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# High Temperature Materials and Processes

Editor-in-Chief: Fukuyama, Hiroyuki

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Volume 35, Issue 8

# Changes in the Material Properties of Steel 1.4762 Depending on the Temperature

Josip Brnic
• Corresponding author
• Department of Engineering Mechanics, University of Rijeka, Vukovarska 58, 51000 Rijeka, Croatia
• Email
• Other articles by this author:
/ Goran Turkalj
• Department of Engineering Mechanics, University of Rijeka, Vukovarska 58, 51000 Rijeka, Croatia
• Other articles by this author:
/ Sanjin Krscanski
• Department of Engineering Mechanics, University of Rijeka, Vukovarska 58, 51000 Rijeka, Croatia
• Other articles by this author:
/ Jitai Niu
• School of Materials Science and Engineering, Harbin Institute of Technology, 92 Xidazhi St, Harbin, China
• Other articles by this author:
/ Qiang Li
• School of Materials Science and Engineering, Henan Polytechnic University, No 2001, Century Avenue, Jiaozuo City, China 454003
• Other articles by this author:
Published Online: 2015-10-16 | DOI: https://doi.org/10.1515/htmp-2015-0075

## Abstract

This paper presents the experimental results and analysis of the behaviour of steel 1.4762 at different temperatures. Tensile mechanical properties and creep behaviour were determined using uniaxial tests. Charpy impact test was used to determine impact energies at different temperatures. The mentioned mechanical properties are displayed as engineering stress–strain diagrams while material resistance to creep is presented in the form of creep curves. Based on Charpy impact energy an assessment of fracture toughness is given. Experimental results show that the values of mechanical properties are continuously decreased by increasing the temperature. After the temperature of 623 K (tensile strength: 468 MPa; yield strength: 298 MPa) the decrease of the ultimate strength and yield strength is especially emphasized and the difference between them becomes smaller. The creep resistance is considered quite small, except at low stress levels.

## Introduction

The design and manufacture of a structure depend on the purpose and service life conditions of the structure. The choice of material plays a leading role in the structural design. Namely, choice of material must correspond to the purpose of the structure. A process of material selection must consider issues of material properties (strength, stiffness, etc.) and analyse them in accordance with the required application [1]. Furthermore, analysis of the structural behaviour of anticipated operating conditions in its service life is usually done using finite element method [2]. Machines, ships, buildings as well as any kind of structure must be designed in such a way that their safety, reliability and economic viability are ensured during their service life and that structural failure will not occur [3]. In spite of that, in the design, manufacture and the use of structure some failures may occur. Generally speaking, failure causes may be structural loading, corrosion, wear, etc. The common causes of failure are poor design, the wrong choice of materials, poor assembling of structure elements, improper use, improper maintenance, unforeseen operating conditions, inadequate control, etc. [4]. In engineering practice a number of failures exist, and some of them are force-induced elastic deformation, yielding, creep, buckling, fatigue, fracture, corrosion, etc. [5]. Each of the mentioned failures has its own cause of origin and the form of its expression. In order to avoid the appearance of failure in the structure, and because of structure safety control, a special discipline was introduced into engineering practice, which is analysis of the structure. It deals with the determination of failure causes. In this sense, it is of great importance to determine why and how a structure has failed. In this paper, creep as one of the possible failure modes will be considered. This phenomenon is usually defined as time-dependent material behaviour where deformation continually increases while the stress (load) is kept constant [6]. The process of creep in metals can be represented by a curve consisting of three stages, which are transient (first), steady- state (second) and accelerating (third) creep stage. In engineering practice only 1–2% of creep strain is allowed. Further, creep in metals can be considered as appreciable at temperatures above 0.4 of melting temperature [7]. The material that is the subject of this paper is widely used in various fields of engineering, mostly those involving high temperature. Published articles on this material can be found in literature, but research presented here shows details of the properties of this material at different temperatures, and its behaviour in terms of creep. This was the main reason to investigate the behaviour of this steel. In accordance with the possible applications of steel 1.4762 (AISI 446) some recent articles can be found in the literature, as can be seen from the further part of this chapter. So, the study of the fatigue behaviour at room and intermediate temperatures of a superferritic stainless steel UNS S 44600 that received different heat treatments is presented in Ref. [8]. Further, an investigation was carried out in the field of microstructure of tubes made of AISI 446 ferritic stainless steel used in petrochemical plant [9]. It was found that, under the working conditions to which the tubes were subjected, precipitation of carbides and sigma phase occurs. Also, in Ref. [10] atomic force microscopy (AFM) was applied to study the surface relief evolution at the emerging persistent slip bands (PSBs) in individual grains of polycrystalline ferritic stainless steel cycled with constant plastic strain amplitude. As stated above, knowledge of the behaviour of material under specific environmental conditions is of utmost importance in the process of material selection. In this sense, it is recommended to gain insight into the following Refs. [1117]. In addition, in a case where the manufacturing procedure of a product is considered, it is of great importance to have appropriate information about used material [18]. Also, considerations may relate to the properties of other materials [19, 20].

