Author: Quentin Scott Allen
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 60
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Book Description
Advanced high strength steels (AHSS) have high susceptibility to hydrogen embrittlement, and are often exposed to hydrogen environments in processing. In order to study the embrittlement and recovery of steel, tensile tests were conducted on two different types of AHSS over time after hydrogen charging. Concentration measurements and hydrogen microprinting were carried out at the same time steps to visualize the hydrogen behavior during recovery. The diffusible hydrogen concentration was found to decay exponentially, and equations were found for the two types of steel. Hydrogen concentration decay rates were calculated to be -0.355 /hr in TBF steel, and -0.225 /hr in DP. Hydrogen concentration thresholds for embrittlement were found to be 1.04 mL/100 g for TBF steel, and 0.87 mL/100g for DP steel. TBF steel is predicted to recover from embrittlement within 4.1 hours, compared to 7.2 hours in DP steel. A two-factor method of evaluating recovery from embrittlement, requiring hydrogen concentration threshold and decay rate, is explained for use in predicting recovery after exposure to hydrogen. Anisotropic hydrogen diffusion rates were also observed on the surface of both steels for a short time after charging, as hydrogen left the surface through 001 and 101 grains faster than grains with 111 orientations. This could be explained by differences in surface energies between the different orientations.
Author: A. R. Elsea
Publisher:
ISBN:
Category : Steel
Languages : en
Pages : 166
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Author:
Publisher:
ISBN: 9789279458200
Category :
Languages : en
Pages : 159
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Book Description
Modern advanced and ultra-high strength steel grades are subject to issues of hydrogen embrittlement (HE), particularly for strength levels above 1000 MPa. The present project has been designed in order to investigate microstructural features and to obtain insights into causes and mechanisms of HE in such steels. Therefore, three material classes with strength levels of 1000, 1200 and 1400 MPa and differing microstructures have been generated through laboratory heat treatments from industrial cast and rolled steels. The samples have been charged with hydrogen and systematic mechanical testing has been performed, thus enabling a comparison of the various microstructures in terms of susceptibility to hydrogen. Accurate microstructure investigations of the uncharged and charged samples, both before and after the mechanical testing have been used as indicators for assessing the embrittlement behaviour. Particularly relevant features include the amount of martensite, non-metallic inclusions, carbides and nitrides within the microstructure. The investigations revealed that there is no direct correlation between the hydrogen uptake and the sensitivity to HE, but significant differences between the embrittlement in tempered and untempered samples. Extensive computer modelling has been performed to support the experimental work and has shed light on some specific mechanisms for H. Examples are indications of a substantial solution of H in the retained austenite of the investigated samples as well as suggesting that there are indications that hydrogen-induced fracture may occur within inclusions rather than at the boundaries to the matrix.
Author:
Publisher:
ISBN: 9789279248344
Category :
Languages : en
Pages : 149
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Book Description
Author: Tuhin Das
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
"Hydrogen Embrittlement (HE) is a serious engineering problem for a wide range of industries starting from fastener, oil & gas to aerospace and nuclear. After several decades of extensive research, the HE problem has not been mitigated to a satisfactory level. HE in general, comprises of numerous layers of complexities involving hydrogen metal interactions, hydrogen diffusion and fracture, where each of these phenomena are entangled with one another to a certain degree. High strength martensitic steels which are known for structural and critical engineering applications suffer premature failure due to HE, to a great extent. The interactions of the hydrogen with the complex microstructure of these martensitic steels further enhances the challenges of mitigation. Therefore, the aim of the current research is to develop a better understanding on the susceptibility and mechanism(s) of HE failure of these materials, by studying the key factors affecting their embrittlement. In order to carry out the investigation, a combined approach based on experiments and numerical modeling has been adopted. As a first step, the HE susceptibility of high strength martensitic steels was evaluated using conventional slow strain rate testing methodology, in bending. A stress coupled hydrogen diffusion finite element analysis (FEA) model integrated with a cohesive zone model was developed to simulate the HE test. The primary factors influencing material susceptibility to HE were studied using the model, and the evaluation of critical hydrogen concentration as a metric of material susceptibility was demonstrated. Following the first study, a new approach involving rapid fracture test in four-point bending was proposed to assess HE susceptibility and mechanism(s). Stress coupled hydrogen diffusion FEA was also performed to calculate both stress and hydrogen concentration distributions in the domain, while simulating the test. A mechanistic description rooted in hydrogen enhanced decohesion (HEDE) mechanism was used to corroborate the mechanical test results, and fundamental understanding on the role of strength, microstructure and plasticity influencing HE susceptibility of materials, was also developed.The difference in susceptibility obtained from the rapid HE test, for two different quench and tempered martensite steels with similar strength level and microstructural features were explained using advanced microstructural characterization techniques, FEA and nanoindentation. The role of local microstructure affecting the micromechanics of HE fracture was discussed.Finally, hydrogen diffusion along the interface boundaries in a typical martensitic microstructure was investigated using centroidal Voronoi based FEA model. The influences of packet boundaries and prior austenite grain boundaries on the output hydrogen flux and concentration were studied. The presence of retained austenite in the microstructure affecting the diffusion of hydrogen was also studied. An overall understanding on hydrogen diffusion characteristics in a martensitic microstructure was demonstrated for better prediction of HE fracture.Thus, the current research provides fundamental understanding on the HE susceptibility of martensitic steels, as well as mechanistic insights, that could be instrumental in tackling the HE problem"--
Author: Andrew W. DeVillier
Publisher:
ISBN:
Category : Materials science
Languages : en
Pages : 286
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Book Description
