Post-Weld Heat Treatment Effects on Impact Toughness of Q690 High-Strength Steel Weld Metal
1. Technical Definition and Fundamental Principles
Q690 is a high-strength low-alloy (HSLA) structural steel with a minimum yield strength of 690 MPa and a tensile strength range of 720–930 MPa. This grade is widely specified in heavy-industry pressure vessels, offshore structural components, bridge construction, and high-pressure hydrogen service equipment. When Q690 steel is joined by fusion welding, the resulting weld metal and heat-affected zone (HAZ) are subjected to complex microstructural transformations during the rapid heating and cooling cycles of welding. Post-Weld Heat Treatment (PWHT) is the controlled thermal process applied after welding to relieve residual stresses, refine grain structure, and—critically—to restore or enhance the Charpy V-Notch (CVN) impact toughness of the weld metal to meet the stringent requirements of design codes.
1.1 Microstructural Mechanisms in Q690 Weld Metal
The weld metal deposited in Q690 joints typically utilizes low-carbon, high-manganese, microalloyed consumables (e.g., E70T-8, E71T-8, or equivalent flux-cored wires) designed to match the parent material strength. During welding, the rapid solidification and cooling produce a mixture of martensite, bainite, and ferrite phases. The high carbon equivalent (CE) inherent to matching a 690 MPa yield strength steel creates a significant hardenability tendency, which directly degrades impact toughness at sub-zero temperatures. The PWHT cycle addresses this by:
- Tempering of martensitic constituents: Reducing the hardness of untempered martensite by precipitating fine carbides and transforming retained austenite into ferrite and cementite, thereby improving ductility and toughness.
- Stress relief: Reducing welding residual stresses from levels approaching yield strength (often 400–600 MPa) to below 100 MPa, which lowers the driving force for hydrogen-induced cracking and improves fatigue performance.
- Grain refinement: Promoting recrystallization and grain boundary spheroidization in the HAZ, reducing the effective grain size and increasing crack arrest capability.
- Diffusion homogenization: Allowing carbon and alloy element redistribution to reduce segregation at prior austenite grain boundaries, which are preferential sites for brittle fracture initiation.
1.2 Thermodynamic Basis of PWHT on Impact Toughness
The Charpy impact energy of weld metal is governed by the ductile-to-brittle transition temperature (DBTT). For Q690 weld metal without PWHT, the DBTT may exceed 0°C, rendering the joint vulnerable to brittle fracture at ambient or sub-ambient service temperatures. A properly designed PWHT cycle—typically performed at 580–650°C for carbon-martensitic weld metals—shifts the DBTT downward by 30–60°C, effectively ensuring adequate impact energy absorption at the required test temperature (commonly −20°C or −40°C per design specification). The Charpy transition curve follows a sigmoidal relationship, and PWHT primarily reduces the upper-shelf energy scatter and steepens the transition slope, indicating improved fracture resistance across the entire transition range.
2. Category and Business Positioning within Cladding Technology Shanxi Co., Ltd.
This technical entry falls under the company's Weld Overlay and Cladding Process Engineering competency domain, specifically within the sub-discipline of Welding Procedure Qualification (WPQ) and Welding Procedure Specification (WPS) development. It represents a critical knowledge asset that directly supports the company's ability to deliver high-integrity cladded products on Q690 base materials, which are increasingly specified in:
- High-pressure hydrogen energy storage vessels and reactors (API 620, ASME VIII Div. 2)
- Offshore platform structural components and subsea piping (NACE MR0175/ISO 15156)
- Heavy-duty wear-resistant cladding on high-strength substrates
- Pressure boundary components in petrochemical and power generation (NB/T 47014, GB/T 150)
Within the company's three primary technology routes, this entry most directly supports the TIG/MIG weld overlay route, as PWHT is a mandatory or recommended post-processing step for high-strength steel weld overlays. It also provides critical input to the hydraulic explosive bonding and explosion welding routes, where understanding of base material weldability and PWHT response informs substrate preparation, cladding material selection, and final mechanical property optimization.
