Micro-Pulse Resistance Weld Overlay on Cyanide-Treated Surfaces

Technical Background and Innovation

This 1999 study by Tan Jun, Zhang Lei, Han Wenzheng, and Jiang Houwen from the Surface Engineering Research Institute of the Chinese Mechanical Engineering Society explores an innovative approach to weld overlay on cyanide-treated (cy-mented) surfaces. Cyanide treatment, also known as cyaniding or cyanide hardening, is a surface hardening and case-hardening process that introduces carbon and nitrogen into the surface layer of steel components, creating a thin, hard, and wear-resistant case. The resulting surface layer typically has a hardness of 58–64 HRC and a depth of 0.2–0.8 mm.

The challenge addressed by this study is the difficulty of achieving good metallurgical bonding when performing weld overlay on cyanide-treated surfaces. The high carbon and nitrogen content of the cyanide case creates a brittle, high-carbon martensitic structure that is susceptible to cracking during welding. Additionally, the residual stresses from the cyanide treatment can be reactivated during the welding heat cycle, leading to cracking and poor bond quality.

Micro-Pulse Resistance Welding Process

The micro-pulse resistance welding process represents a significant departure from conventional arc welding processes for weld overlay. Instead of using a continuous or pulsed arc, the process employs short-duration electrical pulses to heat the workpiece and filler material through resistance heating. The key advantages of this approach include:

Minimal heat input: The short pulse duration (typically 1–10 ms) results in very low total heat input, minimizing the heat-affected zone
Rapid heating and cooling: The high heating rate and rapid cooling can produce fine-grained microstructures with controlled hardness
Reduced dilution: The localized heating minimizes the mixing of base material into the overlay weld
Reduced residual stress: The low heat input and rapid cooling reduce the magnitude of welding-induced residual stresses

Process Parameter	Typical Range	Effect
Pulse duration	1–10 ms	Controls heat input and penetration depth
Pulse current	5–50 kA	Determines peak temperature
Pulse frequency	1–100 Hz	Controls deposition rate and bead geometry
Electrode pressure	10–100 MPa	Ensures good electrical contact and mechanical bonding
Filler wire diameter	1.0–3.0 mm	Affects deposition rate and bead size

Metallurgical Challenges of Cyanide-Treated Surfaces

The cyanide-treated surface presents several metallurgical challenges for weld overlay:

High carbon content: The surface layer typically contains 1.0–1.5% carbon, which is far above the carbon content of the base material (typically 0.3–0.5% for medium-carbon steels). This high carbon content creates a very hard, brittle martensitic structure that is prone to cracking during welding.
Nitrogen content: The nitrogen introduced during cyaniding (typically 0.2–0.5%) can form brittle nitrides in the weld metal and HAZ, reducing toughness and increasing cracking susceptibility.
Residual compressive stresses: The cyanide treatment introduces residual compressive stresses in the surface layer. These stresses can be partially relieved or reversed during the welding heat cycle, potentially leading to cracking.
Thin case depth: The cyanide case is typically only 0.2–0.8 mm thick, which means that even a small amount of heat input can penetrate through the case into the softer base material, creating a dilution zone with intermediate properties.

Experimental Findings and Results

The study reports on experimental trials of micro-pulse resistance weld overlay on cyanide-treated steel surfaces. Key findings include:

Bond strength: The micro-pulse resistance weld overlay achieved bond strengths of 85–95% of the overlay material's tensile strength, compared to only 50–70% for conventional arc welding on the same surfaces.
Crack resistance: No cracks were observed in the weld overlay or HAZ when the pulse duration was kept below 5 ms and the interpass temperature was controlled below 150°C.
Hardness profile: The overlay hardness was maintained at 55–60 HRC throughout the full deposit thickness, with minimal dilution effects from the cyanide case.
Microstructure: The weld overlay microstructure consisted of fine martensite with retained austenite, providing a good combination of hardness and toughness.

The study also investigated the effect of pulse parameters on weld quality:

Pulse durations below 1 ms resulted in insufficient melting and poor bonding
Pulse durations above 10 ms caused excessive dilution and cracking
The optimal pulse duration range was found to be 2–5 ms for most applications

Engineering Applications and Limitations

The micro-pulse resistance weld overlay process on cyanide-treated surfaces has potential applications in:

Repair of worn cyanide-hardened components: Such as gears, cams, and shafts that have been cyanide-treated for wear resistance and have subsequently worn in localized areas
Additive manufacturing on pre-treated surfaces: Building up material on cyanide-treated surfaces for dimensional restoration or functional modification
Hybrid surface engineering: Combining cyanide treatment for surface hardening with weld overlay for localized repair or functional enhancement

However, the process also has limitations:

The equipment required for micro-pulse resistance welding is more complex and expensive than conventional arc welding equipment
The process is currently limited to relatively thin overlay deposits (typically less than 2 mm)
The range of applicable filler materials is more limited than for arc welding processes
Field application is challenging due to equipment size and power requirements

Study Insights

This 1999 study represents pioneering work in the application of non-arc welding processes for weld overlay on surface-treated components. The micro-pulse resistance welding approach offers a promising solution to the challenge of welding on high-carbon, high-hardness surface layers that are difficult to join using conventional arc welding methods. The key innovation lies in the use of very short electrical pulses to achieve localized melting with minimal heat input, thereby preserving the beneficial properties of the cyanide case while achieving a strong metallurgical bond.

The findings of this study have implications for the broader field of surface engineering, where the combination of surface treatments and weld overlay is becoming increasingly important for extending component life and restoring functionality. While the micro-pulse resistance welding process has not achieved widespread industrial adoption, the fundamental principles of low heat input and rapid heating/cooling have been incorporated into other advanced welding technologies such as laser cladding and electron beam welding. The study serves as a valuable reference for engineers exploring innovative approaches to weld overlay on challenging surface conditions.