F92 steel forged Y-type reducer tee welded joints post-weld heat treatment research and practice

According to the successful implementation of post-weld heat treatment of welded joints of F92 steel forged Y-type reducer tee in primary high-temperature reheat steam pipeline, post-weld heat treatment of welded joints of F92 steel forged Y-type reducer tee in primary and secondary high-temperature reheat steam pipeline has been realized, which can provide a reference for the post-weld heat treatment of the same type of parts in the following years. The successful application of the program can provide a reference for the subsequent post-welding heat treatment of the same type of parts.

0. Preface

F92 steel has become the material of choice for main steam valves, high-temperature reheat steam valves, and forged pipe fittings of China’s ultra-supercritical generating units due to its good high-temperature endurance strength, thermal conductivity, high-temperature oxidation resistance, and low coefficient of thermal expansion. The steel is a new type of martensitic heat-resistant steel launched by Japan’s Nippon Steel in the F91 steel based on increased W (add w (W) 1.5% – 2.0%) Mo (reduce w (Mo) 0.5%), add a small amount of B, through the solid solution strengthening of the W and Nb, V and other strong carbon nitride dispersion strengthening, so that the steel has a stronger high-temperature endurance strength and creep properties.

A 1000MW ultra-supercritical secondary reheat unit six pipelines have a total of six Forged Y-type reducer tee; the material is F92, of which the main steam pipeline Forged Y-type reducer tee takeover specifications for ID240 × 84mm/ID343 × 118mm/ID240 × 84mm, the material for the P92 steel; a high-temperature reheat steam pipeline forged Y-type reducer tee Receiver specifications: ID300 × 56mm/ID451 × 83mm/ID300 × 56mm, material P92 steel, Forged Y-type reducer tee structure shown in Figure 1; secondary high-temperature reheat steam pipeline Forged Y-type reducer tee receiver specifications: ID560 × 28mm/ID870 × 42mm/ID560 × 28mm, material P92 steel. The material is P92 steel.

Figure.1 Forged Y-type reducer tee for primary high-temperature reheat steam structure and physical drawing

Forged Y-type reducer tee belongs to the shaped structure; the construction site welding heat treatment is more difficult, especially if the post-weld heat treatment temperature is high; it is very easy to occur over temperature or insufficient temperature. To effectively ensure the performance of the welded joints of the Forged Y-type reducer tee of the six major pipelines of the unit and to ensure the one-time qualification rate of the welded joints of the pipelines, the site has carried out research on the post-weld heat treatment process of Forged Y-type reducer tee welded joints of the primary high-temperature reheat steam pipelines and has applied this technical solution to the welded joints of Forged Y-type reducer tee of the main steam and secondary high-temperature reheat steam pipelines.

1. Analysis of structural characteristics and post-weld heat treatment program of Forged Y-type reducer tee

T819 – 2019 “Thermal Power Plant Welding Heat Treatment Technical Regulations,” Article 6.4.3: post-weld heat treatment when D/δ ≤ 10, the heating width from the center of the weld on each side is not less than 6 times the wall thickness of the pipe (main steam pipeline: when the main pipe D/δ = 4.9, the heating width of 708mm; when the branch pipe D/δ = 4.9, the heating width of 504mm. Primary reheat steam pipeline: when the main D/δ = 7.4, the heating width of 498mm; when the branch D/δ = 7.4, the heating width of 336mm); when 20<D/δ ≤ 40, the heating width from the center of the weld, on each side, is not less than 9 times the pipe wall thickness (secondary reheat steam pipeline: when the main D/δ = 22.7, the heating width of 378mm; when the branch D/δ = 22.7, the heating width of 378mm; when the branch D/δ = 22.7, the heating width of 378mm; when the branch D/δ = 22.7, the heating width of 378mm. 378mm; when the branch pipe D/δ=22, the heating width is 198mm).

From the pre-weld preheating and post-weld heat treatment, the heating width can be seen; it is much larger than the receiver size of the forging tee, so in the pre-weld preheating and post-weld heat treatment process, inevitably needs to be arranged in the forging tee on the heating sheet, forging tee a total of 3 on-site welds, by the conventional program, a weld for 1 time for the pre – welding and post-weld heat treatment. Hence, the forging tee also needs to be heated 3 times. In the process of preheating and post-weld heat treatment, the heating pad to the forging tee heating, preheating before welding, due to the preheating temperature of 200 – 250 ℃, the forging tee temperature will be lower than the preheating temperature, the organization of the forging tee has no effect; post-weld heat treatment, the weld needs to be heated to 750 – 770 ℃, at this time, the temperature of the surface of the forging tee is relatively high, the organization of the forging tee may have a certain effect. Forging tee 3 welds respectively for post-weld heat treatment, forging tee 3 times heating, the organization of the forging tee may have a greater impact on the forging tee and even reduce the service life of the forging tee.