3. Technical Purpose and Strategic Value
3.1 Primary Technical Objectives
- Impact toughness assurance: Ensuring weld metal CVN impact energy meets or exceeds code requirements (typically ≥47 J at −20°C per ASME Section IX, or ≥34 J at −40°C per GB/T 19432) through optimized PWHT parameters.
- Residual stress mitigation: Achieving post-PWHT residual stress levels below 100 MPa (measured by X-ray diffraction or hole-drilling method) to prevent delayed cracking and improve fatigue life.
- Microstructural optimization: Producing a tempered martensite/ferrite-bainite microstructure with controlled grain size (ASTM grain size ≥6) for balanced strength-toughness properties.
- Process qualification: Generating the technical data required for WPS qualification under GB/T 19432, ASME Section IX, or EN ISO 15614-1.
3.2 Strategic Value to the Company
Mastery of PWHT effects on Q690 weld metal impact toughness provides Cladding Technology Shanxi Co., Ltd. with a significant competitive advantage in several dimensions:
- Qualification breadth: Enables the company to qualify WPS for high-strength steel cladding applications that competitors may be unable to deliver, expanding the addressable market in hydrogen energy, offshore, and heavy equipment sectors.
- Product reliability: Reduces field failure risk by ensuring that delivered cladded components meet the full spectrum of mechanical property requirements, including impact toughness at service temperature.
- Engineering credibility: Demonstrates deep metallurgical understanding to end-users and third-party inspection (TPI) agencies, facilitating approval of novel cladding designs and expedited project timelines.
- Cost optimization: Informs optimal PWHT parameter selection that achieves required toughness without excessive energy consumption, minimizing distortion and reducing the need for corrective rework.
4. Key Process and Implementation Points
4.1 PWHT Parameter Selection for Q690 Weld Metal
| Parameter | Recommended Range | Rationale |
|---|---|---|
| PWHT Temperature | 580–650°C | Tempering range for martensitic weld metal; below Ac₁ (≈750°C) to avoid phase transformation and grain coarsening |
| Hold Time | 1 hour per 25 mm thickness (min. 2 h) | Ensures uniform thermal penetration and complete stress relief throughout the weld cross-section |
| Heating Rate | ≤140°C/h (limited by thickness: 1400/t °C/h) | Prevents thermal shock and differential expansion that could cause cracking in the HAZ |
| Cooling Rate | Controlled ≤140°C/h (below 400°C); furnace cooling preferred | Prevents re-hardening and ensures full tempering effect; air cooling above 400°C acceptable for thin sections |
| PWHT Timing | Within 2 hours of weld completion (for HIC/SCC-sensitive applications); before NDT | Minimizes hydrogen diffusion time; ensures NDT reflects final material condition |
| Pre-PWHT Inspection | 100% visual + dye penetrant (PT) inspection of all welds | Prevents PWHT from masking surface defects that could grow during thermal cycling |
4.2 Welding Process Parameters for Q690 Cladding (Pre-PWHT)
| Parameter | Typical Value for Q690 TIG/MIG Overlay | Notes |
|---|---|---|
| Base Material | Q690 (GB/T 1591 / ASTM A710 Gr. 690) | CE ≤ 0.45% preferred for weldability; preheat required if CE > 0.40% |
| Welding Consumable | E71T-8 (MIG) / ER70S-6 (TIG) or equivalent | Low-carbon, high-Mn, microalloyed for toughness matching |
| Preheat Temperature | 100–200°C (depending on CE and thickness) | Controls cooling rate; prevents cold cracking; reduces HAZ hardness |
| Interpass Temperature | ≤250°C | Maintains low cooling rate; prevents localized hardening between passes |
| Heat Input | 1.0–2.5 kJ/mm (TIG); 20–40 kJ/mm (MIG) | Higher heat input = slower cooling = lower HAZ hardness = better toughness |
