Use of the small punch test under static load in hydrogen embrittlement situations. Analysis of the test curve

En este trabajo se analiza un acero de alta resistencia bajo escenarios de fragilización por hidrógeno mediante los últimos avances en el empleo de las técnicas Small Punch en escenarios frágiles. En una primera etapa se fundamenta porqué el ensayo Small Punch (SPT) puede ser una alternativa simple y económica cuando el empleo de ensayos normalizados en ambiente no es viable. A continuación se llevan a cabo ensayos SPT sumergidos bajo carga estática sobre probetas previamente cargadas en hidrógeno. Con ello se comprueba que esta tipología de ensayos SPT da como resultado curvas de que comparten similitudes con las del Small Punch Creep Test, la cual es analizada en profundidad. A partir de ello se lleva a cabo una análisis de la evolución de la probeta a lo largo del ensayo, y se propone un posible camino de trabajo futuro para la determinación de la velocidad de ensayo SPT idónea en condiciones de fragilización por hidrógeno.In this work, a high strength steel is analyzed under hydrogen embrittlement scenarios, employing the latest advances in the use of Small Punch techniques in brittle scenarios. In a first stage it is analyzed why the Small Punch (SPT) test can be a simple and economical alternative method when the use of normalized tests in environment is not viable. Subsequently, SPT tests carried out under static load using pre-charged specimens in hydrogen. It is shown that this type of SPT test give as a result curves that share similarities with those of the Small Punch Creep Test, which is analyzed in depth. From this, an analysis of the evolution of the specimen is carried out throughout the test, and a possible way for the determination of the suitable SPT punch rate in hydrogen embrittlement scenarios is proposed as a future work.The authors of this paper would like to thank the Spanish Ministry of Economy and Competitivity for the support received for the development of the research projects MAT2011-28796, and MAT2014-58738

Analysis of Samples Cleaning Methods Prior to Hydrogen Content Determination in Steel

ABSTRACT: There are multiple references to sample cleaning methods prior to hydrogen content determination, or hydrogen spectroscopy analysis, but there is still no unified criteria; different authors use their own "know-how" to perform this task. The aim of this paper is to solve, or at least clarify, this issue. In this work, the most commonly used sample cleaning methods are compared. Then, five different methodologies are applied on certified hydrogen content calibration pins and on high strength steel concrete-prestressing strands and the three main situations regarding hydrogen content in the microstructural net (non-charged, charged, and charged and uncharged) are studied. It was concluded that the HCl solution C-3.5 cleaning method recommended by ASTM G1 introduces large amounts of hydrogen in the samples; but can be useful for eliminating superficial oxides if necessary. The rest of the methods had similar results; but the more complete ones that involve ultrasounds and last longer than 8 min are not appropriated when important diffusion may occur on the samples during their application. Simple methods that involve acetone or trichloroethylene and last around 1 min are preferable for almost all situations as these are faster, easier, and cheaper. As a final recommendation, as trichloroethylene is toxic, the simple acetone method is, in general, the most convenient one for regular hydrogen content analysis.This research was funded by the UNIVERSITY OF CANTABRIA by means of the “Proyectos Puente 2019” grant program. The APC was funded by papers reviews of the authors for MDPI

Influence of the notch length on the estimation of the reference temperature by means of the small punch test

The Master Curve approach has been widely applied to characterize the ductile to brittle transition region of ferritic steels. In order to further optimise the available material to be tested, great efforts have been recently performed to combine it with miniature testing techniques. One of the most promising is the small punch testing technique, currently under standardisation process in Europe. In this paper, small punch modified specimens with a lateral notch for the estimation of fracture toughness have been employed to obtain the reference temperature, T0, of a pressure vessel steel. The influence of the applied notch length has been analysed and a valid range has been proposed. In addition, the validity criterion of the tests for the estimation of T0 has been further verified, confirming its suitability. Finally, results have been compared with those obtained with conventional fracture mechanics specimens and previous works. As a result, a methodology to estimate the reference temperature by means of small punch tests with notch lengths of approximately 4.4 mm has been proposed, turning it into a promising candidate for the characterisation of the transition regime.This research was supported by the project MAT2014-58738-C3-3-R “Efecto del hidrógeno en aceros de media y alta Resistencia: optimización de los métodos de caracterización para la evaluación de la integridad estructural” financed by the Ministry of Economy, Industry and Competitiveness of the Spanish Government

