Welding of high-pressure boiler heated tubes in power plants— Butt welding of tubes

Welding of high-pressure boiler heated tubes in power plants— Butt welding of tubes

I. Technical status of base metal 15CrMo steel is a low-alloy heat-resistant steel with a pearlitic structure. It has high thermal strength (σb ≥ 420 MPa) and oxidation resistance at high temperatures, along with some resistance to hydrogen corrosion. Its delivery condition is normalized + tempered. Due to the higher content of Cr, Mo, C, and other alloy elements, the tendency for quench hardening is evident, making weldability poor. It is widely used in petrochemicals, power generation, and other industries, often used for manufacturing steam conduits, oil pipelines, and other special components. The chemical composition of 15CrMo steel is shown in Table 1, and the heat treatment parameters of 15CrMo steel are shown in Table 2.

Steel grade	Standard number	Chemical composition (%)
		C
15CrMo	GB/T3077-1999	0.12～0.18

Table 1 Chemical composition w(%) of 15CrMo steel

Table 2 Heat treatment parameters of 15CrMo steel

Steel grade	Standard number	Standard-specified heat treatment	Normalizing or quenching temperature/°C	Tempering temperature/°C	Holding time/h
15CrMo	GB6654-1996	Normalizing + tempering	930～960	680～720	Batch furnace ≥2h
Continuous furnace ≥1h

II. Selection and technical status of welding materials When welding with CO₂ gas shielded arc welding, the Cr, Mo, V in the welding wire basically do not burn off, while Mn, Si burn off significantly. Therefore, welding wires with higher Mn, Si contents than the base metal should be used. According to GB/T8110-1995 ‘Welding wire for carbon steel and low-alloy steel with gas shielded arc welding’, the welding wire brand H08CrMnSiM is selected. The diameter of the welding wire is Φ1.2 solid wire, and the shielding gas is carbon dioxide gas. According to the standard HG/T2537, the purity should be ≥99.5%. CO₂ welding is mainly used for welding carbon steel and low-alloy steel. For stainless steels, due to carburation in the weld metal, which affects intergranular corrosion resistance, it can only be used for welding stainless steel components where weld performance requirements are not high. Additionally, CO₂ welding can be used for surfacing wear-resistant parts, repairing cast steel parts, and electric riveting welding. At present, CO₂ welding has been widely applied in automobile manufacturing, locomotive and vehicle manufacturing, chemical machinery, agricultural machinery, mining machinery, and other sectors.

III. Welding equipment and tools Welding equipment: welding machine model NB-250, name semi-automatic CO₂ welding machine. Welding tools: welding tongs, welding cable; welding gloves, face shield, insulating shoes, work clothes. Auxiliary tools: angle grinder, chisel, wire brush, file, drying oven, welding rod insulation cylinder, slag removal hammer, and universal gauge for welds.

IV. Pre-weld preparation

1. Welder qualification assessment: All welders participating in the welding must receive training according to the rules issued by the Ministry of Labor and Personnel ‘Rules for Examination of Boiler Pressure Vessels’ and the already evaluated and approved welding process. They must hold valid certificates before they can go to work.
2. Before use, welding wires must be cleaned of rust and oil stains, and the metallic luster should be exposed.
3. Suitable welding auxiliary devices must be chosen based on the welding position, joint form, and operational efficiency. A strict inspection of the welding machine and auxiliary equipment must be performed to ensure normal operation of the electrical circuit, water supply, gas supply, and mechanical components.
4. The groove shape and dimensions of the weld can be determined according to GB/T985-88 ‘Basic shapes and dimensions of grooves for gas welding, manual arc welding, and gas shielded arc welding’, combined with specific working conditions, as shown in Figure 1. Before welding, the groove and the area around the groove (10～20mm) must be kept clean, free of rust, oil, water, paint, etc., that may affect welding quality.

Technical requirements: Welding method: CO₂ gas shielded arc welding; Groove type: V-groove, single-side groove angle 30°; Root face 0.5-1mm; Processing method: using groove processing machine; Specimen material: 15CrMo low-alloy heat-resistant steel.

