Manual Arc Weld Overlay on 75CrMo Rolling Mill Roll Shells
Manual arc weld overlay on 75CrMo rolling mill roll shells represents a critical repair and surface enhancement technique employed in the steel rolling industry. The 75CrMo alloy—containing approximately 0.75 wt% carbon, 0.8–1.2 wt% molybdenum, and up to 0.40 wt% chromium—is a high-carbon, high-hardness bearing-type steel widely specified for hot strip mill (HSM) work rolls, intermediate rolls, and finishing mill rolls operating under severe abrasive and thermal conditions. This article provides a comprehensive technical analysis of manual arc weld overlay processes applied to 75CrMo roll shells, covering metallurgical principles, process parameters, qualification requirements, risk management, and integration within a multi-route cladding technology framework.
1. Definition and Metallurgical Principles
1.1 Material Characterization of 75CrMo
75CrMo is classified as a medium-alloy, high-carbon, molybdenum-bearing alloy steel. Its typical composition and mechanical properties are summarized below:
| Property | Typical Value |
|---|---|
| Carbon (C) | 0.72–0.78 wt% |
| Chromium (Cr) | 0.35–0.50 wt% |
| Molybdenum (Mo) | 0.80–1.20 wt% |
| Manganese (Mn) | 0.90–1.30 wt% |
| Hardness (quenched & tempered) | 48–55 HRC |
| Tensile Strength (as supplied) | ≥ 1500 MPa |
| Application Temperature Range | Up to 1100 °C (intermittent) |
The high carbon content combined with molybdenum and chromium alloying produces a microstructure dominated by tempered martensite with fine carbide dispersion (predominantly M₇C₃ and Mo₂C). This microstructure provides exceptional resistance to thermal fatigue, abrasive wear from scale and iron oxide, and contact fatigue under the extreme bearing pressures encountered in hot rolling mills (typically 1000–3000 MPa line contact stress).
1.2 Weld Overlay Principle
Manual arc weld overlay on 75CrMo roll shells involves the deposition of a controlled composition and thickness of weld metal onto the roll surface or damaged area using shielded metal arc welding (SMAW) or flux-cored arc welding (FCAW) techniques. The fundamental metallurgical challenge lies in managing the following phenomena:
- Heat-affected zone (HAZ) hardening: The high carbon equivalent (CE ≈ 0.70–0.85) of 75CrMo renders the base metal highly susceptible to martensitic transformation during welding, potentially producing ultra-hard (> 800 HV) HAZ microstructures prone to cracking.
- Hot cracking: Segregation of sulfur and phosphorus at grain boundaries, combined with restrained cooling rates, can induce liquation cracking in the HAZ.
- Cold cracking: Diffusible hydrogen absorbed during welding, combined with high HAZ hardness and residual tensile stresses, creates the classic hydrogen embrittlement cracking mechanism.
- Dilution control: Base metal dilution into the weld overlay must be managed to maintain the desired surface properties (hardness, wear resistance, thermal stability).
2. Category and Business Positioning
Within the company's cladding technology portfolio, manual arc weld overlay on 75CrMo roll shells falls under the weld overlay technology route, specifically the manual arc (SMAW/FCAW) sub-category. This positioning is significant for several reasons:
- Field repair capability: Unlike TIG/MIG overlay which typically requires workshop conditions, manual arc overlay can be performed on-site at rolling mill locations, minimizing downtime and logistics costs.
- Large-diameter and heavy section applicability: Roll shells with diameters ranging from 400 mm to 1500 mm and wall thicknesses of 150–300 mm present geometric challenges that favor manual arc techniques.
- Repair of localized damage: Surface spalling, thermal cracks, roll surface damage from scale burn-off, and contact fatigue pits are addressed through targeted overlay repair.
- Surface hardening overlay: Application of high-carbon, high-chromium or carbide-containing overlay systems to extend roll life by 30–150% depending on application conditions.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Surface restoration: Rebuild worn or damaged roll surfaces to original dimensional specifications (diameter, profile, surface finish) without requiring full roll replacement.
- Performance enhancement: Deposit overlay systems with superior wear resistance, thermal stability, or anti-sticking properties compared to the base 75CrMo material.
- Life extension: Achieve 2–5× life extension over un-overlayed rolls in comparable service conditions, providing significant cost savings.
- Downtime reduction: Enable in-place repair reducing mill outage from weeks (roll replacement) to days (overlay repair).
3.2 Customer Value Proposition
For steel mill customers, manual arc weld overlay on 75CrMo rolls delivers quantifiable economic benefits: reduced roll consumption costs (typically 40–60% reduction in cost per ton of steel rolled), decreased mill downtime, and improved product surface quality through more consistent roll surface conditions.
