Complete Workflow for Measuring Effective Case Depth of Carburized and Hardened Parts

Applicable Samples and Objectives

This workflow applies to carburized (or carbonitrided) and hardened gears, shafts and pins made from low-carbon alloy steels such as 20CrMnTi and 20CrMo. The objective is to determine the perpendicular distance from the surface to the point of a specified limiting hardness, i.e. the effective case depth (CHD), while confirming the microstructure of the case and core, providing a basis for heat-treatment evaluation and quality acceptance.

The hardness profile method is primary, supported by metallographic examination: the metallographic method visually reveals the structural transition of the case but is influenced by core composition, so it serves as a reference and cross-check; the hardness method, judged against a defined limiting hardness, is stable and reproducible and is the reference method specified by the standard.

Complete Workflow

1. Sampling and Cutting

   Cut a cross-section perpendicular to the carburized surface at the tooth or another representative location. Use a precision cut-off machine with a diamond or resin-bonded wheel, low feed rate and ample coolant, to avoid tempering of the surface structure and falsely low hardness.

2. Mounting

   Cold mount in low-shrinkage epoxy to protect the edges; add filler to the resin or use edge-protection fixtures when necessary, so that edge rounding during grinding does not remove near-surface measurement points or bias the readings low.

3. Grinding and Polishing

   Grind with SiC papers stepwise from P240 to P2000 (starting at P400 if the surface is in good condition), rotating 90 degrees and cleaning thoroughly between steps; then polish with 3 μm and 1 μm diamond suspensions, with an optional final 0.05 μm colloidal silica polish, until the surface is free of scratches and comet tails.

4. Etching

   Etch with 4% nital for a few seconds to about ten seconds until the case appears uniformly dark. Etching supports structural examination; alternatively, complete the hardness test on the as-polished surface first and etch afterwards for structural confirmation.

5. Metallographic Microscope Examination

   At 100×~500×, examine the surface martensite and retained austenite morphology, carbide distribution and core structure, confirm hardening quality against the relevant rating requirements, and record the approximate position of the transition zone as a reference for interpreting the hardness curve.

6. Microhardness Profile Testing

   Per GB/T 9450, make hardness indent rows perpendicular to the surface with a microhardness tester at 1 kgf load (0.5 kgf for thin cases or higher resolution), with a step of 0.1 mm in depth between adjacent indents and the first indent about 0.05 mm from the surface. Normally 2~3 rows are made and the values at corresponding depths are averaged.

7. Data Processing and Determination

   Plot the hardness-depth curve, take 550 HV1 as the limit, and interpolate the depth at which hardness falls to the limiting value; this depth is the effective case depth. When core hardness is high, apply the correction rules specified in the standard.

8. Recording and Reporting

   The report shall state the sampling location, load, step size, number of rows, hardness-depth data and curve, the determined result and micrographs, for reproduction and traceability.

Example hardness-depth data (load 1 kgf, step 0.1 mm):

In this example the hardness crosses 550 HV between 0.9 mm and 1.1 mm; interpolation gives an effective case depth of about 0.95 mm.

Differences Between the Metallographic and Hardness Methods

The metallographic method defines the case depth by the position where the etched case (martensitic region) transitions to the core structure; it is fast, but the reading depends on core carbon content and hardenability and varies between operators, so it is mainly used for process monitoring and quick assessment. The hardness method judges against an objective limiting hardness, offers good repeatability, and is the arbitration basis in disputes. In routine evaluation the two should be combined: report numbers from the hardness method and explain structural causes with the metallographic method.

Key Control Points

• The section must be strictly perpendicular to the carburized surface; any tilt systematically enlarges the measured depth.
• Apply ample coolant during cutting, grinding and polishing to avoid tempering and falsely low near-surface hardness.
• Edge protection must be effective; edge rounding removes the most critical 0.1~0.3 mm of data.
• Indent spacing must be at least 2.5 times the diagonal to avoid work-hardening interference; indents must not fall on defects or inclusions.
• Place the first indents as close to the surface as possible while avoiding rounded edges; for thin cases use 0.5 kgf with a finer step.
• Verify the hardness tester periodically with certified blocks, checking the 1 kgf and 0.5 kgf loads separately.

Common Deviations and Remedies

• Abnormally low near-surface hardness: usually edge rounding or surface decarburization; check mounting quality and furnace atmosphere, and re-sample if necessary.
• Large differences between adjacent rows: check section perpendicularity and grinding parallelism; add rows and average.
• Metallographic depth clearly greater than hardness depth: common in materials with high core hardness; the hardness method result prevails per the standard.
• Irregular indent shapes: indicates poor polishing or uneven loading; re-polish and retest.
• Over-etching obscuring the structure: shorten the etching time, or re-polish and etch again.

Reference Standards

• GB/T 9450 Determination and verification of carburized and hardened case depth of steel parts
• GB/T 4340.1 Metallic materials — Vickers hardness test — Part 1: Test method
• GB/T 13298 Metallographic examination methods for metals
• ISO 2639 Steel — Determination and verification of case hardening depth

The above is a methodological example for technical communication and process discussion, not actual measured data from a specific customer; the actual test plan, sampling locations and acceptance limits are subject to sample evaluation and the procedure confirmed by both parties.