As shown in Figure 1 Assembly dimensions.

V. Welding process parameters According to the basic guidelines and requirements of JB/T9186-1999 ‘CO₂ gas shielded arc welding process regulations’, the welding process parameters are specified as follows:

1. Welding wire diameter: The diameter of the welding wire is Φ1.2 mm. As the diameter of the welding wire increases, the size of spatter particles also increases.
2. Welding speed: The welding speed should meet the requirements of different types of steel for welding line energy. Typically, the welding speed is in the range of 30～60 cm/min.
3. Protrusion length of welding wire: The protrusion length of the welding wire is related to the welding wire diameter, welding current, and welding voltage. It is approximately ten times the diameter of the welding wire. If the protrusion length is too large, the arc becomes unstable, making it difficult to operate, increasing spatter, and deteriorating the weld formation, even leading to porosity due to loss of protection. Conversely, if the protrusion length is too short, it shortens the distance between the nozzle and the workpiece, easily clogging the nozzle with splashed metal. It also obstructs the view of the arc, affecting the welder’s operation. During short-circuit transition welding, the protrusion length of the welding wire has a significant impact on the melting rate of the wire.
4. Power supply characteristics: Direct current reverse polarity. This ensures stable arcs, minimal spatter, good weld formation, and deep penetration, resulting in high productivity.
5. Shielding gas flow rate: Poor shielding effect can lead to porosity and deterioration of weld formation. The shielding gas flow rate ranges from 6～15 L/min.
6. Welding current: The welding current is an important welding parameter and the main factor determining the thickness of the weld bead. The magnitude of the current primarily depends on the feed rate of the wire. Under the principle of ensuring penetration without burning through the base metal, the welding current should be correctly selected based on the thickness of the base metal, joint form, and welding wire diameter. The current range is 90～150 A.
7. Arc voltage: The arc voltage must be reasonably matched with the welding current. The arc voltage is between 19～23 V. Increasing the arc voltage significantly increases the width of the weld bead. If the arc voltage is set too high, regardless of other parameter settings, a stable short-circuit transition process cannot be achieved.

VI. Welding procedure

1. Positional welding: Use a three-point positioning method for welding. The length of the positioning weld is 10 mm; assembly clearance is 2～3 mm; mismatch should not exceed 0.5 mm (see Figure 2).

Figure 2 Horizontal fixed pipe assembly and positioning welding diagram

2. Root pass: Divide the pipe into left and right halves along the centerline of the weld pipe. First, weld the right half counterclockwise, then weld the left half clockwise. The starting and ending points of the arc should extend 5～10 mm beyond the centerline of the pipe. The axis of the welded pipe should be parallel to the horizontal plane and fixed on a jig at a certain height from the ground. The shorter segment should be below. Start welding from the bottom end (from the 6 o’clock direction to the 12 o’clock direction) using the arc extinguishing method (see Figure 3). Before welding the second half, grind the ends of the welds at the 6 and 12 o’clock positions into slopes about 10～20 mm long. Start welding just past the 6 o’clock position after striking the arc, then return to the grinding area to begin welding. At the ending point, continue welding past the grinding area before stopping the arc.

Figure 3 Key points of arc extinguishing welding

Figure 4 Welding direction and angle

3. Filling pass: Before welding the filling layer, clean the surface of the root pass weld to remove spatter and slag. Then, grind the raised parts of the joint flat with an angle grinder. After adjusting the welding parameters, proceed with welding. While welding the filling layer, move the welding rod back and forth in a crescent or zigzag pattern (see Figure 5). The welding rod should pause slightly on both sides of the weld bead, moving a little faster in the center. The swing amplitude should refer to the width of the previous layer of the weld bead.

Figure 5 Welding rod swing during the filling pass of horizontal pipe butt welding

4. Cover pass: Clean the surface of the filling layer weld before welding. The welding operation method for the cover pass is the same as for the filling pass, but the transverse swing amplitude should be larger than that of the filling pass. Ensure that the molten pool penetrates each side edge of the groove by 0.5～1.5 mm. The arc should stay briefly at the edges of the groove and move slowly when returning. The operation at the junction of the second half circle after welding is shown in Figure 6.

