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June 22, 2020

Understanding Non-Destructive Test Results

Scot Forge reviews the numerous applications in forgings and the different methods for testing.

Forging is one of the oldest skills known to man and it’s tough to envision manufacturing and even life without the parts created by forging. Some of the applications for forgings can be straightforward, such as rings for gears and flanges or bars for ship shafts. In contrast, others are very complex, such as hollows for oil country tubular goods (OCTG) and pressure tubing for power boilers or domes for nuclear waste storage.

There are numerous applications for forgings. As a result, there are many manufacturing and testing standards that forgings must meet. Forgings are commonly created to the standards of the following organizations:

  • •  AISI - American Iron and Steel Institute
  • • ANSI - American National Standards Institute
  • • AMS - Aerospace Material Standards
  • • API - American Petroleum Institute
  • • ASTM - American Society for Testing and Materials
  • • ASME - American Society of Mechanical Engineers
  • • NAVSEA - Naval Sea Systems Command
  • • DIN EN - European National Standards

And, just like the applications of the forgings, the methods for testing can be varied. For instance, testing from a ring to a tube for a critical, high-pressure use is not as simple as it first appears. Numerous standards could apply; it depends on the application. ASTM A668, for example, references six different standards.

  1. A275/A275M Practice for Magnetic Particle Examination of Steel Forgings
  2. A370 Test Methods and Definitions for Mechanical Testing of Steel Products
  3. A388/A388M Practice for Ultrasonic Examination of Steel Forgings
  4. A788/A788M Specification for Steel Forgings, General Requirements
  5. E290 Test Methods for Bend Testing of Material for Ductility
  6. E340 Practice for Macroetching Metals and Alloys

Each standard specifies procedures for measurements or test methods. As far as material standards, the mill typically calls out the capabilities of material based on sound forging practices. There are many tests available to meet criteria standards and ensure part quality that helps find indications, or imperfections, in the material – magnetic particle test (MT), dye penetrant test (PT) and ultrasonic inspection (UT).

MT and PT discover surface flaws; they are relatively easy and inexpensive. MT uses a magnetic field with a solution containing iron filing particles that highlight an indication in the forging as the iron particles collect at the magnetic flux that leaks from a crack or void. PT can be more time consuming as it requires applying a liquid penetrant to the forging and then removing it; a developer must then sit on the piece to draw out remaining penetrant from any cracks or crevices. UT is a volumetric test and uses sound waves to find gaps in the forging; when the sound wave hits a void, it ends back a spike in the ultrasonic display, which is called an indication. All these methods require a finished surface to allow for accurate readings.

Performing a test is one process, but evaluating a forging to determine if it is acceptable is a different process altogether. For instance, it’s not uncommon to require a UT inspection by requesting “UT per ASTM A388.” The problem, spelled out in this document, is that “this practice is intended for application to forgings, with a wide variety of sizes, shapes, compositions, melting processes, and applications. It is, therefore, impracticable to specify an ultrasonic quality level which would be universally applicable to such a diversity of products.” The standard does not list exact acceptance requirements other than indications that cannot exceed an acceptable size.

Since metal transitions from a liquid to a solid, there are always some particles that get trapped, much like air bubbles or dirt would in ice. While most particles get eliminated during the forging process, some could remain at an acceptable level. So, this is why it is essential to discuss acceptance criteria, list the voids or flaws that might be found as well as the limits or extent that they are permitted. A good example is specifying the amount of non-metallic inclusions that may be permitted. The specification would include the sizes and spacing between non-metallic inclusions that must be measured and evaluated, which helps set expectations for both the customer and the manufacturer. It is important to note that there are ways to reduce the risk of indications in a forging, such as using refined material like Vacuum Arc Remelt (VAR), but increased purity results in increased cost. 

Finding the balance between acceptable quality and cost-effectiveness is often tricky. Not to mention how the acceptance criteria vary by each forged product. So, it is essential to carefully review documents that support the requirements of the end-use application. From the simple to the complex, a partner like the Scot Forge employee-owners can help guide you through this process.