Abstract:
The application of non-destructive testing methods, particularly ultrasonic testing and magnetic particle testing, in the inspection of steel castings for chemical machinery. By studying the types and distribution characteristics of defects in steel castings, this paper proposes a non-destructive testing method combining “process advancement” and “ultrasonic-magnetic particle” and corresponding measurement technical parameters based on the ASTM A609 standard. Practice has shown that the location and quantitative detection of internal defects in steel castings before high-precision machining and timely disposal can effectively reduce repair costs and improve product acceptance rates.
1. Introduction
Steel castings, used for shaft sleeves, frameworks, etc., are key components in manufacturing industrial structures with high product quality requirements. However, defects such as pores, sand inclusions, cracks, and shrinkage porosity inevitably occur during the processing, leading to a decline in material strength, plasticity, and impact toughness, and resulting in reduced mechanical properties or even failure of the material. To ensure product quality and reduce costs, multiple inspections are conducted during the production process, utilizing various testing methods at different process stages to identify any defects exceeding standards and take timely measures. Since chemical machinery steel castings undergo significant vibrations during use, and their inner bore surfaces are affected by factors such as shaft rotation and friction, the inner bore surfaces and near-surfaces must be free of large defects. Therefore, it is essential to strengthen inspections of the casting surfaces and near-surfaces to ensure that the final product has no large shrinkage or linear defects on its inner bore surfaces and near-surfaces.
2. Application of Ultrasonic Testing in Chemical Equipment
2.1 Necessity of Inspection
Large steel castings for chemical equipment typically undergo visual inspection after pouring at the manufacturing plant, followed by mechanical rough machining. When the inner bore of a shaft sleeve or framework has severe defects, cracks may propagate on its surface under the action of shaft rotational friction and vibration, eventually leading to damage to the shaft sleeve and framework. Therefore, while rough machining the castings to leave a 20-30mm machining allowance in their inner cavities, ultrasonic testing should be employed to identify potential defects in the finished inner bore and address them through chipping, welding, and other methods. Additionally, considering the possible deviation of the inner bore center point, it is crucial to ensure that no defects exceeding the standard exist within a depth of 15-35mm.
Due to the curvature of the inner bore, testing is usually conducted in the near field of the straight beam probe according to ASTM A609, Standard Practice for Ultrasonic Examination of Carbon, Low-Alloy, and Martensitic Stainless Steel Castings. This paper intends to investigate content such as testing frequency, probe type, sensitivity adjustment, curvature compensation (couplant), detection direction, and defect judgment, and establish stricter testing techniques based on the characteristics of the defects to be detected (e.g., shrinkage porosity, pores, inclusions, cracks).
2.2 Selection of Probe Type and Testing Frequency
In casting flaw detection, a combination of longitudinal wave straight beam probes and transverse wave angle beam probes is adopted. Considering that there is still a 20-30mm allowance after rough boring of tail shaft sleeves and rudder frames, combined with the characteristics of the casting process (coarse grains and loose internal structure) and the detection requirements of ASTM A609 for certain depth-related defects, a 4MHz dual-element straight beam probe is used to reduce the near-field length within the machined area, improve sensitivity, reduce blind zones, and decrease noise. However, single-element probes have issues such as low sensitivity, high noise levels, and blind zones. The chip angle of the dual-element probe is selected to be 10°-12°, maximizing the reflection signal within the 20-30mm detection range and achieving the highest detection sensitivity. Transverse wave angle beam probes (70°, 2.5MHz) can be utilized for longitudinal and lateral scanning.
2.3 Sensitivity Adjustment
2.3.1 Sample Selection
According to specifications, considering the significant difference between ordinary reference material and the inspected object, it is difficult to ensure accurate location and quantification of subsequent defects during sensitivity adjustment. Therefore, the same material as the workpiece (sampled from the remaining part of the machined section) is selected to manufacture a reference sample for sensitivity adjustment.
2.3.2 Distance Amplitude Correction (DAC) Curve
Using the standard test piece as a reference, the DAC curve is processed using a dual-element straight beam probe. Through debugging on the probe test block, the display screen shows that the depth range of 10-35mm is the largest segment, indicating the area with the highest detection sensitivity.
2.4 Inspection Conditions
Before inspection, the inner bore should be marked with grid lines to ensure full coverage without omissions. According to specifications, each movement of the probe should cover at least 10% of the area, and the scanning rate should not exceed 100mm/s. For curved detection surfaces, probes with flexible protective films and small chips should be selected, and appropriate curvature compensation should be applied to achieve good coupling between the probe and the workpiece surface.
2.5 Acceptance Criteria Based on ASTM A609
(1) Defect echoes exceeding the DAC curve are evaluated.
