Hardfacing the Inner Surface of Long-Neck Flanges Using CO₂ Gas-Shielded Welding

A corrosion-resistant stainless steel layer is applied to the inner surface of the long weld neck flange on pressure vessels to enhance its resistance to corrosion. This paper discusses the application of CO₂ gas-shielded welding with stainless steel core wire and appropriate welding auxiliary equipment to enable automatic overlay welding of the stainless steel layer in a horizontal position, thereby improving the quality of the overlay welding process.
 

The material used for the long weld neck flange in pressure vessels with high H₂S content is Q345R, with a nominal diameter ranging from DN200 to DN800.
 

The overlay welding layer for pressure vessels with high H₂S content must meet the following technical requirements:
(1) A double-layer overlay welding system should be applied. The overlay layer thickness should be 3 mm, and the surface layer thickness should also be 3 mm. The composite layer thickness after machining must be no less than 3 mm.
(2) The chemical composition of the stainless steel corrosion-resistant layer must comply with the specifications listed in Table 1.
(3) Post-overlay welding testing requirements: The transition and composite layers must undergo penetration or magnetic particle testing, in addition to side-bending tests (d = 4a, A = 180°).
(4) Both macroscopic and microscopic metallographic inspections must show no significant defects.
(5) The hardness of the weld overlay layer (HB) must not exceed 200.
(6) The weld overlay layer must pass the following corrosion tests: stress corrosion (GB/T15970.7), uniform corrosion (JB/T7901-1999), pitting corrosion (GB/T17897), and intergranular corrosion (GB/T4334).
 

The weld seams on the inner surface of the long weld neck flange are typically circumferential welds. To meet the stringent efficiency and quality standards for the overlay welding layer, a CO₂ gas-shielded flux-cored wire overlay welding is employed. This method offers several advantages, such as process stability, enhanced bead formation, low heat input, and the potential for automation.
 

E309LmoT1-1 welding wire is selected for the transition layer, and E316LT1-1 welding wire is used for the surface layer. Based on the specified composition and technical specifications for the welding wire, AFS-309MoL, produced by Kunshan Jingqun Welding Material Co., Ltd., is used for the transition layer of the flux-cored stainless steel wire, while AFS-316L is used for the corrosion-resistant layer.
 

The Panasonic RFII350 MAG welding machine meets the requirements of this welding process.
 

The overlay welding seam follows a spiral pattern with a continuous, uniform bead. To optimize efficiency, the workpiece rotates while the welding gun remains stationary. Therefore, the welding positioning device, designed and built in-house, uses a motor, gearbox, and sprocket-driven transmission system. Additionally, flat welding positions enhance welding seam quality, resulting in savings of over 50% in both labor time and welding materials.
 

To adjust the machine head position according to the surfacing welding process, a welding gun translation device was developed in-house. Its key feature is the threaded track mechanism, which ensures stable, automatic positioning of the welding gun.
 

To maintain consistent horizontal positioning of both sides of the inner surface of the long weld neck flange during the surfacing welding process, and to enable fully automatic overlay welding, the equipment is integrated as shown in Figure 1. The S7-200PL C+ touchscreen provides integrated control of the positioning device and welding gun translation system.


Figure 1 Surfacing Equipment Integration
 
Table 1 Chemical composition of overlay welding layer

 
Table 2 CO2 shielded flux-cored wire stainless steel surfacing process parameters 1.6 mm

 
Table 3 Preheating, inter pass temperature and post-heat treatment

 

In CO₂ gas-shielded flux-cored wire surfacing, correctly matching the arc voltage to the welding current is essential. For a given welding current, if the arc voltage is too low, the weld bead becomes too narrow. This increases the likelihood of slag inclusion and excessive surfacing thickness, leading to wasted welding material. Conversely, if the arc voltage is too high, the weld bead becomes excessively wide, the weld thickness may not meet the required specifications, and bead formation is poor. Experimental results show that optimal bead formation occurs when the arc voltage is between 28 and 34 V.
 

