Methods for determining laser surface hardening effect and hardened layer depth of ductile iron mold

Abstract

The present application belongs to the field of laser strengthening, and specifically discloses methods for determining a laser surface hardening effect and a hardened layer depth of a ductile iron mold. The hardening effect determination method is: performing phase composition analysis on a surface of a strengthened ductile iron mold, and if, among phases other than graphite, martensite content accounts for more than 95% and residual austenite content accounts for less than 5%, the surface hardening effect of the ductile iron mold is determined to meet requirements. The present application proposes using surface phase composition as an evaluation index for the hardening effect, and optimizes the specific evaluation criteria, which can effectively simplify the process of evaluation of the surface hardening effect of the ductile iron mold and provide timely feedback for optimizing process design, and has the advantages of a simple determination process and accurate determination results, thereby having relatively strong industrial application value.

Description

[Corrected 28.03.2025 according to Rule 91] Method for determining the effect of laser surface hardening and the depth of hardened layer of ductile iron mold

Technical field

[0001] [Corrected 28.03.2025 according to Rule 91] The present application belongs to the field of laser hardening and, more specifically, to a method for determining the effect of laser surface hardening of ductile iron moulds and the depth of the hardened layer. [Background Technology]

[0002] Ductile iron (Ductile iron) is a high-strength cast iron material with comprehensive properties similar to those of steel. It is widely used in the manufacture of parts and large molds for automobiles, machinery, engineering machinery, and rail transportation. In particular, high-strength ductile iron is often used as a raw material for automotive panels due to its low cost, ease of processing, and the self-lubricating graphite in its structure, which effectively reduces friction during production. Furthermore, laser surface treatment at key mold locations can further extend mold life.

[0003] [Corrected 28.03.2025, per Rule 91] Hardness testing is commonly used to determine the depth of the laser hardened layer and the hardened layer. However, this method has significant limitations for determining the depth of the laser hardened layer on ductile iron. This is because ductile iron contains a large amount of spherical graphite and some ferrite in the hardened layer. When a hardness indentation strikes graphite or ferrite, it cannot accurately reflect changes in the hardened layer depth. Furthermore, metallographic methods cannot reveal significant changes in the hardened layer. Therefore, using hardness and metallographic methods to determine the effectiveness of laser hardening on ductile iron is not ideal. [Summary of the invention]

[0004] [Corrected 28.03.2025 according to Rule 91] In response to the defects of the prior art, the present application provides a method for determining the effect of laser surface hardening of ductile iron molds, which aims to solve the problem of complex determination processes of the existing hardness method and metallographic method.

[0005] [Corrected 28.03.2025 according to Rule 91] To achieve the above-mentioned purpose, the present application provides a method for determining the effect of laser surface hardening of a ductile iron mold. The determination method is as follows: a phase composition analysis is performed on the surface of the strengthened ductile iron mold. If, except for graphite, the proportion of martensite in other phases is more than 95% and the proportion of retained austenite is less than 5%, then it is determined that the surface hardening effect of the ductile iron mold meets the requirements.

[0006] [Corrected 28.03.2025 according to Rule 91] Compared with the prior art, the above technical solution conceived by the present application only needs to analyze the surface phase composition of the ductile iron mold to directly determine whether its surface hardening effect meets the requirements, which can achieve the beneficial effects of simple judgment process and accurate judgment results.

[0007] [Corrected 28.03.2025 according to Rule 91] As a further preference, the phase composition analysis of the surface of the strengthened ductile iron mold is carried out using SEM and / or EBSD.

[0008] As a further preferred embodiment, a phase composition analysis is performed on the center position of the laser spot.

[0009] [Corrected 28.03.2025 according to Rule 91] Another object of the present application is to provide a method for determining the depth of the hardened layer of a ductile iron mold, aiming to solve the problem that the hardness method cannot correctly reflect the change in the depth of the hardened layer and the metallographic method cannot show significant changes in the depth of the hardened layer.