To reduce the impact on the organization and performance of the forging tee, to ensure the service life of the forging tee, the construction site to determine the use of three welds at the same time post-weld heat treatment program so that the forging tee for the first heating can be completed three welds post-weld heat treatment.

2. Analysis and solution of technical difficulties in post-welding heat treatment

Construction site on the Forged Y-type reducer tee takeover 3 welding seam at the same time post-weld heat treatment, there are the following technical difficulties:

• (1) The forged Y-type reducer tee size is small according to the site construction space. 3 welds cannot be welded at the same time. The site can meet the first main weld to complete the welding. Then the branch pipe 2 welds welded at the same time, the main weld and the branch pipe 2 welds welded to complete the difference in welding time is more than 24h, does not meet the DL/T819 – 2019 “Thermal power plant welding heat treatment” 6.3 Technical Specification. DL/T819 – 2019 “Thermal Power Plant Welding Heat Treatment Technical Regulations” 6.3 requirements. The program adopted at the scene for the main weld first welding, welding is completed after the main welded joints cooled to 90 ℃, insulation 2h, and then post-weld heat treatment, insulation temperature of 350 ℃, insulation 3h, after the end of the heat of the welded joints in the 150 – 200 ℃ between insulation; at the same time on the branch pipe 2 welds for welding.
• (2) Forged Y-type reducer tee takes over 3 welds at the same time post-weld heat treatment due to the forging tee belonging to the anisotropic structure, and wall thickness, and forging tee connected to the wall thickness of the straight pipe there are differences in the post-weld heat treatment process, there will be due to the direction of the heat transfer rate of the highest temperature range of the heat treatment field due to the different temperature zone offset problem. Field to take the program for the forging tee comprehensive auxiliary heating, thermocouples arranged in the weld, base material, heat-affected zone, forging tee base material, and other locations, while in the center of the forging tee arrangement of the two temperature control thermocouples, to prevent the occurrence of over – temperature.
• (3) Forged Y-type reducer tee takeover 3 welds at the same time post-weld heat treatment due to differences in the wall thickness of the straight pipe connected to the forging tee, by DL/T819 – 2019 “Thermal Power Plant Welding and Heat Treatment Technical Specification” 6.4.6 relevant provisions of the main tube and the branch pipe lifting and lowering of the rate of temperature, thermostatic time is not the same. The program adopted at the scene is to raise and lower the temperature rate according to the main, constant temperature time branch pipe, according to DL/T819 – 2019 “Thermal Power Plant Welding Heat Treatment Technical Regulations” 6.4.4 relevant provisions of the implementation of the branch pipe after the completion of the constant temperature that is the beginning of the temperature, the main continues to maintain a constant temperature to the set time, and then begin to lower the temperature.

3. Forged Y-type reducer tee post-weld heat treatment program

To ensure that the Forged Y-type reducer tee 3 welds at the same time post-weld heat treatment program can be effective and safe implementation, the choice of the pipe diameter and wall thickness of a high-temperature reheat steam pipeline Forged Y-type reducer tee as the first workpiece to be dealt with, the specific program of post-weld heat treatment is as follows.

3.1 Post-weld heat treatment process

Heating method: flexible ceramic resistance heating; temperature measurement method: K – type thermocouple temperature measurement; heating width: each side of the main ≥ 498mm, each side of the branch pipe ≥ 336mm; width of the insulation layer: each side of the main ≥ 664mm, each side of the branch pipe ≥ 448mm; constant temperature: 750 – 770 °C; constant temperature time: the main 8.5h, the branch pipe 6h; the number of thermocouples: ≥ 14; temperature rise/temperature drop Speed: ≤ 75 ℃/h; heat treatment heater total power: 110kW.

3.2 Arrangement of thermocouples

To prevent over-temperature, according to the structural characteristics of the Forged Y-type reducer tee, Forged Y-type reducer tee 3 welded joints and forging tee base material are arranged with a total of 23 thermocouples; the specific distribution is shown in Figure 2. Among them, 3 welded joints, forging tee center (front and back) thermocouples are temperature control thermocouples, welded joints temperature control temperature of 760 ℃, forging tee center temperature control temperature of 650 ℃, the rest of the thermocouples for monitoring thermocouples.

Figure.2 Temperature measurement thermocouple arrangement diagram

3.3 heater and insulation cotton arrangement

Thermocouple arrangement is completed after the heater arrangement, the field selection of flexible ceramic resistance heater for heating, heat treatment heater with a total power of 110kW, the heating element layout schematic shown in Figure 3; insulation cotton selection of aluminum silicate insulation cotton, insulation cotton thickness ≥ 50mm, insulation width in the width of the heater based on the unilateral increase in the pipeline width by 2 times the wall thickness. The heater and insulation cotton layout are shown in Figure 4,5.

