The Effect of Delta Ferrite on the Fatigue Behavior of High Resistance Threaded Fastener 2008-36-0063
The formation of structurally delta ferrite in low carbon alloy steels, used to make high resistance bolts, reduces its mechanical properties, due superficial hardness caused by diffusion of phosphorus during quench and tempering process. This phosphorus is present in the surface of roll material as phosphate bound to zinc to reduce friction coefficient during cold forming operations, as wire drawing and extrusions.
In recent years, several recalls have been made by automakers, in order to avoid delayed failure possibility, due embrittlement of the bolts caused by phosphorus diffusion, that became the threaded fasteners susceptible to the retarded fracture for action of hydrogen released in corrosion process when the bolts is simultaneously submitted to tension under corrosion environment.
International standard like ISO 898-1: 1999[E] do not allow metallographic detectable phosphorus enriched layer for bolts over 1220 N/mm2 resistance class, allowing the possibility of the occurrence of it to tensile resistance bellow this range.
The objective of this work is to demonstrate that delta ferrite affects the mechanicals properties of any tensile resistance class, reducing its endurance limit, in function of the micro failures that present in weakest point (root radius). This micro failure occurs due to the low capability of plastic deformation of material's surface (affected by phosphorus diffusion), during of assembly bolts.
A number of fatigue test have been made in M8x1,25 - 10.9 bolts, in samples with and free of phosphorus enriched layer. The procedure used was the Staircase Test as normalized by DIN 969: 1997-12. Along this test by statistics help we can demonstrate that fatigue durability of the bolt is affect by the effect of delta ferrite, causing a reduction of endurance limit independently of the resistance class.