Weld Overlay Repair of Hot Continuous Rolling Conveyor Roll Table Rolls

1. Definition and Technical Principles

Hot continuous rolling (HCR) conveyor roll table rolls are critical mechanical components in steel mill finishing lines, responsible for transporting hot strip products (typically at temperatures between 600°C and 1200°C) between rolling stands and finishing equipment. These rolls are subjected to extreme thermal cycling, mechanical impact loading, abrasion from scale and oxide particles, and chemical oxidation. Over time, the roll surface develops grooves, material loss, pitting, and dimensional deviation that compromise strip flatness, surface quality, and operational safety.

Weld overlay repair of these rolls involves the deposition of specialized alloy weld metal onto the damaged or worn roll surface to restore geometric dimensions, enhance surface hardness, and provide resistance to thermal shock, abrasion, and oxidation. The fundamental principle relies on the metallurgical bonding between the base roll material (typically medium-carbon steel or low-alloy steel such as 40Cr, 42CrMo, or 38CrMoAl) and the overlay weld metal (commonly Ni-Cr-Mo hardfacing alloys, austenitic stainless steels, or martensitic high-carbon steels), achieved through controlled heat input and proper preheating to ensure adequate fusion without excessive dilution or cracking.

The overlay process must account for the residual thermal stresses from the original roll manufacturing (induction hardening, tempering), the thermal expansion mismatch between base and overlay materials, and the operational demands of the HCR line environment where rolls experience rapid temperature gradients during strip transfer.

2. Category and Business Positioning

Within the company's product and service portfolio, roll table overlay repair falls under the Weld Overlay Repair and Restoration business segment, positioned as a value-added aftermarket service that extends component life, reduces capital expenditure on new roll procurement, and minimizes production downtime at steel mills.

This service bridges the gap between preventive maintenance and full component replacement, offering a cost-effective solution where:

The business model targets integrated steel producers, hot rolling mill operators, and roll service centers that manage fleets of conveyor rolls, transfer rolls, and guide rolls in HCR lines operating at capacity rates exceeding 80%.

3. Technical Purpose and Value Proposition

3.1 Engineering Objectives

3.2 Customer Value

4. Key Process and Implementation Points

4.1 Pre-Weld Preparation

Proper surface preparation is the foundation of successful overlay repair. The following sequence must be followed:

  1. Inspection and Assessment: Evaluate roll condition through visual examination, magnetic particle testing (MT) per ASTM E709 for surface-breaking defects, and ultrasonic testing (UT) for subsurface indications. Measure existing wear depth, groove geometry, and dimensional deviation using laser profilometry or coordinate measuring machines.
  2. Defect Removal: Grind away severely damaged material, pitting, and oxidation scale to expose sound base metal. Maintain a minimum of 3 mm of sound material beneath the overlay area. Remove any existing hardening layer (induction hardened case) to a depth of 2–3 mm to eliminate residual compressive stresses that could promote overlay cracking.
  3. Surface Cleaning: Machine the repair area to a smooth finish (Ra ≤ 12.5 μm), remove all scale, oil, and contaminants using wire brushing and solvent cleaning. Ensure the base metal surface is free of hydrogen-contaminating substances.
  4. Fit-Up Design: For deep grooves (>5 mm), design a stepped or V-groove preparation to ensure adequate weld penetration and reduce porosity risk. Multiple passes may be required with build-up layers before final hardfacing.

4.2 Welding Process Parameters

The selection of welding process depends on roll geometry, repair area size, and required overlay properties. The following table summarizes typical parameters:

Parameter TIG Overlay (GTAW) MIG Overlay (GMAW) Submerged Arc Overlay (SAW)
Applicable Scenario Small repair areas, precision work, transition layers Medium to large areas, production efficiency Large flat areas, thick build-up layers
Welding Current 80–160 A 180–320 A 350–600 A
Voltage 12–22 V 18–28 V 28–38 V
Travel Speed 30–80 mm/min 200–500 mm/min 300–800 mm/min
Preheat Temperature 200–350°C 200–350°C 250–400°C
Interpass Temperature ≤300°C ≤350°C ≤400°C
Shielding Gas Ar 99.99% or Ar/He mix Ar 95% + CO₂ 5% or Ar 99.99% Flux-covered (no external gas)
Typical Wire/Filler ER55D2, ER505Ni, ERNiCrMo-3 ER55D2, ER505Ni, ERNiCrMo-3 SAW-55D2, SAW-NiCrMo
Overlay Hardness Target 40–55 HRC 38–52 HRC 35–50 HRC

4.3 Layer Design and Build-Up Strategy

For roll table overlay repair, a multi-layer approach is typically employed:

  1. Transition Layer (Layer 1): Deposit a low-dilution austenitic stainless steel layer (e.g., 309L or 310) to minimize cracking at the base/overlay interface. This layer absorbs thermal stresses and provides a compatible metallurgical transition. Typical thickness: 1.5–2.5 mm.
  2. Build-Up Layer (Layer 2): Fill remaining volume to restore nominal dimensions using a matching or slightly softer alloy. This layer reduces residual stress and provides dimensional accuracy. Typical thickness: 3–8 mm depending on wear depth.
  3. Hardfacing/Functional Layer (Layer 3): Deposit the final wear-resistant, heat-resistant overlay alloy. This layer provides the operational surface properties. Typical thickness: 2–4 mm. Common alloys include Ni-Cr-Mo-B (Stellite-type), high-Cr martensitic (410/420), or austenitic (310 with B/Si additions).

