A method for determining the austenite grain size of spring steel

Abstract

This invention provides a method for determining the austenite grain size of spring steel, specifically including the following steps: (1) heat-treating the metallographic sample of spring steel: after holding at a furnace temperature of 800℃~900℃ for 3~5 minutes, the metallographic sample is taken out of the furnace and air-cooled to room temperature; (2) polishing the polished surface of the metallographic sample again to remove the oxide layer on the surface of the metallographic sample, and then etching to reveal the ferrite network on the surface of the metallographic sample; (3) rating the austenite grain size according to GB / T 6394 method for determining the average grain size of metals by observing the austenite grain boundaries shown by the ferrite network. The technical solution of this invention can clearly display the austenite grains of spring steel and accurately determine the austenite grain size.

Description

Technical Field

[0001] This invention relates to the field of steel testing technology, and more particularly to a method for determining the austenite grain size of spring steel. Background Technology

[0002] Austenite grain size is one of the important technical indicators in automotive suspension spring steel and valve spring steel (including both wire rod and oil-quenched and tempered steel wire). Austenite grain size has a significant impact on the mechanical properties of spring steel, such as strength, shrinkage resistance, and toughness; that is, the larger the austenite grain size, the higher the strength and the better the shrinkage resistance. Therefore, some downstream users have specific technical requirements for the austenite grain size of wire rod and steel wire.

[0003] Since the microstructure of 55SiCr spring steel wire rod is pearlite and a very small amount of ferrite, and the microstructure of 55SiCr oil-quenched and tempered spring steel wire is tempered troostite, neither of these structures can directly show austenite grains. Therefore, the "GB / T 6394 Method for Determination of Average Grain Size of Metals" cannot be used for grain size rating.

[0004] Currently, there is no universally accepted, effective, simple, and easy-to-implement method for displaying and measuring austenite grains in spring steel. Prolonged heating and holding at temperatures above 800℃ (in the austenite region) in 55SiCr spring steel will result in a deep, fully decarburized structure on the steel surface. Therefore, the heat treatment regime (860℃±10℃, holding for at least 30 min) given by the ferrite network method in "GB / T6394 Method for Determination of Average Grain Size of Metals" cannot form a proeutectoid ferrite network along the austenite grain boundaries, thus making it impossible to determine the austenite grain size. Summary of the Invention

[0005] To address the technical problem mentioned above that existing technologies cannot determine the austenite grain size of 55SiCr spring steel, a simple and effective method for determining the austenite grain size of spring steel is provided. This method involves a short-term heating and holding of the 55SiCr spring steel during austenitization to induce slight decarburization on the steel surface (a phenomenon where the carbon content on the steel surface is slightly lower than the carbon content in the matrix during austenitization). This causes a network of pre-precipitated ferrite to form along the austenite grain boundaries on the steel surface, clearly revealing the austenite grains outlined by this network. The austenite grain size can then be determined using the intercept method in "GB / T 6394 Method for Determination of Average Grain Size of Metals".

[0006] The technical means employed in this invention are as follows:

[0007] A method for determining the austenite grain size of spring steel specifically includes the following steps:

[0008] (1) Heat treatment of the metallographic specimens of spring steel: After holding at 800℃~900℃ for 3~5 minutes, take the metallographic specimens out of the furnace and air cool them to room temperature.

[0009] (2) Polish the polished surface of the metallographic sample again to remove the oxide layer on the surface of the metallographic sample, and then etch it to make the ferrite network appear on the surface of the metallographic sample.

[0010] (3) The austenite grain size is rated according to GB / T 6394 Method for Determination of Average Grain Size of Metals, based on the austenite grain boundaries shown by the ferrite mesh.

[0011] Furthermore, the spring steel is 55SiCr spring steel wire rod or 55SiCr oil-quenched and tempered spring steel wire.

[0012] Furthermore, the preparation process of the metallographic sample includes: obtaining a transverse metallographic sample by cutting, and then grinding and polishing it.

[0013] Furthermore, in step (1), the heat treatment is carried out in an air atmosphere.

[0014] Furthermore, in step (1), after the furnace temperature is raised to the set temperature, the polished surface of the metallographic sample is placed into the furnace with the surface facing up for heat preservation.

[0015] Furthermore, in step (2), the metallographic sample is etched using a nitric acid alcohol solution.

[0016] Further, in step (3), the austenite grain size is rated according to the intercept method in GB / T 6394 Method for Determination of Average Grain Size of Metals.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] The method for determining the austenite grain size of spring steel provided by the present invention involves heating and holding 55SiCr spring steel for a short time to induce slight decarburization on the steel surface. After air cooling to room temperature, the austenite grains outlined by the precipitated ferrite network can be clearly seen on the steel surface, thereby accurately determining the austenite grain size.

[0019] Based on the above reasons, this invention can be widely promoted in the field of steel testing and analysis. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a typical original microstructure of 55SiCr spring steel wire rod.

