Experimental Study on Stainless Steel Surfacing of Vessel Head Inner Wall

Literature Overview

This paper by Du Ning from Harbin Electric Group (Qinhuangdao) Heavy Equipment Co., Ltd., published in China Chemical Equipment (2017, Vol. 19, No. 2, pp. 13-16), addresses a practical challenge in chemical equipment manufacturing: the stainless steel cladding of vessel head inner walls. The study investigates two forms of strip electrode surfacing and three deposition trajectories, with detailed discussion of process parameters and operational essentials to ensure weld quality.

Core Technical Content

The study focuses on submerged arc surfacing (SAW) and strip electrode electroslag surfacing (ESS) as two primary methods for depositing stainless steel layers on vessel head inner walls. These methods are particularly relevant for large-diameter pressure vessels where the inner surface requires corrosion resistance while the structural shell can remain as low-carbon or low-alloy steel.

Surfacing Methods Compared

Deposition Trajectories

The three deposition trajectories studied are:

1. Spiral trajectory - suitable for hemispherical heads, where the torch moves in a helical path from pole to pole, providing uniform coverage and minimizing undercut at trajectory transitions.
2. Parallel trajectory - applied to dished heads with parallel passes, requiring overlap of 10-15% between adjacent passes to ensure full coverage without gaps.
3. Circumferential trajectory - used for torispherical heads, where circumferential passes are combined with axial movements to achieve complete coverage of the curved surface.

Process Parameters and Operational Essentials

The study emphasizes several critical process parameters for ensuring sound surfacing:

Key Operational Points

The paper highlights several operational essentials that deserve attention in engineering practice:

• First pass dilution control: The first surfacing pass experiences the highest dilution from the base metal, which can reduce the alloy content of the deposited layer below the required specification. A transition layer or increased alloy content in the first pass is recommended to compensate.
• Trajectory transition management: When transitioning between deposition trajectories, overlapping by 2-3 mm beyond the previous trajectory boundary prevents uncovered areas and ensures continuity of the cladding layer.
• Thermal management on curved surfaces: On highly curved head geometries, the effective arc length varies with the angle of incidence. Operators must adjust the torch angle to maintain a consistent effective gap between the strip electrode and the workpiece surface.
• Flux management: For electroslag surfacing, the slag pool must be maintained at a consistent thickness of 3-5 mm. Too thin a slag layer causes arc breakthrough and surface irregularities; too thick a layer increases dilution and slows the process.

Engineering Practice Implications

From a practical standpoint, this study addresses a common requirement in chemical equipment manufacturing where vessel heads are fabricated from economic steel grades but require stainless steel protection on the inner wetted surface. The choice between strip electrode SAW and electroslag surfacing depends on the head diameter, required cladding thickness, and production volume.

For vessel heads with diameters exceeding 2000 mm, electroslag surfacing offers superior efficiency due to its high deposition rate. However, the equipment investment is substantial, and setup time for each individual head is longer. Strip electrode SAW provides greater flexibility for smaller production runs and more complex geometries.

A critical consideration not fully addressed in the paper is the post-surfacing inspection protocol. According to NB/T 47014 and ASME Section IX, qualification testing of surfacing procedures should include:

• Visual examination (VT) for surface defects, undercut, and porosity
• Magnetic particle testing (MT) for subsurface cracks
• Ultrasonic testing (UT) for lack of fusion and delamination
• Hardness testing to verify the dilution rate and alloy composition
• Corrosion testing (e.g., salt spray or specific media) to confirm the protective function

Key Questions and Reflections

One question that arises from this study is the long-term reliability of the surfacing layer under cyclic thermal and pressure loading. While the paper focuses on deposition quality, the fatigue behavior of the surfacing layer under repeated thermal cycling in service is equally important. Residual stresses from the surfacing process, combined with operational thermal stresses, can lead to interfacial cracking at the surfacing-base metal boundary.

Another consideration is the selection of stainless steel grade for the cladding. For general chemical service, 304 or 316 stainless steel is typically adequate. However, for aggressive environments containing chlorides or sulfur compounds, higher alloy grades such as 316L, duplex stainless steels, or even nickel-based alloys may be required. The dilution from the carbon steel base can significantly reduce the corrosion resistance of the deposited layer, making it essential to achieve a minimum of 2-3 passes to reduce dilution below 10%.

Summary

This study provides valuable practical guidance for the stainless steel surfacing of vessel head inner walls, with clear delineation of two surfacing methods and three trajectory configurations. The process parameters and operational essentials presented are directly applicable to production environments. Engineers should note that while the paper focuses on deposition quality, the overall service reliability depends on comprehensive quality control including post-weld inspection, dilution verification, and corrosion testing. The selection between strip electrode SAW and electroslag surfacing should be made based on a careful evaluation of head geometry, required cladding thickness, production volume, and total cost of ownership including equipment, consumables, and inspection.