Failure modes — white-layer spall, network cracking, distortion, embrittlement

Visual on a polished, Nital-etched microsection: continuous dark lines following the grain boundaries of the diffusion zone, concentrated near corners, edges, and the leading surface. On the surface itself the part often looks normal under low-power optical; the failure is internal until a crack pops out.

Cause: nitrogen activity in the diffusion zone exceeded the alloy capacity to form fine coherent precipitates, so nitrogen instead precipitated as coarse nitrides along grain boundaries. Three things drive it. Cycle temperature too high (above 560°C on most tool steels, above 540°C on grades with lower alloy content). Substrate grain size too coarse from prior heat treatment, since a poorly normalized H13 with grain size ASTM 5-6 instead of ASTM 8-9 has fewer grain boundaries to share nitrogen across, so each one takes a larger share. Geometric concentration at sharp corners and edges, where nitrogen diffuses in from multiple directions and saturates the local microstructure.

The intergranular network is brittle. Under cyclic load the grain-boundary nitrides crack, the cracks link, and the diffusion zone disintegrates along grain boundaries into the substrate. The Engineering Failure Analysis paper on cracking in a multiply gas-nitrided H13 aluminum extrusion mandrel and the related work on shear-driven crack networks in extrusion-die bearing surfaces both document this pattern from real production failures.

The fix has to happen before the cycle, not after. Verify substrate grain size before quoting nitride; ASM Failure Analysis Handbook Volume 11 emphasizes prior heat treatment as a determinant of nitriding outcome and is the standard reference for what an acceptable starting microstructure looks like. Chamfer or radius sharp edges to break the geometric concentration. Run the cycle at the lower end of the temperature window for the substrate. On parts already nitrided into a network, there is no in-service repair; the network runs throughout the diffusion zone and cannot be removed without removing the case.