Analysis of unexpected leaks in AISI 316L stainless steel pipes used for water conduction in a port area

This paper clarifies the causes of a corrosion process observed in austenitic stainless-steel pipes, grade 316L, used for conducting freshwater in a port area. During the pressure test of the installation, before it was put into service, about five months after its construction, a loss of pressure was detected due to leaks of the fluid contained and the presence of corrosion damage on the wall of the tubes, in some cases even passing through the thickness of the tube. An analysis of the chemical composition of the pipe material was carried out, as well as semi-quantitative analysis of the chemical composition of the deposits in the defects, and a culture of sulfate-reducing bacteria (SRB) in Kliguer medium of the stagnant waters within the facility. All this makes it possible to conclude that the observed process fits within the so-called microbiologically induced corrosion (MIC), and, in all probability, it can be affirmed that this process is promoted by the presence and proliferation of sulfate-reducing bacteria (SRB)

Application of the incremental step loading technique to Small Punch Tests on S420 steel in acid environments

ABSTRACT: The Small Punch test has been recently used to estimate mechanical properties of steels in aggressive environments. This technique, very interesting when there is shortage of material, consists in using a small plane specimen and punch it until it fails. The type of tests normally used are under a constant load in an aggressive environment, with the target to determine the threshold stress. However, this is an inaccurate technique which takes time, as the tests are quite slow. In this paper, the Small Punch tests are combined with the step loading technique collected in the standard ASTM F1624 [1] to obtain the value of threshold stress of an S420 steel in a total time of approximately one week. The ASTM F1624 indicates how to apply constant load steps in hydrogen embrittlement environments, increasing them subsequently and adapting their duration until the specimen fails. The environment is created by means of cathodic polarization of cylindrical tensile specimens in an acid electrolyte. A batch of standard tests are performed to validate the methodology.The present work was performed in the University of Cantabria, through the post-doctoral contracts program (budgetary application 62.0000.64251), and supported by the Government of Cantabria and the University of Cantabria in the framework of “Proyectos Puente 2019” call

Análisis de fallo de pernos galvanizados de alta resistencia utilizados en torres metálicas

Este trabajo analiza el fallo de tres tornillos utilizados en las conexiones estructurales de una serie de torres de acero situadas en el norte de Europa. El análisis comprende microscopía óptica y electrónica de barrido, análisis microestructural y de dureza, ensayos mecánicos y evaluación de integridad estructural. Los tres pernos presentaron procesos de fallo muy similares, con una grieta externa circunferencial que condujo al fallo final. La morfología de la propagación de grieta es típica de la corrosión bajo tensión inducida por hidrógeno (HISCC), con micromecanismos intergranulares-transgranulares mixtos, procesos de desgarro y agrietamiento secundario. Las grietas crecieron subcríticamente hasta que alcanzaron su tamaño crítico. Los procesos de agrietamiento por temple o fatiga han sido descartados.This paper analyses the failure of three bolts used in the structural connections of a number of steel towers located in northern Europe. The analysis comprises optical and scanning electron microscopy, microstructural and hardness analysis, mechanical testing and structural integrity assessments.The three bolts presesent very similar failure processes, with a circumferential external crack that led to the final failure. The morphology of the crack propagation is typical of Hydrogen Induced Stress Corrosion Cracking (HISCC), with mixed intergranular-transgranular micromechanisms, tearing processes and secondary cracking. The cracks then grew subcritically until they reached their critical size. Quench cracking or fatigue processes have been ruled out

Caracterización de la fragilización por hidrógeno mediante la aplicación de la técnica de escalones incrementales al ensayo small punch

En este trabajo se aplica al ensayo Small Punch, la técnica de carga por escalones para medir el umbral de fragilización en aceros definida en la noma ASTM Fl624. Para ello se utiliza un acero de media resistencia, simulando el ambiente fragilizante mediante una polarización catódica a través de la aplicación de una corriente de 5mA/cm2 en una solución ácida principalmente 1 N de H2SÜ4 en H2O. En una tarea paralela se llevaron a cabo ensayos normalizados sobre probetas de tracción según la norma ASTM Fl624, observándose las mismas tendencias en ambos casos, lo que valida la metodología propuesta. Para adaptar la citada norma al ensayo Small Punch se modificó la duración de los escalones de carga, proponiendo su reducción en base a las dimensiones de las probetas Small Punch, así como la velocidad de punzonado que tiene lugar en este tipo de escenarios ambientales según la bibliografía. Esta propuesta permite obtener un umbral de carga mediante el empleo de, al menos, 3 probetas en un tiempo total de una semana aproximadamente.Los autores desean agradecer la financiación prestada al Ministerio de Economía y Competitividad a través del proyecto de investigación MAT2014-58738-C3-3-R

