A-TIG Welding Technology for Double Tube Plate Heat Exchangers

Literature Overview

The study by Diao Zhifeng, Yang Shubiao, Huang Renlong, Ji Qinghe, Yan Hua, and Gu Zhimin (2012), published in Hot Working Technology, addresses a critical engineering challenge in the fabrication of double tube plate heat exchangers. Double tube plate heat exchangers are widely used in high-pressure and high-temperature applications, particularly in chemical processing, hydrogenation reactors, and petrochemical industries. The A-TIG welding technique (Automated Tungsten Inert Gas welding) is essential for joining the tube-to-tube plate interfaces in these heat exchangers, where the geometry presents significant access constraints and the material combinations often involve dissimilar metals requiring careful thermal management.

Core Technical Challenges

Double tube plate heat exchangers differ fundamentally from conventional single tube plate designs. The primary technical difficulty lies in the welding of the inner tube plate, which is positioned between two outer tube plates with extremely limited access for torch positioning and filler wire feeding. The A-TIG welding process must achieve full penetration in a confined space while minimizing distortion in the thick-walled tube plates. The authors identify several key challenges:

The restricted accessibility for the welding torch and filler wire in the annular gap between the inner tube plate and the outer tube plate
The requirement for precise control of heat input to avoid excessive distortion in the thick tube plate assembly
The need for consistent weld quality across hundreds of tube-to-tube plate joints in a single heat exchanger shell
The potential for intergranular sensitization when welding austenitic stainless steel tube plates to carbon steel or low-alloy steel shell materials

Process Parameters and Technical Approach

The authors investigated a systematic approach to A-TIG welding parameters for the double tube plate configuration. The key parameters studied include:

Parameter	Typical Range	Purpose
Welding Current (I)	150-250 A	Controls penetration depth
Welding Speed (V)	5-15 cm/min	Balances heat input and deposition rate
Shielding Gas Flow	15-20 L/min	Ensures adequate protection of the weld pool
Arc Length	2-4 mm	Maintains stable arc and penetration
Filler Wire Diameter	1.6-2.4 mm	Adapts to joint geometry
Preheat Temperature	50-150 °C	Reduces cooling rate and cracking susceptibility
Interpass Temperature	100-250 °C	Controls HAZ properties

The A-TIG process parameters were optimized through both experimental welding trials and engineering analysis. The authors emphasized the importance of a push-type or drag-type torch configuration depending on the specific geometry of the tube-to-tube plate joint. For the inner tube plate welding, a specialized torch holder was designed to navigate the annular gap between the tube plate and the shell.

Weld Quality Assessment and Defect Analysis

The study employed a comprehensive quality assessment approach including:

Visual Inspection (VT): Checking for undercuts, overlaps, and surface irregularities
Radiographic Testing (RT): Detecting internal porosity, incomplete fusion, and lack of penetration
Ultrasonic Testing (UT): Evaluating weld thickness and detecting planar defects
Metallographic Examination: Assessing microstructure, penetration profile, and HAZ characteristics

Common defects identified in double tube plate A-TIG welding include:

Defect Type	Root Cause	Countermeasure
Incomplete penetration	Insufficient heat input or misalignment	Increase current, optimize torch angle
Undercut	Excessive welding speed or arc length	Reduce speed, maintain consistent arc length
Porosity	Inadequate shielding gas coverage	Increase gas flow, improve gas nozzle design
Cracking	High cooling rate or hydrogen pickup	Preheat, use low-hydrogen consumables
Distortion	Excessive heat input	Reduce heat input, use backing plates

Engineering Practice Integration

The practical implementation of A-TIG welding for double tube plate heat exchangers requires careful consideration of the overall fabrication sequence. The authors recommend welding the outer tube plate first, followed by the inner tube plate, with controlled cooling between operations. The welding sequence should follow a symmetric pattern to minimize cumulative distortion. In the case of Shuangliang Boiler Company, where this research was applied, the A-TIG welding process was integrated into a production line with automated torch positioning systems to ensure repeatability and consistency across multiple heat exchanger units.

Study Insights and Reflections

This research is particularly valuable because it bridges the gap between laboratory-scale welding studies and actual production requirements for double tube plate heat exchangers. The emphasis on practical process optimization, rather than purely academic parameter studies, makes the findings directly applicable to fabrication engineers. The development of a specialized torch holder for the inner tube plate welding demonstrates the necessity of equipment adaptation when conventional welding configurations cannot access the joint geometry. The study also highlights the importance of welding procedure qualification under NB/T 47014 or ASME IX conditions, particularly for dissimilar metal combinations commonly encountered in heat exchanger fabrication. The work underscores that successful A-TIG welding of double tube plate heat exchangers requires not only proper parameter selection but also careful attention to joint preparation, backing arrangements, and post-weld heat treatment where required by applicable standards such as GB/T 151 or ASME VIII Div.1.