RU2734878C1 - Method of detecting bainite in steel - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to metallographic analysis of structural components of steel of predominantly bainitic type. Method involves preparing sample surface for etching, chemical etching of analysed surface in etching agent, washing in running water and drying in air flow, detecting areas of bainite using an optical microscope, identifying an image using an analyser, fixing and recording using high-resolution optics. As investigated material there used is structural alloyed steel of type H2G2S2MF after formation of lower needle-like morphology of bainite at isothermal holding during hardening, etching involves using a LePera etchant consisting of 50 ml of 1 % aqueous solution of Na₂S₂O₅ and 50 ml of 4 % picric acid in ethyl alcohol, in which ethyl alcohol is used of not less than 95 %. In process of washing and drying of analysed surface, oxide film is stored on etching surface, controlling and optimizing time for detecting a clear image of areas of placing bainite needles and detecting α- and γ -phase is bainite substructures.

EFFECT: invention provides complete information on beinite needle morphology - lower bainite, in the entire range of its formation in isothermal exposures T_ISO = 375–200 °C, in steels of system of alloying H2G2S2MF, related to systems with hard-separated components and phases at detection and identification of them using optical microscope.

1 cl, 6 dwg

Description

The invention relates to metallographic studies of the structural components of steel, mainly of the bainite class.

It can be used to identify and identify lower-needle bainite, mainly carbide-free bainite, in complex structures of medium-carbon steel of the alloying system of the Kh2G2S2MF type, which is an economically alloyed high-strength steel of the third generation, which is a promising structural steel used in the oil and gas industry and as a rail steel.

C. Krempaszky, E. Werner, T. Hebesberger, A. Pichler // international doctoral seminar, Smolenice, SK. 2008. P. 50-54], заключающийся в последовательном травлении несколькими травителями с получением цветных изображений каждой составляющей исследуемых сталей на поверхности шлифа после электротравления. После травления проводят анализ выявленных структурных составляющих на основании различия в окрашивании, в цвете. При этом одним из травителей, используемых при травлении с выявлением бейнита, является состав LePera (к 50 мл 1%-го водного раствора Na₂S₂O₅ добавляют 50 мл 4%-го спиртового раствора пикриновой кислоты). После травления область микроструктуры бейнита окрашивается в темно-коричневый цвет. Травление проводят на низкоуглеродистых высокопрочных автомобильных сталях типа 10ХГ2. Изображение структурных составляющих стали после травления фиксируют и исследуют на оптическом (световом) микроскопе. Идентификацию всех A known method for detecting bainite [Hairer F. Etching techniques for the microstructural characterization of complex phase steels by light microscopy / F. Hairer,

C. Krempaszky, E. Werner, T. Hebesberger, A. Pichler // international doctoral seminar, Smolenice, SK. 2008. P. 50-54], which consists in sequential etching with several etchants to obtain color images of each component of the investigated steels on the surface of the microsection after electric etching. After etching, the identified structural components are analyzed based on the difference in coloration, in color. In this case, one of the etchants used in etching with the detection of bainite is the LePera composition (50 ml of a 4% alcohol solution of picric acid are added to 50 ml of a 1% aqueous solution of Na ₂ S ₂ O ₅ ). After etching, the area of the bainite microstructure turns dark brown. Etching is carried out on low-carbon high-strength automotive steels of the 10HG2 type. The image of the structural components of the steel after etching is recorded and examined on an optical (light) microscope. Identification of all

existing constituents in steel are carried out according to the difference in color of each constituent.

The disadvantages of this method are the impossibility of clearly identifying bainite (as well as other components), especially after isothermal holding of steel with a complex structure organization - acicular bainite. Moreover, to reveal the α- and γ-phases in the substructure of acicular bainite after isothermal holding without taking into account the peculiarities of the interaction of structural and phase components with etchants. The method is functionally limited, a bright field of an optical microscope is used to detect the presence of bainite in general in steel and only by the difference in color. Since there is no information on the optimization of the etching mode, which would make it possible to compare images and carry out primary quality control, and later on the quantity of compared steels, the method cannot be implemented in this direction, especially for acicular bainite in steels of the Kh2G2S2MF type with a fine-needle morphology.

