News - CASE HARDENED ROUND LINK CHAIN HARDENSS TESTING GUIDE

(This guide provides a detailed, engineering-focused procedure for preparing hardness test specimens and conducting Vickers hardness testing on carburized round link chains used in bulk material handling applications such as cement plant bucket elevators and power plant slag conveyors.)

1. Overview and Applicable Standards

The objective of hardness testing on case-hardened round link chains is twofold:

(1) verify that the surface hardness meets the specified minimum (typically ≥800 HV or higher),

(2) determine the effective case depth (CHD) — the perpendicular distance from the surface to the point where hardness drops to a defined limit value, most commonly 550 HV. This is critical because the wear resistance and fatigue life of the chain depend on both the surface hardness and the depth of the hardened layer.

The following standards govern the testing procedure:

| Standard | Title / Application |

| ISO 6507-1 | Metallic materials — Vickers hardness test — Part 1: Test method |

| ISO 2639 | Steels — Determination and verification of the effective depth of carburized and hardened cases |

| DIN 50190-1| Hardness depth of heat-treated parts; determination of the effective depth of carburized and hardened cases |

| SAE J417 | Hardness Tests and Hardness Number Conversions — includes recommendations for surface preparation and testing of case-hardened parts |

2. Specimen Preparation — Step-by-Step Procedure

Proper specimen preparation is the most critical factor for accurate hardness measurement. Surface preparation must prevent damage or alteration of the surface hardness due to excessive heating or cold-working.

Step 1: Sectioning (Cutting)

- Objective: to extract a representative cross-section from the chain link, perpendicular to the case-hardened surface, to reveal the full hardness profile from the surface to the core.

- Method: Use an abrasive cut-off machine with plenty of coolant.

- Critical Warning: Never dry-cut. Heat generated during cutting can temper or alter the hardened case. Thermal cutting methods (burning, plasma) are fundamentally ruled out as they distort the material characteristics.

- Sample Location: Cut at the designated location on the chain link as specified in the engineering drawing. For round link chains, the cross-section is typically taken from the straight leg of the link, away from the weld and the interlink contact zones.

- Edge Retention: Do not chamfer the edges of the detection surface.

Step 2: Mounting (Embedding)

- Objective: Secure the specimen for subsequent grinding and polishing, ensuring the surface remains perfectly flat and the edge is not rounded.

- Method: Embed the cut section in resin using either hot compression mounting or cold mounting.

- Material: Use an edge-retention compound to prevent edge rounding during preparation. This is essential because the hardness profile must be measured from the very edge of the specimen.

Step 3: Grinding

- Objective: Remove the deformation layer left by cutting and produce a flat, progressively smoother surface.

- Procedure: Grind progressively with finer silicon carbide (SiC) papers. A typical sequence:

- Start with 220 or 320 grit to remove cutting damage

- Progress to 400, 600, 800, and 1200 grit

- Final grind with 2400 or 4000 grit

- Critical Requirements:

- Keep the face flat and exactly perpendicular to the surface

- Lubricate with water during grinding

- Ensure the deformation left by cutting is completely removed

- Maintain edge retention — do not allow the edge to become rounded

Step 4: Polishing

- Objective: Achieve a mirror finish (scratch-free, highly reflective surface) so that Vickers indentations have sharp, clearly measurable corners. A surface roughness of Ra ≤ 0.4 μm is typically required.

- Procedure:

- Pre-polishing: Use diamond suspensions, typically 6 μm or 9 μm, on a suitable polishing cloth

- Final polishing: Use diamond suspensions down to 1 μm (or even 0.25 μm) on a napped cloth

- Critical Requirements:

- Avoid relief (differential removal of material between hard and soft phases) and edge rounding

- For automated hardness testing machines, a high-quality polished surface is essential because the software cannot accurately determine the indenter impression if it is surrounded by scratches

Step 5: Etching (Optional)

- For CHD (Effective Case Depth) measurement, the polished surface is sufficient; etching is not required.

- A light etch (e.g., 2–5% Nital — nitric acid in alcohol) is needed only if you also want to reveal the microstructure (e.g., to verify martensitic case, absence of grain boundary carbides) or the visible case boundary.

3. Hardness Testing — Vickers Method

3.1 Tester Verification

- Before measuring, verify the microhardness tester at the chosen load with a certified test block and confirm calibration traceability.

3.2 Test Parameters

3.3 Indentation Traverse (Hardness Profile)

- Pattern: Place a row of Vickers indentations along one or more parallel lines perpendicular to the specimen surface, starting as close as possible to the edge and progressing into the core.

- Spacing Requirements (per ISO 6507):

- Distance between two adjacent indentations: ≥ 2.5 times the diagonal length of the indentation

- Distance from the indentation to the edge: ≥ 2.5 times the diagonal length

- Measurement Range: Continue indentations until the hardness stabilizes at the core hardness level (typically 2× or 3× the effective case depth from the surface).

3.4 Measuring Each Indentation

- For each indentation, measure both diagonals (d₁ and d₂) using the microscope of the hardness tester.

- Record the hardness value (HV) against its distance from the surface (depth).

3.5 Plotting the Hardness Profile

- Plot hardness (HV) versus depth (mm) from the surface.

- The effective case depth (CHD) is read where the hardness curve crosses the limit value of 550 HV (or the specified limit).

- If the difference between two parallel traverse measurements is ≤ 0.1 mm, take their average as the effective case depth. If the difference exceeds 0.1 mm, repeat the test.

4. Acceptance Criteria and Reporting

Report Content:

- Specimen identification (chain size, batch number, link location)

- Test force used (e.g., HV1)

- Hardness profile table (depth vs. HV)

- Plot of hardness vs. depth

- Calculated CHD value (mm)

- Surface hardness value

- Reference to applicable standards (ISO 2639, ISO 6507-1, DIN 50190-1)

5. Common Errors and Preventive Measures

Accurate hardness testing of carburized round link chains demands meticulous specimen preparation — from coolant-rich sectioning through progressive grinding to mirror-finish polishing — followed by a systematic Vickers microhardness traverse per ISO 6507 and ISO 2639. The effective case depth is determined at the 550 HV limit. Proper equipment (micro-Vickers tester with automated software, certified test blocks, and metallographic preparation tools) and strict adherence to spacing and measurement protocols ensure reliable, repeatable results that verify both surface hardness and case depth — the two critical parameters determining chain wear life and performance in abrasive bulk material handling applications.

Post time: Sep-24-2026

CASE HARDENED ROUND LINK CHAIN HARDENSS TESTING GUIDE

1. Overview and Applicable Standards

2. Specimen Preparation — Step-by-Step Procedure

3. Hardness Testing — Vickers Method

4. Acceptance Criteria and Reporting

5. Common Errors and Preventive Measures

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