Complete Workflow for Measuring Depth of Decarburization of Steel Parts

Applicable Samples and Objectives

This workflow applies to the measurement of surface decarburization on hot-rolled, forged or heat-treated steel parts such as spring steels, bearing steels, tool steels and fasteners. The objective is to determine the depth of complete decarburization (the fully ferritic zone) and total decarburization (complete plus partial decarburization, to the point where carbon content recovers to the core level), providing a basis for acceptance and process improvement. The standard specifies two routes — the metallographic method and the microhardness method — selectable according to material condition and the degree of dispute.

Complete Workflow

1. Sampling and Cutting

   Cut a cross-section perpendicular to the product surface, avoiding sharp corners and machining-damaged areas. Use a precision cut-off machine at low feed rate with ample coolant to prevent cutting heat from affecting the surface structure.

2. Mounting

   Because the decarburized layer lies at the very surface, edge protection is the core of this measurement: use low-shrinkage cold mounting resin, with metal protective rings or clamped multi-sample stacks when necessary, so that the edge remains square after grinding and polishing.

3. Grinding and Polishing

   Grind stepwise with SiC papers from P240 to P2000, rotating 90 degrees between steps and keeping the edge free of rounding throughout; then polish with 3 μm and 1 μm diamond to a mirror finish. Alternate the grinding direction relative to the specimen edge to detect and correct edge rounding in time.

4. Etching (Metallographic Method)

   Etch with 4% nital: the completely decarburized zone appears as white ferrite, with pearlite gradually increasing toward the core and a clearly visible transition zone.

5. Microscopic Measurement (Metallographic Method)

   Observe at 100× and measure the depths of complete and total decarburization separately: measure the depth of the fully ferritic white zone along the field, then the depth to where the structure matches the core. Generally measure at least 5 evenly distributed positions and report the maximum or average value as required by the product standard.

6. Microhardness Profile Measurement (Hardness Method)

   For hardened or structurally indistinct materials, make a row of Vickers indents (usually 1 kgf, 0.5 kgf for thin layers) perpendicular to the surface on the unetched polished surface with a step of about 0.1 mm; the decarburization depth is judged where the hardness recovers to the stable core value (or a specified limiting value).

7. Data Processing and Determination

   When the two methods disagree, the arbitration method is agreed upon as specified in the standard; the report states the method used, the number of measurement positions and the determination limit.

Applicability comparison of the two methods:

Key Control Points

• Edge rounding is the largest error source: the complete decarburized layer is often less than 0.1 mm, and rounding distorts readings or causes missed detection.
• The section must be perpendicular to the surface; tilt enlarges the measured value.
• Metallographic measurement should avoid scratched, folded or contaminated surface areas.
• For the hardness method, the first indent should be no more than 0.05~0.1 mm from the surface; for thin decarburized layers, use a finer step and lower load.
• When both methods are combined, complete the hardness test before etching so that etching does not affect indent quality.
• Whether the reported value is the maximum or the average must be agreed in advance per the product standard.

Common Deviations and Remedies

• Rounded edges causing loss of near-surface data: re-mount with stronger edge protection and re-grind.
• Under-etching blurring the boundary between complete and partial decarburization: re-polish and extend the etching time appropriately.
• Large fluctuations in the hardness curve: check surface quality and load stability; increase the number of rows.
• Non-uniform decarburization (locally deep, locally shallow): add measurement positions, report the deepest value and describe the distribution.
• Surface carburization causing misjudgment: assess comprehensively with core hardness and structure; perform carbon analysis if necessary.

Reference Standards

• GB/T 224 Determination of depth of decarburization of steels
• ISO 3887 Steel — Determination of depth of decarburization
• ASTM E1077 Standard Test Methods for Estimating the Depth of Decarburization of Steel Specimens
• GB/T 4340.1 Metallic materials — Vickers hardness test — Part 1: Test method

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.