Microstructure Evolution of Plasma Arc Surfacing Layer Under Different Welding Processes

Literature Overview

This 2018 study by Shi Duanhu, Zhang Wenbo, Sha Jing, and Yang Feng from Xuzhou Institute of Technology investigates the microstructure evolution of plasma transferred arc (PTA) surfacing layers produced under different welding process parameters. Supported by the Jiangsu Provincial Natural Science Foundation (BK20141143) and the Jiangsu Provincial University Natural Science Research Major Project (16KJA430003), the research was published in the Journal of Xuzhou Institute of Technology (Natural Science Edition). The work provides valuable insights into the relationship between welding parameters and the resulting microstructure and properties of PTA overlay welds.

Core Technical Concepts

Plasma transferred arc welding (PTA) is a highly efficient surfacing process that uses a compressed plasma arc to melt a consumable electrode or powder, depositing a wear-resistant or corrosion-resistant overlay on a base metal substrate. The PTA process is characterized by a deep, narrow weld bead with high deposition rates and low dilution, making it ideal for multi-pass surfacing applications where precise control of the overlay composition is required.

PTA Process Parameters

The key process parameters in PTA surfacing include plasma gas flow rate, transfer current, travel speed, electrode feed speed, and shielding gas flow rate. Each of these parameters affects the heat input, weld geometry, and solidification conditions, which in turn determine the microstructure and properties of the deposit.

Microstructure Evolution

The microstructure of the PTA surfacing layer is primarily governed by the solidification conditions, which are determined by the heat input and cooling rate. The heat input is a function of the transfer current, travel speed, and arc voltage, while the cooling rate is influenced by the heat input, base metal thermal conductivity, and the number of passes.

The typical microstructure of a PTA deposit consists of columnar dendrites growing from the weld interface, with equiaxed grains forming in the upper portion of the deposit where the temperature gradient is lower. The dendrite arm spacing (DAS) is a function of the cooling rate and can be used to estimate the solidification conditions. A higher cooling rate results in finer dendrites and smaller grain size, which generally improves mechanical properties.

Process Parameters and Microstructure

Effect of Transfer Current

The transfer current is the primary parameter controlling the heat input in the PTA process. Increasing the current increases the heat input, which reduces the cooling rate and results in coarser microstructure. However, excessive current can also increase the dilution from the base metal, which may alter the deposit composition and microstructure.

The optimal transfer current for PTA surfacing is typically in the range of 200-300 A for a 1.6-2.4 mm electrode diameter. At these current levels, the heat input is sufficient to achieve good wetting and bonding with the base metal while maintaining a reasonable cooling rate for fine microstructure.

Effect of Travel Speed

Travel speed directly affects the heat input per unit length and the cooling rate. Increasing the travel speed reduces the heat input and increases the cooling rate, resulting in finer microstructure. However, excessive travel speed can lead to incomplete melting of the previous pass and poor bonding.

The optimal travel speed for PTA surfacing is typically in the range of 200-400 mm/min, depending on the electrode diameter and the desired bead width. A travel speed of 300 mm/min is often used as a starting point, with adjustments made based on the observed weld geometry and microstructure.

Effect of Electrode Feed Speed

The electrode feed speed controls the deposition rate and the dilution ratio. Increasing the feed speed increases the deposition rate and reduces the dilution, while decreasing the feed speed reduces the deposition rate and increases the dilution. The optimal feed speed is determined by the desired deposition rate and the required dilution level.

The dilution ratio in PTA surfacing is typically in the range of 10-30% for the first pass and 5-15% for subsequent passes. The dilution ratio is influenced by the transfer current, travel speed, and electrode feed speed, and must be controlled to achieve the desired deposit composition.

Microstructure Characterization

Metallographic Analysis

The microstructure of the PTA surfacing layer was characterized using optical microscopy and scanning electron microscopy (SEM). The optical micrographs reveal the columnar dendrite structure growing from the weld interface, with the dendrite arm spacing increasing from the interface to the top of the deposit. The SEM images provide detailed information on the dendrite morphology and the distribution of secondary phases such as carbides and intermetallics.

