Top causes of CRGO lamination performance drift in production batches

You order CRGO 라미네이션 with the same grade, same coating, same drawing. Yet no-load loss goes up 8%. Magnetizing current creeps beyond your design spreadsheet. Noise shifts.

“Same spec, different result.” This article is about that gap.

1. The spec is fixed. The process is not.

On paper, a batch of CRGO laminations is defined by:

• grade (e.g. M3, HI-B, domain-refined, etc.)
• nominal thickness
• coating / insulation class
• guaranteed Epstein core loss and permeability

What’s less visible is the scatter 내부 the mill’s tolerance window and how your own slitting, cutting, stacking, and annealing either amplify or damp that scatter. Grain-oriented steels are highly structure-sensitive; modest shifts in chemistry, texture, grain size, and internal stress give measurable changes in loss and permeability.

So two batches that both “meet spec” can produce noticeably different cores once you’ve punched, stacked, and clamped them. That’s the core of performance drift.

Let’s walk the chain, from slab to finished lamination stack.

2. Mill-side variation inside “good” coils

2.1 Chemistry window and inhibitor system

Even within one grade, mills tune Si, Al, C, N and inhibitor species (MnS, AlN, etc.) to drive secondary recrystallization and Goss texture. Small shifts here affect grain size distribution and final magnetic properties.

You see it as:

• Coil-to-coil variation in Epstein loss, still inside the guarantee
• Slightly different B-H curve slopes
• Different sensitivity to your punching and annealing conditions

Two coils from different heats, same nominal grade, won’t respond identically to the same line.

2.2 Texture, grain size, and domain engineering

Goss texture sharpness and grain size distribution set the baseline loss and permeability. Research continues to show that small changes in recrystallization conditions, inhibitor dispersion, and annealing temperature can shift texture and grain growth behaviour, which then feeds straight into core loss.

You can’t fully “fix” that downstream, only avoid making it worse.

2.3 Coating and insulation class variation

ASTM A976 C-classes (C-0…C-6) differ in chemistry, insulation resistance, friction, and intended stress-relief behaviour.

Two things happen:

• Slitting / punching response – coatings with different friction change strip tension and how burr and rollover form.
• Stack behaviour – insulation resistance and coating thickness change lamination factor and interlaminar loss.

If you change supplier or coating class “but keep everything else”, batch drift is almost guaranteed.

3. Slitting: edge quality is your first hidden variable

By the time the strip leaves the slitter, a lot of the later drift is already baked in.

3.1 Burr height and edge strain

High burr and heavy rollover are not just cosmetic. They:

• reduce stacking factor
• create micro-shorts between laminations
• inject local stress right where flux crowds at the edges

This raises localized eddy current loss and hysteresis loss and often shows up as higher core loss and noisier cores. Industry experience and motor/transformer lamination studies consistently link higher burrs to higher loss and poorer efficiency.

Burr drifts with:

• knife sharpness and clearance
• slitting line setup and tension
• strip hardness and coating (from section 2)

So a batch that happened to be slit late in the knife-life curve will quietly perform worse.

3.2 Camber and width scatter

Strip camber, edge wave, and width variation are usually “within tolerance” until you start stacking and see:

• step-lap gaps not seating repeatably
• uneven clamping pressure
• random micro-air-gaps inside stacks

All of which show up as higher magnetizing current and sometimes “mystery” no-load loss drift.

3.3 Cutting technology choice

As designs move toward thinner gauges and more complex geometries, the cutting process itself becomes a noticeable variable. Mechanical shearing, notching, laser cutting, and advanced core cutting lines impose different stress states and heat-affected zones on the edges.

One batch cut on an older mechanical press line and another on a newer precision cut-to-length line can test differently even with identical coil and drawings.

4. Punching and notching: from sheet to lamination

Now the strip becomes parts. Every tool and press setting starts to matter.

4.1 Tool wear and press settings

As punches wear:

• burr height creeps up
• rollover worsens
• micro-cracking appears at slot edges

Press shut height, die clearance, lubrication, and speed move around as people tweak for throughput. Day-to-day variation here is one of the most common real-world reasons for lamination performance drift. It rarely makes the datasheet, but you’ll see it in microscope photos and in core loss.

4.2 Grain direction mistakes and mixed orientations

CRGO depends on the rolling direction aligning with your main flux paths. Mis-orientation (even a subset of laminations rotated 90°) dramatically increases local loss and magnetizing current.

In production, this can happen when:

• coils are loaded backwards on a line
• blanking tools are shared between different parts and setups shift
• operators mix “left” and “right” parts from two different nests

The batch looks fine visually. The test bench disagrees.

4.3 Local stress raisers

Sharp inside corners, over-tight pilot holes, and heavy forming all concentrate stress. GOES is quite sensitive; local stress shifts the B-H curve and magnetostriction. Even if your drawing is identical, subtle press adjustments change how hard you “work” the steel, and thus the loss.

5. Stacking, joint geometry, and clamping

You can ruin good laminations with sloppy stacking, or make mediocre material look acceptable with disciplined stacking. That alone tells you how strong this link is.

