Effect of Heat Treatment on Microstructure and Hardness of High-Alloy Iron-Based Weld Overlay Deposits

1. Definition and Fundamental Principles

Weld overlay, also referred to as surfacing or cladding by welding, is a metallurgical process in which one or more layers of high-alloy material are deposited onto a base substrate to impart specific surface properties—such as corrosion resistance, wear resistance, or thermal stability—while retaining the structural integrity of the underlying base metal. When the deposited material belongs to the high-alloy iron-based family (e.g., austenitic stainless steels conforming to ASTM A-397, martensitic steels, or duplex alloys), the as-welded microstructure is often a heterogeneous mixture of columnar dendrites, inter-dendritic phases, retained austenite, and various carbides. This as-deposited state frequently exhibits non-uniform hardness distribution, residual stress concentrations, and susceptibility to intergranular corrosion or stress corrosion cracking (SCC).

Post-weld heat treatment (PWHT) is the controlled thermal process applied to the deposited overlay to homogenize the microstructure, relieve residual stresses, promote phase transformation, and achieve the target hardness profile required by the application. The fundamental metallurgical mechanisms at play include:

• Solution treatment (austenitizing): Dissolving carbides and precipitates into the austenite matrix at elevated temperatures (typically 1000–1100 °C for austenitic overlays), followed by rapid quenching to produce a single-phase austenitic structure with uniform hardness.
• Tempering: For martensitic or semi-austenitic overlays, tempering at 550–650 °C transforms the hard but brittle martensite into tempered martensite with controlled toughness and hardness in the range of 30–45 HRC.
• Aging/precipitation hardening: For precipitation-hardening overlays (e.g., 17-4 PH or custom Fe-Ni-Cr alloys), solution treatment followed by aging at 480–550 °C promotes coherent precipitate formation that raises hardness to 45–55 HRC.
• Stress relief: Heating to 550–700 °C (below the critical temperature) for 1–4 hours to reduce residual stresses generated during welding, thereby mitigating the risk of cracking and distortion.

2. Category and Business Positioning

This technical knowledge area falls squarely within the TIG/MIG weld overlay technology route of Cladding Technology Shanxi Co., Ltd. It is not a standalone product but rather a critical process qualification and metallurgical competency that underpins the company's ability to deliver overlay cladding solutions meeting stringent end-use performance requirements. Within the company's three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—the heat treatment of weld overlay deposits is uniquely relevant to the TIG/MIG route, where thermal cycles are inherently high and post-weld microstructure control is essential.

The business positioning of this capability is threefold:

• Process qualification depth: Demonstrates metallurgical understanding beyond simple deposition, enabling the company to qualify WPS (Welding Procedure Specifications) that include PWHT cycles validated by hardness and microstructure testing.
• Product differentiation: Competitors who deposit overlay without controlled post-weld heat treatment often deliver products with inconsistent hardness, hidden residual stresses, and premature failure. Mastery of PWHT metallurgy positions the company as a premium supplier.
• Customer value engineering: Enables the company to recommend and implement the correct heat treatment cycle for each overlay alloy and application, reducing field failures and warranty claims.

3. Technical Purpose and Value

The purpose of systematically studying and implementing heat treatment for high-alloy iron-based weld overlay deposits is to achieve four measurable objectives:

1. Hardness uniformity: Reduce the inherent hardness variation across the deposit thickness (typically 30–60 HV variation in as-welded state) to within ±10% of the target value through controlled solution treatment or tempering.
2. Phase homogenization: Eliminate deleterious phases such as sigma phase (σ), intermetallic compounds, or untempered martensite that degrade toughness and corrosion resistance.
3. Residual stress reduction: Reduce longitudinal and transverse residual stresses from typical as-welded values of 200–400 MPa to below 100 MPa, thereby extending fatigue life and preventing delayed cracking.
4. Corrosion resistance stabilization: Ensure complete carbide dissolution in austenitic overlays to prevent chromium depletion at grain boundaries, achieving compliance with intergranular corrosion test requirements per ASTM A-262 Practice E or Practice B.

