Homogenization evaluation method for hardenability of Ti-containing carburizing steel

Abstract

The invention relates to the field of materials and processes for automobile parts, in particular to a homogenization evaluation method for hardenability of Ti-containing carburizing steel and application. According to different specifications and different looseness and segregation levels of the round steel, sampling regulation is carried out on the center part and the center part of the section of the Ti-containing carburizing steel round steel, and the hardenability and the hardenability bandwidth of the carburizing steel can be more truly reflected. When the hardness of the tail end hardenability sample is detected, two surfaces equidistant from the circle center of the section of the round steel are detected, a telecentric surface far away from the circle center of the section of the round steel and a proximal surface close to the circle center of the section of the round steel are detected, and compared with the current national standard, the detection result is more scientific and accurate. The method can be applied to almost all carburized steel bars with large component fluctuation, the uniformity is truly reflected, technicians can select raw materials with proper hardenability bandwidth to be applied to shaft gear parts with different size characteristics and using performance requirements, and the application prospect is very wide.

Description

Technical Field

[0001] This invention relates to the field of materials and processes for automotive parts, and specifically to a method for evaluating the homogenization of hardenability of Ti-containing carburized steel. Background Technology

[0002] Since the first automobile production in the 1950s, my country's carburizing steel has adopted the grades and performance indicators of the ГOCT standard. The most widely used steel was 18CrMnTiH, which was changed to 20CrMnTiH in the 1980s and has remained in use ever since. Titanium accounts for 0.623% of the Earth's crust, ranking ninth after magnesium, and is higher than common elements such as manganese, copper, nickel, and lead. Industrially used titanium-bearing ores mainly include ilmenite, anatase, and rutile. my country is the country with the richest titanium resources, mainly produced in Panzhihua, Sichuan. In terms of the cost of microalloying steel, ilmenite is about one-tenth that of niobium iron and one-fifth that of vanadium iron, thus developing titanium microalloyed steel has a significant cost advantage. Furthermore, due to its advantages such as uniform and fine grains, low grain growth tendency, good carburizing and quenching performance, and mature and reliable processing properties, titanium steel has been widely used in automobiles, agricultural machinery, and other products in my country. In recent years, with the rapid development of the automobile industry, my country has also vigorously developed and produced gear steels commonly used in advanced countries, including Cr-Mo, Cr-Mn, Cr-Mn-B, Cr-Ni, Cr-Ni-Mo, and Cr-Ni-Mo-Nb series. However, due to smelting cost issues, 20CrMnTiH remains the main carburizing steel used in my country, accounting for approximately 30% or more of the total carburizing steel production.

[0003] However, according to the national standard GB / T 5216 "Structural Steel with Guaranteed Hardenability," the Ti content in Ti-containing carburizing steels such as 20CrMnTiH and 20MnTiBH is typically controlled above 0.04%. Ti readily combines with N to form high-melting-point, large-particle liquid precipitated TiN. Influenced by factors such as the Ti addition process and method, TiN easily forms banded agglomerations, leading to decreased part reliability. Furthermore, austenitic Ti readily combines with C to form TiC, resulting in a reduction in dissolved carbon atoms in local austenite regions, causing a decrease in hardenability. These particles act as nucleation sites for austenite decomposition, reducing austenite stability and accelerating austenite transformation, further decreasing hardenability. This leads to uneven hardenability in the material, increased deformation during heat treatment, and difficulty in controlling the process. Therefore, the problem of homogenizing the hardenability of Ti-containing carburizing steel has become a major factor restricting its development into high-quality gear steel and is one of the key technical challenges urgently needing to be solved in the application of carburizing steel in my country.

[0004] Hardenability refers to the ability of steel to undergo quenching. It can also be considered as the characteristic that determines the hardening depth and hardness distribution of steel under specified conditions, representing the ability to quench austenite into martensite during quenching. Hardenability bandwidth refers to the range of hardness variation measured at a certain distance from the quenching end face during an end-quenching test of a batch of materials. Hardenability and hardenability bandwidth are important indicators for carburized gear steel. The size of the hardenability bandwidth greatly affects the heat treatment deformation of parts. The narrower the hardenability bandwidth and the smaller the dispersion, the more beneficial it is for gear machining and improving its meshing accuracy. In recent years, with the introduction and adoption of advanced international standards, major OEMs have raised higher requirements for the quality of Ti-containing carburized steel products. Hardenability evaluation has become a key inspection item to ensure product quality. Regarding the control range of hardenability bandwidth for carburized steel, domestic requirements generally require a hardenability bandwidth ≤ (5~7) HRC, while international requirements require a hardenability bandwidth ≤ 4 HRC.

