Effect of Tempering Treatment on Microstructure and Properties of Weld Overlay Metals on Large Support Rolls

1. Definition and Technical Principles

1.1 Fundamental Concept

Tempering treatment, also referred to as post-weld heat treatment (PWHT) or tempering annealing, is a controlled thermal process applied to weld overlay metals deposited on large-diameter support rolls used in hot strip mills and cold rolling mills. The process involves reheating the entire assembly—base roll substrate plus weld overlay layers—to a specified temperature below the lower critical temperature (Ac₁), holding for a calibrated duration, and then allowing controlled cooling (either furnace cool, air cool, or controlled-rate cool) to relieve residual stresses, refine microstructure, and optimize the mechanical properties of the overlay deposit.

1.2 Metallurgical Mechanisms

The weld overlay metals on support rolls typically consist of multi-layer deposits of high-alloy austenitic or martensitic stainless steels (e.g., 309, 310, 312, or custom hardfacing alloys). During TIG or MIG welding, rapid solidification produces a columnar dendritic microstructure with retained austenite, martensite, and potential carbide precipitation at grain boundaries. The tempering process achieves the following metallurgical transformations:

• Residual stress relief: Reduction of welding residual stresses (typically 200–450 MPa) to below 100 MPa through stress relaxation at elevated temperature.
• Microstructural homogenization: Dissolution of brittle intermetallic phases (e.g., sigma phase, Laves phase) and redistribution of carbides from grain boundaries into the matrix.
• Retained austenite stabilization: Conversion of unstable retained austenite (ε-martensite prone) into stable equilibrium austenite or controlled transformation to fine tempered martensite.
• Tempered martensite formation: In martensitic overlay systems, primary martensite transforms into tempered martensite with improved toughness and reduced hardness.
• Carbide precipitation control: Controlled precipitation of fine M₂₃C₆ or M₆C carbides that enhance wear resistance without embrittling the matrix.

1.3 Thermodynamic Basis

The tempering process operates on the principle of diffusion-controlled phase transformation. At temperatures between 500°C and 750°C, atomic diffusion rates increase sufficiently to allow: (a) carbon redistribution from supersaturated martensite to carbide nuclei, (b) coarsening of precipitates via Ostwald ripening, and (c) stress-driven dislocation rearrangement and annihilation. The specific temperature window is selected based on the alloy system to avoid sensitization (chromium carbide precipitation at grain boundaries in the 450–650°C range for austenitic stainless steels) while achieving adequate stress relief.

2. Category and Business Positioning

2.1 Process Classification

This technology falls under the category of Post-Weld Heat Treatment for Weld Overlay Systems, specifically targeting large-diameter cylindrical components (support rolls, backup rolls, and work rolls) with diameters typically ranging from 600 mm to 2,500 mm. It is an integral quality assurance step that bridges the gap between the welding fabrication process and the final performance requirements of the roll in service.

2.2 Business Positioning within Cladding Technology Shanxi Co., Ltd.

Within the company's three primary technology routes, tempering treatment is most directly associated with the TIG/MIG weld overlay route, where multi-layer deposits of hardfacing or corrosion-resistant alloys are applied to roll surfaces. However, the principles of controlled thermal treatment also inform the post-bonding annealing procedures used in hydraulic explosive bonding and explosion welding of clad plate assemblies. The technical knowledge captured in this study directly supports:

• WPS (Welding Procedure Specification) qualification and optimization
• WQR (Welder Qualification Record) validation under thermal cycling conditions
• Product acceptance testing and performance guarantee
• Customer technical support and failure analysis

3. Technical Purpose and Value

3.1 Primary Technical Objectives

1. Residual Stress Reduction: Large support rolls subjected to multi-layer weld overlay (typically 3–8 layers with total build-up of 25–80 mm) accumulate severe residual stresses. Without tempering, these stresses can exceed the yield strength of the overlay material, leading to premature cracking during service or during subsequent machining operations.
2. Hardness Uniformity: As-welded overlay metals often exhibit hardness gradients across the deposit thickness and between layers. Tempering equalizes hardness profiles, ensuring consistent wear performance across the roll surface.
3. Toughness Enhancement: The tempering process typically increases impact energy (Charpy V-notch) by 40–120% compared to the as-welded condition, which is critical for rolls operating under impact loading conditions.
4. Dimensional Stability: Elimination of residual stresses prevents post-weld distortion and dimensional drift during subsequent grinding and truing operations.
5. Corrosion Resistance Optimization: For austenitic overlay systems, controlled tempering avoids sensitization while promoting carbide dissolution, maintaining chromium availability in the matrix for corrosion resistance.

