Hardfacing of Tungsten Carbide with Thermal Protection Technology

Overview and Technical Context

Tungsten carbide (WC) is one of the hardest engineering materials, with Vickers hardness exceeding 1500 HV, and is widely used in mining tools, cutting inserts, wear plates, and valve components. However, WC is extremely brittle and susceptible to thermal shock. During welding or hardfacing of WC-containing alloys, the rapid temperature changes can cause catastrophic cracking and spalling of the carbide phase. The study focused on thermal protection technologies designed to mitigate these issues during the hardfacing of WC-based alloys, exploring methods such as preheating strategies, interlayer deposition, controlled cooling, and specialized welding consumables.

Thermal Shock Mechanism and Critical Parameters

The thermal shock vulnerability of WC-based hardfacing alloys arises from the fundamental mismatch in thermal properties between the WC carbide phase and the metallic binder (typically cobalt or nickel). The coefficient of thermal expansion (CTE) of WC is approximately 4.5 × 10⁻⁶ /K, while cobalt has a CTE of about 13 × 10⁻⁶ /K. This mismatch generates internal stresses during cooling that can exceed the fracture strength of the WC particles, leading to microcracking.

The critical cooling rate that triggers cracking in WC-Co hardfacing alloys was found to be approximately 10–15 K/s. Below this threshold, the thermal stresses remain within the elastic limit of the composite structure, and cracking is avoided. This insight directly informed the design of thermal protection strategies.

Thermal Protection Techniques

Several thermal protection approaches were evaluated in the study:

Preheating and Interpass Temperature Control

Preheating the substrate to 300–500 °C before hardfacing reduces the maximum temperature gradient at the weld interface. For thick-section components, a preheat temperature of 400–500 °C was found to be effective. Interpass temperatures of 200–350 °C maintained during multi-pass welding further reduced thermal cycling severity. Post-weld slow cooling in an insulated box or furnace at rates below 5 K/s was recommended for critical applications.

Interlayer Deposition

Depositing a ductile interlayer between the base substrate and the WC-based hardfacing layer is one of the most effective thermal protection strategies. Nickel-based interlayers (e.g., Ni-20Cr-5Fe-3Si or Ni-Cr-Mo) provide excellent ductility and act as a stress-relieving buffer. The interlayer thickness of 2–3 mm was found to be sufficient to prevent crack propagation from the hardfacing layer into the substrate.

Specialized Consumables and Process Selection

The choice of welding process significantly affects the thermal input and thus the cracking tendency. Submerged arc welding (SAW) with flux-cored wire provides high thermal input and deep penetration, which can be beneficial for thick hardfacing layers but requires careful thermal management. Gas metal arc welding (GMAW) with short-circuit or spray transfer offers moderate thermal input and good control. Oxy-acetylene welding, despite being an older technology, provides very high thermal input with slow cooling rates, making it surprisingly effective for WC-based hardfacing when applied with proper technique.

Defect Analysis Using FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) framework to WC-based hardfacing revealed the following critical failure modes:

The highest Risk Priority Number (RPN) of 350 was assigned to interface cracking, confirming it as the primary concern. The combined strategy of preheating, interlayer deposition, and controlled cooling reduced the occurrence probability from 7 to 2, bringing the RPN down to 100, which is within an acceptable risk threshold.

Engineering Practice and Case Study

A practical application involved the hardfacing of a large mining shovel bucket cutting edge originally made of Q345 steel. The bucket experienced severe abrasive wear from coal and rock, with a service life of only 400–500 hours before replacement. The repair procedure involved grinding the worn surface to a smooth finish (Ra ≤ 6.3 μm), preheating to 400 °C using induction heating, depositing a 2.5 mm Ni-20Cr-5Fe-3Si interlayer using GMAW, and then applying a 6 mm WC-17Co hardfacing layer using SAW with a specialized flux. Post-weld, the component was cooled in an insulated box at a rate of approximately 3 K/s.

The result was a dramatic improvement in service life to over 3,500 hours, an eightfold increase. Metallographic examination of the repaired section showed a sound metallurgical bond between the interlayer and substrate, with no cracks at the interface. The hardfacing layer exhibited a uniform distribution of WC particles within the cobalt matrix, with a measured hardness of 1450–1550 HV.

Study Insights and Conclusions

The study demonstrated that thermal protection during WC-based hardfacing is not a single technique but a systematic approach involving multiple complementary strategies. The key insight is that the cracking problem cannot be solved by any single measure alone—preheating reduces the initial thermal gradient, interlayers absorb residual stresses, and controlled cooling prevents secondary cracking during the cooling phase. Engineers must view these measures as an integrated package rather than independent options.

Furthermore, the study emphasized that the quality of surface preparation is often underestimated in the field. Incomplete removal of prior coatings, rust, or oxide scale creates weak interfaces that become initiation sites for cracking and delamination. A disciplined surface preparation protocol, including degreasing, grinding, and inspection, should be mandatory in any production hardfacing operation. The economic benefits of proper thermal protection are substantial, as the cost of preheating equipment and interlayer consumables is negligible compared to the cost of premature component failure and unplanned downtime.