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The fatigue crack propagation and threshold behavior of a C-Mn-B steel has been examined at strength levels appropriate to the use of this material in haulage chain. Two austenitizing temperatures have been used, 1250°C and 900°C, with tempering temperatures up to 400°C. Threshold values at R = 0.2 ranged from about 4 to 6 MPa ∙ m1/2. They tended to be somewhat higher in material austenitized at 900°C, than in material treated at 1250°C. In the material austenitized at 900°C and tempered at low temperatures (up to about 350°C), the fatigue crack fronts were markedly curved, being held up in the center of the fatigue specimens and advanced at the quarter-section positions. This was due to the residual stress distribution resulting from the volume changes associated with cooling and transformation during the quench.
Good agreement was obtained between the residual stress distribution measured by monitoring the change in strain on the bottom face of a fatigue specimen as successive layers of material were removed from the opposite face, and the predicted distribution after the 900°C austenitizing treatment. The predicted residual stress distribution was determined by a viscoelastic-plastic mathematical model. Both methods showed a tensile stress in the region immediately below the surface and a compressive stress towards the center of the fatigue specimens, as anticipated from the crack front curvature.
Testing at R = 0.8 reduced the crack-front curvature very significantly. In material austenitized at 1250°C, the crack front curvature was less marked than it was following the 900°C treatment.
Measured and predicted residual stress distributions for material as-quenched from 900°C and 1250°C are presented, and the relationship between the stress distribution present and the effects of such factors as mean stress and tempering temperature is discussed in terms of crack closure behavior.
boron steels, heat-treatment, martensite, residual stress, fatigue crack propagation, fatigue threshold, crack closure, mining chain
Senior lecturer, Sheffield City Polytechnic, Sheffield S1,
Higher scientific officer, University of Nottingham, Health and Safety Executive, Sheffield,
British Gas/Fellowship of Engineering Senior Research Fellow, University of Cambridge, Cambridge,