Overlay Welding Preparation of Composite Wear-Resistant Materials Using SMAW on Carbon Steel Substrates

Literature Overview

This paper by Zhang Wanhong, Gong Weimin, and Liang Shankun, published in Mining Machinery (2012, Vol. 40, No. 11, pp. 97-101), investigates the fabrication of composite wear-resistant layers on Q235 and 45 carbon steel substrates using SMAW (Shielded Metal Arc Welding) with ER50-6 consumable electrodes. The study focuses on microstructural characterization and hardness measurement of the overlay weld metal, with particular attention to how welding speed and substrate material influence the resulting overlay layer properties. This work is highly relevant to engineers involved in surface hardening of mining equipment, wear plates, and structural components where cost-effective surface protection is required.

Core Technical Content

The researchers employed ER50-6 low-hydrogen, high-tensile-strength electrodes (typically E7018 equivalent per AWS classification) to deposit overlay layers on two different carbon steel substrates. The key findings include:

The overlay weld layer exhibits significant grain refinement compared to the base metal.
Welding travel speed has a pronounced effect on both microstructure and hardness of the overlay layer.
The substrate material (Q235 vs. 45 steel) significantly influences the overlay layer's organizational characteristics and mechanical performance.
The dilution effect between the base metal and overlay deposit creates a gradient in microstructure and hardness from the fusion line outward.

Microstructural Analysis

The grain refinement observed in the overlay layer is attributed to the rapid solidification rate inherent in multi-pass welding, where each subsequent pass acts as a heat sink for the previously deposited layer. This rapid cooling suppresses the growth of coarse austenite grains, leading to a finer ferrite-pearlite microstructure with higher hardness. The microstructure near the fusion line typically shows a mixed grain structure influenced by the base metal composition, while the upper layers of the overlay tend toward the equilibrium microstructure of the ER50-6 weld metal.

Hardness Distribution

Hardness profiles across the overlay layer typically show a gradient from the fusion zone to the surface. Near the fusion line, dilution from the carbon-rich 45 steel substrate introduces more pearlite and potentially bainitic phases, resulting in higher hardness. On Q235, the lower carbon content of the substrate leads to a more uniform ferrite-pearlite structure with moderate hardness values.

Process Parameter Analysis

Parameter	Range Investigated	Effect on Microstructure	Effect on Hardness
Travel speed	Low to high	Lower speed = coarser grains; Higher speed = finer grains	Higher speed generally yields higher hardness due to faster cooling
Substrate material	Q235 / 45 steel	45 steel promotes more pearlite/bainite near fusion line	45 steel substrate yields higher hardness at fusion zone
Electrode type	ER50-6 (E7018)	Low-hydrogen flux provides stable arc and clean weld metal	Baseline hardness ~200-250 HB for single-pass deposit
Number of passes	Multiple	Each pass refines the previous layer's microstructure	Cumulative refinement increases surface hardness
Heat input	Controlled by voltage/current/speed	Lower heat input favors fine grain and higher hardness	Lower heat input correlates with higher hardness

Engineering Practice Implications

The practical significance of this research extends to several industrial applications:

Mining equipment surface protection: Excavator buckets, conveyor rollers, and crusher components can benefit from cost-effective SMAW overlay welding to extend service life without complete part replacement.
Wear plate fabrication: Composite steel plates with hard overlay layers can be produced using this technique, offering a balance between toughness (base metal) and wear resistance (overlay layer).
Repair welding: Field repair of worn components using standard SMAW equipment makes this technique accessible to maintenance workshops without specialized equipment.

Common Defects and Countermeasures

Defect	Cause	Countermeasure
Cracking at fusion line	High carbon equivalent of substrate; high heat input	Preheat substrate; reduce heat input; use low-hydrogen electrode
Porosity	Moist electrode; contamination	Bake electrodes per specification; clean surface thoroughly
Excessive dilution	High travel speed; large electrode diameter	Reduce current; use smaller electrode; increase number of thin passes
Uneven hardness distribution	Inconsistent travel speed; varying interpass temperature	Maintain constant parameters; control interpass temperature below 250°C

Key Technical Insights

The study demonstrates that SMAW overlay welding with ER50-6 electrodes provides a practical and economical solution for surface hardening of carbon steel components. However, the hardness achieved (typically 200-300 HB) is moderate compared to specialized hardfacing alloys which can achieve 500-700 HB. This limitation means that for severe abrasive wear applications, ER50-6 may be insufficient, and engineers should consider hardfacing consumables such as high-carbon, high-chromium, or carbide-containing electrodes.

The grain refinement effect observed is a fundamental metallurgical phenomenon in multi-pass welding. Each subsequent pass heats the previous layer to a temperature below its solidus, causing partial recrystallization and grain refinement. This is analogous to the thermomechanical processing used in modern steel manufacturing, where controlled rolling and cooling achieve fine grain structures.

Study Reflections

This research highlights the importance of understanding the interplay between welding parameters and microstructural evolution in overlay applications. The finding that substrate material significantly affects overlay properties is particularly noteworthy—it reminds engineers that overlay welding is not a standalone process but an integrated system where base metal composition, welding parameters, and consumable selection must be considered holistically. For industrial implementation, engineers should conduct qualification testing on the specific substrate-consumable combination before production deployment, as dilution rates and resulting microstructures can vary substantially between different steel grades.