Effect of Heat Treatment on Microstructure and Properties of D707 Hardfacing Layer

Literature Overview

This study, published in Materials Development and Application in 2002 by Peng Jixiang, Wang Shunxing, and Liu Yong from the Department of Materials Science and Engineering at Luoyang Institute of Technology, investigates the influence of various heat treatment regimes on the microstructure, hardness, and wear resistance of D707 carbide-type hard alloy hardfacing layers. The research was funded by the Henan Provincial Department of Education Key Research Project. The work addresses a practical problem encountered in heavy-duty wear applications where hardfacing deposits must achieve maximum surface hardness and corrosion-abrasion resistance through post-weld thermal processing. The comparative benchmark selected — 35CrMo steel subjected to carbon-nitrogen co-cementation — provides a meaningful industrial reference point for evaluating the effectiveness of hardfacing as a surface engineering strategy.

Core Technical Content and Key Findings

The study examines multiple heat treatment conditions applied to D707 hardfacing deposits, which are based on tungsten carbide (WC) hard alloy composition. The key heat treatment variables investigated include quenching temperature, cooling medium, and tempering temperature. The findings are summarized in the following table.

The study demonstrates that the combination of 850°C oil quenching followed by 400°C tempering yields the optimal balance of hardness and wear resistance. This is attributed to the transformation of retained austenite and the formation of a fine, uniformly distributed carbide network. The tempering at 400°C is critical because it relieves residual stresses introduced during quenching while preserving the hard carbide phase, which is the primary wear-resisting constituent in D707-type deposits.

Interpretation of Microstructural Mechanisms

The microstructure of D707 hardfacing deposits typically consists of WC particles embedded in a binder matrix that may contain martensite, retained austenite, and various carbide phases. The heat treatment process fundamentally alters the binder matrix rather than significantly changing the WC particle morphology. At 850°C, the temperature is sufficient to promote homogenization of the binder phase and partial dissolution of secondary carbides, followed by rapid quenching to form a martensitic matrix that provides additional hardness contribution beyond the WC particles themselves.

The choice of cooling medium is significant. Air cooling provides a slower cooling rate, which allows more time for carbide precipitation and may result in a slightly coarser microstructure. Oil quenching produces a faster cooling rate, which suppresses diffusional transformations and results in a harder martensitic binder matrix. The subsequent tempering at 400°C serves to reduce brittleness without significantly sacrificing hardness, as the temperature is below the threshold for substantial carbide coarsening.

Engineering Practice Implications

For engineers selecting hardfacing materials and processes for critical wear applications, this study provides several actionable insights. First, the post-weld heat treatment is not merely a stress-relief step but a critical process variable that can multiply the performance of the deposit. Second, the 6-12x improvement over chemical surface treatment (C/N co-cementation) of 35CrMo steel justifies the additional cost of hardfacing for severe-duty applications such as mining equipment, cement mill rollers, and pump components operating in corrosive-abrasive slurry environments.

When implementing D707 hardfacing in practice, engineers should ensure that the base material can withstand the thermal shock of quenching without cracking. For thicker deposits or larger components, a preheat temperature of 200-300°C is advisable to reduce thermal gradients. The interpass temperature during multi-pass welding should be maintained below 200°C to prevent grain coarsening in previously deposited layers.

Study Insights and Reflections

This 2002 study, while modest in scope, addresses a fundamental question in surface engineering: how does thermal post-processing interact with the inherent microstructure of a hardfacing alloy to produce the desired performance? The answer — that oil quenching plus moderate tempering optimizes both hardness and toughness — aligns with classical heat treatment principles applied to martensitic steels, but the context of a multi-phase hard alloy deposit adds complexity. The relatively high cost of D707-type consumables and the specialized equipment requirements for controlled quenching mean that this approach is best reserved for high-value components where extended service life justifies the investment. For lower-criticality applications, air cooling alone may provide sufficient performance at reduced processing cost.