3.2 Quantitative Value to Product Delivery

Without proper tempering treatment, the service life of a weld-overlay support roll in a hot strip mill can be reduced by 30–60% due to early fatigue cracking, spalling, or delamination at the weld/base metal interface. Proper tempering extends roll life to design specifications, reducing customer downtime costs by hundreds of thousands of dollars per roll change event. This directly translates to measurable customer value and supports the company's reputation for delivering long-life, high-performance roll products.

4. Key Process and Implementation Points

4.1 Tempering Parameters for Common Overlay Systems

Overlay Alloy System	Tempering Temperature (°C)	Soak Time (per 25 mm thickness)	Cooling Method	Target Hardness (HRC)	Key Objective
309L / 312L Austenitic	870–920	1 hour (solution treatment)	Furnace cool to 600°C, then air cool	25–32	Carbide dissolution, full austenitization
310 / 310S High-Alloy Austenitic	1050–1100	1.5 hours	Furnace cool to 650°C, hold 2h, air cool	20–28	Complete homogenization, sigma phase elimination
410 / 420 Martensitic	400–550	2 hours	Controlled rate cool (≤50°C/h)	42–52	Tempered martensite, stress relief
Stellite 6 (Co-Cr-W Hardfacing)	700–750	1 hour	Furnace cool to 500°C, air cool	40–48	Carbide redistribution, stress relief
Custom High-Silicon Iron	800–900	1 hour	Furnace cool to 600°C, hold, air cool	55–65	Silicon carbide coarsening, ductility improvement

4.2 Heating Rate Control

The heating rate during tempering is critical for large support rolls due to their significant mass and thermal inertia. Key parameters include:

• Initial heating rate (room temperature to 200°C): Limited to 50–80°C/h to prevent thermal shock in the base metal and avoid cracking at the weld interface.
• Intermediate heating (200°C to tempering temperature): 80–150°C/h depending on roll diameter. For rolls exceeding 1,500 mm diameter, rates of 50–100°C/h are recommended.
• Maximum allowable heating rate: Generally calculated as 200–300°C/h per 25 mm of section thickness, but never exceeding 150°C/h for the complete assembly.
• Thermocouple placement: Minimum 4 thermocouples per roll—surface at weld overlay, mid-thickness at weld/base metal interface, surface at non-welded base metal, and core temperature monitoring.

4.3 Soak Time Calculation

The soak time (holding time at tempering temperature) is determined by the following factors:

• Effective thickness: Calculated as the sum of base metal thickness plus total weld overlay thickness, or half the roll diameter for solid rolls. Soak time = 1 hour per 25 mm of effective thickness, minimum 2 hours.
• Layer count effect: Multi-layer deposits (5+ layers) require 20–30% additional soak time to ensure complete stress relief through the full deposit depth.
• Temperature uniformity: The entire roll surface must be within ±15°C of the target tempering temperature before the soak timer begins.

4.4 Cooling Protocol

Cooling is as critical as heating in determining the final microstructure and properties. The cooling strategy must be selected based on the specific alloy system:

1. Furnace cooling (≤50°C/h): Required for high-carbon martensitic overlays and large-diameter rolls to prevent re-hardening or thermal cracking.
2. Controlled-rate cooling (50–100°C/h): Suitable for austenitic systems where some transformation control is needed.
3. Air cooling: Permitted only after the roll has been furnace-cooled to below 400°C (for martensitic systems) or 600°C (for austenitic systems).
4. Prohibited: Water quenching or forced air cooling above 400°C for any overlay system on support rolls.

4.5 Microstructural Evolution During Tempering

4.5.1 As-Welded Condition

The as-welded overlay metal typically exhibits: columnar dendrites growing from the weld interface; retained austenite (20–40% in high-nickel austenitic systems); primary and secondary carbides (Cr₇C₃, Cr₂₃C₆, Ni₃(Fe,Cr)₇) at dendrite boundaries; high dislocation density; and significant residual stresses (300–450 MPa).

4.5.2 After Tempering

Post-tempering microstructure shows: equiaxed grain structure (in austenitic systems after solution treatment); dissolved or coarsened carbides (reduced intergranular precipitation); stabilized retained austenite (reduced ε-transformation susceptibility); tempered martensite with fine carbide precipitates (in martensitic systems); reduced dislocation density; and residual stresses below 100 MPa.

