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Role of Microstructure in Sucker Rod String Failures in Oil Well ProductionSofiane Benhaddad and Glen Lee(Submitted 12 December 2000; in revised form 20 February 2001)Sucker-rod pumps are operating in very aggressive environments in oil well production. The combined effect of a corrosive environment and significant mechanical loads contribute to frequent cases of failure of the rod string during operation. Standards and recommendations have been developed to control and avoid those failures. This study presents various failure cases of sucker rods in different applications. The heat treatment of the steel material and the resulting microstructure are an important factor in the behavior of the sucker rod. A spheroidized microstructure presents a weaker resistance to corrosion affecting the rod life. Non-metallic inclusions are a pitting preferential site leading to fatigue crack initiation. Heterogenous microstructure as banded martensite and ferrite/pearlite decreases the ductility of the material affecting the fatigue propagation resistance.Keywords: corrosion-fatigue, inclusions, pitting, steel microstructure, sucker rod1. IntroductionThe oil production wells in western Canada generally operate in aggressive environments, including corrosive elements, high velocity of the fluids, and considerable depth for completion. Determination and analysis of the necessary data are required for selection of materials for well components. Materials for the tubing, casing, and sucker rod string, as well as a suitable coating or chemical protection program are selected appropriately for each operating well. In such an aggressive environment, the most vulnerable downhole component is the sucker rod string due to its function. Specifications and standards 1-4have been written for the improvement of the performance and life expectation of sucker rods. The recommendations of these codes and standards are directed towards a better choice of material for good corrosion resistance and a better design of the rod string for an even distribution of stresses. The resulting recommendations are restrictive and may pay significant attention to economic factors. Therefore, many operators continue to use carbon or alloy steel components with an appropriate chemical inhibition program. Carbon steel is preferred over other materials due to its machinability, high strength, and low cost.The use of carbon steel rod strings in aggressive environments, without properly considering the complex interactions between the material and its service environment, leads to failures. In this context, numerous rod string failures have been studied through metallurgical investigation that included macro-observation, chemistry, fractography, microscopy, and hardness testing. This paper summarizes several of the studied cases and divides the cases according to the main root cause of failure: mechanically induced failure and environmentally induced failure.2. Mechanically Induced FailuresAll sucker rod failure cases were related to the most commonly used materials: carbon or alloy steel.These rod strings were operating under one of three conditions: 1) in conjunction with a chemical corrosion inhibition program; 2) in conjunction with a protective coating; and 3) without any protection from the service environment. Cases are summarized along with the details of the failure analysis.2.1 Role of the Microstructure2.1.1 Observation. Three sucker rods broke at the pin end (Fig. 1) and produced the same microscopic rupture with identical features. The rods were fabricated from carbon or alloy steel. The observation of the surface of the rupture revealed three distinct zones (Fig. 2). The first zone, corresponding to the crack initiation, revealed pits and corrosion. The second zone was flat and smooth, indicating a macroscopically brittle fracture. The second zone surface was covered with beach marks, suggesting fatigue crack propagation. The third and last zone was a shear lip. The third zone surface was rough and covered with dimples (Fig. 3), showing that the final rupture process was ductile overload. All these features indicated that corrosion-fatigue was the cause of failure. This was confirmed with the presence of fatigue striations on the surface of the second zone (Fig. 4). The pitting of the surface of the rod introduced a stress concentration at the “pit tips” and led to the initiation and propagation of fatigue cracks due to cyclic loads resulting from the operation of the pump. The aggressive down-hole environment may have accelerated the crack growth.2.1.2 Microscopy. Microscopy observations were completed on sections removed from the rod through the surface of the rupture. The microstructure was ferrite and pearlite with numerous inclusions dispersed throughout the structure (Fig. 5). The inclusions were analyzed as MnS particles. The inclusions were elongated parallel to the rod length and contained numerous micro-cracks. Additionally, the inclusions provided preferential sites for pitting in the material (Fig. 6).Several studies have highlighted the detrimental effect of inclusions in the steel 5-8 and shown that MnS particles create a micro-galvanic couple with the steel: MnS being the anode and the steel the cathode. Our observations were consistent with previous works that showed that the presence of MnS inclusions at the surface leads to pit formation in acid environments. The pits formed at the surface of the rod provided local stress concentration that altered the resistance to fatigue initiation. The pits, coupled with the cycles created in the operation of the pump, have served to initiate fatigue. The crack is then propagated through the thickness of the rod.In one of the failure cases studied, elongated bands in the microstructure were