The Role of Weld Seam Heat Treatment Units in Second-Hand Welded Pipe Machines’ Structural Integrity

Weld seam heat treatment units are critical post-welding components in second-hand welded pipe machines, addressing the residual stress and microstructural defects caused by the welding process. Unlike new machines with integrated heat treatment systems, second-hand welded pipe machines often lack this function—leading to weld brittleness, cracking risks, and failure to meet industry standards (e.g., API 5L X80 for high-pressure pipelines). A retrofitted heat treatment unit resolves these issues, improving weld fatigue resistance by 35-45% and ensuring pipes withstand harsh operating conditions (e.g., low temperatures, high pressure).

1. Residual Stress Relief to Prevent Weld CrackingElectric resistance welding (ERW) in second-hand welded pipe machines generates high residual stress (up to 600 MPa) in the weld zone, which can cause cracks during subsequent processing (e.g., bending, hydrotesting) or long-term service. The heat treatment unit uses induction heating (frequency 10-50 kHz) to heat the weld seam to 600-650°C (below the steel’s phase transformation temperature) and hold it for 2-5 minutes, then cool it at a controlled rate (5-10°C/s). This reduces residual stress by 70-80%, lowering crack rates from 12% (without heat treatment) to <1.5%. For DN200 ERW pipes used in Arctic oil pipelines (operating at -40°C), this stress relief ensures the weld meets the Charpy impact test requirement (≥40 J at -40°C), a key standard for low-temperature service.

2. Microstructural Optimization for Uniform Mechanical PropertiesThe welding process in second-hand machines often creates a coarse-grained heat-affected zone (HAZ) around the weld, which has lower tensile strength (e.g., 450 MPa vs. 550 MPa for base metal) and ductility. The heat treatment unit refines the HAZ microstructure: for carbon steel pipes (Q345B), it transforms coarse ferrite-pearlite grains into fine-grained structures, increasing HAZ tensile strength by 20-25% and elongation by 15-20%. This ensures the entire pipe (base metal + weld + HAZ) has uniform mechanical properties, reducing the risk of localized failure under pressure. For example, hydrotesting of DN150 pipes (test pressure 10 MPa) shows a 98% pass rate with heat treatment, vs. 75% without.

3. Compatibility with High-Speed ProductionTraditional off-line heat treatment requires moving pipes to a separate furnace, adding 30-45 minutes per batch and creating a production bottleneck. The in-line heat treatment unit is installed directly after the welding station, operating at speeds up to 80-100m/min (matching the second-hand machine’s maximum pipe output). It uses a closed-loop temperature control system (with infrared sensors sampling at 10 Hz) to adjust heating power in real time, ensuring consistent treatment even for variable pipe diameters (50-300mm). For a second-hand machine producing 500 pipes daily, this in-line integration increases throughput by 20-25% compared to off-line processing.

Practical Operation & Maintenance Guidelines

• Calibrate infrared temperature sensors weekly using a standard temperature source (600-650°C) to ensure accuracy within ±5°C.
• Inspect induction coils monthly for wear or insulation damage; replace coils if power loss exceeds 10%.
• Adjust heating parameters based on pipe material: 620°C for Q345B carbon steel, 650°C for Q690 high-strength steel.

For second-hand welded pipe machine operators targeting high-pressure or low-temperature markets, the heat treatment unit is a mandatory upgrade that bridges the gap between used equipment performance and strict industry standards, while avoiding costly product recalls.

For more information, please pay attention to the website of Jinyujie Mechanical and Electrical Used Pipe Mill Supplier：www.usedpipemill.com