A known method for detecting bainite in steels [George Vander Voort. Identifying isothermally-transformed steel microstructural constituents / George Vander Voort // Tech notes. 2015. V. 5, No. 4. P. 1-5.], Which consists in the fact that the study uses alloyed steels with different carbon content (20KhGNM, 38KhMA, 50KhGA) after isothermal holding during quenching. The main factor affecting the number of phases and components that are etched in three etchants (2% nitale, 4% picric acid, 10% sodium metabisulfite in water) by immersing a sample prepared by preliminary grinding and polishing of the test surface in a solution of 10 15 seconds. At the same time, after etching in 4% picric acid, cementite in ferritic steels, a ferrite-cementite structure, perlite and, in particular, bainite, are perfectly detected. Etching in a 10% aqueous solution of sodium metabisulfite reveals the structural components of many steel compositions, painting

these constituents in very poorly distinguishable colors, which are enhanced by partially crossed polarized light and a sensitive dimming plate. Picric acid etching reveals lower bainite only as one of the structural constituents of steel, but if it is visible visually, i.e. coarse-needle bainite.

The disadvantages of this method are:

- the lower-needle morphology of bainite is detected with 4% picric acid, it is not known in what solution - water or alcohol (and if in alcohol, then what percentage of alcohol and what water), which is important when obtaining a clear image, and etching is performed without controlling the process in time ... Areas where bainite is located are identified, i.e. the general picture and one of the constituent structures - bainite, but if it is "visually distinguishable", i.e. fine-needle bainite is not detected, limiting the range of investigation of detection and identification of bainite by the upper limit of the temperature of lower bainite formation (375 ° C) and is not realized within the range of 300-250 ° C of isothermal holding. Moreover, the effect of revealing lower bainite below Mn is unknown. Etching is carried out in a difficult way, within the search for what etchant and what constituents of the structure in steel it clearly identifies. After 4% picric acid, it is etched in a 10% aqueous solution of sodium metabisulfite to obtain a color image and to view it in polarized light to enhance the color image of large bainite needles;

- the identification of lower bainite is limited to obtaining a color image as a structural component without revealing the α- and γ-phases and substructure, which reduces the quality of information about the state of the phases relative to each other in steel after isothermal holding; the lack of information on the α- and γ-phases of lower bainite and on the orientation of the substructures of bainite needles does not allow a preliminary assessment of the mechanical properties of complexly structured compositions;

- the method uses polarized light only to enhance the color when using a sensitive toning plate in it after viewing in the light field of an optical microscope the structural component poorly detected in the etchant - lower bainite, which does not contribute to the detection of α- and γ-phases of acicular lower bainite and thinner structures - substructures of needles in high-strength steels with an average carbon content, related to economically alloyed, promising, with a mixed, poorly identifiable composition of structural components after isothermal holding. Especially in the region close to the temperature of the onset of martensitic transformation ( _MH ) or below _MH .

The closest technical solution chosen for the prototype is [Method for studying the structure of pipe steels US Pat. No. 2449055, IPC C23F 1/28 dated 18.10.2010]. The method includes the interaction of a steel sample with an aqueous solution of sulfosalts (10% Na ₂ S ₂ O ₅ and K ₂ S ₂ O ₅ ), subsequent washing and drying of the sample, and identifying areas of bainite using an optical microscope. In this case, after applying an aqueous solution of sulfosalts to the sample surface, the film is removed. In this case, during etching, a mechanical action was used on the surface of the sample with a cotton swab in order to remove the film formed during the etching, and the surface was washed that did not contain an oxide film. Dry the surface to be examined in a stream of hot air to keep the surface film-free. For identification and measurements, an automatic image analyzer is used separately from the optical microscope, i.e. a separate program, which complicates the method and does not guarantee the quality of the image, and therefore, they use the quality obtained during etching. Bainite areas are identified using polarized light of an optical microscope, after which the obtained images of the sample are recorded and

quantitatively determine the parameters of the identified areas of bainite lath morphology.

The disadvantages of this method are:

- functional limitation in identifying bainite - only upper bainite and only lath morphology;

- the detection of bainite is limited to hot-deformed pipe steels of the X70 type and the unrealizability of the detection of acicular lower bainite with a lath substructure consisting of α- and γ-phases in promising third-generation steels of the Kh2G2S2MF type having a complex structure;

- to recognize (identify) the structure of the steel under study, separate components are used, which complicates the method, since it is not always possible to have them in conjunction with an optical microscope;

- reveal only the bainite area without phase detail;

- areas of bainite are revealed after removal of the colored oxide film, which makes the composition of the etchant and its staining effect unclaimed in the identification, in the identification of components in the study of steels of the Kh2G2S2MF type with a more complex structure;

- only the upper bainite area is identified by the presence of gray color of these areas compared to the dark background of everything else using an automatic image analyzer equipped with a high-resolution camera and polarized light in an optical microscope - to pre-fix these gray areas, which greatly complicates the identification and narrows research, minimizes information about the phase state of bainite after heat treatment. Moreover, after isothermal holding, steel with a complex structure.

