Teaching how to determine the Charpy impact according to the general requirements defined in part-UG of the ASME VIII code division 1

José L. Meseguer-Valdenebro^a,*, Antonio Portoles^b, Eusebio Martínez-Conesa^a

^aDepartment of Building Technology, Polytechnic University of Cartagena (UPCT), Sq. Cronista Isidoro Valverde, Edif. La Milagrosa, 30202 Cartagena, España

^bDepartment of Applied Physics and Materials Engineering. School of Mechanical Engineering. Technical University of Madrid. St/ José Gutiérrez Abascal 2, 28006 Madrid, España

(*Corresponding author: jlmeseguer507@gmail.com)

1. INTRODUCTIONTOP

Towards the end of the 19th century and the beginning of the 20th, a series of catastrophic boiler failures in the United States led to the first actions aimed at establishing rules for the design and construction of boilers. At that time, there were 350 to 400 boiler explosions per year, in the US alone, with many losses of human life and serious damage to the facilities and plants in which they operated. As a result, the American Society of Mechanical Engineers (ASME) issued its first code in 1914. Fig. 1 shows the structure of the ASME VIII code div. 1.

The parts used in the ASME VIII code div. 1 to determine the Charpy impact are Section A: Part UG; Section B: Part UW and Section C: Parts UCS, UHT and UHA.

It is of great importance that final year students of mechanical engineering degrees have an academic background that deals with the reference design codes used nowadays because these engineers will find them very useful for their professional performance.

There are publications in the field of technical training that cover the ASME IX (Meseguer-Valdenebro et al., 2017) code, the influence of alloy elements on the properties of materials (Meseguer-Valdenebro et al., 2015) and on other engineering issues related to metallurgy and welding (Dugan and Chang, 2010; Bose, 2011; Javier Naranjo and Alejandro Torres, 2015; Meseguer-Valdenebro et al., 2016; Romaní et al., 2017), but there is no academic work in the field of technical training explaining how to determine the Charpy impact according to the ASME VIII code div. 1.

1.1. Background of the Charpy testTOP

The Charpy pendulum is a pendulum designed by Georges Charpy that is used in tests to determine the tenacity of a material. They are impact tests of a test specimen fitted and tested to flex in 3 points. The pendulum hits the back of the test tube and breaks it. The difference between the initial height of the pendulum (h) and the final height after the impact (h’) allows measuring the energy absorbed in the process of fracturing the specimen. Strictly speaking, the energy absorbed in the area under the load curve is measured, which is known as resilience

The energy absorbed in the impact by the specimen is usually calculated as the difference of the initial and final pendulum heights, this implies, obviously, neglecting some friction losses. The calculation formula for the impact energy is Eq. (1):

where:

T, is the energy used in the break in Joules, P is the mass of the pendulum in kg, g is gravity (9.80665 m·s²), h is the initial height of the pendulum, h’ is the final height of the pendulum, l is the length of the pendulum in meters, α and β are the angles that form the pendulum with the vertical before and after releasing it, respectively.

Figure 3 shows the Charpy impact specimen placed on the Charpy pendulum before being broken by the pendulum (Fig. 2).

Toughness in steels is controlled by different microstructural constituents. Some of them, like inclusions, are intrinsic while others happening at different microstructural scales relate to processing conditions (Gutiérrez, 2014).

Figure 4 (Gliha et al., 2004) shows schematically the changes of microstructure that occur in a multiple-pass weld. The zone affected by heat, whose microstructure is thickened by the first cycle, changes to different microstructures according to the thermal sequence of the subsequent passes. When the maximum temperature of the second pass is immediately above the upper critical line (AC3), the microstructure is refined, which is usually called the normalized effect (C in Fig. 3); when the temperature is less than AC1, the microstructure is only relieved (A in Fig. 3). In the event that the reheat temperature is between AC3 and AC1 (called the inter-critical zone of temperatures), local regions enriched in carbon are generated with the risk of forming fragile microstructures (B in Fig. 3). This dual-phase region also exists approximately 3 mm from the fusion line of the first pass. Through the simulation of thermal welding cycles it was determined that, from the point of view of microstructures, the greatest deterioration in the toughness of the HAZ is due to the formation of local fragile zones (LBZ) (other factors are grain growth) and presence of impurities), the authors (Yurioka, 1995; Zalazar et al., 2000) agree in establishing that they originate in a multiple-pass weld, mainly in the region identified as a coarse-grained zone overheated in the intercritical region of temperatures. The LBZ are strongly affected by metallurgical factors such as the martensite islands, so their presence is influenced by the chemical composition of the alloy and by the succession of thermal cycles to which the base metal is subjected. Some authors report equations that link the influence of LBZ on the transition temperature; others, however, consider that the deterioration of the mechanical properties due to the LBZ does not depend on the percentage thereof. The morphological aspect of the LBZ, which may have an elongated shape, should also be considered in the analysis when the length / width ratio of the phase is 4/1. At this point there is a discrepancy as to its influence on the mechanical properties. In steels with nickel, molybdenum and higher percentages of carbon, both types of LBZ are formed, and together with the matrix they deteriorate the tenacity of the base metal. The cooling time, from the second welding cycle, between 800 ºC and 500 °C (t_8/5) influences not only the appearance of these phases, but also the morphology they present (Zalazar et al., 1998).