[0010] [Corrected 28.03.2025 according to Rule 91] To achieve the above-mentioned purpose, the present application provides a method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening, and the determination method is: performing phase composition analysis on the strengthened ductile iron mold layer by layer along the depth direction, and taking the end of the martensite + austenite + Fe3C composite layer formed on the basis of the original pearlite as the sign of the end of the hardened layer, and then calculating the depth of the hardened layer according to the distance from the surface to the end position of the hardened layer.

[0011] Compared with the prior art, the above technical solution conceived by the present application can effectively improve the accuracy of determining the depth of the hardened layer because the present application adopts phase composition as a sign of the end of the hardened layer.

[0012] [Corrected 28.03.2025 according to Rule 91] As a further preferred embodiment, the strengthened ductile iron mold is longitudinally sectioned, and then the phase composition analysis is performed layer by layer on the section.

[0013] As a further preferred embodiment, a longitudinal section is performed with the center position of the laser spot as the section line.

[0014] [Corrected 28.03.2025 according to Rule 91] As a further preference, the phase composition analysis of the strengthened ductile iron mold is carried out using SEM and / or EBSD.

[0015] [Corrected 28.03.2025 according to Rule 91] As a further preferred embodiment, before calculating the hardened layer depth, first observe whether the surface of the ductile iron mold has melted. If there are no protrusions and significant changes in roughness on the surface, it is preliminarily determined that the surface has not melted, and the calculation of the hardened layer depth is continued.

[0016] [Corrected 28.03.2025 according to Rule 91] As a further preferred embodiment, before calculating the hardened layer depth, it is first determined whether the surface of the ductile iron mold has melted based on the surface phase composition. If there is no ledeburite structure in the surface phase, it is determined that it has not melted, and the hardened layer depth calculation is continued.

[0017] In general, the above technical solutions conceived by this application have the following technical advantages compared with the existing technologies:

[0018] [Corrected 28.03.2025 according to Rule 91] 1. This application proposes using surface phase composition as an evaluation index for hardening effect and optimizes the specific evaluation criteria. This can effectively simplify the evaluation process for the surface hardening effect of ductile iron molds and provide timely feedback to optimize process design. It has the advantages of a simple evaluation process and accurate results, and has strong industrial application value.

[0019] [Corrected 28.03.2025 according to Rule 91] 2. At the same time, this application also proposes to use phase composition as a marker for the end of the hardened layer, and then calculate the depth of the hardened layer based on the distance from the unmelted surface to the end position of the hardened layer, which effectively solves the ambiguity between hardness method and metallographic method in determining the depth of the hardened layer of ductile iron molds;

[0020] 3. In addition, this application also optimizes the method for determining the position of the unmelted surface, and proposes to use phase composition as the basis for determination, which can further improve the accuracy of the calculation of the hardened layer depth, and the starting position and ending position of the hardened layer depth both use tissue composition as the determination standard, which has the advantages of simple operation and simplified process.

Brief Description of the Drawings

[0021] [Corrected 28.03.2025 according to Rule 91] FIG1 is a schematic diagram of a method for determining the depth of a hardened layer of a ductile iron mold by laser surface strengthening provided in an embodiment of the present application;

[0022] FIG2 is a microstructure characteristic diagram of the melt layer provided in an embodiment of the present application, wherein (a) is a SEM image 50 μm from the surface, (b) is an EBSD phase distribution diagram 50 μm from the surface, (c) is a SEM image 300 μm from the surface, and (d) is an EBSD phase distribution diagram 300 μm from the surface;

[0023] FIG3 is a microstructure characteristic diagram of the transition zone between the melt layer and the hardened layer provided in an embodiment of the present application, wherein (a) is a SEM image 500 μm from the surface, and (b) is an EBSD phase distribution diagram 500 μm from the surface;

