Effect of Quenching on Microstructure and Properties of Overlay Welding Deposit Formed Parts

Literature Overview

This paper by Chen Jiexiang and Liu Jian, published in the Journal of Shenyang University of Technology (Vol. 35, Issue 6, 2013, pp. 647-651), investigates the influence of quenching conditions on the microstructure and mechanical properties of overlay welding deposit formed parts. The research was funded by the National Natural Science Foundation of China (Grants 50975286 and 51205408) and represents a significant contribution to the field of remanufacturing technology, particularly in the area of directed solidification through controlled cooling.

Core Technical Content

Experimental Methodology

The researchers conducted overlay welding deposit forming experiments on two different substrates: a copper quenching base and a conventional cast iron base. The copper base provides rapid heat extraction (quenching), while the cast iron base allows for natural cooling. This comparison allows for a direct assessment of the cooling rate effect on the resulting microstructure and properties.

Microstructural Analysis

The paper reports several key microstructural findings:

1. Directed solidification tendency – Under quenching conditions, the weld metal exhibits a more pronounced directed solidification pattern, with columnar grains growing perpendicular to the cooling surface. This is a direct consequence of the high thermal gradient established by the copper base.
2. Grain morphology – The quench-formed parts exhibit more ordered, aligned, and uniform grain structures compared to the naturally cooled counterparts. The grains are finer and more homogeneous in size distribution.
3. Grain refinement – The rapid cooling rate suppresses grain growth and promotes nucleation, resulting in a finer grain structure that contributes to improved mechanical properties.

Mechanical Property Comparison

The simultaneous improvement in both strength and ductility is notable, as these properties often exhibit a trade-off relationship in conventional welding. The quenching-induced grain refinement appears to overcome this trade-off by providing a finer, more uniform microstructure that enhances both strength and toughness.

Technical Interpretation

Role of Thermal Gradient in Solidification

The fundamental mechanism behind the observed improvements is the thermal gradient established during solidification. In the copper quenching base, the high thermal conductivity of copper creates a steep temperature gradient at the weld/base interface. This gradient drives directional solidification, where columnar grains grow preferentially in the direction of heat extraction. The resulting microstructure has several advantageous characteristics:

• Reduced number of grain boundaries in the growth direction
• More uniform grain size distribution
• Suppressed formation of coarse, equiaxed grains at the weld centerline
• Enhanced texture that may improve anisotropic properties in the desired direction

Cooling Rate and Microstructural Evolution

The cooling rate directly influences the solidification microstructure through the following mechanisms:

1. Nucleation rate – Higher cooling rates increase the nucleation rate, resulting in a higher number of grains and a finer grain size.
2. Grain growth rate – Higher cooling rates suppress grain growth by reducing the time available for grain boundary migration.
3. Phase transformation – Rapid cooling can suppress certain phase transformations, potentially retaining metastable phases that contribute to mechanical properties.

Engineering Practice Implications

The findings of this paper have direct relevance to remanufacturing applications where overlay welding is used to restore worn components. The ability to control the cooling rate through substrate selection or external cooling techniques offers a practical method for improving the mechanical properties of overlay welds without changing the welding consumables or welding parameters.

In practice, engineers can apply these findings by:

• Selecting high-thermal-conductivity backing plates for critical applications
• Using water cooling or other active cooling methods to enhance the quenching effect
• Monitoring and controlling the cooling rate through thermocouple measurements during welding
• Optimizing the welding sequence to take advantage of the quenching effect in critical areas

Key Reflections

The paper demonstrates a fundamental principle of solidification metallurgy: that the cooling rate is a critical parameter in determining the final microstructure and properties of a weld. While this principle is well-established in casting and solidification theory, its application to overlay welding is less commonly addressed in practical engineering literature. The quantified improvements in mechanical properties (8.8-17.7%) are substantial and demonstrate the practical significance of cooling rate control.

From a remanufacturing perspective, the ability to enhance overlay weld properties through substrate engineering is particularly attractive because it does not require changes to the welding consumables or welding procedures, making it a relatively easy-to-implement process improvement.

Summary

This paper provides valuable insights into the role of cooling rate in overlay welding deposit forming, demonstrating that quenching conditions can significantly improve both the microstructure and mechanical properties of overlay welds. The quantified improvements in tensile strength, yield strength, elongation, and yield-to-tensile ratio are directly applicable to remanufacturing applications, and the underlying metallurgical principles offer a framework for optimizing overlay welding processes in other applications.