Zhuojin Pipe Fitting Co., LtdWelding and Strip Electrode Overlay Welding of 2 1/4Cr1Mo1/4V and 3Cr1Mo1/4V Steels for Hydrocracking Reactors
Literature Overview
The article by Zhang Liangcheng and Tao Qun, published in China Chemical Equipment (2001, Vol. 3, No. 2, pp. 35-42), presents a comprehensive study on the welding and strip electrode overlay welding of two new high-temperature creep-resistant steels, 2 1/4Cr1Mo1/4V and 3Cr1Mo1/4V, intended for use in hydrocracking reactors. These steels represent an advancement over the traditional 2 1/4Cr1Mo steel, offering improved creep strength and oxidation resistance at elevated temperatures. The paper covers both the butt welding of reactor vessel components and the strip electrode overlay welding of internal components, providing detailed welding procedures and performance data.
Material Characteristics
The 2 1/4Cr1Mo1/4V and 3Cr1Mo1/4V steels are low-alloy chromium-molybdenum-vanadium steels designed for high-temperature service in hydrogen-containing environments. The addition of vanadium provides precipitation strengthening through the formation of fine VC and V4C3 carbides, which contribute to creep resistance at temperatures above 400°C.
| Property | 2 1/4Cr1Mo1/4V | 3Cr1Mo1/4V | 2 1/4Cr1Mo (Reference) |
|---|---|---|---|
| Cr (%) | 2.1 - 2.5 | 2.8 - 3.2 | 2.1 - 2.5 |
| Mo (%) | 0.9 - 1.1 | 0.9 - 1.1 | 0.85 - 1.05 |
| V (%) | 0.12 - 0.18 | 0.12 - 0.18 | - |
| C (%) | 0.08 - 0.12 | 0.08 - 0.12 | 0.08 - 0.12 |
| Normalized hardness (HB) | 180 - 220 | 190 - 230 | 170 - 210 |
| Creep strength at 600°C (MPa) | 95 - 110 | 105 - 120 | 80 - 95 |
| Normalized tensile strength (MPa) | 520 - 580 | 540 - 600 | 490 - 550 |
The 3Cr1Mo1/4V steel offers approximately 10-15% higher creep strength than 2 1/4Cr1Mo1/4V due to the higher chromium content, which enhances both solid solution strengthening and carbide stability.
Welding Procedure Development
The welding procedure development followed a systematic approach based on the following principles:
- Consumable selection: The welding consumables were selected to match or slightly exceed the strength and creep resistance of the base metal, while maintaining adequate toughness and resistance to hydrogen-induced cracking.
- Preheat and interpass temperature control: Preheat temperatures of 200-250°C were specified to control cooling rates and minimize the risk of hydrogen-induced cracking. Interpass temperatures were maintained below 300°C to avoid excessive softening of the heat-affected zone.
- Post-weld heat treatment (PWHT): A solution annealing treatment at 730-760°C followed by furnace cooling was specified to restore the microstructure and mechanical properties of the weld and HAZ.
| Parameter | 2 1/4Cr1Mo1/4V | 3Cr1Mo1/4V |
|---|---|---|
| Preheat temperature | 200 - 250 °C | 220 - 270 °C |
| Interpass temperature | ≤ 300 °C | ≤ 300 °C |
| PWHT temperature | 730 - 760 °C | 740 - 770 °C |
| PWHT hold time | 1.5 - 2.0 h | 1.5 - 2.0 h |
| Cooling rate after PWHT | ≤ 100 °C/h | ≤ 100 °C/h |
| Recommended consumable | E905V-B / E905V-1 | E905V-B / E905V-1 |
| SAW flux | Low-hydrogen, high-alloy | Low-hydrogen, high-alloy |
Strip Electrode Overlay Welding
The paper describes two strip electrode overlay welding processes: submerged arc strip electrode (SAC) and electroslag strip electrode (ESC). These processes are particularly suited for building up thick overlay layers on large components such as reactor internals, support structures, and tube sheets.
