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Development of 16MND5 Forging for Nuclear Power Pressure Vessel

Development of 16MND5 Forging for Nuclear Power Pressure Vessel

ABSTRACT: By controlling the content of main chemical components in smelting, reducing the content of gas and inclusions, and adopting appropriate forging and tempering heat treatment process, nuclear safety grade 1 16MND5 forgings were successfully developed at one time.

Key words: 16MND5 forging, hardenability, low temperature impact toughness, RTNDT temperature, ductile-brittle transition curve

1 Preface

With the large-scale construction of nuclear power plants, a large number of 16MND5 forgings are used for civil nuclear pressure equipment. According to the Regulations on Safety Supervision and Administration of Civil Nuclear Pressure Equipment, units applying for manufacturing qualification permits for nuclear pressure equipment are required to select representative simulation parts for the target products and to meet the requirements of nuclear quality assurance system. Conduct trial production activities. Our company's application for nuclear safety grade I forgings manufacturing license, Qinshan Phase two expansion project stabilizer fluctuation tube nozzle as a simulation object, its forging technology conditions for valve nozzle, nozzle nozzle, wave tube nozzle and manhole seat 16MND5 manganese, nickel molybdenum alloy steel forgings, chemical composition, manufacturing Mechanical properties, surface and internal defects inspection, dimension inspection and supplementary tests. 16MND5 manganese-nickel-molybdenum alloy steel forgings are produced for the first time by our company. The hot working process is not familiar with the properties. The mechanical properties of materials are required to be high, and the requirements of ultrasonic flaw detection are strict.

Main Technical Conditions of 216MND5 Steel Forging

2.1 The chemical requirements are shown in Table 1.

Table 1 Chemical Constituents of 16MND5

materialchemical composition
CSiMnPS

Cr

Ni
16MND5≤0.220.10~0.301.15~1.60≤0.012≤0.012≤0.250.50~0.80
materialchemical composition
MoCuVAlCoAsSb
16MND50.43~0.57≤0.10≤0.01≤0.04≤0.08

≤0.0.010

≤0.005

2.2 Metallographic examination

The grade of non-metallic inclusions in forgings is determined according to GB/T 10561 standard. The grade diagram II-ASTM standard grade diagram is used to assess the qualified grade according to the fine series: A < 1.0, B < 1.0, C < 1.0, D < 1.0. According to RCC-M MC1300 or GB/T 6394 standard, the actual grain size of forgings after performance heat treatment should be equal to or less than grade 5.

2.3 The mechanical properties are shown in Table 2.

Table 2 Mechanical properties requirements

Pilot projectsTest temperature(℃)performancePrescribed value
Axial (transverse)(3)Axial (Longitudinal)(3)
Tensile testroom temperatureRp0.2≥400MPa
Rm552~670MPa
A%(5d)≥20
Z%Providing data
360Rtp0.2≥300MPa
Rtm≥510MPa
A%(5d)Providing data
Z%Providing data
KV impact test0Minimum average56J72J
Individual minimum(1)(2)40J56J
-20Minimum average40J56J
Individual minimum(1)(2)28J40J
+20Individual minimum72J88J

Prescribed value
RTNDTMeasurement of TemperatureRTNDTTemperature should not be higher than 0 C, hopefully lower than - 12 C. If the value is between 0 and - 12 C, the actual value should be measured.

(1) Only one of the three samples in each group is allowed to be lower than the minimum average value.

(2) Transverse: perpendicular to the main forging direction

(3) Longitudinal: parallel to the main forging direction

Supplementary impact test and drawing ductile-brittle transition curve

Three impact tests (AKV2) were carried out at - 40, +40, - 60 and + 60 C respectively. The percentage of brittle fracture area and the lateral expansion value of the specimen were measured, which were used to plot the ductile-brittle transition curve of 16MND5 forgings and to determine FATT50 value.

2.4 Ultrasound Flaw Detection

The forgings were examined by 100% UT. Direct shooting test: the range of recorded signal and acceptance limit should be in accordance with the quality level 3 of AFNOR NF A04-308; oblique shooting test: any signal whose echo amplitude exceeds 50% of the height of DAC curve should be recorded; any signal whose echo amplitude exceeds the height of DAC curve should be judged as unqualified.

