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How do you control white etching layer by grinding?
How do you control white etching layer by grinding?
You control it by grading the work to the layer, not by taking a bigger cut. A white etching layer is not a stain on the rail head: it is a different material sitting on top of the parent steel, and in our measurements it reaches about 870 HV0.3 against 340 HV0.3 for the metal below. Its thickness is also an output of the grinding parameters rather than a property of the track. So two numbers decide the plan before a machine moves — how deep the layer is, and how deep the cracks are underneath it.
Three measured facts, and what each one forces into the plan
| Measured property | What the number is | What it forces into the grinding plan |
|---|---|---|
| Hardness of the layer | About 870 HV0.3 at the surface against about 340 HV0.3 in the parent metal, a rise of roughly 155%, with a sharp boundary visible in a section | Removal per pass is not constant through the layer. The wheel meets a much harder material at the top than at the bottom, so the first pass and the last pass do different amounts of work |
| Thickness of the layer | In controlled tests about 45 µm at 1000 rpm, rising to about 320 µm at 3000 rpm, with roughness moving from 6.6 to 8.4 µm over the same range | Thickness is produced by the grinding setting. A setting chosen to save time can generate the very layer the job was sent to remove, so speed and feed are part of the metallurgy, not just of productivity |
| Depth of what sits under it | Contact fatigue cracks; grinding still removes them reliably while they are about 0.2 mm deep or less | The layer is the symptom and the crack depth is the decision variable. Past 0.2 mm the surface can be cleaned while the crack root stays in the rail |
This is why a white etching layer cannot be handled with one setting for the whole network. The same instruction — take off 0.2 mm — is generous on a thin surface film and insufficient on a 300 µm band with cracks below it. Grade the work from the section, and the instruction becomes defensible.
Why one uniform pass presses the layer in instead of taking it off
| Failure mode | How it shows up | What actually happens | What the plan must change |
|---|---|---|---|
| One deep pass over a thin film | Spindle load climbs, the surface goes bright and then tints, and roughness after the job is worse than before it | A hard, brittle film fractures under the grain; the fragments are not carried away, they are rolled into the softer parent metal by the following wheels | Remove the film from the top down in graded passes, so each pass cuts metal rather than fragments |
| Speed raised to shorten the job | The surface looks patchy and spalled rather than cut, and the measured layer is thicker after the campaign than before it | Heat input and grain load rise together; the pass leaves about 320 µm of white layer instead of about 45 µm | Hold the spindle and feed setting, then buy time with passes rather than with speed |
| Repeating a shallow cut | The band looks clean by eye but the measured profile still shows a hollow or a lip | The wheel follows the existing shape, so a shallow pass polishes the layer it failed to remove | A depth decision is also a profile decision: verify per zone, not at one point |
| Working above the temperature limit | Discolouration beyond light straw, and a surface that is soft where it should be hard | The rail head is being tempered or re-hardened by grinding heat rather than cut; the standard working limit for the workpiece is 150 °C | Treat temperature as an acceptance item, reduce depth per pass and let the surface cool between passes |
The reason this matters beyond the day of work is the service behaviour of the layer itself. A hard, brittle white layer breaks up early in traffic, and the fragments are pressed into the matrix beneath; from there they act as crack initiation sites. A campaign that leaves the layer in place does not postpone the problem, it puts a supply of hard fragments directly into the crack path.
One boundary worth stating plainly: which wheel to buy for this duty — abrasive, bond, grade — is a purchasing question answered in our rail grinding wheel answers. What belongs here is the strategy: how deep, in how many passes, and what has to be true before the job is called finished.
A graded plan you can write into a specification
| Grade from the survey | Entry test | Grinding setting | What acceptance must show |
|---|---|---|---|
| Surface film, no cracking | Layer visible on the running band, no crack found under it by the non-destructive check in use | Preventive pass at 0.1 mm or more, inside the profile envelope | Profile inside the tolerance band for every zone, roughness at or under 10 µm, no blue, no abnormal vibration |
| Banded layer with cracks up to 0.2 mm | Crack depth read from a section sample at the affected location, at or under 0.2 mm | Graded: two to three passes, total depth inside the 0.5 mm acceptance ceiling, cooling between passes | The last pass no longer opens cracks, and the contact band measures 25-30 mm on high-speed or 35-40 mm on conventional and heavy-haul |
| Layer with cracks beyond 0.2 mm, or a squat already spalled | Crack depth past 0.2 mm, or metal already missing from the running surface | Grinding becomes a holding action, not a repair; the section is referred for replacement or milling assessment | Do not write a grinding depth that promises removal. Record the referral and the interval to the next inspection |
Two disciplines hold the plan together. The first is the envelope: 0.1 mm preventive, 0.2 mm or more corrective, 0.5 mm as the acceptance ceiling — three numbers describing one finite budget of section, which a graded plan spends slowly and a single deep pass spends at once. The second is position. On a 68 km heavy-haul line we ground an R=800 m curve with six passes at 12 km/h and measured up to 0.3 mm on the high-rail field side against up to 1.8 mm on the low-rail side. The layer sits where the contact sits, so any depth claim that is not positional is a claim about an average nobody can inspect.
That positional detail is also what the wheel has to survive: a Molaton wheel asked to hold a graded cut through a 300 µm hard band on one side of the head and a softer surface on the other is being asked for consistent removal per pass, which is the only way the pass arithmetic holds. Our squat and white layer study sets out the formation mechanism and the graded approach in detail; the plan above is how it turns into a work instruction, and the Molaton range is what has to deliver the depth the plan assumes.
Related questions you may also ask
Rail squat formation, white etching layer and graded grinding What grinding depth is safe before you over-grind a rail? How does rail grinding prevent rolling contact fatigue? How many grinding passes does a rail need? More answers on rail grinding strategy and equipmentFound white etching layer or squat on your line?
Send RailwayCare the rail grade, annual tonnage, the affected locations and any section or crack-depth readings you already have. We will turn them into a graded pass plan and a Molaton wheel schedule that matches the layer you actually have.
Plan the Graded Work