Carlos Eduardo Fortis Kwietniewski

Mechanical and Metallurgical Evaluation of Carburized, Conventionally and Intensively Quenched Steels

Steels subjected to carburizing, quenching, and tempering are widely used for components that require hardness and superficial mechanical resistance together with good core toughness. Intensive quenching is a method that includes advantages including crack prevention, increased mechanical resistance, and improvement in fatigue performance when subjected to very fast (intensive) cooling. However, achieving these advantages requires the formation of sufficiently high surface compressive residual stresses and fine grains at the core of steel components. If the cooling rate is sufficiently high after intensive quenching, then low-hardenability, killed plain carbon steels may be used instead of higher-cost, low alloy steels because compressive residual stresses are formed at the surface of steel parts. The objective of this study was to compare between carburized non-killed AISI 1020 steel samples, which were not modified by Al that were subsequently conventionally and also intensively quenched to determine the effect of quenching on achieving the necessary formation of fine grain size. For comparison, carburized AISI 8620 steel test specimens were conventionally quenched. After quenching, all test specimens were characterized by metallurgical and mechanical analyses. The results of this study showed that when the two quenching methods were compared for carburized non-killed AISI 1020 steel, intensive quenching method was found to be superior with respect to mechanical and metallurgical properties. When comparing the different steels, it was found that intensively quenched, non-killed, AISI 1020 steel yielded grain sizes which were three times greater than those obtained with conventionally quenched, carburized AISI 8620 steel. Therefore, the benefits of intensive quenching were negated. These results show that plain carbon steels must be modified by Al to make fine grains if intensively quenched plain-carbon steel is to replace alloyed AISI 8620 steel.

Evaluation of the Notch Influence on the Stress Corrosion Cracking Susceptibility of the API 5L X70 Steel in Ethanol

The increasing interest in renewable and cleaner fuels has stimulated ethanol production in the last decades. The most cost-effective solution to transport large volumes of it are pipelines. Howeve...

Hydrogen Induced Stress Cracking on Superduplex Stainless Steel Under Cathodic Protection

The optimized and secure operation of oil and gas floating production units depends essentially on the performance of their devices, components and structures. Rigid pipelines are key equipment used in the offshore industry commonly employed as flowlines and risers. Carbon steel such as API 5L X65 is the material of choice for those applications due to its low relative cost and availability. However, for the Brazilian pre-salt it seems unlikely that carbon steels can be applyed, since the oil is contaminated by high concentrations of CO2, which causes generalized corrosion. Therefore, operators in Brazil should consider an alternative solution, such as lined or clad pipes as well as corrosion resistant alloys (CRA). Duplex and super duplex stainless steels (SDSS) have emerged in the last decade or so, as an alternative material for harsh environments. Nevertheless, according to recent studies, SDSS when cathodically protected against corrosion are prone to hydrogen induced stress cracking (HISC). The aim of this investigation is to evaluate through fracture toughness measurements the susceptibility of welded SDSS samples to HISC for two different levels of cathodic protection. For fracture toughness evaluation the step loading test method was selected. This practice is believed to be more realistic because samples are exposed to hydrogen during the entire tests instead of simple hydrogen pre-charging before performing the test in air, as recommended by some procedures. Fracture toughness values are given in terms of both CTOD and J-integral for crack initiation and maximum stress for SENB specimens. The results given here indicates that SDSS are quite susceptible to HISC especially in the heat affect zone even for potentials as negative as −650 mVsce.Copyright

The Effect of Nickel on Fracture Toughness at Low Temperature for Hydrogen Pre-Charged Steel Samples

