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- Hydrogen embrittlement
- Mechanical Properties, Short Time Creep, and Fatigue of an Austenitic Steel
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The correct choice of a material in the process of structural design is the most important task. This study deals with determining and analyzing the mechanical properties of the material, and the material resistance to short-time creep and fatigue. The material under consideration in this investigation is austenitic stainless steel X6CrNiTi The results presenting ultimate tensile strength and 0.
Besides, the creep behavior of the steel is presented in the form of creep curves. The analysis shows that the stress level of The impact energy was also determined and the fracture toughness assessed. The structure is usually designed both in accordance with the purpose for the use intended and with favorable material selection.
Service life conditions are determined by the purpose of the structure and accordingly the properties of the material have to comply with these conditions. It is known that material properties are tied in with the material chemical composition, the processing path, and the resulting microstructure.
The properties depending on the microstructure are called structure—sensitive properties, for example, yield strength, hardness, etc. A material selection process for structural design includes issues such as strength, stiffness, weight, etc. The designing process has to include an efficient selection of materials and it is conducted under the assumption that the engineering material does not contain any failures. Furthermore, the proper use of the structure ensures that no failure will occur in it during its service life [ 3 ].
Any engineering structure or structural element needs to be shaped. This means that the material is subjected to processes called manufacture that include forming i. In all the applied processes mentioned, i. In engineering practice, many failures can occur and any failure has a cause of its origin and the mode of its representation.
The analysis of failure provides an answer to how and why an engineering component has failed, and in this sense such a discipline becomes a very powerful tool in modern structural design [ 5 ]. Common causes of failures are usually mentioned such as pre-existing defects for example pre-existing cracks , defects initiating from imperfections, but also categories such as design errors, misuse, inadequate maintenance, assembly error, etc.
In engineering practice a lot of failure modes have been observed, for example, force induced elastic deformation, creep, yielding, buckling, fatigue, fracture, corrosion, thermal shock, etc.
Creep is mentioned as one of the possible failure modes and in this investigation the short-time creep resistance of the considered material will be examined.
Creep as a thermally activated phenomenon is usually defined as time dependent behavior of the material where at constant stress load the strain continuously increases [ 7 ]. Its occurrence is appreciable at temperatures above 0.
This behavior of the metallic material is usually displayed in the form of a curve consisting of three stages I-transient creep, II-steady—state creep, III-accelerating creep. Material properties, the features of its machinability as well as numerical analysis of the structure made of the considered material belong to the most important information about both material and structure. In this sense, it is also recommended to get an insight into some investigations not only made by the authors of this work but also by other authors [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ].
The possibility of the use of the finite element method in the shear stress analysis of the structure made of any of the mentioned materials is presented in [ 24 ]. The main task of the research presented in this paper is to determine the properties of the materials so as to provide the relevant information about the structure and behavior of the material under certain conditions of exploitation.
In addition, a brief overview of recently published papers relating to steel X6CrNiTi is presented. The material damage and the microhardness variations of X6CrNiTi were analyzed in order to determine the incubation period, performing the tests at different cavitation conditions in a cavitation chamber [ 25 ]. Since the low-cycle fatigue damage evolution in the metastable austenitic steel causes a deformation induced phase transformation from austenite to martensite, the microstructure changes in the pre-crack stage were investigated [ 26 ].
In Ref. Furthermore, the phase formation and annealing behavior are described after nitrogen PIII Plasma immersion ion implantation in two austenitic stainless steels, 1. An optimizing deep drilling parameter based on the Taguchi method for minimizing surface roughness was considered in Ref. The microstructural character of dissimilar welds between Incoloy H and Stainless Steel was discussed in Ref.
The material considered in this research is X6CrNiTi steel, delivered as 18 mm soft annealed round bar. The added titanium content helps in preventing chromium carbide precipitation when exposed to high temperatures, which may occur on account of wrong applications or may be generated from the welding process.
Its maximum service temperature can be significantly reduced in a hot atmosphere due to corrosive compounds such as water and sulfur compounds. However, the grade possesses good creep strength. This steel is known as heat resistant steel and austenitic corrosion resistant steel. The basic properties of steel depend on the chemical composition, resulting microstructure, the state, form, and dimensions of the finished product.
This means that processing as a means to development and control of the microstructure as well as heat treatment are of significance for achieving the required material properties. The properties for example: yield strength, hardness that depend on the microstructure are called structure-sensitive properties. The general properties of this material are: good corrosion resistance, medium values of mechanical properties, average forgeability, poor machinability, excellent weldability TIG, MAG, Arc, laser beam, and submerged arc welding, except gas welding.
