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Wrong Heat Treatment and Appearance of Fractures

It is important how to find out factors that could influence with quality failure. Because some manufacturers are trying to hide the factors to avoid cost impact and responsibility. Cracks were found at job site during UT. Before shipment, an inspector had ...

It is important how to find out factors that could influence with quality failure. Because some manufacturers are trying to hide the factors to avoid cost impact and responsibility.

Cracks were found at job site during UT. Before shipment, an inspector had checked the pipes visually but he couldn’t find it because pipes were covered by varnish coating. And the condition of contract between buyer and MFG was visual and document review. If condition of inspection included portable UT, buyer could avoid such losses.

For Fig.1, a testpiece was taken from crack area and it’s showing crack propagation from first crack area to deap side of pipes. Once a crack found, it’s gradually growing and eventually it causes penetration.

Fig.1

Crack propagation

This issue could be prevented if we took a careful look for the condition of fracture. Fig.2 is the results of mechanical test and the value of result was actually in the range of specification. However, it doesn’t approve that tissue structure is sound. Every tensile test specimen has the same appearance while it has ductile fracture after plastic deformation. However, we found no cup-cone geometry from Fig.2. It means that steel structure itself could have defects which cause crack.

Fig.2

tensile test specimen

 

Duetile Fracture has the typical cup-cone geometry from tensile test specimem. The first feature is the presence of plastic deformation that may be accompanied by neck formation. In tensile testpieces of ductile materials, besides necking, the fracture surface presents a fibrous aspect and a cup-cone geometry, as seen in Fig.3.(comment 1)

Fig.3

tensile test-specimen

 

From the result, if no cup-cone geometry was found during tensile test, microstructure test and ultra sonic test must be performed.

Testpiece took from fracture area(crack area) was tested and found Fe2O3 which cause severe damage of pipes. Fig. 4.

Fig.4

Fe2O3

 

It was examined that thick oxidation layer on crack was formed. That is, it seems that many deformation was made during manufacturing process of pipes. This hard doeformation initiated the crack and crack propagated with deformatio process. Thick oxidation layer seems to be made during post heat treatment process or the next process after finishing pipe manufacturing process.

Comment 1: Failure Analysis of Heat Treated Steel Components by Lauralice de Campos Franceschini Canale,George E. Totten,Rafael A. Mesquita, page 120

The Original Posted by Jethro

MACNET Korea

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Jethro

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