fatigue failure anomaly 3

[thorangle]

I have a 3/4-10 UNC threaded rod, conforming to IEEE standard C135.1-1999, IEEE standard for zinc-coated steel bolts and nuts for overhead line construction.

The failed section of the bolt has striation marks indicative of fatigue failure, and a small zone with typical brittile failure charateristics.

The anomaly:
parallel to the fatigue striations is a zone of discoloration. The zone is approximatly .08" wide, with "clean" normal fatigue striations on either side. The discoloration is blueish rain-bow, similare in apearance to when heat-treated steel is overheated by grinding. The depth of the discoloration appears to be very shallow, i.e. less than .005" into the fatigue fracture face.

Anyone have some thoughts as to what might have caused this anomaly?

Thanks

 

[TVP]

Temperature is the cause of color changes on steel. In order to produce a bluish-type color, a temperature of greater than ~ 250 C is necessary. Time at temperature is also a factor. ASM Volume 4, Heat Treating, has some data on SAE 1035 steel:

Deep Blue: exposed to ~ 310 C for 10 minutes, or ~ 250 C for 1000 minutes.

Blue, purple tinge: ~ 340 C for 10 minutes, or ~ 265 C for 1000 minutes.

Deep grey-blue: ~ 370 C for 10 minutes, or ~ 280 C for 1000 minutes.

 

[gdodd]

Thorangle
From your decription it sounds like fatigue cracks propagated from the discolouration. The discolouration could be caused by a material defect ie inclusion or segregation introduced during casting. Alternatively mechanical damage during bolt manufacture.
Suggest you examine the fracture surface using a low power microscope and look for changes in surface morphology at the discoloured surface and or zinc deposition on the discoloured surface. If present you can trace when the crack occured.
If the bolt was heat treated during processing, crack discolouration could have been caused by oxidation in the furnace atmosphere.