling is about 700-800 mm, while the ring rolling is characterized by continuous small deformation on the inner and outer diameters and end surfaces of the billet. Therefore, the internal quality and performance of the material formed by this method are potentially risky. Compared with ring rolling, although the surface quality of the forging blank formed by free forging is slightly worse, the compaction effect in the center of the forging wall thickness can be significantly improved by a larger amount of press down, which is beneficial to improve the material properties. Considering the advantages and disadvantages of ring rolling and free-forging, the free-forging reaming solution is chosen to ensure the appearance and performance of the flange forgings meet the requirements. However, the outer diameter of the flange forging is close to the crotch limit of our 18500t free forging press, so it is very difficult to control the size, and the free forging forming process needs to be analyzed in depth.

3. Forging process analysis

The forging process is as follows: unloading – upsetting, punching – pre-reaming – flattening end face – fine reaming, trimming; the key forming process is reaming. The arbor reaming used in free forging is to put the arbor through the billet and put it on the horse frame, and the billet is pressed down at every angle, gradually thinning the wall thickness and expanding the inner and outer diameter of the billet [7]. During the reaming process, the metal flows not only in the circumferential direction, but also in the axial direction. Due to the large amount of flange forgings reaming, the reaming process, the amount of elongation along the axial direction, that is, the spread of the distribution of raw materials, the control of the size of each part of the flange is essential. On the other hand, the final forging temperature when reaming out the finished product is often above 800°C, forging blank cooling, the diameter size will be reduced; due to the larger diameter of the flange, the cold shrinkage of the hot forging is often large, there is a significant impact on the final size of the flange forging.

3.1 Analysis of the amount of expansion of the hole

When reaming, the outer surface of the billet is in contact with the hammer anvil, and the inner surface is in contact with the mandrel surface, and the contact part is plastic deformed when pressing down; the metal in the deformation zone flows along the tangential and axial directions, and the metal flow is restricted by the outer end. As the length of the metal deformation zone along the tangential direction is much smaller than the height of the axial direction, and the contact surface of the mandrel and the billet is curved, according to the principle of minimum resistance to plastic deformation, the metal tends to flow along the tangential direction, the wall thickness is gradually thinned, and the inner and outer diameter is expanded; at the same time, the height is slightly increased, and the increased amount is the spreading amount. When the diameter of the ring is small, the amount of expansion is small, and the effect of spreading on the distribution of raw materials is small and negligible. However, the diameter of the flange described in the paper is more than Φ7m, and the reaming volume is larger, so the spreading volume is directly related to whether the size of each part of the forging can meet the requirements after forming. Some scholars have analyzed and calculated the spreading of ring forgings by the energy method [8], which studied the influence of the bore expansion coefficient, average diameter, ring billet height, mandrel diameter and friction factor on the spreading amount respectively, but did not point out the comprehensive influence of each factor on the spreading amount and the quantitative calculation method, so there are some limitations in engineering practice. Mandrel reaming is a local loading and local deformation of the ring billet along the circumferential direction of the draw length, so the research should focus on the key factors affecting the deformation zone of the ring billet during local deformation. From the contact between the ring billet and the mandrel during the reaming process, the shape and size of the deformation zone are mainly influenced by the height of the ring billet and the diameter of the mandrel. According to the principle of minimum resistance, there is a specific pattern of metal flow in the deformation zone and the size of the ring billet height and mandrel diameter. At a certain moment of reaming, the mandrel diameter is certain, the greater the height of the ring billet, the smaller the ratio of the contact width between the mandrel and the inner surface of the ring billet and the height of the ring billet, theoretically the more favorable to the tangential flow of metal; conversely, when the ring height is small, in the reaming process, the height direction resistance becomes smaller, the metal is easy to flow in the height direction. In the whole reaming process, the cumulative effect of metal flow along the tangential direction is much greater than the cumulative effect of flow along the axial direction, when the amount of widening is relatively small. In addition, the cumulative effect of metal flow in the tangential and axial directions during the reaming process is directly related to the amount of reaming. In order to facilitate the calculation, some scholars have converted and simplified this, and studied the effect of the initial diameter-to-height ratio of the ring billet and the reaming volume on the spreading coefficient, and then developed the corresponding calculation method [9]. The process data of eight large ring forgings with a diameter size close to Φ7m produced in the previous period were counted, as shown in Table 3. The analysis found that in the process of reaming of this type of large ring-like forgings, the height growth is roughly in the range of 3.23-3.69mm for every 100mm expansion according to the inner diameter. Forging process design, can be integrated with the aforementioned calculation methods and statistics of the empirical data for spreading parameters set: when the outer diameter and height of the comparison is large, take the large value; when the outer diameter and height of the comparison is small, take the small value.
Table.3 Statistical data of the spreading of large rings (mm)

