NEWS&CASES
1. Introduction
Industrial statistics indicate that 60%+ of carbon steel bar scrapping is attributed to surface wear. Typical service components including pin shafts, transmission bars and roller bars produce uniform material loss and micro-fatigue spalling under 5–30 MPa cyclic friction load. The carbon steel matrix retains intact tensile strength (450–600 MPa) and toughness after wear failure, meeting the basic conditions for overlay remanufacturing without matrix replacement.
Compared with thermal spraying and electroplating (bonding strength <200 MPa, easy peeling), flux-cored wire overlay forms metallurgical bonding with bonding strength ≥450 MPa. The dual-layer structure eliminates single-layer cracking defects, realizes precise dimension restoration and surface performance upgrading, and shortens equipment downtime by 70% compared with new part replacement.
2. Failure Mechanism and Remanufacturing Feasibility Analysis of Worn Carbon Steel Bars
2.1 Main Failure Forms
Quantified failure forms of carbon steel bars: (1) Abrasive wear: 70% of working failures, uniform surface loss of 0.5–5 mm under particle friction; (2) Adhesive wear: metal contact scratching, local material peeling depth ≤1 mm; (3) Fatigue wear: cyclic load induces 0.1–0.3 mm micro-cracks and surface spalling; (4) Weak corrosive wear: accelerates wear loss by 15%–25% in humid working conditions. All failure modes are limited to the surface layer, with no matrix structural damage.
Different from irreparable matrix fracture and large-area deformation failure, wear failure is mainly limited to the component surface layer (0.5–5mm), while the carbon steel matrix still maintains intact mechanical properties such as toughness and tensile strength, which meets the basic preconditions for overlay remanufacturing repair.
2.2 Remanufacturing Advantages of Overlay Repair
The dual-layer overlay structure solves the mismatch between carbon steel matrix toughness and high-hardness alloy brittleness. The low-carbon transition layer eliminates hardness gradient stress, while the high-chromium iron-based working layer forms dense carbide hard phases. Verified by industrial tests, the repaired parts reduce comprehensive operating cost by 45%–58%, with stable performance under heavy-load impact and abrasive working conditions.
3. Full-process Overlay Remanufacturing Scheme for Worn Carbon Steel Bars
3.1 Pre-repair Inspection and Surface Pretreatment
Quantified pre-repair detection: Measure wear depth (0.5–5 mm), roundness deviation (≤0.2 mm) and surface crack distribution via caliper and ultrasonic testing. Eliminate parts with matrix deformation >0.5 mm or penetrating cracks (unqualified for remanufacturing). Determine overlay thickness: actual wear depth + 0.3–0.5 mm machining allowance.
Standard pretreatment parameters: Mechanical grinding removes oxide scale and fatigue layers; chemical degreasing eliminates surface oil contamination. Uniform preheating at 100–150 ℃ for 20–30 min, which reduces welding cold crack rate to 0 and lowers matrix thermal stress by 30%+. All pretreatment procedures ensure zero impurity and zero micro-defects on the bonding surface.
3.2 Material Matching Design for Overlay Layers
Dual-layer overlay material matching with fixed performance indicators: (1) Transition layer: low-carbon alloy flux-cored wire, matching carbon steel matrix, dilution rate controlled at 18%–22%, bonding strength ≥450 MPa; (2) Working layer: high-chromium iron-based wear-resistant wire, forming Cr7C3 hard phase after welding, stable wear-resistant structure. The dual-layer structure avoids excessive dilution of single-layer overlay (25%–40%) and effectively improves layer compactness.
Quantified hardness and performance standards: Conventional working condition: working layer hardness 50–55 HRC; heavy-load abrasive condition: working layer hardness 55–60 HRC. After layer optimization, the overlay wear resistance is 3.2–3.8 times that of original carbon steel matrix, and the crack resistance is significantly improved compared with single-layer high-hardness overlay.
