CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding of Thick-Wall Medium Carbon Steel High-Pressure Piping
Literature Overview
This 1994 paper by Lin Chengde from Ansteel Construction Electromechanical Company, published in Welding Technology, addresses the challenges of TIG welding thick-wall medium carbon steel high-pressure piping. The paper is particularly relevant to engineers working on pressure vessel fabrication because it deals with the same material system—medium carbon steel—and the same service environment—high-pressure applications—that are encountered in the fabrication of cladded pressure vessels and heat exchanger tubesheets. The welding challenges described, including hydrogen-induced cracking susceptibility, residual stress management, and multi-pass weld procedure development, are directly applicable to the root and fill pass welding of clad plate pressure vessels.
Core Technical Content
The paper describes a TIG welding procedure developed for medium carbon steel piping with wall thicknesses in the range of 20-60 mm and operating pressures exceeding 10 MPa. The material specified was a typical medium carbon steel with carbon content of approximately 0.30-0.35 wt%, which falls within the range of steels commonly used as base plates for clad plate pressure vessels. The authors identified hydrogen-induced cracking (HIC) and delayed cracking as the primary welding defects of concern and developed a comprehensive procedure to mitigate these risks.
The welding procedure described includes the following key elements:
- Preheating: Preheat temperatures of 150-250°C depending on wall thickness and carbon equivalent. The authors recommended using the carbon equivalent formula CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 to assess cracking susceptibility and determine the required preheat temperature.
- Interpass temperature control: Interpass temperatures maintained between 150-300°C to prevent excessive cooling rates that could promote hard and brittle microstructures in the heat-affected zone.
- Shielding gas selection: Pure argon at 15-20 L/min for root pass welding, with a backing gas of argon or helium-argon mixture to protect the root side from oxidation.
- Electrode selection: Thoriated tungsten electrodes (WC-20) with diameters of 3-4 mm depending on welding current.
- Filler metal: ER50-6 solid wire or E5018 low-hydrogen electrode for subsequent SMAW passes.
| Wall Thickness | Preheat Temperature | Interpass Temperature | Welding Current | Travel Speed |
|---|---|---|---|---|
| 20-30 mm | 150°C | 150-250°C | 120-160 A | 80-120 mm/min |
| 30-45 mm | 200°C | 200-300°C | 160-200 A | 60-100 mm/mm |
| 45-60 mm | 250°C | 200-300°C | 200-250 A | 50-80 mm/min |
Standards and Code Compliance
The welding procedure described in this paper must comply with applicable pressure vessel codes and standards. For Chinese pressure vessel fabrication, the primary standards include GB/T 150 (Pressure Vessels), NB/T 47014 (Procedure Qualification), and NB/T 47015 (Welding Procedure). For ASME-regulated fabrication, the corresponding standards are ASME VIII Div.1, ASME IX (Welding), and ASME II (Materials).
The paper's emphasis on preheat temperature and interpass temperature control aligns with the requirements of NB/T 47014 and ASME IX, which mandate preheat temperatures based on carbon equivalent and wall thickness to prevent cold cracking. The authors' recommended preheat temperatures are consistent with the minimum preheat requirements specified in these standards for medium carbon steels with carbon equivalent values in the range of 0.4-0.5.
A critical aspect of the procedure qualification process is the demonstration that the welding procedure produces welds with acceptable mechanical properties and microstructure. For medium carbon steel welds, the following tests are typically required:
- Tensile testing: Weld metal tensile strength must meet or exceed the minimum specified tensile strength of the base material.
- Hardness testing: Hardness in the heat-affected zone must not exceed 350 HV for unpeened welds or 375 HV for peened welds, in accordance with ASME VIII Div.1 and NB/T 47014.
- Impact testing: Charpy V-notch impact energy must meet minimum requirements at the service temperature, typically 20-40 J at 0°C or -20°C depending on the design temperature.
- Macrograph and micrograph examination: Weld cross-sections must show complete fusion, no slag inclusions, and acceptable microstructure in the weld metal and HAZ.
Engineering Practice and Defect Analysis
The paper identifies several common welding defects encountered in thick-wall medium carbon steel piping and provides countermeasures for each:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Hydrogen-induced cracking | High diffusible hydrogen, rapid cooling, high restraint | Increase preheat, use low-hydrogen filler, control interpass temperature |
| Porosity | Inadequate shielding, contaminated filler or base metal | Increase gas flow, clean surfaces, use covered electrodes |
| Lack of fusion | Insufficient heat input, poor joint fit-up | Increase current, reduce travel speed, improve fit-up |
| Undercut | Excessive current, improper torch angle | Reduce current, maintain proper torch angle (10-15° from vertical) |
| Excessive dilution | Excessive current, slow travel speed | Reduce current, increase travel speed |
For cladding applications involving medium carbon steel substrates, the defect analysis and countermeasures described in this paper are directly applicable. The root pass of a clad plate pressure vessel weld is essentially the same as the root pass of a thick-wall medium carbon steel pipe weld, and the same principles of heat input control, hydrogen management, and fit-up quality apply. Engineers developing welding procedures for clad plate pressure vessels should reference the process parameters and defect countermeasures described in this paper as a starting point for procedure development.
Key Reflections and Study Insights
This paper provides a practical and thorough treatment of TIG welding for thick-wall medium carbon steel, which is directly relevant to the root and fill pass welding of clad plate pressure vessels. The systematic approach to procedure development—including preheat determination, parameter selection, and defect analysis—provides a useful framework for engineers developing welding procedures for cladding and overlay applications.
One key insight from this paper is the importance of carbon equivalent in assessing cracking susceptibility. For medium carbon steels used as base plates in clad plate pressure vessels, the carbon equivalent value directly influences the required preheat temperature and interpass temperature. Engineers should always calculate the carbon equivalent of the base material before developing a welding procedure, and use this value to determine the minimum preheat temperature in accordance with applicable code requirements.
Another important reflection is the emphasis on fit-up quality. The paper notes that even with proper preheat and process parameters, poor joint fit-up can lead to welding defects such as lack of fusion and excessive dilution. For cladding operations, where the base metal and overlay material are dissimilar, fit-up quality is even more critical because variations in root gap and root face preparation can significantly affect the dilution rate and bonding quality of the overlay layer.
In conclusion, this literature provides valuable practical guidance for engineers involved in the welding of medium carbon steel components, including clad plate pressure vessels. The systematic approach to procedure development, the emphasis on hydrogen management, and the detailed defect analysis all contribute to a deeper understanding of the welding challenges associated with medium carbon steels. Engineers should use this paper as a reference when developing welding procedures for clad plate pressure vessels and should ensure that all procedure qualification tests meet the requirements of applicable codes and standards.