## Investigated material

The material under consideration was ferritic, round 1.4762 steel (X10CrAl24, AISI 446) annealed bar. Its chemical composition in mass (%) is: C (0.102), Cr (23.05), Al (1.23), Si (1.2), Ni (0.6855), Mn (0.519), V (0.201), Mo (0.116), Cu (0.0962), Sb (0.0396), P (0.0217), Ti (0.0129), S (0.01) and rest (72.7161).

Due to high chromium content it offers outstanding scaling resistance in air as well as good resistance to oil ash corrosion. It has low mechanical strength compared to that at austenitic grades. It is used in many fields of engineering, mostly for elements subjected to high temperatures as well as to low tensile stresses. The following applications of this steel can be mentioned: cement, ceramics and chemical industries, furnace technology, metalworking industry, chemical industry, etc.

## Data related to testing procedures

In accordance with the requirements of testing, we used the following equipment: test machine capacity for tensile testing of 400 kN, high-temperature extensometer, furnace (900°C) and Charpy impact pendulum. Engineering stress–strain diagrams were obtained using tensile testing in accordance with ASTM: E8M-11 standard for testing at room temperature and with ASTM: E21-09 standard for testing at elevated temperatures. Creep tensile testing was carried out according to ASTM: E139-11 standard. Testing the impact energy using Charpy impact machine was carried out according to ASTM: E23-07ae1 standard. The aforementioned standards can be found in Ref. [21]. All specimens used in this research were machined from 18 mm round bars.

## The effect of temperature on the properties of material

By applying tensile tests at room temperature and at elevated temperatures, stress–strain diagrams were obtained (Figure 1). The dependence of material mechanical properties and modulus of elasticity on temperature is shown in Figure 2. At the same time, Figure 2 also shows the approximate curves (polynomials) describing the experimental results. Undoubtedly, the tests in engineering practice are often very expensive since days sometimes try to simulate (model) or predict the real test. In the cases presented in Figure 2, the real data (measured data) are approximated by polynomials. However, it is of interest to know the accuracy with which the simulated (approximated) process can replace the actual process (real data). A coefficient of determination (R2) is established, and it is a measure of accordance of real (measured) and simulated (modelled, approximated, predicted) values. The R2 is a statistic that gives information about how fit a model is [22].

Figure 1:

Engineering stress–strain diagrams: steel 1.4762.

Figure 2:

Experimentally obtained properties of the material versus temperature and polynomial approximations: steel 1.4762. (a) Mechanical properties (σm, ultimate tensile strength, σ0.2, 0.2 offset yield strength). (b) Modulus of elasticity (E). (c) Total elongation (ε) and reduction (ψ) of the cross-sectional area of the specimen.

According to the experimentally obtained results (Figures 1 and 2), it is visible that ultimate tensile strength and yield strength of the considered steel at room temperature are quite high (584 MPa, 487 MPa, respectively). By increasing the temperature these properties are continually decreased and at the temperature of 823 K ultimate tensile strength takes the value of 188 MPa, while yield strength takes the value of 179 MPa. Modulus of elasticity also is continually decreased when the temperature is increased. It may be said that request properties related to its use in high-temperature conditions may be treated as satisfactory.

## Strength of the material versus rate of strain

Engineering stress–strain diagrams were obtained in accordance with prespecified strain rates related to material and stages (fields) during the test (the elastic region, the area of flow, etc.). It is known that strain rate affects the level of strength. To determine the dependence of the strength of material on the strain rate, several tests were conducted. In particular, applied strain rate is kept constant throughout the considered test (Figure 3).

Figure 3:

The level of strength (${\mathrm{\sigma }}_{m}$, ultimate strength, ${\mathrm{\sigma }}_{0.2}$, yield strength) versus strain rate ($\stackrel{˙}{\mathrm{\epsilon }}$) at different temperatures: steel 1.4762. (a) Engineering stress–strain diagrams for different strain rates, T = 923 K. (b) Engineering stress–strain diagrams for different strain rates, T = 1,023 K. (c) Engineering stress–strain diagrams for different strain rates, T = 1,123 K. (d) Measured values of the ultimate strength at different strain rates and different temperatures as well as their polynomial approximations. (e) Measured values of the yield strength at different strain rates and different temperatures as well as their polynomial approximations.

As is usually expected, mechanical properties are increased when strain rate is increased. It needs to be said that performed processes related to strain rates, and results of which are stress–strain diagrams, were carried out in accordance with the possibilities of the used machine.

