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Evaluating Sintered Carbide Tools: Key Methods from SAE J439-2018
SAE J439-2018 provides standardized methods for evaluating five essential properties of sintered carbide tools: hardness, specific gravity, apparent porosity, structure, and grain size. These properties directly influence tool performance and reliability. This article outlines the key procedures, noting critical precautions and insights for engineers and quality control professionals.
| Property | Method | Key Requirements |
|---|---|---|
| Hardness | Rockwell A scale (60 kg load, diamond brale) | 220 grit diamond finish, parallelism, calibration with two test blocks |
| Specific Gravity | Immersion method (weight in air and water) | 100 grit diamond grind, analytical balance, temperature correction |
| Porosity, Structure, Grain Size | Metallographic examination (grinding, polishing, etching) | Diamond cutoff, green SiC wheel, 320 grit diamond, 10 and 1 µm diamond paste |
Hardness Testing on the Rockwell A Scale
Hardness is measured using the Rockwell A scale with a 60 kg load and a diamond brale penetrator. Accurate results depend on proper surface preparation: the test surface must have a finish equivalent to a 220 grit diamond grinding wheel, and the opposite faces must be parallel and smooth. Any deviation can introduce significant errors.
⚠️ Critical: Ensure the hardness tester is free from vibration. Vibration can be detected by a bounding needle after the major load is applied.
Calibration is performed using two tungsten carbide test blocks with known hardness values of 90.0 and 92.0 RA. The block closest to the expected hardness of the sample is selected. The average of five readings should check within ±0.2 RA. If needed, adjust the dial before applying the major load.
These steps ensure that hardness readings are accurate and reproducible.
Specific Gravity by the Immersion Method
Specific gravity is determined by weighing the specimen in air and then suspended in distilled water. The density of water at the test temperature is used in the calculation.
The specimen should be surface ground all over with a 100 grit diamond wheel. Use an analytical balance (200 g capacity, 0.1 mg sensitivity). A nonferrous wire suspends the specimen. Take care to remove all air bubbles from the specimen surface after immersion; a wetting agent can be helpful.
The calculation formula is:
Specific Gravity = (Wa × D) / (Wa – Ww)
where Wa is weight in air, Ww is weight in water, and D is the relative density of water at the measured temperature.
💡 Tip: Measure water temperature to the nearest 1°C and use the correct density value for accurate results.
Metallographic Evaluation of Porosity, Structure, and Grain Size
Apparent porosity, structure, and grain size are evaluated by metallographic examination. Sample preparation is critical: section with a diamond cutoff wheel, rough grind with a green silicon carbide wheel, fine grind with a 320 grit diamond wheel, then polish using diamond paste laps (10 µm and 1 µm). Extreme cleanliness is necessary to avoid contamination.
Porosity evaluation is performed on the unetched sample at 200X magnification. Three types are defined:
- Type A: Pores under 10 µm
- Type B: Pores between 10 and 40 µm
- Type C: Cluster porosity or uncombined carbon (most detrimental)
The observed field is compared to standard charts for rating.
Structure and grain size are examined after etching with a solution of 10% potassium hydroxide and 10% potassium ferricyanide (equal parts, fresh) for 2 minutes, at 1500X magnification. This reveals the distribution of metal carbides and binder, and the carbide particle sizes.
Engineering Design Insight
🛠️ Design Insight: Consistent evaluation of these five properties ensures that sintered carbide tools meet performance expectations. For example, maintaining proper hardness correlates with wear resistance, while controlling porosity prevents premature failure. The standardized methods in SAE J439-2018 provide a common language for quality assurance and failure analysis, enabling engineers to make data-driven decisions in tool selection and process improvement.
Frequently Asked Questions
Why is surface preparation so important for Rockwell A hardness testing?
The diamond penetrator has a shallow penetration; any surface roughness or lack of parallelism can cause inaccurate readings. A 220 grit diamond finish and parallel faces are required to ensure reliable data.
How is specific gravity used in quality control of sintered carbides?
Specific gravity reflects the material’s density, which is related to its composition and porosity. Deviations from expected values can indicate issues in the sintering process or material formulation.
What do the porosity ratings (A, B, C) indicate about tool performance?
Type A fine pores generally have less impact, while Type B medium pores can affect strength. Type C cluster porosity or uncombined carbon is most detrimental, often leading to chipping or cracking under load.
Why are two calibration test blocks recommended for hardness testing?
Using two blocks (90.0 and 92.0 RA) ensures accuracy across the typical hardness range of sintered carbides. The block closest to the expected hardness is used, and the adjustment is verified.