4.4 Heat Treatment and Post-Weld Processing

4.5 Quality Control and Inspection

Inspection Stage Method Standard Acceptance Criteria
Pre-weld base metal Magnetic Particle Testing (MT) ASTM E709 / GB/T 26952 No linear indications ≥3 mm; no clusters of round indications
Post-overlay weld Magnetic Particle Testing (MT) ASTM E1444 / GB/T 26952 No cracks, no linear indications ≥2 mm
Post-overlay weld Ultrasonic Testing (UT) - Pulse Echo ASTM E164 / ISO 17640 No volumetric indications above 3 mm equivalent diameter
Overlay surface Hardness Testing ASTM E182 / GB/T 231.1 Within specified range (±3 HRC of target)
Overlay surface Dimensional Measurement ISO 1101 / GB/T 1182 Diameter ±0.05 mm; roundness ≤0.03 mm; runout ≤0.02 mm
Overlay surface Surface Finish ISO 4287 / GB/T 1031 Ra ≤ 6.3 μm (grinding finish)
Overlay microstructure Macro/Micro Examination (cross-section) ASTM E3 / ASTM E4 Full fusion, no lack of penetration, no delamination

5. Applicable Standards and Acceptance Criteria

5.1 Welding Procedure and Qualification Standards

5.2 Material and Performance Standards

5.3 Inspection and Acceptance Standards

5.4 Industry-Specific Standards

6. Common Risks and Controls

6.1 Cracking

Risk: Hot cracking in the overlay weld due to low melting point eutectics (S, P, Si), or cold cracking due to hydrogen diffusion and high carbon equivalent of the base metal. Martensitic transformation cracking can occur in high-alloy hardfacing deposits upon cooling.

Controls:

6.2 Delamination and Spalling

Risk: Poor fusion at the base/overlay interface due to residual stress from prior hardening, scale contamination, or thermal mismatch during service. This leads to overlay spalling during operation at elevated temperatures.

Controls:

6.3 Excessive Dilution

Risk: High dilution from the base metal into the overlay layer reduces hardness, alters microstructure, and compromises wear/thermal resistance properties of the functional layer.

Controls:

6.4 Dimensional Deviation

Risk: Warping, distortion, or uneven build-up leading to out-of-round conditions that compromise roll performance in the conveyor system.

Controls:

6.5 Porosity and Inclusion

Risk: Gas porosity from contaminated base metal, moisture in flux, or inadequate shielding. Slag inclusion from insufficient slag removal between passes.

Controls:

7. Application Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Route

Weld overlay repair of HCR conveyor rolls is the primary application domain for the company's TIG/MIG overlay capabilities. This route provides:

For the specific application of HCR conveyor roll repair, the recommended approach is a hybrid TIG/MIG strategy: TIG for the transition layer and precision repair of localized damage, followed by MIG (or SAW for large areas) for efficient build-up and final hardfacing layers.

7.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding is primarily applied to clad plate and pipe manufacturing, its relevance to roll applications includes:

This route is particularly valuable for high-volume production of new HCR rolls where consistent overlay properties are required and welding dilution variability is unacceptable.

7.3 Explosion Welding Route

Explosion welding (explosive cladding) offers an alternative for manufacturing new overlay rolls with superior metallurgical properties:

For HCR conveyor roll applications, explosion welding is most applicable to new roll manufacturing programs where large quantities are required and the overlay specification calls for materials with high thermal fatigue resistance (e.g., Ni-Cr-Si alloys, high-alloy austenitic steels).

8. Qualification Building and Strategic Value

8.1 WPS/PQR Development

Each HCR conveyor roll overlay repair program should be supported by a qualified Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) developed in accordance with GB/T 9445 or ISO 15614-1. The qualification program should include:

8.2 Customer Qualification and Certification

Establishing a track record of successful HCR roll overlay repairs builds customer qualification for:

8.3 Technical Knowledge Accumulation

The systematic documentation of HCR roll overlay repair projects—including metallurgical analyses, performance tracking, failure analysis, and process optimization—contributes to the company's technical database and intellectual property portfolio. This knowledge base enables:

9. Conclusion and Recommendations

Weld overlay repair of hot continuous rolling conveyor roll table rolls represents a high-value, technically demanding application that leverages the company's core TIG/MIG overlay capabilities while complementing hydraulic bonding and explosion welding routes for new roll manufacturing. Success in this application requires:

  1. Rigorous pre-weld assessment and surface preparation protocols
  2. Multi-layer overlay design with appropriate transition and functional alloys
  3. Controlled heat input and thermal management to prevent cracking and distortion
  4. Comprehensive NDT inspection at each critical stage
  5. Final precision machining to meet tight dimensional tolerances
  6. Documentation and traceability in accordance with applicable standards (ISO 9001, GB/T 9445, ASME Section IX)

By establishing standardized procedures, qualified WPS packages, and demonstrated field performance for HCR roll overlay repair, the company positions itself as a preferred partner for steel mill maintenance operations, generating recurring revenue while building technical credibility across the broader metalworking equipment repair market.