5.1 Lamination factor drift

Standards and mill catalogues talk about lamination factor (stacking factor) for neat, idealized stacks. Real stacks, with burr and coating, rarely match that.

Drivers:

• coating thickness variation
• burr height and edge deformation
• stacking method (manual vs robotic, interleaving consistency)

If your CAD model assumes 100% iron and the actual lamination factor slides from 96% to 93%, flux density moves, and so does loss and magnetizing current.

5.2 T-joint and step-lap accuracy

Local loss distribution in T-joints and overlaps depends strongly on overlap angle, length, and layer pattern. Studies show localized core loss increases from outer to inner edges in mixed-angle step-laps when alignment is off.

Real life sources of drift:

• different assembly teams using slightly different stepping habits
• fixtures wearing over time, so packs can shift
• new core designs reusing old stacking fixtures “that are close enough”

You end up with the same bill of materials, but a different local flux picture.

5.3 Clamping pressure and frame design

Under-clamped cores buzz and move. Over-clamped cores have extra mechanical stress and can show higher loss. Uneven clamping creates spatially varying performance: some legs run closer to spec, some worse.

Batch drift appears when:

• torque sequence changes
• frame design is revised with no matching change in test limits
• pads or insulation under yokes compress differently due to material change

6. Stress relief annealing: the quiet multiplier

Stress relief annealing is one of the strongest levers on CRGO performance, because it relaxes the cold-work from slitting, punching, and stacking. Many datasheets assume strips are stress-relief annealed when quoting best loss values.

Drift shows up when the real process deviates:

• furnace loading gets denser over time
• soak time is trimmed to gain throughput
• thermocouples age, so the actual lamination temperature changes without anyone noticing
• different core sizes share the same cycle, even though thermal mass is not the same

Result: one month the process truly relieves stress; another month it only halfway does.

Finished core tests will reflect that.

There’s also the subtle “do damage after annealing” issue:

• welding or grinding near the core
• rough handling and bending of yokes
• local peening from fixtures

All add fresh stress after you’ve paid for the furnace.

7. Handling, storage, and material mixing

This part feels mundane. It is not.

7.1 Mixing grades or quality levels

Some markets see imports of “seconds and defectives” CRGO material, which come with looser control on flatness, burr, camber, and properties. Industry voices have highlighted how edge burr and camber in such material directly worsen stacking factor and core losses.

If your lamination plant occasionally backfills with this type of material when prime stock is tight, batch-to-batch drift is inevitable, even if the nameplate grade remains the same.

7.2 Rust, moisture, and coating damage

Poor storage – high humidity, condensation, rough stacking – can:

• reduce insulation resistance between laminations
• damage or peel coatings
• introduce corrosion pits and surface roughness

All translate into higher interlaminar loss and sometimes increased noise.

7.3 Reusing or reworking laminations

Re-stamping, re-grinding, or re-stacking laminations from rejected cores or prototype runs saves steel in the short term and injects inconsistency in the long term. Each extra handling step adds stress, possible scratches through coating, and geometry scatter.

8. Measurement and spec illusions

A lot of what is labelled “performance drift” traces back to how you compare test data to mill data.

8.1 Epstein test vs built-core reality

Mill guarantees are usually based on Epstein strips: stress-relief annealed, ideal grain orientation, simple magnetic path.

Your assembled core is:

• punched
• stacked with real burr and coating
• clamped in a frame
• sometimes only partially stress-relieved

Comparing those results one-to-one will always show a gap. What matters is how that gap changes over time.

If your process adds a roughly constant “penalty” to the Epstein result, drift is low. When your own process scatters, drift is high. Many companies don’t track this delta explicitly, which makes root-cause work slower.

8.2 Test set-up drift

Even good labs see shifts in:

• flux density calibration
• core temperature during no-load tests
• sensor placement and lead routing

No-load loss is sensitive to induction, frequency, and temperature, and temperature alone can noticeably change loss in GOES.

Before blaming laminations, it’s worth confirming that the test bench, its wiring, and its software haven’t changed.

9. Quick reference: typical symptoms vs likely root causes

Use this table as a starting filter when a batch of CRGO lamination stacks behaves differently from the previous one.

10. Making CRGO lamination performance more stable on purpose

You can’t remove all variation in grain-oriented electrical steel. But you can design your lamination supply and core production so that most of the variation is upstream and transparent, not hidden inside your own plant.

Typical moves that help:

• Specify process-linked limits, not just grade
  • max burr height and measurement method
  • lamination factor acceptance tests on sample stacks
  • allowed coating classes and suppliers
• Track coil-to-core genealogy
  • know which transformer core used which coil and which line setup
  • when a batch drifts, you can see whether it clusters around a coil, a tool, or a furnace load
• Correlate your own “process signature” against mill data
  • keep a small set of standard test cores built the same way from each batch of laminations
  • compare your penalty vs the mill’s Epstein numbers over time
• Treat cutting and annealing as design parameters, not just production utilities
  • when designs move to thinner gauges or tighter loss targets, review whether existing lines and furnaces are still suitable

Done systematically, this turns “mystery drift” into a set of controlled variables.

FAQ: common questions about CRGO lamination batch drift