Information

Authors: Sean M. Muyskens, Robert C. Goldstein
Location/Venue: IMAT 2022 New Orleans, Louisiana

Overview

• Induction Tube Welding Process
• The problem with welding small tubes
• Impeder core material optimization

Background

• A steel strip (1) is formed into a tube shape with a closing seam
• An induction coil (3) creates eddy currents in the tube
• An impeder (5) is used to force the currents along the weld vee (4)
• The heated tube edges are welded together via the weld rollers (2)

Ref. (1)

What is an impeder?

• A magnetic concentrator that directs current flow
• Typically manufactured using ferrite materials
• Fixtured within the forming tube

Ref. (2)

Induction tube welding magnetic circuit

Ref. (3)

• Inductor drives current around the tube
• Two current pathways around the tube, weld vee and tube ID
• ID becomes more favorable with decreasing tube diameter
• Impeder raises impedance of ID path, forcing current to weld vee

Small diameter thick-walled tube challenges

• Less space fore the impeder core
• ID current path becoming more favorable
• These increase loading of the impeder
• Once the impeder reaches its saturation flux density, process efficiency plummets as current travels on the ID rather than along the weld vee

Solutions

• Add more power in cases where inefficiency is unavoidable
  • Wasted heating away from weld vee
  • Not feasible in many processes
• Increase process efficiency by:
  • Decreasing impeder loading
  • Increasing impeder load capacity

Decreasing impeder loading

Ref. (4)

• Decreasing impeder loading by decreasing weld vee angle
  • Greater proximity effect
  • Lower impeder loading
  • Bigger issue in small tubes

Increasing impeder load capacity

• Increasing impeder load capacity
  • Use larger impeder
  • Greater cooling of the core
  • Use a material with a higher saturation flux density
    • Ferrites are usually in the 0.4-0.5T range
    • Soft Magnetic Composites (SMCs) are usually in 0.9-1.6T range
• SMC permeability is lower, and losses are higher; however, improved saturation flux density is the key to performance

Ref. (5)

New technology adoption/transition

• Increase in process efficiency reduces overall power requirements
  • Process no longer in saturation, focuses power in weld vee
  • Enables increase in process uptime
  • Lower overall powers means less wear and tear
  • Higher saturation flux density drive smaller impeders with less mechanical impact failures
  • Induction coil and impeder changeovers drive downtime

Case study energy savings

• Case setups:
  • ~400kHz 100-300kW power supply
  • 19-20mm OD steel tube
  • 1-3mm thick wall
  • 9-10mm OD impeders
• Compared with the ferrite impeder, the SMC impeder reduced required power by 40-50% for the same line speed
• In cases where induction was limiting max line speeds, increased line speeds over 15% were observed

Case study SMC impeder lifetime

• The higher losses of SMC were of great concern in terms of thermal degradation
• Trials to test SMC impeder lifetime were run
• A few SMC impeders were lost immediately upon startup, as the power was turned on at nearly twice what was required to weld
• Some lost mechanically
• None lost to thermal degradation

Physical testing

• This test stand was developed to determine if additional cooling is required for SMC impeders
• The stand can achieve magnetic loading in the impeder that mimics a real welding installation with impeder loading verified via simulation
• The water flowrate can be adjusted to pinpoint the cooling requirements at various levels of loading in the impeder

Physical testing result

• Shown here are the trials for a 12mm return flow impeder with a Fluxtrol A core
• The process was run at 330kHz and a maximum flux density of ~0.85T was achieved in the core
  • This is near double the typical maximum flux density of ferrites (0.4-0.5T)
• The cooling water supply was decreased until thermal degradation of the impeder was observed
• This test shows that SMC impeders can survive with typical cooling available to ferrite impeders (0.5GPM)
• We want to recreate this for even higher magnetic flux densities

Operational Window

Computational modelling

• Focus on the most heavily loaded section of the impeder
  • Modeling at this location to validate survival at higher loading with 0.5GPM cooling

Computational modelling results

• The trial where thermal degradation was initiated was modeled in CFD
• A temperature of 250°C achieved in the core is near the range for temperatures known to initiate degradation of Fluxtrol A
• This correlates with empirical data and can be used to validate the operational window for SMC impeders

Conclusions

• Small tubes have posed an issue in induction tube welding
• Only small improvements can be made when the impeder is saturated
• A change in impeder material provides much greater improvements in saturation flux density
  • Decreases required power
  • Increases weld performance and line speed
  • Decreases line downtime due to coil and impeder failures and replacement
• More testing needs to be performed on induction tube mills for validation of SMC impeders

Future work

• A new physical simulation setup is being constructed to validated survivability of SMCs at higher flux density
• CFD modelling to be used to expand SMC impeder operational window
• Test impeders are going to be evaluated on production weld lines for better empirical performance data

References

[1] Image taken from UIE book “Induction Heating – Industrial Applications”

[2] https://www.efd-induction.com/en/induction-heating-equipment/weldac-tube-welder

[3] Image taken from Tube & Pipe Technology “Optimizing Efficiency in HF Tube Welding Processes”

[4] Images taken from Thermatool Corp’s YouTube “Proximity Effect for High Frequency Welding of Tube and Pipe – Thermatool”

[5] https://www.researchgate.net/figure/Relative-permeability-vs-saturation-flux-density_fig1_321357089

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