Cracks by zinc-coating

TESTING PROCEDURE
Visual examination
The sample under investigation consists of a mechanical component in galvanized steel broke into operation, which detail is shown in photo No.1.


 
Photo No.1
The Macrographic appearance of the entire fracture surface, shown in photo No.2, is grainy and dull.

Micrograph: 2
Sample:  BLADE SUPPORT
Position: FRACTURE SURFACE

 
Chemical analysis
The chemical analysis allows to classify the material as tempered steel of the type 46Cr2 according to UNI EN 10083.

Hardness
The hardness measured on the component, corresponds to 42 HRC.

Fractographic examination
Each point on the fracture surface is characterized by an intergranular morphology type as shown in photo No.3.
 
Photo No.3
Metallographic Examination
The micrograph No. 4 shows the detected core metallographic structure: it is tempered martensite, typical of this hardened and tempered quenched material.

The micrograph No. 5 shows the structure on the surface, where it is possible to distinguish the zinc coating.
There are no detectable anomalies and defects due to the matrix and/or hardening and tempering heat treatment

Micrograph: 4                               
Etching: NITAL
Magnification: 600X
Sample: BLADE SUPPORT
Position: CORE
Section: CROSS SECTION
 
Photo No.4
Micrograph: 5
Etching: NITAL
Sample: BLADE SUPPORT
Position: SURFACE
Section: CROSS SECTION
DISCUSSION
The fracture occurred by a brittle type mechanism with of intergranular propagation and has involved a border region in which the hole creates a stress concentration.

Given the type of material, its high hardness, the presence of the coating and the morphology of the fracture, the most likely hypothesis is that the component has suffered hydrogen embrittlement, caused by the galvanizing process, which has triggered intergranular cracks which led to failure in the component.

CONCLUSIONS
The failure is due to a mechanism of hydrogen embrittlement due to a lack of or insufficient dehydrogenation of the sample after galvanizing, which is necessary at 200° for 2 hours before putting the particulars into operation.
 
 

Tag: Railways and transportation

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