[0024] FIG4 is a microstructure characteristic diagram of a first area of ​​a hardened layer provided in an embodiment of the present application, wherein (a) is a SEM image 750 μm from the surface, (b) is an EBSD phase distribution diagram 750 μm from the surface, and (c) is an EBSD bcc phase distribution diagram 750 μm from the surface;

[0025] FIG5 is a microstructure diagram of the second zone of the hardened layer provided in an embodiment of the present application, wherein (a) is a SEM image 1500 μm from the surface, (b) is an EBSD phase distribution map 1500 μm from the surface, and (c) is an EBSD bcc phase distribution map 1500 μm from the surface;

[0026] FIG6 is a microstructure characteristic diagram of the transition zone between the hardened layer and the matrix provided in an embodiment of the present application, wherein (a) is a SEM image 1900 μm from the surface, (b) is an EBSD phase distribution map 1900 μm from the surface, and (c) is an EBSD bcc phase distribution map and a local area phase distribution map 1900 μm from the surface;

[0027] FIG7 is a diagram showing the structural characteristics of the matrix provided in an embodiment of the present application. [Specific implementation method]

[0028] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0029] Throughout the description of this application, it should be understood that references to "one embodiment," "one embodiment," "an example," or similar language indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present application. Therefore, appearances of the phrase "in one embodiment" and similar language throughout this specification may, but do not necessarily, refer to the same embodiment.

[0030] [Corrected 28.03.2025 according to Rule 91] An embodiment of the present application provides a method for determining the effect of laser surface hardening of a ductile iron mold, the determination method being: performing a phase composition analysis on the surface of the strengthened ductile iron mold, if, except for graphite, the proportion of martensite in other phases is more than 95%, and the proportion of retained austenite is less than 5%, then it is determined that the surface hardening effect of the ductile iron mold meets the requirements and is close to the best. Ideally, the phases on the surface of the strengthened ductile iron mold, except for graphite, are all martensite structures. In actual circumstances, the proportions of martensite and retained austenite can be further optimized according to needs.

[0031] [Corrected 28.03.2025 according to Rule 91] Compared with the prior art, the above technical solution conceived by the present application only needs to analyze the surface structure of the ductile iron mold to directly determine whether its surface hardening effect meets the requirements, which can achieve the beneficial effects of simple judgment process and accurate judgment results.

[0032] [Corrected 28.03.2025 according to Rule 91] Furthermore, the phase composition analysis of the surface of the strengthened ductile iron mold is carried out by SEM and / or EBSD. Preferably, the phase composition analysis of the surface of the strengthened ductile iron mold is carried out by EBSD, which can more clearly and precisely determine the type and content of each tissue compared with SEM.

[0033] [Corrected 28.03.2025 according to Rule 91] Furthermore, the phase composition analysis is performed on the center position of the laser spot. The phase composition analysis can be performed directly on the surface of the ductile iron mold, or a longitudinal section can be performed along the center position of the laser spot, and then the phase composition analysis is performed on the uppermost layer, which facilitates the subsequent measurement of the hardened layer depth.

[0034] [Corrected 28.03.2025 according to Rule 91] As shown in FIG1 , another embodiment of the present application provides a method for determining the depth of the hardened layer of a ductile iron mold laser surface strengthened. The determination method is as follows: performing a phase composition analysis on the strengthened ductile iron mold along the depth direction, and taking the end of the martensite + austenite + Fe3C composite layer formed on the basis of the original pearlite as a sign of the end of the hardened layer, and then calculating the depth of the hardened layer based on the distance from the surface to the end position of the hardened layer.

[0035] [Corrected 28.03.2025 according to Rule 91] The above technical solution conceived by the present application, compared with the prior art, can effectively improve the accuracy of determining the depth of the hardened layer by using phase composition as a sign of the end of the hardened layer, and solve the problem of inaccuracy of the hardness method and metallographic method in determining the depth of the hardened layer of ductile iron materials.

