Manual Surfacing Welding Process for Industrial Valves

Literature Overview

The paper by Luo Xiaoming and Xu Weipu, published in Chemical Engineering Equipment and Piping (2007, Vol. 44, No. 3, pp. 54–56), addresses the manual surfacing (build-up) welding process applied to industrial valves. Surfacing welding is a critical technique used to restore worn valve seats, stems, and sealing surfaces, or to apply corrosion- and wear-resistant overlay layers on valve bodies. The authors, affiliated with the National Valve Quality Supervision and Inspection Center under the Shanghai Special Equipment Supervision and Inspection Technology Institute, provide a systematic discussion of weld preparation, process parameter selection, and defect prevention measures. This work is particularly relevant for maintenance engineers and quality inspectors responsible for valve repair and reconditioning in petrochemical and process industries.

Core Technical Points

The central argument of the paper is that improper manual surfacing operations lead to a variety of quality-affecting defects, including porosity, slag inclusion, cracking, porosity (loose structure), and uneven hardness distribution across the overlay. The authors emphasize that proper pre-weld preparation combined with a rational and reliable welding procedure specification (WPS) can fundamentally guarantee surfacing quality. However, beyond standard WPS parameters, special operational techniques are required to address each specific defect type.

Weld Preparation and Pre-Heating

Before surfacing, the base metal surface must be thoroughly cleaned of rust, scale, oil, and moisture. The edge of the worn surface should be prepared by machining or grinding to create a suitable groove profile, typically a single-V or U-shaped groove with an included angle of 60°–90° depending on the repair geometry. Pre-heating is essential for valves made of carbon steel (e.g., WCB, WC6) and alloy steel (e.g., CF8M, CF3M) to reduce the cooling rate and prevent hydrogen-induced cracking. Typical pre-heat temperatures are summarized below.

Process Parameters and Operational Techniques

The surfacing process typically employs SMAW (shielded metal arc welding) or GTAW (gas tungsten arc welding), depending on the overlay material and joint geometry. For manual SMAW surfacing, the following parameter ranges are commonly applied:

The authors stress that the electrode should be held at a slight drag angle (10°–15° from vertical) to promote better fusion and a flatter bead profile. Stringer beads should be kept narrow and uniform to minimize dilution with the base metal and ensure consistent overlay composition.

Defect Analysis and Countermeasures

The paper systematically addresses the following common defects and their countermeasures:

Porosity

Porosity in surfacing welds is primarily caused by moisture in the electrode coating, contaminated base metal, or excessive arc length. Countermeasures include strict electrode storage and baking (typically 300–350 °C for 1–2 hours for low-hydrogen electrodes), thorough surface cleaning, and maintaining a short arc length (1.0–1.5 times the electrode diameter).

Slag Inclusion

Slag inclusion occurs when slag from the previous pass is not fully removed before the next layer is deposited. The countermeasure is meticulous slag removal between passes using a wire brush and, if necessary, a grinder. The authors recommend using a slag-peeling electrode with good slag fluidity to facilitate this process.

Cracking

Cracking in surfacing welds can be classified as hot cracks (in the overlay) or cold cracks (in the HAZ or fusion zone). Hot cracks are associated with high sulfur and phosphor content and are mitigated by selecting low-sulfur, low-phosphor electrodes. Cold cracks are hydrogen-related and are prevented by using low-hydrogen electrodes, proper pre-heating, and controlled cooling rates.

Uneven Hardness Distribution

Uneven hardness across the overlay is often caused by inconsistent dilution rates, improper electrode manipulation, or variations in interpass temperature. The authors recommend maintaining a consistent travel speed, keeping the electrode at a uniform angle, and controlling interpass temperature to ensure uniform solidification microstructure.

Engineering Practice Integration

In practical valve maintenance operations, the surfacing process is often performed on valves that have been in service for extended periods and have experienced erosion, corrosion, or mechanical wear. The following FMEA (Failure Mode and Effects Analysis) approach is recommended to manage surfacing quality risks:

After surfacing, the valve should undergo dimensional inspection (seat flatness, stem concentricity), hardness testing (minimum 3 points per overlay area), and, where required, non-destructive testing (MT or PT) to verify the absence of surface defects. The overlay thickness should be verified to meet the minimum specified value, typically 1.5–3.0 mm for valve seat repair.

Study Insights and Implications

This paper, while relatively concise, provides a valuable practical framework for valve surfacing operations. The emphasis on defect-specific countermeasures rather than generic WPS parameters is particularly useful for field engineers who must adapt procedures to varying conditions. One key insight is that the quality of surfacing is not solely determined by the WPS but also by the welder's technique and discipline in following inter-pass procedures. In my experience, the most common cause of surfacing failures in the field is not inadequate procedure design but rather inconsistent execution, particularly in slag removal and interpass temperature control. The paper's systematic approach to defect prevention serves as a useful checklist for quality assurance personnel. Furthermore, the work underscores the importance of post-weld inspection and the need for documented traceability in valve repair operations, which is critical for safety-critical applications in pressure vessels and piping systems.