Static Crack Resistance of 20 and 30KhMA Steels in a Nace Solution
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STATIC CRACK RESISTANCE OF 20 AND 30KHМА STEELS IN A NACE SOLUTION М. R. Chuchman We study the crack resistance of 20 and 30KhМА steels in a NACE hydrogen-sulfide solution under static and repeated static stresses and found their threshold stress intensity factors. A conclusion is made that these steels almost satisfy the criteria of their applicability in hydrogen-sulfide media accepted in the engineering practice. Keywords: steel, crack resistance, hydrogen sulfide, static stresses, repeated static stresses, hydrogenation.
The service life of metal structures and industrial equipment is usually determined by the fracture strength of materials in working media both under static and under cyclic loads. In oil-and-gas extracting and oil refining industries, the working media, as a rule, contain hydrogen sulfide, which accelerates local corrosion and the process of hydrogenation of the metals. The influence of these media on the fracture strength of materials is mainly investigated under static loads in NACE chloride-acetate solutions [1] without taking into account cyclic stresses, although they are always present under the actual conditions. As shown in [2, 3], 20 pipe steel is more susceptible to hydrogen-sulfide corrosion cracking ( σ th /σ 0.2 = 0.6) than 30KhМА steel ( σ th /σ 0.2 = 0.8). However, under the combined action of static and cyclic stresses with a stress ratio R = 0.4–0.7, the durability of 30KhМА steel sharply decreases: For a sufficiently low stress amplitude σ a = 0.2σ 0.2 , there are no mean stresses in a cycle under which this steel does not fail for a base testing time of 720 h. At the same time, for 20 steel, if the mean stress in a cycle is equal to the threshold stress of corrosion cracking σ th = σ m = 175 MPa , and the amplitude is equal to σ a = 0.2σ 0.2 = 54 МРа, then cyclic loads decrease the durability of the specimens by a factor of ∼ 1.2 but, for the same amplitude and σ m = 120 МPа, the specimens do not fail for the base testing time. In other words, the high resistance of steel to corrosion cracking does not, in fact, guarantee its high fatigue strength under cyclic loads. Since the initiation and propagation of cracks determine the fracture strength of materials, in the present work, we study the corrosion crack resistance of these steels in NACE solutions under the action of static and repeated static stresses. Materials and Experimental Procedure We studied 20 steel (wt.%): 0.21 С, 0.23 Si, 0.36 Mn, 0.17 Ni, 0.10 Cr, 0.17 Cu, 0.013 S, and 0.010 P with the following mechanical characteristics: σ u = 474 МРа and σ 0.2 = 290 МPа, and 30KhМА steel (wt.%): 0.29 С, 0.34 Si, 0.57 Mn, 0.13 Ni, 0.92 Cr, 0.19 Cu, 0.008 S, and 0.012 P with σ u = 710 МPа and σ 0.2 = 550 МPа. Their static crack resistance was investigated on beam specimens with preliminarily induced fatigue cracks under loading by cantilever bending [4]. In the course of repeated static loading with trapezoidal cycles, the specimens were unloaded for 1 h/day. Karpenko Physicomechanical Institute, Ukrainian National Academy o
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