Much has been written about the critical nature of steel cleanliness as it relates to the performance of highly stressed, cyclically loaded gear components. Modern steel making technology using ladle metallurgy practices and vacuum degassing enables production of very high-quality steels that approach cleanliness levels historically only achievable by vacuum arc re-melt (VAR) or electro-slag re-melting (ESR) practices.
Previous Materials Matters columnists have detailed current technology used to quantify and evaluate steel cleanliness via metallographic techniques by direct examination of non-metallic inclusions using both light and electron microscope methods in addition to details on the developing ultrasonic cleanliness evaluation techniques. The ASTM E45 (First issued 1942) specification is perhaps the most well-known method of evaluating steel cleanliness. E45 involves metallographic samples that are polished and non-metallic inclusions are examined at 100X magnification using a light microscope and evaluated for size and quantity by comparing with reference charts.
Beyond standard E45 cleanliness requirements, many gear specifications have a call-out requiring the material be “aircraft quality” cleanliness. While the term aircraft quality, or AQ for short, implies that the material will be used in aerospace applications, the reality is that AQ steels are specified in nearly all types of industrial applications including many OEM and industry gear standards. AQ testing is, in fact, another method whereby the cleanliness of steel is evaluated using magnetic particle inspection techniques.
The primary test standards for AQ testing are controlled and distributed by SAE International (www.sae.org) and are labeled AMS (Aerospace, Material, Specifications). There are four AMS specifications that are commonly referenced and detail the testing and requirements for aircraft quality steel cleanliness.
AMS 2300 (First Issued 6/1/1959)
- Steel Cleanliness, Premium Aircraft-Quality Magnetic Particle Inspection Procedure.
- VAR or ESR required, highest quality level “re-melted” steels.
AMS 2301 (First Issued 1/1/1960)
- Steel Cleanliness, Aircraft Quality Magnetic Particle Inspection Procedure.
- Most commonly seen specification.
AMS 2303 (First Issued 11/1/1967)
- Steel Cleanliness, Aircraft Quality Martensitic Corrosion-Resistant Steels Magnetic Particle Inspection Procedure.
- Similar to AMS 2301 but for martensitic stainless steels.
AMS 2304 (First Issued 5/1/1995)
- Steel Cleanliness, Special Aircraft-Quality Magnetic Particle Inspection Procedure.
- “Special” quality to address modern, high quality, non-re-melted steels.
These AMS methods are intended to “qualify material used to produce highly stressed parts where very strict magnetic particle inspection standards are used in final inspection of such parts but may be used for qualification of a heat, melt, or lot of steel.” The testing methodology involves magnetic particle inspection and are therefore different than the ASTM microscopy methods perhaps more well known for evaluation of steel cleanliness.
Each of the four referenced specifications use very similar methodology for sampling, test piece preparation, and magnetic particle inspection. The primary differences between the four specifications are the size of the indications that are recorded and how they are rated based on size and ultimately the acceptance criteria that is applied. The original AMS 2300 specification is the most stringent, as might be expected when evaluating the highest quality VAR or ESR re-melted steels.
The most modern “special quality” specification detailed in AMS 2304 has essentially the same quality requirements as the original AMS 2300 but is intended to evaluate today’s higher quality steels produced without a re-melting process. The AMS 2301 and 2303 specifications are similar to each other in terms of the relevant inclusion sizes counted and the acceptance criteria applied; both have less stringent acceptance criteria as compared to the AMS 2300 and 2304 requirements.
While commonly requested when purchasing steels for a wide variety of applications, the details of how the AQ cleanliness testing is conducted may not be well understood by the industry at large. The goal of this Materials Matters series of columns will be to help readers better understand the process of the aircraft quality testing methodology as specified in the AMS standards — from material sampling and test piece preparation through the magnetic particle inspection to the evaluation and reporting of test results. This column is only intended as a general understanding overview and readers should refer to the official SAE International publications for the specification details.