Production of oil and gas in the Brazilian pre-salt will face several technical challenges. One of them that is a major concern is the presence of CO2 in high concentrations. Indeed, since the local regulatory agency requires increasingly stricter recommendations, it seems unlikely the possibility of simply release CO2 in the atmosphere. Besides that, as it happens since the 1970’s in USA, enhanced oil recovery (EOR) using CO2 might be a great opportunity and has to be considered. If that is the case, as soon as CO2 is separated from the oil in the top side, it has to be pressurized and transported through pipelines into the reservoir. The material of choice for that pipeline would be API 5L X65 since it is widely used and most available. The working pressure can reach up to 500 bar or more and it is an important issue to consider the fact that a suddenly depressurization due to a crack in the pipeline or a failure in a fastener or flange can promote a local abrupt decrease in temperature down to −60°C or even lower. That concern in addition to the fact that cathodic protection has to be used in the pipeline for corrosion control has the potential to produce embrittlement due to a combination of low temperature and hydrogen charging and ultimately lead to a catastrophic failure. The use of nickel to improve steel toughness has been used extensively and some grades or classes of nickel containing steels have been created for special applications. The aim of this work is to evaluate whether a nickel containing steel would be a more suitable material to manufacture high pressure CO2 transporting pipelines, taking in account a possible leak and the corresponding effect of a local and abrupt drop in temperature on the fracture toughness of the material. The effect of hydrogen due to cathodic protection on fracture toughness is also evaluated. The results indicated that under the experimental conditions and materials examined here the 9%wt nickel steel is not sensitive to low temperature and hydrogen, practically maintaining its original fracture toughness.Copyright

Evaluation of the Fatigue Life of High-Strength Low-Alloy Steel Girth Welds in Aqueous Saline Environments with Varying Carbon Dioxide Partial Pressures

High-strength low-alloy steel girth weld specimens were subjected to fatigue tests in saline environments saturated with different carbon dioxide partial pressures. As expected, results show that increases in gas concentration initially affect fatigue life adversely, but when higher partial pressures are associated with low stresses, a reduction in the negative impact of environmental conditions is seen. This may be related to a competition between corrosion rates and mechanisms of crack initiation and propagation. Data is presented with the aim of contributing toward the establishment of a database of results in literature which may lead to better understanding of the phenomena involved through association of these with ongoing research.

Evaluation of Stress Corrosion Cracking Susceptibility of the API 5L X70 Steel in Corn and Sugar Cane Ethanol Environments

The interest in renewable and cleaner fuels has stimulated ethanol production in the last decades. Some of the drivers for that ever increasing production were the Brazilian Alcohol Program, Kyoto Protocol and the replacement in USA of the octane booster MBTE (methyl-tert-buthyl ether) for ethanol. The world’s largest producers of ethanol are The United States of America and Brazil, where the main sources are corn and sugar cane, respectively. Production flow via pipeline is the safest and most cost effective way to connect the producers, usually spread across the country, to the distribution terminals. However, in USA there are evidences that ethanol may have caused stress corrosion cracking (SCC) in pipelines and also in storage tanks. Controversially, in Brazil ethanol has been transported and stored since the 1970’s without any indication of SCC. The aim of this work is to evaluate the susceptibility of the steel API 5L X70 [1] to SCC in different ethanol (corn and sugar cane) using slow strain rate testing (SSRT). These tests were carried out on notched specimens according to NACE TM 0111 [2]. The SSRT results carried out in corn ethanol have shown a considerable reduction of plastic elongation and a mixed fracture micromechanism of quasi-cleavage and intergranular facets clearly indicating a susceptibility of the API 5L X70 steel to SCC. The SSRT also demonstrated that the carbon steel tested here is completely immune to SCC in sugar cane ethanol.© 2015 ASME

New Methodology for Flexible Riser End-Fittings Fatigue Assessment Based on Remote S-N Curves

