Weld Overlay Repair of Continuous Casting Machine Lead-In Chain

Literature Overview and Operational Context

Continuous casting machines (CCMs) are the backbone of modern steelmaking operations, converting molten steel into semi-finished solid shapes. The lead-in chain (also referred to as the lead billet chain or transfer chain) is a critical mechanical component that transports solidified slabs from the casting machine to the downstream finishing line. These chains operate under severe conditions: elevated temperatures (100–400 °C), mechanical abrasion from scale and oxide layers, cyclic loading, and periodic exposure to cooling water. Wear on the chain's load-bearing surfaces—particularly the pin bushings, roller surfaces, and guide contact areas—leads to dimensional degradation, misalignment, and ultimately chain failure. Weld overlay repair offers a viable restoration strategy that avoids the cost and downtime associated with complete chain replacement.

Failure Analysis of Lead-In Chains

Typical Wear Mechanisms

Wear Mechanism	Location	Contributing Factors	Dominant Wear Rate
Abrasive wear	Roller surfaces	Slab scale; oxide particles	0.5–2.0 mm/year
Adhesive wear	Pin-bushing interface	High contact pressure; sliding	0.3–1.5 mm/year
Erosive wear	Guide surfaces	Water droplet impingement	0.2–0.8 mm/year
Corrosive wear	Lower chain sections	Water exposure; acidic scale	0.1–0.5 mm/year
Fatigue spalling	Load-bearing surfaces	Cyclic contact loading	Surface-initiated

Material Degradation Pattern

The lead-in chain is typically fabricated from low-carbon steel (Q235 or 20# steel equivalent) or low-alloy steel (16Mn equivalent) with a base hardness of 150–200 HB. After extended service, the worn surfaces exhibit:

Loss of dimensional tolerance (chain pitch deviation exceeding ±0.5 mm).
Surface roughness increase from Ra 3.2 to Ra 25 μm or higher.
Work-hardened surface layer with microcracking.
Localized material loss at high-contact-stress zones.

Weld Overlay Repair Strategy

Consumable Selection

The selection of overlay consumables for lead-in chain repair must address the specific wear mechanisms identified:

Wear Zone	Recommended Consumable	Overlay Hardness	Key Alloying Elements
Roller surface	H08CrMoSiV (or equivalent)	45–55 HRC	Cr, Mo, Si, V
Pin bushing	H08Cr2Mo (or equivalent)	40–50 HRC	Cr, Mo, Mn
Guide surface	H10Cr2Ni2Mo	45–55 HRC	Cr, Ni, Mo
General surface	H10CrW (or equivalent)	50–60 HRC	Cr, W, Mo

The overlay composition must provide sufficient hardness for wear resistance while maintaining adequate toughness to withstand the cyclic mechanical loading inherent in chain operation.

Process Selection

For lead-in chain repair, the following processes are evaluated:

Process	Suitability	Advantages	Limitations
SMAW (stick welding)	High	Portable; flexible; low cost	Lower deposition rate; higher skill requirement
FCAW (flux-cored)	High	High deposition rate; good efficiency	Requires gas shielding; slag management
GMAW-C (CO2)	Moderate	Good arc stability; low cost	Carbon pickup; cracking tendency
SAW (submerged arc)	Low	High deposition rate	Limited to flat/large surfaces
GTAW (TIG)	Moderate	Precise control; low dilution	Low deposition rate; high cost

SMAW with specialized overlay electrodes is the most commonly employed process for chain repair due to its portability, flexibility for complex geometries, and the availability of purpose-designed overlay electrodes.

