Laser-spot geometry controls where and for how long energy reaches GOES. Compare along-scan length, cross-scan width and through-thickness response separately to select a process on measured magnetic and coating results. [1]
Put the beam and the strip on the same sketch
Define x along the scan, y perpendicular to the scan in the sheet plane, and z through thickness. Mark the rolling direction separately. For an elliptical spot, record the full dimensions dx and dy, the method used to define them and the beam-intensity profile. A diameter defined at one intensity contour cannot be compared directly with a different supplier’s diameter convention.
The footprint sketch should show the ellipse moving along x, its swept band across y, and a section through that band along z. Label optical footprint, visible coating feature and investigated heat-affected region as separate boundaries. Their edges need not coincide.
Match energy before attributing a depth change to shape
For an illustrative continuous-wave screen, incident line energy is e = P/v. Laser power P in watts divided by scan speed v in m/s gives e in J/m. The geometric transit time at the centre of a uniform elliptical footprint is τ = dx/v, in seconds. Actual exposure depends on the intensity distribution. Neither expression accounts for reflected energy or temperature-dependent material response.
Holding P/v constant does not hold the heating history constant. A longer along-scan dimension can distribute the same incident line energy over a longer passage. Changing dy changes the width over which that energy is delivered. Record coating identity and optical state because equal incident energy need not give equal absorbed energy.
| Trial comparison | Held consistent | Response that separates the mechanisms |
|---|---|---|
| Change along-scan dimension | Cross-scan dimension, incident line energy and coating | Transit history and depth-sensitive evidence |
| Change cross-scan dimension | Along-scan dimension and incident line energy | Track width, surface condition and magnetic response |
| Repeat the original geometry | Material position and complete process settings | Repeatability and drift during the trial |
Inspect below the surface that made the photograph
A polished cross-section can establish geometry or microstructural change within its detection capability. It does not directly map residual stress. Choose a separate depth-sensitive method where stress penetration is the question, retaining preparation effects and spatial resolution. Conversely, an unchanged section does not establish that no magnetic-domain change occurred.
Pair the local section with magnetic measurements from identified neighbouring material. The published directional-neutron study used an elliptical beam and found different magnetic outcomes under different treatment conditions; its result is a reason to separate responses, not a shape prescription for another grade. [1]
For catalogue gauge 0.23 mm [2], assume a uniform CW elliptical footprint with full along/cross-scan axes 0.20/0.80 mm and a circular 0.40 mm footprint. Both areas are 0.12566 mm². At assumed P = 10 W and speed 2 m/s, each has line energy 5 J/m and the same mean footprint irradiance, but centre transit times are 100/200 μs. With assumed constant thermal diffusivity α = 1.0 × 10⁻⁵ m²/s, √(ατ) is 31.62/44.72 μm, or 13.75%/19.44% of gauge. Those diffusion scales are not measured stress or coating-damage depths. Equal spot area and line energy do not match local heating history; retain both geometries in a depth-sensitive matched trial before selecting shape.
Measured spot axes, focus, scan direction, speed and coating absorption determine local exposure. Carry these process coordinates into the witness layout and preserve both face and rolling-direction identity.
For procurement, request the spot-axis definition with the magnetic and coating results. Resolve supplier diameter conventions first, then select the process window against the required material response.
References
- P. Rauscher et al., The influence of laser scribing on magnetic domain formation in grain oriented electrical steel visualized by directional neutron dark-field imaging, Scientific Reports 6, 38307, 2016, Table 1. DOI: 10.1038/srep38307.
- Nippon Steel Corporation, Grain-Oriented Electrical Steel Sheets, catalogue D004je, April 2020, specification and typical-property tables.