3. Difficulties in development

3.1 To smelt 16MND5 material, P and S are less than 0.012%, Cu is less than 0.10%, As is less than 0.010% and non-metallic inclusions A, B, C and D are less than 1.0. It is difficult to control the content of harmful elements Cu and As when smelting.

3.2 After the final heat treatment of forgings, high temperature tensile tests at room temperature and 360 are required, and impact tests at 0, -20, -40, -60 are required, and the impact value is high. It is very difficult to manufacture the first forgings without pre-research. RTNDT temperature should be lower than 0 C, and the control of its influencing factors is still in the process of exploration, and there are uncertainties.

3.3 The requirement of ultrasonic flaw detection for internal quality products of forgings is also very strict. Direct shooting method test: the range of recorded signal and acceptance limit should be in accordance with the quality level 3 of AFNOR NF A04-308; oblique shooting method test: any signal whose echo amplitude exceeds 50% of the height of DAC curve should be recorded; any letter whose echo amplitude exceeds the height of DAC curve should be recorded. No. should be judged unqualified.

4. Trial-production process and trial-production process

4.1 Process Flow

Electric Furnace Smelting-LF+Vacuum Refining-Argon Protected Casting-Ingot Red Forging Plant-Forging, Post-Forging Heat Treatment-Rough Truck-Ultrasound Flaw Detection-Heat Treatment-Machinery, Metallographic Inspection-Ultrasound Flaw Detection.

4.2 Measures for Difficulties in Research and Development

4.2.1 Steelmaking

The chemical composition of 16MND5 material requires lower content of harmful residual elements such as Cu, As, P and S than that of common steel. The four harmful residual elements such as Cu, As, P and S are mainly brought in by scrap iron and steel. In the smelting process, copper and As can not be reduced by effective measures. Therefore, the introduction of copper, As, P and S can only be reduced by proportioning. To reduce its content in steel. Specifically: before the production of batching, the contents of reducing iron, pig iron and high-quality scrap steel are sampled and analyzed, and raw materials with lower contents of copper, As, P and S are selected for special use; when batching, 20%-30% reduced iron, 20%-30% pig iron and 40%-60% high-quality scrap steel are added to make the sum of reduced iron and pig iron reach 50%. Sampling analysis after melting in feeding furnace.

The content of non-metallic inclusions in 16MND5 forgings is strictly controlled. The content of gas and inclusions is minimized and the purity of molten steel is improved. The raw materials used in the smelting process, such as ferroalloy and lime, are used after baking to reduce the gas entrance; in the refining process, bottom argon blowing is adopted to strengthen the stirring of molten steel, uniform the temperature and chemical composition of molten steel, so that the gas and inclusions in the steel can be fully floated; in vacuum treatment, the vacuum chamber is made. The vacuum degree is reduced below 133Pa, which greatly improves the degassing efficiency, further uniforms the temperature and chemical composition of molten steel, promotes the floatation of inclusions, and thus improves the purity of molten steel.

4.2.2 Forging

The requirements of ultrasonic flaw detection for 16MND5 forgings are quite strict. In order to ensure the intrinsic quality of forgings, upsetting, punching and drawing are adopted in forging. Upsetting and punching remove the metallurgical defects in the ingot core through punching; then drawing and forming with corresponding mandrel and V-shaped anvil, strictly controlling the size of each part within the technological scope, making the metal fibre streamline of forgings distribute along the axis, improving the stress condition of forgings, and ensuring that both riser end and bottom end are adequate. Enough scrap to exclude defects from the forging body. After the forging is completed, the forging simulator is loaded into the furnace in time, annealed at isothermal temperature, and fully expanded hydrogen. On the one hand, it prevents the generation of white spots, on the other hand, it can refine grains, improve structure, improve metal cutting performance, and lay a good foundation for the final heat treatment.

