Choosing an ASTM X-Ray or Integrity Grade
When specifying requirements for castings, it is impossible to determine a generic requirement for all castings. Because castings are produced for a wide variety of applications (from household items with little requirements beyond visual cosmetics to safety critical automotive and airplane components), the porosity grade specified by a casting user will depend on the end-use requirements of the component. To determine the integrity grade required (such as ASTM grade 3), casting design engineers define the stress level and service loads anticipated for the final cast component, as well as the criticality of any potential non-performance.
Additionally, it is difficult to specify a single radiographic grade for an entire cast part because it is likely that neither the casting mechanical properties nor the in-service stresses will be uniform throughout the component. In practice, this means that different radiographic grades may need to be applied to different areas of the casting. When applying X-ray standards, designers also can specify the technique used (the specific technical settings for the radiographic or fluoroscopic image) to ensure both coverage and image quality.
When creating a casting specification, casting users and designers tend to over-specify requirements. Some designers may be unfamiliar with the casting process and materials and have uncertainty about quality and consistency, or they may have a desire to “pad” their engineering safety margin.
Overly severe specifications that mandate quality beyond the end-use requirements of the component can add cost to the final part. While placing a requirement of grade 1, grade 2 or better on the casting drawing might help assure a designer receives no parts from the metalcaster that would have performance issues, this also could result in a casting specification requiring inspection and validation that add cost but provide no real benefit. The lowest cost specification is one that precludes all things that will harm the necessary component performance but permit everything else.
A good dialogue between the casting designer, specifier and metalcaster will produce the best combination of quality, safety and cost. Users and casting producers should negotiate to better understand the functional requirements and assist in the translation of those needs into appropriate and effective specifications. The dialogue is vital in the development of effective gating and risering, which can determine local mechanical properties, and production process.
ASTM x-ray grades or void sizes should not be accepted arbitrarily as the sole uniform quality criteria. The key issues relating to casting quality are the functional durability and performance reliability of the cast component. For example, cast aluminum automotive wheels (where ASTM E155 is applied) bring with them industry standard durability and performance tests whose results can be correlated for a particular design with an ASTM grade.
Many manufacturers leave the radiographic limit open until the testing process demonstrates what can be accepted safely within the specified performance parameters. This provides for a functional and pragmatic approach to radiographic standard setting. During testing, sample parts can be used to determine the maximum flaw size (amount of mass loss, location, etc.) that can diminish performance and establish a correlation back to x-ray grade.
Radiography may not always be the complete solution for defect identification. Ultrasonic inspection has benefits, and some types of detrimental indications may be discovered through surface analysis, such as dye penetrant inspection. As stresses tend to be conveyed over surfaces, discontinuities in a surface may be more important quality-influencing factors than internal voids. METAL
Sample Guidelines for X-ray Porosity Standards
|Aluminum Castings||Companies that produce aluminum engineered products and structural components should specify anything from a level 1 to a level 4, depending on actual design requirements. Aluminum castings will not generally meet ASTM mechanical property minimums with porosity severity levels greater than a grade 3. A good rule of thumb is for a designer to specify grade 3 if the desire is to meet ASTM property limits for design. This provides a good compromise on cost and performance.|
|Ductile iron castings||Safety critical automotive ductile iron parts (steering knuckles, control arms, brake calipers, etc.) are typically specified as grade 2 or better, unless otherwise designated by the designer. This includes any casting that falls under the National Highway Traffic Safety Administration definition of a component whose failure could result in the loss of vehicle control and potentially life or limb.|