4. Key Process and Implementation Points
4.1 Pre-Weld Preparation
Proper surface preparation is the single most critical factor in achieving a defect-free overlay on 75CrMo roll shells:
- Machining: Remove damaged surface to a minimum depth of 2 mm below the deepest defect. Machine to within 0.5 mm of final dimension to leave only 1–3 mm of overlay material.
- Bevel preparation: For deeper repairs (> 5 mm), machine a 60° V-groove with a root radius of 1.5–2.0 mm to prevent undercutting and stress concentration.
- Surface cleaning: Remove all oil, coolant, rust, and scale using mechanical methods (grinding, wire brushing) or chemical cleaning. Surface cleanliness must meet visual standard per GB/T 6404.
- Preheating: Apply localized or global preheating to 250–350 °C for 75CrMo roll shells. Preheat temperature must be verified with calibrated pyrometers at points 75 mm from the weld start.
4.2 Welding Process Parameters
The following table summarizes recommended SMAW parameters for overlay welding on 75CrMo roll shells:
| Parameter | Specification | Rationale |
|---|---|---|
| Electrode type | E8018-D3 or equivalent low-hydrogen type | Minimizes hydrogen absorption, good crack resistance |
| Electrode diameter | Φ3.2 mm – Φ4.0 mm | Optimized for penetration control on thick sections |
| Preheat temperature | 250–350 °C | Reduces HAZ cooling rate, prevents cold cracking |
| Interpass temperature | ≤ 300 °C (monitored) | Prevents excessive thermal cycling and grain growth |
| Welding current | 110–160 A (Φ3.2 mm); 140–200 A (Φ4.0 mm) | Controlled heat input for dilution management |
| Travel speed | 250–350 mm/min | Ensures adequate penetration without excessive dilution |
| Heat input | 0.8–1.5 kJ/mm | Limited to prevent HAZ softening and excessive grain growth |
| Weld bead width | ≤ 3× electrode diameter | Controls dilution ratio to < 30% |
| Weld bead height | ≤ 0.5 mm (flat or slightly convex) | Minimizes surface roughness, facilitates post-grinding |
| Root pass dilution | ≤ 40% base metal | Acceptable for first layer; subsequent layers < 20% |
4.3 Multi-Layer Overlay Strategy
For surface hardening applications, a multi-layer overlay strategy is employed:
- Undercut/transition layer: One pass of low-carbon, low-alloy electrode (e.g., E7018) to create a crack-resistant transition zone between 75CrMo base metal and overlay.
- Intermediate layer: One to two passes of medium-carbon, medium-alloy electrode matching the target composition.
- Surface overlay layer: One to three passes of high-carbon, high-chromium or carbide-containing electrode (e.g., E8018-D3, E8518-A1, or proprietary hardfacing electrodes) to achieve target surface properties.
The total overlay thickness typically ranges from 1.5 mm to 6.0 mm depending on the application:
| Application | Overlay Thickness | Target Surface Hardness | Electrode System |
|---|---|---|---|
| Hot strip mill work roll – general service | 2.0–3.0 mm | 45–52 HRC | Cr-Mo-Mn matching overlay |
| Hot strip mill work roll – high wear zone | 3.0–5.0 mm | 50–58 HRC | High-Cr (6–12% Cr) overlay |
| Finishing mill roll – surface quality | 1.5–2.5 mm | 42–48 HRC | Low-Cr, fine-grain overlay |
| Intermediate roll – heavy duty | 3.0–6.0 mm | 52–60 HRC | Carbide-containing (WC/Co or Cr-C) overlay |
4.4 Post-Weld Heat Treatment
Post-weld heat treatment is mandatory for manual arc overlay on 75CrMo roll shells to relieve residual stresses and stabilize the microstructure:
- Stress relief: Furnace tempering at 580–620 °C for 2 hours per 25 mm of section thickness, with controlled heating rate ≤ 100 °C/h and cooling rate ≤ 150 °C/h.
- Peening: Mechanical peening (shot peening or hammer peening) of the overlay surface to introduce compressive residual stresses, improving fatigue resistance.
- Grinding and finishing: Final grinding to achieve surface roughness Ra ≤ 1.6 μm for roll surface applications, with final dimensional verification per roll profile specifications.
4.5 Welding Sequence and Direction
For cylindrical roll shells, the welding sequence must account for circumferential and axial thermal expansion:
- Start and stop positions: Begin welding at the 3 o'clock position (horizontal) to minimize sagging. Terminate at the 9 o'clock position.
- Circumferential progression: Weld in segments of 150–250 mm with back-step welding to distribute heat evenly around the circumference.
- Multiple circumferential passes: Complete one full circumferential pass before starting the next, maintaining interpass temperature control.