| Shielding Gas | Ar (TIG); Ar + 5–8% CO₂ (MIG) | Controls arc stability and penetration; minimizes porosity |
| Layer Thickness | 3–6 mm per pass; 8–12 mm total overlay | Multiple thin passes reduce dilution and improve toughness vs. single thick pass |
| Post-Weld Condition | As-welded or PWHT | PWHT recommended for all impact-tested applications |
4.3 Impact Toughness Results: As-Welded vs. PWHT
| Condition | Test Temperature | Typical CVN Energy (J) | Microstructure | Hardness (HV) |
|---|---|---|---|---|
| As-Welded | 20°C | 45–70 | Untempered martensite + retained austenite | 320–380 |
| As-Welded | −20°C | 15–35 | Brittle fracture initiates at grain boundaries | 320–380 |
| As-Welded | −40°C | 5–15 | Full brittle fracture; DBTT exceeded | 320–380 |
| PWHT (620°C/3h) | 20°C | 65–95 | Tempered martensite + ferrite | 260–300 |
| PWHT (620°C/3h) | −20°C | 40–65 | Ductile fracture; improved grain boundary cohesion | 260–300 |
| PWHT (620°C/3h) | −40°C | 25–45 | Partial ductile fracture; DBTT shifted lower | 260–300 |
4.4 Implementation Procedure
- Welding execution: Perform TIG/MIG weld overlay on Q690 substrate per qualified WPS, maintaining strict control of preheat, interpass temperature, and heat input.
- Pre-PWHT NDT: Conduct 100% visual inspection and dye penetrant testing (per GB/T 18851 or ASME Section V Article 6) to identify and repair surface-breaking defects before thermal exposure.
- Instrumentation: Install thermocouples at representative locations (weld centerline, HAZ, and base metal 25 mm from weld) to monitor heating and cooling profiles during PWHT.
- PWHT execution: Ramp temperature at controlled rate to 620°C ± 25°C, hold for calculated duration, then cool at controlled rate to below 100°C before removing from furnace.
- Post-PWHT inspection: Perform volumetric NDT (UT per GB/T 11345 or ASME Section V Article 4; RT per GB/T 3323 or ASME Section V Article 2) to verify no new defects formed during PWHT.
- Mechanical testing: Extract and machine Charpy V-Notch specimens from the weld metal center and HAZ; perform impact testing per GB/T 229 or ASTM E23 at the specified qualification temperature.
- Data evaluation: Compare impact energy results against code acceptance criteria; document all parameters for WPS qualification record.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 1591: Structural steels — High strength low alloy steels (defines Q690 mechanical properties and chemical composition)
- GB/T 19432: Welding procedure qualification and performance qualification (WPS qualification requirements for HSLA steels)
- NB/T 47014: Welding procedure qualification rules for pressure vessels and pressure piping
- ASME Section IX: Qualification rules for welding, brazing, and bonding (PWHT requirements in QW-406; impact testing in QW-420)
- ASME Section VIII Div. 1 & 2: Pressure vessels (impact testing requirements in UG-83; PWHT in UG-120)
- GB/T 229: Metallic materials — Charpy impact test method
- ASTM E23: Standard test methods for notch impact testing of metallic materials
- GB/T 3075: Metallic materials — Charpy impact test method (alternative)
- EN ISO 15614-1: Qualification testing of welding procedures for metallic materials — Welding procedure tests
- GB/T 18851: Non-destructive testing — Penetrant testing of welds
- GB/T 11345: Non-destructive testing of welds — Ultrasonic testing
- GB/T 3323: Non-destructive testing of welds — Radiographic testing
- API 620: Tanks for the storage of low-temperature liquefied gases (impact testing requirements for cryogenic service)
- ASTM A710: Standard specification for low-carbon steel plate suitable for welding (Grade 690 equivalent)
5.2 Impact Toughness Acceptance Criteria
| Standard/Code | Test Temperature | Minimum CVN Energy | Number of Specimens | Applicability |
|---|---|---|---|---|
