Microstructure Evolution of Plasma Arc Welding Cladding Layers Under Different Process Conditions

Literature Overview

The paper by Shi Duanhu, Zhang Wenbo, Sha Jing, and Yang Feng from Xuzhou Institute of Technology, published in the Journal of Xuzhou Institute of Technology (Natural Science Edition) (2018, Vol. 33, No. 2, pp. 47-51), investigates the microstructure evolution of plasma arc welding (PAW) cladding layers deposited on 16 mm thick low-carbon steel plates using Fe90 alloy powder. The research was supported by the Jiangsu Provincial Natural Science Foundation (BK20141143) and the Jiangsu Provincial University Natural Science Research Major Project (16KJA430003).

Core Technical Content

Plasma arc welding is a well-established cladding technique for depositing hardfacing and overlay layers on industrial components. The use of Fe90 alloy powder, a widely used hardfacing material, makes this research particularly relevant to the surface engineering of mining equipment, pipeline components, and structural parts subjected to severe wear.

Experimental Setup and Methodology

The experimental work involved the following key parameters and techniques:

The authors systematically varied the welding current and powder feed rate to investigate their effects on the cladding layer microstructure, providing valuable process-microstructure-property relationships for practical application.

Microstructural Analysis and Results

The study revealed several important microstructural characteristics of the Fe90 plasma arc cladding layers:

1. Primary Phases: The cladding layer was composed of martensite, retained austenite, and eutectic carbides, indicating a complex multiphase microstructure.
2. Microstructural Gradient: A pronounced microstructural gradient was observed through the cladding depth, with different crystal morphologies in different zones.
3. Zone-Specific Morphology:

• Weld zone: Fine-grained structure
• Near-weld zone: Dendritic crystals
• Middle section of cladding layer: Equiaxed crystals

Effect of Welding Current on Microstructure

The study found that increasing the welding current resulted in:

• Larger grain size in the cladding layer
• More uniform grain distribution
• Enhanced thermal input, promoting grain growth and coarsening

This observation is consistent with fundamental welding metallurgy principles: higher thermal input increases the available thermal energy for grain growth and reduces the cooling rate, which generally leads to coarser microstructures.

Effect of Powder Feed Rate on Microstructure

Increasing the powder feed rate produced:

• Smaller grain size
• More uniform microstructure
• Higher dilution of the thermal energy per unit volume of deposited material

The inverse relationship between powder feed rate and grain size can be attributed to the increased volume of solid particles acting as nucleation sites and the reduced thermal energy available per unit volume of deposited material.

Engineering Implications for Pipeline Components

The microstructural gradient observed in the cladding layer has important implications for pipeline component surface engineering:

For pipeline elbows and fittings subjected to erosional wear, the fine-grained weld zone provides excellent surface hardness, while the equiaxed middle section offers good toughness for resisting impact damage. The dendritic near-weld zone serves as a transition region that helps distribute stresses between the hard surface and the ductile substrate.

Process Optimization Guidelines

Based on the findings, the following process optimization guidelines can be derived:

1. For maximum surface hardness: Use higher welding current and lower powder feed rate to promote fine grain formation at the surface.
2. For balanced properties: Use moderate current and higher powder feed rate to achieve uniform microstructure throughout the cladding depth.
3. For deep cladding layers: Consider multiple passes with adjusted parameters for each pass to maintain consistent microstructure quality.

Technical Reflections

The complex microstructure of Fe90 plasma arc cladding, containing martensite, retained austenite, and eutectic carbides, is characteristic of high-carbon, high-alloy hardfacing materials. The retained austenite content is particularly important as it contributes to toughness through transformation-induced plasticity (TRIP) effects during service loading. The eutectic carbides provide the primary wear resistance through their high hardness.

The microstructural gradient phenomenon is a natural consequence of the thermal gradient in the cladding layer. The weld zone, being closest to the heat source, experiences the highest temperatures and fastest cooling rates, promoting fine grain formation. As the distance from the heat source increases, the thermal conditions change, leading to different solidification morphologies. Understanding this gradient is essential for predicting the performance of cladding layers in service.

Integration with Engineering Practice

In the context of pipeline and fitting manufacturing, this research provides practical guidance for:

• Selecting optimal PAW parameters for depositing Fe90 hardfacing on pipeline elbows
• Predicting the microstructural quality of multi-pass cladding layers
• Designing cladding strategies for components with complex geometries
• Quality control procedures based on microstructural evaluation

The systematic investigation of current and powder feed rate effects provides a foundation for developing standardized welding procedures for plasma arc cladding applications in the pipeline industry.

Study Insights and Implications

The research by Shi et al. demonstrates that the microstructure of plasma arc cladding layers is highly sensitive to process parameters, with both welding current and powder feed rate exerting significant influence on grain size and distribution. The microstructural gradient phenomenon, while potentially challenging for quality consistency, can also be exploited to create functionally graded cladding layers with tailored properties.

For engineering practice, the key takeaway is that process parameter optimization must consider the entire cladding depth, not just the surface. The middle section of the cladding layer, which is often neglected in quality assessment, plays a crucial role in the overall performance and durability of the cladded component. Future research should focus on correlating the microstructural characteristics with actual wear performance in pipeline service conditions, bridging the gap between laboratory observations and field performance.