4.6 Process Monitoring and Documentation

Each tempering cycle must be fully documented in accordance with quality management system requirements. The tempering log must include:

• Complete temperature-time curves (heating, soak, and cooling phases)
• Thermocouple calibration certificates (valid within 12 months)
• Furnace uniformity test results (TUS - Temperature Uniformity Survey)
• Atmosphere control records (if inert gas or vacuum is used)
• Roll identification, overlay specification, and layer count
• Operator qualification and shift supervisor sign-off

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

Standard	Scope of Application	Key Requirements for Tempering
ASME Section IX, QW-404	Post-weld heat treatment qualification	Tempering temperature, time, and rate must be within qualified WPS limits; PWHT must be completed within 6 hours of welding completion (for susceptible materials)
ASME Section II Part D	Material properties reference	Minimum hardness, tensile strength, and impact energy after PWHT as specified for each alloy
ASTM A335/A335M	Steel pipe for high-temperature service	Post-weld annealing requirements for welded components
ASTM A276	Stainless steel bars	Heat treatment specifications for austenitic and martensitic grades
NB/T 47014	Pressure vessel welding procedure qualification (China)	Post-weld heat treatment parameters and acceptance criteria for welded components
GB/T 985	Welding procedure qualification (China)	Requirements for post-weld treatment in welding procedure qualification records
GB/T 19542	Welding procedure specification for steel (China)	Thermal treatment parameters for overlay welding on steel components
ISO 15614-1	Qualification of fusion welding procedures	Post-weld heat treatment as a qualifying variable for procedure approval
ISO 3834-2	Quality requirements for fusion welding	Documentation and control of post-weld heat treatment operations
NACE MR0175/ISO 15156	Sulfide-resistant materials	Hardness limits and PWHT requirements for materials in H₂S service
API 670	Hot strip mill rolls	Performance requirements for weld overlay support rolls including PWHT
EN 15614-1	European welding procedure qualification	PWHT parameters as essential variables in procedure qualification

5.2 Acceptance Criteria for Tempered Overlay Metals

5.2.1 Mechanical Properties

• Hardness: Must fall within the specified range for the overlay alloy (typically verified by Vickers or Rockwell C testing at 3 locations across the deposit thickness). Deviation from specification limits indicates improper tempering.
• Tensile strength: Minimum values per ASTM A276 or equivalent for the specific alloy grade. Coupon orientation: transverse to roll axis, through full overlay thickness.
• Impact energy: Minimum 27 J at 25°C (or 40 J at -20°C for cold-service applications) per Charpy V-notch test. Three specimens per heat treatment batch.
• Hardness gradient: Maximum difference of 5 HRC between adjacent measurement points across the overlay thickness (indicating uniform tempering).

5.2.2 Residual Stress

• Maximum residual stress: 100 MPa (compressive or tensile) measured by X-ray diffraction or hole-drilling method at the weld overlay surface and at the weld/base metal interface.
• Stress gradient: No abrupt transitions exceeding 50 MPa/mm across the overlay thickness.

5.2.3 Microstructural Acceptance

• No continuous grain boundary carbide precipitation exceeding 5% of grain boundary length (for austenitic systems).
• No sigma phase, Laves phase, or other brittle intermetallics detectable by optical microscopy at 200× magnification.
• Grain size: ASTM E112 equivalent of 4–8 for austenitic overlays after solution treatment.
• No untempered martensite exceeding 10% of microstructure area (for martensitic systems).

5.2.4 NDT Requirements

• Magnetic particle inspection (MPI) per ASTM E1444: No linear indications exceeding 1.5 mm in length on the overlay surface after tempering.
• Ultrasonic testing (UT) per ASTM E164 or NB/T 47013: No indications exceeding acceptance limits at the weld/base metal interface.
• Dye penetrant inspection (DPI) per ASTM E709: No surface-breaking defects visible under controlled illumination.

6. Common Risks and Controls

6.1 Thermal Cracking During Heating

Risk: Rapid heating of large-diameter support rolls can cause differential thermal expansion between the overlay metal and base metal, generating tensile stresses that exceed the fracture toughness of the weld interface, resulting in circumferential or axial cracking.