| ASME VIII Div. 1 (UG-83) | Service temperature or −20°C | 34 J (25 ft-lbf) average of 3 specimens; no single < 27 J (20 ft-lbf) | 3 (weld metal) + 3 (HAZ) | Pressure vessels with impact testing required |
| ASME VIII Div. 2 (FA-200) | Per design specification | As specified; typically ≥47 J at −20°C or −40°C | 3 (weld metal) + 3 (HAZ) per qualification | F-Category fracture critical applications |
| GB/T 150 | Per design specification | ≥34 J average of 3; no single < 27 J | 3 (weld metal) + 3 (HAZ) | Chinese pressure vessel code |
| NB/T 47014 | Per design specification | Per qualification requirements; typically ≥47 J at −20°C | 3 specimens per condition | Welding procedure qualification for pressure equipment |
| API 620 | Design temperature or −40°C (whichever is lower) | ≥47 J (35 ft-lbf) average of 3 | 3 (weld metal) + 3 (HAZ) | Cryogenic storage tanks |
| EN ISO 15614-1 | Per application specification | Per product standard; typically ≥40 J at −20°C | 3 specimens | European welding procedure qualification |
5.3 Hardness Acceptance Criteria
Post-PWHT hardness of the weld metal and HAZ in Q690 joints must not exceed the following limits to ensure adequate toughness:
- ASME Section IX QW-451.2: Maximum 350 HV (or 360 HV per some interpretations) for weld metal and HAZ
- GB/T 19432: Maximum 350 HV for weld metal; HAZ hardness ≤ base metal hardness + 50 HV
- NB/T 47014: Maximum 350 HV for weld metal; gradient check per 5 mm intervals
6. Common Risks and Control Measures
6.1 Risk Identification and Mitigation
| Risk | Mechanism | Consequence | Control Measure |
|---|---|---|---|
| Insufficient PWHT temperature | Incomplete tempering of martensite; residual stresses not fully relieved | Low impact energy; delayed hydrogen cracking | Verify furnace calibration; use multiple thermocouples; document temperature profile |
| Excessive PWHT temperature (>650°C) | Temper embrittlement (for steels with Sn, P, As impurities); grain boundary weakening | Reduced ductility; potential intergranular fracture | Limit PWHT to ≤650°C; avoid prolonged hold above 540–600°C range |
| Excessive heating rate | Thermal gradients cause differential expansion; HAZ cracking | Cracking in HAZ or weld; PWHT failure | Limit heating rate to ≤140°C/h or 1400/t °C/h; use radiant or indirect heating |
| Inadequate hold time | Insufficient diffusion for stress relief and microstructural equilibrium | Residual stresses remain; impact toughness not fully restored | Calculate hold time per thickness (≥1 h/25 mm); verify with residual stress measurement |
| Uncontrolled cooling | Rapid cooling re-forms martensite; new residual stresses develop | Toughness degradation; potential cracking | Furnace cool to ≤100°C; if air cooling, only above 400°C for sections < 12 mm |
| Hydrogen-induced cracking post-PWHT | Residual hydrogen diffuses to high-stress regions during cooling | Delayed cracking (up to 48 h post-PWHT) | Apply bake-out (250°C/2h) before PWHT; ensure low-hydrogen consumables; wait 24 h post-PWHT before NDT |
| Distortion from PWHT | Thermal expansion/contraction in thick or asymmetric sections | Dimensional non-conformance; fit-up issues in assembly | Use restrained heating; symmetric thermocouple placement; monitor with dial indicators |
| Weld metal dilution exceeding limits | High base metal dilution increases CE of weld metal; higher hardenability | Hardness and toughness outside qualified range | Control dilution per WPS; use multiple thin passes; verify with chemical analysis of weld metal |
6.2 Quality Assurance Protocol
- Prequalification review: Verify base metal CE, welding consumable selection, and PWHT parameters against applicable code requirements before initiating qualification welding.
- Welding log documentation: Record all welding parameters (current, voltage, travel speed, gas flow, preheat/interpass temperature) in a qualified welding log for traceability.
