Effects of Particle Speed on Erosive Wear Behaviour of API X120 Pipeline Steel
Authors: Shakoor R. A. , Okonkwo Paul , Mohamed A. M. 
Published: July 2019

The need of transporting petroleum products has resulted in increased erosion of pipeline steel components. For decades, pipeline systems have been used for transporting petroleum products. Carbon steel is commonly used for constructing long-distance pipeline projects due to its mechanical durability and economic aspect. However, the erosion of oil and gas transmission pipeline continues to be a great concern to the petroleum industry because of increasing pipeline maintenance cost and failure. Pipeline steels are often subjected to severe erosion during transportation of petroleum products containing a broad range of erodent particles. The process of transporting petroleum product through the pipeline often results in mechanical removal of the oxide film from the pipeline surface, leaving the surface directly exposed to stress and degradation.

Material removal due to solid particle erosion is believed to be a series of impact events that occur in pipelines and cause extensive damage due to change in the solid-liquid flow direction. Erosion of oil and gas pipeline is a complex phenomenon, characterized by the impacting erodent particles on the pipeline walls due to solid-liquid flow, flow restrictions or change in flow direction. The erosion of steel surface by stream of solid particles can result in high material loss and maintenance costs. Unfortunately, there is no universal model that can effectively predict all erosion situations and development of a reliable and effective model for solid erosion process still remains a challenge. Several attempts have been made to understand the effect of different parameters, such as; temperature, particles size and microstructure of both the impinging and eroding surface on the solid particles erosion process. However, each parameter behaves peculiar to each process and is often complex due to interrelated variables involved. Most of these works were focused on lower carbon steel. The results obtained from these works showed that the target material, temperature, impact angle, particle velocity, shapes and sizes play critical roles in the erosion mechanisms. The angle at which the erodent particles impinge the target material accounts a greater percentage of the erosion damage. Levy studied the solid particle erosion behaviors of 1020 and 1075 low carbon steels using SiC particles as erodent at different impingement angles and speeds. The result showed that the microstructure of the steel materials had a significant influence on the crack growth observed on the eroded steel surfaces. Similarly, Green et al. investigated the erosion mechanisms of low carbon AISI 1050 steel material in relation to the carbon content and microstructure. The result revealed that thermally hardened martensitic structures behave better than the pearlitic steels of the same carbon content under normal temperature range. McCabe also studied the effect of microstructure on the erosion of AISI 1078 and 1050 steels at different angles and speeds using 240 grit A12O3 erodent particles. The result exhibited that the erosion mechanisms assumed a brittle mode with increase in particle velocity. Liebhard and Levy conducted a study on the influence of the shapes of erodent particle on the erosion of 1018 steel. The result showed that angular particles caused higher order of erosion compared to spherical particles. However, the impact of these parameters on the erosion characteristics and mechanisms significantly depend on the material pairs and testing equipment.

In another direction, significant efforts have been made to improve the erosion resistance of the pipeline steel over the years. Results indicated that micro-alloying of carbon steels with small amount of carbide and nitrate forming elements have achieved significant success in the erosion resistance of the carbon steels. Micro-alloying with application-specific elements in combination with judicious process control (e.g. shape-forming and heat-treatment etc.) provided carbon steels of high yield stresses and desirable toughness, for example, high strength low alloy (HSLA) steels. Interestingly, HSLA steels are becoming the material of choice for the projects requiring larger pipeline because of their appreciably low price-to-yield ratio. API X-70 and API X80 have been of the commonly used pipeline grades steels due their ability of withstanding the basic erosion-corrosive environment. However, recently, petroleum industry has witnessed an outstanding demand for higher strength pipeline steels e.g., API X100 in order to combat more stringent environment in terms of erosion-corrosion. Recently, TransCanada, one of the frontiers amongst the steel manufacturing industries has produced API X120 steel which is considered as highest grade pipeline steel available in today's market. The erosion behaviour of this newly developed pipeline material has not yet been investigated in detail. It is of essence to understand the erosion mechanism of this newly developed high strength steel, and under various incidence angle and erodent particle velocities. Conducting detailed analysis on interaction of API X120 steel with various erodent particles (e.g. aluminum oxide) at different velocities would be worthwhile in order to understand the erosion characteristics and mechanisms. Understanding the effects of particle velocity and erosion behavior associated with the API X120 in simulated pipeline environment is necessary to minimize the rate of erosion in the petroleum industry and would be helpful in efficient pipeline material selection and design. This study has been made to facilitate understanding of erosion mechanism and its transition with particle velocity which has a direct relation with the erosion damage.

In this study, dry erosion test was performed in order to investigate the erosion mechanism of API X120 steel by employing particle velocities over a range of 43–167 m/s at normal impact angle for different durations within 0–10 min. A dry sand blaster erosion tester was used to study the erosion behaviour of API X120 steel impinged with aluminium oxide particles at room temperature. The equipment was designed to impinge the targeted sample surface with solid particles at different velocities under controlled erosion conditions. Scanning electron microscope and profilometry techniques were used to characterize the eroded API X120 steel surface. The results indicated plastic deformation and embedment of the erodent particle on the target material surface to be the predominant erosion mechanism observed at lower speed, while at high particle speeds the dominant erosion mechanism was observed to be metal cutting of the target API X120 steel surface.