CLADDING TECHNOLOGY SHANXI CO., LTD2.25Cr-1Mo Cladding Process Development and Optimization
Literature Overview
This technical paper, published in Petrochemical Equipment in 2007 by researchers from the School of Mechanical Engineering at Liaoning Petrochemical University (Ma Xiangfeng, Gao Lei, and Zhang Yingying), addresses the welding and cladding process development for 2.25Cr-1Mo steel, a critical low-alloy steel used extensively in petrochemical pressure vessels, heat exchangers, and reactor components. 2.25Cr-1Mo steel (equivalent to ASTM A387 Gr.22, EN 1.4542, or GB 1318 15CrMo) is valued for its excellent high-temperature strength, good oxidation resistance, and resistance to high-temperature hydrogen attack (HTHA) at temperatures up to approximately 450°C. The cladding process development for this steel is of significant importance because 2.25Cr-1Mo components are often required to have corrosion-resistant overlay layers (typically 309L, 310L, or 347 stainless steel) to protect against specific service environments such as sulfuric acid, hydrofluoric acid, or high-temperature steam.
Core Technical Content and Interpretation
The welding and cladding of 2.25Cr-1Mo steel presents several unique challenges that distinguish it from carbon steel or austenitic stainless steel welding:
- Crack susceptibility: The high carbon equivalent (CEV ≈ 0.45–0.55) of 2.25Cr-1Mo steel makes it susceptible to cold cracking (hydrogen-induced cracking) during welding, particularly in thick sections.
- Temper embrittlement: The steel is prone to temper embrittlement in the temperature range of 375–525°C, which can significantly reduce toughness if the heat-affected zone (HAZ) is exposed to this temperature range during service or PWHT.
- Dilution control: When cladding with austenitic stainless steel, the dilution from the 2.25Cr-1Mo base metal introduces carbon and alloying elements that can alter the composition and properties of the cladding layer.
- Residual stress management: The high thermal expansion coefficient mismatch between the ferritic base metal and austenitic cladding layer generates significant residual stresses that can lead to distortion or cracking.
Typical Welding and Cladding Process Parameters for 2.25Cr-1Mo Steel
| Process Parameter | Base Metal Welding | Cladding Welding (309L) | Cladding Welding (347) |
|---|---|---|---|
| Welding process | SAW / FCAW / GTAW | SAW / FCAW / GTAW | SAW / FCAW / GTAW |
| Filler metal | E8018-B2 / E81T1-Ni1 | E309L / ER309L | E347 / ER347 |
| Preheat temperature (°C) | 150–250 | 100–200 | 100–200 |
| Interpass temperature (°C) | <250 | <200 | <200 |
| Welding current (A) | 200–400 (depending on process) | 150–350 | 150–350 |
| Travel speed (mm/min) | 200–400 | 150–350 | 150–350 |
| PWHT temperature (°C) | 720–760 | 720–760 | 720–760 |
| PWHT duration (h) | 1 h per 25 mm thickness | 1 h per 25 mm thickness | 1 h per 25 mm thickness |
| Dilution rate (target) | N/A | <5% | <5% |
The preheat temperature of 150–250°C for base metal welding is critical for preventing hydrogen-induced cracking. This temperature reduces the cooling rate of the HAZ, allowing hydrogen to diffuse out of the weld zone before the steel reaches the ductile-to-brittle transition temperature. The interpass temperature must be maintained below 250°C to avoid the formation of brittle microstructures in the weld metal and HAZ.
Cladding Process Development and Optimization
The cladding process development for 2.25Cr-1Mo steel involves several key steps:
- Consumable selection: The choice of filler metal for the cladding layer depends on the service environment. E309L (309L composition) is the most common choice due to its excellent weldability and resistance to dilution from the base metal. E347 (347 composition) is preferred when higher creep strength is required, as the niobium stabilization prevents chromium carbide precipitation at grain boundaries.