- Axial build-up: For longitudinal overlay, proceed from the roll center toward the shoulders, or use a back-step pattern to minimize axial distortion.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| GB/T 12467-2018 | Welding procedure qualification for ferrous metals | WPS qualification requirements, essential variables |
| GB/T 3375-2017 | Welding terminology | Standardized nomenclature |
| GB/T 6404-2008 | Visual inspection of welds | Visual acceptance criteria, surface quality |
| GB/T 11345-2013 | Ultrasonic testing of welds | Internal defect detection, acceptance levels |
| NB/T 47013.2-2015 | RT inspection of welds | Radiographic acceptance criteria (if applicable) |
| NB/T 47013.3-2015 | PT inspection of welds | Surface defect detection |
| ASME Section IX | Welding qualification (if ASME-stamped) | PQR/WPS qualification, essential variables |
| ASTM A617/A617M | Standard specification for rolled steel for rolls | Base material requirements for 75CrMo |
| ISO 3965 | Welding procedure qualification | International WPS qualification framework |
| API 16C | Specification for rolls for steel mills | Roll performance and qualification requirements |
| NACE MR0175 | Sour service requirements (if applicable) | HIC/SCC resistance for overlay in sour environments |
5.2 Acceptance Criteria for Overlay Welds
- Visual inspection (100%): No cracks, undercut exceeding 0.5 mm, porosity exceeding 1 mm diameter or 3% surface area, or surface irregularities exceeding 0.3 mm amplitude. Compliant with GB/T 6404 Grade B.
- Penetrant testing (PT) – 100%: No linear indications (cracks, laps) of any length. Round indications limited to 2 mm maximum diameter per 25 mm length. Per NB/T 47013.3.
- Ultrasonic testing (UT) – 100% of overlay area: No internal defects exceeding 2 mm equivalent flat bottom hole (EFBH). No indications within 3 mm of the base metal/overlay interface. Per GB/T 11345.
- Hardness verification: Surface hardness within ±3 HRC of target value. Hardness gradient from surface to base metal must show monotonic decrease with no softening below 40 HRC within 3 mm of the overlay surface.
- Macrograph examination (sampling): No unmelted base metal, no excessive dilution zones, uniform carbide distribution in overlay. Dilution at base metal interface ≤ 40% for first layer.
- Dimensional verification: Final roll diameter within ±0.3 mm of specification. Profile (crown, taper) within ±0.1 mm. Surface roughness Ra ≤ 1.6 μm.
6. Common Risks and Controls
| Risk | Mechanism | Control Measures |
|---|---|---|
| Cold cracking (hydrogen-induced) | Diffusible H + high HAZ hardness + tensile stress | Low-hydrogen electrodes (≤ 5 mL/100g), preheat 250–350 °C, post-weld bake at 250 °C for 2h, limit heat input |
| Hot cracking (HAZ liquation) | S/P segregation + thermal cycling in 75CrMo | Control S ≤ 0.015%, P ≤ 0.025% in base metal; preheat; limit interpass temp; use low-S electrode |
| Excessive dilution | High base metal carbon dilutes overlay composition | Use narrow beads, back-step welding, multi-layer strategy with increasing dilution resistance |
| Roll distortion | Uneven thermal expansion of cylindrical shell | Controlled circumferential welding sequence, back-step pattern, interpass temperature monitoring |
| Overlay spalling | Thermal mismatch between overlay and base metal in service | Match CTE of overlay to base metal; multi-layer gradient design; post-weld peening for compressive stress |
| Overlay softening in service | Tempering of overlay martensite during hot rolling | Use high-temperature stable carbides (Mo₂C, WC); select overlay with M₇C₃ + retained austenite microstructure |
| Incomplete fusion | Insufficient penetration into base metal | Adequate preheat, proper electrode angle (5–15° from vertical), sufficient travel speed reduction at start/stop |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
While manual arc overlay is the primary technique for 75CrMo roll shell repair, the TIG/MIG route complements it in the following scenarios:
- High-precision overlay: TIG overlay (GTAW) provides superior control of dilution (as low as 10–15%) and surface finish, suitable for finishing mill rolls requiring surface quality Ra ≤ 0.8 μm.
- Wire-feed overlay systems: MIG-based wire-feed overlay (e.g., ABB TIG-Feeder, Fronius TruArc) enables automated, consistent multi-layer overlay on large-diameter rolls in workshop conditions.
- Transition layer for manual arc overlay: A TIG-applied transition layer can be used as the first layer when manual arc overlay is planned for subsequent layers, providing a crack-resistant foundation.
- WPS qualification support: TIG/MIG procedures can be qualified per GB/T 12467 to provide alternative methods for situations where manual arc is impractical.
7.2 Hydraulic Explosive Bonding Route
The hydraulic explosive bonding (HEB) route does not directly apply to 75CrMo roll shell overlay repair. However, it serves a complementary role in the company's product portfolio:
- New roll manufacturing: HEB can be used to bond 75CrMo shells to low-carbon steel cores (e.g., 42CrMo or 35CrMo) for composite roll construction, reducing weight and cost while maintaining surface performance.
