Effect of Tempering Temperature on Wear Resistance of Weld Overlay Molds

1. Definition and Fundamental Principles

1.1 Tempering in Weld Overlay Context

Tempering is a post-weld thermal treatment process applied to weld overlay deposits on tool and die components to modify the microstructure of the weld metal, thereby optimizing the balance between hardness, toughness, and wear resistance. In the context of weld overlay molds — components where a hardfacing or wear-resistant alloy is deposited onto a base substrate to extend service life — the tempering temperature serves as the single most critical process variable governing the final tribological performance of the overlay.

The fundamental metallurgical principle involves the controlled decomposition of retained austenite, martensite, and carbide phases within the weld deposit. During the welding process (typically TIG or MIG), the rapid solidification and subsequent air cooling produce a metastable microstructure dominated by martensite and retained austenite, with dispersed carbide particles. Without proper tempering, this as-welded structure exhibits high hardness but severely compromised fracture toughness, rendering the overlay susceptible to chipping, cracking, and premature failure under cyclic or impact loading conditions typical in molding and forming operations.

1.2 Microstructural Evolution with Tempering Temperature

As tempering temperature increases, a well-documented sequence of microstructural transformations occurs:

• Low-temperature tempering (150–250 °C): Carbon precipitation from supersaturated martensite occurs, forming ε-carbides. Hardness decreases slightly (2–5 HRC) while residual stress relief begins. Retained austenite remains largely untransformed.
• Intermediate-temperature tempering (250–400 °C): ε-carbides transform to cementite (Fe₃C). Dislocation density decreases significantly. Hardness drops more steeply (8–15 HRC) while impact toughness improves substantially. This range is critical for most mold steel overlays.
• High-temperature tempering (400–600 °C): Cementite coarsens and spheroidizes. The matrix transitions toward a tempered martensite or sorbite structure. Hardness drops significantly (20–35 HRC) but fracture toughness and ductility reach peak values.

1.3 Wear Resistance Mechanisms in Tempered Overlays

Wear resistance in weld overlay molds is governed by three primary mechanisms, each affected differently by tempering temperature:

• Abrasion resistance: Dependent on matrix hardness and carbide hardness/distribution. Higher carbide volume fraction and finer dispersion correlate with superior abrasive wear resistance.
• Adhesive wear resistance: Related to the ability of the overlay to maintain surface integrity under sliding contact. Requires adequate substrate support beneath the hard surface layer.
• Impact and fatigue resistance: Governed by the toughness of the matrix between carbides. Insufficient tempering leads to catastrophic brittle failure; excessive tempering reduces the hardness threshold needed to resist deformation.

2. Technical Purpose and Engineering Value

2.1 Process Optimization Objective

The systematic study of tempering temperature effects on wear resistance addresses a fundamental engineering challenge: identifying the optimal tempering window that maximizes service life without compromising structural integrity. This is not a single-variable optimization — the response of wear resistance to tempering temperature is non-linear and material-specific, depending on the overlay alloy chemistry, carbide type, welding process parameters, and base material compatibility.

2.2 Value Contribution to the Company

• Qualification building: Documented tempering studies form the evidentiary basis for Welding Procedure Specifications (WPS) and Welding Procedure Qualification Records (WPQR) under applicable standards. Each validated tempering parameter set constitutes a qualified process variant that expands the company's certified capability envelope.
• Product delivery reliability: Precise tempering temperature control reduces field failure rates, warranty claims, and customer downtime. Predictable performance enables confident specification of service life in contractual deliverables.
• Customer value: Ability to tailor overlay performance to specific wear conditions (abrasive, adhesive, erosive, impact) through controlled tempering demonstrates engineering sophistication and provides measurable ROI through extended component life.
• Knowledge retention: Systematic documentation of tempering trials creates an institutional knowledge base that accelerates future project execution and reduces reliance on individual operator experience.

3. Key Process and Implementation Points

3.1 Critical Tempering Parameters

Parameter	Typical Range	Influence on Wear Resistance	Control Method
Tempering Temperature	200–600 °C	Primary variable; governs hardness-toughness balance	Furnace thermocouple with ±5 °C accuracy
Soak Time	1–4 hours (per 25 mm thickness)	Ensures thermal equilibrium through section	Timer-controlled furnace with logging
Heating Rate	50–150 °C/hour	Prevents thermal shock cracking in overlay	Furnace program ramp control
Cooling Rate	25–75 °C/hour (furnace cool)	Prevents re-tempering effects and distortion	Controlled furnace cool or still air cool
Atmosphere	Inert (N₂/Ar) or vacuum	Prevents oxidation decarburization of overlay surface	Gas purge or vacuum pump
Overlay Thickness	1.5–6.0 mm per pass	Affects heat input and tempering uniformity	Multi-pass deposition with interpass temp control

