Microstructure Evolution of Plasma Arc Cladding Layer Under Different Welding Processes

Literature Overview

The paper by Shi Duanhu, Zhang Wenbo, Sha Jing, and Yang Feng from the School of Mechanical and Electrical Engineering, Xuzhou University of Technology, published in the Journal of Xuzhou University of Technology (Natural Science Edition) (2018, Vol. 33, No. 2, pp. 47–51), investigates the microstructural evolution of plasma arc welding cladding layers deposited on 16 mm thick low-carbon steel plates using Fe90 alloy powder. The research was supported by Jiangsu Provincial Natural Science Foundation (BK20141143) and Jiangsu Provincial Key University Natural Science Research Project (16KJA430003). The study systematically examines how welding current and powder feed speed influence the microstructure of the cladding layer.

Core Technical Approach

Plasma arc welding (PAW) was employed as the cladding process, with Fe90 alloy powder as the consumable material. The substrate was a 16 mm thick low-carbon steel plate. The authors varied two primary process parameters—welding current and powder feed speed—to observe their effects on the resulting microstructure. Characterization was performed using optical metallography and scanning electron microscopy (SEM).

Key Findings on Microstructure

Observation	Description
Primary phases	Martensite, retained austenite, eutectic carbides
Microstructural complexity	High—multiple phases with complex morphologies
Microstructural gradient	Pronounced—three distinct zones identified
Weld zone	Fine-grained structure
Near-weld zone	Dendritic crystals
Mid-cladding zone	Equiaxed grains
Effect of increased current	Larger grain size, more uniform grain distribution
Effect of increased powder feed speed	Smaller grain size, more uniform microstructure

Detailed Microstructural Analysis

The cladding layer exhibited a complex multiphase microstructure consisting of martensite, retained austenite, and eutectic carbides. This phase combination is typical of high-carbon, high-alloy iron-based cladding materials and is responsible for the enhanced wear resistance expected from Fe90-type alloys. The presence of retained austenite is particularly noteworthy, as it can contribute to toughness through transformation-induced plasticity (TRIP) effects during service.

The pronounced microstructural gradient from the weld zone to the mid-cladding zone reflects the varying thermal histories experienced during solidification. The weld zone, being closest to the heat input source, experienced rapid cooling, resulting in fine grains. The near-weld zone, with slightly lower cooling rates, developed dendritic structures. The mid-cladding zone, having the lowest cooling rate and most time for nucleation and growth, produced equiaxed grains.

Process Parameter Effects

Effect of Welding Current

Increasing welding current increases the total heat input, which has two competing effects on microstructure:

Higher temperatures promote grain growth, leading to larger grain sizes.
However, the increased thermal energy also promotes more uniform temperature distribution across the melt pool, resulting in more homogeneous grain distribution.

The net effect, as reported by the authors, is that increased current produces larger but more uniformly distributed grains. This is a critical finding for process optimization: if uniformity is prioritized over fine grain size, higher current settings may be advantageous.

Effect of Powder Feed Speed

Increasing powder feed speed has the opposite effect on grain size:

Higher powder feed rates introduce more material into the melt pool per unit time, increasing the volume of solidification front.
The increased powder volume dilutes the heat per unit mass, effectively reducing the local cooling rate in the deposited material.
The result is smaller grain sizes with more uniform distribution.

This finding is particularly relevant for multi-pass cladding operations, where maintaining consistent microstructure across passes is essential for uniform properties.

Engineering Practice Integration

For piping and fitting engineers, plasma arc cladding is a viable alternative to laser cladding in scenarios where lower equipment costs are preferred or where thicker cladding layers are required. The Fe90 alloy is commonly used for wear-resistant overlay on components such as:

Pump impellers and volutes in slurry service
Valve body seats and trim in abrasive service
Pipeline spools in erosion-prone regions
Reducers and elbows in high-velocity flow lines

The microstructural gradient identified in this study has practical implications for the mechanical properties of the cladding layer. The fine-grained weld zone provides good bonding strength to the substrate, while the equiaxed mid-zone offers balanced toughness and wear resistance. Engineers should consider this gradient when designing multi-pass cladding sequences for thick components.

Process Optimization Recommendations

Based on the findings, the following process windows can be recommended:

Parameter	Recommended Range	Rationale
Welding current	Moderate (balance between grain size and uniformity)	Avoid excessively large grains while maintaining uniform distribution
Powder feed speed	Higher (for finer grains)	Smaller grains improve wear resistance and toughness
Substrate thickness	16 mm (as tested)	Thicker substrates may require preheating to reduce thermal stress
Shielding gas	Argon (typical for PAW)	Ensure adequate protection of molten pool

Defect Analysis and Countermeasures

Potential Defect	Cause	Countermeasure
Excessive grain coarsening	High welding current	Reduce current; increase powder feed speed
Microstructural inhomogeneity	Low powder feed speed	Increase feed rate; optimize current
Cracking at weld zone	High cooling rate, high carbon content	Preheat substrate; reduce current
Excessive dilution	High current, low feed speed	Balance current and feed rate; use larger nozzle
Retained austenite instability	Inadequate tempering	Post-weld heat treatment to stabilize retained austenite

Study Insights and Implications

The systematic investigation of process parameter effects on microstructure provides valuable guidance for plasma arc cladding operations in industrial settings. The finding that powder feed speed is an effective lever for grain refinement is particularly useful, as it allows engineers to adjust microstructure without changing the fundamental heat input parameters that determine dilution and bonding quality.

For piping engineers, the microstructural gradient identified in this study should be considered when evaluating the long-term performance of cladded components. The transition from fine-grained weld zone to equiaxed mid-zone represents a transition from high-strength, high-hardness regions to more ductile, tough regions. This natural gradient can be advantageous in service, as it provides a gradual transition in properties that reduces the risk of stress concentration at the interface between cladding and substrate.

The presence of retained austenite in the microstructure warrants attention from a durability standpoint. While retained austenite can contribute to toughness through TRIP effects, it may also transform to martensite during cold working or low-temperature service, potentially leading to cracking. Engineers should consider post-weld heat treatment to stabilize the retained austenite phase when the cladded components are intended for low-temperature or cryogenic service.

Reference Value and Outlook

This study provides a solid foundation for understanding the process-structure relationships in plasma arc cladding of Fe90-type alloys. Future research should correlate the observed microstructural features with quantitative mechanical properties, particularly hardness profiles, wear resistance, and fatigue performance. For industrial application, the findings should be validated through multi-pass cladding trials on actual piping components, with verification of the microstructural gradient across the full cladding thickness.