How Are Nitrided Crankshaft Surface Hardness and Case Depth Verified?

How Are Nitrided Crankshaft Surface Hardness and Case Depth Verified?

A nitrided crankshaft hardness test needs two separate results: surface hardness at a defined location, and nitriding hardness depth from a cross-sectional profile. A hard surface reading does not establish how deep the hardened zone extends. From a Neway engineering perspective, the inspection plan should settle both requirements before treatment, including whether acceptance applies to the as-nitrided surface or the finished journal. Confirm the governing standard and revision on the inspection plan; do not choose a hardness target from a generic material table when the component drawing requires a different condition.

Define the layer being accepted

Specify the steel grade, prior heat-treatment condition, test locations, hardness scale or force, depth criterion, and acceptance limits. The drawing should distinguish nitriding hardness depth, or NHD, from compound-layer thickness. Under ISO 18203, the usual NHD threshold is core hardness plus 50 HV; a customer specification may prescribe another limit. State that limit explicitly rather than writing only "case depth."

The compound layer, when present, is a surface constituent above the diffusion zone. Its thickness is a separate metallographic measurement if the specification controls it. Neither an NHD result nor a surface indentation gives that thickness. For treatment background, see Neway's explanation of nitriding CNC-machined components; acceptance still depends on the agreed drawing and test method.

Select a surface test the journal can support

Choose the method specified for the material, layer thickness, and journal geometry. Regular Rockwell, superficial Rockwell, and Vickers methods are not interchangeable. A test penetrating too deeply can be influenced by the softer substrate. Superficial Rockwell uses lower forces, but that alone does not prove its suitability for a particular nitrided layer.

Check surface condition, stable support, test-piece thickness, spacing, and the method's requirements for cylindrical surfaces. Record the exact Rockwell scale or Vickers designation, test location, and any permitted curvature correction. Avoid an unexplained converted hardness value. Indentation leaves a local mark, so agree acceptable test positions; do not describe direct hardness testing as completely non-destructive.

Measure depth on a prepared cross-section

NHD verification normally requires sectioning a sacrificial part or approved representative coupon. Prepare the section without rounding the surface edge or introducing damage that distorts the hardness readings. A Vickers traverse places a series of indents at known perpendicular distances from the examined surface, continuing sufficiently far to establish the core hardness.

Plot hardness against depth, identify the specified limiting hardness, and determine the crossing using the applicable method. Retain the individual readings and distances, not just the reported depth. Test force, indent spacing, preparation quality, and optical measurement matter especially near a steep gradient. An isolated microindentation is not a substitute for this profile. Very small indents can increase optical uncertainty, while poor section preparation can alter the apparent response. The laboratory should use a force and preparation procedure suited to the gradient, with spacing that avoids interaction between adjacent indentations.

Establish what the coupon represents

A witness coupon supports production acceptance only within an agreed sampling plan. Its material, prior heat treatment, dimensions, geometry, process exposure, and critical test locations must represent the parts as required by the governing specification. Link it to the actual treatment batch. A convenient flat coupon cannot automatically prove conditions at every journal, fillet, masked region, or finished surface. Where a coupon cannot represent a critical feature, resolve the sampling limitation before processing. The disposition may require a more representative specimen or a sacrificial component, subject to customer approval.

Keep the final surface in the evidence chain

If material is removed after nitriding, identify which surface the reported hardness and depth describe. Grinding or polishing can remove the compound layer and part of the hardness gradient. A pre-removal report is process evidence, not automatic final-journal acceptance; use an approved correlation or final-condition verification. Do not simply subtract stock removal from NHD without an authorized method. This distinction also matters when reviewing heat treatment of carbon steel machined parts.

Request a report that supports release

Include part or coupon identity, batch linkage, specification revision, test stage, locations, hardness method, individual readings, profile data, core hardness, threshold, and reported NHD. Add compound-layer results when required, equipment traceability, deviations, and the acceptance conclusion. These records form part of the inspection evidence for precision-part release. Send Neway the drawing's hardness and layer-depth requirements with the intended finishing sequence so the test locations and sampling basis can be agreed before manufacture.