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Speaking of Precision Blog

Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

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Stress Cracks in Steel Bar Products.

Posted June 05, 2017 10:00 AM by Milo
Pathfinder Tags: cracks machining steel stress

“Stress cracks are defined as transverse or near transverse open crevices created when concentration of residual stresses exceed the local yield strength at the temperature of crack formation. These stresses can be mechanically induced or can be attributable to extreme temperature differences and /or phase transformations. They can originate at almost any point in the manufacture of the steel.”AISI Manual Detection, Classification, and Elimination of Rod and Bar Surface Defects

Stress cracks are often found visually at locations that experience bending or straightening. They are also referred to as “Cross Cracks” or “Transverse Cracks.” Originally they were identified in mill billet and bloom products, prior to rolling.

Micro examination can help determine crack origin by noting:

  • Orientation
  • Intergranular nature
  • Presence of scale
  • Presence of subscale

Additional microstructural characteristics can reveal the thermal history of heating and cooling at the crack location.

This photo shows stress cracks on a conditioned billet.

Causes and Corrective Action

  • Excessive load during straightening can exceed the local yield strength of the material causing it to crack; reduce load applied by machine, or consider tempering or stress relieving material prior to straightening or further cold work.
  • Cooling too quickly can also induce stress cracks. Critical cooling rates are highly dependent on steel chemistry. Crack sensitive chemistries (Medium carbon and high carbon steels; also medium and high carbon steels with straight chromium or straight manganese additions.) These steels should be slowly cooled through transformation temperatures to minimize the occurrence.
  • Design faults such as
    • Heavy sections adjacent to light sections and sharp corners
    • Failure to fillet sharp corners
    • Use of fillets rather than tapers
    • Undercuts
    • Overloading the material during fabrication, processing, or application.

Detection of stress cracks is problematic as their transverse orientation makes them difficult to detect on equipment set up to detect longitudinal defects.

Final caveat: The term stress crack is arbitrarily defined based on industrial usage in the market. It does not necessarily imply anything about the specific metallurgical nature of the crack, I know that a number of people use the term “stress crack” to describe longitudinal cracks on steel bar products as well, which the AISI calls “Strain Cracks.”


Editor's note: CR4 would like to thank Milo for sharing his blog, which can also be read here.

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