The dendrite arm spacing (DAS) was measured using the linear intercept method and was found to be in the range of 10-50 μm, depending on the welding parameters. The DAS is a sensitive indicator of the cooling rate and can be used to assess the solidification conditions. A DAS of 10-20 μm indicates a high cooling rate and fine microstructure, while a DAS of 30-50 μm indicates a lower cooling rate and coarser microstructure.

Hardness Distribution

The hardness of the PTA surfacing layer was measured using Vickers hardness testing (HV0.3) along a traverse from the weld interface to the top of the deposit. The hardness distribution typically shows a gradient from the interface to the top, with the highest hardness at the interface due to the higher cooling rate and lower dilution.

The hardness of the PTA deposit is primarily governed by the microstructure and the volume fraction of hard secondary phases such as carbides. The hardness values typically range from 300-600 HV, depending on the alloy composition and the welding parameters. Higher hardness values are associated with finer microstructure and higher volume fraction of hard phases.

Defect Analysis and Process Optimization

Common Defects in PTA Surfacing

Process Optimization Using PDCA

A systematic PDCA (Plan-Do-Check-Act) approach can be used to optimize the PTA surfacing process:

1. Plan - Define the target properties (hardness, wear resistance, corrosion resistance) and the acceptable defect limits. Select the appropriate filler material and define the initial process parameters based on welding procedure specifications (WPS).
2. Do - Perform the PTA surfacing using the selected parameters and collect data on weld geometry, dilution, hardness, and microstructure.
3. Check - Compare the measured properties with the target values and identify any deviations. Analyze the root causes of any defects or property deviations.
4. Act - Adjust the process parameters based on the findings from the Check phase. Repeat the Do-Check cycle until the target properties are achieved.

Engineering Practice and Application

Application in Wear-Resistant Surfacing

PTA surfacing is widely used for the application of wear-resistant overlays on industrial equipment. Typical applications include:

• Mining equipment: Crusher jaws, conveyor rollers, and bucket teeth
• Cement industry: Kiln liners, grinding mill liners, and transfer chutes
• Power generation: Boiler tubes, heat exchanger tubes, and pressure vessels
• Marine engineering: Propeller blades, shafts, and pump impellers

Application in Corrosion-Resistant Surfacing

PTA surfacing is also used for the application of corrosion-resistant overlays on components exposed to aggressive environments. Typical applications include:

• Chemical processing: Reactors, heat exchangers, and piping systems
• Oil and gas: Downhole tools, wellhead equipment, and subsea connectors
• Pulp and paper: Digester tubes, heat exchangers, and piping systems

Key Questions and Reflections

One of the most important insights from this research is the recognition that the microstructure of the PTA surfacing layer is not solely determined by the welding parameters but is also influenced by the base metal thermal properties and the number of passes. The base metal acts as a heat sink, and the cooling rate is higher for the first pass than for subsequent passes because the previous passes act as additional heat sinks. This results in a gradient of microstructure from the first pass to the last pass, with the first pass having the finest microstructure and the last pass having the coarsest microstructure.

Another critical consideration is the effect of interpass temperature on the microstructure and properties of the PTA deposit. High interpass temperatures can cause grain growth and softening of the previous passes, reducing the hardness and wear resistance. The interpass temperature should be controlled to below 150°C for most PTA applications to prevent grain growth and maintain the desired microstructure.

Study Insights and Implications

The work by Shi and colleagues provides valuable insights into the microstructure evolution of PTA surfacing layers and the influence of welding parameters on the resulting properties. The research demonstrates that the PTA process can produce deposits with fine microstructure and high hardness, making it suitable for wear-resistant and corrosion-resistant applications.

For engineers involved in specifying and qualifying PTA surfacing procedures, this work emphasizes the importance of understanding the relationship between process parameters and microstructure. The research also highlights the challenges associated with PTA quality control, particularly the need for careful control of interpass temperature and the importance of systematic process optimization using PDCA methodology.