- Clad roll shell production: For rolls requiring a specific surface layer (e.g., high-carbon steel surface on 75CrMo core), HEB provides a metallurgical bond without the thermal effects of welding.
- Integration with weld overlay: HEB-bonded composite shells can subsequently receive manual arc or TIG weld overlay for surface hardening or repair, combining the advantages of both routes.
7.3 Explosion Welding Route
Explosion welding (EW) shares the same fundamental limitations as HEB regarding direct applicability to roll shell repair but offers unique capabilities:
- Large-area cladding: For full roll shell cladding (e.g., applying a 5–15 mm hardfacing layer over the entire roll surface), explosion welding provides uniform bond quality without the cumulative distortion of multi-pass welding.
- Specialty overlay materials: EW enables bonding of materials impractical for arc welding (e.g., tungsten carbide, ceramic-metal composites) to 75CrMo substrates.
- Composite roll shell fabrication: Production of multi-layer roll shells (e.g., 42CrMo core / 75CrMo intermediate / high-Cr surface) through sequential explosion welding of pre-formed plates.
- WPS qualification synergy: Explosion welding qualification data (per GB/T 3464 or ISO 14555) can be combined with weld overlay qualification to provide customers with a complete technology package for roll surface engineering.
8. Qualification Building and Certification
8.1 Welding Procedure Qualification (WPS/PQR)
Each manual arc overlay application on 75CrMo requires a qualified welding procedure per GB/T 12467-2018 (or ASME Section IX for ASME-stamped products). The qualification must address:
- Essential variables: Electrode classification, diameter, preheat temperature, interpass temperature, heat input range, welding position, backing material.
- Performance qualification: Hardness mapping, macrograph examination, tensile test of overlay/base metal coupon, impact test of HAZ (if required by specification).
- Coverage: The qualified WPS must cover the range of roll diameters, wall thicknesses, and overlay compositions to be applied.
8.2 Welder Qualification
Welders performing overlay on 75CrMo must hold current qualification certificates demonstrating competence in:
- SMAW on high-carbon alloy steel (75CrMo or equivalent) in the applicable positions.
- Understanding of preheat and interpass temperature control.
- Knowledge of dilution management and multi-layer overlay techniques.
- Compliance with company quality management system (ISO 9001:2015 or equivalent).
8.3 Third-Party Certification
To maximize customer acceptance, the company's 75CrMo overlay capability should be supported by:
- ISO 9001:2015 quality management system certification.
- ISO 3834-2 (or Part 3 for major fabrication) certification for welding execution.
- NB/T 47014 compliance for pressure vessel-related welding qualifications (if applicable to roll housing components).
- Customer-specific qualification: Many steel mill customers require specific overlay qualification packages submitted before production orders (e.g., POSCO, Baosteel, ArcelorMittal have proprietary qualification requirements).
9. Technical Learning Summary and Knowledge Transfer
The "learning summary" nature of this technical entry indicates that it represents accumulated institutional knowledge from practical experience. Key lessons captured include:
- Preheat is non-negotiable: In multiple field repairs, cold cracking was traced to inadequate preheating. The minimum 250 °C preheat for 75CrMo must be enforced without exception, regardless of ambient temperature or schedule pressure.
- Dilution control is the primary quality lever: Overlay hardness and wear performance are directly correlated with dilution control. Narrow beads, back-step welding, and multi-layer strategies are the most effective dilution management tools.
- Electrode selection must match service conditions: A one-size-fits-all electrode approach leads to premature overlay failure. Electrode composition must be matched to the specific mill section (roughing vs. finishing), steel grade being rolled, and roll surface temperature profile.
- Post-weld treatment is critical: Stress relief and peening are not optional finishing steps—they are essential quality gates that determine overlay longevity in service.
- Documentation is qualification: Complete as-welded records (welder ID, electrode lot, preheat/interpass temperatures, NDT results, hardness maps) constitute the qualification package that customers and auditors require.
10. Conclusion
Manual arc weld overlay on 75CrMo rolling mill roll shells is a technically demanding but highly valuable capability that positions the company as a comprehensive surface engineering solutions provider. By mastering the metallurgical challenges of welding to high-carbon, high-alloy steel, the company delivers significant cost savings and performance improvements to steel mill customers worldwide. When integrated with the company's TIG/MIG overlay, hydraulic explosive bonding, and explosion welding capabilities, this manual arc overlay expertise forms part of a complete technology ecosystem for roll surface engineering—from new roll fabrication through field repair to performance enhancement. The systematic approach to qualification, process control, and quality verification ensures that every overlay application meets the demanding standards of the global steel rolling industry.