- Multi-layer deposition strategy: A typical cladding sequence involves:
- Layer 1 (transition layer): E309L deposited with high heat input to achieve good wetting and bonding to the 2.25Cr-1Mo base metal
- Layer 2–n (build-up layers): E309L or E347 deposited with controlled heat input to build up the required thickness
- Final layer (surface layer): E309L or E347 deposited with low heat input to achieve a fine microstructure and smooth surface finish
- Dilution control: The dilution rate in the first layer is typically 10–20%, which is higher than the target of <5%. This is managed by depositing multiple layers, as the dilution decreases with each subsequent pass. The composition of the cladding layer is verified by chemical analysis (spark emission spectrometry or wet chemistry) to ensure that the carbon content is below 0.04% (for 309L) or 0.08% (for 347).
- PWHT optimization: The post-weld heat treatment of 2.25Cr-1Mo components with cladding layers requires careful control of the heating and cooling rates to minimize the risk of temper embrittlement. A slow heating rate (≤100°C/h) is recommended, and the holding time at the PWHT temperature should be minimized to reduce the exposure time in the temper embrittlement range.
Defect Analysis and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cold cracking (HIC) | High CEV, hydrogen embrittlement | MT, PT, UT | Preheat to 150–250°C, use low-hydrogen consumables, post-weld bake |
| Temper embrittlement | Exposure to 375–525°C range | Charpy impact testing | Avoid slow cooling through embrittlement range, minimize PWHT hold time |
| Cladding layer cracking | High residual stress, thermal mismatch | MT, PT | Reduce heat input, use multiple thin layers, consider stress-relief annealing |
| Excessive dilution | High heat input, single-pass deposition | Chemical analysis | Use multi-layer deposition, reduce heat input, verify composition |
| Intergranular corrosion | Carbon precipitation at grain boundaries | ASTM A263 acid test | Use stabilized filler metals (347), control carbon content |
Engineering Practice and Standards Compliance
The fabrication of 2.25Cr-1Mo pressure vessels with cladding layers must comply with the relevant standards, including:
- GB/T 150 (Chinese national standard for pressure vessels): Specifies the design, fabrication, and inspection requirements for pressure vessels, including the requirements for cladding layers.
- NB/T 47014 (Chinese industry standard for welding procedure qualification): Provides the qualification requirements for welding procedures for pressure vessels, including the requirements for cladding welds.
- ASME VIII Div.1 (American standard for pressure vessels): Specifies the construction and inspection requirements for pressure vessels, including the requirements for overlay welds.
- ASME IX (American standard for welding qualification): Provides the qualification requirements for welding procedures and welders.
The welding procedure qualification (WPQ) for 2.25Cr-1Mo cladding must demonstrate that the procedure produces a cladding layer with the required composition, hardness, and bond strength. The qualification test typically involves depositing a test weld on a coupon of the same thickness and composition as the production component, followed by mechanical testing (tensile, bend, hardness) and metallurgical examination (microsection, intergranular corrosion test).
Key Insights and Reflections
The cladding process development for 2.25Cr-1Mo steel highlights the complexity of welding low-alloy steels that require corrosion-resistant overlay layers. The interplay between the base metal metallurgy (crack susceptibility, temper embrittlement), the cladding layer composition (dilution, intergranular corrosion resistance), and the process parameters (preheat, interpass temperature, PWHT) creates a multi-variable optimization problem that requires careful engineering analysis and thorough process qualification. For engineers involved in the fabrication of petrochemical pressure vessels, the key lessons are: (1) the preheat and interpass temperature control are critical for preventing cold cracking in 2.25Cr-1Mo steel; (2) multi-layer deposition is essential for controlling dilution and achieving the required cladding layer composition; (3) the PWHT parameters must be carefully optimized to balance residual stress relief with the risk of temper embrittlement; and (4) rigorous quality control, including chemical analysis, mechanical testing, and metallurgical examination, is essential for ensuring the long-term reliability of the cladded component in high-temperature, high-pressure service.