Microstructure and Hardness of Different Powders Plasma-Cladded on Duplex Stainless Steel

Literature Overview

This 2023 paper by He Tao, Ma Jun, Wang Jianlong, Chen Jiaqi, Meng Fanmin, and Deng Dewei, published in Physical Testing and Chemical Analysis (Physical Section), investigates the microstructure and hardness of various plasma-transferred arc (PTA) cladding powders deposited on duplex stainless steel substrates. The research was conducted at Wuzhong Instrument Co., Ltd. and Dalian University of Technology, with funding from the High-End Control Valve Industry Technology Collaborative Innovation Center and the Liaoning Major Equipment Manufacturing Collaborative Innovation Center. This work is directly relevant to the engineering of high-performance control valves for the oil, gas, and chemical industries, where duplex stainless steel bodies are commonly used for their superior combination of strength, corrosion resistance, and cost-effectiveness.

Duplex Stainless Steel Substrate Characteristics

The duplex stainless steel substrate used in this study is representative of the commonly used grades in control valve applications: UNS S31803 (2205) or UNS S32750 (2507). These materials contain approximately equal amounts of austenite and ferrite phases, providing a balanced combination of mechanical properties and corrosion resistance. The typical microstructure consists of 45-55% ferrite and 45-55% austenite, with a hardness of 250-310 HV and a yield strength of 450-550 MPa.

The key challenge in plasma cladding on duplex stainless steel is maintaining the phase balance during the thermal cycle. The high thermal input of PTA can cause ferrite dissolution and austenite formation, shifting the phase balance toward austenite and potentially degrading the corrosion resistance of the cladded zone. Additionally, the dilution of the substrate into the overlay can alter the composition of the cladding layer, affecting its wear and corrosion properties.

Cladding Powder Selection and Process Parameters

The study evaluates multiple cladding powder compositions, which can be categorized as follows:

The PTA process parameters used in this study were typical for industrial cladding applications:

Microstructural Analysis

The microstructure of the cladding layers varies significantly depending on the powder composition. For the Ni-based powders, the microstructure consists of a dendritic matrix of austenite or austenite-martensite with carbide phases (M7C3, M6C) distributed at the dendrite boundaries. The cooling rate in PTA cladding is typically 5-20 K/s, which is faster than electroslag welding but slower than laser cladding, producing a moderate grain refinement.

For the WC-Co composite powder, the microstructure is more complex. The tungsten carbide particles are partially dissolved during melting, with the dissolved tungsten and carbon forming new carbide phases (WC, W2C, and W6C3) in the solidified microstructure. The undissolved WC particles act as hard reinforcement phases, providing exceptional wear resistance. However, excessive dissolution of WC particles leads to the formation of brittle iron carbides (Fe3C), which reduce the toughness of the cladding layer.

The dilution ratio—the fraction of substrate material melted into the cladding layer—is a critical parameter that directly affects the final composition and properties. For PTA cladding, the dilution ratio is typically 15-30% for the first pass and 5-15% for subsequent passes. The dilution ratio can be controlled by adjusting the powder feed rate relative to the heat input: higher powder feed rates produce lower dilution ratios.

Hardness Distribution and Phase Transformation

The hardness profile across the cladding layer shows characteristic variations. For the Ni-based claddings, the surface hardness is typically 350-450 HV, decreasing to 280-320 HV at the interface due to substrate dilution. The hardness gradient is relatively gentle, reflecting the gradual compositional change from pure cladding material to substrate material.

For the WC-Co composite cladding, the surface hardness is exceptionally high (800-1200 HV) due to the presence of undissolved WC particles. However, the hardness drops sharply to 300-400 HV at the interface, creating a steep hardness gradient that can promote crack initiation at the interface under thermal or mechanical cycling.

The hardness of the duplex stainless steel substrate near the cladding interface is affected by the thermal cycle. In the heat-affected zone (HAZ), the phase balance may shift toward austenite due to ferrite dissolution at elevated temperatures, resulting in a localized softening of 20-40 HV. This softening can be detrimental in applications where the substrate must maintain its mechanical integrity under pressure loading.

Engineering Application to Control Valves

The practical application of this research is in the refurbishment and enhancement of control valve internals. In the oil and gas industry, control valve trim components—particularly valve plugs, seats, and guides—experience severe erosion from high-velocity fluid flow containing solid particles. Plasma cladding with Ni-based or WC-Co composite powders can extend the service life of these components by 3-10 times compared to bare duplex stainless steel.

The selection of cladding powder must be based on the specific service conditions:

• For clean hydrocarbon service with moderate erosion: Ni-based Stellite 6 equivalent powder provides adequate protection with good corrosion resistance.
• For high-temperature service above 400°C: Ni-based Stellite 21 equivalent powder offers superior thermal stability.
• For severe abrasive service with hard solid particles: WC-Co composite powder provides the highest wear resistance but requires careful control of dilution to maintain toughness.
• For severely corrosive environments (H2S, chlorides): Cr-Ni-Mo alloy powder provides the best corrosion resistance.

Key Questions and Study Insights

The most significant finding from this research is that the dilution ratio is the single most important parameter controlling the final properties of the PTA cladding layer. Engineers who specify PTA cladding without controlling the dilution ratio are essentially specifying an uncontrolled process, and the resulting properties will vary from lot to lot and even from pass to pass within the same component. The recommendation is to always specify and verify the dilution ratio through chemical analysis of the cladding layer, particularly at the interface.

Another important insight is that the phase balance of the duplex stainless steel substrate must be monitored after cladding. If the HAZ experiences significant phase transformation, the corrosion resistance of the base material may be degraded, leading to intergranular corrosion or pitting in the HAZ even if the cladding layer itself is performing well. This requires post-cladding inspection of the substrate HAZ, which is often overlooked in standard quality control procedures.

This 2023 paper represents the current state of the art in PTA cladding for duplex stainless steel applications and provides valuable data for engineers designing cladding specifications for control valve refurbishment programs. The systematic evaluation of multiple powder compositions under controlled process parameters establishes a foundation for rational material selection that goes beyond empirical trial-and-error approaches.