I plant nitrogen transformation in the reactor, 105D 123C of the pipeline exports to a number of high pressure welded joints, made of 15CrMo steel, specifications for the φ14 "× 3 / 4", tube medium for hydrogen, ammonia nitrogen gas mixture, the working pressure Up to 150Kg, operating temperature up to 500 ℃. In order to ensure the welding quality, use a different welding consumable  and welding technology of the welding procedure qualification test for welding spot welding process provides a reliable data.
1 15CrMo steel welding
15CrMo Department of pearlite steel heat resistant steel at high temperatures with high thermal strength of the (δb ≥ 440MPa) and oxidation resistance, and has some resistance to hydrogen corrosion. Because the steel contains a higher content of Cr, C and other alloying elements, the more obvious tendency of hardened steel, welding of the poor. 15CrMo chemical composition of steel is as follows:
C: 0.16%, Si: 0.27%, Mn: 0.55%, Cr: 0.95%, Mo: 0.50%, S ≤ 0.40%, P ≤ 0.035%, its carbon equivalent (according to the International Welding Association 11W) recommended the formula:
Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 = 0.545%
Experience: When Ceq> 0.4%, the hardening tendency of welded joints large, cold cracking may occur, and 15CrMo Ceq value of 0.545 percent steel, so the tendency of large 15CrMo hardened steel, welding is poor, so 15CrMo steel welding, welding material selection and rigorous measures to prevent cracks and ensure performance is critical.
2, Welding
2.1 Welding
15CrMo steel for spot welding and working characteristics of high-pressure pipeline, based on past experience, the light of the welding process cards outside the country, we have chosen two options for welding test.
Program â… : welding preheating, using ER80S-B2L wire, T1G welding backing, E8018-B2 electrode surface covered arc welding electrodes, local heat treatment after welding.
Program â…¡: using ER80S-B2L welding wire , T1G welding backing, E309Mo-16 electrode surface covered arc welding electrodes filled, no heat treatment after welding. Wires and welding rods in the chemical composition and mechanical properties in Table 1.
Table 1 Chemical composition of welding and mechanical properties
Type C Mn Si Cr Ni Mo S P δb / Mpa δ,%
ER80S-B2L ≤ 0.05 0.70.41.2 <0.20.5 ≤ 0.025 ≤ 0.025 ≤ 500 25
E8018-B2 0.070.7 0.3 1.1 0.5 ≤ 0.04 ≤ 0.03 550 19
E309Mo-16 ≤ 0.12 0.5 ~ 2.5 0.9 22.0 ~ 25.0 12.0 ~ 14.0 2.0 ~ 3.0 ≤ 0.025 ≤ 0.035 550 25
2.2 welding preparation
Specimen with 15CrMo steel tube, specification of φ325 × 25, the type and size of groove shown in Figure 1.
With the angle grinder before welding the outside groove and the groove edge of the range 50mm to the exposed metallic luster polished, then cleaned with acetone.
Fixed position for the level of the specimen, corresponding gap of 4mm, with manual TIG welding along the park even six weeks, the length of each fixed at the point of not less than 20mm. Electrode according to the specifications in Table 2 baking.
Table 2 electrode baking specification
Model Baking temperature holding time of electrode
E8018-B2 300 ℃ 2h
E309Mo-16 150 ℃ 1.5h
2.3 The welding parameters
Required by program â…  preheating before welding, according to Tto-Bessyo et al preheating temperature calculation formula:
To = 350 √ [C] -0.25 (℃) where, To - preheating temperature, ℃.
[C] = [C] x + [C] p [C] p = 0.005S [C] x
[C] x = C + (Mn + Cr) / 9 + Ni/18 +7 Mo/90 where
[C] x - component of carbon equivalent;
[C] p - size of carbon equivalent; S - specimen thickness (this paper S = 25mm);
[C] x = C + (Mn + Cr) / 9 +7 / 90Mo = 0.361
[C] p = 0.045 则 To = 138 ℃
Therefore chosen as the preheating temperature 150 ℃. With oxygen - acetylene flame for heating the specimen, first determine the temperature pen rough surface temperature of the specimen (in handwriting, to estimate the speed of color change), and finally determined by semiconductor point thermometer, measuring points should be selected at least three o'clock to ensure that the sample as a whole have reached the required preheating temperature.
