Tensile Performance Assessment of Q345R Pressure Vessel Steel Under Simulated Fire Exposure
1. Definition and Technical Principles
Q345R is a low-carbon low-alloy pressure vessel steel specified under GB/T 1591 and widely referenced in GB 150 (Pressure Vessels) and NB/T 47003 (Steel Plates for Pressure Vessels). Its nominal yield strength is 345 MPa, and it is the most commonly used base material in China for pressure vessels, heat exchangers, reactors, and storage tanks in the petrochemical, power generation, and nuclear industries.
The study titled "Tensile Performance Testing of Q345R Simulated Post-Fire" investigates how the mechanical properties of Q345R steel—specifically tensile strength, yield strength, elongation, and reduction of area—degrade when the material is subjected to elevated temperatures simulating fire exposure conditions. The fundamental principle is that fire events in industrial facilities can raise structural steel temperatures well above 600°C, causing microstructural transformations (ferrite-pearlite to austenite recrystallization), grain coarsening, and loss of dislocation density, all of which degrade ductility and strength.
Post-fire tensile testing is a critical component of structural fire engineering assessment. By conducting controlled thermal exposure tests (heating the coupon to target temperatures such as 300°C, 400°C, 500°C, 600°C, and 700°C, followed by controlled cooling) and subsequent room-temperature tensile testing, engineers can establish quantitative degradation curves that inform structural integrity judgments and repair decisions.
2. Category and Business Positioning
This technical capability falls under the company's Quality Assurance and Engineering Support division, specifically within the Post-Service Assessment and Structural Integrity competency. It serves as a knowledge foundation that supports:
- Cladding product reliability validation — Understanding how base metal degrades under fire helps design cladding systems that maintain integrity during fire events.
- Weld overlay qualification support — When TIG/MIG weld overlay layers are applied to Q345R substrates, understanding post-fire behavior of the base metal informs HAZ (Heat Affected Zone) design and intermetallic compound mitigation strategies.
- Explosion welding quality assurance — For explosion-welded clad plates on Q345R, post-fire tensile data validates that the metallurgical bond retains adequate strength under thermal abuse.
- Customer engineering support — Providing post-fire assessment data to end-users (refineries, power plants) to support insurance claims, regulatory compliance, and return-to-service decisions.
3. Technical Purpose and Value
3.1 Establishing Degradation Curves
The primary technical output is a set of quantitative degradation curves mapping tensile properties (UTS, yield strength, elongation) as a function of peak exposure temperature. These curves enable:
- Engineering judgment on whether a fire-damaged Q345R structure requires replacement, repair, or can be returned to service.
- Definition of critical temperature thresholds beyond which structural integrity is compromised.
- Validation of fire-resistant cladding and insulation system design margins.
3.2 Supporting WPS and PQ Qualification
When welding procedures are qualified for Q345R substrates (per ASME Section IX or GB/T 19866), understanding post-fire mechanical behavior ensures that the qualified weld procedure produces joints that maintain acceptable performance even under fire exposure scenarios. This is particularly relevant for nuclear applications governed by NB/T 20043 and ASME BPV Code Section VIII.
3.3 Customer Value
For end-users in petrochemical and power industries, post-fire tensile assessment data provides:
- Objective engineering basis for structural integrity decisions post-fire incident.
- Reduced risk of catastrophic failure from premature return-to-service of fire-damaged equipment.
- Compliance with regulatory requirements under API 579-1/ASME FFS-1 (Fitness-for-Service).
4. Key Process and Implementation Points
4.1 Test Specimen Preparation
Test specimens are prepared in accordance with GB/T 228.1 (Metallic Materials — Tensile Testing — Part 1: Method of Test at Room Temperature) and ASTM E8/E8M. Standard round or flat tensile specimens are machined from Q345R plate material (typically 6-20 mm thickness, representing common pressure vessel wall thicknesses).
| Parameter | Specification | Reference Standard |
|---|---|---|
| Specimen Geometry | Round or flat, gauge length 5d or 5W | GB/T 228.1 / ASTM E8 |
| Material Grade | Q345R (GB/T 1591) | NB/T 47003 |
| Plate Thickness | 6 mm, 10 mm, 16 mm, 20 mm | GB 150.2 |
| Specimen Orientation | Longitudinal (L), Transverse (T), Short-Transverse (ST) | GB/T 228.1 |
| Surface Finish | Machine-ground to remove machining marks | ASTM E8 |
4.2 Thermal Exposure Protocol
Simulated fire exposure is conducted using controlled heating furnaces. The protocol follows recognized fire engineering methodologies, including those referenced in EN 1993-1-2 (Eurocode 3: Design of Steel Structures — Fire Design) and ASTM E119.
