The Art of Repair: EN 16725 Manganese Crossing Restoration

Introduction to EN 16725

The “frog” (crossing point) of a turnout is the most abused piece of steel on the railway. To withstand the violent impact of wheels jumping the gap, these components are cast from Austenitic Manganese Steel (AMS), also known as Hadfield Steel. This material is magical: the more you hit it, the harder it gets (work hardening).

However, this material has a fatal flaw: it hates heat. If you weld it like normal steel, it becomes brittle as glass and crumbles. EN 16725, titled “Railway applications – Track – Restoration and repair of manganese crossings,” is the handbook for navigating this metallurgical minefield. It specifies the strict procedures for grinding out defects and rebuilding the worn surface using electric arc welding without destroying the casting’s crystal structure.

Snippet Definition: What is EN 16725?

EN 16725 is a European standard specifying the requirements for the restoration and repair of cast austenitic manganese steel crossings (frogs). It defines the qualifications for welders, the preparation of the base metal (removal of fatigued layers), the selection of consumables, and most critically, the thermal regime (interpass temperature) to prevent carbide precipitation and embrittlement.

The Material Challenge: Hadfield Steel

EN 16725 is built around the unique chemistry of AMS (11-14% Manganese).

• The Strength: Under impact, the surface hardness jumps from ~200 HB to >500 HB.
• The Weakness: Prolonged heating between 300°C and 800°C causes “Carbide Precipitation.” The carbon migrates to the grain boundaries, causing the steel to lose its toughness.
• The Rule: Unlike normal rail steel (which needs pre-heating), Manganese steel must be kept COLD.

The Repair Process

The standard outlines a surgical procedure:

1. Preparation (The Excavation)

Before adding metal, the “dead” metal must be removed.

• Fatigue Removal: The work-hardened layer is often full of micro-cracks. This must be ground away until sound base material is reached.
• Dye Penetrant: EN 16725 mandates Non-Destructive Testing (NDT) to prove all cracks are gone before welding starts.

2. Welding (The Restoration)

This is where the standard is strict.

• No Pre-heat: Never heat the casting before welding.
• Interpass Temperature: The temperature of the casting must typically stay below 250°C. Welders must weld short beads, stop, and wait for the part to cool down (or use compressed air cooling) before the next pass.
• Hammering (Peening): Each weld bead is often hammered immediately after laying to relieve shrinkage stress and prevent cracking.

3. Finishing (The Profile)

The weld is ground flush to restore the original geometry of the wing rails and the point. A smooth transition is vital to prevent renewed impact damage.

Inspection and Acceptance

How do you know the repair is good?

• Visual Inspection: Checking for porosity or undercut.
• Dye Penetrant (PT): The primary NDT method for AMS (Magnetic Particle inspection doesn’t work because AMS is non-magnetic!).
• Hardness Testing: Ensuring the weld metal matches the hardness of the base casting.

Comparison: EN 16725 vs. EN 14730 (Aluminothermic)

Different welding for different parts of the track.