Controls:

• Strict adherence to maximum heating rate limits (≤80°C/h initial, ≤150°C/h subsequent)
• Preheating to 200–300°C before beginning the main heating cycle
• Use of multiple thermocouples to monitor temperature gradients across the roll cross-section
• Limiting maximum temperature differential between inner and outer surfaces to 50°C at any point during heating

6.2 Over-Tempering (Excessive Softening)

Risk: Excessive tempering temperature or prolonged soak time leads to carbide coarsening, grain growth, and significant hardness reduction, compromising wear resistance of the overlay.

Controls:

• Temperature control accuracy of ±10°C throughout the soak period
• Strict adherence to calculated soak time (no exceeding by more than 10%)
• Post-tempering hardness verification at multiple locations
• Documentation of actual temperature-time curves for traceability

6.3 Under-Tempering (Incomplete Stress Relief)

Risk: Insufficient temperature or soak time results in residual stresses remaining above acceptable limits, leading to dimensional instability during subsequent machining and potential in-service cracking.

Controls:

• Verification that minimum temperature was achieved at all thermocouple locations (not just the hottest point)
• Residual stress measurement post-tempering to confirm stress relief effectiveness
• Dimensional stability check after tempering (measure roll diameter at multiple points before and after)
• Charpy impact testing to verify toughness improvement

6.4 Sensitization (Austenitic Systems)

Risk: For chromium-nickel austenitic overlays (309, 312, 316), prolonged exposure to the 450–650°C range causes chromium carbide precipitation at grain boundaries, depleting chromium from adjacent regions and reducing corrosion resistance.

Controls:

• Minimize time spent in the sensitization range (450–650°C) by rapid passage during heating and cooling
• For austenitic systems requiring solution treatment, perform full solution treatment (870–1100°C) followed by controlled cooling through the sensitization range at rates exceeding 200°C/h
• Post-tempering corrosion testing (salt spray per ASTM B117 or intergranular corrosion per ASTM A262 Practice E) for critical applications

6.5 Distortion and Dimensional Change

Risk: Differential contraction during cooling can cause roll barrel distortion, affecting the geometric accuracy required for mill operation.

Controls:

• Controlled cooling rates to minimize thermal gradients
• Use of cooling fixtures or supports to maintain roll geometry during cooling
• Post-tempering dimensional inspection (diameter, taper, and out-of-round per API 670)
• Allowance for post-tempering grinding/truing in the manufacturing sequence

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Route

The tempering treatment technology is most directly and extensively applied in the TIG/MIG weld overlay route for support roll fabrication. Key application scenarios include:

• Hot strip mill backup rolls: Large-diameter (1,200–2,500 mm) solid rolls with 40–80 mm multi-layer weld overlay of 309L/310L transition followed by Stellite or high-silicon iron wear layers. Tempering is mandatory after the final overlay layer is completed, typically requiring 8–16 hours total cycle time.
• Cold rolling mill work rolls: Smaller diameter (300–600 mm) rolls with 10–25 mm overlay of martensitic or austenitic hardfacing. Tempering at 400–550°C for 2–4 hours to achieve target hardness of 55–65 HRC while maintaining impact toughness.
• Ring rolls for plate mills: Large forged rings with weld overlay for wear protection. Tempering must account for the complex geometry and thick section, requiring extended soak times (12–24 hours).
• Repair and refurbishment: Tempering of field-repaired rolls returned from customer service. Critical for restoring mechanical properties after localized re-welding.

7.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding (HIB) produces a metallurgical bond without melting, the subsequent thermal treatment of HIB-clad assemblies follows principles analogous to tempering treatment:

• Post-bond annealing: HIB-clad plate assemblies (e.g., 316L/SAE 1010 or Inconel 625/SAE 4140) are annealed at 600–900°C for 2–4 hours to relieve bonding-induced residual stresses and improve interfacial toughness.
• Stress relief for clad plate: Similar to tempering of weld overlay, the goal is to reduce residual stresses at the bond interface from 200–350 MPa (as-bonded) to below 80 MPa (annealed).
• Microstructural stabilization: Annealing stabilizes the deformed microstructure at the bonding interface, preventing delayed cracking during subsequent cold forming or machining operations.