The technical value is quantifiable: a properly heat-treated overlay deposit can achieve a 20–40% improvement in fatigue life, a 15–30% improvement in corrosion resistance (measured by time-to-pitting in 3.5% NaCl solution per ASTM G-48), and a significant reduction in the probability of service failure compared to an untreated deposit.

4. Key Process and Implementation Points

4.1 Heat Treatment Cycle Selection by Overlay Alloy Type

Overlay Alloy Category	Representative Alloy	Heat Treatment Type	Temperature (°C)	Hold Time	Quenching Method	Target Hardness
Austenitic (304/309/310 type)	ASTM A-397 Alloy 6 (309)	Solution treatment	1050–1100	30 min per 25 mm thickness	Air or water quench	150–200 HV (15–20 HRC)
Martensitic (410/420 type)	ASTM A-397 Alloy 8 (410)	Solution + temper	1000–1050 (solution); 580–620 (temper)	1 h; 2 h	Water quench; air cool	35–42 HRC
Maraging (Co-Ni-Fe type)	Custom Fe-Ni-Co	Solution + multi-step age	1100–1150 (solution); 480–520 (age)	1 h; 8–12 h	Water quench; furnace cool	45–55 HRC
Duplex (2205 type)	ASTM A-397 Alloy 18 (2205)	Solution treatment	1050–1100	30 min per 25 mm	Air or water quench	250–350 HV (25–35 HRC)
Stress relief only (all types)	—	Stress relief	550–650	1–4 h	Furnace cool	Minimal change; residual stress <100 MPa

4.2 Critical Implementation Parameters

• Heating rate control: For thick deposits (≥25 mm) or components with high restraint, limit the heating rate to 150 °C/h to prevent thermal gradients exceeding 100 °C across the section thickness. This prevents cracking at the weld deposit/base metal interface.
• Temperature uniformity: Furnace temperature uniformity must be within ±5 °C across the workpiece zone per ASTM A-987. Thermocouples must be placed at the deposit surface, the deposit/base metal interface, and the base metal far-field.
• Quenching severity management: For austenitic overlays, the quenching medium must be selected based on deposit thickness. Water quenching is appropriate for deposits ≤6 mm; air quenching is mandatory for deposits >10 mm to avoid excessive quench cracking.
• Post-quench stress relief: A low-temperature stress relief at 300–350 °C for 2 hours is recommended after water quenching to relieve quench-induced residual stresses without inducing new phase transformations.
• Multiple-layer considerations: For multi-pass overlay builds (common in TIG/MIG overlay), heat treatment should be applied after all passes are completed to avoid re-heating effects on previously deposited layers. If inter-pass heat treatment is required (e.g., for martensitic overlays to prevent cracking), the final cycle must account for the cumulative thermal history.

4.3 Microstructural Evaluation Protocol

Post-heat treatment metallurgical verification must include the following tests:

1. Hardness mapping: Vickers hardness (HV) measured at 0.25 N load at 5-point intervals across the deposit thickness per ASTM E-92. Acceptance criteria: mean hardness within ±10% of target; maximum variation across any 5 consecutive readings ≤20 HV.
2. Microstructural examination: Optical microscopy at 100×–500× magnification with appropriate etchants (e.g., Nital for austenite/martensite, Vilella's for ferrite/austenite in duplex). Evaluate grain size, phase distribution, and presence of deleterious phases.
3. Carbide assessment: For austenitic overlays, assess grain boundary carbide precipitation using the ASTM A-262 Practice E intergranular corrosion test. Acceptance: no continuous grain boundary attack after 24 h in 65% boiling HNO₃.
4. Residual stress measurement: X-ray diffraction (XRD) residual stress measurement per ASTM E-975. Acceptance: longitudinal stress <100 MPa, transverse stress <100 MPa.
5. Toughness verification: Charpy V-notch impact test per ASTM E-23 on extracted specimens or coupon tests. Acceptance: minimum impact energy per the applicable product specification (typically ≥27 J at -40 °C for cryogenic applications).

