Quality Influencing Factors in Strip Cladding for Petrochemical Equipment

Literature Overview

This study, conducted by Han Yue, Tang Chuanjian, and Bi Xiaomin from Lanzhou Lanchi Company (Lanchi Petrochemical Division) and published in 2006, addresses the critical quality influencing factors in strip cladding (electroslag welding overlay) processes applied to petrochemical equipment. Strip cladding via electroslag welding (ESW) is one of the most widely used methods for producing large-format clad plate used in hydrogenation reactors, distillation columns, and heat exchangers in petrochemical service. The paper systematically identifies and analyzes the key variables that govern cladding quality, providing a framework for process optimization and defect prevention.

Core Technical Analysis

Strip cladding by electroslag welding involves the consumption of a metal strip electrode through an electrically conductive slag pool to deposit a corrosion-resistant or wear-resistant overlay layer onto a carbon steel or low-alloy steel base plate. The process is characterized by high deposition rates (typically 30-50 kg/h), excellent bond strength, and the ability to produce uniform overlay thickness across large plate dimensions. However, the process is sensitive to multiple interrelated parameters that must be carefully controlled to ensure metallurgical quality.

Key Quality Influencing Parameters

Parameter	Typical Range	Influence on Quality
Slag ratio	1.0-1.5	Controls heat input and dilution; affects carbon content in overlay
Stripping speed	0.2-0.6 m/min	Determines cooling rate; too fast causes incomplete melting, too slow causes excessive dilution
Current	800-2000 A	Higher current increases dilution; affects microstructure
Slag composition	CaO-SiO2-Al2O3-MnO system	Controls slag viscosity, wetting, and deoxidation
Base plate preheat	100-250 °C	Prevents cracking; controls cooling rate
Interpass temperature	≤350 °C	Prevents grain coarsening and cracking
Strip composition	304/316L/321/625	Determines corrosion resistance of overlay

Dilution Control

The most critical quality factor in ESW strip cladding is the dilution rate, which represents the proportion of base plate material melted and alloyed into the overlay layer. For austenitic stainless steel overlays, dilution must typically be controlled below 30% to maintain adequate corrosion resistance. Excessive dilution introduces carbon and manganese from the base plate, promoting sigma phase formation and reducing pitting resistance. The authors emphasize that dilution is governed by the combined effect of current density, stripping speed, slag ratio, and the thermal conductivity of the base material.

Slag System Optimization

The slag composition in ESW cladding serves multiple functions: it acts as a heat source to melt the strip electrode, provides a protective atmosphere, and acts as a flux to remove impurities. The study highlights that the slag must maintain appropriate viscosity (typically 0.5-1.5 Pa·s at operating temperature) to ensure stable slag pool formation and uniform heat distribution. The CaO/SiO2 ratio is particularly important for controlling slag basicity and deoxidation efficiency.

Engineering Practice Integration

In petrochemical applications, strip-clad plates are commonly used for hydrogenation reactor shells, where the overlay must withstand high-temperature hydrogen attack and catalytic poisoning. The base plate is typically 16MnR or 18MnMoNbR, while the overlay is 304L or 321 stainless steel. The following quality control measures are derived from the study's recommendations:

Base plate conditioning: Ensure flatness within 2 mm/m and remove scale, oil, and moisture from the cladding surface.
Slag preheating: Slag mixture must be preheated to 200-300 °C to remove moisture and ensure consistent composition.
Parameter stability: Maintain constant current and speed throughout each pass; avoid interruptions that cause cold shuts.
Multi-pass strategy: For thick overlays (>6 mm), use 2-3 passes with the first pass at lower current to minimize dilution.
Post-cladding heat treatment: Solution treatment at 1050-1100 °C followed by water quenching to eliminate sigma phase and restore corrosion resistance.

Common Defects and Countermeasures

Defect Type	Root Cause	Countermeasure
Cold shut	Low current or high speed	Increase current by 5-10%, reduce speed
Incomplete melting	Insufficient heat input	Increase current, reduce stripping speed
Excessive dilution	High current, low speed	Optimize current-speed ratio; use lower carbon strip
Slag inclusion	Unstable slag pool	Improve slag composition; ensure proper slag ratio
Cracking	High hydrogen, restricted cooling	Preheat base plate; use low-hydrogen slag; control interpass temperature
Sigma phase	Excessive heat input	Reduce interpass temperature; apply solution heat treatment

Study Insights and Reflections

The significance of this study lies in its systematic approach to identifying quality factors in a process that is inherently variable. In my engineering experience, the challenge with ESW strip cladding is not the individual parameter control but rather the interaction between parameters across long production runs. The paper's emphasis on the combined effect of current, speed, and slag composition on dilution reflects a mature understanding of the process. For modern petrochemical applications involving high-pressure hydrogen service, the dilution control requirement becomes even more stringent, as even small amounts of carbon in the overlay can lead to hydrogen attack susceptibility. The study's recommendations for slag system optimization remain highly relevant, as slag chemistry is often the most overlooked variable in production environments where consistency is paramount.