3.2 Alloy-Specific Tempering Windows

Overlay Alloy Type	As-Welded Hardness	Optimal Tempering Temp	Post-Temper Hardness	Wear Resistance Rating
High-Carbon Martensitic (e.g., D2, M2)	58–62 HRC	200–300 °C	55–60 HRC	Excellent abrasive, moderate impact
Medium-Carbon Martensitic (e.g., H13, 4Cr5MoSiV)	52–58 HRC	540–620 °C	45–50 HRC	Good abrasive, excellent impact/hot
High-Speed Steel (e.g., M2, M42)	62–68 HRC	540–580 °C (double temper)	60–65 HRC	Superior hot wear, good abrasive
Leaded/Cobalt Alloy (e.g., Stellite 6)	40–48 HRC	700–800 °C (solution) or no temper	40–45 HRC	Exceptional erosion/corrosion
Nickel-Aluminum (e.g., NiAl 5)	30–35 HRC	1000–1100 °C (solution) + water quench	45–50 HRC	Outstanding thermal fatigue
High-Chromium Cast Iron (e.g., HCR)	50–55 HRC (as-cast)	550–600 °C	48–52 HRC	Good abrasive, good impact

3.3 Implementation Protocol

1. Pre-tempering inspection: Verify overlay quality — no cracks, porosity, or undercut. Confirm overlay thickness uniformity within ±0.5 mm of nominal. Document as-welded hardness at minimum 3 locations across the overlay surface.
2. Fixturing: Support component to prevent distortion. Use ceramic or refractory packing for critical geometries. Ensure thermocouple placement at the thickest section of the overlay, not on the base material surface.
3. Furnace loading: Load only after furnace reaches 100 °C to minimize thermal shock. Position component centrally with adequate clearance for uniform heat circulation.
4. Heating cycle execution: Follow programmed ramp rate. Log temperature continuously. Allow soak time calculated as 1 hour per 25 mm of maximum section thickness (minimum 2 hours).
5. Cooling: Furnace cool to below 150 °C before removal. For critical components, cool in still air to room temperature. Avoid forced air cooling above 300 °C.
6. Post-tempering verification: Measure hardness at the same locations as pre-tempering. Perform visual and dimensional inspection for distortion. Conduct magnetic particle inspection (MPI) if applicable.

3.4 Wear Testing Methodology

Validation of tempering temperature effects requires standardized wear testing. The following methods are recommended for qualification trials:

• Abrasive wear: ASTM G65 (Pin-on-Disk) or ASTM G99 (Reciprocating Sliding) using standardized alumina or carbide counterfaces. Report wear volume in mm³ per unit load-distance.
• Impact wear: ASTM G105 (Ball-on-Ring Impact) simulating chipping and plastic deformation under impact loading.
• Erosion wear: ASTM G74 or ASTM G76 (Sand/Rubber-wheel) for erosive service conditions.
• Field-equivalent testing: Accelerated service simulation using actual production molds under controlled conditions with periodic weight-loss measurements.

4. Applicable Standards and Acceptance Criteria

4.1 Welding Procedure Standards

• ASME Section IX: Governs qualification of welding procedures for overlay welds. Qualification group for overlay depends on the filler metal classification. Post-weld heat treatment (PWHT) parameters including tempering must be within the qualified range.
• ISO 15614-1: Qualification of welding procedures for metallic materials. Defines essential and non-essential variables for overlay welding. Tempering temperature is classified as a non-essential variable for procedure qualification but critical for performance qualification.
• GB/T 985.1 / GB/T 985.2: Chinese national standards for welding procedure qualification testing. Applies to domestic projects requiring Chinese regulatory compliance.
• NB/T 47014: Chinese standard for qualification of welding procedures for pressure equipment. Relevant when overlay is applied to pressure vessels or piping components.

4.2 Performance and Acceptance Standards

• ASTM A220: Standard specification for cast-iron overlays for valves, pumps, and similar hardware. Defines minimum hardness, composition, and performance requirements for overlay materials.
• ASTM B777: Standard specification for nickel-chromium-tungsten-cobalt alloy (Stellite-type) castings. Provides performance benchmarks for cobalt-based overlays.
• ASTM G65 / G99 / G74: Wear testing standards referenced above for quantitative performance evaluation.
• GB/T 16491: Chinese standard for wear testing of materials. Applicable for domestic qualification documentation.
• API 571: Damage mechanisms in petroleum refining — provides context for overlay selection and performance expectations in petrochemical service.
• NACE MR0175 / ISO 15156: Materials for H₂S-containing environments — relevant when overlay must resist sulfide stress cracking in addition to wear.