| Exposure Temperature | Soak Time | Cooling Method | Representative Scenario |
|---|---|---|---|
| 300°C | 30 min | Air cool | Low-intensity fire / insulation failure |
| 400°C | 30 min | Air cool | Moderate fire exposure |
| 500°C | 30 min | Air cool | Sustained hydrocarbon fire |
| 600°C | 30 min | Water quench / Air cool | Severe fire / jet fire |
| 700°C | 30 min | Water quench / Air cool | Extreme fire / pool fire |
4.3 Tensile Testing Procedure
Post-fire tensile testing is performed at room temperature after controlled cooling. Key parameters include:
- Strain rate: 0.5–2.0 mm/min (crosshead speed), per GB/T 228.1
- Extensometer: Contact-type or non-contact optical, measuring uniform elongation and total elongation
- Data acquisition: Load-displacement and load-strain curves recorded continuously
- Replicates: Minimum 3 specimens per temperature condition for statistical validity
4.4 Typical Results and Degradation Trends
| Peak Temperature | UTS Retention (%) | Yield Strength Retention (%) | Elongation Change | Engineering Judgment |
|---|---|---|---|---|
| 300°C | 95–100% | 90–98% | Minimal change | Safe for return-to-service |
| 400°C | 90–97% | 85–95% | Slight decrease | Acceptable with monitoring |
| 500°C | 75–90% | 65–85% | Significant decrease | Requires detailed assessment |
| 600°C | 50–75% | 40–70% | Severe decrease | Replacement recommended |
| 700°C | 30–55% | 20–50% | Catastrophic loss | Immediate replacement required |
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
- GB/T 1591 — Low-alloy high-strength structural steels (Q345R grade)
- NB/T 47003 — Steel plates for pressure vessels
- GB 150.2 — Technical code for pressure vessels — Materials
- ASTM A516 — Carbon steel plates for pressure vessels (equivalent reference)
5.2 Test Standards
- GB/T 228.1 — Tensile testing at room temperature
- ASTM E8/E8M — Standard test method for tension testing of metallic materials
- EN 1993-1-2 — Eurocode 3: Fire design of steel structures
- ASTM E119 — Fire endurance tests of building construction
5.3 Fitness-for-Service and Assessment Standards
- API 579-1/ASME FFS-1 — Fitness-for-Service
- NB/T 47013 — Non-destructive testing of pressure vessels
- ASME BPV Code Section VIII, Division 1 — Rules for construction of pressure vessels
5.4 Acceptance Criteria
For post-fire assessment of Q345R structures, acceptance is typically based on:
- Yield strength retention ≥ 85% of original value (for continued service at full design pressure)
- Elongation retention ≥ 80% of original value (for ductility assurance)
- No evidence of microstructural damage (confirmed by metallographic examination per GB/T 19540)
- Compliance with API 579-1 Part 6 (Material Properties) for fitness-for-service determination
6. Common Risks and Controls
| Risk | Impact | Control Measure |
|---|---|---|
| Non-uniform heating during thermal exposure | Inaccurate degradation data | Use calibrated furnaces with thermocouple monitoring; maintain temperature uniformity within ±5°C |
| Oxidation and scale formation on specimens | Surface cracking; erroneous tensile results | Test in inert atmosphere (Ar/N₂) or apply anti-oxidation coating; remove scale before tensile testing |
| Inappropriate cooling rate | Unrealistic microstructural transformation | Standardize cooling protocol (air cool vs. water quench); document cooling rate |
| Specimen orientation bias | Non-representative results for rolled plate | Test in all three orientations (L, T, ST) per GB/T 228.1 |
| Failure to account for thickness effect | Incorrect extrapolation to thick-section components | Test multiple thicknesses; apply thickness correction factors |
| Extrapolation beyond tested temperature range | Unsafe engineering decisions | Clearly define valid temperature range; do not extrapolate beyond tested conditions |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Applications
In TIG/MIG weld overlay of corrosion-resistant alloys (e.g., 309L, 310L, Inconel 625) onto Q345R substrates, post-fire tensile data is critical for:
- HAZ assessment: The Heat Affected Zone of Q345R is susceptible to softening at elevated temperatures. Post-fire tensile data establishes the minimum acceptable HAZ strength after fire exposure, informing the design of multi-pass overlay procedures that minimize thermal input.
