CN114964983A - Method for detecting austenite grain size of 38MnS6 steel - Google Patents

Method for detecting austenite grain size of 38MnS6 steel Download PDF

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CN114964983A
CN114964983A CN202210473646.6A CN202210473646A CN114964983A CN 114964983 A CN114964983 A CN 114964983A CN 202210473646 A CN202210473646 A CN 202210473646A CN 114964983 A CN114964983 A CN 114964983A
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38mns6
austenite grain
detection surface
pearlite
grain size
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黄永建
孙晓明
戴观文
刘运娜
郝彦英
刘献达
张红卫
王昊
相楠
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Shijiazhuang Iron and Steel Co Ltd
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    • G—PHYSICS
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    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
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    • G—PHYSICS
    • G01—MEASURING; TESTING
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    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
    • G01N1/286—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q involving mechanical work, e.g. chopping, disintegrating, compacting, homogenising
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    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
    • G01N1/286—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q involving mechanical work, e.g. chopping, disintegrating, compacting, homogenising
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Abstract

The invention discloses a method for detecting austenite grain size of 38MnS6 steel, which comprises the following steps: (1) cutting a metallographic specimen; (2) heating the metallographic specimen to a temperature higher than the austenitizing temperature, keeping the temperature for a certain time, taking out, and cooling in the air; (3) fully removing an oxide layer on the detection surface of the metallographic sample subjected to the heat treatment in the step (2), and then carrying out grinding and polishing treatment on the detection surface; (4) eroding the polished metallographic specimen detection surface in the step (3) by using 2-4% nitric acid alcohol, and then washing and drying by using alcohol; (5) and evaluating the grains of the pearlite/pearlite colony according to the GB/T6394-2017 map comparison as the austenite grain size grade of the pearlite/pearlite colony. The method does not need picric acid, is simple and easy to operate, and can quickly and accurately evaluate the 38MnS6 austenite grain grade.

