When you see AMS 5629 on a drawing, and therefore go in search of a datasheet, you know that the spec number provides very little information. However, it will point to 13-8 PH. If you understand what that particular alloy does, the call out will make perfect sense. 13-8 PH is typically specified for applications requiring toughness.
What the spec actually includes
AMS 5629 is an aerospace material specification that covers 13-8 PH in bar, wire, forgings, forging stock, tube, and rings of up to 12” (305 mm) in section. The material is supplied in solution heat-treated condition, so it is soft enough for machining prior to a hardening process. The softening process is a single low-temperature treatment, and the hardness selected, H950, H1000, H1025, etc. to H1150, indicates the balance of strength, hardness, and toughness. Hence, a single process provides several properties.
Where the toughness comes from
Here is the important information. 13-8 PH is melted twice in a process consisting of vacuum induction melting and a consumable electrode remelting. Double melting incorporates all of the inclusions and refines the chemistry. The alloy retains an excellent transverse toughness meaning it is capable of retaining strength along and across the grain. That is the whole purpose of this grade/type of stainless steel. There are many stainless steels that have good hardness, but there are few that retain excellent toughness when load is applied short transversely. 13-8 stainless steel contains about 13% chromium, 8% nickel, a couple of percent molybdenum and aluminum, unlike other similar stainless steels having a low level of carbon, all of which helps explain why it remains very tough.
The places it is used
The first place this stainless steel was used was in the manufacture of certain components for the landing gear of various types of aircraft. Other components include structural elements, valves, shafts and various fittings and fasteners. It is used in various types of equipment for the petrochemical and nuclear industries as well as in components of motorsport equipment and in injection molding tools where it provides the required strength combined with corrosion resistance. It loses strength at about 600°F (316°C), which means it is ideal for much of the work carried out in the engine bay and process equipment.
A caveat about super strength: strength comes at a cost. Machineability is better with 15-5 or 17-4PH. It’ll weld and cut just fine, but strength makes it more difficult to machine. If you can position the piece to machine it solution treated then aging is possible.
This isn’t anything new. 13-8 PH has been doing the same work for years. However, an AMS 5629 callout over a cheaper PH grade is almost always for the transverse toughness. There is no replacement for this.