Corrosion-Fatigue Crack Propagation in Exploited Pump Rods Made of 20N2M Steel
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CORROSION-FATIGUE CRACK PROPAGATION IN EXPLOITED PUMP RODS MADE OF 20N2M STEEL N. V. Kret,1, 2 L. M. Svirska,1 and T. P. Venhrynyuk3
UDC 620.193: 620.17
We analyze the influence of model stratal water with low acidity on the cyclic crack resistance of 20N2M steel of a pump rod after operation for 5.5 yr. The state of the metal of a pump rod head is conventionally regarded as the reference state. We revealed insignificant differences between the basic mechanical properties and the characteristics of cyclic crack-growth resistance of steel in air for different states of the material. However, under the action of corrosive media, the fatigue crack-growth rate for the exploited steel is much higher than for steel in the reference state. This means that the in-service degradation of the rod steel manifests itself mainly in its sensitivity to the action of corrosive media. Keywords: pump rod, stratal water, fatigue crack-growth rate, in-service degradation.
Introduction Pump rods are among the most crucial structural elements in the equipment used for the oil and gas extraction because their failures (breaks) may have severe consequences. These elements operate under the conditions of high-cycle loading and under the action of aggressive stratal waters. Therefore, the characteristics of corrosion-fatigue strength are decisive for guaranteeing the integrity of rods [1, 2]. The failures of the rods are caused by the initiation of surface fatigue cracks in the process of operation. This follows from the analysis of their state after exhaustion of the designed service life [3–5]. Therefore, as an important characteristic of the serviceability of rods, we can use the corrosion-fatigue crack-growth resistance. On the other hand, in the course of long-term operation and, in particular, under the action of cyclic loading, the mechanical properties of structural steels are deteriorated [6–11], [12, 13]. Hence, it is necessary to take into account the possibility of in-service degradation of steels and the mechanical and stress-corrosion properties of the pump rods. In what follows, we present the results of comparative studies of the kinetics of fatigue and corrosionfatigue crack growth for steel taken form different sections of pump rods after operation under different conditions of force loading, which may lead to different degrees of the in-service degradation of the metal. Object and Methods of Tests We studied pump rods made of 20N2M thermally hardened steel with a diameter of the working part equal to 19 mm. The rods were studied after operation for a period of ∼ 5.5 yr (about 2 ⋅10 7 cycles), which corresponds to the conditions of high-cycle loading. 1 2 3
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding author; e-mail: [email protected].
Ivano-Frankivsk National Technical University of Oil and Gas, Ivano-Frankivsk, Ukraine.
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 56, No. 2, pp. 130–134, March–April, 2020. Original article submitted January
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