Improvement of Heat Treatment Process for Weld Overlay Rolls
1. Definition and Technical Principles
Weld overlay rolls (also referred to as surfacing rolls or cladded rolls) are critical rotating equipment components used extensively in steel mills, paper machines, mining operations, and material handling systems. These rolls consist of a base steel substrate with a hardfacing or wear-resistant overlay deposited on the working surface through arc welding processes. The heat treatment process for weld overlay rolls refers to the post-weld thermal treatment applied to the entire roll assembly—typically normalizing, annealing, or quench-and-tempering cycles—designed to relieve residual stresses, homogenize microstructure, and optimize the mechanical properties of both the base material and the overlay layer without compromising the integrity of the cladding interface.
The fundamental metallurgical principles governing this heat treatment include:
- Residual stress relief: Weld overlay processes (TIG, MIG, or submerged arc) introduce significant thermal gradients that generate tensile residual stresses in the overlay, heat-affected zone (HAZ), and base material. These stresses can reach 200–400 MPa and are the primary driver for cracking and premature fatigue failure.
- Microstructural refinement: The HAZ in the base steel undergoes grain growth during the overlay welding cycles. Post-weld heat treatment at appropriate austenitizing temperatures (typically 850–950°C for medium-carbon steels) promotes recrystallization and grain refinement, restoring ductility and toughness.
- Overlay layer property stabilization: Hardfacing alloys (e.g., austenitic Cr-Ni-Mo, martensitic Cr-V, or high-alloy Ni-Cr-Mo) may develop brittle phases or excessive hardness during welding. Controlled cooling rates and tempering cycles ensure the overlay retains its designed wear resistance while achieving acceptable toughness.
- Interface integrity preservation: The heat treatment parameters must be carefully calibrated to avoid interfacial decohesion, delamination, or excessive softening at the overlay-base metal bond line.
2. Category and Business Positioning
This technical capability falls within the post-weld processing and quality assurance domain of Cladding Technology Shanxi Co., Ltd.'s overall service portfolio. It bridges the gap between the primary overlay welding operation and final product delivery, serving as a critical value-add process that differentiates the company's offerings from basic welding contractors.
| Dimension | Positioning |
|---|---|
| Technology Route | Post-processing complement to TIG/MIG weld overlay |
| Service Category | Engineering process improvement and optimization |
| Value Chain Role | Quality assurance, product qualification, and customer specification compliance |
| Industry Sector | Steel industry, mining, paper manufacturing, bulk material handling |
| Competitive Advantage | Extended roll service life, reduced downtime, compliance with OEM specifications |
3. Technical Purpose and Value
The improvement of the heat treatment process for weld overlay rolls addresses several critical engineering objectives:
3.1 Primary Technical Objectives
- Crack elimination: Reduce or eliminate cold cracking and fatigue cracking in the HAZ and overlay layer by relieving residual stresses to below 50 MPa.
- Mechanical property optimization: Achieve a balanced combination of hardness (HRC 45–62 for hardfacing) and impact toughness (≥27 J at -20°C per ASTM A703) in the overlay and transition zone.
- Dimensional stability: Minimize post-treatment distortion to within ±0.2 mm/m for roll geometry, ensuring proper running clearance in mill equipment.
- Service life extension: Increase roll life by 30–60% compared to untreated overlay rolls through improved fatigue resistance and reduced spalling.
3.2 Business and Customer Value
- Specification compliance: Many OEM specifications (particularly for hot strip mill rolls, cold finishing rolls, and mining conveyor rolls) mandate post-weld heat treatment as a non-negotiable requirement. Process improvement ensures consistent compliance and avoids costly rework or rejection.
- Qualification building: Documented, repeatable heat treatment procedures support WPS/PQR qualification packages required under ASME IX, AWS D10.9, or customer-specific qualification programs.
- Cost reduction: Optimized thermal cycles reduce furnace time, energy consumption, and post-treatment machining allowance while improving first-pass yield.
- Customer trust: Demonstrable process improvement with quantifiable performance data strengthens the company's position in competitive bidding for high-value roll refurbishment and new roll fabrication contracts.
