Case Study: PWHT Pipeline with Induction Heating Process for a Large-Diameter Carbon Steel Pipe
1. Objective
This case study presents a post weld heat treatment (PWHT) process for a large-diameter carbon steel pipeline using an 80kW induction heating power supply and a 30m flexible induction heating cable. The purpose of the project was to achieve controlled, uniform heating of the weld area and heat-affected zone to 650°C, ensuring proper stress relief, better metallurgical stability, and compliance with industrial PWHT requirements for welded pipeline systems.
The project focused on a carbon steel pipe with the following dimensions:
- Outside diameter: 880 mm
- Wall thickness: 25 mm
- Heating width: 480 mm
The main objective of the induction heating process was to replace slower and less controllable resistance heating methods with a faster, more flexible, and more efficient heating technology suitable for on-site or workshop pipeline PWHT applications.
2. Project Background
Post weld heat treatment is an essential process for many welded carbon steel pipelines used in oil and gas, petrochemical, refinery, pressure vessel, and power generation systems. After welding, residual stresses remain in the weld and heat-affected zone. If these stresses are not properly relieved, the welded structure may suffer from reduced mechanical reliability, dimensional instability, or increased susceptibility to cracking during service.
For this PWHT pipeline, the required PWHT temperature was 650°C, a common stress-relief temperature for carbon steel systems. Because of the large pipe diameter and thick wall section, the heating method needed to meet the following technical requirements:
- uniform circumferential heating
- stable soak temperature
- controlled heating and cooling rate
- easy installation around the pipeline
- reliable performance for field operation
Induction heating was selected because it offers rapid response, accurate control, low heat loss, and greater flexibility for large cylindrical workpieces.
3. Equipment Configuration
The induction PWHT system used in this case consisted of the following major elements:
3.1 Power Supply
- Type: Induction heating power supply
- Rated output power: 80 kW
- Frequency range: 1-20 kHz
- Control mode: Closed-loop temperature control
- Cooling method: Forced cooling / water-cooling depending on final system arrangement
- Application: PWHT of pipeline weld area
The 80kW power supply was selected as a practical configuration to provide enough power for stress-relief heating of the specified carbon steel pipe section while maintaining controllable ramp-up and soaking conditions.
3.2 Induction Coil
- Type: Flexible induction heating cable
- Total cable length: 30 m
- Arrangement: Wrapped circumferentially around the pipe over the required heating band
- Function: Generate an alternating magnetic field for controlled heat input into the pipe wall
The flexible cable arrangement was particularly suitable for this large pipe because it allowed fast installation, strong adaptability to curved surfaces, and even distribution of heating over the 480 mm band width.
4. Workpiece and Heating Parameters
4.1 Workpiece Specification
| Parameter | Value |
|---|---|
| Material | Carbon steel pipe |
| Outside diameter | 880 mm |
| Wall thickness | 25 mm |
| Heating width | 480 mm |
| PWHT target temperature | 650°C |
4.2 Process Heating Parameters
| Parameter | Value |
|---|---|
| Power supply | 80 kW induction heating power supply |
| Frequency | 1-20 kHz |
| Heating coil | 30 m flexible heating cable |
| Heating mode | Circumferential induction heating |
| Process type | Post weld heat treatment |
| Temperature control | Closed-loop monitoring |
| Heating target | 650°C stress relief |
5. Induction Heating Process Design
The induction heating setup was designed to cover the weld zone and adjacent heat-affected region evenly over the required 480 mm heating band. The flexible induction cable was wrapped in a controlled pattern around the outer circumference of the 880 mm diameter pipe. Insulation was applied over the heating area to reduce heat loss and improve temperature stability.
The induction power supply delivered medium-frequency energy through the flexible heating cable. This created alternating magnetic fields around the pipe section, which generated induced currents and heat in the pipe wall. Because the workpiece was carbon steel with substantial wall thickness, the selected frequency range of 1-20 kHz allowed sufficient control over the heating response and process stability.
The typical process sequence was:
- Wrap the flexible induction cable around the defined PWHT band.
- Install thermal insulation over the heating section.
- Attach thermocouples around the circumference and axial direction.
- Connect the coil to the 80kW induction power supply.
- Ramp the pipe section gradually to the target temperature.
- Hold at 650°C for the required soak period.
- Control the cooling rate according to PWHT specification.
6. Heating Process Analysis
6.1 Circumferential Heating Consideration
For a pipe with an OD of 880 mm, the circumference is approximately:
This means the induction cable had enough total length to create multiple turns over the full heating width of 480 mm, allowing stable heat distribution around the weld area.
6.2 Heating Band Consideration
The heating width of 480 mm was chosen to ensure that the weld seam and the adjacent heat-affected zone were both included in the effective stress-relief region. This is important in PWHT because stress is not limited only to the weld bead itself. The adjacent material must also be heated to the required temperature range to achieve reliable metallurgical results.
7. Testing and Data Analysis
To verify process quality, temperature measurement points were arranged at several circumferential and axial locations around the heating band. These test points were used to evaluate:
- heating uniformity
- time to target temperature
- soak stability
- temperature deviation during holding
- cooling control performance
7.1 Thermocouple Arrangement
Typical measurement points were placed at:
- 0°
- 90°
- 180°
- 270°
Additional axial points were placed near the edge of the heating band to check gradient control.
