Carbon Steel Bold fracture

[Hercules28]

Dear all,

I got this bolt that fractured during service. The screw does see some low cyclic tensile loads in the orders of ~10- 100 /day. Environment unknown. From the degradation of the zinc coating I would think some moisture but nothing severe.

It is Carbon steel 1025 Galvanized.
Take a look at the attached pics and let me know what you think.

 

[Hercules28]

The other side

 

[Hercules28]

Zoom on a feature with some rust on it.

 

[Hercules28]

Zoom on a feature that looks like striations...?

 

[Hercules28]

The other side of the fracture. I did a mistake on Pic #2. All the previous ones are from the same side.

 

[Hercules28]

Side view of the "other"side

 

[CoryPad]

Are you sure it is 1025 steel? What is the hardness or strength level? The fracture looks like it initiated from the thread root at multiple locations. What you called striations may be crack arrest marks from step growth of the cracks. It looks like hydrogen embrittlement may be a possibility. Hard to say much more without more information on materials and environment.

 

[mrfailure]

You should gently clean one of the fracture surfaces so you can better see fracture morphology better. The side with rust could be the origin side where there are ratchet marks indicating multiple origins (rust indicates that surface had existed longest). Look at the morphology under a stereobinocular microscope (try both the ring light and fiberoptic lighting) and look for evidence of whether fatigue had occurred. You will get good guidance on on the meaning of observed fatigue characteristics if you look at the schematic diagram in ASM Handbook Volume 11 Page 632 (I have also seen this same chart reproduced elsewhere).

Regarding material: you should look at the bolt head markings, which will tell you what material and associated properties you should expect.

Aaron Tanzer

http://www.lehightesting.com

 

[metengr]

Not enough background information and no way to tell from the macrophotographs. Stop guessing and have it sent to a competent met lab.

PS; striations caused by fatigue crack propagation require much higher magnification to view. Crack arrest lines could be visible at this magnification.