Effect of Welding Current on Microstructure and Mechanical Properties of Fe5 Cladding Layer

Introduction and Material Background

Fe5 is a widely used nickel-iron-based hardfacing alloy containing approximately 2.5% carbon, 10% chromium, 1% molybdenum, and 25% nickel. It is extensively applied in coal mining equipment, cement industry components, and steel mill equipment where high abrasion resistance combined with moderate toughness is required. The welding current used during Fe5 overlay deposition directly controls the heat input, dilution rate, and solidification behavior of the overlay, thereby determining the final microstructure and mechanical properties. This study note examines the systematic influence of welding current on Fe5 cladding layer performance.

Experimental Design and Parameters

The study varied welding current from 180 A to 320 A in increments of 35 A, while maintaining constant arc voltage (26 V) and travel speed (300 mm/min). The welding process was submerged arc welding (SAW) with a standard Fe5 self-shielded electrode and matching flux. The base metal was SA 106 carbon steel with hardness of 180 HV.

Microstructural Evolution with Current Variation

At low current (180 A), the microstructure consisted of a martensite matrix with fine carbide particles (primarily M7C3 type) dispersed throughout. The low heat input produced rapid solidification, resulting in fine grain structure (ASTM No. 8) and minimal dilution. The martensite was predominantly acicular type with high dislocation density, contributing to the elevated hardness of 620 HV.

As current increased to the optimal range of 215–250 A, the microstructure evolved to a mixed martensite-cementite (ledeburite-type) structure with coarser carbide particles. The increased heat input promoted more complete austenite solidification before martensitic transformation, resulting in a more homogeneous distribution of carbides within the martensitic matrix. The hardness peaked at 650 HV in this range due to the synergistic effect of martensite hardness and carbide dispersion strengthening.

At high current (285–320 A), excessive dilution introduced carbon-depleted regions in the overlay, and the cooling rate decreased sufficiently to allow pearlite formation. The microstructure became a mixture of martensite, pearlite, and coarse cementite networks. The presence of soft pearlitic phases significantly reduced the overall hardness to 530 HV at 320 A.

Mechanical Property Relationships

The relationship between welding current and mechanical properties was characterized by distinct regimes:

1. Low current regime (180–215 A): Increasing current improves hardness through enhanced carbide formation and refinement of martensite morphology. Toughness remains moderate (KIC = 28–32 MPa·m^0.5).
2. Optimal regime (215–250 A): Maximum hardness achieved (640–650 HV) with acceptable toughness (KIC = 30–35 MPa·m^0.5). The dilution rate remains below 16%, preserving the alloy chemistry of the overlay.
3. High current regime (285–320 A): Hardness drops rapidly due to dilution and pearlite formation. Toughness initially improves slightly due to reduced residual stresses but then decreases as crack susceptibility increases from coarse carbide networks.

The abrasion resistance, measured by pin-on-disk testing against SiC abrasive paper, showed maximum wear life at 230 A welding current, which corresponds to the hardness peak. The wear rate at this condition was 0.012 mm³/N·m, representing a 40% improvement over the 320 A condition.

Practical Recommendations for Fe5 Cladding

Based on the systematic investigation, the following practical guidelines are recommended for Fe5 overlay welding:

1. For maximum wear resistance, use welding current in the range of 215–250 A with heat input of 1.1–1.3 kJ/mm.
2. For applications requiring higher toughness (impact loading), reduce current to 180–200 A to minimize dilution and maintain fine microstructure.
3. Never exceed 270 A for Fe5 overlay on carbon steel base metals, as dilution above 20% fundamentally changes the overlay composition and degrades wear performance.
4. Monitor dilution rate through periodic cross-section hardness profiling; if base metal hardness appears in the first 0.5 mm of the overlay, reduce current immediately.
5. Consider multi-pass deposition with lower current per pass to achieve thicker overlays while maintaining optimal microstructure in each pass.

Summary and Technical Reflection

The systematic investigation of welding current effects on Fe5 cladding layer properties reveals a clear non-linear relationship with a well-defined optimal window. The fundamental insight is that welding current controls a coupled set of metallurgical variables—heat input, dilution rate, solidification rate, and cooling rate—that collectively determine the final microstructure. Engineers must recognize that there is no single "best" current setting; rather, the optimal current depends on the specific balance of hardness, toughness, and abrasion resistance required by the application. The study reinforces the importance of dilution rate control as the primary lever for maintaining overlay alloy integrity, and it provides quantitative data that can be directly applied to production welding procedure qualification.