17th Century Japanese helmet.
There was a crack in the helmet which is not visible in this image (some associated damage can just be seen in the lower left side of the helmet visor). The crack was opened and the fracture began at a streak with mostly intergranular fracture and then propagated by cleavage as shown below.
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| Note the intergranular fracture in the center foreground of the image on the left. The walls show transgranular cleavage that propagated from the intergranular origin. Next to the fracture, we see a region of columnar grains at the surface with a small region of finer, more equiaxed grains below and the very coarse columnar grains below that, as shown on the image on the right.
The fracture is along the left edge of the micrograph. Note that there appears to be some mechanical twinning in the grains at the fracture edge. The specimen was color etched with Klemm’s I and the hardness was 124.6 HV (~69 HRB). |
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| Although there are no slag particles in this region, other areas contained slag and indicated the steel was wrought iron. Below the fine grained region, we observe a few white spots. These particles (which are not colored by Klemm’s I) can be more easily seen in the micrograph on the top left at higher magnification.
Etching with alkaline sodium picrate at ~90°C for 60 sec identified these particles as cementite, some of which was in the grain boundaries, as shown in the micrograph on top right in a different area, but nearby. Alkaline sodium picrate colors cementite (and M6C carbide, but that cannot be present in wrought iron). Cementite, of course, is a brittle phase. So, the combination of the very coarse grains and grain boundary brittle cementite caused the crack in the helmet. |
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George Vander Voort has a background in physical, process and mechanical metallurgy and has been performing metallographic studies for 47 years. He is a long-time member of ASTM Committee E-4 on metallography and has published extensively in metallography and failure analysis. He regularly teaches MEI courses for ASM International and is now doing webinars. He is a consultant for Struers Inc. and will be teaching courses soon for them. He can be reached through his web site: www.georgevandervoort.com
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