(2) On the DAC curve, the indicated length of a defect is consecutive and exceeds the defect echo on the DAC curve, and the length of the defect is measured using the 6dB method centered on the probe.
(3) The total area of the defect is evaluated using the average length/width. The evaluation frame is placed over the area with the heaviest defects, and then the sum of the defect sizes within the evaluation frame is used for assessment according to Table 1.
(4) The presence of cracks renders the piece unqualified.
Table 1. Quality Grades for Ultrasonic Testing
| Quality Grade | Parameters | Ultrasonic Testing Results |
|---|---|---|
| Grade 1 | Maximum indicated length of single defect (mm) | ≤30 |
| Maximum indicated area of single defect (mm²) | ≤200 | |
| Total defect area (mm²) | ≤5000 |
Table 2. Inspection Methods
| Item Name | Parameters or Requirements |
|---|---|
| Test Piece Name | Stern Shaft Sleeve / Rudder Frame |
| Specification | ∮865/∮6670 |
| Material | Carbon-Manganese Steel Casting |
| Surface State | Machined |
| Inspection Ratio | 100% of Inner Bore |
| Inspection Timing | After Rough Machining |
| Instrument Model | EPOCH4B |
| Couplant | Motor Oil + Grease (Beef Tallow) |
Table 3. Probe Specifications
| Item Name | Parameters |
|---|---|
| Probe Type | Dual-Element Probe / Angle Beam Probe |
| Model | MSEB4H 4MHz 3.5×10 / 2.5P8×10 70° |
| Reference Test Block | ∮2.4mm Flat-Bottomed Hole RB-2; φ3×40 Transverse Through-Hole |
| Coupling Compensation and Scanning Method | 6dB Grid Line Scanning / 6dB Multi-Direction |
| Sensitivity | DAC Curve Production / φ3~12dB (Adjustment) |
3. Application of Magnetic Particle Testing in Chemical Equipment
After initial ultrasonic testing, most defects exceeding standards generated during casting are removed. To better control product quality and eliminate tiny defects caused by acoustic wave attenuation due to coarse grain structure, making them difficult to detect, magnetic particle testing is conducted when there is still a 0.2-0.3mm allowance after finishing. Defects such as linear or dense porosity can also be eliminated through chipping and welding.
By selecting appropriate instruments, magnetic particles, and scanning methods, the surface defects of the workpiece can be accurately detected.
3.1 Equipment
3.1 Equipment for Magnetic Particle Testing
Magnetic particle testing (MT) can be selected for objects with large volumes, complex shapes, and harsh testing environments. Therefore, in practical applications, portable AC/DC yokes, cross-rotating yokes, and excitation probe contacts are more commonly used.
3.1.1 Selection of Equipment Based on Field Tests
Field tests revealed that both tail shaft sleeves and rudder seat inner bores possess curvature, leading to a significant detection area. Initially, a cross-rotating yoke was considered due to its ability to satisfy both longitudinal and lateral magnetization requirements, coupled with its own rollers for accelerated detection speed. However, during practical testing, several issues were identified:
- Deformation and Instability: The cross-rotating yoke experienced bending deformation, with its four legs unable to change direction and position instability, resulting in point contact with the workpiece. This reduced detection sensitivity and impacted the detection results.
- Contact and Damage Issues: The support contact method failed to effectively magnetize the entire area and was prone to metal impact and sparking when the contacts excited the workpiece, potentially causing burns on the machined surface.
Given these challenges, the portable AC/DC yoke was ultimately selected for its effective contact with the workpiece, adjustable leg spacing, and lightweight design.
3.1.2 Advantages and Applications of Portable AC/DC Yokes
- Portable AC/DC Yoke: This equipment can be effectively utilized for contact with the workpiece. Its lightweight design and adjustable leg spacing make it suitable for various testing scenarios.
- AC Power Supply: It generates a strong surface magnetic field due to the skin effect, which is conducive to the migration of magnetic particles and has a sensitive response to surface defects. It is easy to demagnetize and has a relatively shallow detection depth. Since there is only a 0.3mm allowance left in the final processing, AC power is preferred to avoid seeking deep defects with DC power.
- DC Power Supply: It has a large penetration depth and can detect defects buried deeper within the material. However, it has low pulsating intensity during magnetic processing, poor fluidity, large residual magnetism, and is not easy to demagnetize.
- Application: Given the specific testing requirements and the characteristics of the tested objects, the portable AC/DC yoke was selected for its ability to detect linear or denser porosity defects effectively, especially when there is a 0.2~0.3mm allowance left after polishing.
In summary, based on field tests and the characteristics of the tested objects, the portable AC/DC yoke was chosen as the primary equipment for magnetic particle testing, ensuring effective detection of defects while minimizing potential damage to the workpiece surface.