## Short-time creep tests

Several short-time creep tests have been carried out. Tests were conducted at different stress levels and different temperatures (Figures 4 and 5). The level of the stress in each creep process was defined in relation to the appropriate yield strength at the considered temperature at which the process was carried out.

Figure 4:

Short-time creep tests at 823 K: steel 1.4762.

Figure 5:

Short-time creep tests at 1,023 K: steel 1.4762. (a) Creep test at 8.32 MPa. (b) Creep test at 20.82 MPa.

The considered material is used in high-temperature conditions and it has good mechanical properties at room temperature. However, it can be said that it may not be subjected to high tensile stress level at high temperatures as well as that it cannot be referred to the material which is creep resistant. So, when it is subjected to high temperatures and high stress levels, its creep strain becomes too high.

## Effect of high temperature on the microstructure of material

As stated, the choice of materials is a very important task in the design of structures. In this way, material selection needs to be done in accordance with the intended purpose of the structure. It is known that material properties of steels are linked to the chemical composition, processing path, etc. For a particular steel composition, the properties depend on the microstructure and these properties are so-called structure-sensitive properties, like yield strength.

This paper presents the results of research related to the microstructure of the as-received material and that which has previously been subjected to creep (Figure 6).

Figure 6:

Optical micrograph of steel 1.4762, aqua regia. (a) As-received material, cross-section of the specimen, 200×. (b) After creep process (823 K/89.5 MPa/125 min), cross-section of the specimen, 500×.

At room temperature, the base phase with the light colour is the ferrite (sign A). The phase with dark colour (sign B), different orientation, is ferrite as well. The phase with middle light colour (sign C) is the austenite. The phase of big black point in picture is AlN (sign D) and the phase with characteristics of white stick is the intermetallic compound of Fe-Cr (sign E). At the temperature of 823 K, the specimen was subjected to stress of 89.5 MPa for a duration of 125 min. The grains are obviously elongated.

Meanwhile, the big Fe-Cr phase is reduced to a smaller size and elongated at the same time. The phase of big black point, the AlN compound, has no evident changes.

## Experimental determination of Charpy impact energy

The use of material is governed by its properties. Yield strength is usually said to be selected as a criterion in structure design against plastic deformation, and at the same time fracture toughness is selected as criterion in structure design that defines structure resistance to crack propagation [23]. However, fracture toughness (KIc) is usually experimentally determined and can be defined as critical value of stress intensity factor (SIF). Although experimentally determined value is the most reliable information, because of possible difficulties in manufacturing of the specimen as well as identity between the real structures where crack may occur and manufactured notch in the specimen, there are some reservations about the direct application of these results in engineering practice. To avoid these difficulties related to the manufacture of the specimen, but also to use some experiment to evaluate material toughness, some other methods were developed. Charpy impact energy method is one of them. Namely, some correlation between experimentally obtained fracture toughness and calculated value of fracture toughness based on Charpy impact energy was developed. Usually, Roberts–Newton formula is used for the assessment of fracture toughness. It is based on Charpy impact energy and its application is independent of temperature level [24]: ${K}_{\mathrm{I}\mathrm{c}}\text{\hspace{0.17em}}=8.47{\left(\mathrm{C}\mathrm{V}\mathrm{N}\right)}^{0.63}.$(1)The measured values of Charpy V notch (CVN) impact energy are given in Table 1, and these values are related to the specified temperatures and modulus of elasticity.

Table 1:

Charpy impact energy and fracture toughness calculation: steel 1.4762.

As is visible, the obtained data related to fracture toughness are in accordance with the properties of considered material which belongs to the ferritic materials.

## Conclusion

The paper presents the results relating to the properties of the material, the creep resistance and the impact energy. These results may be useful to structure designers. The results show that the investigated material has a high level of strength at room temperature and this is also valid up to a temperature of 623 K. After this temperature, ultimate tensile strength and yield strength of the considered steel decrease quite fast. Also it is evident that resistance to creep of this material is quite low. Its impact energy corresponds to the material of this kind. In accordance with machine possibilities, an impact of strain rates on the level of strength is also shown.

## Acknowledgments

The authors are grateful to the staff of the School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, China, as well as to the staff of the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, China, for their efforts in the experimental analysis and explanation of the microstructure.

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Accepted: 2015-07-28

Published Online: 2015-10-16

Published in Print: 2016-09-01

Funding: This work has been fully supported in part by the Croatian Science Foundation under the project 6876 – Assessment of structural behaviour in limit state operating conditions, and by the University of Rijeka under the project 13.09.1.1.01.

Citation Information: High Temperature Materials and Processes, Volume 35, Issue 8, Pages 761–767, ISSN (Online) 2191-0324, ISSN (Print) 0334-6455,

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