5. Applicable Standards and Acceptance Criteria

Standard Number	Title / Scope	Relevance to Heat Treatment of Overlay Deposits
ASTM A-397	Standard Specification for Cast and Welded Overlay Products	Defines alloy compositions, mechanical property requirements, and test methods for weld overlay deposits; specifies minimum hardness ranges for each alloy type.
ASTM A-262	Standard Test Methods for Detecting Susceptibility to Intergranular Corrosion in Austenitic, Ferritic, and Duplex Stainless Steels	Practice E (24 h HNO₃) and Practice B (5% CuSO₄/H₂SO₄) are mandatory for verifying carbide-free grain boundaries after solution treatment.
ASTM A-987	Standard Practice for Heat Treatment of Ferrous Castings	Provides furnace temperature uniformity requirements, thermocouple placement guidelines, and cooling rate specifications applicable to overlay heat treatment.
ASTM E-92	Standard Test Method for Vickers Hardness of Metallic Materials	Governs hardness measurement procedure for verifying heat treatment effectiveness.
ASTM E-975	Standard Practice for X-Ray Diffraction Determination of Residual Stress in Welds	Primary standard for verifying residual stress reduction after PWHT.
ASME B31.3	Process Piping	Section 331.5 specifies PWHT requirements for welded overlays on process piping; defines temperature ranges, times, and acceptance criteria based on alloy group and thickness.
ASME Section IX	Welding, Brazing, and Fusing Qualifications	QP-32 and QW-32 cover PWHT requirements for weld procedure qualification; QW-33 covers PWHT effects on mechanical properties.
NACE SP0388	Guide for Applying Cathodic Protection in Internal Corrosion Control of Refinery Storage Tanks	Relevant when overlay deposits are applied to tank internals; specifies PWHT requirements to ensure compatibility with cathodic protection systems.
ISO 9015	Welding — Classification of Welding Processes	Provides the framework for documenting and classifying the heat treatment process as part of the overall weld overlay procedure.
GB/T 17955	Welding Consumables — Classification and Designation	Chinese national standard for weld consumables used in overlay applications; relevant for domestic qualification and customer compliance.
NB/T 47014	Welding Procedure Qualification Rules for Steel Pressure Vessels	Chinese industry standard for welding procedure qualification in pressure vessel applications; specifies PWHT requirements for overlay welds on pressure vessels.

6. Common Risks and Controls

6.1 Thermal Cracking of the Deposit

Risk: Rapid heating or excessive heating rates can cause thermal cracking in the overlay deposit, particularly in high-sulfur or high-carbon alloys where low-melting-point eutectics form at grain boundaries. This is a significant risk for martensitic overlays with carbon content >0.4%.

Controls: Limit heating rate to 150 °C/h for deposits >15 mm; preheat the component to 200–300 °C before introducing it to the furnace; use a controlled-ramp furnace program with stepwise temperature increases.

6.2 Delta (δ) Ferrite Formation in Austenitic Overlays

Risk: If the solution treatment temperature is too high or the cooling rate is too slow, δ-ferrite can form in austenitic overlays (particularly 309/310 type). Excessive δ-ferrite (>10%) degrades corrosion resistance and ductility.

Controls: Optimize solution treatment temperature to 1050–1080 °C for 309-type alloys; ensure adequate cooling rate (air quench for deposits <10 mm, water quench for deposits <6 mm); verify δ-ferrite content by metallographic examination per ASTM E-490.

6.3 Sigma Phase Precipitation

Risk: Prolonged exposure to temperatures in the 600–870 °C range during stress relief or tempering can cause sigma phase (Cr₂Mo) precipitation in high-chromium overlays, leading to severe embrittlement.

Controls: Strictly limit stress relief temperature to below 600 °C for high-Cr overlays; avoid holding in the 600–870 °C range for more than 30 minutes; document thermal history to demonstrate avoidance of the sigma phase formation window.

6.4 Distortion and Dimensional Change

Risk: Differential thermal expansion between the overlay deposit and the base metal during heat treatment can cause warping, particularly in thin-walled components or asymmetric geometries.

Controls: Use fixture-supported loading in the furnace; limit heating rate to minimize thermal gradients; perform post-heat treatment dimensional verification per the applicable product drawing tolerances; allow for post-PWHT machining allowances in the design.