- PWHT cycle monitoring: Continuously record temperature profiles at minimum three thermocouple locations; flag any deviation exceeding ±25°C from the prescribed cycle.
- Post-PWHT hardness survey: Perform Vickers hardness traverse across the weld (base metal → HAZ → weld center → HAZ → base metal) at 5 mm intervals; verify maximum hardness ≤350 HV.
- Impact testing protocol: Machine Charpy specimens from the weld center (transverse orientation) and HAZ (1–2 mm from fusion line); test at the qualification temperature per ASTM E23 or GB/T 229.
- Residual stress verification: Measure residual stress at weld centerline using X-ray diffraction or strain gauge hole-drilling method; confirm ≤100 MPa post-PWHT.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
In the TIG/MIG weld overlay route, PWHT is the most directly applicable and frequently required post-processing step. Q690 base materials are commonly used as substrates for wear-resistant or corrosion-resistant cladding layers (e.g., Stellite, 309L/316L stainless, or high-chromium cast irons). The knowledge gained from this technical study directly informs:
- WPS development: Defining PWHT parameters as an integral part of the welding procedure, ensuring that the qualified procedure produces acceptable impact toughness in both the weld metal and the cladding interface.
- Multi-layer overlay design: Determining whether PWHT is required after each layer or only after the final layer, based on the cumulative heat input and resulting microstructure.
- Transition layer optimization: When overlaying dissimilar materials on Q690, the transition layer (e.g., 309L between Q690 and 316L) must be evaluated for PWHT response to ensure interface toughness is maintained.
- Production scaling: Applying PWHT knowledge to large-format overlay operations (e.g., large diameter pipe cladding, vessel head overlay) where uniform PWHT is challenging and localized PWHT may be required.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (water-jet-assisted explosive cladding) does not involve fusion welding, the understanding of Q690 weld metal PWHT effects is relevant in the following ways:
- Substrate weld repair qualification: Q690 substrates used in hydraulic explosive bonding may require weld repairs (e.g., for NDE-identified defects or post-forming crack repair). The PWHT knowledge ensures these repairs maintain the substrate's impact toughness integrity.
- Post-bonding heat treatment: Some hydraulic explosive bonded joints undergo post-bonding annealing to relieve bonding-induced residual stresses. Understanding PWHT effects on Q690 informs the temperature and time selection for this annealing step.
- Interface toughness evaluation: The bonding interface in hydraulic explosive bonded Q690 joints must meet impact toughness requirements. Knowledge of PWHT effects on Q690 microstructure helps predict how post-bonding thermal treatments will affect interface cohesion.
- Component integration: When hydraulic explosive bonded Q690 cladding is subsequently welded to other structural components, the PWHT knowledge ensures that the welding procedure accounts for the modified microstructure at the bonding interface.
7.3 Explosion Welding Route
In explosion welding of Q690 cladding, the PWHT knowledge contributes to:
- Post-explosion welding annealing: Explosion welding introduces significant residual stresses and plastic deformation in the substrate. A controlled annealing treatment (analogous to PWHT) is often applied to relieve these stresses. The knowledge of optimal PWHT parameters for Q690 directly translates to the annealing cycle design for explosion-welded Q690 components.
- Welded joint qualification on explosion-welded substrates: When explosion-welded Q690 cladding is subsequently machined and welded to form pressure boundary components, the welding procedure must account for the PWHT-annealed condition of the substrate. Impact toughness data from this study provides the baseline for welding procedure qualification.
- Hydrogen embrittlement assessment: Q690 steel used in explosion welding may be susceptible to hydrogen embrittlement during subsequent welding operations. Understanding of PWHT effects on hydrogen diffusion and retention informs the bake-out and PWHT protocols for post-explosion welding operations.
- Microstructural compatibility: The knowledge of Q690 microstructure evolution during PWHT helps ensure that the explosion-welded interface microstructure remains compatible with subsequent welding and heat treatment operations.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This technical knowledge directly enables Cladding Technology Shanxi Co., Ltd. to:
- Qualify new WPS: Develop and qualify welding procedures for Q690 weld overlay that include PWHT as a defined process step, with documented impact toughness data demonstrating compliance with code requirements.
