Quality Influencing Factors in Strip Cladding Process

Overview and Scope of Study

Strip cladding, also referred to as strip submerged arc welding overlay, is one of the most widely adopted methods for producing bimetal clad plates in the pressure vessel and heat exchanger fabrication industries. This study note focuses on the systematic analysis of quality influencing factors that govern the metallurgical bonding, mechanical integrity, and service reliability of strip-clad products. The discussion draws upon standards such as ASTM A263, A264, A265, NB/T 47014, and ASME IX, as well as decades of field experience in clad plate manufacturing and pressure vessel fabrication.

Core Quality Factors

The quality of strip cladding is determined by an interplay of material selection, process parameters, base metal preparation, and post-weld treatment. Each factor contributes to the final clad plate performance in terms of bond strength, overlay layer composition, hardness profile, and resistance to cracking.

Material Selection and Compatibility

The selection of the strip electrode (clad strip) and the flux is the first critical decision. The strip electrode composition must be compatible with the base metal in terms of thermal expansion, carbon equivalent, and metallurgical bonding characteristics. For example, when clad with 304 stainless steel strips on carbon steel, the carbon equivalent of the base steel must be controlled below 0.40 to minimize the risk of cold cracking in the bond line.

Base Metal	Clad Strip Material	Carbon Equivalent Limit	Recommended Flux Type
Q235 / A36	304 SS	CE ≤ 0.40	Low-hydrogen flux
16Mn / A516-70	316 SS	CE ≤ 0.45	Low-hydrogen flux
15CrMo / A387-11	Inconel 625	CE ≤ 0.55	Low-hydrogen flux
20G	321 SS	CE ≤ 0.40	Low-hydrogen flux

Process Parameter Control

The welding current, voltage, travel speed, and strip width directly influence the penetration depth, dilution rate, and overlay layer homogeneity. Excessive current leads to over-penetration and increased dilution, which degrades the corrosion resistance of the overlay layer. Insufficient current results in incomplete bonding and lack of fusion defects.

Parameter	Typical Range	Effect of Deviation
Welding current	800–1400 A	High: over-penetration; Low: lack of fusion
Voltage	30–38 V	High: wide bead, low penetration; Low: narrow bead
Travel speed	300–600 mm/min	High: thin bead, low deposition; Low: excessive dilution
Strip width	25–50 mm	Wider strips require higher currents
Flux coverage thickness	8–12 mm	Too thin: spatter; Too thick: slag inclusions

Base Metal Preparation

Surface cleanliness, preheating, and interpass temperature control are essential for preventing hydrogen-induced cracking and ensuring proper metallurgical bonding. The base metal surface must be free of rust, oil, scale, and moisture. Preheating temperatures are determined by the carbon equivalent of the base steel and the thickness of the plate.

Base Steel Grade	Plate Thickness (mm)	Preheat Temperature (°C)	Interpass Temperature (°C)
A36	≤ 25	50–80	≤ 150
A516-70	≤ 25	80–120	≤ 200
A516-70	26–50	120–150	≤ 250
A387-11	≤ 25	150–200	≤ 300
A387-11	26–50	200–250	≤ 350

Post-Weld Heat Treatment

For low-alloy steel base metals with higher carbon equivalents, post-weld heat treatment (PWHT) is often required to relieve residual stresses and reduce the risk of delayed cracking. The PWHT temperature is typically 580–620 °C for low-alloy steels, with a controlled cooling rate to avoid re-hardening.

Defect Analysis and Countermeasures

Common defects in strip cladding include lack of fusion at the bond line, porosity in the overlay layer, cracking (both hot and cold), and excessive dilution leading to reduced corrosion resistance. Each defect has identifiable root causes and can be mitigated through process optimization.

Defect Type	Root Cause	Detection Method	Countermeasure
Lack of fusion	Low current, high travel speed, poor flux coverage	MT, UT	Increase current, reduce travel speed
Porosity	Moist flux, contaminated base metal	RT, UT	Bake flux, clean base surface
Cold cracking	High CE, low preheat, high hydrogen	MT, UT	Increase preheat, use low-hydrogen flux
Hot cracking	High sulfur/phosphorus in base metal	MT, PT	Use appropriate strip composition
Excessive dilution	High current, low voltage, slow travel speed	Spectroscopy, hardness test	Optimize parameters, use multiple passes

Engineering Practice Insights

In practice, the most critical lesson learned from strip cladding quality management is that no single parameter operates in isolation. The interplay between base metal chemistry, process parameters, and environmental conditions determines the final quality. A systematic approach using FMEA (Failure Mode and Effects Analysis) is recommended to identify potential failure modes at the planning stage. For example, when clad plates are fabricated in winter with low ambient temperatures, the effective cooling rate increases, raising the risk of cold cracking even if the preheat temperature meets the minimum specified value. In such cases, local heating of the weld area to maintain the interpass temperature is essential.

Another practical insight is that the bond line quality is more sensitive to the first pass than subsequent passes. The first pass establishes the metallurgical bond between the clad strip and the base metal, and any defect introduced at this stage is difficult to correct in later passes. Therefore, the first pass parameters should be conservatively set and verified through destructive testing on coupon specimens before production welding begins.

Standards and Inspection Requirements

Strip cladding products must comply with the applicable standards for both the clad plate and the welding procedure qualification. ASTM A263 specifies the requirements for single-layer steel-clad plate with stainless steel or nickel alloy cladding. NB/T 47014 provides the qualification requirements for welding procedures in pressure vessel fabrication. The inspection requirements typically include visual examination, magnetic particle testing (MT) of the overlay surface, and ultrasonic testing (UT) of the bond line.

Summary

The quality of strip cladding is governed by a complex set of interdependent factors, including material selection, process parameters, base metal preparation, and post-weld treatment. A systematic approach to quality control, supported by proper welding procedure qualification, thorough non-destructive testing, and continuous process monitoring, is essential for producing reliable bimetal clad plates. The key takeaway is that process discipline and parameter control are far more important than any single technological advancement, and that a deep understanding of metallurgical principles underpins every successful cladding operation.