4.3 Acceptance Criteria for Tempered Overlays

Criterion	Acceptance Requirement	Verification Method
Hardness uniformity	Within ±3 HRC of nominal specified value across overlay surface	Rockwell C hardness testing, minimum 3 points per 100 cm²
Hardness gradient	No more than 5 HRC change per 1 mm depth from surface	Micro-indentation hardness profile
Crack-free condition	No cracks visible at 5× magnification in overlay or heat-affected zone	Magnetic Particle Inspection (ASTM E1444) or Visual (ASTM E947)
Distortion	Within original machining tolerance ±0.05 mm	Coordinate measuring machine (CMM) or precision gauge
Wear resistance	Minimum 1.5× improvement over base material under specified test conditions	ASTM G65/G99 wear testing per qualified procedure
Impact toughness	Minimum 20 J at 23 °C (Charpy V-notch) for impact-critical applications	ASTM E23 Charpy impact testing

5. Common Risks and Controls

5.1 Technical Risks

Risk	Cause	Consequence	Mitigation
Under-tempering (excessive hardness, low toughness)	Temperature too low, insufficient soak time, poor thermocouple placement	Brittle fracture, chipping, catastrophic mold failure	Calibrated thermocouple at overlay thickness, extended soak, hardness verification before release
Over-tempering (excessive softening)	Temperature too high, extended soak, furnace calibration error	Reduced wear resistance, plastic deformation under load, shortened service life	Furnace calibration verification, temperature logging, hardness verification, secondary temper if needed
Temper embrittlement	Slow cooling through 250–400 °C range in susceptible alloys	Reduced fracture toughness despite acceptable hardness	Controlled cooling rate, avoid dwelling in embrittlement range, impact testing for critical applications
Overlay cracking during tempering	High residual stress from welding, thermal mismatch with base material	Service failure, component rejection	Stress-relief anneal before final temper, low heating rate, interpass temperature control during welding
Decarburization	Tempering in oxidizing atmosphere (air)	Softened surface layer, dramatically reduced wear life	Inert atmosphere (N₂/Ar) or vacuum tempering, protective coating for non-critical areas
Distortion	Thermal expansion mismatch, inadequate fixturing	Out-of-tolerance geometry, re-machining required, possible rejection	Proper fixturing, symmetric heating, controlled ramp rates, post-temper dimensional verification

5.2 Process Control Measures

1. Furnace calibration: Annual calibration of tempering furnaces per ISO/IEC 17025 requirements. Daily verification with certified reference thermocouples.
2. Temperature uniformity survey (TUS): Annual TUS per ASTM E2207 to verify ±5 °C uniformity within the working zone of the furnace.
3. Heat treatment documentation: Complete cycle logs including temperature-time curves, furnace identification, operator sign-off, and traceable to individual component serial numbers.
4. First-article hardness verification: For each new tempering parameter set, perform full hardness profile testing before production release.
5. Statistical process control: Track hardness values across production runs using control charts. Investigate and correct any trend toward specification limits.

6. Application Across Company Technology Routes

6.1 TIG/MIG Weld Overlay Route

The tempering temperature study directly supports the TIG/MIG weld overlay route, which is the primary method for applying wear-resistant overlays to molds, dies, and tool components. Key integration points include:

• WPS development: Each qualified tempering temperature constitutes a distinct PWHT parameter within the WPS. Multiple tempering variants can be qualified under a single WPQR envelope, expanding the procedure's applicability.
• Multi-pass overlay tempering: For thick overlays (3–6 mm), interpass tempering between deposition passes controls residual stress and prevents cracking. The final tempering temperature determines the ultimate wear performance.
• Transition layer compatibility: When a transition layer (e.g., 309L stainless) is deposited between base and overlay, the tempering temperature must not exceed the solution temperature of the transition layer. This constrains the maximum tempering temperature for multi-layer overlays.
• Post-weld machining: For precision molds requiring dimensional accuracy, tempering is performed before final machining. The tempering temperature determines the machinability of the overlay material.

