RU2521786C1 - Method for determining phase composition of bainitic steels - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: x-ray diffraction spectrum is obtained; qualitative phase analysis is performed and content of phases is determined quantitatively by Rietveld method considering convergence factor GOF; with that, bainitic steel in the form of a metallographic polished section is chosen as a sample; reflexes belonging to alpha-phase are separated on a diffractogram and divided into components - peaks of ferrite and bainitic ferrite; degree of tetragonal ability of a bainitic ferrite pattern is set; quantitative and qualitative phase composition is calculated and adjusted.

EFFECT: providing the possibility of determining qualitative and quantitative phase composition of bainitic steels with determination of ratio of bainite and ferrite.

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Description

The invention relates to a method of x-ray analysis and can be used to study the structure and phase composition of metals in materials science.

A known method of studying the structure of pipe steels, including the interaction of a sample of pipe steel with an aqueous solution of sulfosalts, subsequent washing and drying of the sample, and identifying areas of bainite rack morphology using an optical microscope [patent No. 2449055]. The disadvantage is that the method is intended only to determine the structure, without dividing the ferrite into its components.

Known x-ray method for quantitative determination of the phase composition of Portland cement clinkers, adopted as a prototype [RU Patent No. 2461817]. The method consists in obtaining an X-ray diffraction spectrum from a powder preparation made from the Portland cement clinker under study, using the X-ray diffraction spectrum obtained, the phase composition of the powder preparation is determined by the Rietveld method. Preliminarily, a polished section is prepared from the studied Portland cement clinker, by which the phases present in the polished section are visually revealed, after which the phase compositions are compared and the phase composition, which is obtained by the X-ray diffraction spectrum, is corrected by the phases that are detected in the smallest quantities, and then the ratio of the two monoclinic modifications is determined Alite, which is contained in the largest amount, by analyzing the asymmetry of superimposed reflections in the range of angles 2Θ _Cu = 31.5-33 °. Next, the quantitative content of all detected phases is determined by the Rietveld method, then the quantitative content of all phases in the clinker under study is determined in the interval between their average content and the content obtained from monoclinic alite modification present in large quantities. The X-ray diffraction spectrum can be measured in a powder automated X-ray diffractometer using copper radiation in the range of certain angles, at a voltage of 45 kV and a current strength of 35 mA.

This method does not allow to study the structure of bainitic steels with the separation of the alpha phase into components - ferrite and bainite.

The objective of the invention is to develop a method for determining the qualitative and quantitative phase composition of bainitic steels with the identification of the ratio of bainite and ferrite.

An X-ray automatic diffractometer is used to record the X-ray diffraction spectrum of a metallographic section of bainitic steel. The resulting spectrum determines the phase composition of steel, Figure 1 - the result of a qualitative phase analysis of bainitic steel. Reflections belonging to the alpha phase are distinguished on the diffractogram and separated into components — peaks of ferrite and bainite. The separation technique is based on a visual assessment of the asymmetry of the α-Fe diffraction peak associated with the presence of ferrite with a carbon content of about 0.02%, and bainite, which has a distorted crystalline body-centered cubic lattice due to the increased carbon content in it. Set the degree of tetragonality of the bainite lattice. The parameters of the atomic structure of the phases are selected from the ICSD [Inorganic Crystal Structure Database] for calculating the quantitative composition (relative coordinates of atoms, population of positions and displacement parameters, as well as the expected average sizes and shapes of crystallites), the value of the GOF convergence factor is calculated according to the Rietveld method, and, if necessary, make adjustments to the qualitative and quantitative phase composition of the steel by repeatedly selecting the parameters of the atomic structure of the phases until the GOF convergence factor is reached by the Rietveld method less than 1.4.

During metallographic microsection analysis (. - Microstructure (× 500), and b - panoramic view of the structure after etching in polarized light and 2) can be produced.

