Strip Electrode Surfacing Process for Lock Hopper Cone Body

Literature Overview

The paper by Guo Xiaochun, published in Welding Technology (Vol. 41, No. 2, 2012, pp. 23-25), presents a process study on strip electrode surfacing of 00Cr17Ni14Mo2 austenitic stainless steel onto a 16MnR carbon steel substrate for a lock hopper cone body. The work was conducted by Daqing Oilfield Engineering Construction Co., Ltd. Building Materials Company. The study addresses the practical challenges of surfacing a large, curved component with a corrosion-resistant overlay, including the selection of surfacing method, design of welding fixtures, and determination of welding parameters through process qualification.

Lock hoppers are critical components in oil and gas processing facilities where they handle corrosive fluids, particularly sour gas containing hydrogen sulfide (H2S). The cone body, which is exposed to continuous fluid flow and corrosion, requires a high-quality corrosion-resistant overlay to ensure long-term service integrity. The 16MnR substrate is a low-alloy pressure vessel steel, while the 00Cr17Ni14Mo2 overlay is a low-carbon austenitic stainless steel (equivalent to 316L) with excellent resistance to chloride pitting and crevice corrosion.

Core Technical Analysis

Substrate and Overlay Material Compatibility

The combination of 16MnR (a low-alloy steel with yield strength of approximately 345 MPa) and 00Cr17Ni14Mo2 (a low-carbon austenitic stainless steel with approximately 17% Cr, 14% Ni, and 2% Mo) presents a classic dissimilar material welding challenge. The primary concerns include:

Concern	Description	Mitigation Strategy
Dilution	Carbon steel dilution into the stainless steel deposit reduces corrosion resistance	Use strip electrode with high Cr and Ni content, multiple passes
Thermal mismatch	Different thermal expansion coefficients (16MnR: ~12 µm/m·K, 316L: ~16 µm/m·K)	Control heat input, use interpass temperature limits
Cracking susceptibility	Carbon steel HAZ may be susceptible to cold cracking	Preheat substrate, use low-hydrogen consumables
Carbon migration	Carbon from substrate may diffuse into the overlay during service	Low-carbon overlay (00Cr17Ni14Mo2 has C ≤ 0.03%)

Strip Electrode Surfacing Method Selection

The study evaluates and selects strip electrode surfacing (SES) as the preferred method for this application. SES offers several advantages for this specific application:

High deposition rate: SES can deposit 5-10 kg/h of weld metal, significantly faster than GTAW or GMAW.
Low dilution: The slag pool limits base metal dilution to typically 5-15%, preserving the corrosion resistance of the overlay.
Good penetration: SES provides deep penetration, ensuring adequate metallurgical bonding between the overlay and substrate.
Flat profile: The process produces a flat deposit profile, which is beneficial for subsequent machining or finishing.

Fixture Design for Curved Surfaces

The cone body geometry presents a significant challenge for strip electrode surfacing, which is traditionally designed for flat or slightly curved surfaces. The study addresses this by designing specialized welding fixtures (jigs) that:

Support the cone body in a stable position during welding.
Allow access for the welding head to approach the surface at the correct angle.
Enable rotation of the component to achieve uniform coverage.
Provide cooling to control interpass temperatures and reduce distortion.

The fixture design is a critical aspect of the process, as improper support can lead to distortion, poor weld quality, or safety hazards. The study demonstrates that with proper fixture design, SES can be successfully applied to curved surfaces with acceptable quality.

Welding Process Qualification and Parameters

Welding Parameter Determination

The study conducts a comprehensive welding procedure qualification (WPQ) to determine the optimal welding parameters. The qualification follows the relevant standards (such as NB/T 47014 or ISO 15614) and includes:

Parameter	Typical Value	Rationale
Current	250-400 A (DC, electrode negative)	Ensures stable arc and adequate penetration
Travel speed	150-300 mm/min	Controls deposition rate and heat input
Electrode feed speed	Matched to travel speed	Maintains consistent arc length
Flux coverage	Full coverage, 20-30 mm ahead of arc	Ensures adequate gas protection and slag formation
Interpass temperature	Below 150°C	Prevents excessive heat buildup and distortion
Number of passes	2-3 passes	Achieves required overlay thickness (typically 3-5 mm)
Preheat temperature	100-150°C	Reduces cold cracking susceptibility

Quality Assessment

The qualified process was validated through the following tests:

Chemical analysis: Verification of Cr, Ni, Mo, and C content in the deposited metal.
Hardness testing: Hardness profile across the weld, HAZ, and base metal.
Mechanical testing: Tensile strength and impact toughness of the deposited metal.
Corrosion testing: Pitting and crevice corrosion resistance of the overlay.
Non-destructive testing: Visual inspection (VT), magnetic particle testing (MT), and ultrasonic testing (UT) for defects.

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Poor slag release	Inadequate flux coverage or wrong flux type	Ensure proper flux distribution, use compatible flux
Cracking in HAZ	Excessive restraint, high carbon equivalent	Preheat to 150°C, reduce heat input
Excessive dilution	Low electrode feed speed, high travel speed	Optimize parameter balance, increase passes
Distortion	High heat input, asymmetric welding	Use back-up plate, control interpass temperature
Porosity	Moist flux, inadequate shielding	Dry flux per standard, ensure proper flux coverage

Engineering Practice Integration

Application to Lock Hopper Manufacturing

Lock hoppers in oil and gas processing facilities operate under demanding conditions, including high pressure, elevated temperature, and corrosive fluid environments. The cone body, which directs fluid flow, is particularly susceptible to erosion-corrosion. The application of a 316L overlay provides a corrosion-resistant barrier that extends the service life of the component significantly.

The manufacturing process for a lock hopper cone body with SES overlay typically involves:

Fabrication: Forming the cone body from 16MnR plate by rolling and welding.
Surface preparation: Grinding the interior surface to be surfaced, removing scale and contaminants.
Fixture setup: Mounting the cone body in the specialized welding fixture.
Surfacing: Applying 2-3 passes of 00Cr17Ni14Mo2 strip electrode SES.
Post-weld treatment: Stress relief annealing if required by the design specification.
Inspection: Comprehensive NDT and dimensional verification.
Hydrostatic testing: Pressure testing to verify leak tightness.

Standards Compliance

The surfacing process must comply with relevant standards, including:

NB/T 47014: Qualification of welding procedures for pressure vessels.
GB/T 150: Pressure vessel design and fabrication.
ASME B31.3: Process piping (if applicable).
ISO 15614: Qualification testing of welding procedures.
API 5L: Line pipe (if the lock hopper is part of a pipeline system).

Study Insights and Implications

This study demonstrates the practical application of strip electrode surfacing to a complex curved component, highlighting the importance of fixture design in enabling the process for non-standard geometries. The key insight is that SES, while traditionally associated with flat surfaces, can be successfully adapted to curved components with proper engineering of the welding fixture and process parameters.

The study also underscores the importance of process qualification in ensuring consistent quality. The comprehensive WPQ, including chemical analysis, mechanical testing, and NDT, provides confidence that the surfacing process meets the required performance standards. For manufacturers of pressure vessels and process equipment, this approach offers a reliable method for applying corrosion-resistant overlays to complex geometries, reducing maintenance costs and extending equipment life in aggressive service environments.

Future work should focus on optimizing the fixture design for even more complex geometries, exploring the use of robotic automation for SES on curved surfaces, and developing standardized procedures for SES qualification on dissimilar material combinations in accordance with relevant pressure vessel codes.