observed (Fig. 7). These bands corresponded to the rupture features of the fracture surface. Microhardness testing of the bands and the bulk of the structure showed a significant variance, with the hardness of the bulk being 293 HV and the band significantly harder at 411 HV. The high hardness of the bands is indicative of a brittle material. It corresponds to the location of inclusions and micro-cracks in the material. Additionally, the band areas show segregation of chrome, which leads to the higher hardness. The banding resulted from partial austenitizing, which caused the original banded ferrite and pearlite microstructure to transform to bands of very hard martensite and bands of ferrite/pearlite. The presence of the hard martensite bands combined with MnS inclusions to promote the initiation of fatigue cracks and lead to a decreased resistance to crack propagation. A high number of inclusions in the steel is extremely detrimental and the use of higher quality steel is preferred. Changes in the fabrication process for the rods, including improved quality control, resulted in homogeneous microstructure with an even chemistry and hardness and decreased the tendency for failure. Fig.1 Sucker rod broken at the pin endFig.2 Surface of the rupture of the sucker rodFig.3 Surface of the last stage of the ruptureFig.4 Fatigue striations on the surface of the ruptureFig.5 Inclusions in the microstructureFig. 6 Pits at initiation stage of the ruptureFig. 7 Bands in the microstructure2.2 Coating and DesignTwo of the major factors contributing to the failure of sucker rods are:1. Corrosion at the surface of the rods and,2. Stress concentrations induced by mechanical design. The elimination of these factors is achieved by:a) Protection of the surface with the application of either a corrosion resistant coating or chemical inhibition of the oil field environment, and/orb) Better design of the sucker rod to remove stress concentrations.2.2.1 Observation. In the following case, although design improvements were made, the rod failed in service. A socket was tightened around the pin to reduce the stress concentrations applied on the pin end of the rod. However, the rod broke right at the edge of the socket after a few months of operationin sour service (Fig. 8). The surface of the rupture was examined after acid cleaning. The fracture surface contained the same features as previously discussed: three distinct zones corresponding to the initiation, propagation, and final fracture portion of the failure (Fig. 9). Cracks and corrosion deposits were observed on the initiation zone. Striations and hammering marks were observed in the propagation stage zone. Dimples covered the final rupture zone. The sucker rod failed by corrosion-fatigue mode. The corrosion particles were determined to be iron sulfide (x-ray diffraction) indicating the aggressive nature of the service environment.2.2.2 Microscopy. A transverse cross section through the diameter of the rod showed that crack initiation was followed by the propagation of a nearly straight crack. Small curves were observed as the crack passed through the coating due to the presence of voids in the coating layer (Fig. 10). Micro-cracks were observed near the surface and a large secondary crack was observed at the end due to the high deformation (Fig. 11). The rod material was of type 8610 and spray coated with a Ni-Cr-B eutectic alloy. Since the material was operating in sour service, it should conform to the requirements of NACE Standard MR01761.The composition of the coating was slightly different than the minimum NACE recommendation. 1 Both chrome and boron contents were just over the recommended maximum. The hardness of the coating was 63 HRC, while the minimum required1 is 55 HRC, and the coating thickness was 0.3 mm while the minimum required1 is 0.25 mm.These results lead to the conclusion that the sprayed metal coating conformed to the reference requirements. The main cause of the failure was the stress concentration at the end of the socket where the rod failed. This stress concentration along with the accumulation of abrasive corrosion by-products, iron sulfide, led to the creation of a physical notch at the surface of the rod. The cyclic load on the rod caused the initiation and propagation of the crack and resulted in a fatigue failure.The protective coating on the surface of the rod increased the corrosion resistance of the rod material but did not prevent the failure. This failure case confirms the fact that both chemical protection of the surface and design must be considered to prevent failure.Fig. 8 Sucker rod broken at the edge of the socket Fig. 9 Surface of the rupture of the sucker rodFig. 10 Initiation stageFig. 11 Propagation and final stage of the rupture3. Environmental Induced FailuresAlthough the sucker rods failed mainly in corrosion fatigue mode, other types of failure of sucker rods are also encountered. The following case describes a sucker rod downhole tubing failure where a long rod string composed of rods of varying diameters was operating in sour conditions with the presence of CO2.3.1 ObservationsThe rod string consisted of the rod diameters 24.9 mm, 22.12 mm, and 19.1 mm with the largest diameter rod sections being near the well surface. Two distinct types of surface damage were observed on the rod sections immediately after removal from service:1. Rod sections #1 and #3 (Fig. 12): Large corrosion imperfections aligned along the length of the rod on the same side with no apparent corrosion attack adjacent to the corroded regions.2. Rod section #2 (Fig. 13): Total erosion along the length of the rod. The production tubing revealed severe corrosion attack with pinholes aligned along one side of the length of the tubing. After acid cleaning, the I.D. surface revealed large corroded areas with a flat bottom and stepped edges. Most of the corroded areas were hollow. No corrosion attack was observed in the areas adjacent to the region of extensive corrosion (Fig. 14). The corrosion deposits found on both corroded surfaces of the tubing and the sucker rod #3 were primarily iron sulfide and some iron oxide (Fig. 15).The deposits collected from the sucker rod section #3 were composed of iron sulfide, iron carbonate, plus some manganese.Fig. 12a Rod section #1 Fig. 12b Rod section #3 Fig. 13 Rod section #2 Fig. 14 Tube sectionsFig. 