The selection of alpha-phase reflections, their separation into components, and the determination of the degree of tetragonality of the bainite lattice make it possible to reveal differences in the structural characteristics of ferrite and bainite, which as a result makes it possible to use the Rietveld method to determine the qualitative and quantitative composition of bainitic steel. The combination of distinctive features is necessary and sufficient to solve the problem.

Three metallographic sections were made from samples of bainitic steels of strength class K65 with a carbon content of up to 0.07%.

An X-ray study of the obtained samples was performed on a Bruker D8 Advance automated diffractometer in Θ-Θ geometry using copper radiation, mounted on a diffracted (i.e. reflected) beam of a graphite crystal monochromator, in the angle range 2Θ = 34-120 °, at a voltage of 40 kV and a current strength of 40 mA, with rotation and registration of diffracted rays by a scintillation counter. Scanning was carried out with a step of 0.02 ° in 2Θ and a signal accumulation time at each point of 9 s. According to the obtained spectrum, the phase composition of the steel was determined, including α- and γ-Fe and cementite (Figure 1. The result of a qualitative phase analysis of bainitic steel). Qualitative phase analysis was determined by comparing the set of interplanar distances of the experimentally obtained x-ray diffraction pattern with x-ray phase data of the ICDD powder base [International Center for Diffraction Data - PDF2008].

Reflections belonging to the alpha phase were isolated on the obtained diffractograms and divided into components — peaks of ferrite and bainite, suggesting that the bainite lattice is similar to the martensite lattice, but with an extremely small (up to 1.004) tetragonality value (Figure 3. Example of peak separation α -phases on the components - reflexes of ferrite and bainite). The degree of tetragonality of the bainite lattice was set.

The structural parameters for calculating the quantitative phase composition according to the Rietveld method (relative atom coordinates, position occupancy and displacement parameters) were taken from the ICSD [Inorganic Crystal Structure Database]. The value of the convergence factor GOF = 4.68 was calculated; R _wp = 47.41.

The qualitative and quantitative phase composition of the steel was adjusted by re-selecting the parameters of the atomic structure of the phases until the GOF convergence factor was reached by the Rietveld method of less than 1.4 - GOF = 1.31; R _wp = 11.65. (Figure 4. a - View of the initial diffractogram; b - an intermediate result of processing the diffraction pattern according to Rietveld; c - the final result of the calculation according to Rietveld of the number of phases.)

As an additional criterion for the correspondence between the model and the experiment, the shape of the difference curve can also be taken (Figure 5. The decomposition of the experimental diffraction profile corresponding to the 110 α-Fe line into the components of ferrite (Iron alpha) and bainite (Bainite)).

In the process, a metallographic analysis of the studied samples can be carried out, which will visually determine the type and morphology of the structural components - ferrite and bainite crystallites.

The results of the qualitative and quantitative phase analysis of the studied metallographic sections and the parameters of the identified phases are presented in the table.

Sample one 2 3 Austenite,% 0.30 0.09 1.25 Bainitic ferrite,% 48.39 44.57 43.06 Ferrite,% 50.39 54.95 55.40 Cementite,% 0.92 0.38 0.29 The lattice parameter of austenite a , Å 3.61128 (5) 3,60871 (5) 3.60852 (7) The dispersion of carbides, nm 308 45 67 GOF 1.31 1.36 1.29

Thus, the proposed method made it possible to establish a qualitative and quantitative phase composition of high-strength pipe steels with determination of the ratio of ferrite and bainitic ferrite.

Claims (1)

A method for determining the phase composition of bainitic steels, which consists in obtaining an X-ray diffraction spectrum with a qualitative phase analysis and quantitative determination of the phase content by the Rietveld method, taking into account the GOF convergence factor, characterized in that bainitic steel is selected as a metallographic thin section as a sample, reflections are distinguished on the diffractogram, belonging to the alpha phase and divide them into components - peaks of ferrite and bainitic ferrite, specify the degree of tetragonality of the lattice of bainitic ferrite, calculate and correct the quantitative and qualitative phase composition.

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