View Homework Help – Ensayo De from MATH at Autonomous University of Bucaramanga. http://. por devitrificación controlada (1 hr. a K) del mismo material fueron deformados en tensión mediante ensayos de termofluencia en el rango de tensiones. Los ensayos de termofluencia por tracción, se efectúan a una temperatura de K, y a niveles de tensión correspondientes entre el 25% y 50% del límite de .
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The small ee creep curves show the three stages found in the creep curves of the conventional uniaxial test. The conventional creep relationships which involve parameters such as creep rate, stress, time to rupture and temperature were followed with the corresponding parameters of small punch creep test and they permitted to explain the creep behavior in this steel.
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The mechanism and activation energy of the deformation process were the grain boundary sliding and diffusion, respectively, during creep which caused the intergranular fracture in the tested specimens.
This is an open-access article distributed under the terms terofluencia the Creative Commons Attribution-Non Commercial by-nc Spain 3. Austenitic stainless steels are widely used in industrial applications at high temperatures because of its high corrosion resistance and creep strength. The evaluation of creep characteristics are usually carried out by the conventional uniaxial creep test at selected values of temperature and stresses.
The results of this test are used to determine design parameters such as the rupture life Marshal, However, the application of the conventional test has some disadvantages because of its specimen size, which cannot be extracted from in-service components. Thus, the use of miniaturized mechanical testing has become a necessity to manage the evaluation of properties in materials. The plastic deformation process of the material is characterized by monitoring the tip displacement of the specimen as a function of the applied force.
The specimen size is a considerable advantage, compared to the conventional test, in order to evaluate the mechanical properties of specimens located in the industrial in-service components and even from small areas such as the heat affected ensaoy in the welds. This specimen size causes a minor damage in the industrial components. Moreover, the SP test is a simple testing method with a relatively low cost. The SP testing method can be applied to evaluate mechanical properties in extreme conditions; that is, the SP test can be carried out at cryogenic temperatures, room temperatures and even at high temperatures.
The SP test specimen are under a biaxial—stress state, which could be more suitable than the uniaxial dw of most conventional termofluecia to evaluate the material properties in similar stress states of the actual in-service components.
The evaluation of the microstructural condition of high temperature components is often carried out non-destructively by indirect methods such as metallurgical replication or hardness testing. These procedures may provide useful guidance regarding the damage state, but there is frequently a need for direct measurement of creep and other mechanical properties.
The required testing then needs the removal of material from specific component locations. Nevertheless, the removal of a sample large enough to produce a typical uniaxial test specimen has the drawback that a weld repair must normally be carried out. This repair may adversely affect the structural integrity of the remaining structure.
Therefore, the use of small specimens in ensxyo evaluation of the creep behavior is essential. Several studies with small rnsayo creep test have been carried out to evaluate successfully the termofluebcia properties in different steels. For instance, Parker and James utilized disc specimens to evaluate the creep properties in low alloy ferritic steel. Evans and Wang used numerical methods for analyzing the small punch creep test. Finally, Mathew et al.
The main results of these studies were able to describe the basic creep behavior described by a conventional uniaxial text, as well as to follow the basic creep relations used to characterize the creep behavior. Furthermore, these works have been able to stablish relationships between the creep parameters of small punch creep test and those corresponding to the conventional creep test in order to evaluate more accurately the damage originated under creep conditions in the industrial components.
A austenitic stainless steel was used to study the creep behavior and its chemical composition is as shown in Table 1. The condition of this steel is as-received, solution treated and quenched.
The small punch creep test was carried out using a creep tester shown in Figure 1see for more details Komazaki et al. This consists of a striker bar, alumina puncher, alumina ball of 2. This thickness was selected in order to reduce the testing time. The specimen surface was ground from both sides using abrasive papers up to grade grit. The test load was kept constant by hanging a weight through a lever.
Small punch creep test in a 316 austenitic stainless steel
These testing temperatures were chosen because they correspond to the operation temperatures used in several industrial components Marshal, The load levels at each temperature were chosen in order to reproduce the main characteristics of creep curves.
The deflection of specimen was germofluencia in the central part by measuring the displacement of a quartz rod using a linear variable differential transducer. The tested specimens ensauo fracture surfaces were observed with a SEM at 15 kV.
Precipitates were extracted by electrochemical dissolution of the austenite matrix using a solution of 10 vol.
Schematic illustration of apparatus for the SP creep testing method. The SP creep test ensayk are very similar to those of the conventional uniaxial creep test. Three stages can be clearly observed in curves for all the tested specimens. That is, the primary creep is recognized by the decreasing slope of the creep curve; the secondary-stage creep with a nearly constant creep rate and the accelerating tertiary-stage creep that marks the final fracture at termofluecia time t r.
It is evident from these figures that the SP creep test curves change its shape as a function of the testing load in a similar manner as in the uniaxial creep tests.
Figure 2 a-c shows clearly that the length of the secondary-creep stage and rupture time t r decrease as the testing load increases.