[0036] [Corrected 28.03.2025 according to Rule 91] Furthermore, the strengthened ductile iron mold was longitudinally sectioned with the center of the laser spot as the section line, and then the microstructure composition analysis was performed on the section. Since the energy at the center of the laser spot is the highest, the center of the laser spot is used as the section line to obtain the maximum depth of the hardened layer.

[0037] [Corrected 28.03.2025 in accordance with Regulation 91] Furthermore, the microstructure of the strengthened ductile iron mold was analyzed using SEM and / or EBSD. Because both SEM and EBSD methods can perform rapid, continuous scanning, they offer not only increased efficiency but also more accurate results compared to dot-scan analysis.

[0038] [Corrected 28.03.2025 in accordance with Rule 91] Furthermore, considering that surface melting of ductile iron molds significantly affects their surface roughness, and that their poor surface quality makes them unsuitable for final heat treatment, it is necessary to determine whether the surface has melted before calculating the hardened layer depth. Visual inspection can be used to determine whether the surface of the ductile iron mold has melted. If there are no protrusions or significant changes in surface roughness, it is preliminarily determined that the surface has not melted, and the calculation of the hardened layer depth can be continued. More preferably, the phase composition of the surface can be used to determine whether the surface of the ductile iron mold has melted. If the surface phases do not contain ledeburite, it is determined that the surface has not melted, and the calculation of the hardened layer depth can be continued. Using phase composition as the basis for judgment not only effectively solves the problem of inaccurate visual observation, but also eliminates the need for any additional testing, effectively improving the efficiency of hardened layer depth detection and reducing testing costs.

[0039] [Corrected 28.03.2025 according to Rule 91] The method for determining the laser surface hardening effect and hardened layer depth of ductile iron molds provided in this application is mainly based on the following principles. After ductile iron is melted by laser surface hardening, the changes from the surface to the matrix are divided into five stages, as follows:

[0040] Stage 1: Melting Layer

[0041] After the melt zone undergoes self-quenching, the graphite completely dissolves due to the high surface temperature. As shown in Figure 2, the main structural feature of the melt layer is a structure with some acicular martensite distributed within the austenite in the ledeburite.

[0042] Stage 2: Transition zone between melt layer and hardened layer

[0043] As shown in Figure 3, the amount of ledeburite gradually decreases in the transition zone between the melt layer and the hardened layer. In this region, due to the laser action, graphite partially dissolves, and the original ferrite and pearlite completely transform into austenite. The high carbon content in the austenite reduces the start and end temperatures of the martensite transformation, retaining a large amount of retained austenite. Consequently, a transition layer composed primarily of retained austenite and martensite is generated during the self-quenching process. Near the hardened layer, as the quenching temperature gradually decreases, less graphite dissolves, and the start and end temperatures of the martensite transformation increase, leading to a gradual increase in martensite.

[0044] Stage 3: Hardened layer

[0045] The hardened layer can be divided into two areas, as shown in Figure 4. The first area of ​​the hardened layer is mainly composed of graphite, a large amount of martensite and a small amount of retained austenite (except for graphite, martensite accounts for more than 95% and retained austenite accounts for less than 5%). This layer has excellent hardness. Therefore, the organizational characteristics of graphite, a large amount of martensite and a small amount of retained austenite represent the sign that the surface layer can obtain close to the highest hardness. Gradually transitioning to the interior of the material, as shown in Figure 5, the second area of ​​the hardened layer retains a large amount of ferrite, and the hardness gradually decreases. At this time, the martensite is mainly formed after the original pearlite structure is transformed into austenite after quenching.

[0046] Stage 4: Transition zone between hardened layer and matrix

[0047] As shown in Figure 6, the transition zone near the hardened layer is a composite structure of graphite, ferrite, martensite + austenite + Fe3C. Considering that the ledeburite after quenching is also a composite structure of martensite + austenite + Fe3C, but its micromorphology is fishbone-shaped, and the martensite + austenite + Fe3C in the hardened layer near the matrix transition zone is a composite lamellar structure formed on the basis of original pearlite. Therefore, the end of the martensite + austenite + Fe3C composite lamellar layer formed on the basis of original pearlite is used as the sign of the end of the hardened layer.