4. Key Process and Implementation Points
4.1 Heat Treatment Cycle Design
The improved heat treatment process incorporates optimized thermal parameters based on material characterization, finite element thermal analysis, and empirical validation. The following table summarizes typical parameters for common roll configurations:
| Parameter | Conventional Process | Improved Process | Rationale |
|---|---|---|---|
| Heating Rate (to austenitizing) | 100–150°C/hr | 60–100°C/hr (below 600°C); 80–120°C/hr (above 600°C) | Slower initial heating reduces thermal gradient-induced cracking in overlay |
| Austenitizing Temperature | 900–950°C (fixed) | Material-specific: 850°C (low-carbon base); 920°C (medium-carbon base); 880°C (high-alloy overlay) | Prevents overlay softening while ensuring base steel grain refinement |
| Hold Time | 1–2 hr | 0.5–1 hr per 25 mm of max section (minimum 30 min) | Eliminates over-tempering of overlay while achieving uniform austenitization |
| Cooling Method (Furnace) | Furnace cool to 600°C | Controlled furnace cool at 50–80°C/hr to 650°C, then air cool | Minimizes differential contraction between overlay and base; reduces distortion |
| Tempering Cycle | Single temper at 580°C/2hr | Dual temper: 550°C/1.5hr + 600°C/1hr | Dual temper eliminates retained austenite in martensitic overlay and stabilizes carbide distribution |
| Post-Treatment Stress Relief | Not performed | 350°C/2hr final stress relief (where overlay permits) | Final stress relief brings residual stresses below 30 MPa without affecting overlay hardness |
4.2 Critical Implementation Steps
- Pre-treatment inspection: Complete NDT (MPI per ASTM E1444, UT per ASTM E164 for delamination detection) of the overlay weld before heat treatment. Document all indications for post-treatment comparison.
- Thermal barrier application: Apply ceramic fiber blanket or refractory coating at the overlay-to-base transition zone to manage thermal differential during heating. This is particularly critical for thick overlays (>15 mm) on thin-walled roll shells.
- Instrumentation: Embed thermocouples at minimum three locations: overlay surface, overlay/base interface (or nearest accessible point), and base material core. Record continuous temperature profiles for process traceability.
- Atmosphere control: Maintain a neutral or slightly reducing atmosphere (dew point ≤ -40°C or use endothermic gas) to prevent scale formation on the overlay surface. For austenitic overlay alloys, avoid oxidizing conditions above 600°C to prevent intergranular carbide precipitation.
- Post-treatment verification: Perform hardness mapping (overlay, HAZ, base), tensile testing of interface coupons, and residual stress measurement (XRD per ASTM E975 or hole-drilling per ASTM E837) to confirm process effectiveness.
4.3 Process Improvement Methodology
The improvement program follows a structured approach:
- Baseline characterization: Analyze historical failure data (cracking locations, service life statistics, customer complaints) to identify process deficiencies.
- Thermal simulation: Use finite element analysis (FEA) to model thermal gradients, residual stress evolution, and microstructural transformation during the heat treatment cycle.
- Small-scale validation: Conduct trial heat treatment on representative coupons or test rolls with instrumented monitoring. Compare microstructural and mechanical property results against baseline.
- Scale-up and standardization: Translate validated parameters into a documented WPS (Welding Procedure Specification) with defined control limits, inspection requirements, and acceptance criteria.
- Continuous improvement: Implement statistical process control (SPC) on key parameters (heating rate, peak temperature, cooling rate) and track field performance data for ongoing optimization.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application |
|---|---|
| ASME Section IX | WPS qualification and heat treatment requirements for weld overlay |
| AWS D10.9 | Specification for Welding and Surfacing of Cast Iron and Steel |
| ASTM A703 | Standard Specification for Steel Billets and Bars for Hot Rolling Mill Rolls |
| ASTM A453 | Standard Specification for Steel Forgings for Hot Working and Hot Rolling Mill Rolls |
| ASTM E1444 | Magnetic Particle Testing (crack detection pre/post heat treatment) |
| ASTM E164 | Ultrasonic Examination of Welds (interface delamination detection) |
| ASTM E975 | X-ray Diffraction Determination of Residual Stress |
| ASTM E837 | Determination of Residual Stress by the Hole-Drilling Strain Gage Method |
| ISO 9013 | Metallurgical Examination of Welds |
| GB/T 11352 | Non-destructive Testing of Castings (where applicable to roll bodies) |
| NB/T 47013 | NDE methods for pressure equipment (applicable to pressure vessel roll applications) |
5.2 Acceptance Criteria
- Residual stress: Maximum principal tensile residual stress ≤ 50 MPa in the overlay and HAZ (verified by XRD or hole-drilling method).
