CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding of Medium Carbon Steel Thick-Wall High-Pressure Piping
Literature Overview
This 1994 technical paper by Lin Chengde from Angang Construction Electromechanical Company documents the application of gas tungsten arc welding for thick-walled medium carbon steel high-pressure piping systems. The research addresses one of the most challenging welding applications in petrochemical and power generation construction, where piping sections with wall thicknesses exceeding 30 mm must achieve full penetration with controlled microstructure to withstand high pressures and elevated temperatures.
Core Technical Parameters and Process Design
Medium carbon steel piping, typically of grades such as 20G, 15CrMo, or 25Cr2MoV, presents significant weldability challenges due to the elevated carbon equivalent and susceptibility to cold cracking in the heat-affected zone. The researcher documented a multi-pass TIG welding procedure designed for pipe diameters ranging from DN100 to DN300 with wall thicknesses of 25 to 45 mm.
| Parameter | Typical Value | Rationale |
|---|---|---|
| Base metal grade | 20G / 15CrMo | High-pressure service |
| Wall thickness | 25-45 mm | Heavy-wall application |
| Preheat temperature | 150-250 °C | Hydrogen crack prevention |
| Interpass temperature | 200-250 °C | Controlled cooling rate |
| Shielding gas | Pure argon | Stable arc, oxide protection |
| Tungsten electrode | 2% thorium (WC-20) | High current density |
| Filler wire | ER70S-6 / ER80S-6 | Matched tensile strength |
| Travel speed | 30-50 mm/min | Controlled heat input |
| Current range | 180-280 A (DCEN) | Deep penetration per pass |
Multi-Pass Strategy and Microstructure Control
The welding sequence employed a root pass technique using TIG with a backing bar or internal gas shield, followed by fill passes using either continued TIG or transition to SMAW for efficiency. The root pass was executed at reduced current (150-180 A) with a travel speed of 40-50 mm/min to achieve full penetration with a controlled bead profile. Subsequent fill passes increased current progressively to 220-280 A while maintaining interpass temperatures below 250 °C to avoid excessive grain growth in the HAZ.
A critical observation from this research was the relationship between heat input and the formation of tempered martensite in the HAZ. Heat inputs exceeding 25 kJ/mm produced coarse-grained HAZ regions with reduced impact toughness, while heat inputs below 12 kJ/mm risked incomplete grain boundary carbide dissolution, leading to potential intergranular cracking under thermal cycling.
Hydrogen-Induced Cracking Prevention
The study emphasized a comprehensive hydrogen management program including low-hydrogen filler metal selection, thorough surface preparation to remove moisture and hydrocarbon contamination, and controlled post-weld cooling. The researcher recommended that for wall thicknesses exceeding 30 mm, a post-weld heat treatment at 620-680 °C for 2 hours per 25 mm of thickness was mandatory to relieve residual stresses and temper any martensitic structures formed during welding.
Engineering Practice Integration
This work directly informs modern practices for welding high-pressure hydrogenation reactor piping and thick-walled pressure vessel nozzles. The principles of controlled heat input, hydrogen management, and post-weld stress relief remain unchanged in contemporary standards such as GB/T 150 and ASME VIII Div.1. Engineers fabricating bimetal pressure vessels with thick carbon steel backing plates would find the thermal management strategies documented here directly applicable to their overlay welding operations, particularly when using SAW or FCAW for multi-pass buildup of the backing layer prior to cladding.
Study Insights
The paper demonstrates that successful thick-wall welding is fundamentally a thermal management exercise rather than merely an arc stability challenge. The systematic approach to preheat selection, interpass control, and post-weld treatment represents a mature engineering methodology that should guide all similar applications. The emphasis on metallurgical control rather than purely mechanical qualification reflects a sophisticated understanding of long-term service reliability in high-pressure environments.