Manual Welding Overlay on the Inner Wall of Stainless Steel Pipes

Literature Overview

The manual welding overlay of stainless steel on the inner wall of pipes is a specialized technique used in the fabrication of heat exchangers, condensers, and chemical processing equipment where corrosion resistance is required on the bore surface. Unlike external overlay, which can be performed with mechanized equipment, inner-wall overlay requires manual welding techniques that accommodate the confined geometry, limited access, and restricted electrode reach. This literature examines the welding procedures, filler metal selection, and quality control measures for GTAW (gas tungsten arc welding) and GMAW (gas metal arc welding) overlay of austenitic stainless steel on carbon steel pipe inner walls.

Core Technical Points

Geometric Challenges of Inner-Wall Overlay

The primary challenge of inner-wall overlay is the restricted access to the weld zone. For a typical 89 mm outer diameter pipe with a 6 mm wall thickness, the available working space inside the pipe is approximately 77 mm in diameter, which limits the torch angle and electrode reach. The welder must maintain a consistent torch angle of 15 to 25 degrees from the vertical to achieve adequate penetration and a uniform bead profile. The limited visibility also makes it difficult to monitor the weld pool, increasing the risk of burn-through and incomplete fusion.

Filler Metal Selection

The selection of filler metal is critical for ensuring compatibility between the stainless steel overlay and the carbon steel substrate. The literature recommends using 309L or 310L stainless steel filler metal for the first pass to accommodate the dilution from the carbon steel substrate. The 309L composition, with approximately 23% chromium and 13% nickel, provides a sufficient dilution margin to maintain a single-phase austenitic microstructure in the first pass. Subsequent passes can use 304L or 316L filler metal to achieve the desired corrosion resistance in the final overlay.

Welding Position and Technique

Inner-wall overlay is typically performed in the overhead position, with the pipe supported on a fixture that allows rotation. The welder uses a push technique with a torch angle of 20 degrees and a travel speed of 150 to 200 mm/min for GTAW. The arc length is maintained at 2 to 3 mm, and the welding current is set at 100 to 130 A for a 2.4 mm tungsten electrode. For GMAW, a short-circuit transfer mode is used with a current of 120 to 150 A, voltage of 18 to 22 V, and wire feed rate of 3 to 4 m/min.

Process Parameter Optimization

Heat Input Control

The heat input for inner-wall overlay must be carefully controlled to minimize distortion of the thin-walled pipe and to prevent burn-through. The recommended heat input range is 0.5 to 1.5 kJ/mm for GTAW and 1.0 to 2.0 kJ/mm for GMAW. Excessive heat input causes pipe ovalization and can lead to post-weld distortion that affects the dimensional accuracy of the assembled equipment. The literature recommends performing a test weld on a coupon of the same thickness before production welding to optimize the heat input parameters.

Preheating and Interpass Temperature

Preheating is generally not required for inner-wall overlay of stainless steel on carbon steel, as the thin wall thickness provides adequate heat dissipation. However, for thicker-walled pipes (above 10 mm), a preheat temperature of 50 to 100 °C is recommended to reduce residual stress and minimize cracking risk. The interpass temperature must be kept below 150 °C to prevent excessive grain growth in the overlay and to maintain the corrosion resistance of the stainless steel.

Gas Shielding

Adequate gas shielding is essential for inner-wall overlay to prevent contamination of the weld pool and the hot back side of the pipe. For GTAW, a combination of external argon shielding at 15 to 20 L/min and internal argon backing at 5 to 8 L/min is used. The internal gas flow is introduced through a purge fitting at the weld start point and exits at the weld end. For GMAW, the shielding gas flow is increased to 18 to 25 L/min to compensate for the turbulent flow conditions inside the pipe.

Common Defects and Countermeasures

Burn-Through

Burn-through is the most common defect in inner-wall overlay of thin-walled pipes. It occurs when the heat input is too high, causing the base metal to melt completely and creating a hole in the pipe wall. Countermeasures include reducing the welding current by 10 to 15%, increasing the travel speed by 20 to 30%, and using a smaller diameter tungsten electrode or welding wire. The literature recommends performing a burn-through test on a coupon before production welding to establish the maximum allowable heat input.

Incomplete Fusion

Incomplete fusion at the interface between the overlay and the pipe wall is a critical defect that compromises the corrosion barrier function of the overlay. Causes include insufficient current, excessive travel speed, and inadequate torch angle. Countermeasures include increasing the current by 10 to 15%, reducing the travel speed by 15 to 20%, and ensuring the torch angle is maintained at 15 to 25 degrees from the vertical. The literature recommends using a welder qualification test according to ASME Section IX to verify that the welder can consistently produce sound welds in the inner-wall position.

Surface Porosity

Surface porosity in the overlay is caused by gas entrapment from moisture in the filler metal or from inadequate gas shielding. Countermeasures include using low-hydrogen filler metal with moisture content below 0.1%, ensuring the welding wire is stored in a dry environment, and maintaining a minimum gas flow rate of 15 L/min for GTAW and 18 L/min for GMAW. The literature recommends performing visual inspection of the overlay surface after each pass and grinding out any porosity before proceeding to the next pass.

Quality Control and Inspection

The quality of inner-wall overlay is verified through visual inspection, dye penetrant testing, and ultrasonic testing. Visual inspection checks for surface defects, undercut, and bead profile consistency. Dye penetrant testing is applied to the exposed overlay surface to detect surface-breaking cracks and porosity. Ultrasonic testing is performed from the outer surface to assess the bond strength and detect subsurface defects at the interface. The acceptance criteria for inner-wall overlay are typically more stringent than for external overlay, requiring no indications exceeding 2 mm in length for PT and no indications exceeding 3 mm in length for UT.

Engineering Practice Integration

In the fabrication of condensers for the power generation industry, inner-wall overlay of stainless steel on carbon steel pipes is used to provide corrosion resistance against condensing steam and flue gases. A case study from a 600 MW coal-fired power plant demonstrated that a three-pass overlay of 309L, 304L, and 316L on SA179-T2 condenser tubes achieved a uniform overlay thickness of 2.0 to 2.5 mm with no burn-through or incomplete fusion defects. The overlay was verified by PT and UT inspection, and the assembled condenser passed a hydrostatic test at 1.5 times the design pressure without leakage.

Key Questions and Reflections

The literature raises several practical questions for engineers involved in inner-wall overlay. First, the welder skill level required for consistent quality in the confined geometry is high, and the qualification and certification process must be rigorous to ensure that only experienced welders are assigned to this task. Second, the inspection of inner-wall overlay is challenging due to the limited access for UT probes and the difficulty of applying PT to curved surfaces. Third, the long-term performance of the overlay under cyclic thermal loading is not well documented, and there is a risk of intergranular corrosion or stress corrosion cracking if the overlay is sensitized during welding.

Study Insights and Implications

The study of manual welding overlay on the inner wall of stainless steel pipes provides a practical framework for achieving high-quality corrosion-resistant overlays in confined geometries. The key insight is that the combination of proper filler metal selection, controlled heat input, and adequate gas shielding is essential for producing sound welds with acceptable bond strength and corrosion resistance. Engineers must adopt a systematic approach to welder qualification, process parameter optimization, and quality control to ensure that every overlay meets the stringent requirements of the end application. The study also highlights the importance of post-weld inspection and the need for non-destructive testing methods that are suitable for the confined geometry of inner-wall welds.