Effect of Molybdenum on Microstructure and Wear Resistance of High-Hardness Open-Arc Overlay Alloys

Literature Overview

The 2016 paper by Gong Jianxun, Liu Jiangqing, and Li Yi from Xiangtan University's School of Mechanical Engineering investigates the role of molybdenum (Mo) in high-hardness open-arc overlay alloys, specifically examining its influence on microstructure evolution and wear resistance. Supported by the Hunan Natural Science Joint Fund (Grant 2015JJ5031), this work addresses a practical challenge in open-arc (GMAW/SAW) overlay welding where atmospheric exposure and high heat input create unique microstructural challenges that differ from shielded or high-energy beam processes.

Core Technical Content

Role of Molybdenum in Overlay Alloys

Molybdenum serves multiple functions in iron-based overlay alloys, each contributing to the overall performance:

Function	Mechanism	Threshold Content
Solid solution strengthening	Mo atoms in bcc/fcc matrix lattice	> 1 wt%
Secondary carbide formation	Mo₂C, Mo₄C₃ precipitation	> 3 wt%
Matrix hardening	Supersaturation of austenite/martensite	> 2 wt%
Retained austenite stabilization	Expansion of γ-field	> 5 wt%
High-temperature strength	Creep resistance enhancement	> 4 wt%
Hot shortness control	Sulfide modification	> 0.5 wt%

The paper demonstrates that molybdenum's contribution to wear resistance is non-linear — the improvement is modest below 3 wt%, significant between 3–8 wt%, and exhibits diminishing returns above 8 wt% due to the onset of brittle intermetallic phases.

Microstructural Effects of Molybdenum Addition

The open-arc process environment introduces unique challenges that molybdenum helps address:

Carbide modification — Mo addition transforms the carbide morphology from coarse M₇C₃ plates to a mixture of M₇C₃ + Mo₂C, with the latter providing superior wear resistance due to its higher hardness (1600–1800 HV versus 1200–1400 HV for M₇C₃).
Martensite stabilization — Mo increases the Ms temperature depression and stabilizes retained austenite, creating a dual-phase microstructure of tempered martensite + retained austenite that provides both hardness and toughness.
Grain refinement — Mo₂C particles act as heterogeneous nucleation sites during solidification, reducing grain size by 20–35% compared to Mo-free alloys of equivalent carbon content.
Segregation control — Mo reduces the degree of microsegregation of carbon and chromium at interdendritic boundaries, producing a more homogeneous microstructure that improves mechanical property consistency.

Wear Resistance Mechanisms

Wear Condition	Dominant Mechanism	Mo Contribution	Optimal Mo Content
Dry sliding (steel pin)	Adhesive + abrasive	Matrix hardening, carbide refinement	4–6 wt%
Abrasive (SiC paper)	Micro-ploughing	Hard carbide resistance	5–8 wt%
Erosive (airborne particles)	Impact + cutting	Toughness + hardness balance	3–5 wt%
High-temperature sliding	Oxidative + adhesive	Oxidation resistance, creep strength	6–10 wt%
Corrosive-abrasive	Corrosion-assisted wear	Passivation enhancement	4–8 wt%

Open-Arc Process Specific Considerations

The open-arc environment (GMAW with flux-cored wire or SAW with open flux) introduces specific challenges:

Challenge	Impact	Mo Mitigation
Atmospheric oxygen	Oxide inclusion formation	Mo forms stable MoO₃ that can be slagged
Nitrogen pickup	Nitride formation, embrittlement	Mo competes for nitrogen, forming stable MoN
High heat input	Coarse microstructure	Mo promotes grain refinement despite high heat
Dilution variability	Property inconsistency	Mo's strong solid solution effect maintains hardness
Spatter	Material loss, porosity	Mo improves fluidity, reduces spatter tendency

Process Parameter Optimization

Parameter	Without Mo	With 5% Mo	With 8% Mo
Optimal current (A)	380–420	400–450	420–480
Optimal travel speed (mm/min)	180–220	200–250	220–280
Optimal wire feed rate (m/min)	4.5–5.5	5.0–6.0	5.5–6.5
Achievable hardness (HRC)	58–62	62–66	65–69
Crack susceptibility	Moderate	Low	Moderate-high

Defect Analysis

The addition of molybdenum introduces specific defect risks that must be managed:

Defect	Cause	Prevention
Mo₂C network embrittlement	Excessive Mo content (>10%)	Limit Mo to 8% maximum
Hot cracking	Mo segregation at grain boundaries	Control interpass temperature < 300°C
Cold cracking	Hydrogen sensitivity increase	Preheat > 250°C, use low-hydrogen consumables
Porosity	MoO₃ slag entrapment	Ensure proper slag removal between passes
Bond line cracking	Dilution-induced phase transformation	Control dilution rate, use interlayer if needed

Engineering Practice Integration

Typical Application Scenarios

Mining equipment — Excavator buckets, crusher hammers, and conveyor components where severe abrasive wear occurs. Mo-containing overlays extend service life by 3–5 times compared to unalloyed steel.
Cement industry — Mill liners, kiln wear plates, and grinding media where combined abrasive and impact wear occurs. Mo additions improve resistance to both wear mechanisms simultaneously.
Power generation — Boiler tubes, furnace components, and steam turbine parts where high-temperature wear and oxidation occur. Mo provides creep resistance and oxidation resistance at temperatures up to 650°C.
Oil and gas — Drill bits, valve components, and pipeline fittings where erosive wear in corrosive environments occurs. Mo enhances both wear and corrosion resistance.

Quality Control Considerations

For production implementation, the following quality control measures are recommended:

Composition verification — Spectroscopic analysis of overlay wire/feedstock to ensure Mo content is within ±0.5% of nominal.
Hardness mapping — Systematic hardness testing across the overlay cross-section to verify the hardness profile meets specifications (typically > 60 HRC at surface, gradual transition to substrate).
Metallographic examination — Verification of carbide distribution, absence of brittle phase networks, and adequate matrix microstructure.
Bond strength testing — Shear or tensile bond strength testing to ensure adequate metallurgical bonding (minimum 200 MPa for most applications).
Wear testing — Representative wear testing under simulated service conditions to validate performance claims.

Study Insights and Reflections

This research provides practical guidance for overlay alloy designers working with open-arc processes, where the process constraints are less favorable than high-energy beam or plasma processes. The key finding that molybdenum optimally contributes between 4–8 wt% for most wear applications is directly actionable for alloy selection and process development. The identification of 8 wt% Mo as the practical upper limit — beyond which brittle phase formation and crack susceptibility increase — provides a clear design boundary.

The paper's treatment of the open-arc environment as a distinct process category is particularly valuable, as much of the overlay literature focuses on laboratory conditions that do not reflect production reality. The atmospheric effects (oxygen, nitrogen pickup) and high heat input characteristic of open-arc processes create microstructural challenges that molybdenum uniquely addresses through its ability to stabilize beneficial phases while suppressing detrimental ones.

For practitioners, the most important takeaway is that molybdenum is not merely a hardening element but a microstructure modifier that improves the overall quality of the overlay deposit. The grain refinement, segregation control, and carbide modification effects collectively produce overlays with more consistent and predictable properties than Mo-free alternatives. This consistency is particularly valuable in production environments where process parameter variation is inevitable, as the Mo-containing alloy maintains acceptable performance over a wider parameter range. The research effectively demonstrates that alloy design and process design must be considered together — the optimal Mo content depends on the specific process conditions, not just the target properties.