- Intermetallic compound evaluation: At fire temperatures, intermetallic phases (e.g., Ni-rich phases in Ni-alloy overlays) may form at the weld interface. Tensile data from post-fire testing of overlay joints validates whether these phases compromise joint integrity.
- Procedure qualification: WPS qualification per ASME Section IX or GB/T 19866 requires demonstrating that the weld joint retains adequate tensile properties. Post-fire data extends qualification validity to fire-exposed service conditions.
- Repair procedure development: When fire-damaged clad equipment requires repair overlay, post-fire tensile data guides the selection of repair overlay material and preheat parameters.
7.2 Hydraulic Explosive Bonding Applications
For hydraulic explosive bonding of clad plates (e.g., 304/316 stainless steel onto Q345R), post-fire tensile assessment is relevant in the following ways:
- Interface integrity validation: The metallurgical bond formed during hydraulic explosive bonding must retain sufficient strength after fire exposure. Post-fire tensile and shear test data confirm that the bond interface does not become a preferential failure path.
- Delamination risk assessment: Thermal expansion mismatch between the cladding layer and Q345R substrate during fire exposure can induce interfacial stresses. Tensile data, combined with finite element analysis, quantifies delamination risk.
- Design margin determination: Post-fire degradation curves for Q345R inform the minimum substrate thickness required to maintain structural integrity of hydraulically bonded clad plates under fire scenarios.
7.3 Explosion Welding Applications
Explosion welding produces clad plates with a distinctive wave-pattern interface. Post-fire tensile testing contributes to:
- Interface bonding quality verification: Post-fire tensile and peel testing of explosion-welded samples confirms that the wave-pattern interface maintains cohesive failure (not interfacial failure) even after thermal abuse.
- Clad plate qualification for fire-exposed service: For explosion-welded clad plates intended for use in fire-prone environments (e.g., flare stacks, emergency relief systems), post-fire tensile data is a prerequisite for qualification per ASTM A283 or GB/T 13112.
- Post-fire NDT protocol development: Understanding post-fire mechanical degradation informs the selection and calibration of NDT methods (ultrasonic, magnetic particle) for inspecting explosion-welded joints after fire exposure.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQ extension: Post-fire tensile data allows the company to extend existing welding procedure qualifications to include fire-exposure scenarios, reducing the need for separate qualification testing for each fire-service application.
- Material qualification databases: Systematic post-fire tensile data for Q345R (and other base materials) builds a proprietary material database that supports rapid engineering response for customer inquiries.
- Regulatory compliance: Demonstrated post-fire assessment capability supports compliance with NB/T 47013, API 579-1, and ASME FFS-1 requirements for fitness-for-service evaluations.
8.2 Product Delivery
- Enhanced product reliability: Clad products delivered with post-fire performance data provide customers with verified performance envelopes, reducing warranty claims and enhancing brand reputation.
- Accelerated project approvals: Providing post-fire tensile data as part of product documentation accelerates customer engineering review and regulatory approval for fire-prone applications.
- Customized fire-resistant solutions: Post-fire data enables the design of clad products with specific fire-resistance targets, differentiating the company's offerings in competitive bids.
8.3 Customer Value
- Reduced lifecycle cost: Customers benefit from data-driven decisions on repair vs. replacement of fire-damaged equipment, minimizing unnecessary replacement costs.
- Insurance and regulatory support: Post-fire tensile assessment reports provide objective evidence for insurance claims and regulatory compliance submissions.
- Safety assurance: Verified post-fire mechanical properties ensure that returned-to-service equipment maintains adequate safety margins, protecting personnel and the environment.
- Technical partnership: Providing post-fire assessment capability positions the company as a technical partner rather than merely a supplier, deepening customer relationships and enabling long-term contracts.
9. Conclusion
The study of Q345R tensile performance under simulated fire exposure represents a foundational engineering competency that directly supports the company's cladding technology offerings across all three manufacturing routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. By establishing quantitative degradation curves, validating cladding interface integrity under thermal abuse, and providing actionable fitness-for-service data, this capability enhances product qualification, accelerates customer approvals, and delivers measurable value in terms of safety, cost optimization, and regulatory compliance. The integration of post-fire assessment data into WPS qualification, NDT protocols, and product design margins represents a systematic approach to delivering fire-resilient clad products that meet the demanding requirements of petrochemical, power generation, and nuclear industries.