Welding, the first layer using GTAW base, in order to avoid overhead position welding seam at the back of a dip, wire filled wire method used, that wire through the tube into the gap from their counterparts. SMAW using other layers of welded 6 layers, each layer of a weld bead. â…  â…¡ programs and program the welding process parameters in Table 3,4. â…  welding by program
Table 3 program welding parameters â… 
Name weld welding consumables welding specifications / mm Welding current / A arc voltage / V preheat and interpass temperature of heat treatment specifications
A bottom tungsten plate welding ER80S-B2L φ2.4 110 12
SMAW fill layer E8018-B2 φ3.2 5 85 ~ 90 23 ~ 25150 ℃ 715. × 75min
SMAW covered surface E8018-B2 φ3.2 5 85 ~ 90 23 ~ 25
Table 4 program welding parameters â…¡
Name weld welding consumables welding specifications / mm Welding current / A arc voltage / V preheat and interpass temperature of heat treatment specifications
A bottom tungsten plate welding ER80S-B2L φ2.4 110 12
SMAW fill layer E309Mo-16 φ3.2 90 ~ 95 22 ~ 24 / /
SMAW covered surface E309Mo-16 φ3.2 90 ~ 95 22 ~ 24
Then, the layer temperature should be not less than 150 ℃, in order to prevent disruption caused by welding specimen cooling, when the welding operation by the two alternating welders, welding heat should be taken immediately after the slow cooling measures.
2.4 The post-weld heat treatment
Scheme â…  welding using specimens with local temperature post-weld tempering should be carried out. Heat treatment process as follows: heating rate of 200 ℃ / h, rose to 715 ℃ ℃ for 1 hour and 15 minutes, cooling rate 100 ℃ / h, down to 300 ℃, air cooling. JL-4 specific type of track with electric heaters (1146 × 310) surrounding the weld, with silicate cotton layer insulation, insulation thickness 50mm, the temperature is controlled by DJK-A-type electric heater automatic temperature control instrument.
3 Welding procedure qualification test
Specimen after welding by JB4730-94 "Pressure Vessel NDT" standard of 100% of the ultrasonic testing, welding â…  level qualified. By JB4708 "steel pressure vessel welding procedure qualification" standard welding procedure qualification test. Assessment of results in Table 5.
Table 5 the results of welding procedure qualification test
Tensile test bend test pilot program of testing in toughness aky (J/cm2)
Tensile strength δb / Mpa bending fracture surface of the back part of the bending angle bend weld fusion line heat affected zone (HAZ)
Program â…  550/530 base material 50. Qualified Qualified 162 135.6 84.8
Program â…¡ 525/520 base material 50. Qualified Qualified 96.7 79.4 109.2
From the tensile test results, the two programs off all the tensile samples in the base metal, indicating that the tensile strength higher than the base metal weld; bending test all qualified, that weld the plastic better. According to Table 5, the impact toughness test results, the program was significantly higher than the impact toughness â…  program â…¡, â…  welding certificate program is ideal norms after the heat treatment, tempering not only met the purpose of improving the joint microstructure and properties, but also the toughness and with the appropriate strength. Mechanical properties from room temperature results, the recommended two welding programs can be used for the construction site. Scheme â…  and base metal components used close to the electrode, weld properties match with the base metal, weld should have a higher thermal strength of the weld is not easy in the long-term use under high temperature damage. Difficulty is more stringent post-weld heat treatment specifications, tempering temperature and holding time and heating and cooling speed control but will lead to improper welding performance. Scheme â…¡ by welding of austenitic stainless steel electrode, although the post-weld heat treatment can save, but the expansion coefficient of the weld and base metal different from the long-term high temperature can occur when the diffusion of carbon in the phenomenon, easily lead to weld in the fusion zone damage occurred. Therefore, from the use of reliability considerations, field welding by more steady Scheme â… .
4 Conclusion
15CrMo steel welding thick-walled high-pressure pipe welding using two options are feasible. To ensure the weld properties match with the base metal and has a high thermal strength, the adoption of the program â…  better, the key is to strictly control the post-weld heat treatment process.
Although the program â…¡ weld heat treatment can be avoided, but the welds occurred at a high temperature diffusion of carbon migration possibility of damage caused by the weld can not be ignored, therefore, not only in the post-weld heat treatment Shicai carefully used.

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