Zhuojin Pipe Fitting Co., LtdDevelopment of PLC-Based MIG/MAG Automatic Welding Equipment for Stainless Steel Sanitary Tanks
Literature Overview and Industrial Application Context
This paper by Song Jinhua, published in Welding Machine (2012, Vol. 42, No. 7, pp. 44-47), describes the design and development of a fully automatic welding system for circumferential seam welding of stainless steel sanitary tanks. The author is affiliated with Shandong Jiaotong Vocational College, indicating an educational and technical training context for the work. The application domain is the food processing and pharmaceutical industries, where stainless steel sanitary tanks are used for storing, mixing, and processing food-grade and pharmaceutical liquids. The requirement for high weld quality, smooth internal surfaces, and compliance with sanitary standards makes this an excellent case study in process-controlled welding of austenitic stainless steels.
The stainless steel sanitary tank described in the paper is fabricated from cylindrical shell sections (barrel segments) and left and right heads (end caps), joined by circumferential butt welds. The welding of these circumferential seams is a repetitive, high-volume operation that demands consistency, precision, and productivity. Manual welding, while flexible, is prone to operator fatigue, inconsistent parameter application, and quality variation. The development of an automatic welding system addresses these challenges by providing repeatable, parameter-controlled welding with reduced labor intensity and improved throughput.
System Design and Technical Architecture
The automatic welding system described in the paper comprises four major subsystems: the machine frame, the workpiece clamping and rotation mechanism, the welding carriage, and the wire feeding mechanism. The system employs MIG/MAG welding (Gas Metal Arc Welding with either inert or active shielding gas) and is equipped with two welding torches, enabling simultaneous welding of two circumferential seams. The control system is centered on a Programmable Logic Controller (PLC), supplemented by a touch screen display, variable frequency drive, and relay logic components.
| Subsystem | Key Components | Function |
|---|---|---|
| Machine Frame | Welded steel structure, guide rails | Provides structural rigidity and alignment for all subsystems |
| Clamping and Rotation | Hydraulic or pneumatic clamps, motorized turntable | Secures the tank section and rotates it at controlled speed |
| Welding Carriage | Linear guide, positioning motor, torch mount | Translates the torch along the circumferential seam with precise speed control |
| Wire Feeding | Servo-driven wire feed motor, drive rollers | Delivers filler wire at consistent speed synchronized with travel speed |
| Control System | PLC, touch screen HMI, VFD, relays | Orchestrates all subsystems, manages welding parameters, and provides operator interface |
The dual-torch configuration is a notable design feature that enables simultaneous welding of two circumferential seams, effectively doubling productivity without increasing cycle time. This is particularly advantageous for tank fabrication shops that produce large quantities of similar tanks, as it allows parallel processing of two shell-to-head joints in a single setup. The PLC-based control architecture provides several advantages over hardwired relay logic: flexibility in programming different welding sequences, easy parameter adjustment via the touch screen interface, built-in diagnostic capabilities, and the ability to implement complex interlocks and safety logic.
The welding parameters are managed through multiple preset modes, each corresponding to a specific combination of current, voltage, travel speed, wire feed speed, and shielding gas flow rate. The operator selects the appropriate mode based on the plate thickness, joint geometry, and quality requirements of the specific tank section being welded. The variable frequency drive controls the rotation speed of the workpiece turntable, which is synchronized with the linear travel speed of the welding carriage to maintain a constant circumferential welding speed.
Welding Quality and Process Control
The quality of circumferential welds in stainless steel sanitary tanks is governed by several critical factors. First, the internal weld surface must be smooth and free of spatter, undercut, and other surface defects to facilitate cleaning and prevent bacterial contamination. Second, the weld must achieve full penetration without excessive reinforcement, as excessive weld metal can create crevices that trap residues. Third, the heat-affected zone must be controlled to minimize sensitization (chromium carbide precipitation) and intergranular corrosion susceptibility in austenitic stainless steels such as 304 and 316L.
| Quality Requirement | Control Method | Acceptance Criteria |
|---|---|---|
| Surface quality | Clean shielding gas, proper wire stick-out, post-weld grinding | No spatter, smooth contour, no undercut > 0.5 mm |
| Penetration | Adequate current and travel speed combination | Full penetration confirmed by visual inspection and UT |
| HAZ control | Low interpass temperature, controlled heat input | No sensitization, intergranular corrosion test passed |
| Geometric accuracy | Precise alignment and clamping, consistent travel speed | Weld reinforcement within 1-3 mm, no offset > 0.5 mm |
| Corrosion resistance | Appropriate filler metal (e.g., ER308L, ER316L), post-weld passivation | Salt spray test > 500 hours without pitting |
The PLC control system plays a crucial role in maintaining process consistency. By automating the welding sequence, the system eliminates operator-dependent variations in parameter application and travel speed. The touch screen interface allows the operator to monitor welding parameters in real time and make adjustments as needed, while the PLC ensures that all safety interlocks are enforced, including emergency stop, gas flow verification, and torch height control.
Study Insights and Engineering Practice Implications
This paper provides a comprehensive example of how PLC-based automation can be applied to improve welding productivity and quality in a specific industrial application. The dual-torch design is particularly innovative, demonstrating that productivity gains can be achieved through mechanical design innovation in addition to process parameter optimization. The application of MIG/MAG welding to stainless steel sanitary tanks is well-established, but the integration of PLC control with multi-torch operation represents a step forward in automation capability.
From a metallurgical perspective, the welding of austenitic stainless steel requires careful attention to filler metal selection, heat input control, and post-weld treatment. The use of low-carbon grades (e.g., 304L, 316L) or stabilized grades (e.g., 321, 347) minimizes sensitization risk, while the application of ER308L or ER316L filler wire ensures compatibility of the weld metal composition. Post-weld passivation and pickling are essential to restore the corrosion resistance of the weld and HAZ, particularly for food and pharmaceutical applications where hygienic standards are stringent.
The paper also highlights the importance of ergonomic and operational considerations in welding equipment design. The touch screen interface, multiple preset modes, and simplified operator controls reduce the cognitive load on the operator and minimize the potential for human error. This is consistent with the broader trend in manufacturing toward user-friendly, intuitive control interfaces that enable operators to focus on quality monitoring rather than parameter management.