Forging No	Blank process size	Height before reaming	Height after reaming	Hole expansion after leveling	Reaming 100mm and widening
1-1	Φ6880/Φ5510/H1125	1010	1135	3510	3.56
1-2	Φ6950/Φ5590/H1375	1270	1390	3590	3.34
1-3	Φ6770/Φ5420/H1035	915	1045	3520	3.69
1-4	Φ6620/Φ5310/H1275	1180	1290	3410	2.23
1-5	Φ6620/Φ5320/H1105	995	1115	3370	3.56
1-6	Φ6925/Φ5570/H1220	1105	1230	3570	3.50
1-7	Φ6100/Φ4915/H1140	1050	1150	3015	3.32
1-8	Φ6960/Φ5635/H1305	1075	1195	3595	3.3

3.2 Prediction of cold shrinkage rate of hot forgings

The size of hot forgings is generally larger than the size of cold forgings, and the size change is mainly due to the thermal expansion and contraction of the metal material and the phase change during the cooling process of the metal material. For sub-eutectoid steel, when the metal is below the critical phase change temperature A1, no phase change occurs during cooling, and the cold shrinkage rate is approximately linear with the temperature; while above the A1 temperature, phase change occurs during cooling, so the relationship between cold shrinkage rate and temperature is more complex. Some scholars through the hot die forging press production of various types of forgings for hot and cold state of a large number of measurements, statistical analysis of the forging final forging temperature at 900-1050 ° C, the hot forging cold shrinkage rate by 1.5%-1.75% to control more appropriate [10]. The limitation of this method is that it ignores the influence of forging type and size on the cold shrinkage rate, especially the difference between solid and hollow forgings is often large. Therefore, the study of cold shrinkage of hot forgings should be distinguished at least from the type of forgings, so as to ensure smaller errors and more operability. For large ring-type forgings, the difference between cold and hot form dimensions is large, especially the hot form dimensions are difficult to estimate, causing great difficulties in size control during forging and forming. The larger the size of the forging, the more obvious the problem. Influenced by various factors such as ring diameter, height, wall thickness, material and forging process, it is difficult to derive a universally applicable formula for calculating the cold shrinkage rate of hot forgings, therefore, the current research on the cold shrinkage rate of hot forgings is generally based on a large number of cold and hot state data measurements. In order to guide the actual production and find the change law of cold shrinkage rate of hot forgings of such large ring forgings, the cold and hot state dimensions are tracked, measured and systematically studied for the same type of products produced in the previous period with the material of sub-eutectoidal steel — rotary kiln wheel belt forgings, by forging and forming the forgings after The cold shrinkage rate of large ring forgings is calculated by collecting and analyzing the hot and cold dimensions after forging and forming. The statistical data of cold and hot dimensions and cold shrinkage rate of large ring forgings are shown in Table 4. Integrating and calculating the data in Table 4, the average cold shrinkage of the outer diameter of the wheel and belt forgings is 0.861% and the average cold shrinkage of the inner diameter is 0.856%. However, due to the large size of the wheel belt forgings, there is an error in measuring the hot state blank size after forging and forming, which leads to some error in the cold shrinkage rate as well.
Table.4 Cold and hot state data statistics of large ring forgings

Forging No	Hot size / mm	Cold size / mm	Outer diameter shrinkage /%	Inner diameter shrinkage /%
2-1	Φ7025/Φ5660	Φ6965/Φ5610	0.862	0.891
2-2	Φ6845/Φ5450	Φ6785/Φ5405	0.884	0.832
2-3	Φ6685/Φ5345	Φ6630/Φ5300	0.83	0.849
2-4	Φ6695/Φ5350	Φ6635/Φ5305	0.904	0.848
2-5	Φ6980/Φ5625	Φ6920/Φ5575	0.867	0.897
2-6	Φ6170/Φ4940	Φ6120/Φ4900	0.817	0.816