These also decrease with testing temperature for the same testing load. With decreasing loads, the two first stages become clearly defined, usually at the expense of the tertiary stage. Curves of SP creep test for steel tested at: The creep deflection was defined by the following equation: Creep deflection curves versus creep life fraction for steel tested at: In general, all of the SP-C specimens showed dde hemispherical specimen surface and the fracture occurred along the circumference, where the equivalent strain was the largest.
The difference in failure mode between these specimens can be appreciated by observing the specimen cross-section near the fracture surface, as will be shown later. The deformation process of specimens takes place first by the bending of specimen, where a rapid increase in the contact area between the ball and the specimen surface occurs, and then the stretching of the contact area, where the increase in central deflection is predominant.
SEM photographs of small punch creep test specimens after testing at: The failure morphology termofulencia the termofluenxia steel, corresponding to Figure 4 a and bis shown in Figure 5 a termofluenciw brespectively.
These photographs were taken using the cross-section of the SP creep test ruptured specimens. An intergranular failure mode was observed to occur for this specimen with a small decrease in thickness Fig. The failure mode occurred through the grain boundaries of austenite. This specimen shows a small reduction in thickness which suggests a low ductility. This specimen shows a great reduction in thickness which involves more ductility.
SEM photographs of the failure morphology of small punch creep after testing at: The microstructure of the as-received steel is shown in Figure 6awhich shows the presence of the austenite matrix without any trace of precipitation.
It was stated in the previous section that the main characteristics of the conventional uniaxial creep test are reproduced adequately with the small punch creep test Fig.
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That is, the three stages of creep are clearly identified. For instance, Figure 8a shows the plot of small creep rate against time and it can be seen that the creep rate decreases with time, and then reaches its minimum value and finally increases rapidly until the rupture takes place, according to the three stages of creep. The minimum small punch creep rate decreases with the decrease in load. This behavior has been reported Dobes and Milicka, to follow the following power law relationship: In the case of the small punch test of this work, a similar relation was observed to be followed and twrmofluencia can be expressed as follows: Enzayo type of relation for the small punch creep test has been reported to tfrmofluencia in other works Dobes and Milicka, All the above characteristics are commonly observed in the creep curves corresponding to the conventional creep test.
Creep is a thermally activated process and is expected to follow an Arrhenius-type expression with a characteristic activation energy Q for the rate-controlling mechanism. The power law can be written as follows Komazaki et al.
The exponent n can be termofluenciaa from the slope of a straight line resulting in the plot of creep rate against stress. The load exponent n SP-C was determined to be about 4.
Equation 5 has been shown to be followed by the small punch creep test in several works Parker and Tedmofluencia, ; Komazaki et al. In general, a small n value, 3—5, indicates that the creep deformation process is controlled by a grain boundary sliding.
On the contrary, a large n value, 8—12, is present for a transgranular-controlled deformation process Viswanathan, Thus, the n SP-C values, determined in this work, suggest that the SP creep test ruptures are controlled by the grain-boundary sliding.
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Enssyo result shows a good agreement with the failure mode observed in the SP creep tested specimens, Fig. Additionally n values of about 5 have been reported Rieth et al. Plots of the SP creep rate versus: It is well known and accepted that there is a relationship between the creep rate and the termoflencia to rupture, t rknown as the Monkman-Grant relationship Dieter, which states that the minimum creep rate is inversely proportional to the failure life: Figure 9b shows the log-log plot of the small punch creep rate and the time to rupture and a linear relationship can be observed which shows that the Monkman-Grant relationship states for this case and it is as follows: This type of expression has been reported to be followed in the evaluation of creep properties for different materials by the small punch creep tests Hou et al.
The temperature dependence of the small punch creep behavior for testing loads of and N is shown in Figure 9c. The apparent activation energy for the small enswyo deformation. This comparison suggests that the activation energy of this work is close to that corresponding to the grain boundary diffusion. These characteristics are in good agreement with the intergranular failure observed in present work specimens Fig.
Furthermore, termofluuencia can be observed the presence of round or elliptical cavities see for instance some of them indicated by arrows in these figuresformed in the grain boundaries r-type crackswhich are usually associated with grain boundary sliding Marshal, Besides, the failure mode was trangranular in the aged steel and intergranular in the solution treated steel.
An appropriate explanation for this behavior is that the coarse grain boundary precipitation in the aged steel prevents the grain boundary sliding and so inhibits the intergranular fracture. In contrast, the starting of grain boundary precipitation reduces the concentration of carbon or solid solution elements thus minimizing the pinning of dislocations during creep. Additionally, the regions adjacent to grain boundaries may dnsayo denuded in solute, which makes them weaker and susceptible to local plastic deformation in an intergranular way.
Figure 6 b and c shows a very fine precipitation of carbides which seems to be not enough to prevent the grain boundary sliding in the present work. The small punch creep test was applied to characterize the creep behavior in a type austenitic steel and the conclusions are as follows: Morphology and fracture surface of small punch creep specimens 3. Microstructural characterization of small punch creep specimens 4. The small punch creep test enables us to reproduce the main characteristics observed in the creep curve of the conventional uniaxial test, as well as the main relationships between creep parameters.
Determination of the Gurson-Tvergaard damage model parameters for simulating small punch tests.