6.5 Incomplete Stress Relief

Risk: Insufficient hold time or temperature during stress relief can leave residual stresses above acceptable limits, leading to delayed cracking, stress corrosion cracking, or premature fatigue failure in service.

Controls: Follow the minimum hold time per ASTM A-987 (typically 1 hour per 25 mm of maximum section thickness, minimum 1 hour); verify residual stress by XRD per ASTM E-975; if measured stress exceeds 100 MPa, re-heat treat with extended hold time.

7. Application Across the Three Technology Routes

7.1 TIG/MIG Weld Overlay (Primary Application)

Heat treatment is most directly applicable to the TIG/MIG weld overlay route, where the thermal cycle of the welding process creates an as-deposited microstructure that is inherently non-equilibrium. The following scenarios illustrate the application:

• Multi-layer corrosion-resistant overlay on carbon steel piping: A 309L transition layer followed by a 316L or 310 overlay on carbon steel pipe (per ASME B31.3) requires post-weld solution treatment at 1050 °C to homogenize the deposit and achieve the required corrosion resistance. Without heat treatment, the as-welded 310 deposit may exhibit 40–60 HV hardness variation and incomplete carbide dissolution.
• Hardfacing overlay for wear-resistant components: A martensitic hardfacing alloy (e.g., 440C type per ASTM A-397) deposited by TIG onto a pump impeller or valve seat requires solution treatment at 1020 °C followed by tempering at 600 °C to achieve a hardness of 40–45 HRC with acceptable toughness. The tempering cycle directly determines the hardness-toughness balance.
• Multi-pass overlay build on thick sections: For overlay deposits exceeding 15 mm in thickness (e.g., on large pump housings or reactor internals), inter-pass temperature control during welding (typically 150–250 °C) combined with final PWHT is essential to prevent cold cracking and ensure uniform microstructure throughout the deposit thickness.

7.2 Hydraulic Explosive Bonding (Complementary Application)

In hydraulic explosive bonding (also known as hydraulic explosion welding or explosive bonding by hydraulic pressure), the bonding interface is formed through high-velocity impact rather than thermal fusion. While the bonding process itself does not create a weld microstructure requiring heat treatment, the following scenarios create a need for heat treatment expertise:

• Post-bonding stress relief: The high-strain-rate deformation during hydraulic explosive bonding generates significant residual stresses in both the cladding layer and the base metal. A controlled stress relief cycle (550–650 °C, 2–4 h, furnace cool) is often required to reduce these stresses to below 100 MPa, ensuring dimensional stability and fatigue performance. The heat treatment parameters must be carefully selected to avoid adverse effects on the metallurgical bond interface.
• Hybrid bonding-welding processes: In some applications, a thin weld overlay is applied to the bonded interface to enhance bonding strength or seal the interface. In this case, the heat treatment of the weld overlay directly affects the overall component performance, and the metallurgical knowledge from weld overlay heat treatment is essential.
• Cladding layer property optimization: When the cladding layer is a high-alloy iron-based material (e.g., 316L or 2205 duplex), the as-bonded microstructure may contain deformation-induced martensite or strain-induced phases. A solution treatment can restore the equilibrium microstructure and achieve the target hardness and corrosion resistance of the cladding layer.

7.3 Explosion Welding (Complementary Application)

Explosion welding produces a characteristic wavy metallurgical bond interface with minimal heat-affected zone (HAZ) due to the extremely short duration of the collision event. However, heat treatment remains relevant in the following contexts:

• Post-explosion stress relief: The collision event generates residual stresses of 150–350 MPa in the explosion-welded cladding. A stress relief cycle at 550–650 °C is typically required to bring residual stresses within specification limits for pressure vessel or piping applications per ASME Section VIII or NB/T 47014.
• Cladding layer microstructure stabilization: For high-alloy cladding materials (e.g., 310SS, 2205 duplex), the explosion welding process can introduce strain-induced martensite in austenitic layers or alter the ferrite/austenite ratio in duplex layers. A solution treatment at the appropriate temperature can restore the target microstructure and properties.
• Integration with subsequent welding operations: When explosion-welded clad components require additional welding (e.g., attachment welding, repair welding), the heat treatment of the weld overlay deposit and the overall component becomes a critical consideration to ensure that the metallurgical bond interface is not degraded.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