The technical result is aimed at obtaining complete information about the lower bainite formed in the process of isothermal holding, in

steels of the alloying system Kh2G2S2MF with difficult to distinguish structures in the entire range of formation.

The objective of the invention is to obtain complete information about bainite of acicular morphology (lower bainite in the entire range of its formation during isothermal holdings (T _ISO = 375-200 ° C)) in steels of the alloying system Kh2G2S2MF, related to systems with intractable components and phases upon detection and their identification using an optical microscope.

The technical solution is achieved by the fact that a method for detecting bainite in steels, which consists in preparing the surface of the test sample for etching, chemical etching of the studied surface in an etchant containing an aqueous solution of a sulfosalt Na ₂ S ₂ O ₅ , with the formation of an oxide film during etching, rinsing in running water and drying in a stream of air, identifying areas of bainite using an optical microscope, identifying an image using an analyzer, fixing and shooting using high-resolution optics, according to the invention, mainly bainitic steels of the alloying system Kh2G2S2MF are studied after the formation of the lower-acicular morphology of bainite during isothermal exposure in the hardening process, during etching, an etchant of the LePera composition is used (50 ml of a 1% aqueous solution of Na ₂ S ₂ O ₅ and 50 ml of 4% picric acid in ethyl alcohol), while using ethyl alcohol at least 95%, while maintaining the oxide film on the etching surface in the process of pr washing and drying the investigated surface, controlling and optimizing the time for identifying a clear image of the areas where the needles of bainite are placed and for identifying the α- and γ-phases - the bainite substructure, and then the images are identified by the difference in color and then the obtained color images are recorded in the bright field of an optical microscope using high resolution, then identify the α- and γ-phases in the areas of placement on

the etched surface of acicular bainite, using polarized light, passed through the crystal analyzer in the same optical microscope, in the form of a luminous image of the α-phase and a dark image of the γ-phase, the image of the phases is recorded and the shooting is carried out.

Comparative analysis of the claimed invention with the prototype showed that the claimed method has the novelty of the materials used as research, the sequence of operations to identify bainite and the used etchant composition, has significant differences in solving the problem and the functionality of research combinations to identify and identify poorly studied at this level of the lower carbide-free bainite, has industrial applicability, since the method uses existing devices and the known composition of the etchant in the study of promising steels related to economically alloyed high-strength alloying systems Kh2G2S2MF, which after isothermal holding have difficultly identifiable structural components and even more difficult to detect and release α- and γ- phases of acicular bainite, formed during isothermal holding, in an optical microscope in a bright field and polarized light, especially the γ-phase. Combined approach for detecting and identifying bainite: at the beginning, etching in a selected known etchant to obtain a colored film, which is preserved throughout the entire study period, but the time of its appearance is controlled and optimized. This is the most important process that made it possible to solve the most difficult task of identifying and identifying lower bainite and expanded the possibilities of research. Etching in the well-known LePera composition containing 50 ml of a 1% aqueous solution of Na ₂ S ₂ O ₅ and 50 ml of a 4% solution of picric acid in ethanol is the result of a long selection of an etchant for medium-carbon steel that can clearly identify needle lower bainite after isothermal exposure.

