Surface Formation Improvement of Large-Area Wear-Resistant Alloy Hardfacing Layer

Literature Overview

This paper by Wang Zhihui, Yan Yuqin, and He Dingyong (1993), published in the Journal of Beijing University of Technology, addresses a practical manufacturing challenge in the production of large-area wear-resistant composite plates. The study investigates the effect of adding silicon (Si) and boron (B) to alloy powder used in hardfacing on the surface formation, hardness, and microstructure of the overlay. The research is particularly relevant to industries that require large-area wear-resistant surfaces, such as mining, cement, and material handling, where the surface quality of the hardfacing layer directly impacts both aesthetics and functional performance.

Core Technical Findings

The researchers identified that the use of ordinary chromium iron as the outer filler alloy powder for hardfacing on A3 steel (equivalent to AISI 1006/1010 low-carbon steel) resulted in poor surface formation of the hardfacing layer. Through systematic experimentation, they demonstrated that the addition of appropriate amounts of silicon and boron could effectively improve the surface formation, increase the hardness, and enhance the hardenability of the overlay.

Effect of Silicon and Boron Additions

Additive	Surface Formation	Hardness (HV)	Microstructure	Hardenability
No addition (baseline)	Poor, rough	Moderate	Ferrite + pearlite + carbides	Low
Silicon addition	Improved, smoother	Increased	Fine pearlite + martensite	Moderate
Boron addition	Improved, smoother	Increased	Martensite + carbides	High
Silicon + Boron	Best, smooth	Highest	Fine martensite + carbides	Highest

The study demonstrates that boron is particularly effective in promoting martensite formation in the hardfacing layer even under air cooling conditions. This is attributed to boron's ability to partition to grain boundaries and inhibit the formation of austenite, thereby promoting the transformation of austenite to martensite during cooling. The resulting martensitic microstructure provides significantly higher hardness and wear resistance compared to the ferrite-pearlite microstructure obtained without boron.

Welding Metallurgy Analysis

The surface formation of hardfacing layers is influenced by several factors, including the fluidity of the molten pool, the solidification behavior, and the cooling rate. Poor surface formation is typically characterized by:

Rough surface: Due to uneven solidification and shrinkage.
Craters and depressions: Resulting from hot tearing and inadequate filling.
Excessive spatter: From violent arc action and gas evolution.
Uneven bead profile: Due to inconsistent weld pool dynamics.

The addition of silicon and boron improves surface formation through the following mechanisms:

Silicon as a deoxidizer: Silicon reacts with dissolved oxygen in the molten pool to form SiO2, reducing the oxygen content and preventing gas porosity. This results in a cleaner, denser weld metal with better surface formation.
Silicon as a grain refiner: Silicon promotes the formation of finer grains by increasing the nucleation rate and inhibiting grain growth. The finer grain structure results in more uniform solidification and reduced shrinkage, improving surface formation.
Boron as a hardenability enhancer: Boron increases the hardenability of the weld metal by inhibiting the formation of austenite and promoting martensite transformation. The martensitic microstructure has a finer, more uniform structure that solidifies with less shrinkage, resulting in better surface formation.
Boron as a grain boundary segregant: Boron segregates to grain boundaries, where it inhibits the growth of austenite grains and promotes the formation of fine martensite. This results in a more uniform microstructure with reduced internal stresses and improved surface formation.

Effect on Microstructure Evolution

The microstructure of the hardfacing layer evolves through the following stages during solidification and cooling:

Stage	Temperature Range	Microstructure	Influence of Si and B
Liquid	Above liquidus	Homogeneous liquid	Si and B dissolved in liquid
Primary solidification	Liquidus to solidus	Dendrites (austenite or ferrite)	Si promotes finer dendrites; B inhibits austenite
Solid-state transformation	Below solidus	Martensite, bainite, pearlite	B promotes martensite; Si promotes fine pearlite
Air cooling	Room temperature	Final microstructure	B promotes martensite even in air cooling

Engineering Practice Considerations

The production of large-area wear-resistant composite plates requires careful attention to several process parameters to achieve both good surface formation and adequate hardness:

Process Parameters

Parameter	Typical Range	Effect on Surface Formation	Effect on Hardness
Welding current	150–300 A	Higher current improves wetting but may cause roughness	Higher current increases dilution, reducing hardness
Travel speed	100–400 mm/min	Faster speed reduces heat input, improving hardness but may cause poor wetting	Faster speed increases cooling rate, improving hardness
Wire feed speed	3–8 m/min	Affects deposition rate and bead profile	Indirect effect through heat input
Shielding gas flow	10–20 L/min	Adequate shielding prevents oxidation and porosity	Indirect effect through weld quality
Interpass temperature	<200°C	Lower temperature promotes martensite formation	Lower temperature increases hardness

Surface Formation Quality Criteria

Quality Level	Surface Roughness (Ra)	Visual Appearance	Acceptability
Excellent	<10 μm	Smooth, uniform	Acceptable for most applications
Good	10–25 μm	Slightly rough but uniform	Acceptable for general wear applications
Fair	25–50 μm	Rough with minor irregularities	May require grinding or machining
Poor	>50 μm	Very rough with craters and depressions	Unacceptable, requires rework

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Poor surface formation	Inadequate deoxidation, high oxygen content	Add silicon as deoxidizer, use dry shielding gas
Low hardness	Excessive dilution, slow cooling rate	Reduce heat input, add boron for hardenability
Cracking	High residual stress, low toughness	Preheat base metal, control interpass temperature
Porosity	Gas evolution, inadequate shielding	Add silicon as deoxidizer, improve shielding gas coverage
Incomplete fusion	Poor wetting, excessive travel speed	Increase welding current, reduce travel speed

Key Questions and Reflections

This study from 1993 provides valuable insights into the metallurgical effects of silicon and boron in hardfacing alloys, but several aspects have evolved significantly since then. Modern hardfacing alloys are much more sophisticated, with multi-element compositions designed for specific wear environments. The role of silicon and boron in modern hardfacing alloys may differ from their role in the simple chromium iron alloys studied in this paper.

From my perspective, the most significant finding of this study is the demonstration that boron can promote martensite formation under air cooling conditions. This is particularly important for field applications where controlled cooling is not possible, and the hardfacing layer must achieve adequate hardness without post-weld heat treatment. However, the use of boron in hardfacing alloys must be carefully controlled, as excessive boron can lead to embrittlement and cracking due to its tendency to segregate to grain boundaries.

Study Insights and Implications

The work by Wang Zhihui et al. is a foundational study that demonstrates the effectiveness of simple alloying additions in improving the performance of hardfacing layers. The clear demonstration that silicon and boron can improve both surface formation and hardness provides practical guidance for engineers working with hardfacing alloys.

For modern hardfacing applications, the insights from this study remain relevant. Engineers developing new hardfacing consumables should consider the role of silicon and boron in their alloy design, particularly for applications where surface formation is critical and controlled cooling is not available. The study also highlights the importance of understanding the fundamental metallurgical mechanisms behind alloying effects, rather than relying solely on empirical trial and error.

The practical implications of this research extend beyond hardfacing to other surface engineering applications, including thermal spraying and plasma coating, where the addition of silicon and boron may similarly improve the properties of the deposited layer. Engineers working in surface engineering should consider the lessons learned from this study when developing new coating processes and consumable formulations.