TIG Welding in Large Forging Oil Press Piping Systems

Overview of the Study

This literature, authored by Zhang Shenpu and Sheng Xuebing from Taiyuan Heavy Industry Co., Ltd. Technical Center (2013), addresses the application of gas tungsten arc welding (GTAW/TIG) in the fabrication of piping systems for large forging oil hydraulic presses. These presses are critical heavy equipment used in aerospace, shipbuilding, and power generation industries, where hydraulic piping must withstand extreme pressures (often exceeding 40 MPa), cyclic loading, and harsh service environments. The study examines the challenges of joining thick-walled, high-strength alloy steel pipes under field and workshop conditions, where access is limited and distortion control is paramount.

Technical Challenges and Process Parameters

Large forging oil press piping typically involves carbon-manganese steels (Q345R, 16Mn) or low-alloy steels (15CrMo, 12Cr1MoV) with wall thicknesses ranging from 12 mm to over 40 mm. The primary challenge lies in achieving full-penetration welds with acceptable dilution, minimizing residual stress, and ensuring long-term fatigue resistance under cyclic hydraulic loading. The authors evaluated a multi-pass TIG welding approach with careful attention to root preparation, interpass temperature control, and post-weld treatment.

Key Technical Insights

The study emphasizes several critical process considerations. First, root pass quality is paramount in thick-wall pipe welding; the authors recommend using a backing bar technique with copper backing strips to ensure full root penetration and prevent backside oxidation. Second, the choice of electrode material significantly affects arc stability and penetration characteristics—WCer-40 electrodes provide deeper penetration suitable for root passes, while WCer-20 electrodes offer better bead appearance for fill and cap passes. Third, the study highlights the importance of maintaining low interpass temperatures to avoid excessive grain growth in the heat-affected zone (HAZ), particularly for low-alloy steels susceptible to temper embrittlement.

Defect Analysis and Countermeasures

A notable finding from the study is the correlation between hydrogen-induced cracking (HIC) susceptibility and inadequate preheating in high-strength steels. The authors recommend strict control of electrode moisture (drying at 250°C for 2 hours before use) and limiting the hydrogen content in the weld metal to below 5 mL/100g. Porosity formation was identified as a secondary concern, primarily caused by inadequate shielding gas flow rates in restricted-access areas; the recommended flow rate was 15–20 L/min with appropriate gas nozzles to ensure laminar flow over the weld pool.

Engineering Practice Integration

From a fabrication standpoint, the study's recommendations align with NB/T 47014 and ASME IX qualification requirements for weld procedure specification (WPS) development. The TIG-only approach, while slower than hybrid processes, offers superior control over weld geometry and dilution—critical when welding dissimilar material transitions (e.g., 15CrMo pipe to carbon steel flanges). For pressure vessel applications governed by GB/T 150 or ASME VIII Div.1, the study's emphasis on PWHT is essential for relieving residual stresses and achieving acceptable toughness in the HAZ. The fatigue life implications of weld quality in cyclic-pressure applications warrant careful consideration of surface finish and weld toe geometry, as these features act as stress concentrators under hydraulic pressure cycling.

Study Reflections and Implications

The 2013 publication predates many modern advances in TIG welding automation, yet its fundamental principles remain highly relevant. The emphasis on process discipline—controlled preheat, monitored interpass temperatures, and thorough post-weld treatment—reflects the philosophy that in heavy equipment fabrication, weld quality is achieved through systematic process control rather than reliance on operator skill alone. For engineers currently involved in large hydraulic press piping fabrication, this study serves as a reminder that even in an era of advanced hybrid welding technologies, the foundational understanding of GTAW metallurgy and heat input management remains indispensable. The study also implicitly highlights the importance of weld procedure qualification under actual service conditions, as laboratory-qualified procedures may not fully capture the geometric constraints and access limitations encountered in field fabrication environments.