Core X6CrNiTi steel is titanium-stabilized steel with improved intergranular corrosion resistance and may be used in an extended temperature range. With respect to well-balanced material properties, it is suitable for many applications and can be used at elevated temperatures. As regards processing, it can be said that it is suitable for machining but not automated , hammer and die forging, cold forming, but it is not suitable for polishing, etc.
However, due to formation of titanium carbo-nitrides, its machinability differs from other low carbon stainless steels with titanium free variants. Its main applications are in: the automotive industry, the chemical industry chemical processing equipment , building and construction industries, food and beverage industries, and aviation and aerospace industries used for aircraft parts, such as exhaust systems , jet engine parts, weld equipment, power station constructions, furnace heat treated parts, oil refiners, as well as general use in mechanical engineering.
Equipment used in these investigations can be divided into equipment used to perform uniaxial tests at room and high temperatures and equipment used to determine impact energy. Specimens used in uniaxial tests to determine engineering stress-strain diagrams and in creep testing were machined from 18 mm steel rod.
The geometry of each specimen used in the uniaxial test room and high temperature was defined according to that specified in ASTM: E 8Ma, and its ends were threaded to match the holders of the testing machine, Figure 1.
Test procedure and standards in accordance with which uniaxial tests were carried out are as follows. Tensile tests related to determination of engineering stress-strain diagrams at room temperature were performed according to ASTM: E 8Ma, while those related to high temperatures were performed in accordance with ASTM: E standard [ 35 ].
Charpy tests were carried out according to ASTM: Ec standard and specimens used in these tests were also manufactured in accordance with the same standards. All of the mentioned standards can also be found in the Annual Book, Ref. Engineering stress-strain diagrams were obtained by performing uniaxial tests at room and high temperatures.
A minimum of five tests was performed for each test temperature. Since engineering stress-strain diagrams carried out at the considered temperature do not differ from each other at all, or a very little, only one diagram is shown for each of the considered temperatures the first conducted test , Figure 2. Engineering stress-strain diagrams at room and high temperatures: X6CrNiTi steel. The modulus of elasticity continuously decreases with an increase in temperature.
As a consequence of dynamic strain aging, which is treated as hardening phenomenon, in the stress-strain curves some effects can arise. Serrations in the stress-strain curves are the most visible effects of dynamic strain aging. When this effect is not seen other effects can be present.
Usually, if serrations are not seen, dynamic strain aging can be marked by lower strain rate sensitivity. Also, dynamic strain aging causes a minimum variation of ductility with temperature, a plateau in strength as well as a peak in work hardening. Experimentally determined mechanical properties such as ultimate tensile strength, yield strength 0. Approximation curves related to the same properties are presented using solid or dashed lines.
These approximation curves relating to the mentioned properties describe their experimentally obtained values with greater or lower accuracy. To determine the accuracy of approximation, the coefficient of determination R 2 is used as a measure of accordance between experimentally obtained results and polynomial approximation and serves as a statistic that gives information on the fit of a model [ 36 ].
Dependence of mechanical properties on temperature: X6CrNiTi steel. The structure operating under certain environmental conditions is to be subjected to certain loads. It is also necessary to choose a material whose properties will meet the structure service life requirements, i.
Besides, very important aspects, e. In accordance with this, several uniaxial tensile tests regarding determination of ultimate tensile strength were performed. In the so called descriptive error bars, Figure 4 , an analysis of ultimate tensile strength is shown where range R and standard deviation SD are used. The standard deviation was calculated as given in Ref. To assess the short-time creep behavior of the considered material or to predict its creep resistance, several uniaxial short-time creep tests were carried out.
Data defining creep tests as well as material creep responses are shown in Figure 5 , Figure 6 , Figure 7 and Figure 8. In these investigations short-time creep behavior was considered since most materials can be subjected to such temperature conditions occurrence of high temperature due to error in cooling, hazard, fire, etc.
Only some of the special materials are intended to be used in structures designed for long term operation at high temperatures. In long term creep processes, however, special equipment for creep process monitoring is to be used.
In these tests, stress levels are selected in accordance with the 0. To perform a creep test, appropriate but usually expensive equipment is indispensable.
Although the creep test shows deformation behavior of the material realistically, sometimes it is possible, based on known data of the behavior from similar conditions, to predict the creep behavior for the prescribed conditions.