- Hardness: Overlay hardness within specified range (typically HRC 45–62 depending on alloy); base material hardness deviation ≤ ±10% from pre-treatment value.
- Impact toughness: Charpy V-notch impact energy ≥ 27 J at -20°C for the overlay/HAZ region (per ASTM A703 for hot mill roll applications).
- NDT: No indications exceeding acceptance limits per ASTM E1444 (MPI) or ASTM E164 (UT). Zero new indications compared to pre-treatment baseline.
- Dimensional tolerance: Post-treatment roll runout ≤ 0.05 mm TIR; diameter tolerance within ±0.1 mm of specified dimension.
- Interface integrity: No interfacial cracking, delamination, or excessive softening (hardness drop >15% at bond line) confirmed by metallographic examination of test coupons.
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Overlay cracking during heating | Excessive thermal gradient between overlay and base; high heating rate | Limit heating rate to ≤100°C/hr; apply thermal barrier at transition zone; pre-heat to 200°C before ramping |
| Interface delamination | Differential thermal expansion; excessive austenitizing temperature causing overlay softening | Cap austenitizing temperature at 900°C for austenitic overlays; use graded thermal barrier; verify interface with UT pre/post treatment |
| Excessive distortion | Non-uniform cooling; asymmetric roll geometry; inadequate support during treatment | Use controlled cooling rates; support roll on V-blocks or cradle fixtures; monitor with dial indicators during cooling |
| Overlay softening | Prolonged hold at high temperature; temperature exceeding overlay recrystallization range | Minimize hold time; use material-specific temperature caps; verify hardness post-treatment |
| Scale and oxidation | Oxidizing furnace atmosphere; inadequate protective coating | Use controlled atmosphere furnace or vacuum furnace; apply ceramic coating; maintain dew point ≤ -40°C |
| Retained austenite instability | Incomplete tempering of martensitic overlay; single-temper cycle insufficient | Implement dual-temper cycle; verify retained austenite content by metallography or XRD |
| Hydrogen-induced delayed cracking | Residual hydrogen from welding; insufficient bake-out before heat treatment | Perform 200°C/4hr hydrogen bake-out of overlay before heat treatment; monitor hydrogen content by gas chromatography |
7. Application Across Technology Routes
7.1 TIG/MIG Weld Overlay Route
For TIG and MIG weld overlay operations, heat treatment improvement is particularly critical because:
- Layer-by-layer stress accumulation: Multi-pass overlay builds up residual stresses incrementally. The improved heat treatment cycle is designed to accommodate the higher stress levels typical of multi-layer TIG/MIG builds (up to 25–40 passes for thick overlays).
- HAZ sensitivity: TIG/MIG processes produce narrower HAZs with steeper thermal gradients, making the transition zone more susceptible to cracking during thermal cycling. The improved process uses slower heating rates and thermal barriers specifically at this zone.
- Alloy dilution management: Post-weld heat treatment can alter the dilution characteristics at the overlay-base interface. The improved process includes microstructural verification to ensure dilution remains within acceptable limits (typically ≤15% base metal dilution into the first overlay layer).
- WPS qualification integration: The improved heat treatment parameters are incorporated into the WPS as a post-weld heat treatment (PWHT) step, enabling full qualification under ASME IX or AWS D10.9 requirements. This is essential for customer audits and regulatory compliance in pressure equipment or safety-critical applications.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (waterjet-assisted explosive cladding) produces a metallurgical bond through controlled shock wave interaction, heat treatment improvement contributes in the following ways:
- Post-bond stress relief: Even though explosive bonding produces lower residual stresses than welding, the shock wave induces complex stress states in both the cladding layer and base material. A controlled stress relief cycle (typically 550–650°C for 2–4 hours) optimizes the stress state without disrupting the cold-welded interface.
