GENERAL INFORMATION

SPALLS & SHELLED TREAD

The AAR does not distinguish between shells and spalls in Rule 41.

Although the two defects appear similar, the conditions required for each of the defects to materialize is entirely different.

Spalling is the most prevalent defect accounting for approximately 70% of all shell/spall defects.

Conditions to form a spall: picture

• The Wheel slides.
• The slid portion heats up to temperatures > 1350 degrees Fahrenheit in a matter of seconds.
• The slid portion changes structure from the as-manufactured pearlite to a brittle martensite.
• This brittle material then cracks under normal rolling loads to form a spall.

Conditions to form a shell: picture

• Entire tread surface (>600 Degree Fahrenheit) is heated to high temperatures for long periods of time.
• Steel surface is weakened.
• Contact rolling stresses cause tiny cracks called heat checks on the tread surface.
• If the cracks are not worn out, the cracks will eventually link together, pit out, and form a shell.

Visual differences between Shelling and Spalling.

Shelling:

• Heat checks or tiny lines running on angles may be present on the tread surface.
• Defect is usually around the entire tread surface.
• There is no martensite present on the tread surface.

Spalling:

• Defect is usually not uniform around the tread surface.
• There may be slid marks on the surface.
• The mate wheel may have a spall or slid mark at the same location.
• Martensite is present on the tread surface.

Possible causes of shelling and spalling.

Shelling:

• Trains running on incline for long periods of time with brakes applied.
• Stuck brake shoes
• Heavy grade braking services.

Spalling:

· Running empty cars and brake pressure is not reduced.

· Malfunctioning brake system.

· Handbrakes left applied.

· Foreign substances such as water, ice or oil present on the rail.