2. SECTION A OF THE ASME VIII CODE, DIV. 1TOP

Section A of the ASME VIII code div. 1 that determines the Charpy impact describes the general requirements (UG) and the points UG-20 and UG-84.

2.1. UG-20: Design temperatureTOP

The design temperature is the temperature at which the pressure equipment is designed. The design temperature will determine the energy required for the Charpy impact that will be carried out according to the operating temperature of the pressure vessel. The structure of paragraph UG-20 is shown in Fig. 5.

Paragraph UG-20 is then broken down to explain each of its parts.

Maximum temperature used in design: The maximum temperature defined in the design will not be less than the average temperature under operating conditions; it is considered the average temperature of the metal (through the thickness). If the pressure vessel is directly subjected to the flame, then point UW-2 section d) -3 will be applied, which indicates that the welded joints under pressure, thus the affected joints will be designed at temperatures not lower than the maximum metal temperature on its surface under operating conditions. The maximum temperatures will be determined by calculations or by direct measurement on the pressure vessel in service.

Minimum design temperature: The minimum design temperature is also known as MDMT. The minimum operating temperature is the minimum temperature in service. The MDMT will be determined either numerically or experimentally and will be registered in the registration tag of the pressure vessel together with the maximum admissible working pressure, also known as MAWP. If lower temperatures are allowed by paragraphs UCS-66 and UCS-160 then the MDMT temperature will be in accordance with these paragraphs. These paragraphs will be discussed later.

Impact test: The impact test according to UG-84 is not mandatory for materials subjected to pressure that meet the following conditions:

2.2. UG-84: Charpy impact testTOP

In paragraph UG-84 all the points that affect the Charpy test are developed according to the general design rules.

The structure of paragraph UG-84 is schematized in Fig. 6.

3. STUDENT FEEDBACKTOP

This course was taught to two groups, A and B, with 29 and 33 students, respectively. The knowledge of both groups was assessed upon performing an impact test according to the general requirements as per the ASME VIII div. 1 part UG code. To check the effectiveness of the learning procedure, the tables and figures explaining the ASME VIII div. 1 part. UG code provided in this work were only introduced to group A, while the ASME VIII div. 1 part. UG code was presented directly to group B, without these figures and tables. To be pragmatic, we will call them the test sample and the control sample, respectively. Once the antecedents and the methodology were presented to the students, a short multiple-choice questionnaire was presented to both groups at the same time. In addition, once the results comparing groups A and B were obtained, the lecturer carried out a survey among group A to estimate group satisfaction with the new approach. In the questionnaires, the correct answers are measured over a given period of time. If the time established for the evaluation is exceeded, only those answers answered within the time established in the test will be evaluated. It has been proven that those students who answered the most correct questions were those who used the tables and diagrams presented in this article, however, those students who did the test directly using the code ASME VIII div. 1, in addition to answering fewer questions, they made more mistakes. As an example, Fig. 6 relates the MDMT to UW-2 (d) (3), the question that addressed this question was not answered or answered erroneously by those students who used the ASME VIII code, div. 1.

The main advantage of these schemes is shown in Fig. 6, where the article UG-84 is structured in all the subsections that this article is formed. By means of a swipe of view you can see the structure and be able to choose the best option in front of a question formed in a test of evaluation type of ASME VIII div. 1.

Table 4 shows the results of the questionnaire for the test and the control groups. Although the total sample size is small, the comparison between groups suggests that the proposed method increases the comprehension level of the students related to their knowledge about the role of the alloy elements in steels, which can lead to a better understanding of this subject matter. Lecturers are continuing to gather more data for a more reliable statistical study for further conclusions in future works.

There are other works that have used other different teaching methodologies (Vergara and Rubio, 2012; López-Martínez et al., 2014; Vergara et al., 2016; Jurado-Navas and Munoz-Luna, 2017) from that used in the workplace but which have helped me to create this new method of teaching the complex but very useful ASME VIII code div. 1.

4. CONCLUSIONSTOP

REFERENCESTOP

Nomenclature (Abreviations)TOP