In the following part there are two rheological models and one analytical method formula proposed to be used in creep modeling.
All of the proposed tools can be used for modeling the first and second creep stages. The Burgers model is represented in Refs. All of the mentioned Equations 3 — 5 can be used for three different types of modelling, namely, for:.
The first type of modeling, Equation 6a , denotes the modeling of one exactly defined creep curve that is described by the defined creep temperature and the defined stress level at this temperature. The last type of modeling, Equation 6c , is the most useful and applicable one. Namely, this modeling covers an entire range of stress levels and temperature levels for the considered time range. In Table 2 , data relating to creep modelling are presented, while creep modelling curves are presented in Figure 9.
Experimental and modeled creep curves: steel X6CrNiTi Modeling was performed using Equation 5 by applying the principle 6c. In general, all of the above mentioned models for simulating creep behavior are considered to be satisfactory.
However, an analytical formula is proposed as the best model. When rheological models are considered, then the Burgers model seems to be more suitable for the creep processes where the dominated creep phase is the steady-state phase, while for creep process with dominated transient phase more expressed parabolic shape , the SLS model is considered to be more suitable.
The correct choice of a material in the process of structural design is the most important task. This study deals with determining and analyzing the mechanical properties of the material, and the material resistance to short-time creep and fatigue. The material under consideration in this investigation is austenitic stainless steel X6CrNiTi The results presenting ultimate tensile strength and 0. Besides, the creep behavior of the steel is presented in the form of creep curves. The analysis shows that the stress level of
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Temperature Effects Mechanical characteristics of most materials are greatly influenced by the operating temperature. Stress-strain diagrams are obtained at specific temperatures. High temperature reduces material stiffness and strength, while low temperature increases material stiffness and strength. Almost all materials creep over time if exposed to elevated temperatures under applied load.
ASTM's steel standards are instrumental in classifying, evaluating, and specifying the material, chemical, mechanical, and metallurgical properties of the different types of steels, which are primarily used in the production of mechanical components, industrial parts, and construction elements, as well as other accessories related to them. The steels can be of the carbon, structural, stainless, ferritic, austenitic, and alloy types. These steel standards are helpful in guiding metallurgical laboratories and refineries, product manufacturers, and other end-users of steel and its variants in their proper processing and application procedures to ensure quality towards safe use.
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Mechanical Properties, Short Time Creep, and Fatigue of an Austenitic Steel
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Hydrogen embrittlement HE also known as hydrogen assisted cracking or hydrogen-induced cracking , describes the embrittlement of a metal by diffusible hydrogen. The essential facts about the nature of the hydrogen embrittlement of steels have now been known for years. In steels, diffusible hydrogen ions come from water that is typically introduced by a wet electrochemical process such as electroplating. Diffusible hydrogen can be introduced during manufacture from operations such as forming , coating, plating or cleaning. The most common causes of failure in practice are poorly-controlled electroplating or bad welding practice with damp welding rods. Both of these introduce hydrogen ions which dissolve in the metal.
Quenching and tempering treatments were applied to samples removed from different bars of commercial SAE steel with different P content. Therefore, the heat treatments cycles were applied to specimens containing low 0. The Charpy tests results showed that the phosphorus content analyzed in this work has caused embrittlement, even in the bars with the lowest P content, leading to intergranular fracture. However, if the nucleation life is taken into consideration, this embrittlement has no effect on the nucleation fatigue life of the component. Keywords: steel, phosphorus, austenitizing, tempering. It was observed that in the SAE steel steels used to make springs, the P content may ranges from 0.
Fatigue and Durability of Metals at High Temperatures (#G) This publication is being made available in PDF format as a benefit to members and.
High temperature degradation in power plants and refineries. Thermal power plants and refineries around the world share many of the same problems, namely aging equipment, high costs of replacement, and the need to produce more efficiently while being increasingly concerned with issues of safety and reliability. For equipment operating at high temperature, there are many different mechanisms of degradation, some of which interact, and the rate of accumulation of damage is not simple to predict. The paper discusses the mechanisms of degradation at high temperature and methods of assessment of such damage and of the remaining safe life for operation. Keywords: degradation mechanisms, high temperature, life assessment, power plants, refineries. Thermal power plants and refineries around the world are aging and need to be assessed to ensure continued safe operation. Replacement is frequently not an option because of high capital costs, and the much lower cost of continuing the operation of the older plant.
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