The ASTM A388/A388M-95 Standard Practice for Ultrasonic Inspection of Heavy Steel Forgings, developed by the American Society for Testing and Materials (ASTM), describes inspection procedures for contact, impact-echo ultrasonic inspection of heavy steel forgings using straight and angled beam techniques. This standard includes the use of straight beam techniques and the distance gain-size method. This practice should be used when an inquiry, contract, order, or specification states that forgings will be subjected to ultrasonic inspection in accordance with the ASTM A388/A388M-95 standard.
An ultrasonic, pulsed, reflection type instrument is used for this application. The system is capable of inspecting at frequencies from 1 to 5 MHz. When inspecting austenitic stainless forgings, the system is capable of inspecting at frequencies as low as 0,4 MHz.
The ultrasonic instrument shall provide linear presentation (within 75 percent) for at least 5 percent of the display height (scan line to top of screen). The 5 percent linearity referred to is descriptive of the display presentation of amplitude. Instrument linearity shall be verified in accordance with the intent of ASTM E317. Any block set processed in accordance with ASTM E317 or ASTM E428 may be used to determine the specified 65 percent instrument linearity.
The electronics include an attenuator (with accuracy within the useful range of plus/minus 10 percent (+1 dB) of the amplitude ratio) to allow measurement of readings beyond the linear range of the instrument.
Search units with a transducer having a minimum dimension of 20 mm and a maximum active area of 30 mm650 are used for straight beam scanning. Search units equipped with dimensions between 2 mm x 13 mm and 25 mm x 25 mm are used for angled beam scanning. Transducers are used at their nominal frequencies. Other search units can be used to evaluate and determine indicators.
Couplings with good wetting properties such as SAE number 20 or SAE number 30 motor oil, glycerin, pine oil or water should be used. Couplings may not be comparable to each other and the same coupling should be used for calibration and inspection.
Reference blocks containing flat-bottomed holes may be used for calibration of equipment in accordance with this standard and may be used to determine recording levels for flat beam inspection if specified in the order or contract.
Distance gain-size (DGS) scales appropriate to the ultrasonic testing unit and transducer to be used may be used to determine registration levels for straight beam inspection when specified in the order or contract. The DGS scale range should be selected to include the full thickness section of the forging to be inspected.
Personnel performing ultrasonic inspections in this application must be qualified and certified in accordance with a written procedure that complies with this standard or another national standard acceptable to both the purchaser and the supplier.
In general, whenever possible, the entire volume of the tattoo is subjected to ultrasonic examination. Due to variations in the radii and other local configurations of the sections, it may be impossible to examine some sections of a tattoo.
Ultrasonic inspection for mechanical properties shall be carried out after heat treatment (excluding stress relieving operations) but before drilling holes, cutting keyways, tapers, grooves or machining sections for contouring. If the configuration of the forging required for the mechanical properties process prevents a complete inspection of the forging later, inspection prior to machining is permitted. In such cases, the forging shall be re-inspected ultrasonically as completely as possible after heat treatment.
To ensure complete coverage of the tattoo volume, the search unit is aligned with at least 15 percent overlap on each pass. For manual scanning, a scanning speed of 150 mm/s should not be exceeded.
For automatic scanning, the scan rate or instrument repetition rate, or both, are adjusted to permit detection of the smallest interruptions specified in the specification and to permit operation of the recording or signaling device. At no time should the scan rate exceed the rate at which an acceptable calibration is made.
If possible, all sections of the forgings are scanned in two perpendicular directions. Where possible, disc forgings are scanned using the straight beam technique from at least one flat surface and radially from the periphery. Cylindrical sections and hollow forgings are scanned using the angled beam technique. Where possible, the forging is also examined in the axial direction. Additionally, hollow forgings are examined using the angled beam technique from the outer diameter surface.
In re-checking or re-evaluating by the manufacturer or the purchaser, comparable equipment, search units, frequency and coupler are used. Forgings may be inspected stationary or on lathes or while rolling on rollers. If not specified by the purchaser, either method may be used at the manufacturer's option.
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