Electrode Strip Surfacing Process for Lock Hopper Cone

Literature Overview

This 2012 paper by Guo Xiaochun from Daqing Oilfield Engineering Construction Co., Ltd. documents the development of an electrode strip surfacing process for applying 00Cr17Ni14Mo2 (equivalent to Hastelloy C-276) to a 16MnR lock hopper cone component. The study addresses the practical challenges of surfacing a highly alloyed nickel-based superalloy onto a carbon-manganese pressure vessel steel, including process selection, fixture design, and parameter optimization. The work represents a typical engineering challenge encountered in oilfield equipment where aggressive process fluids require corrosion-resistant linings on pressure-containing components.

Core Technical Analysis

Material Compatibility Challenges

The combination of 00Cr17Ni14Mo2 (C-276 equivalent) overlay on 16MnR substrate presents several metallurgical challenges:

Challenge	Description	Mitigation Strategy
Dilution	Base metal dilution reduces corrosion resistance	Backing layer with intermediate alloy
Cracking	High thermal expansion mismatch	Controlled heat input, post-weld stress relief
Intermetallic phases	Sigma phase at interface	Limit interpass temperature
Porosity	Hydrogen pickup from nickel alloy	Consumable drying, low hydrogen process
Hot cracking	Low melting point eutectics at grain boundaries	Controlled cooling rate

00Cr17Ni14Mo2 is a nickel-chromium-molybdenum alloy with approximately 57% Ni, 16% Cr, 3% Mo, and 2% W. Its exceptional corrosion resistance makes it suitable for hydrofluoric acid, hot concentrated sulfuric acid, and chloride-containing environments. However, its high nickel content makes it prone to hot cracking during solidification, particularly when deposited on dissimilar substrates.

16MnR is a low-alloy pressure vessel steel with 0.5-0.7% C, 1.2-1.6% Mn, and good weldability. Its carbon equivalent (CE) of approximately 0.45% places it in a moderate hydrogen cracking susceptibility range.

Process Selection Rationale

The selection of electrode strip surfacing (ESES) over alternative processes is justified by several factors:

Deposition efficiency: ESES provides deposition rates of 4-8 kg/h, significantly higher than SMAW (0.5-1 kg/h) or FCAW (2-4 kg/h).
Dilution control: The slag pool in ESES provides a barrier that limits base metal dilution to typically 5-10%, compared to 15-30% in arc processes.
Surface quality: ESES produces smooth, uniform deposits suitable for direct service without extensive post-weld machining.
Geometry suitability: The cone geometry of the lock hopper requires continuous, overlap-free surfacing that ESES can provide with proper fixture support.

Fixture Design and Geometry Considerations

The cone geometry of the lock hopper presents unique challenges for surfacing:

Curvature variation: The cone angle creates varying standoff distances and slag pool stability requirements along the surface.
Access limitations: Internal surfacing of a cone requires careful fixture design to support the electrode holder and wire feed while allowing access to all areas.
Thermal distortion: Differential heating of a cone can cause warping, particularly if the component is not properly restrained.

The paper describes the development of a dedicated fixture that allows the cone to be rotated while maintaining consistent electrode positioning. This fixture design is critical to achieving uniform deposit thickness and quality across the entire cone surface.

Process Parameter Development

The welding process qualification (WPS/PQR) development involved systematic variation of key parameters:

Parameter	Range Tested	Selected Value	Rationale
Current	350-500 A	420 A	Adequate penetration, controlled dilution
Voltage	28-35 V	32 V	Stable arc, proper slag pool
Traverse speed	200-400 mm/min	280 mm/min	Uniform deposit thickness
Electrode feed rate	8-15 m/h	11 m/h	Balanced with traverse speed
Flux coverage	Continuous	Optimized	Prevent oxidation
Interpass temperature	<150°C	<100°C	Minimize dilution, control HAZ

The interpass temperature control is particularly important for C-276 surfacing, as elevated temperatures increase dilution and promote the formation of brittle intermetallic phases at the weld interface.

Quality Control and Verification

For a component of this nature—surfacings on a pressure-containing oilfield component—the quality control protocol must be rigorous:

Visual inspection: All surfaces must be free from cracks, porosity, undercut, and incomplete fusion per ISO 17637 or ASME B31.3 Table 6.61.
Ultrasonic testing: Full coverage UT per ISO 17640 or ASME V Article 4 to detect lack of fusion and internal porosity.
Hardness testing: Overlay hardness should be 200-250 HV (typical for C-276); HAZ hardness should not exceed 250 HV to avoid excessive hardening.
Metallographic examination: Cross-sectional examination to verify deposit thickness uniformity, absence of intermetallic phases, and sound bonding to the base material.
Corrosion testing: Potentiodynamic polarization in representative service solution to verify corrosion resistance of the deposit.

Engineering Practice Implications

The successful development of this surfacing process for the lock hopper cone demonstrates the feasibility of applying nickel-based superalloys to carbon-manganese pressure vessel components in oilfield service. The economic case is compelling: rather than fabricating the entire hopper cone from C-276 (which would be prohibitively expensive), surfacing the critical internal surface provides equivalent corrosion protection at a fraction of the material cost.

For engineers considering similar applications, the following lessons from this work are particularly relevant:

The fixture design is as important as the welding parameters; a well-designed fixture ensures consistency and repeatability.
The dilution rate must be carefully controlled; even small variations in dilution can significantly affect the corrosion performance of a nickel-based alloy overlay.
Process qualification testing should include not only mechanical properties but also corrosion testing in the actual service environment.
The transition zone between the overlay and the base material should be examined for intermetallic phase formation, particularly in areas of high thermal cycling.

Study Insights and Reflections

This paper, while concise, captures the essence of practical welding engineering: the systematic approach to solving a real manufacturing challenge. The integration of process selection, fixture design, parameter optimization, and quality verification into a cohesive process package represents the kind of comprehensive engineering that distinguishes successful industrial applications from laboratory demonstrations. The choice to apply C-276 to a 16MnR component reflects a mature understanding of materials selection—using the most expensive alloy only where it is truly needed, while leveraging the strength and economy of the base material for structural purposes. This philosophy of strategic alloy placement is fundamental to cost-effective engineering design in the petrochemical and oilfield industries.