Influence of drill pipe manufacturing technology on fracture mechanisms and fatigue properties

An experimental investigation was carried out to assess the impact of manufacturing technologies on the fracture mechanisms and fatigue mechanical properties of drill pipe N (in its normalized state) and pipe U (following improvement) under cyclic loads. Two strength groups of drill pipe, N and U, were subjected to study. The presence of a highly tempered fine-dispersed structure in the material of pipe U, in comparison to the ferrite-perlite structure of the material of pipe N, provides enhanced resistance to fatigue crack initiation. This is confirmed by the values of the endurance limits, which are 503 MPa for pipe U and 294 MPa for pipe N. The fatigue tests carried out in the air revealed an inflection in the fatigue curves. For the metal of drill pipe N, the inflection corresponds to a value of 343 MPa, while for the metal of pipe U, it corresponds to a value of 522 MPa. Such inflection is associated with the formation of a barrier in the form of a hardened surface layer that arises during testing.