Description

Method for detecting austenite grain size of 38MnS6 steel
Technical Field
The invention belongs to the field of metal material physical and chemical detection, relates to a method for detecting austenite grain size of steel, and particularly relates to a method for detecting austenite grain size of 38MnS6 steel.
Background
The size of the metal material austenite grain is an important index for measuring the material use performance, and the austenite grain size detection belongs to a conventional project for detecting the steel material performance. GB/T6394-2017 metal average grain size determination method prescribes various austenite grain size detection methods.
The direct hardening method is a method of heat-treating a metallographic specimen at a required heating temperature for a heat preservation time, then obtaining a martensite structure by water cooling or oil cooling, corroding by using supersaturated picric acid, and displaying an austenite crystal boundary. The direct hardening method is the most widely used austenite grain size detection method at present, and is the most accurate austenite grain size detection method recognized at home and abroad. However, the direct hardening method requires that supersaturated picric acid solution is heated for erosion to display austenite grain size, picric acid belongs to one type of explosive, and the use and purchase are limited; meanwhile, because of the difficulty in corrosion of the austenite grain boundary due to the chemical component characteristics of part of the materials, the austenite grain boundary cannot be clearly displayed, and particularly, the austenite grain of the silicon-manganese spring steel is displayed by adopting a direct hardening method, which always belongs to the industrial problem.
The carburizing method is a method of placing a metallographic sample in a carburizing atmosphere, keeping the temperature for a certain time according to a specified heating temperature, enabling the surface of the sample to obtain a carburized layer with a certain thickness, slowly cooling to enable cementite to be separated out along an austenite crystal boundary to form a cementite network, and corroding by using a nital solution to display the austenite crystal boundary. The carburizing method is an austenite grain size display method for changing the original chemical components of steel, and can only display the austenite grain size in the range of a carburized layer because of limited depth of the carburized layer; the simulated carburizing method adopts the same heating temperature and heat preservation time as the carburizing method, then obtains the martensite structure in the heat treatment state by water cooling or oil cooling, uses supersaturated picric acid solution to corrode austenite crystal boundary, does not change the chemical composition of a sample, has larger detection area, but needs to use supersaturated picric acid to corrode as the direct hardening method. Both methods are applicable to steels used for late carburization.
The oxidation method is that the metallographic specimen is treated according to the specified heating temperature and heat preservation time, and then is rapidly cooled, so that the austenite grain boundary on the surface of the steel is oxidized, and the austenite grain boundary is displayed by using 15% hydrochloric acid solution for corrosion. The principle of detecting the austenite grain size by an oxidation method is that the surface of a sample is oxidized when the sample is heated and insulated at high temperature; because the oxidation resistance of the grain boundary is poorer than that of the grain boundary, the austenite grain boundary is preferentially oxidized to form an oxide network at high temperature; along with the prolonging of the heating and heat preservation time, fine austenite grains grow gradually, and an oxide layer on the surface of the sample is thickened continuously to reflect that the oxide network of the austenite grains forms iron scale at the previous moment; the oxide network reflecting the austenite grains at the present moment is a brand new grain boundary which is equivalent to the size of the grown austenite grains in the sample. The oxidation method is also a display method of austenite grains with changed original chemical components of steel, only a layer of oxidized austenite grain boundary which is close to the surface iron scale of a sample with the austenite grain size detected by the oxidation method can be used as a detection surface of the austenite grains, and meanwhile, the oxidation method has higher requirement on sample preparation, is easy to completely remove the grain boundary oxide layer on the surface of the sample in the sample preparation process, and can display that the area of the austenite grain boundary is smaller.
The austenite grain size detection of 38MnS6 steel basically uses a direct hardening method at present, but due to chemical composition reasons, when the martensite structure of the steel is eroded by using a supersaturated picric acid solution, the structure is relatively easy to erode, grain boundaries are not easy to erode, an erosion test needs to be repeatedly carried out, and the display effect of the austenite grain boundaries is poor. The operation steps of detecting the austenite grain size of the 38MnS6 steel by a direct quench hardening method are as follows:
cutting a metallographic specimen;
II, treating the metallographic sample according to a specified heat treatment system (heating at 860 +/-10 ℃ and keeping the temperature for 1 h), cooling with water or oil, and keeping the temperature at 230 +/-10 ℃ for 15 min;
III, fully removing an oxide layer from the metallographic specimen detection surface subjected to heat treatment according to the step II, and grinding and polishing the detection surface;
IV, using supersaturated picric acid solution and a proper amount of white cat detergent on the detection surface of the metallographic sample treated according to the steps II and III, heating and eroding for 3-5min, scrubbing the detection surface and drying;
and V, evaluating the austenite grain size grade according to a GB/T6394-2017 map comparison method.
The existing 38MnS6 austenite grain size detection method has the problems that:
i, a supersaturated picric acid solution is used for erosion, and the picric acid belongs to one kind of explosive, so that the purchase is limited along with stricter and stricter national regulation;
II, because of chemical components, the austenite grain boundary is difficult to display, repeated tests are needed, and the display effect is poor;
III the successful display of their austenitic grain boundaries is difficult for inexperienced operators.
Therefore, a method which does not use picric acid, is simple and easy to operate and can clearly display 38MnS6 austenite grain boundary needs to be found to replace a direct quenching method for detecting the austenite grain size.
Disclosure of Invention
The invention aims to provide a method which does not use picric acid, is simple and easy to operate and can clearly display 38MnS6 austenite grain boundaries.
In order to solve the technical problems, the technical scheme provided by the invention is as follows:
a method for detecting austenite grain size of 38MnS6 steel comprises the following steps:
