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Cleanliness Rating of Steels by the Magnetic Particle Method đ ī¸
This recommended practice from SAE J421-2018 provides a structured approach for rating steel cleanliness using magnetic particle inspection. It outlines procedures for sampling, specimen preparation, magnetization, and indication counting to assess non-metallic inclusions.
Sample Selection and Specimen Preparation
Sampling should be agreed upon between producer and consumer. For heat qualification, samples are taken from specific locations such as the top and bottom of ingots. Specimens are prepared from blooms, billets, or bars. For cross sections over 230 cm², a quarter section is cut and machined to a cylinder. The original stock center must be near the specimen surface, and its location should be identified. The following table specifies minimum stock removal based on specimen size:
| Nominal Size of Specimen (mm) | Nominal Size of Specimen (in) | Minimum Stock Removal (mm) | Minimum Stock Removal (in) |
|---|---|---|---|
| To 13 | To 1/2 | 0.76 | 0.030 |
| 13-19 | Over 1/2 to 3/4 | 1.1 | 0.045 |
| 19-25 | Over 3/4 to 1 | 1.5 | 0.060 |
| 25-38 | Over 1 to 1-1/2 | 1.9 | 0.075 |
| 38-51 | Over 1-1/2 to 2 | 2.3 | 0.090 |
| 51-64 | Over 2 to 2-1/2 | 3.2 | 0.125 |
| 64-89 | Over 2-1/2 to 3-1/2 | 4.0 | 0.156 |
| 89-114 | Over 3-1/2 to 4-1/2 | 4.7 | 0.187 |
| 114-152 | Over 4-1/2 to 6 | 6.4 | 0.250 |
Engineering Design Insight: Ensuring the original stock center is at the specimen surface allows evaluation of the worst-case inclusion distribution, critical for quality assessment.
Heat Treatment and Surface Preparation
After rough machining, specimens are heat-treated to a hardness of 250 HB minimum for carbon content of 0.25% or over, using oil or water quenching followed by tempering between 200 and 650°C. This improves magnetic retention and uniformity. 🔍
⚠️ Care must be taken to avoid quenching cracks, which can produce false indications. Transverse grinding is recommended to prevent scratches that retain magnetic powder.
Surface finish should be at least 1 µm (40 µin), and specimens must be cleaned with solvent to remove grease and marks before testing.
Magnetization and Indication Evaluation
Magnetization is performed with direct current for 0.2 to 0.5 seconds at 300 to 500 A per cm of diameter. The wet continuous method is standard, where the specimen is covered with magnetic particle suspension during magnetization. Indications are examined under diffused light, and the severity factor is computed based on indication frequency and weighted length.
🛠️ For hardened steel (Rockwell C 50+), the residual method may be used, applying suspension after magnetization.
FAQs:
- How are steel samples selected for cleanliness rating?
- Sampling follows a plan agreed between producer and consumer, typically from top and bottom of ingots for heat qualification.
- What is the severity factor?
- It is calculated from the frequency and weighted length of indications, providing a quantitative cleanliness measure.
- Why is transverse grinding important?
- Longitudinal scratches can mimic inclusions, leading to false readings; transverse grinding minimizes this risk.
- What precautions are needed during magnetization?
- Current and duration must be controlled; over-magnetization can obscure indications, while under-magnetization may miss them.