[0005] The end-hardenability test is a commonly used method for determining the hardenability of steel materials. Major steel mills and main engine manufacturers in China generally adopt the testing method in the national standard GB / T 225-2006, "End-hardening Test Method for Hardenability of Steel," to evaluate the hardenability and hardenability bandwidth of carburized steel. The specific procedure involves heating a cylindrical specimen to austenitize it at a specified temperature, holding it at that temperature for a specified time, then quenching it with water starting from one end of the specimen. Hardness is measured at two points or at specified points along the length of the specimen, and the hardenability and hardenability bandwidth are determined based on the change in hardness value. However, after testing the hardenability of carburized steel according to the GB / T 225-2006 method, Ti-containing carburized steel with a hardenability bandwidth ≤4HRC still exhibits existing production problems such as uneven material composition distribution and excessive deformation of parts after heat treatment. Therefore, the test method in GB / T 225-2006 cannot accurately measure the hardenability and hardenability bandwidth of Ti-containing carburized steel, thus making the hardenability test results of this method lose the authenticity of the evaluation of the homogenization of Ti-containing carburized steel.

[0006] Besides the aforementioned end-hardenability test method for determining the hardenability of metallic materials, other methods for determining the hardenability of materials have emerged in recent years. For example, the invention patent application CN101477073A, "Method for Rapidly Detecting the Hardenability of Aluminum Alloys Using Electrical Conductivity," takes large-section aluminum alloy plates or forgings as the research object and uses the material's electrical conductivity to rapidly determine its hardenability. The main basis is that "the electrical conductivity of solution-aged aluminum alloys is closely related to the degree of solid solution in the alloy. The more complete the solid solution in the alloy, the more severe the lattice distortion, and the greater the obstruction to electron movement, which is reflected in a lower electrical conductivity value; conversely, when the degree of solid solution in the alloy is smaller, the lattice distortion..." The smaller the crystal density, the less resistance electrons encounter during movement within the crystal lattice, resulting in higher electrical conductivity. When an alloy has good hardenability, the overall solid solution degree is similar during quenching, so the electrical conductivity changes little across different parts of the alloy. Conversely, if the alloy has poor hardenability, the ability to retain solid solution in the core is weaker during quenching, leading to less lattice distortion, less resistance to electron movement, and thus higher electrical conductivity. This characteristic of electrical conductivity variation can be used to quickly and effectively detect the hardenability of aluminum alloys. However, this method of determining hardenability using material conductivity is not suitable for Ti-containing carburized steel. This is because Ti-containing carburized steel does not undergo solid solution aging strengthening during quenching, and therefore there is no difference in electrical conductivity. If the material's electrical conductivity is used to determine hardenability, the phenomenon of increasing conductivity with increasing distance from the quenching end will not be observed, making this method unsuitable for determining the hardenability of Ti-containing carburized steel. The invention patent application CN116397078A, entitled "A Method for Testing the Hardenability of Steel," uses steel as the research object and employs a test involving heating and holding a round bar with a diameter greater than 40 mm and a height ≥ twice the diameter, followed by water quenching, to determine the hardenability of the steel. The specific testing steps are as follows: "After quenching, using the cross-section at half the height of the sample as the test surface, a sheet-like sample with a thickness of at least 10 mm is machined. Based on the circle containing the test surface, along the two endpoints of the diameter segment on it, towards the center point of the test surface, the Rockwell hardness of the sample is tested at preset points. The statistical data is then used to plot a hardness-distance curve from the surface. The preset points are located at 1.5 mm, 3 mm, 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, 15 mm, 20 mm, 25 mm… from the center of the surface, respectively, to test the Rockwell hardness of the sample." The hardenability testing method in this patent is closely related to the maximum water quenching diameter of the steel itself and is not applicable to the hardenability determination of Ti-containing carburized steel.This is because Ti-containing carburizing steel generally has low hardenability, with a maximum water-quenched diameter ≤25mm (maximum water-quenched radius ≤12.5mm). This means that if the test method in the invention patent CN116397078A is used to determine hardenability, the sample only shows a decrease in hardness from high to low at depths of 1.5mm, 3mm, 5mm, 7mm, 9mm, 11mm, and 13mm from the surface towards the center. From a depth of 13mm, the hardness of the sample tends to stabilize and there will be no further substantial difference. Therefore, this method cannot determine key hardenability indicators such as J15, J20, and J25 of Ti-containing carburizing steel, thus failing to reflect the true hardenability of Ti-containing carburizing steel and providing no basis for selecting Ti-containing carburizing steel.

