Effect of Tempering Temperature on Wear Resistance of Overlay Welded Microcar Intermediate Shaft Molds

Literature Overview

The paper by Xu Wujiang, Ding Yongfeng, and Wang Pengcheng from Chongqing University, published in Heat Treatment of Metals (Volume 38, Issue 4, 2013, pages 85–87), investigates the influence of tempering temperature on the wear resistance of overlay-welded layers applied to intermediate shaft molds for microcar manufacturing. The study employs universal testing machines, optical microscopy, hardness testing, and pin-on-disk friction-wear testing to characterize the mechanical and tribological performance of overlay layers subjected to different tempering temperatures. The classification number TG156.5 places this work squarely in the domain of surface hardening and heat treatment of weld overlay deposits.

Core Technical Findings

The central conclusion of the research is that a tempering temperature of 500 °C yields the optimal combination of tensile strength and hardness in the overlay layer, resulting in the best wear resistance among all tested conditions. At this temperature, the microstructure of the overlay exhibits uniformly and finely dispersed carbide particles within the matrix, forming a particle-reinforced composite layer. This microstructural configuration provides superior resistance to abrasive wear through the combined mechanisms of carbide hardening and particle dispersion strengthening.

Microstructural Evolution with Tempering Temperature

The tempering process fundamentally alters the phase composition and morphology of the overlay weld metal. During the welding process itself, a complex mixture of martensite, bainite, and various carbide phases (such as M7C3, M23C6, and MC-type carbides) is typically formed. The subsequent tempering treatment governs the transformation and coarsening behavior of these phases.

The critical insight is that 500 °C represents a thermodynamic and kinetic balance point. Below this temperature, the carbides remain in a metastable state with limited dispersion refinement. Above this temperature, Ostwald ripening and coarsening mechanisms become dominant, reducing the number density of effective reinforcing particles and softening the matrix.

Process Analysis and Engineering Implications

Welding Process Considerations

Overlay welding of intermediate shaft molds typically employs processes such as SMAW (Shielded Metal Arc Welding) or GTAW (Gas Tungsten Arc Welding) with appropriate hardfacing consumables. The consumable selection must be compatible with the base material of the mold, which is commonly a medium-carbon alloy steel (e.g., 45 steel or 40Cr). The welding parameters—current, voltage, travel speed, and interpass temperature—directly influence the dilution ratio and the initial microstructure of the overlay deposit.

Tempering Treatment Protocol

The tempering treatment is typically conducted in a controlled-atmosphere furnace to prevent oxidation. The recommended protocol based on this study would include:

1. Preheat the mold assembly to 200–300 °C to reduce residual stress and minimize thermal shock.
2. Heat to 500 °C at a controlled rate (typically 100–150 °C/hour for thick sections).
3. Hold at 500 °C for 1.5–2.5 hours depending on section thickness.
4. Cool in air to room temperature; avoid furnace cooling to prevent secondary softening.

Defect Analysis and Countermeasures

Study Insights and Reflections

The finding that 500 °C tempering produces the optimal particle-reinforced composite structure is consistent with established metallurgical principles regarding tempered martensite and carbide dispersion. The particle-reinforced composite layer concept draws a direct analogy to metal matrix composites (MMCs), where the distribution, size, and volume fraction of reinforcing particles are the primary determinants of wear resistance. The uniform and fine dispersion achieved at 500 °C suggests that this temperature is near the nose of the TTT (Time-Temperature-Transformation) curve for carbide precipitation, promoting nucleation over growth.

From a practical standpoint, this study provides a clear and actionable guideline for mold shops involved in microcar intermediate shaft production. The tempering temperature window is narrow enough that precise furnace control is essential. Deviations of even 50 °C can significantly alter the wear life of the overlay, translating directly into production cost and tool life metrics. Engineers should also consider that the tempering response may vary with the specific overlay consumable composition, as the carbon and alloying element content of the weld metal determines the carbide type and precipitation kinetics.

A notable limitation of the study is the absence of long-term wear testing under actual production conditions. Laboratory pin-on-disk tests, while valuable for comparative evaluation, do not fully replicate the complex loading conditions encountered in intermediate shaft forming operations, which involve cyclic plastic deformation, elevated temperatures, and lubricant interactions. Future work should incorporate field trials with instrumented monitoring of mold wear over production cycles.

The practical recommendation derived from this literature is to standardize the post-weld tempering process at 500 °C for overlay-welded intermediate shaft molds, with strict furnace temperature control within ±10 °C, and to implement routine hardness and microstructural verification as part of the quality assurance protocol. This approach maximizes the wear life of the overlay while maintaining sufficient toughness to resist cracking under cyclic loading.