Heat Treatment Process Optimization and Wear Resistance Enhancement for 65Mn/Q235 Bimetallic Composite Pipe
1. Definition and Technical Principles
The 65Mn/Q235 bimetallic composite pipe is a functionally graded tubular product in which a wear-resistant 65Mn manganese spring steel layer is metallurgically bonded to a Q235 mild carbon steel structural base pipe. The 65Mn alloy (typically 0.62–0.70% C, 0.90–1.20% Mn per GB/T 699) provides high hardness, excellent abrasion resistance, and good spring characteristics, while the Q235 base (0.14–0.22% C per GB/T 700) supplies structural integrity, formability, and weldability at a lower cost. The composite interface is formed through high-energy bonding processes, and subsequent heat treatment is critical to achieving the target hardness gradient, residual stress relief, and long-term wear performance.
The fundamental principle of the heat treatment optimization revolves around three objectives: (a) tempering the 65Mn layer to a controlled hardness range of 38–52 HRC while avoiding brittle martensite structures; (b) relieving residual stresses introduced during the explosive bonding or hydraulic bonding process without degrading the metallurgical bond integrity; and (c) establishing a hardness gradient at the interface that prevents crack initiation under cyclic wear loading.
2. Category and Business Positioning
This technology entry falls under the company's Explosion Welding and Hydraulic Explosive Bonding product routes, with downstream thermal processing as a value-added step. In the company's business portfolio, the 65Mn/Q235 composite pipe serves as a cost-effective wear-resistant solution for mining, aggregate handling, and bulk material transfer applications where the premium cost of fully alloyed pipes is unjustifiable but the base Q235 pipe alone cannot meet service life requirements.
The "learning notes" nature of this entry indicates it represents a knowledge consolidation and process improvement initiative — translating field experience into documented, repeatable WPS (Welding Procedure Specification) or HPS (Heat Treatment Procedure Specification) protocols that can be scaled across production batches.
3. Technical Purpose and Value
The optimization of heat treatment for 65Mn/Q235 composite pipes delivers measurable value across multiple dimensions:
- Extended service life: Properly tempered 65Mn achieves 2–3× the wear life of untreated as-bonded pipe in abrasive slurry applications.
- Reduced cracking risk: Controlled tempering eliminates the high residual stresses (often exceeding 300 MPa) left by explosive bonding, reducing the probability of interface delamination under thermal cycling.
- Cost efficiency: Using Q235 as the structural base reduces material cost by 40–60% compared to using 65Mn throughout the entire pipe wall thickness.
- Process qualification: Documented and optimized heat treatment parameters support WPS/HPS qualification per applicable codes, enabling product certification and customer acceptance.
4. Key Process Implementation Points
4.1 Heat Treatment Route Selection
Two primary heat treatment routes are employed for 65Mn/Q235 composite pipes, depending on the bonding method used and the target hardness profile:
| Parameter | Route A: Full Annealing + Tempering | Route B: Stress Relief Only |
|---|---|---|
| Austenitizing Temperature | 820–860°C | Not applicable |
| Quenching Medium | Oil quench (No. 2 or No. 3 quenching oil) | Not applicable |
| Tempering Temperature | 420–560°C (step-controlled) | 550–620°C (single stage) |
| Holding Time | 1.5–2.5 hours per 25 mm wall thickness | 1.0–1.5 hours per 25 mm wall thickness |
| Cooling Rate | Furnace cool to 300°C, then air cool | Furnace cool |
| Target 65Mn Hardness | 42–52 HRC | 38–45 HRC |
| Target Q235 Hardness | ≤ 130 HBW | ≤ 120 HBW |
4.2 Critical Process Control Variables
- Heating rate control: The heating rate must not exceed 150°C/hour for pipes with wall thickness greater than 50 mm to prevent differential thermal expansion between the 65Mn layer and Q235 base, which could cause interface separation.
- Temperature uniformity: The furnace must maintain ±15°C uniformity across the entire pipe length. For pipes exceeding 3 meters in length, multi-zone furnace control with independent thermocouple monitoring is required.
- Atmosphere control: During austenitizing, a neutral or slightly reducing atmosphere is required to prevent surface oxidation of the 65Mn layer. Nitrogen or endothermic gas is recommended. Oxidation of the manganese-rich surface would form MnO inclusions that degrade wear performance.
- Quenching severity management: For Route A, the oil temperature at quench initiation must be maintained between 40–80°C. Overheated quench oil (>100°C) will reduce quench severity, resulting in incomplete martensite transformation and suboptimal hardness.
- Tempering step strategy: For high-hardness targets (>48 HRC), a two-step tempering approach is recommended: first temper at 420°C for 1.5 hours, then second temper at 500°C for 1.0 hour. This eliminates retained austenite while maintaining adequate hardness.
4.3 Interface Considerations
The metallurgical bond interface between 65Mn and Q235 undergoes complex phase transformations during heat treatment. During austenitizing, carbon diffusion from the 65Mn layer into the Q235 base creates a decarburized zone and a carbon-enriched zone. This diffusion zone typically extends 0.3–0.8 mm into each material and must be accounted for in hardness profiling.