Production of oil and gas in the Brazilian pre-salt will face several technical challenges. One of them that is a major concern is the fatigue life of top risers end-fittings. The new ultra-deep water fields will require a complete new fatigue assessment methodology with higher accuracy. Besides that, the historical data of failure for this sort of equipment shows that the current methodology is not quite optimized for floating units operating in deep water fields. With this, even worst results are expected in ultra-deep water fields.During its assembly inside the end-fitting, the tensile armor wires need to be folded, unfolded and set in position using epoxy resin. This assembly process involves localized plastic deformation and as consequence of this produces a complex residual stress field and also introduces an elastic stress to maintain the wires in position. Both stresses are not actually taken into account in the current fatigue assessment methodology.Therefore, the aim of this work is to develop and evaluate a new fatigue assessment methodology for top risers end-fittings based on remote S-N curves. The proposed methodology takes into account both residual and assembly elastic stresses. The effect of stress concentration on the tensile armor wire at the region with localized plastic deformation is also evaluated. Basically, the objectives of this investigation will be achieved through the construction of remote S-N curves using a test box that contains the deformed wire embedded in resin representing a single wire physical 2D model of an end-fitting, the so called mid-scale testing. For this investigation, a six inches API end-fitting was selected because it is widely used and most available in the market.The results produced here indicated that the localized plastic deformation imposed during the folding and unfolding process has a very important detrimental effect on the flexible risers end-fittings tensile armor wires fatigue life, which makes mandatory a revision of the current four-point bending fatigue assessment methodology.Copyright

Validation Methodology of Crack Growth Measurement Using Potential Drop Method on SENB Specimens

The Brazilian pre-salt oil and gas discoveries brought technical challenges as impressive as the reserves themselves. Besides the concerns with exploration, the oil contamination with CO2, H2S and chloride enriched seawater combined with critical cyclic loads due to the relative movement of the production vessel and high water depths imposes an environment chemically and mechanically aggressive. Suitable materials to work on such harsh conditions are few and one should consider the use of special materials, such as supermartensitic and superduplex stainless steels. Although the corrosion and mechanical properties of these materials are well established, still additional crack growth data in specific environments should be provide to the subsea equipment designer. Indeed, due to the combination of cyclic loading and corrosive ambient the corrosion fatigue phenomenon is a major concern. In order to evaluate the effect of oil contaminants on the corrosion fatigue resistance of candidate materials, one should provide methods for crack growth measurement other than the use of crack gauges since those can not be used in chemically aggressive solutions. The present work aims to validate the potential drop crack growth measurement method comparing the results obtained by this technique with those produced by crack gauges on SEN(B) (Single Edge Notch Bending) specimens in air. This validation effort is essential because the ASTM E647 standard only consider the use of C(T) (Compact Tension) specimens which actually does not represents the real cracks propagation path in crucial subsea equipment, such as risers, drill pipes et cetera, that is through the wall thickness. The results produced by the two tested methodologies have an excellent agreement which makes reliable the use of the potential drop method as an alternative to monitor and measure crack growth in corrosive media.Copyright

Preliminary evaluation of fatigue in carburized, conventionally and intensively quenched steels

Steels treated through carburizing thermochemical treatment and quenching and tempering thermal treatment are broadly used in components that need to have hardness and superficial mechanical resistance together with good toughness in the core of the component. Additionally, it is possible to produce surface compressive residual stresses that normally improve fatigue resistance. Relatively unknown, the intensive quenching thermal treatment is a method that presents some advantages, one of them being the possibility of avoiding cracking by distortion due to extreme cooling. Other advantages are the increase in mechanical resistance, the use of shorter carburizing cycles, improvement of fatigue performance, among others. Once the cooling rate is high, low carbon steels can be used instead of low alloy steel such as AISI 8620. This way, it is possible to use less costly steel and to obtain the advantages of intensive quenching. The present work aims to compare samples carburized during 6 hours at a temperature of 920oC and carbon potential of 0,9% both for AISI 1020 and AISI 8620 samples, through mechanical and metallurgical analyses, being the principal the production of Wohler curves together with fractographic analysis in low magnifying glass and scanning electron microscope. Results pointed out that the AISI 1020 steel presented grain size which is three times bigger than AISI 8620 steel grain size. The effect of optimizing intensive quenching when applied to AISI 1020 steel is practically covered by the fact that AISI 8620 steel presents a more refi ned structure, with smaller grain size comparatively and therefore better mechanical properties. This way, intensive quenching treatment can provide superior performance to non-alloy steels relatively to alloy steels only if grain size is equal or inferior. Key words: carburizing, intensive quenching, fatigue.