The result showed that, unexpectedly, etching is possible with obtaining a color image of structural components, including lower bainite, but with the observance of the fact that picric acid is dissolved in ethyl alcohol not lower than 95%. A lower concentration of ethyl alcohol worsens its solubility and, probably, reduces the ability to influence the process. Color etching made it possible to clearly identify bainite needles and observe them in the bright field of an optical microscope at various magnifications and for the first time to reveal the substructure of needles (α- and γ-phases), which accelerated the process of studying the morphological features of lower bainite in more detail, not being limited to the areas where bainite is located on the surface sample (as in the prototype), and that allowed after color etching and color preservation on the bainite needles to fix the image clearly and to shoot it to increase preliminary information on the image quality of both the general structure of the needles and the substructure of lower bainite in the entire range of studies of bainite formation at isothermal exposure and the amount of carbon in steels in the bright field of an optical microscope. Also, in order to select the area (areas) on the etched surface with needles, in this part of the study to identify the α- and γ-phases in the needle and their position relative to each other in polarized light. At the same time, when passing polarized light in an optical microscope through a crystal analyzer in this microscope, it was for the first time found that the α-phase glows, while the γ-phase remains dark, which simplifies the identification process, provides complete information about the phases and their state. This luminescence effect of the α-phase made it possible for the first time to detect, fix and carry out the survey of acicular bainite in an optical microscope after isothermal holding below _MH . Probably, the appearance of the luminescence effect can be explained by the presence in the studied steels of an increased amount of silicon as an optically active component, voltage, and a crystallographic feature.

The method is carried out as follows.

For the study, a rod with a diameter of 22-23 mm of steel system alloying Kh2G2S2MF and a carbon content in the range of 0.40-0.50% is used. A sample in the form of a cylinder 10 mm long and 3-3.5 mm in diameter is cut from the rod. Isothermal treatment is carried out on a high-speed quenching dilatometer according to the following mode: heated at a rate of 1.5 ° C / s to the austenitizing temperature T _AUST = 1000 ° C, held for 15 minutes, cooled to the holding temperature at a rate of 1.5 ° C / s, holding in the field of bainite transformation, cooling at a rate of 1.5 ° C / s to room temperature. The steels have a structure that is difficult to detect and difficult to identify. Previously, numerous attempts to separate the constituents have only led to the identification of lower bainite regions on an optical microscope. A ready-made etchant has been found and used, which can clearly reveal the structural components by chemical etching at room temperature. It turned out to be the well-known etchant LePera, which contains 50 ml of a 1% aqueous solution of Na ₂ S ₂ O ₅ and 50 ml of a 4% solution of picric acid. In this case, picric acid is dissolved in a sleep containing a minimum amount of water (95-99%), which makes it possible to improve the quality of the color image during the etching of structural components and to reveal the difference in the color of the bainite needle in the light field of an optical microscope, i.e. to identify clearly the phases in the needle of the lower bainite by their difference in color.

Etching is carried out after preparation of the end surface of the test sample by grinding and polishing the surface. To do this, immerse the sample in the etchant for an optimal time, at which a clear color image of the structural components appears. Over-etching leads to a strong darkening of the colors and complicates the separation of the constituents from each other: martensite and bainite are difficult to distinguish when over-etched. To obtain quality

Images of the revealed structural components and substructure of bainite after etching, the sample is carefully washed, excluding mechanical impact on the surface under study, under running water. Dry with a stream of warm air. Then the image revealed by etching is fixed in the light field of an Olympus GX-51 optical microscope and shooting is carried out at different magnifications to obtain complete information on the shape and color difference: martensitic needles - light brown; acicular lower bainite is blue, while the α-phase in the needle is blue, and the γ-phase in the needle is light; austenite remains light. Then, the phases of bainite are identified in polarized light. To do this, allocate for identification of the area, when fixing the image in a bright field, placing the needles of the lower bainite in the same optical microscope in polarized light, passing light through the crystal analyzer [Methods of research of non-metallic materials. Volume III / Edited by B.I. Panshina, B.V. Perov and M. Ya. Sharova. - M: Mechanical engineering. 1973. S. 284]. In this case, it is found that one of the phases is lit. It turned out that this is the α-phase, and the second remains dark. It turned out that this is the γ-phase. Thus, the combination of the optimally selected components in the etchant, bright field, color etching, and luminescence of the α-phase in polarized light provides complete information on the presence of both structural components and phases that facilitate further research and obtain more reliable quantitative data. Especially for lower bainite, formed hammered to M and _H newly diagnosed lower M _N.

The α- and γ-phases are located in the needle parallel to each other. Images are taken at a magnification of 1000 times (× 1000). The etching time for the studied samples is optimized - 30 sec. Large-needle bainite can be detected at 600 times magnification (identified at 1000 times magnification), and small-needle bainite - by 1000-1500 times, and identified and identified (Fig. 1 and 2 and Fig. 3-6, respectively).

In the light field of an optical microscope, after etching and revealing the structural components of steel in color, an image in the color of the needles of the lower bainite is revealed: placement areas, the needles are identified (by color), α- and γ-phases in the needle (by the difference in color).