[0007] The objective drawbacks of existing technologies are: ① National standard GB / T 225 only specifies the testing of eccentric specimens for round steel sections, without specifying sampling for the central part where compositional fluctuations are large. ② When testing end-hardenability specimens, only two surfaces equidistant from the center of the round steel section are tested. However, the compositional fluctuations on these two equidistant surfaces are often very small, failing to effectively reflect the material's compositional uniformity. ③ The hardenability bandwidth of Ti-containing carburized steel obtained using the existing national standard testing methods cannot truly reflect the material's compositional uniformity, making it impossible to distinguish the quality of uniformity in Ti-containing carburized steel and thus unable to guide the selection of materials for parts with different dimensional characteristics.

[0008] Therefore, there is an urgent need to develop a precise hardenability testing method specifically for Ti-containing carburized steel. This method should be able to effectively capture the influence of compositional inhomogeneity caused by TiN banding and TiC formation on hardenability, overcome the limitations of existing national standard methods in terms of sampling location and testing range, and overcome the inapplicability of other testing methods to Ti-containing carburized steel. Summary of the Invention

[0009] To achieve one of the above objectives, this invention proposes a method for evaluating the homogenization of hardenability of Ti-containing carburized steel. This method can significantly improve the evaluation accuracy of the hardenability bandwidth of Ti-containing carburized steel, directly and accurately reflecting the quality of Ti-containing carburized steel with different homogeneities, thereby allowing for the selection of materials with appropriate hardenability bandwidths based on the dimensional characteristics of the parts. The technical solution of this invention is implemented as follows:

[0010] In a first aspect, the present invention proposes a method for evaluating the homogenization of hardenability of Ti-containing carburized steel, comprising the following steps:

[0011] S1. Take samples of Ti-containing carburized steel round bars to obtain end-quenched sample blanks;

[0012] S2. The end-quenched blank is normalized and machined to obtain the sample;

[0013] S3. Place the processed sample into a heating furnace, heat and hold it at that temperature, then transfer it to a quenching device for end-hardenability test.

[0014] S4. Grind the cooled sample to obtain a flat surface. On the flat surface, take a preset point from the quenched end to the far end along the sample axis and measure the hardness of the preset point.

[0015] S5. Record the average hardness of the corresponding points on the four sides of the sample, calculate the hardness value and hardness range of the end-quenched carburized steel sample corresponding to each distance, and evaluate the hardenability and hardenability bandwidth of the carburized steel.

[0016] Further preferably, in step S1, the sampling method specifically includes: when the diameter of the Ti-containing carburized steel round bar is less than 30 mm, it is directly forged into an end-quenched sample blank; when the diameter of the Ti-containing carburized steel round bar is 30-32 mm, it is directly used as an end-quenched sample blank; when the diameter of the Ti-containing carburized steel round bar is 32-40 mm, it is directly processed into an end-quenched sample blank, the axis of the end-quenched sample blank coincides with the axis of the Ti-containing carburized steel round bar; when the diameter of the Ti-containing carburized steel round bar is 40-60 mm, sampling is performed on a central sample blank and an eccentric sample blank respectively, the axis of the central sample blank coincides with the axis of the Ti-containing carburized steel round bar, and the distance between the axis of the eccentric sample blank and the surface of the Ti-containing carburized steel round bar is 20-25 mm.