Welding Current: The welding current affects arc stability, bead formation, penetration depth, dilution rate, and mechanical properties. The voltage (U = 28–34 V), gas flow (Q = 15–20 L/min), and welding speed (T = 200 mm/min) were applied during testing with a standard welding current. The optimal welding current for this process is 200 A.
 

Welding speed primarily affects the weld bead shape and the dilution rate of the weld deposit. If the welding speed is too high, the weld bead becomes too narrow and thin, increasing the risk of overlap during subsequent passes. If the speed is too low, the weld bead may have excessive stack height and insufficient width, causing nozzle clogging and slag inclusion. Testing results indicate that the optimal welding speed for this process is 200 mm/min.
 

The wire extension length influences arc stability, penetration, and heat input. If the extension is too long, the arc becomes unstable, leading to increased spatter. If it is too short, excessive spatter may block the nozzle, reducing the effectiveness of gas shielding and increasing the likelihood of pore formation during overlay welding. Based on testing results, the optimal wire extension length is between 10 and 15 mm. During overlay welding, the welding gun should be positioned at an angle of 80°–85° to the horizontal and 25°–30° to the vertical axis of the weld bead.
 

The overlap between overlay welding weld passes (the distance between the welding wire and the preceding weld) directly affects the thickness and uniformity of the overlay welding layer. Improper overlap can negatively affect the mechanical properties of the overlay welding, such as its bending strength. To optimize the process parameters, a series of tests were conducted. When the welding wire overlapped 50% of the previous weld's edge, the bead formed smoothly and exhibited good mechanical properties.
 

The recommended CO₂ gas flow rate for flux-cored wire welding of stainless steel is 15–20 L/min. If the flow rate is too low, the shielding gas will be inadequate, compromising the gas shielding effectiveness. Conversely, an excessively high flow rate may lead to waste.
 

Based on the test results, the optimal welding process parameters—preheating, interpass temperature, and post-heat treatment—were identified for the inner surface cladding of long-neck flanges using CO₂ gas-shielded stainless steel flux-cored wire (see Tables 2 and 3).
 

(1) Start by cleaning the surface to remove oil, rust, and oxide scale, exposing the metallic surface. Then, perform 100% PT flaw detection.
(2) Apply the fatigue layer using AFS-309MoL welding wire, maintaining a welding height of 3 mm. After processing, conduct 100% PT flaw detection, then apply the corrosion-resistant layer using AFS-316L welding wire. The welding height is controlled at approximately 3 mm, and welding the inner surface should ideally be done in a horizontal position.
(3) Use narrow, non-swinging continuous pressure welds with a 45%–50% overlap of the weld width (align the welding wire with 50% of the previous weld's edge), welding from the inside out.
(4) After welding each flange, clean the welding slag and remove any spatter from the nozzle. Fill the arc pit as the arc closes.
(5) During welding, the welding gun should be tilted at an angle of 80°–85° to the horizontal and 25°–30° to the vertical axis of the weld.
 

After the process test, the test plate was welded using the selected process parameters. The test plate, made of Q345R (R-HIC), measured 1000 × 200 mm and was welded circumferentially. After welding, the plate was heat-treated at 620°C for 3 hours, and various tests were conducted as required for composite plate manufacturing. The test results are as follows:
(1) The transition and composite layer color inspection, along with the ultrasonic inspection of the welded parts, were successful.
(2) The bending test and chemical composition analysis conformed to the relevant standards.
(3) The four corrosion tests—stress corrosion, intergranular corrosion, uniform corrosion, and pitting corrosion—complied with the test standards and technical specifications.
 

To verify the feasibility of the developed automatic flux-cored wire CO₂ gas-shielded welding process for the inner surface of the long-neck flange, a verification part was produced.
 

After lathe machining, 100% UT and PT inspections were conducted on the surface of the welded long-neck flange, and the results were classified as Level I. The verification results confirm that the developed process is feasible.
 

For stainless steel overlay welding of long-neck flanges on high-H₂S pressure vessels, the use of appropriate tooling equipment enables automatic overlay welding with CO₂ gas-shielded flux-cored wire, thereby improving overlay welding quality and reducing costs. This is particularly advantageous for overlay welding the inner surfaces of large-diameter long-neck flanges.