Figure 6 Operation diagram for the filling pass

VII. Other considerations

1. Preheating before welding (according to ISO 13916:1996) (1) Preheating requirements should be comprehensively considered based on the chemical composition, weldability, thickness, restraint degree of the welding joint, welding method, and welding environment of the base metal. Preheating temperatures are shown in Table 3.

Table 3 Preheating temperatures

Steel grade	Preheating temperature	Remarks
15CrMo	150-250	Below 0°C take upper limit

(2) Preheating methods should include electric heating, infrared heating, etc. Flame heating can also be used, but the flame should not directly touch the weld groove. The preheating should be gradual and uniform to prevent local overheating. Contact-type thermometers should be used, and measurement points should be evenly distributed. (3) The welding joint should be completed in one continuous operation. After completing the root pass, immediately start the next layer of welding. If interrupted, immediate post-heating should be performed. Before resuming welding, inspect the weld to confirm the absence of cracks or other defects.

2. Post-weld heat treatment Post-weld heat treatment should be carried out immediately after welding to reduce residual stress and obtain the desired properties. The heat treatment process is specified as follows: (1) Use electric heating method. (2) Heating and cooling rates should be controlled at ≤220°C/h. Cooling below 300°C can be uncontrolled. (3) Heat treatment temperature and holding time are shown in Table 3.

Table 3 Heat treatment temperature and holding time

Steel grade	Heat treatment temperature (°C)	Holding time (h)
15CrMo	670～700	1 h

(4) The heating width should be no less than three times the wall thickness of the pipe and no less than 60 mm from the center of the weld. (5) The heating width should be no less than five times the wall thickness of the pipe and no less than 100 mm from the center of the weld to reduce the temperature gradient. (6) The heating method should aim for uniform temperatures on the inner and outer walls and both sides of the weld. During constant temperature holding, the temperature difference between any two measuring points within the heating range should be less than 50°C. (7) Temperature measurement must be accurate and reliable, using automatic recording. Instruments, thermocouples, and accessories should be calibrated. (8) Measuring points should be symmetrically arranged on both sides of the center of the weld, with no fewer than two points. On horizontal pipes, measuring points should be arranged symmetrically up and down. (9) Records and markings of heat treatment must be made.

VIII. Welding quality inspection (according to JB/T4709-2007 standards)

1. Visual inspection of weld appearance (according to ISO 17637:2003 ‘Visual testing of fusion welds’) The weld edges should smoothly transition to the base metal. The weld dimensions should comply with the design drawings and process documents. The weld height should not be lower than the surface of the base metal, ideally 0～2 mm. There should be no cracks, slag inclusions, arc pits, or porosity on the surface of the weld and heat-affected zone. The depth of undercut should be ≤0.5 mm, and the total length of undercut on both sides of the pipe weld should not exceed 20% of the circumference or 40 mm. Passageway testing should be conducted on heated surface pipes.
2. Non-destructive testing (NDE) inspection (according to GB/T12605-90 and GB/T15830-1995 standards) (1) The unevenness of the weld surface should not affect NDE evaluation. (2) NDE should be carried out more than 24 hours after welding. (3) Radiographic testing should be evaluated according to GB3323-87 ‘Radiographic testing of fusion welds in steel butt joints and classification of quality’. Class I welds require level II qualification, and class II welds require level III qualification. Ultrasonic testing should be evaluated according to GB11345-89 ‘Manual ultrasonic testing of steel welds and grading of test results’. Class I welds require B level I qualification, and class II welds require B level II qualification. Table 4 NDE inspection rate table

Weld category	Class I	Class II
Ultrasonic testing inspection rate (%)	100	100
Radiographic testing inspection rate (%)	25	10

(4) If unacceptable defects are found during partial NDE, additional inspections should be carried out in the direction of the defect or in suspicious areas. If unacceptable defects are still found after additional inspections, NDE should be performed on all welds completed by the same welder. (5) The results of NDE must be reported to the supervisor within 48 hours after completion.