Nickel-Based Alloy Powder Plasma Arc Hardfacing of Control Valves

Literature Overview

This technical paper by Xu Hongbo, published in 1994 from Nanchang Valve Factory, addresses the application of nickel-based alloy powder plasma arc hardfacing (PTA) technology to control valve components. Control valves are critical components in process industries including petrochemicals, power generation, and pharmaceuticals, where they must withstand severe erosion, corrosion, and cavitation conditions. The valve trim components, particularly the valve seat and plug, are subject to high-velocity fluid flow, pressure differentials, and chemically aggressive media, making them ideal candidates for surface engineering through hardfacing.

Technical Rationale and Material Selection

Nickel-based alloys are widely used in control valve hardfacing applications due to their excellent combination of corrosion resistance, erosion resistance, thermal fatigue resistance, and hot hardness. The most common nickel-based alloy systems used for valve hardfacing include:

The PTA process is particularly well-suited for control valve hardfacing because it provides a dilution-free or near-dilution-free overlay with excellent metallurgical bonding, uniform composition, and fine microstructure. The powder feedstock allows precise control over the overlay composition, and the process can be performed on complex geometries typical of valve trim components.

PTA Process Parameters and Welding Strategy

The PTA hardfacing of control valve components requires careful process planning to ensure uniform overlay coverage, adequate bond strength, and minimal thermal distortion. The following table summarizes the typical process parameters:

The welding strategy for valve components typically involves multiple thin passes to build up the required overlay thickness while minimizing thermal input and distortion. For valve seats, the overlay must be applied in a pattern that ensures complete coverage of the sealing surface, often requiring careful manipulation of the torch to achieve uniform deposition on the conical or spherical geometry.

The powder composition is critical to the final overlay properties. A typical nickel-based powder for valve seat hardfacing may contain 55-65% Ni, 15-20% Cr, 4-6% Mo, 2-4% W, and balance Fe, C, Si, and Mn. The carbon content is carefully controlled to balance hardness and toughness; excessive carbon can lead to brittle carbide networks and reduced toughness, while insufficient carbon results in inadequate hardness and wear resistance.

Microstructure and Performance Characteristics

The microstructure of the PTA nickel-based overlay on control valve components typically consists of an austenitic matrix with dispersed carbides and intermetallic phases. The primary phases are:

• Austenite (γ): The matrix phase providing toughness and corrosion resistance
• M6C carbides: Chromium-rich carbides providing wear resistance
• Laves phase (Mo, W, Cr)₂C: Hard intermetallic phase contributing to erosion resistance
• σ phase: A brittle phase that should be minimized as it reduces toughness

The relative proportion of these phases is controlled by the powder composition and the cooling rate during solidification. A slower cooling rate promotes the formation of larger, more coherent carbides and reduces the amount of brittle σ phase, while a faster cooling rate produces finer microstructures with potentially higher hardness but reduced toughness.

The performance of the hardfaced valve components is evaluated through several key tests:

Engineering Applications and Performance

The application of PTA nickel-based hardfacing to control valve components has demonstrated significant performance improvements in several industrial applications. In petrochemical service, hardfaced valve seats and plugs have shown 3 to 5 times longer service life compared to unclad components, particularly in applications involving high-velocity hydrocarbon flow and corrosive media. In power generation applications, hardfaced valve trim components have proven effective in reducing maintenance intervals and improving plant availability.

The paper's contribution to the field lies in demonstrating the practical feasibility of PTA hardfacing for valve components, addressing concerns about process control, quality consistency, and economic viability. The Nanchang Valve Factory's experience with this technology provided valuable data on the long-term performance of hardfaced valve components in real industrial service conditions.

Defect Analysis and Quality Control

Common defects in PTA hardfacing of valve components include:

Quality control for PTA hardfaced valve components includes visual inspection for surface quality and coverage, hardness survey to verify uniformity, magnetic particle testing for cracks, and ultrasonic testing for subsurface defects. The bond strength is verified through a destructive bond test on a witness coupon.

Key Reflections and Study Insights

A key insight from this study is the importance of powder quality and consistency in PTA hardfacing. The powder feedstock must be of high purity, with tight particle size distribution and consistent chemical composition, to ensure uniform overlay properties. Any variation in powder quality can lead to significant variation in overlay hardness, microstructure, and performance.

The study also highlights the challenges of applying PTA to complex valve geometries. The torch manipulation required to achieve uniform coverage on curved and contoured surfaces demands skilled operators and careful process planning. In modern practice, this challenge has been partially addressed through the use of robotic PTA systems with programmed torch paths, although the fundamental principles of process control remain the same.

Another important consideration is the post-weld heat treatment. Some nickel-based overlays require solution treatment and aging to achieve optimal properties, particularly for precipitation-hardened alloys such as Inconel 718. The heat treatment must be carefully controlled to avoid distortion and to maintain the dimensional accuracy of the valve components.

Summary

This 1994 paper by Xu Hongbo provides valuable technical insights into the application of PTA nickel-based hardfacing to control valve components. The paper demonstrates that PTA is a viable and effective technology for extending the service life of valve trim components in severe service conditions. The key success factors identified include careful powder selection, precise process parameter control, skilled torch manipulation, and rigorous quality control. These principles continue to form the basis of modern PTA hardfacing practice for control valve components, and the engineering approach of tailoring the overlay solution to the specific service conditions remains the most effective strategy for valve surface engineering.