7.3 Explosion Welding Route

Explosion welding produces clad assemblies with unique interfacial microstructures characterized by adiabatic shear zones, vortex patterns, and high residual stresses. Post-explosion thermal treatment is essential:

• Stress relief annealing: Explosion-welded clad plates (e.g., 304L/SAE 1045, Inconel 718/SAE 4130) are stress-relieved at 650–800°C for 2–6 hours depending on plate thickness. This reduces explosion-induced residual stresses (typically 300–500 MPa) to acceptable levels.
• Heat-affected zone optimization: The thin HAZ produced during explosion welding contains refined grains and precipitate-free zones. Controlled annealing restores precipitate distribution in age-hardenable alloys (e.g., Inconel 718) while maintaining interface integrity.
• Interfacial toughness improvement: Annealing improves peel test strength and shear bond strength by 15–30% compared to the as-explosion-welded condition, directly improving product qualification for demanding applications.

8. Contribution to Qualification Building and Customer Value

8.1 Welding Procedure Specification (WPS) Qualification

The tempering treatment parameters (temperature, time, rate, atmosphere) constitute an essential variable in welding procedure qualification per ASME Section IX, QW-404 and ISO 15614-1. Mastery of tempering technology enables the company to:

• Qualify WPS for complex multi-layer overlay systems with specific PWHT requirements
• Demonstrate process capability for critical applications requiring verified PWHT (nuclear, aerospace, oil and gas)
• Expand qualification scope to include new alloy systems and component geometries
• Provide customers with fully qualified procedures that guarantee performance

8.2 Product Certification and Traceability

Complete tempering documentation forms an integral part of the product certification package, including:

• Material test reports (MTR) with post-tempering mechanical properties
• Welding Procedure Qualification Records (WPQR) with PWHT variables
• Heat treatment logs with complete temperature-time curves
• NDT reports confirming absence of tempering-induced defects
• Compliance declarations per applicable standards (ASME, API, NB/T)

8.3 Customer Value Proposition

The technical competence in tempering treatment directly translates to measurable customer benefits:

1. Extended roll life: Properly tempered overlay metals exhibit 40–80% longer service life compared to untempered equivalents, reducing customer change-out frequency and mill downtime.
2. Predictable performance: Consistent tempering produces uniform properties across the roll surface, ensuring predictable wear patterns and maintenance scheduling.
3. Reduced warranty claims: Elimination of tempering-related defects (cracking, spalling, premature failure) reduces after-sales service costs and enhances customer confidence.
4. Competitive differentiation: Demonstrated expertise in PWHT for large rolls provides a competitive advantage over suppliers who lack comprehensive thermal treatment capability.
5. Technical consulting capability: Ability to recommend optimal tempering parameters for customer-specific applications enhances the company's role as a technical partner rather than a mere supplier.

8.4 Quality Management System Integration

The tempering treatment process must be fully integrated into the company's quality management system per ISO 9001 and ISO 3834-2 requirements. This includes:

• Documented procedures for all tempering operations (heating, soak, cooling, testing)
• Qualified personnel with demonstrated competence in thermal treatment operations
• Calibrated and verified equipment (furnaces, thermocouples, data loggers)
• Internal audit programs covering thermal treatment operations
• Corrective and preventive action systems for tempering-related nonconformities
• Supplier qualification for external tempering services (when in-house capacity is exceeded)

9. Summary and Forward Recommendations

The tempering treatment of weld overlay metals on large support rolls represents a critical process step that determines the final performance, service life, and reliability of the delivered product. Mastery of this technology—encompassing metallurgical understanding, process parameter optimization, equipment capability, and quality documentation—provides Cladding Technology Shanxi Co., Ltd. with a significant technical advantage in the competitive roll refurbishment and fabrication market.

Key recommendations for continued development include:

1. Investment in large-capacity furnaces: Capable of accommodating rolls up to 3,000 mm diameter with ±10°C temperature uniformity.
2. Development of process simulation capabilities: Finite element analysis (FEA) of thermal treatment cycles to predict residual stress evolution and optimize parameters before physical trials.
3. Expansion of metallurgical testing laboratory: In-house capability for X-ray diffraction (residual stress), SEM/EDS (microstructure), and advanced mechanical testing (fatigue, fracture toughness).
4. Standardization of tempering databases: Systematic collection of tempering parameters, microstructural outcomes, and service performance data to build a proprietary knowledge base.
5. Personnel development: Training and certification of thermal treatment specialists with deep metallurgical understanding of overlay alloy systems.

Technical Note: The knowledge captured in this study on tempering effects on weld overlay microstructure and properties directly supports the company's qualification portfolio, enables delivery of certified products meeting international standards (ASME, API, NB/T, ISO), and provides a foundation for customer technical support that enhances long-term business relationships in the metallurgical equipment market.