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

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 (σ_m, ultimate tensile strength, σ_0.2, 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 (σm, ultimate strength, σ0.2, yield strength) versus strain rate (ε˙) 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 (K_Ic) 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]:

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.

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.