Effect of Vanadium on Microstructure and Properties of Iron-Based Tungsten Carbide Wear-Resistant Overlay Layers

Literature Overview

This study, authored by Wei Wei, Huang Zhiquan, Zhang Haiyan, and Yang Wei from Zhengzhou Machinery Research Institute Co., Ltd., was published in the Journal of Welding in 2019. The work investigates the influence of vanadium addition on the microstructure and mechanical properties of iron-based tungsten carbide (WC) wear-resistant overlay layers. Iron-based WC overlay alloys are widely used in applications requiring extreme wear resistance, such as mining equipment, cement mill liners, and hydraulic pump components. The addition of alloying elements such as vanadium is a common strategy to enhance the properties of these overlay layers, but the specific effects of vanadium on the microstructure and performance have not been extensively studied.

Technical Background

Iron-based tungsten carbide overlay alloys typically contain 15–30 wt% tungsten, 5–15 wt% chromium, and 2–5 wt% carbon, with iron as the balance. The primary wear-resistant phase in these alloys is tungsten carbide (WC), which has a hardness of approximately 2300 HV and provides excellent resistance to abrasive wear. However, WC is susceptible to oxidation and corrosion, and the WC particles can fracture and detach from the matrix during wear, leading to premature failure of the overlay layer.

The addition of vanadium to these alloys is intended to improve the properties through several mechanisms:

Carbide formation: Vanadium forms vanadium carbides (VC, V4C3) that are hard and wear-resistant, providing additional strengthening phases.
Matrix strengthening: Vanadium is a strong ferrite former and increases the hardness and strength of the iron matrix.
Grain refinement: Vanadium acts as a grain refiner, reducing the grain size of the matrix and improving toughness.
Oxidation resistance: Vanadium improves the oxidation resistance of the overlay layer by forming a protective oxide scale.

Experimental Design and Key Findings

The study investigated the effect of vanadium addition at concentrations of 0, 2, 4, and 6 wt% on the microstructure and mechanical properties of iron-based WC overlay layers. The overlay layers were deposited using submerged arc welding (SAW) with a consumable flux, and the following welding parameters were used:

Parameter	Value
Welding current	350–400 A
Arc voltage	28–32 V
Travel speed	200–250 mm/min
Wire diameter	2.5 mm
Number of passes	3–5
Interpass temperature	200–250 °C

The microstructure of the overlay layers was examined using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The mechanical properties were evaluated using hardness testing (Vickers), tensile testing, and abrasive wear testing (pin-on-disk tribometry).

Microstructural Evolution with Vanadium Addition

The study found that vanadium addition significantly altered the microstructure of the overlay layers:

Vanadium Content (wt%)	Primary Phases	Matrix Structure	WC Particle Morphology	Hardness (HV)
0	WC, Cr7C3, Fe3C	Martensite + retained austenite	Large, irregular	850–950
2	WC, VC, Cr7C3, Fe3C	Martensite + bainite	Medium, more uniform	950–1050
4	WC, VC, V4C3, Cr7C3	Martensite + bainite + ferrite	Fine, well-dispersed	1050–1150
6	WC, VC, V4C3, Cr7C3	Ferrite + pearlite + carbides	Fine but coalesced	950–1050

The optimal vanadium content was identified as 4 wt%, which produced a microstructure with a fine dispersion of WC, VC, and V4C3 carbides within a martensitic-bainitic matrix. This microstructure exhibited the highest hardness (1100–1150 HV) and the best wear resistance.

Wear Performance

The abrasive wear performance was evaluated using a pin-on-disk tribometer under dry sliding conditions at a normal load of 20 N and a sliding distance of 1000 m. The specific wear rate (volume loss per unit distance per unit load) was measured for each composition:

Vanadium Content (wt%)	Specific Wear Rate (mm³/N·m)	Wear Mechanism
0	1.2 × 10⁻⁵	Abrasive + adhesive
2	0.8 × 10⁻⁵	Abrasive
4	0.5 × 10⁻⁵	Abrasive (minimum)
6	0.7 × 10⁻⁵	Abrasive + matrix softening

The results clearly demonstrate that 4 wt% vanadium addition provides the optimal wear resistance, with a specific wear rate 60% lower than the unalloyed condition. The wear scar morphology analysis revealed that the 4 wt% vanadium alloy exhibited shallow grooves with minimal material transfer, indicating that the fine carbide dispersion effectively resisted ploughing and adhesion.

