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Author Topic: Examination of Fractured Bren Gun Piston Post - Sheerness  (Read 2162 times)

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Offline kyn

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Re: Examination of Fractured Bren Gun Piston Post - Sheerness
« Reply #2 on: December 28, 2012, 20:41:01 »

Offline kyn

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Examination of Fractured Bren Gun Piston Post - Sheerness
« Reply #1 on: December 28, 2012, 17:20:06 »
Bragg Laboratory, N.O.I.D.

April, 1951.

Examination of Fractured Piston Post from
Bren Gun 1 MG. .303.  Mk. 1 ex R.N. Rifle Range Sheerness

A piton post, Bren gun 1 MG, .303 Mk. 1. which failed after firing a burst of 20 rounds at Rifle range Sheerness, was received from N.O.I.O. Chatham for examination.  The total number of rounds fired by the component was 250.

Fracture occurred at the position shown in Fig. 1. resulting  in a small portion of the rear end of the component being broken off from the main body.  The fracture which is illustrated in Fig 2. was bright and of fatigue type, and there was no evidence of any original defect.  Failure originated at the radius blending the bottom of the recess, and a silhouette of a longitudinal section taken to show this position is illustrated in Fig. 3.

Composition
The component was of the following composition:-

Carbon0.16%
Silicon0.26%
Manganese0.46%
Sulphur0.015%
Phosphorus0.012%
Nickel3.03%
Chromium0.92%
Molybdenum0.07%

This analysis is normal for 3% nickel-chromium case hardening steel and corresponds to specification B.S. 970 En. 36.

Structure
Microexamination of sections cut from the piston post adjacent to the fracture showed the component to be case carbonised to a depth of .01/.012.  The case was martensitic and free from cementite.  The core was of fine grained martensite.  The steel was generally clean and no large inclusions were detected.

Hardness
Diamond pyramid hardness tests showed the hardness of the carburised layer to be uniformly 792 V.P.N.  The core hardness was 385 V.P.N.

Conclusions
Failure of the Bren gun piston post examined was due to fatigue stressing at the root of the rear slot.

F. H. Edwards
Superintending Scientist.

 

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