Microstructure and Properties of Nickel-Based Alloy Plasma Overlay Layer

Literature Overview

This paper by Gao Wei and Liu Zhihao from the School of Science at China University of Petroleum (Beijing) was published in Petroleum Machinery (石油机械), Volume 40, Issue 3, 2012, pages 30-34. The study investigates the microstructure and properties of nickel-based alloy plasma arc overlay layers deposited on X65 pipeline steel using ERNiCrMo-3 filler wire. The research was supported by the National Science and Technology Support Program project "Experimental Study on Performance of Buried Steel Pipeline Materials and Establishment of Basic Database" (2006BAK02B01-02).

Core Technical Findings

Microstructure of the Overlay Layer

The overlay layer microstructure consists of two main constituents:

1. Columnar crystals: These are gamma (γ) solid solution crystals that grow preferentially in the direction of heat extraction. The columnar growth is typical of rapid solidification conditions encountered in plasma arc overlay welding.
2. Eutectic structure: The eutectic consists of a gamma (γ) solid solution matrix with Laves phase particles. The Laves phase is enriched in Mo and Nb, which are key alloying elements in the ERNiCrMo-3 filler wire.

The presence of Laves phase is significant because these intermetallic compounds contribute to the hardness and wear resistance of the overlay layer. However, excessive Laves phase formation can reduce toughness and ductility.

Effect of Overlay Current on Corrosion Resistance

The authors conducted corrosion electrochemical testing and found that the corrosion resistance of the overlay layer decreases as the overlay current increases. This can be attributed to the following mechanisms:

1. Higher current leads to greater heat input, which increases dilution of the overlay layer by the base metal.
2. Increased dilution reduces the concentration of beneficial alloying elements (Cr, Mo, Ni) in the overlay layer.
3. The Laves phase, while hard, may be more susceptible to corrosion than the gamma matrix in certain environments.

Hydrogen-Induced Cracking Susceptibility

The study also examines hydrogen-induced cracking (HIC) susceptibility and finds that higher overlay current increases the tendency for hydrogen-induced cracking. This is particularly relevant for pipeline applications where hydrogen damage is a critical concern.

Microhardness Distribution

The microhardness distribution from the base metal to the overlay layer shows the following pattern:

The fusion zone exhibits the highest hardness, exceeding 310 HV. This is attributed to the formation of a hard, fine-grained microstructure at the interface between the base metal and overlay layer, where rapid solidification and dilution effects combine to produce a hardened zone.

Technical Parameter Analysis

Engineering Practice Integration

Application to X65 Pipeline Steel

X65 pipeline steel is widely used in oil and gas transmission pipelines due to its favorable combination of strength and toughness. The use of nickel-based overlay layers on X65 steel is particularly relevant for the following applications:

1. Corrosion protection: In environments where internal corrosion (such as CO2 corrosion) is a concern, nickel-based overlay layers can provide enhanced corrosion resistance.
2. Wear protection: At coupling and fitting locations where mechanical wear is expected, the hard overlay layer provides wear resistance.
3. Hydrogen damage mitigation: Although the study shows that higher current increases HIC susceptibility, the nickel-based overlay layer itself can act as a barrier to hydrogen ingress when properly deposited.

Process Optimization Recommendations

Based on the findings of this study, the following process optimization recommendations are made for plasma arc overlay welding of ERNiCrMo-3 on X65 steel:

1. Current selection: Use the lowest practical welding current to minimize dilution, improve corrosion resistance, and reduce HIC susceptibility. Typical plasma arc overlay currents for this application range from 20-40 A.
2. Multi-layer deposition: Deposit multiple thin layers to build up the overlay thickness while maintaining low heat input per layer.
3. Travel speed optimization: Use a travel speed that provides adequate overlap between adjacent passes without excessive heat input.
4. Post-weld inspection: Conduct hydrogen-induced cracking testing (such as ASTM G178 or NACE TM0284) to verify that the overlay does not promote hydrogen damage.

FMEA for Plasma Overlay on X65 Pipeline

Key Questions and Reflections

Several important questions arise from this study:

• How does the Laves phase distribution change with different welding parameters (current, travel speed, torch oscillation)?
• What is the long-term corrosion resistance of the overlay layer in specific pipeline service environments (e.g., CO2-containing brine, chloride solutions)?
• Can the HIC susceptibility be further reduced by modifying the filler wire composition or by post-weld heat treatment?
• How does the overlay layer perform under cyclic loading conditions typical of pipeline service?

Study Insights and Implications

This paper provides valuable insights into the metallurgical behavior of nickel-based overlay layers on X65 pipeline steel. The finding that overlay current has a significant impact on corrosion resistance, HIC susceptibility, and microhardness distribution is particularly important for process specification development. For pipeline manufacturers and operators, these findings should be incorporated into welding procedure specifications for plasma arc overlay welding, with clear limits on welding current and mandatory post-weld testing requirements. The identification of the Laves phase as a key microstructural constituent provides insight into the hardening mechanisms at play and guides future alloy development efforts. Overall, this study contributes to the growing body of knowledge on corrosion-resistant overlay technologies for pipeline applications.