CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Alloying Elements on the Hardness of Weld Overlay Deposits
Literature Overview
This study note focuses on the systematic investigation of how various alloying elements influence the hardness of weld overlay deposits. The literature reviewed covers a broad range of alloying additions including carbon, chromium, molybdenum, tungsten, vanadium, cobalt, and nickel in weld overlay consumables applied to carbon and low-alloy steel substrates. The research employs metallographic analysis, microhardness profiling, X-ray diffraction, and scanning electron microscopy to correlate microstructural evolution with hardness variation across the overlay layer. The primary welding methods examined include submerged arc welding, gas metal arc welding, and gas tungsten arc welding, with particular attention paid to multi-pass overlay procedures where thermal cycling effects are significant.
Core Technical Findings
The fundamental mechanism by which alloying elements affect overlay hardness can be categorized into three principal mechanisms: solid solution strengthening, precipitation hardening, and carbide formation. Carbon remains the most potent hardening element in steel-based overlay deposits, with each 0.1 wt% increase in carbon content typically raising hardness by approximately 25 to 40 HV in the as-welded condition. However, excessive carbon promotes brittle cementite networks and increases susceptibility to cracking during cooling.
Chromium plays a dual role in overlay deposits. At levels below 8 wt%, chromium contributes to solid solution strengthening and promotes the formation of M7C3 type carbides which provide moderate hardness enhancement. Above 12 wt%, chromium favors austenitic microstructures stabilized by nickel additions, resulting in lower as-welded hardness but superior corrosion resistance. The critical threshold around 10 to 12 wt% Cr represents a microstructural transition zone where ferrite-austenite balance shifts dramatically.
Molybdenum and tungsten are high-temperature strengthening elements that form extremely hard MC and M2C carbides. A single addition of 3 to 5 wt% Mo can increase overlay hardness from approximately 250 HV to 380 HV in the as-welded condition. These elements also retard softening during subsequent thermal exposure, making them indispensable for overlay applications in elevated temperature service environments.
Vanadium forms the hardest carbide phase (VC, approximately 2800 HV intrinsically) among common alloying additions. Even modest vanadium additions of 0.5 to 1.5 wt% produce significant hardness improvements through fine dispersion of carbide particles. The challenge lies in maintaining uniform vanadium distribution throughout multi-pass overlays, as vanadium tends to segregate at interpass regions during rapid solidification.
Cobalt enhances hardness through solid solution strengthening and improves the thermal stability of carbide phases. Cobalt additions of 5 to 20 wt% in high-speed steel type overlay deposits produce hardness values exceeding 500 HV while maintaining reasonable toughness. Nickel, conversely, tends to stabilize austenite and reduce hardness in the as-welded condition, but it becomes beneficial when combined with chromium to produce duplex microstructures with balanced properties.
| Alloying Element | Typical Addition Range | Hardness Contribution | Primary Mechanism |
|---|---|---|---|
| Carbon | 0.5–3.0 wt% | +25–40 HV per 0.1% C | Carbide formation, solid solution |
| Chromium | 4–20 wt% | +10–30 HV per 1% Cr (below 12%) | Solid solution, carbide formation |
| Molybdenum | 2–8 wt% | +15–25 HV per 1% Mo | MC/M2C carbide formation |
| Tungsten | 3–12 wt% | +12–20 HV per 1% W | MC/M6C carbide formation |
| Vanadium | 0.3–2.0 wt% | +30–50 HV per 0.5% V | VC carbide precipitation |
| Cobalt | 5–20 wt% | +8–15 HV per 1% Co | Solid solution strengthening |
| Nickel | 3–12 wt% | -5 to -15 HV per 1% Ni | Austenite stabilization |
Microstructural Analysis and Heat Treatment Effects
The as-welded microstructure of overlay deposits is governed by the cooling rate, which depends on the welding method, substrate thickness, interpass temperature, and number of passes. Rapid cooling rates characteristic of GTAW produce fine dendritic structures with high hardness but potential brittleness. Slower cooling in SAW deposits yields coarser microstructures with lower hardness but improved toughness.
The literature emphasizes that post-weld heat treatment can significantly modify overlay hardness. Tempering at 550 to 650°C for 1 to 2 hours in high-carbon, high-alloy overlay deposits converts retained austenite to tempered martensite and spheroidizes cementite, increasing hardness from approximately 350 HV to 550 HV. However, excessive tempering temperatures above 700°C cause carbide coarsening and hardness degradation.
Interpass temperature control is critical in multi-pass overlay procedures. Maintaining interpass temperatures below 200°C for hard-facing deposits preserves the fine microstructure achieved during individual pass solidification. Temperatures exceeding 350°C between passes promote grain growth in previously deposited layers, reducing overall overlay hardness by 20 to 40 HV.
Engineering Practice Implications
In practical application, the selection of alloying additions must balance hardness requirements against weldability, corrosion resistance, and service environment. For example, in coal mill roller overlay applications requiring 500 HV hardness, a typical composition of 2.5% C, 12% Cr, 5% Mo, 1% V, and 8% Co produces reliable results with GTAW single-pass or multi-pass procedures. In contrast, overlay deposits for corrosion-resistant service on hydrogenation reactor internals may require 18% Cr, 8% Ni, 2% Mo with lower carbon content, accepting hardness values of 200 to 280 HV in exchange for superior resistance to hydrogen-induced cracking and sulfide stress corrosion.
The literature also highlights the importance of matching overlay composition to substrate compatibility. Applying high-carbon overlay deposits directly to high-strength low-alloy steel substrates creates steep hardness gradients that promote cracking at the bond line. A transition layer of moderate alloy content (such as 8% Cr, 0.3% C) between the base metal and the hard overlay deposit effectively reduces thermal mismatch and prevents interfacial cracking.
Key Questions and Reflections
One persistent challenge identified in the literature is the reproducibility of overlay hardness across different manufacturing facilities. The same consumable composition welded under different conditions can produce hardness variations of 50 to 100 HV. This variation stems from differences in shielding gas purity, electrode dryness, travel speed consistency, and interpass temperature discipline. The study reinforces the need for strict welding procedure qualification per NB/T 47014 or ASME IX, with hardness testing as a mandatory acceptance criterion for each production batch.
Another important reflection concerns the relationship between hardness and wear resistance. While higher hardness generally correlates with better abrasive wear resistance, the literature demonstrates that this relationship is non-linear. Beyond approximately 600 HV, marginal hardness increases produce diminishing returns in wear life. The microstructural architecture—specifically carbide size, shape, distribution, and matrix ductility—becomes more important than absolute hardness values at high hardness levels. This insight is critical for engineers selecting overlay specifications for demanding wear applications.
Study Insights and Practical Recommendations
The comprehensive study of alloying element effects on overlay hardness provides a rational basis for consumable selection and welding procedure optimization. The key takeaway is that overlay hardness is not determined by any single element but by the synergistic interaction of multiple alloying additions, processing conditions, and microstructural features. Engineers must adopt a systems-based approach, considering the complete service environment—including temperature, corrosive media, contact pressure, sliding velocity, and abrasive particle characteristics—when specifying overlay compositions.
In conclusion, the systematic understanding of alloying element effects on weld overlay hardness enables more rational and economical design of cladding systems, reducing reliance on trial-and-error approaches and improving the reliability and service life of clad components in industrial applications.