Clever sampling techniques highlight AMS aircraft quality testing

AMS 23XX specs can qualify entire heats of steel or individual steel products to comply with ‘aircraft quality’ cleanliness requirements.

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In this edition of Materials Matter, we review the sampling and test piece production required for the AMS aircraft quality (AQ) testing methods specified in AMS 2300, 2301, 2303, and 2304. While each of these standards has variations in the quality levels and testing sensitivity, the overall sampling methodology and test sample geometry requirements are essentially the same for all four specifications.

The AMS 23XX specifications can be used to qualify entire “heats” of steel or individual steel products to comply with “aircraft quality” cleanliness requirements. Once a heat is tested and qualified, any products produced from the qualified heat do not require additional sampling or testing. For this reason, many parts manufacturers will purchase raw material (ingots, blooms, or billets) that is supplied, tested, and qualified to the applicable AMS 23XX specification.

The samples are “hand-forged” using an open-die hammer to produce a straight cylinder forging where the outer diameter of the sample represents material from the surface, mid-radius, to center location.

The specific heat lot sampling requirements for top-poured, bottom-poured, or strand-cast heats are specific to the cast steel production process used. In general, for ingot cast material, samples are specified to be taken from material near the top and near the bottom location of each individual ingot. These top and bottom location samples will qualify the material for the entire ingot. When multiple ingots are cast in a heat, normally three ingots are sampled, again using samples taken at the ingot top and bottom locations. Strand-cast heats require samples to be taken from the first, middle, and back of up to two strands produced from a given heat of steel. The test results of all samples and tests from a heat are averaged and a final frequency (F) and severity (S) rating is produced that becomes the AMS 23XX rating.

Figure 1: Schematic of a stepped-cylinder test sample.

Interesting aspects of these AQ testing methods are the clever sampling techniques used to produce the test samples that enable inspection of the entire cross-section of the parent material quality. There are two different sampling techniques that can be used, depending on the size of the starting material, that both enable qualification of the material as AQ.

Perhaps the most straightforward sample to understand is the stepped cylindrical, whereby a full-cross section of a round bar (>1.5 inches diameter) or flat-plate material (>1 inch thick) is machined with five equal steps. Smaller cross-section bars or plates require fewer step downs and all the details tabulated within the specifications. Each of the five steps represents the parent material at progressively increasing distance from the outer surface. The machined surface finish must be very fine, and the specifications require a surface roughness of 40 microinches AA or better in accordance with ANSI B46.1. The outer surface of each step on the sample is tested using fluorescent magnetic particle inspection and non-metallic inclusions will be illuminated under ultraviolet light. Thus, the non-metallic inclusions present at the various depths represented by the steps can be evaluated and included in the final rating.

After the hand-forging operation, this is what the samples look like before machining.

The stepped sample is typically used for material where the material size enables machining and easy handling of a sample for testing. Typically, sections less than about five inches may use a stepped sample.

In larger sections, typically greater than six inches up to the largest ingots being qualified, a different approach is used. A straight cylindrical or rectangular sample from a quarter section of the parent material can be produced by simply cutting a quarter section, machining to the required surface finish and tested. In large sections a full quarter section of the parent material is sampled and is then re-forged to produce a more manageable cylindrical sample size. The resulting straight cylinder must have a length of five inches and a diameter up to four inches. Following this approach, the outer diameter of the resulting cylinder continuously represents material from the outer diameter through the mid-radius to the centerline of the parent material. Following the forging process, the sample is then finish machined and inspected using fluorescent magnetic particle inspection to reveal the non-metallic inclusions. The indications are measured, and the size and quantities are recorded. More details on the magnetic particle inspection, recording, and reporting will be discussed in next month’s Materials Matter column.

Finish machined AMS Aircraft Quality test samples ready for inspection.

To see the sample preparation and testing first hand, I had the opportunity to visit Solmet Technologies Inc. in Canton, Ohio. Solmet has been providing AMS Aircraft Quality testing services for 30 years and has tested nearly 400,000 samples. The company receives material from many different steel producers that submit material for testing and certification to the AMS specifications. The quarter sections are torch cut from the incoming material and then heated in a forge furnace. Next, the samples are “hand-forged” using an open-die hammer to produce a straight cylinder forging where the outer diameter of the sample represents material from the surface, mid-radius, to center location.

In next month’s installment of Materials Matter, we will review the fluorescent magnetic particle inspection, indication measurement, and reporting methodology required for the AMS 23XX Aircraft Quality specifications. 

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Guy Brada is a metallurgical engineer with more than 25 years in the steel industry. He received his Bachelors and Masters degrees in Metallurgical and Materials Engineering from the Colorado School of Mines. During his career, he has worked in steelmaking, the heavy forging industry, at an independent metallurgical test laboratory, and at a commercial heat treater. He has authored seven steelmaking and steel product patents. Currently he is technical sales service manager for Ellwood City Forge the open-die forging division of the Ellwood Group.