(for round link chains used in demanding applications like bucket elevators in cement plants and ash/scraper conveyors in power plants. These components require a unique combination of high surface hardness for wear resistance and a tough, ductile core to withstand impact and fatigue)
The objective is to create a deep, metallurgically sound case that is well-bonded to the core. The process involves several critical steps:
Step 1: Pre-Treatment (Optional)
- Process: Normalizing.
- Purpose: To refine the grain structure and improve machinability/weldability of the raw chain links.
- Reference Parameter: Heat the links to 880–920°C and allow them to cool in air .
Step 2: Carburizing
This is the core process where carbon is diffused into the surface. Gas carburizing is the most common and controllable method for these applications.
- Purpose: To enrich the surface carbon content, enabling it to become extremely hard after quenching.
- Temperature: 880–930°C. Consistent temperature control is vital for uniform case depth.
- Atmosphere: A carbon-rich atmosphere, typically endothermic gas enriched with a hydrocarbon like methane or propane. The carbon potential must be carefully controlled.
- Carbon Potential: Maintain at 0.8–1.0% to achieve the optimal surface carbon concentration for maximum hardness without forming excessive carbides.
- Time: Determined by the desired case depth. Diffusion is time-dependent. For example:
- For a 1.0 mm case depth: Approximately 8–10 hours.
- For a 1.5 mm case depth: Proportionally longer time.
- Depth Specification: For heavy-duty chains, a substantial case depth is required.
- Rule of Thumb: A minimum carburizing depth of 0.1 times the bar diameter up to 0.21 times the bar diameter is often specified by manufacturers.
- Absolute Depth: Typically ranges from 0.5 mm to 2.0 mm, with 1.0–1.5 mm being common for slag and cement applications.
Step 3: Quenching
- Purpose: To transform the high-carbon surface layer into a hard, wear-resistant martensitic structure.
- Medium: Oil is the preferred quenchant for these alloy steels. Oil quenching provides a fast enough cooling rate to achieve high hardness while minimizing the risk of distortion and cracking associated with water quenching.
- Temperature: Pre-heated oil at 60–80°C is often used for a more uniform cooling rate.
Step 4: Tempering
- Purpose: To relieve the internal stresses induced by quenching, reduce brittleness, and achieve the final balance of hardness and toughness.
- Temperature & Time:
- For maximum surface hardness (e.g., 58-62 HRC), temper at a low temperature of 150–200°C for 1-2 hours.
- If a slightly lower hardness but higher toughness is required, a tempering temperature of 400–450°C can be used.
Step 5: Post-Treatment (Optional but Recommended)
- Shot Peening: This process bombards the chain surface with small spherical media, inducing compressive residual stresses. This significantly improves fatigue strength, which is critical for chains subjected to repeated cyclic loading.
Case Depth Measurement
This is the most critical test to ensure the carburized layer is deep enough to withstand wear without the case collapsing under load.
- Effective Case Depth: This is defined as the perpendicular distance from the surface to a point where the hardness falls to a specific value, typically 550 HV (or 52 HRC) .
- Procedure: A cross-section of a chain link is polished, etched (often with nital), and examined under a microscope. Microhardness indentations are made to determine the exact depth at which the hardness drops to 550 HV.
- Acceptance Criteria: The measured effective case depth must meet the minimum specified value (e.g., ≥1.0 mm or as per the `0.1 x diameter` rule) and be uniform around the circumference of the link .
Metallurgical Analysis
- Microstructure: A metallurgical microscope is used to examine the etched cross-section. The goal is to verify a fine-grained, martensitic case with a gradual transition to a tough core structure. There should be no significant network of grain boundary carbides, which can cause brittleness .
Mechanical Testing
- Breaking Force: Sample chains are pulled to destruction in a tensile testing machine to verify they meet or exceed the minimum breaking load specified by standards like DIN 764 or DIN 766 for the relevant grade (e.g., Grade 2 or 3) .
Post time: Mar-23-2026
