PLC-Based Steel Pipe TIG Welding Machine Control System Analysis

Literature Overview

This 2015 publication in the journal Welding, authored by Li Dandan and Zhang Kai from the Department of Mechanical and Electrical Engineering at Zhengzhou Tourism College, presents a programmable logic controller (PLC)-based control system for a TIG welding machine designed specifically for steel pipe welding operations. While the academic depth of the work is moderate, its practical engineering value lies in the systematic implementation of automated process control for a widely used welding configuration in piping and pressure vessel fabrication.

Control System Architecture

The PLC-based control system addresses the fundamental challenge of maintaining consistent weld quality across varying pipe diameters, wall thicknesses, and positional angles. In pressure vessel fabrication, pipe-to-flange welding, pipe-to-nozzle welding, and pipe butt welding are performed under ASME VIII Div.1 or GB/T 150 requirements, where weld quality directly impacts the structural integrity of the vessel.

System Components and Functions

Component	Function	Typical Specification
PLC Controller	Process sequencing and parameter control	Siemens S7-200 or equivalent
Inverter Power Supply	TIG arc current regulation	200-400 A, DC
Servo Motor	Torching head positioning	0.1 mm resolution
Gas Flow Controller	Shielding gas regulation	8-15 L/min Ar
Position Sensors	Pipe joint detection	Inductive proximity sensors
HMI Interface	Parameter input and monitoring	7-inch touchscreen

Process Control Logic

The PLC implements a multi-stage welding sequence:

Pipe alignment verification: Proximity sensors detect the joint gap and misalignment, triggering automatic correction before welding commences.
Preheating stage: A reduced current (30-50% of full welding current) is applied for a controlled duration to preheat the joint area, particularly critical for thick-walled pipes (≥ 10 mm) where cold cracking susceptibility increases.
Root pass welding: Low current (60-100 A), high travel speed, and precise torch oscillation produce a clean root with full penetration and minimal spatter.
Fill and cap passes: Increasing current (150-300 A) with programmed torch weaving patterns ensure uniform weld bead width and adequate fusion.
Post-heat treatment: Controlled cooling rate management through residual heat dissipation or auxiliary heating to prevent hydrogen-induced cracking in low-alloy steels.

Relevance to Pressure Vessel Fabrication

In the context of bimetal pressure vessel fabrication, pipe welding quality is critical for nozzle attachments, manway connections, and internal piping within hydrogenation reactors and heat exchangers. The PLC-based control system ensures repeatability of weld parameters—a prerequisite for meeting the requirements of NB/T 47014 welding procedure qualification. The system's ability to store and recall parameter sets for different pipe specifications directly supports the engineering practice of maintaining qualified WPS (Welding Procedure Specifications) for various combinations of base metals and fillers.

Quality Assurance Integration

The control system can be configured to implement real-time monitoring of critical parameters:

Arc voltage monitoring: Deviations beyond ±5% trigger alarm and automatic correction, preventing arc instability that leads to lack of fusion or porosity.
Travel speed verification: Encoder feedback ensures the programmed speed is maintained, preventing excessive heat input (which causes excessive grain growth and reduced toughness) or insufficient penetration.
Gas flow verification: Mass flow controllers ensure consistent shielding gas delivery, preventing atmospheric contamination and nitrogen/oxide inclusions.

Defect Prevention Through Process Control

The systematic parameter control enabled by the PLC system directly addresses common weld defects:

Defect	Root Cause	PLC Control Countermeasure
Porosity	Inadequate shielding gas	Flow rate monitoring and alarm
Lack of fusion	Excessive travel speed	Speed verification and correction
Undercut	Excessive arc length	Voltage monitoring and torch height control
Cold cracking	Rapid cooling rate	Post-weld heat treatment sequencing
Excessive reinforcement	Excessive current	Current regulation and bead width monitoring

Study Insights

While the publication is primarily focused on control system implementation, the underlying engineering principle—that consistent process parameter control is the foundation of weld quality assurance—is directly applicable to all aspects of cladding and overlay welding. In my experience with GTAW overlay of nickel-based alloys on carbon steel pressure vessel shells, the reproducibility of arc parameters is as critical as the selection of filler metal composition. The PLC-based approach demonstrates that automation of even basic parameter control significantly reduces the variability that leads to weld rejection and rework. For engineers developing welding procedure specifications, this reinforces the importance of defining not only the target parameters but also the acceptable tolerance bands and the monitoring methods for real-time process verification.