(1) cutting a metallographic specimen;
(2) heating the metallographic specimen to a temperature higher than the austenitizing temperature, keeping the temperature for a certain time, taking out, and cooling in the air;
(3) fully removing an oxide layer on the detection surface of the metallographic specimen subjected to the heat treatment in the step (2), and then grinding and polishing the detection surface;
(4) eroding the polished metallographic specimen detection surface in the step (3) by using 2-4% nitric acid alcohol, and then washing and drying by using alcohol;
(5) and evaluating the grains of pearlite/pearlite groups according to the GB/T6394-2017 map comparison to obtain the austenite grain size grade.
In the step (2), the austenitizing temperature is 860 +/-10 ℃, and the heat preservation time is 1-1.5 h.
The specific operation of removing the oxide layer in the step (3) of the invention is as follows: and milling the detection surface of the metallographic sample by more than 3mm to fully remove the oxide layer.
In the step (4), the time for eroding the detection surface of the metallographic specimen by using 2-4% nitric acid alcohol is 5-10 seconds.
Compared with a common direct hardening method, the grain size detection method provided by the invention has the advantages that after a metallographic sample is processed according to the specified heating temperature and heat preservation time, air cooling is carried out to replace a water cooling or oil cooling processing method specified by the original standard; replacing supersaturated picric acid solution recommended by the original standard with 2-4% nitric acid alcohol in metallographic sample erosion; when a metallographic specimen is detected, a method for comparing the sizes of pearlite (agglomerate) grains by using a map is adopted to replace a method for comparing the sizes of martensite grain boundaries by using a map specified by the original standard; the method is simple and easy to operate, and can clearly display 38MnS6 austenite grain boundaries.
The invention principle of the invention is as follows:
after the steel is heated to a temperature above the austenitizing temperature and is kept warm for a certain time, the steel is in a complete austenite state, pearlite generally forms nuclei along an austenite crystal boundary or a triple grain boundary when the steel is cooled in air, cementite branches grow forwards, ferrite is formed among cementite branches in a coordinated mode, and therefore the steel grows into a pearlite field, a pair of cementite and ferrite crystal nuclei in different directions can be generated beside the pearlite field, and the other pearlite field can be formed. One pearlite area generally grows into only one austenite grain, and generally grows into a coronary pearlite colony; even when the supercooling degree is high, nuclei are formed in austenite grains, and at this time, two phases of pearlite grow from the inside of austenite to form pearlite nodules in a nearly spherical shape. The austenite grain size grade of the 38MnS6 steel can be obtained by evaluating the grains of the pearlite/pearlite clusters.
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in:
the method of the invention is adopted to lead the austenite grain size detection of the 38MnS6 steel to be simple and easy; the austenite grain size detection does not use picric acid solution for erosion, and simultaneously avoids the influence of the formula of the erosion liquid, the erosion temperature and the erosion time on the erosion effect; directly using 2-4% nitric acid alcohol solution, eroding for 5-10 seconds at normal temperature, washing with alcohol and drying, and has clear erosion structure and large display area.
Drawings
FIG. 1 shows austenite grains of example 1 and comparative example 1;
FIG. 2 is an austenite grain shown in example 2 and comparative example 2;
FIG. 3 is an austenite grain shown in example 3 and comparative example 3;
FIG. 4 shows austenite grains of example 4 and comparative example 4;
FIG. 5 is an austenite grain shown in example 5 and comparative example 5;
FIG. 6 shows austenite grains of example 6 and comparative example 6;
FIG. 7 shows austenite grains of example 7 and comparative example 7;
FIG. 8 shows austenite grains of example 8 and comparative example 8.
Detailed Description
The present invention will be described in further detail with reference to specific examples.
8 groups of 38MnS6 samples with different chemical compositions are taken, the compositions of the samples are shown in Table 1, two samples are taken from each group, one sample is treated by the method of the invention (examples 1-8), and the other sample is treated by a direct hardening method (comparative examples 1-8); the sample size was 15mm 20mm, with the 15mm face being the detection face.
Examples 1 to 8
A method for detecting austenite grain size of 38MnS6 steel comprises the following steps:
(1) cutting a metallographic specimen;
(2) heating the metallographic specimen to 860 ℃, preserving heat for 1-1.5h, taking out, and cooling in air;
(3) milling the detection surface of the metallographic sample subjected to heat treatment in the step (2) by more than 3mm, fully removing an oxide layer, and grinding and polishing the metallographic sample to 2.5 microns of grinding agent;
(4) corroding the polished metallographic specimen detection surface in the step (3) by using 2-4% nitric acid alcohol for 5-10s, and then washing and drying by using alcohol;
(5) the grain size of pearlite/pearlite colony is evaluated according to the GB/T6394-2017 map comparison and is taken as the austenite grain size grade of 38MnS6 steel.
Comparative examples 1 to 8
A method for detecting austenite grain size of 38MnS6 steel comprises the following steps:
(1) cutting a metallographic specimen;
(2) heating the metallographic sample to 860 ℃, preserving heat for 1h, taking out, cooling with water, and preserving heat at 230 +/-10 ℃ for 15 min;
(3) milling the detection surface of the metallographic sample subjected to heat treatment in the step (2) by more than 3mm, fully removing an oxide layer, and grinding and polishing the metallographic sample to 2.5 microns of grinding agent;
(4) eroding the detection surface of the metallographic sample ground and polished in the step (3) by using a supersaturated picric acid solution and a proper amount of white cat detergent for 3-5min, scrubbing the detection surface and drying;
(5) and (4) evaluating the austenite grain size grade according to a GB/T6394-2017 map comparison method.
The heat treatment system, the etchant, and the austenite grain size measurement results of each example and comparative example are shown in Table 2.
TABLE 1 chemical composition (%)
Figure DEST_PATH_IMAGE002
TABLE 2 Heat treatment system, etchant, and austenite grain size measurement results of examples and comparative examples
Figure DEST_PATH_IMAGE004
As can be seen from Table 2, the austenite grain size grade measured by the method of the present invention is consistent with that of the direct quench method. The method of the invention does not need to use picric acid solution for erosion, directly uses 2-4% nitric acid alcohol solution, erodes for 5-10 seconds at normal temperature, and is carried out by alcohol rinsing and blow-drying, the eroded tissue is clear, and the display area is large, as shown in figures 1-8.

