CLADDING TECHNOLOGY SHANXI CO., LTD2.25Cr-1Mo Cladding Process Technology
Literature Overview
This paper by Ma Xiangfeng, Gao Lei, and Zhang Yingying, published in Petrochemical Equipment in 2007, originates from the School of Mechanical Engineering at Liaoning Petrochemical University. The study focuses on the cladding process technology for 2.25Cr-1Mo steel, a widely used low-alloy steel in high-temperature pressure vessels, heat exchangers, and piping systems in the petrochemical industry. The 2.25Cr-1Mo steel offers excellent creep resistance and hydrogen resistance at elevated temperatures, but requires careful cladding when corrosion-resistant overlay layers are needed.
Material Characteristics and Cladding Challenges
2.25Cr-1Mo steel has a base composition of approximately 2.25% Cr and 1.0% Mo with a balanced amount of Mn, Si, and other alloying elements. The material is typically supplied in the normalized and tempered condition, with a hardness of 150–200 HB. Cladding this steel presents several challenges including susceptibility to hydrogen cracking, high dilution effects, and the need for controlled cooling rates to prevent brittle microstructures in the heat-affected zone (HAZ).
| Property | 2.25Cr-1Mo Steel | Typical Overlay (e.g., Inconel 625) |
|---|---|---|
| Carbon | 0.10–0.20% | < 0.10% |
| Chromium | 2.0–2.5% | 20.0–23.0% |
| Molybdenum | 0.85–1.05% | 8.0–10.0% |
| Hardness (HB) | 150–200 | 250–350 |
| Dilution tolerance | < 15% | < 20% |
| PWHT requirement | Mandatory | Recommended |
Process Selection and Parameters
The study evaluates multiple cladding processes including submerged arc welding (SAW), gas metal arc welding (GMAW), and gas tungsten arc welding (GTAW). Each process has distinct advantages and limitations. SAW offers high deposition rates and good penetration, making it suitable for thick overlay layers. GMAW provides good process control and versatility. GTAW offers the highest precision and is ideal for thin overlay layers or critical applications.
| Process | Deposition Rate | Penetration | Cost | Best Application |
|---|---|---|---|---|
| SAW | High | Deep | Low | Thick overlay layers |
| GMAW | Medium | Moderate | Medium | General purpose cladding |
| GTAW | Low | Shallow | High | Thin overlay, critical joints |
| ESW | Very high | Deep | Medium | Large vessel shells |
Process Parameters and Heat Input Control
The heat input during cladding is a critical parameter that affects the microstructure of both the overlay and the HAZ. Excessive heat input promotes grain growth and can lead to brittle phases in the HAZ of 2.25Cr-1Mo steel. The study recommends controlling the heat input to below 25 kJ/mm for SAW processes and below 10 kJ/mm for GMAW processes. Preheating to 200–300 °C is recommended to reduce cooling rates and minimize the risk of hydrogen-induced cracking.
| Parameter | SAW | GMAW | GTAW |
|---|---|---|---|
| Current (A) | 300–500 | 200–350 | 100–200 |
| Voltage (V) | 25–35 | 22–30 | 12–20 |
| Travel speed (mm/min) | 200–400 | 150–300 | 50–150 |
| Wire/feed diameter (mm) | 3.2–4.0 | 1.2–1.6 | Strip 6–10 |
| Shielding gas | Flux | Ar + CO₂ | Ar |
| Preheat (°C) | 200–300 | 200–300 | 150–250 |
Defect Analysis and Countermeasures
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Hydrogen cracking | High hydrogen absorption | Use low-hydrogen consumables, preheat |
| Hot cracks | High carbon dilution | Use transition layer, control dilution |
| Cold cracks | High cooling rate | Increase preheat, reduce heat input |
| Lack of fusion | Insufficient heat | Increase current, improve fit-up |
| Porosity | Gas entrapment | Clean surface, use proper shielding |
| Excessive HAZ hardening | High heat input | Reduce heat input, apply PWHT |
Post-Weld Heat Treatment
PWHT is mandatory for 2.25Cr-1Mo cladding applications to relieve residual stresses and stabilize the microstructure. The typical PWHT temperature is 720–760 °C with a holding time of 2 hours per 25 mm of thickness. The cooling rate should be controlled to avoid excessive hardness in the HAZ. The PWHT cycle must be carefully designed to avoid sensitization of the overlay layer, particularly for stainless steel overlays.
Engineering Practice and Standards Compliance
The cladding process must comply with relevant standards including NB/T 47014 for procedure qualification, GB/T 150 for pressure vessel design, and NB/T 47002 for material specifications. The welding procedure specification (WPS) must be qualified through mechanical testing, including tensile tests, bend tests, and hardness surveys. The overlay layer must meet specified hardness and composition requirements, and the bond line must be free of defects.
Summary
The cladding of 2.25Cr-1Mo steel requires a careful balance of process parameters, consumable selection, and heat treatment to achieve a reliable overlay that maintains the structural integrity of the base material. The key challenges include controlling dilution, preventing hydrogen cracking, and ensuring proper PWHT. Engineers should adopt a systematic approach that includes thorough procedure qualification, rigorous quality control, and adherence to applicable standards to ensure the long-term reliability of clad components in petrochemical service.