CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Application in Large Forging Oil Press Machine Piping
Literature Overview
This paper by Zhang Shenpu and Sheng Xuebing from the Technical Center of Taiyuan Heavy Industry Co., Ltd., published in Welding Technology in 2013, describes the application of TIG welding in the fabrication of piping systems for large forging oil press machines. Large forging presses are critical equipment in the heavy industry sector, used for producing large forgings for power generation, shipbuilding, and oil and gas industries. The piping systems for these presses carry high-pressure hydraulic oil and must withstand extreme pressure cycles and thermal loading.
Core Technical Content
Piping System Requirements
The piping systems for large forging oil press machines have stringent requirements:
- Pressure rating: Typically 20–35 MPa (200–350 bar) for hydraulic oil circuits.
- Material: Carbon steel (20# or Q345R) or stainless steel (304, 316) depending on the service environment.
- Diameter range: 50–300 mm for main hydraulic lines.
- Wall thickness: 8–25 mm depending on pressure rating.
- Service temperature: -20°C to +150°C.
- Fatigue life: ≥10⁶ cycles for pressure cycling.
- Leak tightness: No visible leakage at 1.5 times working pressure.
TIG Welding Process Parameters
| Parameter | Typical Value | Notes |
|---|---|---|
| Polarity | DCEN | Deep penetration, stable arc |
| Current range | 150–400 A | Depends on wall thickness |
| Travel speed | 200–600 mm/min | Controlled heat input |
| Shielding gas | 100% Argon or Argon-Helium mix | Inert atmosphere |
| Gas flow rate | 10–15 L/min | Adequate coverage |
| Tungsten electrode | 2.4–4.0 mm, pure tungsten or thoriated | Arc stability |
| Filler wire | ER70S-6 (carbon steel) or ER308L (stainless steel) | Matched composition |
| Preheat temperature | 100–200°C for carbon steel | Reduce cracking risk |
| Interpass temperature | ≤250°C for carbon steel | Control cooling rate |
Welding Procedure Development
The authors describe a systematic approach to welding procedure development for large-diameter piping:
- Joint preparation: The joint is prepared with a V-groove or U-groove depending on the wall thickness. The groove angle is typically 60–75° for V-grooves and 40–60° for U-grooves. The root gap is 2–4 mm for full penetration welds.
- Weld sequence: The weld is deposited in multiple passes: root pass, fill passes, and cap pass. The root pass is critical for achieving full penetration and preventing root defects.
- Torch technique: The torch is held at a 10–15° angle to the direction of travel, with the tungsten electrode slightly offset to one side of the weld groove. The filler wire is introduced from the opposite side at a 30–45° angle.
- Heat input control: The heat input is controlled by adjusting the current, travel speed, and arc length. Excessive heat input can lead to grain coarsening, distortion, and reduced mechanical properties.
Quality Control and Inspection
| Test Method | Purpose | Frequency |
|---|---|---|
| Visual inspection (VT) | Surface defects, bead profile | 100% of welds |
| Dye penetrant testing (PT) | Surface-breaking defects | 100% of welds |
| X-ray radiography (RT) | Internal defects, porosity, lack of fusion | 10–100% depending on class |
| Ultrasonic testing (UT) | Lack of fusion, cracks | 10–100% depending on class |
| Hydrostatic pressure test | Leak tightness | 100% of piping systems |
| Tensile testing | Mechanical properties | Qualification |
| Bend testing | Ductility | Qualification |
| Hardness testing | HAZ hardening | Qualification |
Common Defects and Countermeasures
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Porosity | Hydrogen absorption, surface contamination | Thorough cleaning, preheat, high-purity shielding gas |
| Lack of fusion | Insufficient current, excessive travel speed | Adjust parameters, ensure proper joint fit-up |
| Cracking | Excessive cooling rate, constrained joint | Preheat, reduce travel speed, use low-hydrogen filler wire |
| Distortion | Asymmetric heat input | Double-sided welding, fixture design, post-weld straightening |
| Undercut | Excessive current or travel speed | Adjust parameters, use proper torch angle |
Engineering Practice Integration
In my experience with large forging press fabrication, the piping systems are one of the most critical components for ensuring the reliability and safety of the equipment. A single piping failure can result in catastrophic hydraulic oil leakage, equipment shutdown, and significant production losses. The TIG welding process described in this paper provides a reliable method for producing high-quality piping welds that meet the stringent requirements of large forging press applications.
Key Design Considerations
- Material selection: The piping material must be selected based on the service environment, pressure rating, and temperature range. Carbon steel is suitable for general hydraulic oil circuits, while stainless steel is required for high-temperature or corrosive environments.
- Welding procedure qualification: The welding procedure must be qualified in accordance with applicable standards such as ASME IX, GB/T 150, or NB/T 47014 before production application. The qualification must include mechanical property testing, non-destructive testing, and metallographic examination.
- Welder qualification: The welders must be qualified to perform the specific welding process, material, and joint configuration. The qualification test must include practical tests that demonstrate the welder's ability to produce sound welds under production conditions.
- Quality control: A comprehensive quality control plan must be implemented, including visual inspection, non-destructive testing, hydrostatic pressure testing, and mechanical property testing. The quality control plan must be documented and followed throughout the fabrication process.
Study Insights
This paper provides a practical guide to TIG welding for large forging press piping systems. The authors' experience with heavy industry fabrication is evident in the detailed process parameters, quality control measures, and defect analysis presented. For engineers involved in large equipment fabrication, the principles described here are directly applicable to the welding of high-pressure piping systems in various industrial applications.
The paper also highlights the importance of welding procedure development and qualification in ensuring the reliability of piping systems. The welding procedure must be designed to achieve full penetration, minimize heat input, and prevent defects such as porosity, lack of fusion, and cracking. The qualification process must include mechanical property testing, non-destructive testing, and metallographic examination to verify the weld quality and mechanical properties.
The integration of TIG welding with large forging press fabrication requires careful attention to the interaction between the piping material, weld metal, and heat-affected zone. The welding procedure must be designed to minimize the width of the HAZ and to maintain the mechanical properties of the base metal. This balance is critical for ensuring the structural integrity and service life of the piping system.
The paper also emphasizes the importance of welder training and qualification in producing high-quality welds. Unlike automated welding processes, manual TIG welding requires a high degree of skill and discipline to maintain consistent weld quality. Welders must be trained in proper joint preparation, torch technique, and visual assessment of weld quality. The qualification process should include practical tests that demonstrate the welder's ability to produce clean, sound welds under production conditions.
In conclusion, the TIG welding process described in this paper provides a reliable and efficient method for fabricating high-quality piping systems for large forging oil press machines. The detailed process parameters, quality control measures, and defect analysis presented in the paper provide valuable guidance for engineers involved in heavy industry fabrication. The integration of TIG welding with large equipment fabrication requires careful attention to material selection, welding procedure development, welder qualification, and quality control to ensure the reliability and safety of the equipment.