[0048] Stage 5: Matrix

[0049] As shown in Figure 7, the matrix is ​​composed of graphite, ferrite, and pearlite.

[0050] [Corrected 28.03.2025 according to Rule 91] Based on the above principles, this application uses the surface structure of graphite, martensite, and a small amount of retained austenite, where, in addition to graphite, martensite accounts for more than 95% and retained austenite accounts for less than 5%, as an evaluation standard for determining whether the laser quenching process used has achieved a good surface hardening effect. This method can directly judge the process effect from the surface structure, thereby providing a key reference basis for process optimization. At the same time, this application also uses the end of the martensite + austenite + Fe3C composite layer formed on the basis of the original pearlite as a sign to determine the end of the hardened layer. The total hardened layer depth is the distance from the surface to the end position of the hardened layer. This method can solve the problem of ambiguity in determining the hardened layer depth of ductile iron materials using hardness method and metallographic method.

[0051] It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. [Corrected 28.03.2025 in accordance with Rule 91] A method for determining the effect of laser surface hardening of ductile iron molds, characterized in that: The determination method is: performing a phase composition analysis on the surface of the strengthened ductile iron mold. If, except for graphite, the martensite accounts for more than 95% of the other phases and the retained austenite accounts for less than 5%, it is determined that the surface hardening effect of the ductile iron mold meets the requirements.

2. [Corrected 28.03.2025 in accordance with Rule 91] The method for determining the effect of laser surface hardening of ductile iron molds as claimed in claim 1, characterized in that: The phase composition of the surface of the strengthened ductile iron mold was analyzed using SEM and / or EBSD.

3. [Corrected 28.03.2025 according to Rule 91] A method for determining the effect of laser surface hardening of a ductile iron mold as claimed in claim 1 or 2, characterized in that: Phase composition analysis is performed at the center of the laser spot.

4. [Corrected 28.03.2025 in accordance with Rule 91] A method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening, characterized in that: The determination method is as follows: the phase composition of the strengthened ductile iron mold is analyzed layer by layer along the depth direction, the end of the martensite + austenite + Fe3C composite layer formed on the basis of the original pearlite is used as the sign of the end of the hardened layer, and the depth of the hardened layer is calculated based on the distance from the surface to the end position of the hardened layer.

5. [Corrected 28.03.2025 according to Rule 91] The method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening according to claim 4, characterized in that: The strengthened ductile iron mold was longitudinally sectioned, and then the phase composition of the section was analyzed layer by layer.

6. [Corrected 28.03.2025 according to Rule 91] The method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening according to claim 5, characterized in that: The center position of the laser spot is used as the section line for longitudinal sectioning.

7. [Corrected 28.03.2025 according to Rule 91] The method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening according to claim 4, characterized in that: The phase composition of the strengthened ductile iron mold was analyzed using SEM and / or EBSD.

8. [Corrected 28.03.2025 according to Rule 91] A method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening according to any one of claims 4 to 7, characterized in that: Before calculating the depth of the hardened layer, first observe whether the surface of the ductile iron mold has melted. If there are no protrusions or significant changes in roughness on the surface, it is preliminarily determined that the surface has not melted, and the calculation of the hardened layer depth can be continued.

9. [Corrected 28.03.2025 according to Rule 91] A method for determining the depth of the hardened layer of a ductile iron mold by laser surface strengthening according to any one of claims 4 to 7, characterized in that: Before calculating the hardened layer depth, first determine whether the surface of the ductile iron mold has melted based on the surface phase composition. If there is no ledeburite structure in the surface phase, it is determined that it has not melted, and the hardened layer depth calculation continues.

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