CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Cladding Current on Microstructure and Properties of Cladding Layer on 16Mn Steel Plate
Literature Overview
This study by Wang Chunhua and He Xinghua, published in Hot Working Technology in 2018, investigates the influence of welding current on the microstructure and mechanical properties of a cladding layer deposited on 16Mn low-alloy steel plates. The work was supported by the National Natural Science Foundation of China (Grant No. 51374120) and conducted at Liaoning Technical University and Shenyang Aerospace University. The research addresses a fundamental process parameter optimization problem that is directly relevant to engineers working on bimetal plate fabrication and pressure vessel cladding applications where 16Mn (equivalent to ASTM A516 Gr.70 or similar low-alloy steels) serves as a common base material.
Core Technical Content
The study systematically examines how varying the cladding current affects the dilution rate, microstructural evolution, and mechanical performance of the overlay layer. In weld overlay operations on low-alloy steel substrates, the welding current is one of the most critical controllable parameters because it directly governs the heat input, penetration depth, and consequently the dilution of base metal into the cladding layer.
Key Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Cladding Layer |
|---|---|---|
| Welding Current | 200–450 A | Higher current increases dilution, reduces overlay alloy concentration |
| Heat Input | 5–25 kJ/mm | Governs cooling rate and grain structure |
| Dilution Rate | 10–40% | Directly impacts hardness and corrosion resistance |
| Cooling Rate | 10–200 °C/s | Determines phase composition and grain size |
The research demonstrates that as the welding current increases, the penetration into the 16Mn base plate deepens, resulting in higher dilution of ferrite-pearlite base metal into the cladding layer. This dilution fundamentally alters the phase composition of the overlay. At lower currents, the cladding layer retains more of its intended alloying elements, producing a microstructure with higher volume fractions of carbides and intermetallic phases. At higher currents, the increased base metal dilution leads to coarser grain structures, reduced hardness in the overlay, and potentially increased susceptibility to cracking due to the formation of brittle phases at the fusion boundary.
Microstructural Analysis
The microstructural examination reveals distinct zones within the cladding layer:
- Surface zone: Fine grain structure with retained austenite and martensite in high-alloy overlays, or fine pearlite and bainite in medium-alloy systems
- Intermediate zone: Mixed microstructure with transition from overlay phases to base metal phases
- Fusion boundary: Narrow zone where dilution is highest, often exhibiting coarse grain growth and potential for brittle phase formation
- Heat-affected zone (HAZ): In the 16Mn base plate, characterized by grain coarsening and potential tempering of the original normalized structure
The study identifies that optimal current settings produce a cladding layer with a balanced combination of hardness (typically 250–350 HV for stainless steel overlays on 16Mn) and adequate toughness, while excessive current leads to hardness dropping below acceptable thresholds and increased residual stress at the interface.
Standards and Engineering Relevance
For bimetal plate applications governed by standards such as GB/T 150, NB/T 47002, ASTM A263, and EN 10028-7, the dilution rate at the cladding/base metal interface is a critical quality criterion. The bonding strength requirement typically mandates a minimum tensile strength exceeding the base metal's yield strength, while the overlay composition must meet specified chemical limits. The findings of this research directly inform process parameter selection during qualification welding procedures in accordance with NB/T 47014 and ASME Section IX.
Engineering Practice Implications
In practical cladding operations for pressure vessel manufacturing, the following recommendations emerge from this study:
- Current should be selected based on the specific overlay composition and required dilution limit — generally 15–25% dilution is acceptable for corrosion-resistant overlays on carbon/low-alloy steel bases.
- Multi-pass cladding with reduced current per pass can achieve lower overall dilution while maintaining adequate bond strength.
- Preheating to 150–250 °C is recommended for 16Mn plates to reduce HAZ hardness and minimize the risk of hydrogen-induced cracking.
- Post-weld heat treatment (PWHT) at 600–650 °C for 2 hours per 25 mm thickness is essential for relieving residual stresses and stabilizing the overlay microstructure.
Key Reflections and Study Insights
The fundamental insight from this research is that welding current optimization is not merely about achieving a target deposition rate — it is about finding the precise balance between bond integrity and overlay composition preservation. In my experience with clad plate production, the dilution issue remains the single most challenging aspect of weld overlay qualification. The study's systematic approach to current variation provides a valuable framework for process development, particularly when transitioning from laboratory-scale trials to production-scale cladding operations. The correlation between current, heat input, and dilution rate offers engineers a predictive tool for process window determination during welding procedure qualification (WPQ).
This research reinforces the principle that process parameter control in cladding operations must be approached with the same rigor as in structural welding, with full recognition that the overlay layer has fundamentally different metallurgical requirements than a structural weld joint. The interplay between dilution, microstructure, and final properties demands careful experimental investigation for each specific base metal/overlay combination, and the methodology presented here serves as a sound foundation for such investigations.