Microstructure and Properties Analysis of SMAW Surfacing Metal on 45 Steel

Literature Overview

This study by Zhong Yu, Qu Jinshan, Chen Wenjing, Pan Quanxi, and Luo Chaoyu from Xihua University, published in Welding Technology (2007, Vol. 36, No. 3, pp. 10-12), investigates the microstructure and mechanical properties of surfacing metal deposited using CHR172 surfacing electrode by Shielded Metal Arc Welding (SMAW) on 45 steel substrate. The research systematically examines how welding heat input and alloy composition influence the microstructure, microhardness, and hard particle formation in the surfacing layer. This work provides fundamental understanding of the metallurgical behavior of hardfacing deposits, which is essential for optimizing surfacing processes in industrial applications.

Experimental Methodology and Microstructure Analysis

The CHR172 electrode is a high-carbon chromium-based surfacing electrode designed to produce hard, wear-resistant deposits. The study examined surfacing layers deposited under different welding conditions, analyzing both the Heat-Affected Zone (HAZ) and the deposited metal.

Microstructure Characteristics

The surfacing layer microstructure exhibits a complex arrangement of:

A martensitic or austenitic matrix depending on the cooling rate and alloy composition.
Dispersed hard carbide particles of various types, including Cr7C3, Cr23C6, and possibly other chromium carbides.
A gradient microstructure from the fusion line to the surface, reflecting the varying cooling rates and solidification conditions.

The HAZ of the 45 steel substrate shows typical transformation products influenced by the welding thermal cycle, with potential hardening due to rapid heating and cooling.

Influence of Heat Input

The welding heat input is a critical parameter affecting the surfacing layer properties:

Heat Input Level	Microstructure Characteristics	Microhardness Trend	Hard Particle Distribution
Low	Coarse grains, more martensite	Higher hardness	Larger, more dispersed
Moderate	Balanced grain size	Optimal hardness-toughness	Uniform distribution
High	Coarser grains, possible tempering	Lower hardness	Coarser particles, possible coalescence

Higher heat input promotes grain growth, reduces cooling rates, and can lead to tempering of the as-quenched microstructure. While this may reduce peak hardness, it can improve toughness and reduce the risk of cracking.

Alloy Composition and Hard Particle Analysis

The study highlights that the surfacing layer properties are governed by multiple factors related to alloy composition:

Alloy element types and contents: Chromium, carbon, and other alloying elements determine the thermodynamic stability and morphology of carbide phases.
Hard particle types: The specific carbide phases formed depend on the local chemistry during solidification, which varies with cooling rate and heat input.
Particle-matrix bonding: The interface between hard particles and the matrix determines the effectiveness of the hard particles in resisting wear and deformation.
Particle distribution: Uniform dispersion maximizes the wear-resistance benefit, while agglomeration creates local weak points.

The microhardness measurements reveal significant variation within the surfacing layer, reflecting the heterogeneous microstructure. Hard particles exhibit substantially higher hardness than the matrix, and the overall wear resistance depends on the volume fraction, distribution, and bonding quality of these particles.

Engineering Practice Considerations

For engineers applying CHR172-type surfacing electrodes, the following practical considerations emerge from this study:

Heat input control: Maintaining moderate heat input through appropriate electrode diameter selection, current level, and travel speed is essential for achieving optimal microstructure and properties.
Preheating considerations: Preheating can reduce thermal gradients and cracking risk but may affect the as-deposited microstructure and hardness.
Multi-pass strategies: When building up thick surfacing layers, each subsequent pass re-heats the previous pass, potentially altering its microstructure. The final properties depend on the thermal history of the entire deposit.
Base material preparation: The condition of the 45 steel substrate, including its hardness and surface preparation, influences the fusion characteristics and transition zone properties.

Study Insights and Reflections

This study underscores a fundamental principle in surfacing metallurgy: the deposited metal properties are not solely determined by the electrode composition but are equally influenced by the welding process parameters. Two deposits from the same electrode can exhibit significantly different microstructures and properties if deposited under different thermal conditions.

The emphasis on hard particle characteristics—type, distribution, and bonding—provides a valuable framework for evaluating surfacing material performance. Engineers should consider that wear resistance is a system property dependent on the interaction between matrix and reinforcement phases, not merely a function of peak hardness.

The study also highlights the importance of characterizing the HAZ, as the transition between base metal and surfacing layer can be a critical location for crack initiation or delamination under service loading. Future work should incorporate mechanical testing of the transition zone and evaluate the surfacing layer performance under specific wear conditions to correlate microstructure with functional performance. This research provides essential metallurgical understanding for optimizing SMAW surfacing processes on carbon steel substrates.