Research on TiC-NbC Super Hard Phase Wear-Resistant Overlay Welding Electrodes

Literature Overview and Core Concept

This study focuses on the development of a specialized overlay welding electrode incorporating TiC-NbC composite hard phases to achieve exceptional wear resistance in demanding industrial applications. The fundamental principle relies on the synergistic effect between titanium carbide and niobium carbide, which together form a multi-phase microstructure with hardness values exceeding 1500 HV in the as-welded condition. The research addresses a critical gap in conventional overlay electrodes where single-carbide systems often suffer from thermal cracking due to excessive brittleness and poor fracture toughness. By introducing NbC as a secondary hard phase, the electrode achieves a balanced combination of hardness, toughness, and thermal stability that surpasses traditional CrC or WC-based systems.

The study examines electrode compositions ranging from 25 to 45 wt% total carbide content, with the TiC:NbC ratio varied between 1:1 and 3:1 to evaluate the influence on microstructure evolution and tribological performance. The base alloy matrix was designed around a high-chromium austenitic stainless steel (Cr 22-26%, Mo 4-6%) to ensure adequate corrosion resistance at the working interface while providing sufficient ductility to accommodate thermal cycling stresses.

Microstructure and Hardness Analysis

The microstructure of the weld overlay deposit reveals a complex arrangement of primary and secondary carbides distributed within an austenitic-ferritic matrix. Metallographic examination at 500x magnification shows that TiC particles appear as angular blocky phases with sizes ranging from 5 to 25 micrometers, while NbC particles exhibit a more rounded morphology with sizes between 2 and 12 micrometers. The distribution pattern follows a dendritic arrangement where TiC preferentially nucleates at dendrite arms and NbC precipitates in inter-dendritic regions.

The hardness profile across the overlay thickness demonstrates a layered characteristic: the surface layer achieves 1500-1650 HV due to the highest carbide concentration near the cooling surface, while the bond line region transitions to 800-950 HV. This gradient provides an effective wear-resistant surface while maintaining adequate bonding strength to the substrate. The bond strength test results show values exceeding 180 MPa for a 3-pass overlay with proper preheat and interpass temperature control, meeting the requirements specified in API 934 for high-performance overlay deposits.

Process Parameters and Welding Procedure Development

The welding procedure development followed a systematic approach using SMAW (Shielded Metal Arc Welding) with DCEP polarity. The electrode diameter was standardized at 4.0 mm to balance deposition rate with arc stability. Key process parameters were optimized through orthogonal experimental design (L16) to minimize cracking tendency while maximizing carbide retention.

A critical finding from the study is that the arc voltage directly influences the degree of carbide dissolution. At voltages above 28 V, significant melting of TiC particles occurs, reducing the effective hard phase content by 15-20%. Conversely, at voltages below 22 V, incomplete fusion leads to lack-of-bonding defects at the interface. The optimal window of 24-26 V provides the best compromise between carbide retention and wetting characteristics.

Defect Analysis and Countermeasures

The primary defects encountered during welding include hot cracking in the final pass, carbide agglomeration, and porosity at the bond line. Hot cracking is attributed to the high carbon equivalent (CE ≈ 0.65-0.78) of the deposit alloy combined with the restraint from the substrate. The countermeasure involves using a low-hydrogen electrode flux coating with controlled moisture content (≤1.0% by weight) and maintaining strict interpass temperature discipline.

Carbide agglomeration occurs when travel speed drops below 50 mm/min, allowing excessive local heat input and coalescence of unmelted carbide particles. This defect is identified through hardness mapping showing localized peaks exceeding 2000 HV with adjacent soft zones below 600 HV. The remediation strategy requires maintaining consistent travel speed and using a weave pattern with amplitude controlled at 1.5 times the electrode diameter.

Engineering Application and Performance Validation

Field trials were conducted on a slurry pump impeller operating in a coal washing plant with abrasive slurry containing 35% solid content and particle sizes up to 2 mm. The overlay deposit thickness was 8 mm applied in 4 passes. After 6,000 hours of continuous service, the measured wear rate was 0.012 mm per 1000 hours, representing a 4.5-fold improvement over the baseline high-chromium cast iron surface. The deposit maintained its microstructure integrity with no evidence of intergranular corrosion or stress corrosion cracking under the alkaline (pH 9.5) operating conditions.

Study Insights and Implications

The TiC-NbC composite hard phase approach represents a significant advancement over single-carbide systems, particularly for applications requiring both extreme wear resistance and moderate toughness. The key engineering insight is that the NbC phase serves not only as a secondary hard phase but also as a crack-arresting mechanism that disrupts the continuity of TiC-induced crack paths. This finding has direct implications for the design of overlay electrodes for mining equipment, cement mill liners, and hydraulic components operating in highly abrasive environments. The systematic process parameter optimization methodology presented in the study provides a transferable framework for developing specialized overlay consumables for other hard phase systems, including Mo2C-TaC and Cr3C2-HfC combinations.