Mastery of heat treatment metallurgy for high-alloy iron-based weld overlay deposits directly strengthens the company's qualification portfolio in the following ways:

• WPS qualification with PWHT: Each WPS qualified per ASME Section IX or NB/T 47014 can include a PWHT cycle as part of the essential variables. Having documented, qualified PWHT cycles for multiple overlay alloys significantly expands the range of applications for which the company can submit qualified procedures to customers and third-party inspection agencies.
• Material qualification packages: Customers in the oil, gas, and power generation industries require comprehensive material qualification packages that include heat treatment records, hardness maps, microstructure photographs, and corrosion test results. The ability to produce these packages systematically demonstrates process control and compliance.
• ISO 3834 and EN 15085 compliance: These international welding quality standards require documented procedures for post-weld heat treatment, including temperature control, thermocouple verification, and post-treatment verification testing. Proficiency in overlay heat treatment metallurgy is a prerequisite for maintaining these certifications.

8.2 Product Delivery

The technical competency in heat treatment of overlay deposits translates directly into improved product delivery in the following respects:

• Reduced rework rates: Understanding the metallurgical effects of heat treatment enables the company to predict and prevent defects (cracking, distortion, hardness non-conformance) before they occur, reducing rework and scrap rates.
• Faster cycle times: Optimized heat treatment cycles (correct temperature, hold time, and cooling rate) minimize the time spent in the furnace while achieving full property conformance, reducing overall project lead times.
• Consistent quality: Standardized heat treatment procedures with documented parameters and verification protocols ensure that every overlay deposit meets the same quality standard, regardless of the production shift or operator.

8.3 Customer Value

The ultimate value delivered to customers through heat treatment expertise includes:

• Extended service life: Properly heat-treated overlay deposits exhibit 20–40% longer fatigue life and 15–30% improved corrosion resistance compared to untreated deposits, directly reducing customer maintenance costs and unplanned shutdowns.
• Regulatory compliance: Customers in regulated industries (nuclear, pharmaceutical, food processing) require documented PWHT records and verification testing. The company's ability to provide complete, audit-ready documentation reduces customer compliance burden.
• Engineering partnership: The company's metallurgical expertise enables it to function as a true engineering partner rather than a simple fabrication supplier. Customers value the ability to consult with the company on optimal alloy selection, overlay thickness, and heat treatment strategy for their specific application.

9. Summary and Actionable Recommendations

1. Develop a standardized PWHT database: Compile qualified heat treatment cycles for each overlay alloy in the company's product catalog, including temperature, hold time, quenching method, and verification test results. This database should be maintained and updated as new alloys and applications are qualified.
2. Implement thermocouple verification programs: Calibrate and verify all thermocouples used in heat treatment furnaces per ASTM E-2208 at intervals not exceeding 12 months. Document calibration records and maintain a traceable chain of measurement.
3. Establish a post-heat treatment verification protocol: Mandate hardness mapping, microstructural examination, and residual stress measurement for all critical overlay components. Define clear acceptance criteria and document all results in the product data package.
4. Cross-train operators across technology routes: Ensure that operators working on hydraulic explosive bonding and explosion welding projects also understand the heat treatment requirements for post-bonding stress relief and cladding layer microstructure stabilization.
5. Pursue customer-specific qualifications: Proactively offer to develop and qualify custom heat treatment cycles for key customers' specific applications, using the metallurgical knowledge gained from this study as the technical foundation for these qualifications.

Key Takeaway: Heat treatment is not merely a post-processing step—it is a critical metallurgical control variable that determines whether a weld overlay deposit achieves its designed performance in service. For Cladding Technology Shanxi Co., Ltd., systematic mastery of heat treatment metallurgy for high-alloy iron-based overlays is a foundational competency that differentiates the company in the competitive cladding market, strengthens its qualification portfolio, and delivers measurable value to customers through improved product performance and reliability.