Microstructure and Properties of Boride-Strengthened Iron-Based Overlay Alloys

Literature Overview

This 2024 publication in Hot Working Technology, authored by researchers from Shenyang University of Chemical Technology and China Construction Installation Group, investigates the role of boride formation in strengthening iron-based overlay alloys. The research, supported by the National Natural Science Foundation of China (Project 51901141) and the Liaoning Provincial Department of Education (Project LJ2020034), addresses a practical challenge in wear-resistant overlay welding: achieving high hardness without sacrificing toughness or causing excessive cracking.

Core Technical Contributions

Borides, particularly FeB and Fe₂B phases, are well-known for their high hardness and chemical stability. However, their incorporation into overlay alloys is challenging because borides tend to form in continuous networks that can serve as crack initiation sites. This study systematically examines how boron content, welding process parameters, and microalloying elements influence the morphology, distribution, and volume fraction of borides in the overlay deposit.

Comparative Analysis of Boride Morphologies

Boride Phase	Hardness (HV)	Morphology	Effect on Toughness
FeB	1600-1800	Needle-like, network	Significantly reduces toughness
Fe₂B	1200-1400	Plate-like, isolated	Moderate effect on toughness
M₇B₃ (M = Cr, Mo)	1000-1300	Spheroidal, dispersed	Minimal effect on toughness

The study demonstrates that the key to successful boride-strengthened overlays lies in controlling the boride morphology rather than simply maximizing the boron content. When boron is present at levels above 0.5 wt%, the formation of continuous FeB networks becomes inevitable, leading to brittle fracture. By introducing chromium and molybdenum as microalloying elements, the borides transform into more complex M₇B₃-type phases that are spheroidal and dispersed rather than network-forming.

Process Parameters and Their Influence

The welding process parameters play a critical role in determining the boride characteristics. The study compares submerged arc welding (SAW) and gas metal arc welding (GMAW) processes and finds that GMAW produces finer boride morphology due to its higher cooling rate and smaller heat input. The heat input directly affects the solidification microstructure and, consequently, the boride precipitation pattern.

Process Parameter	SAW Range	GMAW Range	Effect on Boride
Heat Input	25-40 kJ/cm	8-15 kJ/cm	Higher heat input promotes coarser borides
Welding Current	300-450 A	180-280 A	Higher current increases dilution and boride coarsening
Travel Speed	200-400 mm/min	300-600 mm/min	Higher speed refines boride morphology
Wire Feed Rate	N/A (strip)	5-8 m/min	Higher rate increases deposition rate

The optimal boron content for achieving a balance between hardness and toughness is identified as 0.3-0.4 wt% in the overlay alloy composition. At this level, the hardness of the overlay reaches 60-70 HRC while maintaining acceptable impact toughness values above 20 J at room temperature. Exceeding 0.5 wt% boron leads to a sharp decline in toughness due to continuous FeB network formation.

Engineering Application Considerations

For engineers selecting overlay alloys for wear-resistant applications, this study provides clear guidance on boride engineering. The boride-strengthened iron-based overlay alloys are particularly suitable for applications requiring resistance to abrasive wear in moderate corrosive environments, such as mining equipment, cement mill liners, and pump impellers. The alloys are not recommended for applications involving severe thermal cycling or high-temperature service above 500°C, where boride coarsening and subsequent embrittlement become significant concerns.

The study also notes that the base metal selection is important. When overlaying boride-strengthened alloys onto low-carbon steel substrates, the dilution effect during welding can alter the effective boron content in the weld zone. Engineers should account for a dilution rate of 20-40% when designing the overlay alloy composition, adjusting the nominal boron content upward to compensate for dilution by the base metal.

Study Reflections

This research represents a valuable contribution to the field of wear-resistant overlay welding, particularly in its systematic approach to boride morphology control. The insight that microalloying with chromium and molybdenum can transform detrimental network-forming FeB into beneficial spheroidal M₇B₃ phases is a practical tool that can be directly applied in consumable development. The study also underscores the importance of welding process selection: GMAW is preferred over SAW for boride-containing overlays because its lower heat input produces finer, more beneficial boride morphologies. Engineers working on wear-resistant overlay applications should carefully consider boron content, microalloying strategy, and process selection to achieve the desired balance of hardness and toughness.