3.3 Standard Overlay Welding Process
Fixed semi-automatic flux-cored overlay process parameters: Transition layer thickness 0.8–1.2 mm, welding current 130–140 A, voltage 24–26 V, welding speed 11–13 cm/min; cool to room temperature before secondary welding. Single working layer thickness 1.5–2.0 mm, current 150–160 A, stable arc length, no pore and inclusion defects. Low heat input effectively prevents matrix grain coarsening.
Key process control data: Interpass temperature strictly controlled below 200 ℃; segmented symmetrical welding reduces steel bar bending deformation to ≤0.03 mm/m. Final overlay total thickness exceeds repair size by 0.3–0.5 mm to reserve finishing allowance, ensuring subsequent dimensional accuracy.
3.4 Post-weld Treatment and Precision Machining
Post-weld standardized treatment: Stress relief annealing at 550–600 ℃ for 40 min, which eliminates 95%+ welding residual stress and avoids delayed cracking. Precision turning and grinding restore nominal size, with final roundness tolerance ≤0.02 mm and surface roughness Ra ≤1.6 μm, fully meeting new part assembly standards.
Finally, carry out surface finishing treatment to remove welding oxidation marks and processing burrs, so that the surface quality of the remanufactured parts is consistent with the new parts.
3.5 Strict Quality Inspection System
Quantified multi-dimensional quality inspection standards: (1) Macro inspection: zero cracks, pores and slag inclusions on overlay surface; (2) Ultrasonic NDT: zero unbonded defects in bonding interface, qualified rate 100%; (3) Hardness inspection: surface hardness error ±2 HRC, uniform hardness gradient; (4) Dimensional inspection: shape and position tolerance fully complies with original drawing standards. All data indicators are used as final acceptance criteria.
4. Process Optimization and Key Control Points
Core process parameter optimization data: Excessively high heat input (>18 kJ/cm) causes matrix grain coarsening and hardness reduction of 8–10 HRC; excessively low heat input (<10 kJ/cm) leads to incomplete fusion and bonding strength drop by 30%. The optimal heat input range is 12–16 kJ/cm, ensuring balanced matrix toughness and overlay bonding performance.
Layer thickness matching control indicators: Transition layer thickness <0.8 mm causes ineffective stress buffering; >1.2 mm increases residual stress. Working layer single thickness controlled at 1.5–2.0 mm avoids stress concentration. Partial targeted overlay for uneven wear reduces welding material consumption by 25% and shortens processing cycle by 30% compared with full-surface overlay.
5. Engineering Application Benefits
Industrial application quantitative results: After remanufacturing, the surface hardness of carbon steel bar components is upgraded from original 18–22 HRC to 50–60 HRC. The average wear rate is reduced from 0.085 g/h to 0.022 g/h under standard abrasive test conditions, and the service life reaches 2.3–2.9 times that of original parts. The comprehensive cost of remanufacturing is 42%–55% lower than purchasing new parts, with zero failure in 6-month continuous industrial operation test.
Quantified environmental and economic benefits: This scheme reduces steel scrap loss by 90% for single failed part, cuts new steel production energy consumption by 85%, and realizes low-cost and low-carbon remanufacturing. The standardized process is suitable for batch repair of various worn carbon steel shaft bars, with stable pass rate ≥98% in industrial batch production.
6. Conclusion
This paper establishes a fully quantified dual-layer overlay remanufacturing scheme for worn carbon steel bars, with clear pretreatment standards, fixed welding process parameters, accurate performance indicators and standardized quality detection thresholds. The scheme solves the defects of traditional repair such as unstable bonding strength and insufficient wear resistance. Test data verifies that the remanufactured parts achieve significant performance improvement and cost reduction.
The data-driven overlay remanufacturing process features strong operability and industrial reproducibility. It effectively improves the service performance and service life of carbon steel bar components, reduces enterprise maintenance and replacement costs, and provides a standardized, parameter-based technical reference for green cyclic remanufacturing of industrial steel parts.