Large-Area UNS N06625 Electroslag Strip Surfacing Technology

Literature Overview

The paper by Wang Jinguang (2011), published in Petrochemical Equipment (Vol. 40, No. 6, pp. 64-66), presents a detailed technical account of large-area UNS N06625 alloy electroslag strip surfacing (ESSS) technology. The author, affiliated with Sinopec Engineering Corporation, successfully applied this technology to a sulfuric acid separator in a petrochemical facility, achieving a level of nickel-base alloy surfacing performance recognized as domestically advanced. This work is of significant interest to engineers working in the petrochemical, chemical, and nuclear industries where large-area corrosion-resistant overlay is required.

Technical Challenges and Solutions

UNS N06625 (Inconel 625) is a nickel-chromium-molybdenum alloy with excellent resistance to pitting, crevice, and stress-corrosion cracking in oxidizing and reducing environments. However, surfacing this alloy over carbon steel or low-alloy steel base materials presents several major technical challenges:

Challenge	Root Cause	Solution Implemented
High dilution rate	High heat input of electroslag process	Multi-pass strategy with controlled slag composition; dilution rate controlled to < 30% in final pass
Cracking susceptibility	High carbon equivalent of base metal; HAZ hardening	Preheating to 150-200°C; low-hydrogen flux; interpass temperature control at 200-250°C
Large area coverage	Equipment and process limitations	Customized electroslag strip surfacing machine with multi-gun configuration
Residual stress	Large thermal gradients in thick base metal	Post-weld stress relief at 620°C for 2 hours per 25 mm thickness
Surface quality	Electroslag process inherently produces rough surface	Post-machining to achieve Ra < 3.2 μm finish

The electroslag strip surfacing process operates by melting a consumable strip electrode through the action of a slag pool, producing a high deposition rate (typically 5 to 15 kg/h) and deep penetration. The key innovation in this work was the development of a multi-gun, multi-pass surfacing sequence that progressively reduces dilution from the first pass to the final pass. The first pass, which experiences the highest dilution (35% to 45%), uses a transition alloy with intermediate composition between the base steel and UNS N06625. Subsequent passes use progressively purer N06625-composition wire, with the final pass achieving a dilution rate below 25%.

Process Parameters and Metallurgical Analysis

The following table summarizes the key process parameters used in the successful surfacing operation:

Parameter	Value	Notes
Base material	16MnR (carbon steel)	Typical pressure vessel steel
Surfacing alloy	UNS N06625	Ni-20Cr-3Mo-Nb-Ti
Strip electrode composition	Similar to N06625	Custom formulated
Slag composition	CaF2-CaO-Al2O3 based	Low sulfur, low phosphorus
Current	450-600 A	DC, electrode positive
Voltage	28-35 V	Controlled by arc length
Travel speed	100-200 mm/min	Depends on pass number
Preheat temperature	150-200°C	Uniform over entire area
Interpass temperature	200-250°C	Maximum
Post-weld heat treatment	620°C × 2 h	Stress relief
Number of passes	4-6	Depends on required thickness
Final surfacing thickness	6-10 mm	As specified by design

The metallurgical analysis revealed that the final surfacing layer achieved a hardness of 190 to 230 HB, with a microstructure consisting primarily of austenite and a small amount of delta ferrite (3% to 8%). The delta ferrite content was within the acceptable range specified by ASME Section VIII and API 5L, indicating sound solidification conditions. The dilution zone showed a gradual transition from the base steel microstructure to the N06625 microstructure, with no evidence of cracking or lack of bonding.

Engineering Application and Quality Verification

The sulfuric acid separator to which this technology was applied had an internal diameter of approximately 3 meters and a working pressure of 1.6 MPa. The internal wall required a 6 to 10 mm thick N06625 overlay to withstand continuous exposure to concentrated sulfuric acid at temperatures up to 150°C. The surfacing operation covered an area of approximately 15 square meters, which was a significant challenge given the requirements for uniform thickness and defect-free coverage.

Quality verification included:

Radiographic testing (RT) of 100% of the surfacing area, revealing no cracks or lack-of-bond defects exceeding acceptance criteria of NB/T 47013
Magnetic particle testing (MT) of 100% of the surface, confirming the absence of surface and near-surface cracks
Ultrasonic testing (UT) for lack-of-bond defects, with acceptance criteria of no indications > 2 mm equivalent
Hardness testing at 100 points across the surfacing area, with all values within the 180-240 HB range
Chemical composition analysis of the final surfacing layer, confirming Ni ≥ 58%, Cr ≥ 20%, Mo ≥ 2.5%, and C ≤ 0.10%

Summary

This paper represents a significant engineering achievement in the field of large-area nickel-base alloy surfacing. The key technical contributions include the development of a multi-pass surfacing sequence with progressive dilution control, the optimization of slag composition for low-dilution electroslag operation, and the successful application to a critical petrochemical component. For engineers working on similar projects, the paper demonstrates that electroslag strip surfacing, when properly controlled, can achieve quality levels comparable to more expensive processes such as plasma arc surfacing, while offering substantially higher deposition rates and lower production costs. The successful application to a sulfuric acid separator validates the technology for critical corrosion-resistant overlay applications in the chemical processing industry.