Thus, examining the structural state of steel 44Kh2G2S2MF after isothermal holding T _ISO = 375 ° C in the light field of an optical microscope, the general picture of the components of the steel is clearly revealed in color: martensite, acicular bainite, austenite (by light brown color, blue color and light image, respectively) (Fig. 1). And the α- and γ-phases in the bainite needles are identified: blue color - alpha-phase, light color of the plates. Magnification 1000 times. Then it is identified in the needles of the α- and γ-phases in polarized light, since plates of α- and γ-phases are more clearly detected in it, regardless of the size of the needles (Fig. 2). In addition, in polarized light, it is found that the α-phase seems to glow and appears during shooting in the form of light plates, and dark plates of the γ-phase are placed between them.

Investigating the structural state of 44Kh2G2S2MF steel after isothermal holding T _ISO = 250 ° C in the same order as before, we obtain a color image in a bright field (Fig. 3), and in polarized light: α-light areas (in black and white image) and, as it were, luminous plates - visually (Fig. 4). The γ-phase is also placed parallel to the α-phase plates both visually, and at 1000 times magnification is visible as dark plates. The same effect is also observed at T _ISO = 200 ° C (i.e. below Mn and found due to the claimed method). In this case, in the bright field, small-needle blue bainite is detected (Fig. 5), and in polarized light, the α-phase is also clearly identified - a luminous (light) image, and the γ-phase is dark plates (Fig. 6) between the plates of the α-phase ...

The inventive method has the following advantages:

the method clearly reveals and identifies the lower acicular bainite in complex structured systems of alloy steels;

the method, using both a bright field and polarized light transmitted through an analyzer crystal of an optical microscope, allows one to obtain all information about the state of the lower bainite: the area where it is concentrated in steel, the α- and γ-phases - their presence and ratio in bainite, which it is important to establish the proportion of phases in the needles (brittle and plastic components), to determine the degree of reliability of steel relative to the area and mode of its operation;

the method expands the functionality of both etching, and detection, and identification with the detection of the substructure and the appearance of luminescence of the α-phase when using polarized light transmitted through the crystal analyzer;

the method allows, using the known composition of the etchant, to find the enhancing effect of coloration of the lower bainite during chemical etching, which simplified the process of identifying needles and substructure in color, which makes it possible to record this effect already in the light field of an optical microscope;

identification by the method of the α- and γ-phases of lower bainite - the substructure of the needles, is of predetermined importance for the formation of the operational properties of high-strength steels with a complex structure, such as economically alloyed steels with a difficult-to-separate set of structural components;

the method made it possible to discover something that was not possible before - acicular bainite below the temperature of the beginning of martensitic transformation;

The method makes it possible to carry out preliminary control of the quality of steel by the presence of all information about the structural components by color, their clear identification and luminescence of the α-phase, which is responsible for the strength of the steel, and the γ-phase, which is responsible for the plasticity of the steel.

Claims (1)

A method for detecting bainite in structural alloy steels, including preparing the surface of the test sample for etching, chemical etching of the studied surface in an etchant containing an aqueous solution of Na ₂ S ₂ O ₅ sulfosalt, with the formation of an oxide film during etching, rinsing in running water and drying in an air stream , identification of bainite areas using an optical microscope, image identification using an analyzer, fixation and shooting using high-resolution optics, characterized in that structural alloy steel of the Kh2G2S2MF type is used as a test material after the formation of the lower acicular morphology of bainite during isothermal holding during quenching , when etching, an etchant LePera is used, consisting of 50 ml of a 1% aqueous solution of Na ₂ S ₂ O ₅ and 50 ml of 4% picric acid in ethyl alcohol, in which ethyl alcohol is used at least 95%, in the process of washing and drying the studied surface of the oxide The film is preserved on the etching surface, controlling and optimizing the time for revealing a clear image of the areas where bainite needles are placed and for revealing the α- and γ-phases - the bainite substructure, and then the images are identified by the difference in color and the obtained color images are recorded in the bright field of an optical microscope using high resolution, then identify the α- and γ-phases in the areas of placement on the etched surface of acicular bainite using polarized light passed through the crystal analyzer in the same optical microscope in the form of a luminous image of the α-phase and a dark image of the γ-phase, fix image of phases and take pictures.

Images