Claims (4)

1. A method for detecting austenite grains of 38MnS6 steel is characterized by comprising the following steps:
(1) cutting a metallographic specimen;
(2) heating the metallographic specimen to a temperature higher than the austenitizing temperature, keeping the temperature for a certain time, taking out, and cooling in the air;
(3) fully removing an oxide layer on the detection surface of the metallographic sample subjected to the heat treatment in the step (2), and then carrying out grinding and polishing treatment on the detection surface;
(4) corroding the polished detection surface of the metallographic specimen in the step (3) by using 2-4% nitric acid alcohol, and then washing and drying by using alcohol;
(5) and evaluating the grains of the pearlite/pearlite colony according to the GB/T6394-2017 map comparison as the austenite grain size grade of the pearlite/pearlite colony.
2. The method for detecting the austenite grains of the 38MnS6 steel according to claim 1, wherein the austenitizing temperature in the step (2) is 860 +/-10 ℃, and the holding time is 1-1.5 h.
3. The method for detecting austenite grains of 38MnS6 steel according to claim 1, wherein the step (3) of removing the oxide layer comprises the following specific operations: and milling the detection surface of the metallographic sample by more than 3mm to fully remove the oxide layer.
4. The method for detecting austenite grains of 38MnS6 steel according to claim 1, wherein the time for eroding the detection surface of the metallographic specimen with 2-4% nitric alcohol in the step (4) is 5-10 seconds.
CN202210473646.6A 2022-04-29 2022-04-29 Method for detecting austenite grain size of 38MnS6 steel Pending CN114964983A (en)

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