Effect of Quenching Action on Microstructure and Properties of Overlay Deposited Components

Literature Overview

The paper by Chen Jiexiang and Liu Jian, published in the Journal of Shenyang University of Technology in 2013, investigates the effect of quenching action on the microstructure and mechanical properties of overlay deposited components used in remanufacturing applications. The study compares the microstructure and properties of overlay deposited components produced on a copper quenching base with those produced on a conventional cast iron base. The results demonstrate that the use of a copper quenching base significantly improves the microstructure and mechanical properties of the overlay deposited components, with increases in tensile strength, yield strength, elongation, and yield-to-tensile strength ratio of 8.8%, 17.7%, 16.2%, and 8.3%, respectively.

This study is of particular interest to me because it addresses a fundamental aspect of welding metallurgy — the influence of cooling rate on the microstructure and properties of weld metal. The cooling rate is one of the most important parameters in welding, as it determines the solidification mode, the grain structure, and the phase composition of the weld metal. The use of a copper quenching base provides a simple and effective method for controlling the cooling rate, and the results of this study provide valuable insights into the optimization of overlay deposited components for remanufacturing applications.

Fundamental Principles of Quenching in Welding

Influence of Cooling Rate on Microstructure

The cooling rate of the weld metal is determined by the heat input, the thermal conductivity of the base material, and the geometry of the weld. A higher cooling rate results in a finer grain structure and a higher hardness, while a lower cooling rate results in a coarser grain structure and a lower hardness. The relationship between the cooling rate and the microstructure is governed by the solidification behavior of the weld metal, which is influenced by the composition, the solidification mode, and the solidification sequence.

In overlay deposited components, the cooling rate is particularly important because it determines the degree of directional solidification, which is a key factor in the mechanical properties of the deposited metal. Directional solidification results in a columnar grain structure that is oriented in the direction of heat extraction, which can provide improved mechanical properties in the direction of the applied stress.

Role of Copper in Quenching

Copper is an excellent choice for a quenching base because of its high thermal conductivity, which is approximately 400 W/(m·K) at room temperature. This is significantly higher than the thermal conductivity of cast iron, which is approximately 50 W/(m·K). The high thermal conductivity of copper allows for rapid heat extraction from the weld pool, resulting in a higher cooling rate and a finer grain structure.

However, the use of copper as a quenching base also introduces the challenge of thermal expansion mismatch between the copper base and the deposited metal. The coefficient of thermal expansion of copper is approximately 17 × 10⁻⁶ /°C, which is higher than that of most steels. This mismatch can introduce residual stresses in the deposited metal, which must be managed through appropriate welding parameters and post-weld heat treatment.

Experimental Methodology

Test Setup

The study employs a simple and effective experimental setup that compares the microstructure and properties of overlay deposited components produced on two different bases: a copper quenching base and a conventional cast iron base. The overlay deposition is performed using the same welding parameters for both bases, ensuring that the only variable is the cooling rate.

The welding parameters used in the study include the welding current, the voltage, the travel speed, and the wire feed speed. The heat input is calculated as the product of the voltage, current, and time divided by the travel speed. The cooling rate is estimated based on the thermal conductivity of the base material and the heat input.

Microstructural Analysis

The microstructure of the overlay deposited components is analyzed using optical microscopy and scanning electron microscopy. The optical microscopy provides an overview of the grain structure and the phase composition, while the scanning electron microscopy provides detailed information about the microstructure at the sub-grain level.

The key microstructural features of interest include the grain size, the grain morphology, the degree of directional solidification, and the phase composition. The grain size is measured using the standard linear intercept method, and the grain morphology is characterized in terms of the aspect ratio and the orientation.

Mechanical Property Testing

The mechanical properties of the overlay deposited components are tested using standard tensile testing procedures. The tensile test provides the tensile strength, the yield strength, the elongation, and the yield-to-tensile strength ratio. The hardness is also measured using the Vickers hardness test, which provides a measure of the local hardness and can be used to infer the local microstructure.