- Interface microstructure stabilization: The explosive bonding interface contains adiabatic shear zones with unique microstructures. The improved heat treatment ensures these zones are stabilized without excessive grain growth or phase transformation that could weaken the bond.
- Composite roll fabrication: For rolls requiring both explosive-bonded cladding and weld overlay (hybrid approach), the heat treatment must accommodate both interfaces. The improved process defines sequential treatment steps: first stress relief for the explosive bond, then overlay welding, followed by final PWHT.
- Thick cladding applications: Hydraulic explosive bonding can produce cladding layers of 10–50 mm thickness. The thermal mass of thick cladding creates unique heating and cooling challenges. The improved process incorporates section-dependent temperature and time parameters.
7.3 Explosion Welding Route
For explosion-welded clad rolls and pipes, heat treatment improvement addresses:
- Wavy interface preservation: The characteristic wavy interface produced by explosion welding is a key indicator of bond quality. Excessive heat treatment temperatures can flatten this interface through recrystallization. The improved process caps temperatures to preserve the wavy morphology while achieving stress relief objectives.
- Multi-material compatibility: Explosion welding enables bonding of dissimilar metals (e.g., stainless steel to carbon steel, copper to steel, titanium to steel). The improved heat treatment defines material-specific temperature limits to prevent intermetallic compound formation at the interface.
- Post-explosion residual stress management: Explosion welding produces very high residual stresses (up to 800 MPa in some configurations). The improved heat treatment cycle is optimized for complete stress relief while maintaining bond integrity, validated by interface shear testing per ASTM E8.
- Large-diameter roll applications: Explosion welding is commonly used for large-diameter rolls (up to 1500 mm) where welding is impractical. The improved heat treatment accounts for the large section thickness through extended heating and cooling times, with instrumentation at multiple radial positions.
8. Qualification Building and Certification
The improvement of the heat treatment process for weld overlay rolls directly contributes to the company's qualification and certification framework:
- ISO 9001:2015 compliance: Documented process improvement with defined control limits, inspection records, and traceability satisfies the Quality Management System requirements for controlled processes.
- ASME "U" Stamp or "R" Stamp support: For rolls used in pressure equipment applications, the improved PWHT procedure supports ASME Section IX qualification, including procedure qualification records (PQR) demonstrating heat treatment effectiveness.
- Customer-specific qualification: Major steel mill operators (Baosteel, Shagang, Angang, etc.) require supplier qualification that includes documented heat treatment capability. The improved process, with supporting test data and NDT records, provides the evidence base for these qualification programs.
- NACE/AMPP compliance: For corrosion-resistant overlay rolls in chemical or marine environments, the improved heat treatment ensures compliance with NACE MR0175/ISO 15156 requirements for HIC/SOHIC resistance in the overlay and HAZ.
- API standards alignment: For oil and gas industry applications (conveyor rolls, pump shafts with overlay), the process supports API 5L and API 670 compliance through verified mechanical properties and NDT acceptance.
9. Conclusion and Strategic Significance
The improvement of the heat treatment process for weld overlay rolls represents a systematic engineering advancement that elevates Cladding Technology Shanxi Co., Ltd. from a welding service provider to a full-cycle engineering partner capable of delivering qualified, specification-compliant overlay products. By addressing the metallurgical challenges inherent in post-weld thermal processing—residual stress management, microstructural optimization, dimensional stability, and interface integrity—the improved process directly translates to:
- Higher product quality: Reduced defect rates, improved mechanical properties, and extended service life.
- Stronger market position: Ability to qualify for demanding OEM specifications and competitive bidding for high-value contracts.
- Technical leadership: Demonstrated process knowledge that differentiates the company in a market where many competitors offer only basic welding services without post-processing expertise.
- Customer value creation: Quantifiable benefits including reduced roll change frequency, lower maintenance costs, and increased production uptime for end users.
This capability, when integrated across all three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding), establishes a comprehensive process improvement framework that supports the company's strategic growth in premium cladding and overlay markets.