- Expand qualification database: Accumulate a comprehensive database of PWHT parameter sets (temperature, time, cooling rate) correlated with impact toughness results, enabling rapid WPS selection for future Q690 projects without redundant qualification testing.
- Support third-party certification: Provide the technical documentation required by certification bodies (e.g., TUV, DNV, CCS, ABS) for product type approval and manufacturing facility qualification.
- Enable multi-code compliance: Demonstrate PWHT capability that satisfies simultaneous requirements of GB, ASME, and European standards, broadening the company's market access.
8.2 Product Delivery Enhancement
For product delivery, this knowledge ensures:
- First-time-right execution: Optimized PWHT parameters reduce the probability of impact test failure, minimizing rework cycles and delivery delays.
- Consistent quality: Standardized PWHT procedures ensure that every Q690 cladded product delivered meets the same mechanical property targets, regardless of production shift or operator.
- Traceability: Complete documentation of PWHT parameters, temperature profiles, and mechanical test results provides full traceability from raw material to finished product, satisfying customer quality audit requirements.
- Efficient scheduling: Knowledge of optimal PWHT cycle times enables accurate production planning and furnace scheduling, improving throughput without compromising quality.
8.3 Customer Value Creation
The ultimate value delivered to customers includes:
- Service life assurance: Products with verified impact toughness at service temperature deliver predictable fatigue life and fracture resistance, reducing unplanned shutdown risk in critical service.
- Regulatory compliance: Products meeting the full impact testing requirements of applicable codes (ASME, GB, NB, API) ensure customer compliance with regulatory inspection and certification requirements.
- Design flexibility: Demonstrated PWHT capability allows customers to specify Q690 cladding for demanding applications (low-temperature service, high-pressure hydrogen, seismic zones) that would otherwise require alternative and more expensive materials.
- Cost optimization: By achieving required toughness through optimized PWHT rather than material upgrades, customers benefit from lower material costs while maintaining equivalent or superior mechanical performance.
- Technical partnership: The company's deep metallurgical expertise positions it as a technical partner rather than a simple manufacturer, enabling collaborative design optimization and value engineering with end-users and EPC contractors.
9. Conclusions and Recommendations
The systematic study of post-weld heat treatment effects on Q690 high-strength steel weld metal impact toughness represents a foundational technical competency for Cladding Technology Shanxi Co., Ltd. The key findings and actionable recommendations are:
- PWHT is essential for Q690 weld applications requiring impact toughness below 20°C: As-welded Q690 weld metal consistently fails to meet code impact requirements at sub-ambient temperatures. PWHT at 620°C for adequate hold time is mandatory for −20°C and −40°C qualification.
- Optimal PWHT window is 580–650°C: Temperatures below 580°C provide insufficient tempering; temperatures above 650°C risk temper embrittlement and grain boundary weakening. The recommended setpoint is 620°C ± 25°C.
- Hold time must scale with thickness: A minimum of 1 hour per 25 mm of section thickness ensures complete stress relief and microstructural transformation throughout the weld cross-section.
- Controlled cooling is non-negotiable: Furnace cooling to below 100°C prevents re-hardening and ensures the full benefit of PWHT is realized. Air cooling above 400°C is acceptable only for sections under 12 mm.
- Integration across all three technology routes: While PWHT is most directly applied in the TIG/MIG weld overlay route, the underlying metallurgical knowledge is transferable to hydraulic explosive bonding and explosion welding applications involving Q690 substrates.
- Continuous qualification database development: Systematic documentation of PWHT parameters, temperature profiles, and mechanical test results should be maintained as a living database to support rapid WPS development for future projects.
By institutionalizing this knowledge and integrating it into the company's standard operating procedures, WPS development protocols, and quality management system, Cladding Technology Shanxi Co., Ltd. positions itself as a technically differentiated supplier capable of delivering high-integrity Q690 cladded products for the most demanding industrial applications.