All-Position Pulse TIG Welding of Stainless Steel Tube Sheets in Condensers

Literature Overview

This paper by Sun Lizhong and Zhao Qiaoliang, published in Foundry Technology in 2012 (Vol. 33, No. 5, pp. 606-607), describes the application of deep-penetration all-position pulse TIG multi-pass welding technology for stainless steel tube-to-tubesheet fillet welds in condensers. The work was conducted at Zhejiang Vocational and Technical College of Industry, reflecting the practical, industry-oriented nature of the research.

Core Technical Content

Application Context

Condensers in power plants, chemical processing, and marine applications rely on the integrity of tube-to-tubesheet joints to prevent leakage between the tube side and shell side fluids. These joints are typically fillet welds connecting hundreds or thousands of small-diameter tubes (commonly 12–25 mm OD) to a thick tubesheet (15–50 mm). The welding must be performed in all positions—flat, horizontal, vertical, and overhead—as the tubes are arranged radially around the tubesheet center.

Pulse TIG Welding for Tube Sheet Applications

Pulse TIG welding offers several advantages for tube sheet welding:

Reduced heat input: The pulsed current alternates between a high peak current (for penetration) and a low background current (for maintaining the arc), resulting in lower total heat input compared to DC continuous TIG.
Improved weld pool control: The periodic cooling effect of the background current helps control weld pool flow in vertical and overhead positions.
Reduced distortion: Lower heat input minimizes tubesheet warpage, which is critical for maintaining tube straightness and expansion fit.
Deep penetration: The peak current provides sufficient penetration for root pass quality without excessive dilution.

Welding Sequence and Direction Strategy

Welding Position	Recommended Direction	Key Considerations
Flat (bottom)	Clockwise or counterclockwise	Easiest position; establish baseline parameters
Horizontal	Upward or downward	Control weld pool sagging
Vertical	Upward (preferred)	Minimize weld pool overflow
Overhead	Upward	Maximum challenge; smallest parameters

The authors emphasize the importance of a rational welding sequence to minimize cumulative distortion. A common strategy is to weld in a balanced, symmetric pattern—welding opposite tubes simultaneously or in alternating sequences—to counteract thermal distortion.

Combined Welding and Expansion Approach

A distinctive feature of this work is the combined approach of welding and tube expansion ("贴胀"). The tubes are first mechanically expanded into the tubesheet holes to achieve an interference fit, then welded in place. The welding is performed in a manner that complements the expansion:

Pre-expansion: Tubes are mechanically expanded to 1.0–1.5% interference fit.
Root welding: Pulse TIG root pass establishes the weld metal bond.
Post-expansion: Additional expansion may be applied after welding to ensure tight seal.
Fill and cap passes: Additional passes build up the fillet weld to the required geometry.

Process Analysis and Engineering Implications

Typical Pulse TIG Parameters for Tube Sheet Welding

Parameter	Typical Range	Notes
Peak current	60–120 A	Depends on tube diameter and tubesheet thickness
Background current	20–40 A	Must maintain stable arc
Pulse frequency	5–20 Hz	Higher frequency for better pool control
Duty cycle	30–50%	Ratio of peak to total current time
Travel speed	50–150 mm/min	Slower for deeper penetration
Shielding gas	Ar or Ar/He mix	He mix for higher penetration
Tungsten electrode	2.4–3.2 mm diameter	Ceriated or lanthanated for stability

Common Defects in Tube Sheet Welding and Countermeasures

Defect	Cause	Detection Method	Countermeasure
Root undercut	Excessive peak current	Visual/RT	Reduce peak current or increase travel speed
Burn-through	Excessive heat input	RT/PT	Reduce peak current, increase frequency
Lack of fusion	Insufficient penetration	RT/UT	Increase peak current or reduce travel speed
Porosity	Gas absorption from tube surface	RT/PT	Clean tube surfaces, improve shielding
Distortion	Uneven welding sequence	Dimensional inspection	Use balanced welding sequence
Cracking	High residual stress	MT/PT	Apply preheat, reduce heat input

Quality Control Considerations

For condenser tube sheet welds, the quality control regime must include:

Visual inspection (VT): 100% inspection of all welds for surface defects.
Penetrant testing (PT): 100% inspection for surface-breaking defects, particularly at the tube-to-tubesheet junction.
Radiographic testing (RT): 10–100% depending on application criticality and code requirements.
Hydrostatic testing: Pressure testing of the condenser after fabrication to verify joint integrity.
Dimensional inspection: Verification of tubesheet flatness and tube alignment after welding.

Study Insights and Reflections

This paper addresses a highly practical and critical welding application. Condenser tube sheet joints are among the most numerous and most failure-prone welds in power generation and process industries. A single tube failure can lead to condenser water contamination, turbine damage, and unplanned shutdowns with costs exceeding millions of dollars.

The emphasis on pulse TIG welding for all-position tube sheet welding is well-founded. In my experience, the transition from manual DC TIG to pulse TIG for tube sheet welding has been one of the most impactful improvements in the past two decades. The reduced heat input and improved pool control have significantly reduced distortion-related rework and improved overall joint quality.

The combined welding-and-expansion approach described in this paper is a pragmatic solution to the challenge of achieving both metallurgical bonding and mechanical sealing in tube sheet joints. However, I would emphasize that the expansion parameters must be carefully controlled to avoid over-expanding the tubes, which can cause plastic deformation and reduce fatigue life.

The practical orientation of this research—focused on weld sequence, welding direction, and parameter selection rather than fundamental metallurgy—is exactly what is needed for field-level improvement of welding quality. Engineers working on condenser fabrication should pay close attention to the welding sequence strategy, as this is often the most neglected aspect of tube sheet welding quality.