Microstructure and Properties of CHR Series Surfacings on A3 and 45 Steel

Literature Overview

This research by Zhang Xin and Qu Jinshan, published in the journal Hot Working Technology in 2006, investigates the microstructure and mechanical properties of surfacing layers deposited on A3 and 45 carbon steel substrates using CHR series surfacing electrodes via manual metal arc welding (SMAW). The study addresses the fundamental relationship between electrode type, welding parameters, and the resulting microstructure and hardness of the overlay deposit, with particular attention to the role of hard phases in determining wear resistance.

Core Technical Content

The authors systematically deposited CHR series surfacing electrodes on A3 and 45 steel substrates using conventional SMAW technique. Metallographic examination revealed that the microstructure and hardness of the surfacing layer are governed by three interrelated factors: the type of surfacing electrode used, the welding process parameters applied, and the characteristics of the hard phases formed during solidification. The hard phases, including their type, properties, quantity, and distribution, are identified as the primary determinants of the overlay's wear resistance and mechanical performance.

The study demonstrates that different CHR electrode compositions produce distinctly different microstructures in the surfacing layer. The variation in microstructure is reflected in measurable differences in hardness, with certain electrode types producing significantly harder overlays than others. The distribution of hard phases within the matrix is particularly critical, as uniform distribution contributes to consistent wear performance, while localized clustering can create weak points susceptible to premature failure.

Microstructural Analysis

Feature	Description	Impact on Performance
Hard Phase Type	Carbides, intermetallics, borides	Determines hardness level and wear mechanism
Hard Phase Quantity	Volume fraction of hard phases	Higher quantity generally increases hardness
Hard Phase Distribution	Uniform vs. clustered	Uniform distribution ensures consistent wear resistance
Matrix Microstructure	Ferrite, pearlite, martensite	Influences toughness and ductility of the overlay
Grain Size	Fine vs. coarse	Finer grains improve both hardness and toughness

The metallographic examination of the surfacing layers reveals that the solidification microstructure is strongly influenced by the cooling rate, which is in turn determined by the welding parameters such as current, voltage, and travel speed. Higher cooling rates promote the formation of harder phases such as martensite and fine carbides, while lower cooling rates allow for the formation of softer phases such as ferrite and pearlite.

Welding Parameter Effects

The welding parameters used in the SMAW process directly influence the thermal cycle experienced by the surfacing layer and subsequent weld passes. The following parameters were examined:

Welding Current: Higher current increases the heat input, resulting in a larger weld pool and slower cooling rates. This can lead to coarser microstructures and lower hardness, but may also improve fusion and reduce the risk of cracking.
Travel Speed: Faster travel speeds reduce the heat input per unit length, promoting higher cooling rates and finer microstructures. However, excessively fast travel speeds can lead to incomplete fusion and porosity.
Electrode Type: Different CHR electrode compositions produce different alloying effects and hard phase formations. The selection of the appropriate electrode type is critical for achieving the desired combination of hardness and toughness.

Hard Phase Characteristics

The hard phases formed in the surfacing layer are the primary contributors to wear resistance. The type of hard phase is determined by the alloying elements present in the electrode composition. Common hard phases in surfacing alloys include:

Cementite (Fe3C): Moderate hardness, common in carbon steel surfacing alloys
M7C3 and M2C carbides: Higher hardness, formed in high-carbon and alloyed surfacing alloys
Chromium carbides (Cr7C3, Cr23C6): Excellent wear resistance and corrosion resistance
Borides (Fe2B, FeB): Very high hardness, formed when boron is present in the alloy

The quantity and distribution of these hard phases are influenced by the welding parameters and the number of surfacing passes. Multiple passes can alter the microstructure of previously deposited layers through re-melting and re-solidification, potentially improving the homogeneity and distribution of hard phases.

Engineering Practice Implications

For engineers selecting surfacing electrodes for wear-resistant applications on carbon steel components, the following considerations emerge from this study:

The electrode type must be selected based on the specific wear mechanism involved (abrasive, adhesive, erosive, or corrosive wear).
Welding parameters should be optimized to promote the formation of fine, uniformly distributed hard phases within the overlay.
The number of surfacing passes and the inter-pass temperature should be controlled to ensure adequate fusion between passes and prevent excessive grain growth.
Metallographic examination of test coupons is essential for verifying that the desired microstructure has been achieved before proceeding with production welding.

Key Questions and Reflections

The study raises important questions about the optimal balance between hardness and toughness in surfacing layers. While higher hardness generally improves wear resistance, it can also reduce toughness and increase susceptibility to cracking. The authors note that the hard phase characteristics are the primary determinants of performance, but they do not provide detailed quantitative data on the specific hard phase types, their volume fractions, or their precise morphologies. This limitation makes it difficult to directly apply the findings to specific electrode selections without additional testing.

The use of SMAW for surfacing applications, while practical and widely available, has inherent limitations in terms of process control and reproducibility compared to more advanced processes such as submerged arc welding or plasma arc surfacing. The manual nature of the process introduces variability in bead geometry and heat input, which can lead to inconsistencies in microstructure and properties. Engineers must be aware of these limitations and implement appropriate quality control measures, such as regular hardness testing and periodic metallographic examination, to ensure consistent performance.

Study Insights and Implications

This study provides a foundational understanding of how electrode type and welding parameters influence the microstructure and properties of surfacing layers on carbon steel substrates. The emphasis on hard phase characteristics as the primary determinants of wear resistance is a valuable insight that guides both electrode selection and process optimization. The comparative approach, examining multiple electrode types and process conditions, offers practical guidance for engineers facing specific surfacing challenges in the field.

The work also underscores the importance of metallographic analysis in surfacing process development and quality control. Without detailed examination of the microstructure, it is impossible to predict or explain the mechanical performance of the overlay. The study demonstrates that even small variations in welding parameters can produce significant changes in microstructure and hardness, highlighting the need for careful process control and documentation.