[0022] Figure 2 The microstructure of a 55SiCr spring steel wire rod produced by a certain factory after being treated using the method described in Example 1.

[0023] Figure 3 The microstructure of a 55SiCr spring steel wire rod produced by a certain factory after being treated using the method described in Example 2.

[0024] Figure 4 This is a typical original microstructure of 55SiCr oil-quenched and tempered spring steel wire.

[0025] Figure 5 The microstructure of 55SiCr oil-quenched and tempered spring steel wire produced by a certain factory after being treated using the method described in Example 3.

[0026] Figure 6 The microstructure of 55SiCr oil-quenched and tempered spring steel wire produced by a certain factory after being treated using the method described in Example 4.

[0027] Figure 7 The microstructure of 55SiCr oil-quenched and tempered spring steel wire produced by a certain factory after being treated using the method described in Example 5. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Given that the heat treatment regime (860℃±10℃, holding time of at least 30 min) given by the ferrite mesh method in "GB / T 6394 Method for Determination of Average Grain Size of Metals" has a long holding time, a deep decarburized layer will form on the surface of 55SiCr steel, making it impossible to display austenite grain boundaries through the ferrite mesh, and thus the austenite grain size cannot be determined by the ferrite mesh method, this invention provides a method for determining the austenite grain size of spring steel, specifically including the following steps:

[0030] (1) Heat treatment of the metallographic specimens of spring steel: After holding at 800℃~900℃ for 3~5 minutes, take the metallographic specimens out of the furnace and air cool them to room temperature.

[0031] (2) Polish the polished surface of the metallographic sample again to remove the oxide layer on the surface of the metallographic sample, and then etch it to make the ferrite network appear on the surface of the metallographic sample.

[0032] (3) The austenite grain size is rated by the intercept point method in GB / T 6394 "Method for Determination of Average Grain Size of Metals" based on the austenite grain boundaries shown by the ferrite mesh.

[0033] Furthermore, the spring steel is 55SiCr spring steel wire rod or 55SiCr oil-quenched and tempered spring steel wire.

[0034] Furthermore, the preparation process of the metallographic sample includes: obtaining a transverse metallographic sample by cutting, and then grinding and polishing it.

[0035] Furthermore, in step (1), the heat treatment is carried out in an air atmosphere.

[0036] Furthermore, in step (1), after the furnace temperature is raised to the set temperature, the polished surface of the metallographic sample is placed into the furnace with the surface facing up for heat preservation.

[0037] Furthermore, in step (2), the metallographic sample is etched using a nitric acid alcohol solution.

[0038] Further, in step (3), the austenite grain size is rated according to the intercept method in GB / T 6394 Method for Determination of Average Grain Size of Metals.

[0039] Because the room temperature microstructure of 55SiCr spring steel wire rod consists of pearlite and a very small amount of ferrite precipitated along the grain boundaries, but without forming a ferrite network, the austenite grain size cannot be determined; the microstructure of 55SiCr oil-quenched and tempered spring steel wire is tempered troostite, which also makes it impossible to display and determine austenite grains; and the composition of 55SiCr steel is close to that of eutectoid steel. The technical solution provided by this invention aims to generate a proeutectoid ferrite network precipitated along the austenite grain boundaries in spring steels of the above two microstructures by re-austenitizing the steel (the austenitizing temperature should not be too high). To prevent grain growth, a short-term heat treatment is used to induce slight decarburization on the steel surface (a phenomenon where the carbon content on the steel surface is slightly reduced compared to the carbon content in the matrix during austenitization). Based on the lever principle in the iron-carbon phase diagram, the amount of pre-precipitated ferrite on the steel surface will increase compared to the matrix under equilibrium conditions. Due to the increased pre-precipitated ferrite content on the steel surface, after the steel is austenitized and air-cooled to room temperature, a pre-precipitated ferrite network will form along the austenite grain boundaries on the steel surface. Based on the above principle, the austenite grains outlined by the pre-precipitated ferrite network can be clearly seen. Then, the austenite grain size of the steel is determined according to the intercept method in "GB / T 6394 Method for Determination of Average Grain Size of Metals".

[0040] The following examples illustrate the method for determining the austenite grain size of spring steel and its technical effects provided by this invention.

[0041] Example 1

[0042] In the method for determining the austenitic grain size of spring steel provided in this embodiment, the spring steel is 55SiCr spring steel wire rod produced by a steel plant on a large scale. Based on the technical solution provided by this invention, the only difference is that in step (1), the metallographic sample of spring steel is heat-treated: after the furnace temperature is raised to 800℃, it is kept at 800℃ for 5 minutes, and the metallographic sample is taken out of the furnace and air-cooled to room temperature.

[0043] The microstructure of 55SiCr spring steel wire rod treated using the method provided in this embodiment is as follows: Figure 2 As shown.

