Multi-Element Composite Reinforced Iron-Based High-Temperature Wear-Resistant Plasma Arc Overlay Alloys and Wear Mechanisms

Literature Overview

This paper presents a comprehensive study on the development and characterization of iron-based high-temperature wear-resistant alloys deposited by plasma transferred arc (PTA) cladding. The research addresses a significant industrial challenge: the degradation of wear-resistant surfaces at elevated temperatures (typically 400–800°C) where conventional carbide-based overlay alloys suffer from thermal softening, oxidation, and accelerated wear. The authors propose a multi-element composite reinforcement strategy, incorporating multiple alloying additions and second-phase reinforcements into the iron-based matrix to achieve synergistic strengthening effects at elevated temperatures.

Core Technical Content

Alloy Design Philosophy and Multi-Element Reinforcement Strategy

The alloy design philosophy centers on the concept of multi-element composite reinforcement, where several strengthening mechanisms are simultaneously activated to produce a synergistic effect. The base matrix is an iron-based alloy modified with strategic additions of chromium (Cr), molybdenum (Mo), vanadium (V), tungsten (W), niobium (Nb), and carbon (C). Each element serves a specific metallurgical purpose:

The multi-element approach is designed to address the limitations of single-element reinforcement. For example, while vanadium carbides (VC) provide excellent dispersion strengthening, they are prone to coarsening at temperatures above 500°C. The addition of molybdenum and tungsten carbides, which have higher thermal stability, compensates for this coarsening. Similarly, niobium carbides (NbC) have extremely high melting points and thermal stability, providing long-term strength retention even at temperatures approaching 800°C.

Microstructural Characterization and Phase Analysis

Metallographic examination and X-ray diffraction (XRD) analysis of the PTA overlay layer reveal a complex microstructure consisting of:

1. Matrix phase: A tempered martensite structure with retained austenite, providing a combination of hardness and toughness. The retained austenite fraction (typically 10–20%) is beneficial for wear resistance as it can transform to martensite under contact stress, providing additional hardening (TRIP effect).
2. Primary carbides: Coarse M7C3-type carbides (Fe, Cr, Mo, W)7C3 formed during solidification, typically 5–15 μm in size, distributed along prior austenite grain boundaries. These provide primary wear resistance through their high hardness (approximately 1800–2200 HV).
3. Secondary carbides: Fine MC-type carbides (VC, NbC, Mo2C) precipitated during cooling and subsequent tempering, typically 0.1–0.5 μm in size, dispersed within the martensitic matrix. These provide dispersion strengthening and resistance to micro-plowing wear.
4. Carbide network: A continuous or semi-continuous network of mixed carbides at grain boundaries, which provides crack resistance and prevents grain boundary sliding at elevated temperatures.

The synergistic effect of these multiple carbide types is the key to the alloy's high-temperature wear resistance. At room temperature, the fine MC carbides dominate the wear resistance through dispersion strengthening. As temperature increases, the coarsening of fine carbides is compensated by the thermal stability of NbC and Mo2C, while the primary M7C3 carbides provide continued hard particle reinforcement.

Wear Mechanism Analysis

The paper identifies three primary wear mechanisms operating at different temperature regimes:

At elevated temperatures, the formation and spallation of oxide scales becomes a critical wear mechanism. The Cr content in the alloy promotes the formation of a protective Cr2O3 scale, which can significantly reduce oxidative wear. However, at temperatures above 600°C, the oxide scale may become brittle and spall due to thermal expansion mismatch with the underlying metal, exposing fresh metal to oxidation and accelerating wear.

The multi-element reinforcement strategy addresses this challenge by:

• Maintaining matrix hardness at elevated temperatures through solid solution strengthening (Mo, W) and precipitation hardening (Mo2C, NbC)
• Promoting stable oxide scale formation through Cr and Al additions
• Providing thermal stability through high-temperature-stable carbides (NbC, Mo2C)

Engineering Practice Implications

Process Parameters for PTA Cladding

The PTA cladding process parameters are critical for achieving the desired microstructure and wear performance. The following parameter ranges are recommended:

The dilution rate between the overlay layer and the base metal is a critical parameter. For iron-based high-temperature wear-resistant alloys, the dilution rate should be controlled below 15% to maintain the carbide formation capacity of the overlay alloy. Excessive dilution with carbon steel or low-alloy steel base metals reduces the carbon and alloying element content in the overlay, leading to fewer and coarser carbides, and consequently reduced wear resistance.

Application Scenarios

The developed alloy is particularly suitable for the following applications:

• Cement kiln components: Kiln shells, seals, and wear plates exposed to temperatures of 400–700°C with abrasive cement clinker
• Coal-fired boiler components: Burner tips, air preheater tubes, and furnace walls exposed to 500–800°C with ash erosion
• Metallurgical equipment: Ladle linings, tundish roofs, and continuous casting molds exposed to 600–800°C
• Power plant components: Steam turbine blade tips, exhaust duct linings, and heat exchanger tubes exposed to 400–600°C with particulate erosion

Study Insights and Reflections

The multi-element composite reinforcement approach presented in this paper represents a significant advancement in the design of high-temperature wear-resistant overlay alloys. The key insight is that no single strengthening mechanism is sufficient for high-temperature applications; instead, a combination of solid solution strengthening, precipitation hardening, carbide dispersion strengthening, and thermal stability of oxide scales must be simultaneously optimized.

One area that merits further investigation is the long-term thermal stability of the multi-carbide system. While the paper demonstrates excellent wear resistance at elevated temperatures, the behavior after prolonged exposure (hundreds to thousands of hours) is less clear. The coarsening kinetics of different carbide types at various temperatures should be quantified to predict long-term performance. Additionally, the interaction between different carbide types during prolonged thermal exposure—such as the possible dissolution of fine MC carbides into the matrix or the transformation of M7C3 carbides into more stable phases—should be studied.

From a practical standpoint, the powder composition for PTA cladding must be carefully controlled to ensure consistent microstructure and performance. The powder should be atomized to a uniform particle size distribution (typically 75–150 μm) to ensure stable arc and consistent deposition. The powder should also be stored under controlled humidity conditions to prevent oxidation, which can adversely affect the overlay microstructure.

This work provides a valuable framework for the design of next-generation high-temperature wear-resistant overlay alloys. The multi-element reinforcement strategy can be adapted for specific applications by adjusting the relative proportions of different alloying elements and carbide-forming elements. For example, for applications where oxidation resistance is more critical than pure wear resistance, the Cr and Al content can be increased at the expense of some wear resistance. For applications where thermal shock resistance is critical, the Mo and Ni content can be increased to improve the toughness of the overlay layer.