Degradation of a low-carbon steel in long operation in an oil-hydraulic unit
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DEGRADATION OF A LOW-CARBON STEEL IN LONG OPERATION IN AN OIL-HYDRAULIC UNIT A. D. Markov,1, 2 L. M. Svirs’ka,1 O. Z. Student,1 and H. M. Nykyforchyn 1
UDC 621.181:669.018
We present an atypical example of corrosion fracture of a low-strength high-plasticity steel from an internal surface formed by a hole in a choke of an oil-hydraulic unit, operated for 35 years. The conclusion that, in terms of the effect on the mechanical behavior of the metal operated for a long time, oil can be considered to be an aggressive medium is made. Its influence is enhanced by the preliminary plastic deformation of subsurface layers of the metal using force drilling of the hole in the choke. We detected fractographically the subcritical growth of a crack from the internal surface of the choke with multiple secondary cracking, which is interpreted as a manifestation of the mechanism of hydrogen embrittlement of the metal degraded during long operation.
The major factor of possible fracture of the structural materials of oil-hydraulic units is cyclic load caused by a periodic change in the pressure of oil in the system. Since oil is assumed to be a nonaggressive medium, the problem of corrosion fatigue of the metal is usually not considered. At the same time, oil changes its properties in long operation as a result of destruction and may become so aggressive that it will be able to cause corrosionmechanical fracture [1]. Critical elements of an oil-hydraulic system are pressure hoses with special chokes with external thread (Fig. 1a) intended for joining constructively the units of the system. The chokes are made of low-carbon steels 35 and 20 in the as-delivered state of rod rolled products [2]. The structural embodiment of such chokes assumes a large strength margin. Moreover, taking into account the low strength and high plasticity of such steels, they are considered to be unsusceptible to the corrosion-hydrogenating influence of media [3]. We investigated a choke of an oil-pressure hose with a taper inch tread, which was operated for 35 years and fractured nearly perpendicularly to the axis at the outlet from the site of twisting in a base member (Fig. 1). The choke was made of steel 20, which, after operation, had the following mechanical properties in tension: σ0.2 = 272 MPa, σu = 488 MPa, δ = 25%, and ψ = 65%. The working pressure of oil in the pressure system was 9 MPa. If the stress concentration caused by the external thread is neglected, the calculated axial stresses in the transverse net section of the choke generated by the internal pressure of oil must not be larger than 18 MPa. A feature of the fractured choke is the substantial roughness of the surface of the internal hole (Fig. 1c), which was evidently caused by force drilling in its making. This is confirmed by the characteristic orientation of projections on the surface of the hole along the helical line. The violation of the manufacturing process caused not only the stress concentration in the vicinity of hollows on the internal surface of the hole, but also the s
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