Figure.3 Schematic layout of the heater

Fig.4 Heater arrangement

Figure.5 Insulation cotton layout

4. Forged Y-type reducer tee post-weld heat treatment

Thermocouples, heaters, and insulation cotton are arranged after the completion of the branch pipe welded joints for insulation, insulation temperature 90 ℃, insulation time 2h; insulation after the end of the start of heating, heating speed is 75 ℃/h, 3 weld temperature control temperature of 760 ℃, forging tee center temperature control temperature of 650 ℃. In the actual heat treatment process, 3 welds at the same time to reach 760 ℃, at the same time, by the heat treatment process card insulation time, 2 branch pipe weld insulation time for 6h, the main weld insulation time for 8.5h, insulation 6h after the branch pipe weld began to cool down, the main weld continues to be a constant temperature, the constant temperature of 8.5h, the main weld began to cool down the temperature reduction rate of 75 ℃/h, the temperature of 300 ℃ or below Temperature reduction rate is not controlled, branch pipe and the main post-weld heat treatment curve shown in Figure 6,7. The heat treatment process, technical personnel throughout the tracking, the process of temperature control points, and monitoring points have not been found to have over-temperature conditions, while the site has no anomalies.

Fig.6 Heat treatment curve of 2 branch pipes after welding

Fig.7 Heat treatment curve after welding of main pipe

5. Forged Y-type reducer tee welded joint post-weld heat treatment test

5.1 Hardness test

The hardness test is carried out when the temperature of welded joints after weld heat treatment is reduced to room temperature; to effectively evaluate the feasibility of the above post-weld heat treatment program, the hardness test is carried out on the weld seam, base material, and heat – affected zone of the three welded joints, and at the same time, the hardness test is carried out on the base material of forging and forging tee, the location of hardness test of Forged Y-type reducer tee after post-weld heat treatment is shown in Fig.8, and the test data is shown in Table 1.

Figure.8 Schematic diagram of hardness testing position of welded joints

Table.1 Forged Y-type reducer tee hardness test data after heat treatment after welding

According to DL/T438 – 2016 “Thermal Power Plant Metal Technology Supervision Regulations” Appendix C: P92 steel pipe parent material hardness range HB185 – HB250, P92 steel weld hardness range HB185 – HB270, F92 forged tee’s hardness range HB180 – HB269, after post-weld heat treatment, weld, Parent material (including pipe, forged tee parent material), heat – affected zone hardness is in line with the standard requirements.

5.2 Metallographic Organization Analysis

The three welded joints of base metal, heat-affected zone, weld for metallurgical analysis, as shown in Figures 9, 10, and 11, the metallurgical organization is tempered martensite, uniform organization, no cracks, and other welding defects.

Figure.9 Metallographic organization of primary branch pipe welded joints

Figure.10 Secondary branch pipe welded joints metallurgical organization

Fig.11 Metallographic organization of welded joints of the main pipe

6. Forged Y-type reducer tee welded joints after welding heat treatment application

According to the post-weld heat treatment technology plan for the welded joint of the forged tee of the high-temperature reheater, two Y-shaped reducing forged tees (welded joint numbers ZQ6, ZQ12, ZQ13, ZQ29, ZQ30, ZQ31). One Y-shaped reducing forged tee (welded joint numbers YR15, YR20, YR25) of the main steam pipeline of the unit was installed the welded joints of two Y-shaped reduced-diameter forged tees (welded joint numbers ER5, ER10, ER15, ER20, ER21, ER29) in the secondary high-temperature reheater pipeline were subjected to heat treatment. After heat treatment, the hardness of the base material, heat affected zone, and weld seam of the welded joints were tested. The hardness values of the welded joints are shown in Figure 12.

Figure.12 Hardness value of welded joints

Figure 12 shows that the hardness value of welded joints at the weld seam is HB215 – HB240, moderate hardness value; the base material, heat-affected zone hardness value of HB180 – HB210; the base material of the welded joints, heat-affected zone, weld hardness value after heat treatment is in line with the requirements.

7. Conclusion

• (1) Forged y-type reducer tee of three welded joints at the same time post-weld heat treatment, the process of the temperature at the weld can reach the set temperature of 760 ℃ constant temperature, welded joints and forged tee of each part of the wall temperature was not found to be over – temperature.
• (2) 6 Forged Y-type reducer tees of 3 welded joints to take the same time after welding heat treatment, the hardness testing of its weld, heat-affected zone, the hardness of the base material is within the standard range; at the same time a high-temperature reheater forged tee of the welded joints for the metallurgical organization of the test, the metallurgical organization of the tempered martensite, in line with the requirements.
• (3) 6 Forged Y-type reducer tees of 3 welded joints taken at the same time after post-weld heat treatment, its heat treatment process did not appear abnormal, the hardness of the welded joints and metallurgical meet the requirements, the program can effectively improve the forged tee 3 welded joints of the heat treatment efficiency, to the similar forging tee of the post-weld heat treatment provides a program.

Author: Zhang Jie