Preparation Study of Wear-Resistant and Heat-Resistant Overlay Welding Electrodes

Literature Overview

This paper by Ma Jianghong, Yu Yueguang, Xue Wentao, and Huang Jingyong from the Beijing Research Institute of Mining and Metallurgy, published in Surface Technology of China (Vol. 19, Issue Z1, 2006), presents a comprehensive study on the preparation of wear-resistant and heat-resistant overlay welding electrodes. The research covers the entire process chain from powder preparation through electrode fabrication to weld deposit characterization.

Core Technical Content

Alloy System and Preparation Process

The authors selected the Ni-Co-Cr-B-C alloy system as the base composition for the overlay welding electrode. The preparation process involves:

1. Mechanical ball milling — For homogeneous mixing of constituent powder elements
2. Extrusion forming — To shape the electrode rod
3. Vacuum sintering — To achieve full densification while preventing oxidation

Weld Deposit Composition and Structure

The overlay was applied using GTAW (Tungsten Inert Gas Arc Welding) onto a high-temperature cast alloy substrate. The characterization results are summarized below:

Phase Analysis

The weld deposit microstructure consists of:

• Matrix phase: Face-centered cubic (FCC) solid solution of Co and Ni — provides good toughness and thermal stability
• Primary hard phase: M23(C,B)6 or M23C6 — provides primary wear resistance through high hardness
• Secondary phases: C7M3 (iron carbide type) and M3B2 (boride) — contribute additional hardness and may influence high-temperature stability

Technical Interpretation

Alloy Design Philosophy

The Ni-Co-Cr-B-C system represents a carefully balanced alloy design for combined wear and heat resistance:

• Ni and Co — Form the FCC matrix, ensuring good ductility and thermal stability up to approximately 800–900°C
• Cr — Promotes carbide formation (M23C6 type) and enhances oxidation resistance through Cr2O3 scale formation
• B — Forms boride phases (M3B2) that provide high hardness; also modifies carbide morphology
• C — Essential for carbide formation; controls the volume fraction and size of hard phases

The interplay between borides and carbides is particularly important. While both phases are hard, borides tend to be more brittle and can be detrimental to toughness if present in excessive quantities. The optimal B/C ratio must be carefully controlled to balance wear resistance against mechanical integrity.

Hardness and Wear Resistance Correlation

The measured hardness of 49.5 HRC (approximately HV 600) places this overlay in the category of hard-facing deposits suitable for severe abrasive wear conditions. The hardness is achieved through:

1. Solid solution strengthening of the FCC matrix by Co and Cr
2. Dispersion strengthening from fine M23C6/M23(C,B)6 carbides
3. Possible precipitation hardening from C7M3 and M3B2 phases

High-Temperature Oxidation Resistance

The good high-temperature oxidation resistance is attributed to:

• Chromium content promoting protective Cr2O3 scale formation
• Nickel and cobalt stabilizing the oxide scale at elevated temperatures
• The presence of borides potentially contributing to scale adhesion

Engineering Practice Integration

Welding Process Selection

The use of GTAW for applying this overlay is significant for several reasons:

• Low dilution — GTAW provides minimal dilution with the base metal, preserving the designed overlay composition
• Clean weld — No slag inclusions, ensuring clean microstructure
• Precise control — Excellent control over heat input and weld geometry
• Limitation — Lower deposition rate compared to SMAW or FCAW, making it less suitable for thick overlay requirements

For production applications requiring thicker overlays, a hybrid approach may be considered:

1. First pass using GTAW to establish a clean, well-bonded root layer
2. Subsequent passes using FCAW or SMAW with compatible electrodes for increased deposition rate

Application Suitability

This type of overlay is particularly suitable for:

• Hot working tools — Dies, punches, and forming tools operating above 500°C
• Turbine components — Compressor blades and casings in gas turbines
• Furnace components — Pusher bars, roller tables, and heat exchanger tubes in high-temperature furnaces
• Pipe manufacturing equipment — Mandrels, rollers, and forming tools in pipe mills processing hot steel pipe

Quality Assurance Protocol

A recommended quality assurance protocol for overlay welding with this electrode type:

1. Electrode storage — Store in dry conditions (dew point control below -20°C) to prevent moisture absorption
2. Pre-weld preparation — Clean base metal to bare metal within 25 mm of weld area; preheat if required by base metal specification
3. Welding parameters — Control current density (typically 200–350 A/cm² for Ni-based alloys in GTAW), travel speed, and arc length
4. Interpass temperature — Maintain below 150°C to prevent grain growth in previously deposited layers
5. Post-weld inspection — Visual inspection, hardness profiling, and periodic XRD or metallographic examination

Key Questions and Reflections

Several important considerations emerge from this study:

• Phase stability at service temperature — While the as-deposited microstructure shows favorable phases, prolonged exposure at service temperatures may cause phase transformations (e.g., carbide coarsening, boride dissolution). Long-term stability data would be valuable for life prediction.
• Thermal cycling effects — In applications involving repeated heating and cooling, thermal fatigue may degrade the overlay. The coefficient of thermal expansion mismatch between the overlay and substrate should be evaluated.
• Cost-benefit analysis — The Ni-Co-Cr-B-C system is significantly more expensive than iron-based alternatives. Engineers must justify the premium through demonstrated performance advantages in the specific application.

Study Insights

This paper provides a complete process-structure-property framework for sintered overlay electrode development. The systematic approach — from powder design through fabrication to characterization — serves as an excellent model for similar development programs. The emphasis on vacuum sintering as the densification method is particularly noteworthy, as it ensures oxygen-free processing that is critical for maintaining the integrity of boride and carbide phases. For engineers in the steel pipe and fitting industry, this research demonstrates that advanced overlay solutions exist for the most demanding thermal and wear environments, and that the key to success lies in careful alloy design and process control throughout the entire manufacturing chain.