Properties of C-Cr-Nb-Ni Alloy Powder Plasma Shallow-Penetration Surfacings

Literature Overview

This paper by Li Guodong, Li Zhuoxin, Yang Jinyu, and Feng Jicai, published in Hot Working Technology in 2014 (Vol. 43, No. 23, pp. 196–198), examines the properties of a self-developed C-Cr-Nb-Ni alloy powder deposited by powder plasma surfacing with shallow penetration. The research was conducted at Beijing University of Technology, China Petroleum Pipeline Machinery Manufacturing Co., Ltd., and the State Key Laboratory of Advanced Welding and Joining at Harbin Institute of Technology, supported by the open research fund of the State Key Laboratory. The study investigates the effect of surfacing current on penetration depth and characterizes the microhardness, microstructure, and phase composition of the deposited layers.

Core Technical Findings

The primary objective of this research is to achieve a low-dilution surfacing layer by controlling the plasma arc current to minimize base metal melting. The self-developed C-Cr-Nb-Ni alloy powder is designed to provide high hardness and wear resistance through a combination of solid solution strengthening, carbide precipitation, and grain refinement. The study systematically varies the surfacing current and evaluates the resulting dilution ratio, microhardness distribution, microstructure, and phase composition.

At a surfacing current of 120 A, the base metal is essentially not melted, resulting in a surfacing layer with a very low dilution ratio. This is a critical finding for engineers who require the deposition of alloy compositions with minimal alteration from the base material composition. The microhardness measurement reveals that beyond a distance of 0.5 mm from the fusion line, the hardness increase becomes negligible, indicating that the dilution effect is confined to a narrow zone near the interface.

The microstructure of the surfacing layer consists primarily of martensite with a small amount of retained austenite. The hard phase is identified as hexagonal Cr₃C₂ carbide. Notably, despite the high carbon and niobium content in the deposited layer, no niobium carbide precipitates were observed. Instead, niobium-containing compounds function as nucleation sites for grain refinement. This is an important metallurgical insight: niobium in this context acts not as a carbide-former but as a grain refiner, which contributes to the overall hardness through the Hall-Petch mechanism.

Process Analysis and Microstructure Interpretation

The shallow-penetration approach in plasma surfacing is a deliberate strategy to minimize dilution while maintaining a metallurgical bond with the substrate. The current of 120 A represents a carefully selected operating point where the arc energy is sufficient to melt the powder and form a pool but insufficient to significantly penetrate into the base metal. This is consistent with the well-established principle that lower current in plasma surfacing reduces penetration but may increase porosity if not properly managed.

The formation of martensite as the primary phase is a direct consequence of the rapid cooling rate in plasma surfacing combined with the high carbon content of the alloy powder. The carbon content, which is not diluted away due to the low dilution ratio, provides the necessary driving force for martensitic transformation. The small amount of retained austenite is expected in high-carbon martensitic systems and can contribute to toughness through transformation-induced plasticity under certain conditions.

The absence of NbC is metallurgically significant. In many high-speed steels and wear-resistant alloys, niobium is expected to form NbC or Nb₂C precipitates. The fact that this does not occur in this system suggests that the carbon activity is preferentially consumed by chromium to form Cr₃C₂, or that the cooling rate and solidification sequence prevent Nb carbide nucleation. The role of Nb as a grain refiner through compound formation is a valuable insight for alloy design, as it suggests that niobium can be used to refine grain structure without necessarily forming discrete carbide phases.

The hexagonal Cr₃C₂ phase is a relatively soft carbide compared to M₂C or M₇C₃ carbides, but its hexagonal morphology and distribution within the martensitic matrix provide effective hardening through dispersion strengthening. The combination of martensitic matrix hardening and carbide dispersion hardening produces a synergistic effect on the overall hardness of the deposit.

Engineering Practice Implications and Reflections

For engineers designing surfacing processes for wear-resistant applications, this study provides a practical framework for achieving low-dilution deposits through current control. The 120 A current setting for the specific powder and torch geometry used should serve as a starting point, but must be adjusted for different torch designs, powder feed rates, and substrate materials. The key principle is to minimize the heat input into the base metal while maintaining sufficient arc stability for consistent powder melting.

The finding that niobium acts as a grain refiner rather than a carbide former in this system has implications for alloy design. Engineers developing new surfacing powders may consider incorporating niobium specifically for grain refinement purposes, with the understanding that the hardening contribution will come from the martensitic matrix and chromium carbides rather than from niobium carbides. This distinction is important for predicting the response of the coating to thermal cycling, as NbC-free coatings may exhibit different thermal stability compared to NbC-containing systems.

A potential concern with the shallow-penetration approach is the bond strength between the surfacing layer and the substrate. While the low dilution ratio preserves the alloy composition, it may also reduce the metallurgical bonding area. Engineers should verify the bond strength through peel testing or micro-indentation testing, particularly for applications involving cyclic loading or thermal shock. Additionally, the absence of NbC means that the coating may not exhibit the same thermal stability as NbC-containing coatings at elevated temperatures, which is a consideration for applications in high-temperature environments.

This study is a focused and practical investigation into the process-parameter-microstructure-property relationships in powder plasma surfacing. The emphasis on shallow penetration as a means to control dilution is a straightforward but effective strategy that can be readily applied in industrial settings. The insight into the role of niobium as a grain refiner adds a layer of alloy design sophistication that extends beyond simple hardening considerations.