Iron-Based Alloy Plasma Surfacing on Martensitic Stainless Steel: Microstructure and Wear Performance

Literature Overview

This study by Zhang Song et al. (2019), published in the Journal of Shenyang University of Technology (Vol. 41, No. 2), investigates the microstructure, composition, hardness, and tribological performance of iron-based alloy coatings produced by plasma transfer arc (PTA) surfacing on Z5CND16-04 martensitic stainless steel substrates. The research was conducted at Shenyang University of Technology in collaboration with Shenyang Blower Group Nuclear Pump Industry Co., Ltd., and supported by the National Key R&D Program of China (2016YFB1100204) and the National Science and Technology Major Project (2013ZX06002-002). The application context is nuclear pump components requiring enhanced surface wear resistance while maintaining the corrosion resistance of the martensitic stainless steel substrate.

Substrate Material and Application Context

Z5CND16-04 is a martensitic stainless steel (equivalent to AISI 410/420 grade) with approximately 13–14% Cr, used extensively in nuclear pump impellers, wear rings, and valve components. The material offers good strength and moderate corrosion resistance but suffers from relatively poor wear resistance compared to higher-alloy materials. The requirement for surface enhancement is driven by the need to extend component life in nuclear service, where:

• Component replacement is costly and time-consuming due to regulatory requirements
• Radiation environments may accelerate wear mechanisms
• The substrate must maintain its corrosion resistance and radiation resistance
• The surfacing process must not introduce radioactive contamination or degrade the base material properties

Surfacing Process and Consumable Design

The PTA process was selected for its advantages over conventional arc surfacing:

The iron-based alloy consumable was designed with Cr and Mo additions to form complex carbide phases, and the effect of cerium oxide (CeO2) rare earth addition was investigated as a microstructure refiner.

Microstructural Analysis

Phase Composition

XRD analysis identified the following phases in the iron-based alloy deposit:

The addition of CeO2 did not significantly alter the phase composition but produced a marked refinement of the microstructure. The rare earth element cerium acts as a heterogeneous nucleation site and grain refiner during solidification, producing a finer distribution of carbide particles within the matrix.

Microstructural Comparison

The refinement achieved by CeO2 addition is consistent with the well-documented rare earth effect in steel metallurgy: CeO2 particles at the melt pool boundary provide nucleation sites that reduce the grain size and promote uniform carbide precipitation. The increased hardness (approximately 20% improvement) is attributed to both the Hall-Petch strengthening from grain refinement and the increased density of hard carbide particles.

Wear Performance Evaluation

Pin-on-disc wear testing was conducted under dry sliding conditions:

The wear resistance improvement is approximately 6–8× for the iron-based alloy coating and 10–12× for the CeO2-modified coating relative to the uncoated substrate. The transition from adhesive-abrasive wear to mild abrasive wear indicates that the hard carbide particles effectively resist material removal, while the refined microstructure reduces crack initiation at the coating surface.

Dilution and Interface Quality

PTA surfacing achieves a dilution rate of 10–20%, which is significantly lower than conventional arc surfacing (30–60%). This low dilution is critical for:

1. Composition control: The deposit chemistry closely matches the designed alloy composition
2. Property preservation: The martensitic substrate retains its original mechanical and corrosion properties
3. Bond quality: The metallurgical bond between the iron-based deposit and the stainless steel substrate is strong, with no evidence of interfacial cracking or delamination

The interface between the iron-based alloy and the Z5CND16-04 substrate shows a smooth transition with minimal intermetallic formation, as the similar iron-based chemistry of both materials promotes homogeneous solidification at the fusion boundary.

Engineering Application and Quality Control

For nuclear pump applications, the following quality control measures are recommended:

Key Technical Insights and Process Optimization

The study demonstrates several important technical principles:

1. Rare earth addition as microstructure refiner: The CeO2 addition is an effective and economical method for improving the microstructure and properties of PTA coatings without requiring changes to the base alloy composition or process parameters.
2. Carbide type and distribution: The wear resistance is governed primarily by the volume fraction and dispersion of hard carbide phases rather than by the matrix hardness alone. The (Fe,Cr,Mo)7C3 and (Fe,Cr,Mo)23C6 phases provide the necessary hardness, while their fine dispersion prevents crack initiation.
3. PTA process suitability: The low dilution, high deposition rate, and excellent process repeatability of PTA make it the preferred surfacing method for nuclear-grade components where property consistency and traceability are paramount.

Study Insights and Reflections

This study is particularly significant for the nuclear industry, where component life extension through surface engineering must meet the stringent requirements of regulatory qualification. The combination of PTA surfacing with iron-based alloy consumables and rare earth modification provides a practical solution for enhancing the wear resistance of martensitic stainless steel pump components without compromising their corrosion resistance or radiation tolerance. The findings have direct applicability to other nuclear components such as pump wear rings, valve seats, and bearing surfaces where tribological degradation limits service life.

The remaining challenges for industrial implementation include: qualification of the surfacing process according to nuclear industry standards (such as RCC-M or ASME NQA-1), development of non-destructive inspection methods suitable for thin coatings on complex geometries, and long-term performance validation under actual nuclear service conditions including radiation exposure and thermal cycling. The CeO2 modification approach is particularly promising because it leverages established metallurgical principles and can be implemented with existing PTA equipment without major capital investment. For engineers working in nuclear equipment maintenance and life extension programs, this study provides a scientifically grounded basis for specifying and qualifying plasma surfacing solutions that meet the demanding requirements of nuclear service.