[0044] Example 2

[0045] The only difference between this embodiment and embodiment 1 is that in step (1), the metallographic sample of spring steel is heat-treated: after the furnace temperature is raised to 860°C, it is kept at that temperature for 4 minutes, and then the metallographic sample is taken out of the furnace and air-cooled to room temperature.

[0046] The microstructure of 55SiCr spring steel wire rod treated using the method provided in this embodiment is as follows: Figure 3 As shown.

[0047] Figure 1 This is a typical original microstructure of 55SiCr spring steel wire rod produced on a large industrial scale in a steel mill. The microstructure consists of pearlite and a very small amount of ferrite. Figure 1It can be seen that the austenite grains cannot be clearly displayed and the grain size cannot be rated.

[0048] Depend on Figure 2 and Figure 3 The austenite grains outlined by the ferrite mesh can be clearly seen. The austenite grain size is rated according to the intercept method in "GB / T6394 Method for Determination of Average Grain Size of Metals". The austenite grain size corresponding to the metallographic sample of Example 1 is grade 8, and the austenite grain size corresponding to Example 2 is grade 7.

[0049] Example 3

[0050] In the method for determining the austenitic grain size of spring steel provided in this embodiment, the spring steel is 55SiCr oil-quenched and tempered spring steel wire produced by a downstream user of a steel plant. Based on the technical solution provided by this invention, the only difference is that in step (1), the metallographic sample of spring steel is heat-treated: after the furnace temperature is raised to 900℃, it is kept at 900℃ for 3 minutes, and the metallographic sample is taken out of the furnace and air-cooled to room temperature.

[0051] The microstructure of 55SiCr oil-quenched and tempered spring steel wire treated using the method provided in this embodiment is as follows: Figure 5 As shown.

[0052] Example 4

[0053] The difference between this embodiment and embodiment 1 is only in step (1) heat treatment of the metallographic sample of spring steel: after the furnace temperature is raised to 870°C, it is kept at that temperature for 4 minutes, and the metallographic sample is taken out of the furnace and air-cooled to room temperature.

[0054] The microstructure of 55SiCr oil-quenched and tempered spring steel wire treated using the method provided in this embodiment is as follows: Figure 6 As shown.

[0055] Example 5

[0056] The difference between this embodiment and embodiment 1 is only in step (1) heat treatment of the metallographic sample of spring steel: after the furnace temperature is raised to 850°C, it is kept at that temperature for 4 minutes, and the metallographic sample is taken out of the furnace and air-cooled to room temperature.

[0057] The microstructure of 55SiCr oil-quenched and tempered spring steel wire treated using the method provided in this embodiment is as follows: Figure 7 As shown.

[0058] Figure 4 The typical original microstructure of 55SiCr oil-quenched and tempered spring steel wire produced for downstream users of steel mills shows that the structure is tempered martensite, and it is impossible to directly display austenite grains and perform grain size rating.

[0059] Depend on Figure 5-7The microstructure shows that after processing with the method provided by the present invention, the austenite grains outlined by the proeutectoid ferrite network can be clearly displayed. According to the intercept method in "GB / T 6394 Method for Determination of Average Grain Size of Metals", the austenite grain size corresponding to Examples 3-5 is grade 8, grade 9 and grade 10, respectively.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the austenite grain size of spring steel, characterized in that, Specifically, the following steps are included: (1) Heat treatment of the metallographic specimens of spring steel: After holding at a furnace temperature of 800℃~900℃ for 3~5 minutes, take the metallographic specimens out of the furnace and air cool them to room temperature. (2) Polish the polished surface of the metallographic sample again to remove the oxide layer on the surface of the metallographic sample, and then etch it to make the ferrite network appear on the surface of the metallographic sample. (3) The austenite grain size is rated according to GB / T6394 Method for Determination of Average Crystallinity of Metals by the austenite grain boundaries shown by the ferrite mesh. The spring steel is 55SiCr spring steel wire rod or 55SiCr oil-quenched and tempered spring steel wire. The preparation process of the metallographic sample includes: obtaining a transverse metallographic sample by cutting, and then grinding and polishing.

2. The method for determining the austenite grain size of spring steel according to claim 1, characterized in that, In step (1), the heat treatment is carried out in an air atmosphere.

3. The method for determining the austenite grain size of spring steel according to claim 1, characterized in that, In step (1), after the furnace temperature is raised to the set temperature, the polished surface of the metallographic sample is placed in the furnace with the surface facing up for heat preservation.

4. The method for determining the austenite grain size of spring steel according to claim 1, characterized in that, In step (2), the metallographic sample is etched using a nitric acid alcohol solution.

5. The method for determining the austenite grain size of spring steel according to claim 1, characterized in that, In step (3), the austenite grain size is rated according to the intercept method in GB / T6394 Method for Determination of Average Grain Size of Metals.

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