The optimal interface hardness gradient should show:
- 65Mn core: 42–52 HRC
- 65Mn near-interface (0–0.5 mm): 38–45 HRC
- Interface zone: 32–38 HRC (transition)
- Q235 near-interface (0–0.5 mm): 100–120 HBW
- Q235 core: ≤ 120 HBW
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application |
|---|---|
| GB/T 699-2015 | Chemical composition and mechanical properties of 65Mn carbon spring steel |
| GB/T 700-2006 | Chemical composition and mechanical properties of Q235 carbon structural steel |
| GB/T 8163-2018 | Seamless steel tubes for fluid transport (base pipe specification) |
| GB/T 13384-2017 | Heat treatment terminology and process classification |
| GB/T 18170-2008 | Explosion welding of metals — general specification |
| GB/T 32043-2015 | Explosion-welded cladding steel plates and pipes — technical conditions |
| NB/T 47015-2011 | Welding procedure qualification for pressure vessels (if applicable) |
| ASTM E10/E92 | Rockwell hardness test methods (HRC verification) |
| ASTM E18/E10 | Brinell hardness test methods (HBW verification) |
| ISO 9075 | Rockwell hardness testing — calibration and verification |
| ASTM G65 | Pin-on-disc wear testing methodology |
| ASTM G99 | Tabulated wear rates and coefficients for abrasive wear |
5.2 Acceptance Criteria
- Hardness verification: Minimum 3 test points per 300 mm pipe length on the 65Mn surface, measured per ASTM E18 (HBW) or ASTM E10/E92 (HRC). All values must fall within the specified range of the HPS.
- Interface integrity: Transverse cross-sections must show continuous metallurgical bonding with no voids, cracks, or unmelted regions at the interface. Macrographic examination per GB/T 32043-2015 Section 7.
- Residual stress: Post-heat treatment residual stress at the interface must not exceed 150 MPa (measured by X-ray diffraction per ASTM E975).
- Microstructure: The 65Mn layer must exhibit tempered martensite or sorbite structure. No untempered martensite or retained austenite exceeding 10% volume fraction is permitted.
- Wear test: Pin-on-disc wear testing per ASTM G65 shall demonstrate specific wear rate not exceeding 8×10⁻⁶ mm³/N·m for the optimized condition.
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Interface delamination | Excessive heating rate causing differential thermal expansion | Limit heating rate to ≤150°C/hour; implement multi-zone furnace control |
| Over-tempering (excessive softening) | Tempering temperature too high or holding time excessive | Calibrate furnace thermocouples per ISO 9075; use thermocouple-coupled test coupons |
| Under-tempering (brittle fracture risk) | Insufficient tempering temperature or premature cooling | Verify tempering temperature with independent pyrometer; maintain minimum holding time |
| Surface oxidation/decarburization | Exposure to oxidizing atmosphere during austenitizing | Use endothermic or nitrogen atmosphere; limit oxygen potential below 10⁻¹⁴ atm |
| Quench cracking | Excessive quench severity on thick 65Mn sections | Use preheated quench oil (60–80°C); consider stepped quench for wall thickness >15 mm |
| Carbon segregation at interface | Prolonged austenitizing at upper temperature limit | Limit austenitizing time; monitor carbon profile via metallographic analysis |
| Retained austenite instability | Incomplete tempering of 65Mn martensite | Implement two-step tempering; verify with Rockwell hardness stability test after 48-hour aging |
7. Application Across Company Technology Routes
7.1 Explosion Welding Route
In the explosion welding route, the 65Mn/Q235 composite pipe is fabricated by detonating a shaped explosive charge between a 65Mn inner pipe (or strip wound into a tube) and a Q235 outer pipe. The resulting plastic wave interaction creates a metallurgical bond with characteristic wave morphology at the interface. Post-bonding, the composite pipe typically contains high residual stresses (250–400 MPa) and a martensitic structure in the 65Mn layer due to localized plastic deformation. The optimized heat treatment serves as the essential post-processing step to:
- Relieve explosive-bonding-induced residual stresses
- Temper the deformation-hardened martensite to a controlled toughness level
- Stabilize the wave-pattern interface against micro-crack propagation
For explosion-welded pipes, Route A (full annealing + tempering) is generally preferred because the as-bonded microstructure is highly heterogeneous, requiring complete recrystallization followed by controlled transformation.
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding uses water as a confinement medium for the explosive charge, producing a cleaner bond interface with reduced spatter contamination compared to air-confinement explosion welding. The residual stress levels are typically 10–20% lower than air-confinement explosion welding. For this route, Route B (stress relief only) may be sufficient when the target hardness is in the lower range (38–45 HRC), as the as-bonded 65Mn hardness from deformation is already elevated. However, for higher hardness targets or when the 65Mn layer thickness exceeds 8 mm, Route A remains necessary.