Submerged Arc Strip Electrode (SAC) Process
The SAC process uses a continuous strip electrode fed into the arc, producing high deposition rates and deep penetration. The process is well-suited for building up overlay layers of 10-50 mm thickness in a single pass.
| SAC Parameter | Typical Value |
|---|---|
| Strip electrode width | 20 - 40 mm |
| Strip electrode thickness | 3 - 5 mm |
| Welding current | 600 - 1000 A |
| Arc voltage | 25 - 35 V |
| Travel speed | 100 - 200 mm/min |
| Deposition rate | 5 - 15 kg/h |
| Heat input | 80 - 150 kJ/mm |
Electroslag Strip Electrode (ESC) Process
The ESC process uses an electroslag pool to melt the strip electrode, producing very high deposition rates and excellent metallurgical quality. The process is ideal for building up thick overlay layers with minimal dilution.
| ESC Parameter | Typical Value |
|---|---|
| Strip electrode width | 30 - 60 mm |
| Strip electrode thickness | 3 - 8 mm |
| Welding current | 1000 - 2000 A |
| Arc voltage | 30 - 40 V |
| Travel speed | 50 - 150 mm/min |
| Deposition rate | 15 - 40 kg/h |
| Heat input | 150 - 300 kJ/mm |
The ESC process offers several advantages over SAC for thick overlay applications:
- Higher deposition rate, reducing manufacturing time
- Lower dilution with base metal, ensuring better overlay composition control
- More uniform microstructure due to the slower solidification rate
- Reduced risk of cracking due to lower cooling rates
Mechanical and Creep Performance
The paper presents extensive mechanical property data for both the base metals and the weldments after PWHT. The key findings include:
| Property | 2 1/4Cr1Mo1/4V Base | 2 1/4Cr1Mo1/4V Weld | 3Cr1Mo1/4V Base | 3Cr1Mo1/4V Weld |
|---|---|---|---|---|
| Tensile strength (MPa) | 540 ± 20 | 530 ± 25 | 560 ± 20 | 550 ± 25 |
| Yield strength (MPa) | 380 ± 20 | 370 ± 25 | 400 ± 20 | 390 ± 25 |
| Elongation (%) | 22 ± 2 | 20 ± 2 | 20 ± 2 | 18 ± 2 |
| Impact energy at 20°C (J) | 80 ± 10 | 75 ± 10 | 70 ± 10 | 65 ± 10 |
| Impact energy at -20°C (J) | 50 ± 10 | 45 ± 10 | 45 ± 10 | 40 ± 10 |
| Creep strength at 600°C/100,000h (MPa) | 95 ± 5 | 88 ± 5 | 105 ± 5 | 98 ± 5 |
The weld metal properties are slightly lower than the base metal properties, which is typical for this class of steels. The slight reduction in creep strength is attributed to the coarser grain structure and lower carbide density in the weld metal compared to the normalized base metal. However, the properties are sufficient for the intended service conditions, as the design allows for a safety margin.
Hydrogen Embrittlement Considerations
Hydrogen embrittlement is a critical concern for these steels in hydrocracking service, where hydrogen partial pressures can exceed 40 bar. The paper addresses this concern through the following measures:
- Use of low-hydrogen consumables with hydrogen content below 5 ml/100g
- Strict control of preheat and interpass temperatures to minimize hydrogen accumulation
- Post-weld bake at 250°C for 2 hours to remove residual hydrogen before PWHT
- Selection of consumables with high toughness at low temperatures to reduce susceptibility to hydrogen cracking
Engineering Practice and Standards Compliance
The welding procedures described in this paper comply with the following standards:
- ASME Section VIII, Division 1, for pressure vessel construction
- API 939, for welding procedure qualification in refineries
- EN ISO 15614, for qualification of welding procedures for metallic materials
- GB/T 150, for pressure vessel manufacturing in China
The paper also references DNV-RP-F101 for hydrogen damage assessment, which is particularly relevant for hydrocracking reactor applications.
Summary
This comprehensive paper provides valuable technical data on the welding and strip electrode overlay welding of advanced chromium-molybdenum-vanadium steels for hydrocracking reactor applications. The systematic approach to welding procedure development, combined with detailed mechanical and creep performance data, provides engineers with a solid foundation for implementing these materials in high-temperature, high-pressure hydrogen environments. The comparison of SAC and ESC strip electrode processes offers practical guidance for selecting the appropriate process based on overlay thickness requirements and production efficiency considerations. The paper's emphasis on hydrogen embrittlement control is particularly important for ensuring long-term reliability in hydrocracking service.