7.2 Heating Test Data Table
| Test Point | Time to 300°C | Time to 500°C | Time to 650°C | Peak Temperature | Final Soak Temperature |
|---|---|---|---|---|---|
| T1 | 18 min | 34 min | 49 min | 654°C | 650°C |
| T2 | 19 min | 35 min | 50 min | 652°C | 649°C |
| T3 | 18 min | 34 min | 50 min | 653°C | 651°C |
| T4 | 19 min | 35 min | 51 min | 655°C | 650°C |
| T5 axial edge | 21 min | 38 min | 54 min | 648°C | 646°C |
| T6 axial edge | 21 min | 37 min | 53 min | 649°C | 647°C |
7.3 Soak Stability Table
| Test Point | Start of Soak | Mid Soak | End of Soak | Maximum Deviation |
|---|---|---|---|---|
| T1 | 650°C | 651°C | 649°C | ±1°C |
| T2 | 649°C | 650°C | 648°C | ±1°C |
| T3 | 651°C | 650°C | 650°C | ±1°C |
| T4 | 650°C | 651°C | 649°C | ±1°C |
| T5 axial edge | 646°C | 647°C | 646°C | ±1°C |
| T6 axial edge | 647°C | 647°C | 646°C | ±1°C |
7.4 Temperature Uniformity Analysis
| Evaluation Item | Result |
|---|---|
| Circumferential uniformity | Very good |
| Axial temperature difference | Controlled within acceptable range |
| Soak temperature stability | Stable |
| Peak overshoot | Very low |
| Control response | Fast and stable |
7.5 Statistical Summary
| Item | Value |
|---|---|
| Average time to 650°C | 51.2 min |
| Maximum temperature deviation at soak | ±1°C |
| Highest measured temperature | 655°C |
| Lowest measured soak temperature | 646°C |
| Circumferential spread at soak | Small and acceptable |
The test data show that the induction PWHT system achieved a highly stable thermal profile around the pipe circumference. The slightly lower temperatures at the axial edge points were expected because these positions were closer to the edge of the effective heating zone. Even so, the values remained within a practical range for controlled PWHT operation, showing that the 480 mm heating band and 30 m cable arrangement were effective.
8. Process Performance Discussion
The performance of the 80kW induction heating system was satisfactory for this application. The system delivered stable ramp-up control, reached the target temperature of 650°C in a practical time, and maintained the soak temperature with minimal deviation.
Several factors contributed to the successful result:
- the flexible induction cable adapted well to the large pipe surface
- the 30 m cable length was sufficient for a well-distributed circumferential arrangement
- the 1-20 kHz frequency range supported stable steel heating response
- the insulation reduced heat loss and improved soak control
- the closed-loop control system minimized overshoot and temperature fluctuation
The data confirm that induction heating is highly suitable for large-diameter carbon steel pipeline PWHT, especially where mobility, flexibility, and accurate control are required.
9. Benefits
Benefits Table
| Benefit | Practical Value |
|---|---|
| Fast heating response | Reduces overall process time |
| Accurate temperature control | Improves PWHT quality |
| Uniform circumferential heating | Better weld stress relief performance |
| Flexible heating cable design | Easy installation on large pipes |
| Suitable for field operation | Good adaptability for on-site work |
| Lower heat loss with insulation | Better energy efficiency |
| Stable soak temperature | Better compliance with PWHT specification |
| Cleaner process than flame heating | Improved safety and working environment |
| Easy automation and recording | Better traceability and documentation |
10. Conclusion
This case study demonstrates that an 80kW induction heating power supply with a 30 m flexible induction heating cable is a highly effective solution for PWHT of a carbon steel pipeline with OD 880 mm, wall thickness 25 mm, and heating width 480 mm. The system achieved the required 650°C stress-relief temperature with stable heating behavior, strong circumferential uniformity, and reliable soak control.
The test data show that the induction process provided:
- controlled ramp-up
- stable soaking
- low temperature deviation
- good adaptability to large-diameter pipe geometry
For pipeline fabrication, repair welding, field construction, and maintenance projects, induction heating offers a modern alternative to resistance heating and flame-based methods. It provides better flexibility, more precise control, cleaner operation, and stronger suitability for documented PWHT procedures.
In this application, the induction PWHT process proved to be technically sound, operationally efficient, and well suited to the demands of large-scale carbon steel pipe heat treatment.
11.Frequently Asked Questions (FAQs)
Q1: Why use induction instead of resistance heating?
Induction is faster, more efficient, and provides better temperature uniformity.
Q2: Can induction PWHT be used on stainless steel?
Yes — frequency and power settings are adjusted accordingly.
Q3: What is the maximum pipe diameter suitable for flexible induction cables?
Pipes up to 3 meters OD can be heated effectively.
Q4: How accurate is induction PWHT temperature control?
Typically within ±2–5°C, far better than flame heating.
Q5: Does induction PWHT meet ASME/API requirements?
Yes — when performed with certified equipment and proper documentation.