The cold shrinkage law of the forgings has yet to be verified and further corrected in the subsequent production. The final forging temperature of the wheel and belt forgings is 800-900°C, so the cold shrinkage rate is applicable to large sub-eutectoid steel ring forgings with the final forging temperature at 800-900°C. Other temperature ranges and steel grades need to be studied separately. In addition, in order to control the accuracy of the measurement of the inner and outer diameter of large ring parts, the measurement should be done by arranging the data in such a way that every 45° angle is recorded and then statistically analyzed. This measurement is not only more accurate, and can be found in a timely manner whether the shape of the forgings are regular, such as oval, oversize and other problems, can be timely repair. 4 heat treatment process test flange forgings wall thickness up to 480mm, and requires a sample at the wall thickness of 1/2 for performance testing. Quenching, the heart cooling rate is less than the critical cooling rate is prone to ferrite organization, the risk of unqualified performance is great. Studies have shown that the material sometimes fails in low-temperature impact values during performance testing, and the test values fluctuate widely [11]. In response to such problems, microscopic analysis of the fracture was performed using scanning electron microscopy, and it was concluded that the impact failure was mainly due to metallurgical defects [12]. However, the current technology of pure steel is more mature, and the performance is not only related to the material composition factors, but also the heat treatment process is crucial. In order to find out the best heat treatment process, a process test was conducted by smelting and casting ingots of 09MnNiD material in a vacuum induction furnace. As shown in Fig. 2, the cast ingot was removed from the water spout to ensure that the ingot body was about 50 kg, forged into a rectangular test block by a free forging hammer, and split into strips for the process test. Quenching temperatures of 840, 880, 920 and 950°C were selected; since NB/T47009-2017 [3] requires the tempering temperature of the material to be no less than 620°C, and also to ensure the strength, the test tempering temperature was selected to be 630°C; each quenching temperature included one tensile test and three low-temperature impact tests. Table 5 shows the process test results. The test results show that, in the temperature range of 840-950 ° C, the strength of the forgings under the four quenching temperatures without obvious patterns, are in line with the process requirements; comparison of impact toughness, 950 ° C quenching + 630 ° C tempering heat treatment state of the impact toughness is the most stable. Overall, 950°C quenching + 630°C tempering process state has the best performance, so the best heat treatment process for this material forgings is 950°C quenching + 630°C tempering.

Figure.2 Test material preparation process
(a) Ingot demoulding (b) Sawing and preparation (c) Forging of test pieces (d) Cutting of test specimens

Table.5 Process test results

Process scheme	Tensile strength Rm / MPa	Yield strength Rel / MPa	Elongation A /%	Impact absorbed energy Kv2 / J (- 70 ° C)
840 ° C quenching + 630 ° C tempering	511	409	28	147.9、99.3、156.1
880 ° C quenching + 630 ° C tempering	526	430	26	154.2、43.6、177.3
920 ° C quenching + 630 ° C tempering	522	420	27.5	130.8、127.1、59.6
950 ° C quenching + 630 ° C tempering	532	436	28.5	144.2、128.9、124.7

5. Production verification

The outer diameter to height ratio of the flange forging is 4.8. In the design of forging and forming process, with reference to the aforementioned analysis statistics, the spreading amount is calculated according to the average height growth of 3.3mm when the hole is expanded by 100mm; the cold shrinkage rate is estimated according to 0.860% when calculating the hot state dimension. In the actual forging, considering the pulling and drawing of the punch on the end of the billet during punching, the billet is upsetting and spinning to a height of 1450mm when punching, and the punching diameter is controlled by Φ1300mm. According to this calculation, the upsetting ratio is 2.6 and the reaming ratio is 2.75. The leveling height and hot forging size are strictly controlled by the forming process. The actual production process is shown in Figure 3. The dimensions of the cold state are Φ7345-Φ7360mm in outer diameter, Φ6380-Φ6395mm in inner diameter and 1525mm in height, measured after the forging preparation heat treatment. Heat treatment, armored thermocouple completely heat preservation to the time required by the process after quenching out of the furnace; insulation process, armored thermocouple temperature difference in each area is controlled within 10 ° C. From the start of the furnace door to lifting to the water tank all into the water at the fastest speed to fully ensure the cooling effect. The metallographic organization of the flange after tempering treatment is shown in Figure 4, which is a typical bainite organization with grain size up to grade 6. As we all know, bainite usually has excellent comprehensive mechanical properties. After actual inspection, the mechanical properties of the forgings meet the requirements, see Table 6 for details.

Figure.3 09MnNiD low-temperature steel flange manufacturing process

(a) reaming (b) forging and forming (c) quenching (d) semi-finishing

Fig. 4 microstructure of 09MnNiD low temperature steel flange after quenching and tempering treatment

Table.6 Mechanical properties of forgings and grain size test results

Parameter	Tensile strength Rm / MPa	Yield strength Rel / MPa	Elongation A /%	Impact absorbed energy Kv2 / J (- 70 ° C)	Grain size / grade
Numerical value	515	408	29.5	166.5、172.7、195.9	6

6. Conclusion

• (1) When designing forging process for large flange forgings of Φ7.3m grade 09MnNiD low-temperature steel, the amount of hole expansion is calculated according to the increase of 3.23-3.69mm in height direction for every 100mm of inner diameter expansion; when the comparison of outer diameter and height is large, the larger value is taken; when the comparison of outer diameter and height is small, the smaller value is taken.
• (2) The final forging temperature in the range of 800-900 °C, 09MnNiD low-temperature steel and other sub-eutectic steel large ring hot forging blank average cold shrinkage rate of 0.860% reference.
• (3) The best heat treatment process for 09MnNiD low temperature steel forgings is 950°C quenching + 630°C tempering, quenching to take accelerated cooling measures to strengthen the cooling effect, can ensure that the material forgings have a better overall performance.

Author: Li Changyi, Chen Mingming, bojielu, bokong, Yuting, Li Xue

Source:  China Flanges Manufacturer: www.epowermetals.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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