Evaluación de las propiedades de tracción mediante ensayos Small Punch en la zona afectada por los cortes térmicos

Los sistemas de corte térmico provocan una alteración de la zona de material anexa a los bordes de corte, pudiendo así afectar a su comportamiento en servicio. Este artículo pretende aplicar la técnica Small Punch para comparar las propiedades a tracción en el material base y en la Zona Afectada Térmicamente (ZAT) generada por los distintos procesos de corte. El material seleccionado es un acero estructural S460M y se consideran tres métodos de corte térmico con gran trascendencia en ingeniería (oxicorte, corte por plasma y corte por láser). En los bordes de corte resultantes tras cada uno de estos tres procedimientos se ha obtenido la microestructura, se han realizado perfiles de dureza y se han determinado las propiedades de tracción mediante probetas Small Punch. Se ha observado una modificación significativa de las propiedades en la ZAT respecto de las obtenidas en el material base y una variación de las mismas en función de la técnica de corte seleccionada.Thermal cutting processes cause variations in the material next to cutting edges, which may affect its subsequent inservice behaviour. The aim of this paper is to apply the Small Punch technique in order to compare the tensile properties of the base material and the Heat Affected Zone (HAZ) generated by different cutting processes. S460M structural steel has been chosen for this purpose, analysing three different thermal cutting methods of great influence in engineering (oxy-fuel, plasma and laser cutting). Microstructure, hardness profiles and tensile properties by means of Small Punch specimens have been obtained at the resulting cutting edges after the employment of each of these three processes. As a result, a significant deviation of the HAZ properties from the ones obtained for the base material has been observed, as well as an influence of the applied cutting method on the alteration of these properties

Revisión de metodologías para la determinación de la tensión umbral en ambientes agresivos mediante ensayos Small Punch

RESUMEN: El ensayo Small Punch es una técnica reciente para estimar propiedades a tracción, fluencia y fractura de materiales metálicos en ambientes agresivos, y es considerada como una de las más adecuadas para evaluar propiedades mecánicas cuando hay escasez de material. Las primeras experiencias se llevaron a cabo pre-fragilizando las muestras en ambiente y ensayándolas posteriormente al aire. Metodologías posteriores propusieron llevar a cabo los ensayos en el mismo medio agresivo en que se habían pre-fragilizado las probetas para no perder parte de su capacidad fragilizante durante el ensayo; en estos escenarios la velocidad de solicitación tiene una enorme influencia. También otros trabajos han explorado la utilización de cargas constantes gradualmente aumentadas hasta provocar el fallo de la probeta para la determinación del umbral. Finalmente, los trabajos más recientes proponen aplicar al ensayo Small Punch la técnica de carga escalonada recogida en ASTM F1624. En este trabajo, se presentan las guías de aplicación de los ensayos Small Punch para determinar la carga umbral en ambiente, basándose en los resultados en ambientes con fragilización por hidrógeno. Se analiza el rango de velocidades de punzonado a emplear, así como los tiempos de escalones a emplear y una correlación para estimar la tensión umbral obtenida exclusivamente con ensayos Small Punch cuando se aplica la metodología de carga por escalones

Analysis of the environmental degradation effects on the cables of La Arena bridge (Spain)

After nearly 25 years of service, some of the wires of the tendons of ?La Arena? bridge (Spain) started to exhibit the effects of environmental degradation processes. ?La Arena? is cable-stayed bridge with 6 towers and a reference span between towers of about 100 meters. After a maintenance inspection of the bridge, evidences of corrosion were detected in some of the galvanized wires of the cables. A more in-deep analysis of these wires revealed that many of them exhibited loss of section due to the corrosion process. In order to clarify the causes of this degradation event and to suggest some remedial actions, an experimental program was designed. This program consisted of tensile and fatigue tests on some strand samples of the bridge together with a fractographic analysis of the fracture surfaces of the wires, its galvanized layer thickness and some hydrogen measurements (hydrogen embrittlement could be another effect of the environmental degradation process).Once the type and extension of the flaws in the wires was characterized, a structural integrity assessment of the strands was performed with the aim of quantifying the margins until failure and establishing some maintenance recommendations