Repair Procedure

Step-by-Step Methodology

Inspection and measurement: Document wear depth, dimensional deviation, and surface condition using coordinate measuring or profilometry.
Surface preparation: Grind worn surfaces to remove contaminated oxide layers and establish a clean base for overlay adhesion. Remove any existing cracks by machining or gouging.
Crack repair: If transverse or longitudinal cracks are present, machine them out to a rounded-bottom groove and fill with a compatible low-hydrogen electrode before applying the overlay.
Preheating: Apply localized preheat of 100–200 °C to the repair area to reduce hydrogen cracking susceptibility and minimize thermal stress in the chain structure.
Overlay application: Deposit 2–4 passes of overlay material to achieve a total overlay thickness of 3–6 mm, rebuilding the worn surface to exceed the original dimension by 1–2 mm for subsequent machining.
Post-weld heat treatment: Temper the overlay at 200–300 °C for 1–2 hours to relieve residual stresses and stabilize the microstructure.
Machining: Machine the overlay surface to final dimensional specifications and surface finish requirements.
Inspection: Perform MT or PT examination of the overlay surface for cracks or porosity. Verify dimensional accuracy and hardness profile.

Quality Requirements

Inspection Method	Acceptance Criteria	Frequency
Visual examination (VT)	No visible cracks, undercut, or excessive spatter	100% of repair areas
Magnetic particle testing (MT)	No indications exceeding 0.5 mm length	100% of overlay surfaces
Hardness testing	40–55 HRC within overlay; gradient at interface acceptable	3 points per repair zone
Dimensional check	Pitch deviation ≤ ±0.3 mm; surface finish Ra ≤ 6.3 μm	100% of critical dimensions
Bond strength test	Peel strength ≥ 30 MPa (coupon test)	Per batch

Case Study: Chain Roller Surface Restoration

A representative case involved a 500-tonne CCM lead-in chain with 120 rollers showing 1.5–3.0 mm of surface wear after 18 months of service. The repair program was as follows:

Preparation: Each roller surface was ground back 0.5 mm to remove the work-hardened and contaminated layer.
Overlay: Applied 3 passes of H08CrMoSiV electrode (3.2 mm diameter, DCEN, 160–180 A) to build 4.0 mm total overlay thickness.
Tempering: Batch tempering at 250 °C for 2 hours in a furnace.
Machining: CNC turning to restore roller diameter to nominal +0.5 mm, achieving Ra 3.2 surface finish.
Result: Overlay hardness measured 48–52 HRC; chain pitch accuracy restored to ±0.2 mm.

Post-repair service life was extended to 24 months (compared to 18 months for the original), representing a 33% life extension. The overlay exhibited uniform wear without spalling or delamination, confirming adequate bond strength and microstructural compatibility.

Economic and Operational Benefits

Parameter	Chain Replacement	Overlay Repair
Direct material cost	100% (baseline)	8–15%
Labor cost	40–60 hours	20–30 hours
Downtime	8–16 hours	4–8 hours
Service life	18 months	24 months
Total cost of ownership	100%	25–35%

The economic case for overlay repair is compelling, particularly when considering the cumulative cost savings over multiple repair cycles before eventual chain replacement.

Study Insights and Practical Recommendations

The successful application of weld overlay repair to continuous casting machine lead-in chains demonstrates that even complex mechanical components with demanding service conditions can benefit from strategic surface restoration. The key success factors identified are:

Thorough wear analysis: Understanding the dominant wear mechanism is essential for selecting the appropriate overlay composition.
Adequate surface preparation: The quality of the base surface directly determines overlay bond strength and durability.
Controlled thermal management: Preheating and tempering are not optional steps but essential elements of a reliable repair procedure.
Dimensional discipline: The overlay must be deposited with sufficient excess thickness to allow machining back to precise tolerances.

For cladding engineers working in the metallurgical industry, this case study illustrates the principle that weld overlay is not merely a surface hardening technique but a complete restoration methodology capable of returning worn components to better-than-original condition. The overlay layer, when properly designed and applied, can outperform the original base material in terms of wear resistance while maintaining the structural integrity of the underlying component. This philosophy of selective surface enhancement rather than wholesale component replacement should be embraced as a standard practice in industrial maintenance engineering.