[0017] Specifically, when the diameter of round steel exceeds 40mm, if only "eccentric samples" are processed according to national standards, the central area, which may have component segregation, porosity, and inclusion enrichment, will be avoided, and the true level of the Ti-containing carburized steel round steel as a whole cannot be represented. Adding "central samples" or "core samples" can evaluate the hardenability level of the near-center area of ​​the Ti-containing carburized steel round steel. By comparing the test results of "central samples" or "core samples" with "eccentric samples", the performance uniformity of large-diameter round steel can be judged more comprehensively, avoiding misjudgment due to the defects of a single eccentric sample, and ensuring the safety and reliability of Ti-containing carburized steel round steel in practical applications.

[0018] Preferably, in step S4, the grinding method specifically includes: grinding four mutually perpendicular planes along the entire length of the sample at positions parallel to the sample axis and at 90° to each other.

[0019] Further preferably, in step S1, the sampling method further includes: when the diameter of the Ti-containing carburized steel round bar exceeds 60mm, and it meets the following requirements: center porosity ≤ 0.5 grade, general porosity ≤ 0.5 grade, ingot segregation ≤ 0.5 grade, and center segregation ≤ 0.5 grade, sampling is performed on the central sample billet and the eccentric sample billet, respectively. The axis of the central sample billet coincides with the axis of the Ti-containing carburized steel round bar, and the distance between the axis of the eccentric sample billet and the surface of the Ti-containing carburized steel round bar is 20-25 mm. mm; When the diameter of the Ti-containing carburized steel round bar exceeds 60 mm, and meets any one of the following conditions: center porosity ≥ 1.0 grade, general porosity ≥ 1.0 grade, ingot segregation ≥ 1.0 grade, or center segregation ≥ 1.0 grade, sampling shall be performed on the center sample and the eccentric sample respectively. The distance between the axis of the center sample and the axis of the Ti-containing carburized steel round bar is 12.5 to 15 mm, and the distance between the axis of the eccentric sample and the surface of the Ti-containing carburized steel round bar is 20 to 25 mm.

[0020] Specifically, when the porosity and segregation are mild (center porosity ≤ 0.5 grade, general porosity ≤ 0.5 grade, ingot segregation ≤ 0.5 grade, center segregation ≤ 0.5 grade), the porosity / segregation area is small, and the cross-section of the center sample billet can cover this area. In this case, using the machined center sample to evaluate the hardenability of the steel can truly reflect the influence of porosity and segregation in the center of the round steel on the hardenability value. When the porosity and segregation are severe (center porosity ≥ 1.0 grade / general porosity ≥ 1.0 grade / ingot segregation ≥ 1.0 grade / center segregation ≥ 1.0 grade), because the porosity / segregation influence area is large, it is necessary to adjust the sampling position and increase the sample coverage area. By using the machined center sample to cover the key porosity / segregation area near the center of the round steel, the influence of porosity / segregation on the hardenability of Ti-containing carburized steel round steel can be accurately assessed.

[0021] Preferably, the end-quenched blank in step S1 is a cylinder with a diameter of 30-32 mm and a length of 120 mm.

[0022] Preferably, in step S2, the length of the sample is 101.5 to 102.5 mm; the sample includes a main body segment and an annular flange located at one end of the main body segment, the diameter of the main body segment is 24.5 to 25.5 mm; the thickness of the annular flange is 5 mm, the outer diameter of the annular flange is larger than the diameter of the main body segment and is coaxially arranged with the main body segment, and the outer diameter of the annular flange is 30 to 32 mm.

[0023] Specifically, the length of the sample is 1.5 to 2.5 mm longer than the length specified in GB / T 225-2006, which facilitates the rapid transfer of the red-hot end-quenched sample to the end-quenching equipment for testing and improves the success rate of manual or robotic transfer operations.

[0024] Preferably, in step S2, the normalizing temperature is 940–960°C and the holding time is 30–35 min.

