Forged Retained Rings

High strength non-magnetic forgings. Resistant to stress corrosion cracking.

High strength non-magnetic forgings resistant to stress corrosion cracking

API Engineering has over a decade of experience as a manufacturer of high quality forged retaining rings. As one of the main uses of this type of forgings is rotor end caps for turbo generators, quality and strength is of the essence. Our austenitic steel forgings are produced to customers’ requirements and dimensions.

The forging material preferred by API Engineering is Chromium, Manganese steel; a high strength steel that is non-magnetic and resistant to stress corrosion cracking. This material is commonly referred to as 18-18 steel forging due to the steel type being an example of x8 Cr MN N 18.18 (18 percent being the nominal percentage of Chromium and Manganese).

Manufacturing process

The steel is produced in an electric furnace and undergoes a refining process before being cast into an ingot. The chemical composition of the steel is checked during the molten stage, to ensure the quality prior to the ingot being formed. The ingot is then hot-forged into the approximate required shape prior to an initial machining operation to produce a uniform forging. Heat treatment then gives the material its strength characteristics, which are further increased by a highly specialised cold expanding technique. The stresses that have developed in the forging by the preceding processes are then removed by further heat treatment.

At this point a sample of the forging, normally a 25mm square section ring, is machined from one end of the ring for the necessary mechanical and quality checks to be carried out. Final machining to the customer’s requirements finishes the forging. The sample ring is cut up into small segments and further machined to produce test pieces for Tensile Tests, Impact Tests, Hardness Tests, Chemical Analysis, Grain Structure and Magnetic Permeability tests. The final machined forging also undergoes Dye Penetrant Testing to detect any surface defects on all surfaces, as well as 100% Ultra Sonic Testing to detect any material defects with in the forging.

Technical specification

Our forgings are manufactured to the suppliers’ standard specifications, but can also be produced to meet relevant international standards.

Chemical composition

















Mechanical properties

Heat treatment

The forging undergoes a solution heat treatment cycle typical 1,050°C for 2 hours.

Stress relieving is carried out at above 400°C for 3 hours.

Non-destructive testing (NDT)

Surface Defects: 100% Surface Area by Dye Penetrant Method

Internal Defects: 100% of the forging by Ultra Sonic Testing.


Every forging has a unique identification number engraved on the surface. This allows full traceability. Additional information including the cast /ingot number, the manufacturers mark, material type and purchase order number is also displayed on the forging.

Inspection & quality control

Every forging manufactured by API Engineering is fully inspected by our inspectors and when accepted given a coat of anti-rust protection and packed for transport. Every forging is supplied with a certificate of conformity and dimensional report. The certificate of conformity contains the results of all the mechanical tests, chemical analysis and heat treatment cycles. NTD and test reports will be also provided.