High-strength, high-toughness steels are used in critical components of Naval aviation systems. These critical applications include landing gear components in tactical aircraft and rotary components in helicopters where large stresses must be sustained and the probability of failure due to flaws in the part are kept minimal. To prevent corrosion and wear, high strength steels are plated. When parts undergo repair/rework, the plating is stripped in an acidic bath, and the part is re-plated. During stripping and plating, as well as in-service duty, the steel is exposed to hydrogen-rich environments, which have been identified as a potential cause of premature failures in aviation system components. A high strength, precipitation hardened steel, PH 13-8Mo, one of six candidate steels, was examined in three different conditions - solution annealed (SA), H1000 (538°C aged), and H1150 (621°C aged) in order to compare the susceptibility of the different microstractures to hydrogen embrittlement and determine the stress-life curve for PH 13-8 Mo in the H1000 condition. The PH 13-8 Mo was selected as a resuh of a recent inservice failure of a reworked component. Fatigue pre-cracked compaction-tension (CT) specimen were loaded in displacement control and seated in a 10% strength hydrochloric acid bath. The load and crack mouth opening displacement (CMOD) were monitored as a function of time. From the load and CMOD versus time behavior, failure criterion were developed to determine points of crack initiation. Fractured samples were examined via scanning electron microscopy (SEM). Polished samples of the H1000, SA and H1150 condition were also analyzed by electron backscatter diffraction (EBSD). EBSD analysis showed that the packets of reverted austenite that grew in the H1150 condition material possessed an orientation that tended towards [111], which may indicate a Kurdjumov-Sachs or Nishiyama-Wasserman orientation relationship. In addition, packets of austenite within a local region also shared a common orientation with one another. An explanation for this behavior is proposed. The hydrogen embrittlement results show that the H1000 condition is highly susceptible to crack initiation over a wide range of applied stress values. A comparison of the different conditions showed that the H1150 and SA conditions possessed similar equal or better resistance to crack initiation in the hydrogen-rich environment at equivalent values of applied stress intensity (K) than the H1000 condition. Comparing equal percentages of applied (apparent) fracture toughness (KsubscriptQsubscript or KsubscriptICsubscript), only the SA condition had better resistance to initiation than the H1000 condition. Resistance to crack propagation for the H1150 and SA conditions was superior to that of the H1000 condition. Finally, the threshold stress intensity, Ksubscriptthsubscript, below which no cracking occurs for the H1000 condition, is on the order of 10% KsubscriptICsubscript or less, which is in good agreement with what has been established in literature.
Author: Metals Technical Committee
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
This standard describes a test method for evaluating the susceptibility of uncoated cold rolled and hot rolled Ultra High Strength Steels (UHSS) to hydrogen embrittlement. The thickness range of materials that can be evaluated is limited by the ability to bend and strain the material to the specified stress level in this specification.Hydrogen embrittlement can occur with any steel with a tensile strength greater than or equal to 980 MPa. Some steel microstructures, especially those with retained austenite, may be susceptible at lower tensile strengths under certain conditions. The presence of available hydrogen, combined with high stress levels in a part manufactured from high strength steel, are necessary precursors for hydrogen embrittlement.Due to the specific conditions that need to be present for hydrogen embrittlement to occur, cracking in this test does not indicate that parts made from that material would crack in an automotive environment. Results from this test should be considered in conjunction with the strain state of the material and the operating environment of the part when selecting any UHSS. Since this test method is comparative, the most information can be gained if a control sample of known performance is evaluated along with the material being studied. Aggressive mass reduction targets for ground vehicles have led to the development of a new generation of high strength steels. Very high tensile strength as well as the presence of retained austenite as a major microstructural constituent make these steels susceptible to hydrogen embrittlement.
Author: Tuhin Das
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
"Hydrogen embrittlement (HE) is identified as a serious problem even after decades of extensive research, in various industries starting from aerospace to fastener. HE is primarily responsible for the degradation of mechanical performances of a number of metals and alloys that are used nowadays. Among them, high strength steels owing to their widespread applications also suffer from HE failures to a great deal. Hence, this thesis focuses on fundamental issues influencing the susceptibility of three different grades of high strength steels designed for fastener applications.The present work includes investigation of mechanical properties both in presence and absence of hydrogen, microstructural characterisations and direct hydrogen quantification. The mechanical property test methodology is a modification of the ASTM F1624 standard test method to determine hydrogen embrittlement threshold in steels, which facilitates a unique approach to investigate the HE susceptibility of these steel grades. The microstructural characterisation process involves scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) to identify the potential hydrogen trap states in microstructure, both qualitatively and quantitatively. Microstructural characterisation also provides useful information to predict local microstructural evolution in the presence of hydrogen and stress. Altogether, a structure-property relationship has been developed for these materials employing a comprehensive investigation process. In addition to it, fracture surface mapping using SEM indicates the mechanisms of failure associated with HE process in these materials. And finally, direct hydrogen quantification methodology using thermal desorption spectroscopy (TDS) provides further experimental evidence to the above-mentioned facts. Thus, in this work a combined approach involving various research techniques and knowledge has been utilised to investigate the susceptibility of the steel grades and rank them according to their performances under the influence of hydrogen." --
Author: Joseph A. Ronevich
Publisher:
ISBN:
Category : Steel, High strength
Languages : en
Pages : 130
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Book Description
Author: Mahmoud Y. Demeri
Publisher: ASM International
ISBN: 1627080058
Category : Business & Economics
Languages : en
Pages : 312
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Book Description
Examines the types, microstructures and attributes of AHSSAlso reviews the current and future applications, the benefits, trends and environmental and sustainability issues.