Mechanical Properties

The tensile properties of the overlay layers were also evaluated, although these are less critical than the wear properties for most applications. The results showed that vanadium addition improved the yield strength but reduced the elongation, as expected for a wear-resistant alloy:

Vanadium Content (wt%)	Yield Strength (MPa)	Elongation (%)
0	850	5.2
2	950	4.5
4	1050	3.8
6	1000	3.2

Engineering Practice and Process Considerations

From a practical fabrication standpoint, the addition of vanadium to iron-based WC overlay alloys introduces several process considerations:

Weldability: Vanadium is a strong carbide former and can reduce the weldability of the alloy by increasing the tendency for hot cracking. The welding parameters must be carefully optimized to minimize the risk of cracking, particularly at the overlay/base metal interface.
Heat input control: Higher heat input promotes carbide coalescence and matrix softening, which can degrade the wear properties. The heat input should be kept as low as practical while maintaining adequate fusion and bond strength.
Preheat and interpass temperature: A preheat temperature of 150–200 °C and an interpass temperature of 200–250 °C are recommended to control the cooling rate and reduce residual stresses.
Post-weld heat treatment: A stress-relief anneal at 600–650 °C for 2–4 hours is recommended to relieve residual stresses without significantly affecting the microstructure and hardness of the overlay layer.

Defect Analysis and Countermeasures

The study identified several defects that can occur during the welding of vanadium-containing overlay alloys:

Defect Type	Root Cause	Detection Method	Countermeasure
Hot cracking	High vanadium content, high heat input	MT, RT	Reduce heat input, control cooling rate, optimize composition
Cold cracking	Hydrogen embrittlement, high residual stress	MT, UT	Preheat, low-hydrogen filler, post-weld stress relief
Excessive dilution	High heat input, thin overlay	Chemical analysis	Use lower heat input, increase pass thickness, use bond layer
Carbide coalescence	Excessive heat input, slow cooling	Metallography	Reduce heat input, increase travel speed

Key Questions and Reflections

One important question that arises from this study is the effect of vanadium on the corrosion resistance of the overlay layer. While vanadium improves the wear resistance and hardness, it may have adverse effects on the corrosion resistance, particularly in acidic or oxidizing environments. The formation of vanadium carbides and the modification of the matrix microstructure can alter the electrochemical behavior of the overlay layer, potentially increasing the susceptibility to pitting corrosion or stress corrosion cracking. Further investigation of the corrosion properties is necessary to fully assess the applicability of vanadium-containing overlay alloys in corrosive environments.

Another consideration is the cost-benefit analysis of vanadium addition. Vanadium is a relatively expensive alloying element, and its addition increases the cost of the filler material. The improvement in wear resistance must be weighed against the increased cost, and the optimal vanadium content should be selected based on the specific application requirements and service conditions. For applications where the wear life is the primary concern, the additional cost of vanadium may be justified. However, for applications where the overlay layer is replaced frequently, the cost savings from using a lower-cost alloy without vanadium may be more important.

The study also raises questions about the long-term stability of the microstructure under service conditions. The fine carbide dispersion that provides superior wear resistance may coarsen over time at elevated temperatures, leading to progressive loss of hardness and wear resistance. This phenomenon is particularly relevant for applications where the overlay layer is exposed to temperatures above 300 °C, such as cement mill liners or hydraulic pump components.

Study Insights and Implications

The fundamental contribution of this research is the systematic evaluation of the effect of vanadium addition on the microstructure and properties of iron-based WC overlay alloys. The identification of 4 wt% vanadium as the optimal concentration provides a clear composition recipe that can be directly applied in the development of new filler materials for wear-resistant overlay welding.

From a materials design perspective, this work demonstrates that the addition of vanadium is an effective strategy for enhancing the wear resistance of iron-based WC overlay alloys. The formation of vanadium carbides and the refinement of the matrix microstructure contribute to a synergistic improvement in hardness and wear resistance. This finding is consistent with the general principle that the optimal wear resistance of a composite material is achieved when the hard phase is finely dispersed within a tough matrix.

In summary, this study demonstrates that the addition of 4 wt% vanadium to iron-based WC overlay alloys significantly improves the microstructure, hardness, and wear resistance, with a specific wear rate reduction of 60% compared to the unalloyed condition. The practical implementation of these findings requires careful attention to welding parameters, heat input control, and post-weld heat treatment, but the performance gains are substantial and well-documented. Engineers and fabricators working with wear-resistant overlay alloys should consider vanadium addition as a viable strategy for enhancing the performance of their products, particularly in applications where extreme wear resistance is required.