6.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding (HEB) produces metallurgical bonds without melting, the resulting clad plate may require heat treatment to optimize properties. Tempering temperature considerations in this context include:

• Post-bond heat treatment: Clad plates produced by HEB may be tempered to relieve residual stresses from the bonding process and to optimize the mechanical properties of both base and cladding layers. The tempering temperature must be compatible with both materials — typically constrained by the lower-temperature limit of the cladding layer.
• Wear component fabrication: Clad plates from HEB can be fabricated into mold inserts, where the clad surface serves as the wear face. Subsequent tempering of the complete assembly must preserve the bond integrity while optimizing wear properties.
• Hybrid processing: In some applications, HEB produces the base clad structure, and TIG weld overlay adds a final wear layer on top. The tempering sequence must account for both the clad bond and the weld overlay, requiring careful thermal cycle design.

6.3 Explosion Welding Route

Explosion welding (EW) produces clad plates with high bond quality and minimal dilution. Tempering considerations for EW-produced components include:

• Microstructure optimization: The cladding layer in EW typically has a cold-worked, deformed microstructure from the high-velocity collision. Tempering can partially anneal this cold work to improve ductility while maintaining sufficient hardness for wear applications.
• Tempering temperature constraints: The maximum tempering temperature is limited by the bond interface. Excessive temperatures can cause intermetallic formation at the interface, degrading bond strength. Typical maximum tempering temperatures for EW clad plates are 550–600 °C for steel-steel bonds and lower for dissimilar metal combinations.
• Wear component qualification: When EW clad plates are used for mold components, the tempering temperature is selected to achieve the required hardness-toughness balance in the cladding layer while maintaining bond strength above the minimum specified value (typically 200–300 MPa shear strength per ASTM A491).
• Post-welding tempering: If EW clad plates are subsequently machined into components that receive additional weld overlay, the final tempering must be compatible with both the EW bond and the weld overlay. This requires comprehensive thermal compatibility analysis.

7. Qualification Building and Organizational Impact

7.1 WPS/WPQR Expansion

Systematic tempering temperature studies directly expand the company's qualified procedure library. Each validated tempering temperature range, when documented through proper testing and evaluation, constitutes a qualified PWHT parameter that can be incorporated into WPS documentation. This expansion has direct commercial value:

• Broader alloy coverage — ability to qualify more overlay materials for customer applications
• Expanded thickness ranges — understanding of how section thickness affects tempering requirements
• Multi-material capability — documented compatibility of tempering parameters across base/clad/overlay combinations
• Regulatory acceptance — qualified procedures meet regulatory requirements for pressure equipment, nuclear components, and critical industrial applications

7.2 Customer Deliverables Enhancement

Documented tempering qualification provides customers with:

• Predictable performance: Quantified wear resistance data allows customers to model expected service life and plan maintenance intervals with confidence.
• Traceable quality: Each delivered component can be traced to a specific qualified tempering procedure, providing full quality documentation for customer audits.
• Performance guarantees: With validated tempering parameters, the company can offer performance-based warranties (e.g., "minimum X hours of service life") backed by qualification data.
• Customization capability: Ability to tailor tempering to specific customer wear conditions demonstrates engineering value-add beyond commodity welding services.

7.3 Continuous Improvement Cycle

The tempering temperature study establishes a foundation for ongoing process improvement:

1. Baseline establishment: Current tempering parameters documented as baseline performance.
2. Variable optimization: Systematic variation of tempering temperature (and secondary variables like soak time, cooling rate) to identify performance improvements.
3. Standardization: Optimized parameters incorporated into standard operating procedures and WPS documentation.
4. Technology transfer: Documented knowledge enables training of new personnel and consistent execution across shifts and facilities.
5. Customer feedback integration: Field performance data feeds back into tempering parameter refinement, creating a closed-loop improvement system.

8. Conclusion and Recommendations

The systematic study of tempering temperature effects on weld overlay wear resistance represents a high-value technical capability that differentiates the company in the competitive weld overlay market. Tempering is not merely a post-weld cleanup operation — it is the process step that determines whether an overlay will perform reliably in service or fail prematurely. Mastery of this variable, documented through proper qualification testing and incorporated into WPS/WPQR documentation, provides:

• Technical authority in customer qualification reviews
• Reduced field failure rates and warranty costs
• Expanded product capability through qualified parameter ranges
• Institutional knowledge that survives personnel changes
• Competitive differentiation in bids and tenders requiring documented process qualification

It is recommended that the company maintain an active tempering qualification program, periodically re-validate existing procedures as materials and testing capabilities evolve, and extend the study to include combined effects of tempering with other process variables (welding parameters, base material preheat, interpass temperature) for comprehensive process understanding.