TIG Welding of Medium Carbon Steel Thick-Walled High-Pressure Piping

Literature Overview

This 1994 paper by Lin Chengde from Ansteel Construction Electromechanical Company, published in Welding Technology (Vol. 23, No. 1, pp. 42–43), addresses the challenging application of TIG welding to thick-walled high-pressure piping fabricated from medium carbon steel (45 steel). The specific application is the high-pressure water descaling pipeline for the Ansteel wide and heavy plate project, with pipe specifications of φ325×45 mm, a working pressure of 250 MPa, and a test pressure of 310 to 350 MPa. The pipe material has a carbon equivalent (CE) of 0.56 to 0.65%, which places it in a high weldability risk category. This paper is classified under TG444 (arc welding) and represents a significant engineering challenge in high-pressure piping fabrication.

Material and Process Challenges

The combination of medium carbon steel, thick wall, and extremely high pressure creates a unique set of welding challenges. The 45 steel (equivalent to AISI 1045 or ASTM A105) has a carbon content of approximately 0.45%, which results in a high carbon equivalent and consequently high susceptibility to cold cracking (hydrogen-induced cracking) in the heat-affected zone. The thick wall (45 mm) requires multiple welding passes, creating a complex thermal cycle with multiple heat-affected zone interactions.

The carbon equivalent calculation typically follows the formula CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. For 45 steel, the CE value of 0.56 to 0.65% exceeds the conventional threshold of 0.45% above which preheating becomes mandatory for thick sections. This means that without proper preheating and interpass temperature control, the weld is highly susceptible to hydrogen-induced cracking.

Welding Process Design

TIG welding of a 45 mm thick pipe wall requires approximately 8 to 12 passes depending on the groove preparation and welding parameters. The root pass is critical because it establishes the weld's internal geometry and must achieve full penetration with a smooth, uniform root profile. The root pass is typically performed with a smaller tungsten electrode (2.4 to 3.2 mm diameter), lower current (80 to 120 A), and higher travel speed to limit heat input and minimize HAZ width.

The fill passes use progressively larger electrodes and higher currents to build up the weld volume efficiently. The cap pass requires careful control of bead profile to meet non-destructive testing (NDT) acceptance criteria and to ensure smooth stress transition at the weld toe.

Preheating and Interpass Temperature Control

Given the high CE value, preheating is mandatory. The preheat temperature should be at least 200°C for the first pass, and the interpass temperature should be maintained between 150°C and 250°C throughout the multi-pass welding sequence. This temperature control serves two purposes: it slows the cooling rate of the weld metal and HAZ, reducing the formation of hard martensitic microstructures, and it allows trapped hydrogen to diffuse out of the weld metal before solidification.

Hydrogen Control

Hydrogen is the primary contributor to cold cracking in high-CE steel welds. Sources of hydrogen include moisture in the base metal, shielding gas contamination, and flux or coating moisture. For TIG welding, the primary hydrogen sources are base metal surface contamination and tungsten electrode contamination.

Hydrogen control measures include:

• Thorough cleaning of the weld preparation surfaces to remove oil, rust, and moisture
• Use of high-purity argon shielding gas (99.99% or higher)
• Use of dry, uncontaminated tungsten electrodes
• Avoidance of excessive arc length, which increases atmospheric pickup

Quality Assurance and Inspection

For a pressure vessel or high-pressure piping application at 250 MPa working pressure, the quality assurance requirements are stringent. The following inspection methods are typically required:

The hydrostatic test at 310 to 350 MPa is an extremely demanding requirement. At this pressure level, any weld defect—even a small porosity or incomplete fusion—can lead to catastrophic failure. The weld metal must achieve a tensile strength comparable to or exceeding the base metal, and the HAZ must maintain adequate toughness to prevent brittle fracture under high hoop stress.

Engineering Practice and Reflections

The welding of thick-walled medium carbon steel high-pressure piping is a high-risk application that demands rigorous process control and quality assurance. The paper's focus on TIG welding for this application is notable because, in modern practice, TIG is often combined with other processes (such as SAW or FCAW) for thick-section welding to improve productivity. However, for high-pressure applications where weld quality is paramount, TIG welding provides superior control over heat input and weld geometry.

The 45 mm wall thickness presents a significant productivity challenge for TIG welding. At typical TIG deposition rates of 1 to 3 kg/h, filling a 45 mm thick V-groove requires substantial welding time. A single joint may require 15 to 30 hours of welding time depending on the groove geometry and welding parameters. This is why hybrid approaches (TIG root plus SAW or FCAW fill) are commonly used in industry.

However, the paper's approach of using TIG for the entire weld sequence has merit in terms of quality consistency. A single welding process throughout the weld sequence eliminates the process transition zone that can create mechanical property discontinuities. The TIG process also produces a more uniform weld metal microstructure because the thermal cycle is more consistent across passes.

The key lessons from this paper for modern engineering practice are:

1. Preheating and interpass temperature control are non-negotiable for high-CE steel welding, regardless of the welding process used.
2. Hydrogen control through surface preparation and shielding gas purity is critical for preventing cold cracking.
3. Multi-pass welding of thick sections requires careful planning of the welding sequence to minimize residual stress and distortion.
4. For high-pressure applications, 100% NDT coverage and hydrostatic testing are essential quality assurance measures.

The paper also highlights the importance of welding procedure qualification (WPQ) for such demanding applications. Each welding parameter combination must be qualified through coupon testing and destructive examination to ensure that the weld meets the required mechanical properties and NDT acceptance criteria.

Summary

TIG welding of medium carbon steel thick-walled high-pressure piping at 250 MPa working pressure represents one of the most demanding welding applications in pressure vessel and piping fabrication. The combination of high carbon equivalent (0.56–0.65%), thick wall (45 mm), and extreme pressure requirements demands rigorous control of preheating, interpass temperature, hydrogen levels, and welding parameters. Engineers must recognize that weld quality in such applications is not merely a matter of meeting NDT acceptance criteria but of ensuring long-term structural integrity under sustained high-pressure loading. The TIG welding approach, while slower than alternative processes, provides the precision and control necessary to achieve the required weld quality for this critical application.