4.2.3 Heat Treatment

After the forgings are roughed and qualified by ultrasonic inspection, the quality control is carried out. Considering that there are many test items for mechanical properties, it is required to do high temperature tensile tests at 360 (?) C, low temperature impact tests at - 20 (?), - 40 (?) C and - 60 (?) C, low content of alloy elements and poor hardenability of 16MND5. Therefore, cooling should be strengthened during quenching to obtain bainite structure so as to improve strength and low hardness at high temperature. Temperature impact toughness. Quenching temperature is 900-910 C, cooling mode is water quenching oil cooling, tempering using high temperature tempering, tempering temperature is 640-650 C, mainly to obtain comprehensive mechanical properties with good combination of strength, plasticity and toughness.

5. Test results and analysis

5.1 The chemical constituents are shown in Table 3.

Table 3 Chemical Constituents

materialchemical composition
CSiMnPSCrNi
16MND50.170.251.420.0080.0070.150.72
materialchemical composition
MoCuVAlCoAsSb
16MND50.550.080.010.0230.0050.010.001

5.2 The results of mechanical test are shown in Table 4.

Table 4 Organic test results

Pilot projectsTest temperature(℃)performancePrescribed value
Axial (transverse)Axial (Longitudinal)
RequirementTransverse X-ray measurementTransverse Y-measurementRequirementactual measurement
Tensile testroom temperatureRp0.2                                                    (MPa)≥400460474//
Rm(MPa)552~670602618//
A%(5d)(%)≥20%28.528//
Z%(%)Providing data7574//
360Rtp0.2                                                    (MPa)≥300417407//
Rtm                                                    (MPa)≥510592580//
A%(5d)                                                    (%)Providing data27.526.0//
Z(%)Providing data7679.0//
KV impact test0KV(J)≥5621727025523224124072261277207
-20KV(J)≥4022524823017019422856146171125
+20KV(J)≥7225925426425026025888200222232
-40KV(J)Providing data171131162////
+40KV(J)Providing data220226227////
-60KV(J)Providing data114110118////
+60KV(J)Providing data242241218////

5.3 The metallographic examination results are shown in Table 5.

Table 5 Metallographic examination results

project

Non-metallic Inclusions

Grain size

A

B

C

D

Result

1

0.5

0.5

0

6

5.4 Draw FATT curve and measure FATT50 temperature.

The percentage of brittle area of impact fracture is shown in Table 6.

Table 6 Percentage of brittle area of impact fracture

temperature(℃)

-60

-40

-20

0

20

Percentage of brittle fracture area(%)

88%

60%

20%

0%

0%

5.5 According to RCCM MC100 and ASTME 208a-95a standard, RTNDT temperature was determined with P-2 test block (-12 C).

According to the test results, after quenching and tempering, the high temperature tensile test at room temperature and 360 is qualified, the impact value at 0, - 20, 20 is qualified, and relative to the technical conditions, the RTNDT temperature is below - 12, the grain size is 6, and the non-metallic inclusions are under 1 without ultrasonic flaw detection. It is found that the defect exceeding the standard exists.

From the test results, the C content is limited in smelting, and the main alloying elements Mn, Ni, Mo and V content is controlled in the upper limit, which can refine grain size and increase hardenability, and improve low temperature impact toughness and strength index of forgings. The content of harmful elements in steel can be effectively reduced by using raw materials with low contents of Cu, As, P and S and bottom argon gas refining in smelting process; upsetting, punching and drawing process are adopted in forging process to ensure the intrinsic quality and fiber direction of forgings, thus ensuring the quality of forgings'flaw detection; water quenching is used in heat treatment. Oil-cooled quenching and tempering process makes forgings have good comprehensive mechanical properties.

6 Concluding remarks

The results of ultrasonic inspection, metallographic examination and mechanical properties test of forgings are all qualified, and the chemical composition of forgings also meets the technical requirements. The forgings have passed the on-site acceptance of the National Nuclear Safety Administration once. This shows that the measures we have taken to solve the manufacturing difficulties of 16MND5 forgings are correct and provide technical support for our company to produce nuclear first-class forgings for nuclear pressure equipment of similar materials.

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