[0025] Specifically, the normalizing treatment ensures full austenitization of the end-quenched billet. For Ti-containing carburized steel, normalizing at 940–960℃ is chosen, primarily to balance microstructure optimization and performance requirements. The reasons are as follows: ① Matching material composition to ensure complete austenitization: This ensures that alloying elements such as Cr, Mn, Ti, and Al in the Ti-containing carburized steel are fully dissolved into austenite, avoiding undissolved ferrite residue. This lays the foundation for subsequent end-quenchability tests and reduces performance fluctuations caused by microstructure inhomogeneity. ② Controlling grain size to avoid overheating: Temperatures below 940℃ may lead to insufficient austenitization, while temperatures above this level tend to result in coarse austenite grains. At 940℃, the grain size remains moderate, satisfying the microstructure stability requirements of subsequent end-quench tests while avoiding hardenability fluctuations caused by coarse grains. ③ Improving the original microstructure: Normalizing eliminates defects such as banded structures and Widmanstätten structures after rolling / forging of Ti-containing carburized steel, forming a uniform, fine, and thin pearlite microstructure. This microstructure reduces hardenability fluctuations during subsequent end-quenchability tests.

[0026] Preferably, in step S3, the heating temperature is 925-935℃, the heating time is not less than 20 minutes, and the holding time is 30-35 minutes.

[0027] Specifically, it is best to conduct end-hardenability tests on Ti-containing carburized steel at 925–935℃. This temperature is slightly higher than the Ac3 point of carburized steel, resulting in a single-phase austenitic structure, which ensures that the hardenability test reflects its true performance. At the same time, it avoids excessively high temperatures that could lead to coarse austenite grains, thus preventing any impact on the true hardenability value.

[0028] Specifically, during heating, the sample needs to be covered with a heat-resistant steel sleeve, and fine charcoal should be added inside the sleeve to prevent oxide scale from forming on the quenched surface of the sample and to eliminate the effects of oxidation and decarburization.

[0029] Preferably, step S3 specifically includes: transferring the heated and heat-preserved sample to the quenching device and placing it vertically on the sample support, wherein the time from when the sample is taken out of the heating furnace to when the quenching begins does not exceed 5 seconds.

[0030] Preferably, step S3 further includes: performing end-spray water quenching on the sample in the quenching device, with a spraying time ≥10 min, a free height of 55-75 mm, and the free height remaining constant during the spraying process.

[0031] Preferably, in step S4, the preset points are points on the plane along the axial direction of the sample, ranging from 1.5mm, 3mm, 5mm, 7mm, 9mm, 11mm, 13mm, 15mm, 20mm, and 25mm from the quenching end; the grinding depth is 0.4 to 0.5mm.

[0032] Specifically, when measuring hardness, use an appropriate sample holder to ensure the correctness of each measurement position and minimize the test time. V-blocks are not recommended because they will cause the sample to tilt. When measuring the hardness of the back of the sample, care must be taken not to be affected by the measurement indentation on the previously measured side.

[0033] Specifically, when measuring the end hardenability of carburized steel, an appropriate sample holder should meet the requirements of accurate positioning, stability and reliability, and avoidance of indentation interference.

[0034] Secondly, the present invention provides an application of the homogenization evaluation method described in the first aspect in evaluating the hardenability of Ti-containing carburized steel.

[0035] Compared with the prior art, the advantages of the present invention are as follows:

[0036] (1) In addition to sampling the end-quenched sample at the eccentric position of the cross section of Ti-containing carburized steel round steel, this invention also specifies the sampling of the central part and the core part of the cross section of Ti-containing carburized steel round steel with different specifications and different porosity and segregation levels. Compared with the current national standard GB / T 225-2006, it can more accurately reflect the hardenability and hardenability bandwidth of Ti-containing carburized steel.

[0037] (2) When testing the hardness of the end-hardenability sample, this invention not only tests the two surfaces equidistant from the center of the round steel section, but also tests the distal surface far from the center of the round steel section and the proximal surface close to the center of the round steel section. Compared with the current national standard, the test results are more scientific and accurate.

[0038] (3) The method for evaluating the hardenability uniformity of Ti-containing carburized steel provided by the present invention can truly reflect the hardenability fluctuation of Ti-containing carburized steel and the uniformity of raw materials, thereby helping technicians to select raw materials with appropriate hardenability bandwidth for use in shaft and gear parts with different size characteristics and performance requirements, and has broad application prospects. Attached Figure Description

[0039] 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 only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of the machining sampling positions of the center sample and the eccentric sample end quenched billet of the Ti-containing carburized steel round bar in this invention.

