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What’s up guys? This is Michael with Michlin Metals back today to continue our converstaion (albeit one sided) about Aircraft Alloy Steel. Before we jump in….how about subscribing or a quick like? No big deal but it helps the channel! Let’s get into it!

Well if you have been watching our video series we now know that what differentiates an aircraft quality  “AQ” alloy from a commercial quality “CQ” alloy is the internal cleanliness of the metal and the steps taken during manufacture to assure its quality through testing.

Today’s video will be about some of the most popular alloys and common AMS specifications and MIL standards that apply to them.

In chemistry, we are focusing on the carbon content and the primary alloying elements. All have a maximum Phosphorus and Sulfur of  point 025% each. AMS 2301 magnetic particle cleanliness specification also applies to all of the alloys.

Alloy steels develop their best properties through heat treatment. Most of the mill treatments in specs are not the final required condition for the parts and additional heat treatment is required during the manufacture of parts.

First up are the direct hardening steels. Heat treatment by heating to high temperatures , quenching, and then tempering to the desired hardness. Carbon content affects the hardness that can be achieved; higher carbon -higher hardness.

4130 with a carbon content of point 28 to point 33%.  Chromium at point 80 to 1.10 % and Molybdenum at  point 15 to point 25 % are the primary alloying elements.   Typically used in parts at lower hardness or when welding is required.

Common specifications for bars and forgings are AMS6348, AMS 6370 and MIL-S-6758.  AMS 6371 and MIL-T-6376  for mechanical tubing.

4140 has a carbon content of point 38 to point 43% and the same Chromium and Molybdenum. The higher carbon content will achieve higher hardness as compared to 4130.

Common specifications are AMS 6349, AMS 6382 and MIL-S-5628 for bars and forgings. AMS 6381 for mechanical tubing.

Alloy 4340  has the same carbon content, but has point 70 to point 90% Chromium, 1 point 65 to 2 % Nickel, and point 20 to point 30% Molybdenum. The higher alloy content gives a greater depth of hardening and additional toughness as compared to 4140.

Common specs for bars, forgings, and tubing are AMS 6415, AMS 6484 and MIL-S-5000.

Alloy 52100 has a Carbon at point 95 to 1.10 %, and  1.30 to 1.60% Chromium. Most commonly used for bearing applications, it is capable of high hardness in heat treatment.Common specifications are AMS 6440 for bars, forgings and tubing. MIL-S-7420 for bars and forgings.

Carburizing is a heat treating process where a part is exposed to a carbon rich environment at high temperature,  Over many hours,Carbon is absorbed into the surface to create a  much harder and wear resistant surface layer over a softer and tougher core.

8620  is an alloy most often used for applications involving a carburizing heat treatment. It has a carbon content of point18 to point 23% Carbon, point 40 to point 60% Chromium, point 40 to point 70% Nickel, and point 15 to point 25% Molybdenum.

Common specification is AMS 6274 for bars, forgings , and mechanical tubing.

9310  is also a carburizing grade and has a carbon at point 07 to point 13% Carbon, 1 to 1 point 40% Chromium,3 point 0 to 3.50% Nickel, and point 08 to point 15% Molybdenum. The high nickel content adds toughness to both the carburized case and the core as compared to 8620.

Specifications AMS 6260 applies to bars, forgings and tubing. MIL-S-7393 to bars.

Nitriding is another technique for creating a hard and wear resistant surface. Long exposure to a nitrogen rich environment forms a thin layer of metallic nitrides on the surface. The surface layer is harder but much thinner than carburized surfaces.

True to its name, Nitriding #3 (aka 135 modified) is used for applications where parts are nitrided to achieve a hard wear resistant surface. It  has a carbon content of point 38 to point  43% carbon, 1.40 to 1.80% Chromium, point 30 to point 40% Molybdenum, and point 95 to 1.30 % Aluminum. The aluminum facilitates the formation and bonding of nitrogen at the surface. Specifications for bars, forgings, and tubing is AMS 6470.  AMS 6472, and MIL-S-6709 apply to bars and forgings.

There are many individual aircraft company specifications, too many to address here. Each of the Individual specifications have further requirements for the product form and thermal condition as well as additional restrictions or modifications to the chemistry. The devil is in the details, and there are lots of details in aircraft specifications, so as we always say….. check the specs!

Stay tuned for the next video… The premium aircraft quality alloys and specs. If you have missed any previous videos, click here. If you made it this far, subscribe! I know you like metal. Thanks for tuning in, this is Michael with Michlin Metals. I’m out.