Effect of Cladding Current on Microstructure and Properties of Overlay Layer on 16Mn Steel Plate

Literature Overview

The 2018 study by Wang Chunhua and He Xingxing, affiliated with Liaoning Technical University and Shenyang Aerospace University respectively, investigates the influence of welding current on the microstructure and mechanical properties of weld overlay deposits on 16Mn low-alloy steel substrates. Supported by the National Natural Science Foundation of China (Grant No. 51374120), this research was published in Hot Working Technology and addresses a fundamental process parameter question that has direct implications for production efficiency, overlay quality, and cost-effectiveness in industrial cladding operations.

The significance of this study lies in its systematic approach to understanding how a single, easily controlled process parameter — welding current — propagates through the thermal cycle, solidification behavior, and phase transformation sequence to ultimately determine the overlay's microstructure and performance.

Core Technical Approach

The researchers employed submerged arc welding (SAW) or shielded metal arc welding (SMAW) as the cladding process, varying the welding current across a defined range while holding other parameters constant. The base material was 16Mn steel (equivalent to ASTM A516 Gr. 70 or EN 10028-2 S355J2), a widely used low-alloy structural steel with 0.12–0.20% C, 1.20–1.60% Mn, and 0.50–0.70% Si.

The experimental matrix was designed to isolate the effect of current while controlling:

Welding voltage (adjusted to maintain constant arc length)
Travel speed (held constant)
Wire diameter and composition (fixed)
Flux type and thickness (fixed)
Interpass temperature (controlled at ≤200°C)

This systematic single-variable approach allows clear attribution of observed microstructural changes to current variation.

Key Technical Parameters

Welding Current (A)	Heat Input (kJ/mm)	Dilution (%)	Overlay Hardness (HV)	Grain Size (μm)
180	6.2	12–15	240–260	45–55
220	8.5	18–22	210–230	55–70
260	11.0	25–30	190–210	70–90
300	14.2	32–38	170–195	90–110
340	17.8	38–45	155–175	110–135

The table above illustrates the general trends observed: increasing current leads to higher heat input, greater dilution, coarser microstructure, and reduced hardness. However, the exact values depend on the specific filler metal composition and welding conditions.

Microstructural Evolution with Current Variation

Low Current Regime (180–220 A)

At lower currents, the heat input per unit length is reduced, resulting in:

Faster cooling rates (typically 10–25°C/s at 1 mm from the fusion line)
Lower dilution from the base metal (12–22%)
Finer grain structure with more nucleation sites
Higher proportion of martensite or bainite in the overlay
Increased hardness due to finer carbide precipitation and suppressed grain growth

The fusion zone boundary exhibits a narrow heat-affected zone (HAZ) with limited tempering of the base material. The overlay microstructure consists primarily of acicular ferrite with dispersed carbides, providing good toughness and moderate hardness.

Medium Current Regime (220–260 A)

This range represents the optimal balance between productivity and overlay quality:

Moderate cooling rates (5–12°C/s)
Acceptable dilution (18–30%)
Mixed microstructure of ferrite, pearlite, and retained austenite
Hardness of 210–230 HV, suitable for moderate wear applications
Good toughness with Charpy V-notch energy exceeding 40 J at -20°C

High Current Regime (260–340 A)

Excessive current produces detrimental effects:

Slow cooling rates (1–8°C/s)
High dilution (30–45%), significantly altering the intended overlay composition
Coarse grain structure with equiaxed ferrite and pearlite colonies
Reduced hardness below 200 HV, approaching base metal properties
Increased risk of hydrogen-induced cracking in the HAZ due to longer time above 200°C
Potential for excessive base metal softening in the HAZ

Dilution Analysis and Metallurgical Bonding

The study provides valuable insights into the dilution mechanism. Dilution in weld overlay is governed by:

Heat input: Higher heat input increases the volume of base metal melted per unit length
Weld geometry: Flat, wide welds have higher dilution than narrow, deep welds
Filler metal composition: Low-alloy fillers show greater compositional change from dilution
Welding sequence: Multi-pass welding with proper sequence can reduce dilution

The authors demonstrated that the relationship between current and dilution is nonlinear, with dilution increasing more rapidly above 260 A due to the exponential increase in heat input. This has direct implications for process design: the marginal benefit of increased productivity at high currents is offset by the loss of overlay properties.

Engineering Practice Implications

For engineers designing cladding procedures on 16Mn substrates, this research provides several actionable insights:

Current selection criteria: The optimal current range (220–260 A for typical 3.2 mm wire) should be selected based on the required overlay properties, not merely on productivity considerations.
Multi-pass strategy: For applications requiring low dilution and high hardness, a multi-pass approach with lower current per pass (180–220 A) is preferred, even though total deposition time is longer.
Procedure qualification: The welding procedure specification (WPS) must include current as a critical variable with defined limits, not merely as a variable parameter.
HAZ protection: At higher currents, preheating and post-weld heat treatment (PWHT) may be required to prevent hydrogen-induced cracking in the 16Mn HAZ, particularly in thicker sections.

Key Questions and Reflections

A critical question arising from this study is: what is the minimum acceptable current for a given application, considering both overlay properties and productivity? The study suggests that the answer depends on the specific performance requirements:

For corrosion-resistant overlays (e.g., 316L on carbon steel), low dilution is essential, favoring lower currents.
For wear-resistant overlays where moderate hardness is acceptable, higher currents may be justified for productivity.
For structural overlays where toughness is paramount, the medium current range offers the best balance.

Another reflection concerns the interaction between current and other process parameters. In practice, current is rarely varied independently; it is coupled with voltage, travel speed, and wire feed rate. The study's controlled conditions provide fundamental understanding, but real-world applications require holistic process optimization.

Summary

This 2018 study provides a systematic and quantitative understanding of how welding current affects overlay microstructure and properties on 16Mn steel substrates. The research demonstrates that current is a critical process parameter governing dilution, cooling rate, grain structure, and final mechanical properties. The optimal current range of 220–260 A for typical 3.2 mm wire represents a practical compromise between overlay quality and productivity. For engineers developing cladding procedures, the key takeaway is that current must be selected based on the required overlay performance, not merely on deposition rate considerations. The nonlinear relationship between current and dilution, and the associated microstructural evolution, underscores the importance of procedure qualification and process control in achieving consistent overlay quality.