7.3 TIG/MIG Weld Overlay Route
While the primary bonding mechanism for 65Mn/Q235 composite pipes is explosive bonding, the TIG/MIG weld overlay route can be used as a supplementary or alternative process for small-diameter pipes or repair applications. In this case, 65Mn-flux-cored or solid wire is deposited onto the Q235 pipe interior via TIG or MIG welding. The weld overlay deposit inherently contains a weld microstructure that requires post-weld heat treatment (PWHT) to:
- Temper the weld metal and heat-affected zone (HAZ)
- Relieve welding residual stresses
- Reduce HAZ hardness to prevent cold cracking
For the weld overlay route, the heat treatment parameters must be adjusted to account for the weld metal chemistry (which may differ slightly from wrought 65Mn due to dilution with Q235 base metal). Typical PWHT parameters are 580–620°C for 2 hours per 25 mm wall thickness with furnace cooling, per NB/T 47015-2011.
8. Wear Performance Characterization
8.1 Testing Methodology
Wear resistance of the optimized 65Mn/Q235 composite pipe is evaluated through multiple test methods:
- Pin-on-disc testing (ASTM G65): Using 100Cr6 hardened steel pins or alumina (Al₂O₃) ceramic pins against the 65Mn surface under controlled normal load (20–50 N) and sliding distance (1000–5000 m).
- Abrasive slurry testing: Simulating mining or slurry pump conditions with silica sand (60–80 mesh) in water at controlled flow velocity (3–8 m/s) through a pipe section.
- Impact abrasion testing: Using rubber-wheel abrasion tester (ASTM G117) to simulate conditions where material impacts the pipe surface, such as in bucket elevator discharge chutes.
8.2 Performance Benchmarks
| Condition | 65Mn Hardness (HRC) | Specific Wear Rate (×10⁻⁶ mm³/N·m) | Relative Wear Life vs. Q235 |
|---|---|---|---|
| As-bonded (untreated) | 45–55 (variable) | 12–18 | 1.5–2.0× |
| Stress relief only (550°C) | 38–44 | 8–12 | 2.0–2.5× |
| Optimized tempering (Route A) | 42–50 | 5–8 | 3.0–4.0× |
| Over-tempered (>560°C) | 32–38 | 15–22 | 1.0–1.5× |
| Baseline Q235 | — | 25–35 | 1.0× (reference) |
9. Contribution to Qualification Building and Customer Value
9.1 Qualification and Certification
The documented heat treatment optimization process directly supports the company's qualification building in the following ways:
- HPS qualification: The optimized parameters form the basis for a qualified Heat Treatment Procedure Specification that can be referenced in product certifications and customer audits.
- WPS/HPS traceability: Each production batch can be traced to a qualified procedure, satisfying customer requirements for NACE MR0175 or API 5L compliance in downstream applications.
- Third-party inspection readiness: Well-documented procedures enable efficient third-party inspection per ISO 9001 quality management requirements and customer-specific quality plans.
9.2 Product Delivery Value
- Consistent quality: Optimized and documented heat treatment eliminates batch-to-batch variability, ensuring every delivered pipe meets the same wear performance specification.
- Reduced rejection rate: Process optimization reduces non-conformance from 8–12% (unoptimized) to <2% (optimized), directly improving cost efficiency and delivery schedule reliability.
- Extended warranty confidence: Quantified wear performance data enables the company to offer extended warranty periods (24–36 months) with confidence, differentiating from competitors who cannot substantiate performance claims.
9.3 Customer Value Proposition
The 65Mn/Q235 composite pipe with optimized heat treatment delivers a compelling value proposition to end customers in mining, cement, aggregate, and bulk material handling industries:
"A 40–60% reduction in material cost compared to fully alloyed wear-resistant pipes, combined with a 3–4× extension in service life over standard carbon steel pipes, yielding a total cost of ownership reduction of 50–70% over the equipment lifecycle."
The documented process optimization provides customers with verifiable performance data, reducing their procurement risk and enabling data-driven maintenance planning rather than reactive replacement strategies.
10. Summary and Recommendations
The heat treatment process optimization for 65Mn/Q235 bimetallic composite pipes represents a critical value-adding step that transforms a bonded composite into a high-performance wear-resistant product. Key recommendations for implementation include:
- Establish a formal HPS for each heat treatment route (A and B) with full parameter documentation and qualification testing.
- Implement furnace calibration programs per ISO 9075 with quarterly verification of temperature uniformity and accuracy.
- Use thermocouple-coupled test coupons in every heat treatment batch for traceable hardness verification.
- Develop a wear performance database correlating heat treatment parameters with field service life data for continuous improvement.
- Extend the optimized process to other bimetallic combinations (e.g., 45Mn2/Q345, 65Mn/20#) to broaden the product portfolio while leveraging the same thermal processing infrastructure.
- Integrate the heat treatment process into the company's digital quality management system for real-time monitoring, automated data capture, and predictive maintenance of thermal processing equipment.