[0041] Figure 2 This is a schematic diagram of the machining sampling positions of the core sample and the eccentric sample end quenched blank of the Ti-containing carburized steel round bar in this invention.

[0042] Figure 3 This is a schematic diagram of the sampling location for end-quenched specimens in the national standard GB / T 225-2006.

[0043] Figure 4 This is the end-quenched sample size design drawing used in this invention;

[0044] Figure 5 This is a schematic diagram of the quenching device used in the end-quenching test of the present invention;

[0045] Figure 6 This is a schematic diagram of the hardness testing surfaces of the center sample and the eccentric sample of the end-quenched sample in this invention.

[0046] Figure 7 This is a schematic diagram of the hardness testing surfaces of the center and eccentric specimens of the end-quenched sample in this invention.

[0047] In the figure, 1-sample positioning and centering device; 2-sample position; 3-disc; 4-water spray nozzle; 5-quick switch valve; 6-water supply pipe. Detailed Implementation

[0048] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0049] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0050] In this document, the terms “containing,” “comprising,” or “including” are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects.

[0051] In this document, the terms “optional,” “optionally,” or “optional” generally refer to an event or condition that may, but may not, occur, and the description includes both cases in which the event or condition occurs and cases in which the event or condition does not occur.

[0052] It should be noted that this invention creatively designs a homogenization evaluation method for the hardenability of Ti-containing carburized steel, enabling accurate evaluation of its hardenability. Compared with existing technologies, this invention, in addition to sampling end-quenching specimens at the eccentric position of the Ti-containing carburized steel round steel section, also specifies sampling at the center and core of the Ti-containing carburized steel round steel section according to different specifications and different porosity and segregation levels, thus more accurately reflecting the hardenability and hardenability bandwidth of the carburized steel. When testing the hardness of the end-quenching specimens, this invention not only tests two surfaces equidistant from the center of the round steel section, but also tests the distal surface far from the center and the proximal surface close to the center. Compared with current national standards, the test results are more scientific and accurate. The method for evaluating the hardenability homogenization of Ti-containing carburizing steel provided by this invention can be applied to almost all carburizing steel bars with large compositional fluctuations. It can truly reflect the hardenability fluctuation of carburizing steel and the uniformity of raw materials, thereby helping technicians to select raw materials with appropriate hardenability bandwidth for use in shaft and gear parts with different size characteristics and performance requirements. Its application prospects are very broad.

[0053] 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0054] Example 1

[0055] This embodiment provides an application of a method for evaluating the uniformity of hardenability of Ti-containing carburized steel to 20CrMnTiH2 round steel (specification: Ф75mm), including the following steps:

[0056] (1) For 20CrMnTiH2 round steel with a specification of Ф75mm, the low-magnification microstructure test results are: central porosity 0.5, general porosity 0, ingot segregation 0, and central segregation 0. Based on its porosity and segregation levels, sampling of central and eccentric ingots is required, such as... Figure 1As shown, along its rolling direction, two end-quenched sample blanks are wire-cut from two 120mm long round steel bodies: a) center sample blank: the center of the sample coincides with the center of the round steel; b) eccentric sample blank: the distance between the center of the sample and the outer circle of the round steel is 20mm; the dimensions of the end-quenched sample blanks are: diameter 30mm and length 120mm.

[0057] (2) Normalizing treatment of end-quenched blanks: Place the center blank and eccentric blank processed in (1) in a heating furnace and keep them at 940℃ for 30 minutes to fully austenitize them;

[0058] (3) End-quenched sample processing: The normalized center blank and eccentric blank are processed according to... Figure 4 The end-quenched specimen size design drawing was cut and machined into end-quenched specimens. The dimensions of the center specimen and the eccentric specimen are: length 102mm and diameter 25mm.

