Discontinuous surface cracks appeared on steel rebars

[Engkzd77]

I'm working as a quality control engineer in EZDK company (Egypt) which produce deformed steel rebars. We use QTB technology for producing rebars. Recently, discontinuous cracks appeared on the Deformed 12 mm rebars surface. Micro structure test showed that the crack depth is about 0.8 mm. I need to know the possible reasons could lead to existence of such cracks.

 

[metengr]

This is one of those questions that can only be a answered by a proper metallurgical failure analysis of the surface cracks. There are various possibilities for such cracks and to narrow one or more down without a proper metallurgical analysis is foolish.

 

[racookpe1978]

Where did you get the steel from?
If you rolled the rebars yourself, where did the raw material come from, and how do you know absolutely that it (the raw material that was melted into your rebar) is any good? What was your material QA and lab test on the source?

 

[weldstan]

As metengr states, a metallurgical investigation is required. Possible culprits are: cooling rate, last hot forming temperature too low, non homogeneity. I assume tha you have a properly staffed met lab, since you manufacture rebar.

 

[Engkzd77]

Thank you all for your replays. For more details and clarification about the problem I attached three microstructure test results (two types of cracks were found)

 

[Engkzd77]

Other type of crack found in same sample

 

[Engkzd77]

This is the original defective sample

 

[TVP]

I wouldn't characterize either of these defects as cracks. The first is called a lap, which is a defect that occurs during the hot rolling process. The hot-rolled wire rod should be inspected for this type of defect. It is prevented by good billet surface quality and correct roll alignment, among other things. I'm not sure what the second defect is, but it doesn't appear to be a crack, since it has a rounded appearance. The following link has some information that you may find useful:

http://pmpaspeakingofprecision.com/2012/05/15/laps-on-rolled-steel-products/

 

[metengr]

Surface seams and slivers are also common defects in rolled products. On-line eddy current testing will catch these types of defects.

 

[Engkzd77]

Thanks for your reply metengr, I need to know the possible reasons for such cracks and the expected sources to occur.

 

[metengr]

To add to what TVP provided;

http://pmpaspeakingofprecision.com/2012/05/24/seams-on-rolled-steel-products/

http://pmpaspeakingofprecision.com/tag/steel-defects/

http://www.ijmer.com/papers/Vol3_Issue2/AW32736738.pdf

http://www.siderurgia.org.ar/pdf/32.pdf

 

[Engkzd77]

Thanks a lot metengr for your help