Effect of Molybdenum on Microstructure and Properties of Plasma Cladding of Cobalt-based Alloys

Literature Overview

This study investigates how varying molybdenum content influences the microstructure, hardness, wear resistance, and corrosion resistance of cobalt-based alloys produced by plasma transferred arc (PTA) powder cladding. Cobalt-based alloys, particularly those in the Stellite family, have long served as benchmark materials for severe thermal and chemical environments in aerospace engines, chemical processing, and mining equipment. The addition of molybdenum is a deliberate metallurgical strategy to enhance solid-solution strengthening, modify carbide precipitation behavior, and improve resistance to pitting and crevice corrosion in oxidizing-acid environments.

Core Technical Points

Role of Molybdenum in Cobalt-based Solid Solution

Molybdenum is a strong solid-solution strengthening element in the FCC cobalt matrix. Its atomic radius (139 pm) is significantly larger than that of cobalt (126 pm), creating substantial lattice distortion that impedes dislocation motion. The study demonstrates that as Mo content increases from approximately 2 wt% to 12 wt%, the microhardness of the cladding layer rises progressively from around HV 420 to HV 580. This strengthening is primarily attributed to the increase in solute drag stress and the associated reduction in dislocation mobility. The relationship between Mo content and hardness follows a sub-linear trend, consistent with the Cottrell solid-solution strengthening model where hardness increment is proportional to the square root of interstitial substitutional concentration.

Carbide Precipitation Behavior

A critical finding of this research is the transformation in carbide morphology and distribution as Mo content increases. At low Mo levels (below 4 wt%), the dominant carbide phase is Co₃(W,Mo)₄C, which forms as coarse blocky particles at grain boundaries and within the dendritic structure. As Mo content rises to 8–12 wt%, the carbide volume fraction increases significantly, and the morphology transitions toward finer, more uniformly distributed particles. This refinement is attributed to Mo's effect on the carbon activity in the liquid phase and its interaction with tungsten in stabilizing the M₄C carbide type. The increased carbide volume fraction provides additional dispersion strengthening, but excessive carbide formation beyond 12 wt% Mo leads to interconnected networks at grain boundaries that compromise toughness.

Corrosion Resistance Enhancement

Molybdenum is well established as a pitting-corrosion inhibitor in austenitic and cobalt-based systems. The study's electrochemical testing in 3.5% NaCl solution and 10% H₂SO₄ demonstrates that Mo addition shifts the pitting potential (E_pit) to more noble values. For a baseline Co-Cr-W alloy without Mo, E_pit in chloride solution is approximately -0.15 V vs. SCE. With 8 wt% Mo, E_pit improves to approximately +0.08 V vs. SCE, representing a gain of roughly 230 mV. This improvement is attributed to Mo's ability to promote selective passivation film enrichment at defect sites, analogous to its mechanism in austenitic stainless steels.

Process Parameters and Their Interaction with Mo Content

The PTA process parameters interact significantly with the metallurgical outcome. The following table summarizes the typical process windows examined in the study and their effects:

Parameter	Range Examined	Effect on Microstructure
Powder feed rate	80–150 g/min	Higher rates increase dilution, reducing effective Mo in the cladding
Travel speed	300–600 mm/min	Faster speeds produce finer dendrites but increase porosity risk
Arc current	250–400 A	Higher current increases heat input and base metal dilution
Mo content in powder	2, 6, 8, 12 wt%	Controls carbide volume fraction and solid-solution strengthening
Interpass temperature	<200°C (multi-pass)	Prevents grain coarsening and carbide coarsening at interfaces

A key practical observation is that dilution from the base substrate (typically a carbon steel or nickel-based superalloy) can reduce the effective Mo content in the cladding layer by 15–30%. Engineers must account for this dilution when specifying powder composition to achieve the desired final chemistry.

Defect Analysis and Countermeasures

Common Defects Observed

Defect Type	Root Cause	Countermeasure
Cracking at HAZ	High Mo promotes retained austenite instability and thermal stress	Preheat substrate to 150–250°C; use multi-pass with low interpass temperature
Porosity	Inadequate shielding at high travel speeds; powder feed instability	Verify gas flow rate (>20 L/min for Ar); stabilize powder feed system
Excessive dilution	High current or low travel speed	Reduce current by 20–30%; increase travel speed; use backing plate
Carbide network at grain boundaries	Mo content >12 wt% with high carbon	Limit Mo to 8–10 wt%; control carbon content below 1.0 wt%

Engineering Practice Integration

In practical application, the findings of this study directly inform the selection of PTA cladding alloys for specific service conditions. For instance, in the chemical processing industry where equipment handles hot concentrated sulfuric acid, a Co-Cr-W-Mo alloy with 8–10 wt% Mo provides superior pitting resistance compared to the conventional Stellite 6 (which contains approximately 2.5 wt% Mo). However, in high-temperature tribological applications such as turbine blade tips, the optimal Mo content is lower (4–6 wt%), because excessive Mo promotes carbide coarsening during thermal exposure above 900°C.

The study's methodology of systematically varying a single alloying element while maintaining constant process parameters is a rigorous approach that engineers should adopt in their own cladding qualification programs. This controlled variable approach enables clear attribution of property changes to composition rather than process artifacts.

Key Reflections and Study Insights

The most valuable insight from this research is the recognition that molybdenum serves a dual role in cobalt-based PTA cladding: it is simultaneously a solid-solution strengthening agent and a corrosion inhibitor. These two functions operate through fundamentally different mechanisms but are synergistically enhanced by the same element. The challenge for the practicing engineer is to find the optimal Mo content that balances these competing demands without introducing detrimental side effects such as intergranular carbide networks or increased susceptibility to thermal cracking.

From a standards perspective, the findings align with the composition ranges specified in ASTM B408 for cobalt-base alloys and are consistent with the qualified procedures documented in ASME Section IX for PTA overlay welding. The study reinforces the principle that cladding alloy selection should be driven by the dominant failure mode in service—whether it is wear, corrosion, or a combination thereof—and that alloying element optimization must always be considered in conjunction with process parameter control.

Reference Value and Outlook

This research provides a quantitative foundation for alloy design in cobalt-based PTA cladding applications. Future work should extend the investigation to multi-element interactions, particularly the combined effects of Mo, W, and Cr on phase stability during thermal cycling. Additionally, in-situ process monitoring techniques such as thermocouple arrays and high-speed imaging could provide real-time feedback on the interaction between Mo content, solidification rate, and microstructure formation. The integration of these findings into digital twin frameworks for cladding process optimization represents a promising direction for the next generation of overlay welding technology.