TIG Welding Application in Large Forging Oil Press Pipe Fabrication

Literature Overview

The study by Zhang Shenpu and Sheng Xuebing (2013), published in Welding Technology, documents the application of TIG (tungsten inert gas) welding in the fabrication of large-diameter pipes for heavy forging oil presses manufactured by Taiyuan Heavy Industry Group. This is a significant engineering case study because large forging oil presses represent some of the most demanding pressure-containing equipment in heavy industry, where pipe integrity directly affects operator safety and production continuity. The authors provide a comprehensive account of welding process selection, defect analysis, shielding techniques, and quality assurance methods tailored to this specific application.

Welding Process Selection and Rationale

The authors begin by reviewing the origins and working principles of TIG welding, then systematically compare it with alternative welding methods for this application. The key advantages of TIG welding identified in the study include:

Criterion	TIG Welding	SMAW	GMAW	SAW
Slag production	None	Significant	Moderate	Significant
Weld quality	High	Moderate	Good	Good
Penetration control	Excellent	Limited	Moderate	Good
Suitability for thin-wall large-diameter pipe	High	Low	Moderate	Low
Atmospheric contamination risk	Low	Moderate	Moderate	Low
Cost per meter	High	Low	Moderate	Moderate

For large forging oil press pipes, the wall thickness is typically in the range of 6–25 mm, and the diameter can exceed 500 mm. The combination of thin-to-moderate wall thickness, large diameter, and the requirement for high-quality welds makes TIG welding the preferred process. The absence of slag eliminates the risk of slag inclusion defects, and the precise control of the arc and heat input allows for excellent weld geometry and penetration control.

Process Parameters and Technique

The authors emphasize the importance of developing a rational welding procedure. For the specific application of large forging oil press pipes, the following process considerations are critical:

Joint preparation — V-groove or U-groove preparation with precise root gap control (typically 1–3 mm) to ensure full penetration without excessive burn-through.
Shielding gas delivery — The study highlights the use of a bladder-type shielding gas device to provide effective protection of the weld pool. This is particularly important for large-diameter pipes where conventional nozzle shielding may be insufficient due to the large cross-sectional area of the weld zone.
Travel speed and current — Balancing penetration depth with heat input to avoid distortion in the large-diameter pipe geometry.
Interpass temperature — Maintaining interpass temperatures below 150 °C to prevent excessive grain growth in the HAZ.

Defect Analysis and Quality Control

The study provides a thorough analysis of welding defects encountered during production and their root causes. This defect analysis follows a systematic approach consistent with FMEA (Failure Mode and Effects Analysis) principles:

Defect Type	Root Cause	Detection Method	Countermeasure
Lack of fusion	Insufficient heat input, poor joint fit-up	RT, UT	Increase current, improve fit-up
Porosity	Inadequate shielding, contaminated base metal	RT, PT	Improve gas coverage, clean surfaces
Cracking	Residual stress, hydrogen embrittlement	MT, PT	Preheat, post-weld stress relief
Excessive reinforcement	Excessive travel speed variation, filler wire excess	Visual, dimensional	Stabilize technique, control wire feed
Undercut	Excessive arc length, travel speed too high	Visual, UT	Reduce arc length, slow travel speed

The bladder-type shielding gas device deserves particular attention. In large-diameter pipe welding, the weld pool is exposed to atmospheric contamination from multiple directions. Conventional TIG nozzles with gas flow rates of 8–15 L/min may not provide adequate coverage for the extended weld length and large cross-section. The bladder-type device wraps around the pipe circumference, creating a sealed gas environment that ensures uniform protection of the entire weld pool. This technique is analogous to the internal gas shielding used in stainless steel pipe welding but adapted for external weld pool protection on large-diameter components.

Non-Destructive Testing Strategy

The authors specify ultrasonic testing (UT) as the primary volumetric inspection method for weld quality verification. This choice is well-justified for the application:

UT advantages for this application: High sensitivity to internal defects such as lack of fusion, cracks, and porosity; ability to inspect 100% of the weld length; minimal interference from the large-diameter pipe geometry; real-time feedback during production.
Supplementary methods: Visual inspection (VT) for surface defects, magnetic particle testing (MT) for surface-breaking cracks, and radiographic testing (RT) for spot-check verification of UT results.

The inspection protocol should follow relevant standards such as ASME B31.3 (Process Piping), ASME B31.1 (Power Piping), or API 5L, depending on the specific service conditions of the forging oil press pipes.

Engineering Practice Integration

From an engineering practice perspective, this study offers several valuable lessons for the fabrication of large-diameter pressure-containing pipes:

Welding procedure qualification (WPQ) — The study implicitly demonstrates the importance of formal WPQ testing before production welding. Each change in welding parameters, joint geometry, or material grade requires requalification.
Welder certification — Given the criticality of the application, welders must be certified to the specific welding procedure and qualified on the actual pipe geometry and position.
Production monitoring — Real-time monitoring of welding parameters (current, voltage, travel speed, gas flow) is essential to maintain consistent weld quality throughout production runs.
Traceability — Each weld should be traceable to the specific welder, welding procedure, electrode lot, and shielding gas lot for quality assurance and failure investigation purposes.

The application of TIG welding to large forging oil press pipes also raises the question of productivity. TIG welding is inherently slower than GMAW or SAW for thick sections, but the superior weld quality and reduced rework rates often result in lower total cost. For critical applications where weld failure has catastrophic consequences, the premium for TIG welding quality is justified.

Key Questions and Reflections

A notable aspect of this study is the practical orientation — it addresses real production challenges rather than theoretical welding metallurgy. The use of the bladder-type shielding device is an innovative solution to a common problem in large-diameter pipe welding. However, the study does not provide detailed quantitative data on welding parameters (current, voltage, travel speed, gas flow rate), which limits its direct applicability to other similar projects. Engineers adapting this approach to their own work should develop their own parameter matrices through WPQ testing.

Another reflection: the study does not discuss post-weld heat treatment or stress relief. For large forging oil press pipes subjected to cyclic loading and high internal pressures, residual welding stresses can be a significant concern. Engineers should evaluate whether post-weld stress relief (PWSR) is required based on the stress level, material toughness, and service conditions.

Summary

This study demonstrates the successful application of TIG welding to a demanding heavy industry application — large-diameter pipe fabrication for forging oil presses. The key technical contributions include the use of bladder-type shielding gas devices for effective weld pool protection on large-diameter pipes, systematic defect analysis with root cause identification, and UT-based quality assurance. The study reinforces the principle that welding process selection must be driven by the specific requirements of the application, not by convenience or cost alone. For engineers working on similar heavy industrial piping projects, the lessons of this case study — particularly regarding shielding technique and defect prevention — are directly transferable and highly valuable.