15 (a) XRD analysis of corrosion deposits in tube; (b) XRD analysis of corrosion deposit on rod #33.2 MicroscopyMetallographic sections were prepared from the tubing and the rods. The tube microstructure was composed of ferrite and poorly formed pearlite (Fig.16). Numerous inclusions were also observed and were most likely MnS. The hardness of the tubing material was 176 HV corresponding to 565 MPa in tensile strength. The tube material conformed to API Grade J559. The rod section #1 had a microstructure of pearlite with pro-eutectoid ferrite and some martensite indicating a partial austenitizing heat treatment, temperature between A1 and A3, with moderate cooling (Fig. 17). The hardness value was 280 HV corresponding to 899 MPa in tensile strength.The rod section #2 had a microstructure of ferrite and martensite indicating fast cooling from a temperature higher than A1 (Fig. 18). The hardness of the rod section #2 was 292 HV corresponding to 930 MPa in tensile strength. The rod section #3 revealed a similar microstructure (Fig. 19) to the tube material (Fig. 16), but with a larger volume of pearlite in the rod, indicating higher carbon content or a faster cooling through the austenite ferrite region. The hardness of rod section #3 was 179 HV corresponding to 576 MPa in tensile strength.Fig. 16 Tube microstructureFig. 17 Microstructure of rod #1 Fig. 18 Microstructure of rod #2 Fig. 19 Microstructure of rod #33.3 CommentsThe conditions of service were aggressive, with the water containing H2S and CO2 at a down-hole temperature greater than 50 C. In a wet environment, CO2 becomes aggressive. CO2 corrosion can be distinguished as one of three types, depending on the flow: Pitting in moderate flow, mesa-type attack in medium flow, or flow induced localized corrosion (FILC) in critical flow. 10 The observation on the tubing and on rod sections #1 and #3 suggests a mesa-type corrosion or FILC, as grooving was observed on the ID of the tube. The mesa-type corrosion is influenced by the following factors: 101. Environmental: water composition and partial pressure of CO2, temperature and corrosion product deposits2. Metallurgical: steel chemistry and heat treatment3. Hydrodynamics: gas/fluid velocityThe x-ray analysis indicated an iron sulfide scale. The adhesion of iron sulfide to surfaces is weaker than carbonate scale and spalling of scale is caused by high velocity fluids. The corrosion attack of the tubing and rod sections was consistently along one side. This is most likely due to the local turbulence at the effected sections of the tubing or rods. The variation of sucker rod diameters in the rod string contributed to the turbulence. The metallurgy and heat treatment of the steel are important factors. 11 Both tubing and rod section #3 were heat treated with a fast cooling rate, leading to a soft material with a poorly formed pearlite. The lamellae of cementite in the pearlite were not long and thin to provide good corrosion resistance. The cementite lamellae were short shaped lamellae. Some bainite was also present in the microstructure of rod section #3. The observed bainite has cementite lamellae with large spacing, offering weak resistance to acid attack of the ferrite. Rod section #1 had a duplex microstructure of ferrite/pearlite and martensite. The material was hard. The microstructure was obtained by partial austenitizing followed by moderate cooling from a temperature between A1 and A3. During cooling, austenite bands transformed to martensite, leading to heterogeneity of the microstructure and micro-hardness.Rod section #2 had a hard microstructure obtained after moderate cooling from a temperature between A1 and A3 followed by tempering. The cementite lamellae are short and wide due to tempering. The hard microstructure was subject to erosion corrosion of the high velocity fluid. The erosion corrosion led to the removal of a thick layer of material.For this case, it is recommended to use a normalized microstructure, if strength requirements can be accomplished.12 It is also recommended to use steel with less inclusions and specify at least 1% Cr in the composition of both the tubing and sucker rod materials. It is also recommended to “design out” flow disruption in the downhole assembly and apply a corrosion inhibitor program following NACE Standards RP01953 and MR0174. 24. ConclusionThe studied failure cases revealed the importance of the material microstructure on the service life ofsucker rods. Other factors contributing to failure include the fabrication process, the lack of corrosion inhibition, and the design of the string.The most common failure encountered for the sucker rods was corrosion-fatigue. The initiation was caused by pitting of the surface leading to a stress concentration at the pits. The pits initiated primarily at inclusions sites. The propagation of the crack was caused by cyclic operations of the pump. To avoid this problem, a clean steel is recommended. The microstructure of the rod should be nearly free of inclusions. Corrosion protection of the surface with application of a suitable chemical program and/or coating of the surface with a corrosion resistant material is also recommended.In an aggressive environment, the failure mode was localized corrosive attack caused by turbulence anderosion cor