Structural and Fractographic Features of Gas Pipeline Steel Degradation

Abstract

Structural and fractographic features of degradation are analyzed for pipe steels after their operation on main gas pipelines. The structural feature of steel degradation is damaging along the boundaries between pearlite and ferrite grains manifested by more intensive etching the boundaries with extraction of cementite particles from the matrix. The most obvious effect of degradation is revealed for the steel 17H1S, and the least one for the steel X70. It is concerned with steel texture peculiarities, namely, different sizes (thickness and length) of strips of pearlite, and with dispersion of its components (i.e. cementite lamellae) which control to a large extent hydrogen permeability in a pipe wall and its trapping at the ferrite–pearlite boundaries. Structural peculiarities of steel degradation revealed themselves clearer fractographically in a form of delaminations at the fracture surfaces of the operated steels. The hydrogen absorbed by metal during pipe operation and concentrated at the structural defects along the boundaries between interlayers of ferrite and pearlite grains led to the occurrence of these delaminations and their extension. Therefore, these delaminations are considered as the fractographic features of embrittlement for the operated steels. The relationship between mechanical properties of pipeline steels and metallographic and fractographic parameters is obtained. The critical state of degraded steels is substantiated by the change of the crucial element of embrittlement on the fracture surfaces from delaminations (in the subctitical state) to cleavage (in the overcritical one).