[0059] (4) End hardenability test: a) Put the central sample and the eccentric sample from (3) into a heating furnace that can reach the specified quenching temperature of 925℃. The heating time is 30min. After the core is uniformly heated, keep it at the temperature for 30min. During heating, put on a special heat-resistant steel sleeve and add fine charcoal inside the sleeve. This will prevent the quenching surface of the sample from developing oxide scale and eliminate the effects of oxidation and decarburization. b) Quickly transfer the red-hot sample after heating and holding to the quenching device and place it vertically on the sample support. The time interval between taking the sample out of the heating furnace and starting quenching is 4s. c) Immediately perform end water spray quenching on the red-hot sample until the entire sample is cooled. Keep the cooling time at 15min. Then put it in water to cool it completely. After the water cooling starts, the water spray height should immediately reach the specified free height of 65mm and keep the free height unchanged during cooling.

[0060] (5) Hardness testing of end-quenched specimens: a) Grinding of test surfaces: After cooling, the central specimen and the eccentric specimen are ground along the entire length of the specimen at corresponding positions at a relative 90° angle. The grinding depth is 0.45 mm, completing the grinding of 4 planes, such as... Figure 6 As shown, there are 8 planes for the co-centered specimen (planes A, B, C, and D) and the eccentric specimen (planes A, B, C, and D). When grinding the specimens, care must be taken to avoid changes in the microstructure caused by grinding heat. b) Take points along the axial direction of the specimen from a distance of 1.5 mm or more from the quenched end, selecting points at 1.5 mm, 3 mm, 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, 15 mm, 20 mm, and 25 mm from the quenched end, and measure the hardness of each point on each of the 8 planes. c) Record the test results and calculate the average hardness of the corresponding points on the four sides of the specimen. Record the axial hardness and distance, calculate the hardness value and hardness range of the end-quenched carburized steel specimens corresponding to each distance, and evaluate the hardenability and hardenability bandwidth of the carburized steel.

[0061] The hardenability test results of 20CrMnTiH2 round steel (specification: Ф75mm) were obtained by using the evaluation method of the present invention. The results are shown in Table 1.

[0062] Table 1. Hardenability test results of 20CrMnTiH2 round steel in Example 1

[0063]

[0064] As can be seen from the end-quenching test results of the round steel in Table 1, if the test method in the national standard GB / T 225-2006 is strictly used to evaluate the hardenability bandwidth of the carburized steel, its bandwidth is ≤1.3HRC, which is obviously inconsistent with the actual situation of the material. However, using the hardenability evaluation method of the carburized steel of this invention, the bandwidth is ≤4.9HRC, which can truly and effectively evaluate the uniformity of the carburized steel round steel, thereby scientifically judging the application range of the material and reducing the risk of substandard production quality when using this material in shaft and gear parts.

[0065] Example 2

[0066] This embodiment provides an application of a method for evaluating the uniformity of hardenability of carburized steel to 20CrMnTiH3 round steel (specification: Ф120mm).

[0067] The evaluation process for the hardenability of carburized steel in this embodiment is as follows:

[0068] (1) For 20CrMnTiH3 round steel with a specification of Ф120mm, the low-magnification microstructure test results are as follows: center segregation grade 1.5, ingot segregation grade 0, center porosity grade 0, and general porosity grade 0. Based on its porosity and segregation levels, sampling of center-mounted and eccentric-mounted blanks is required: For 20CrMnTiH3 round steel with a specification of Ф120mm, two end-quenched blanks are wire-cut along its rolling direction on a 120mm long section of the round steel body, such as... Figure 2 As shown, the dimensions of the sample are: a) center blank: the distance between the axis of the sample and the axis of the round steel should be 15mm; b) eccentric blank: the distance between the center of the sample and the outer circle of the round steel should be 25mm; the dimensions of the end-quenched blank are: diameter 32mm and length 120mm.

[0069] (2) Normalizing treatment of end-quenched blanks: Place the center blanks and eccentric blanks processed in (1) in a heating furnace and keep them at 950℃ for 35 minutes to fully austenitize them;

[0070] (3) End-quenched sample processing: The normalized center blank and eccentric blank are processed according to... Figure 3 The end-quenched specimen size design drawing was cut and machined into end-quenched specimens. The dimensions of both the center specimen and the eccentric specimen are: length 102mm and diameter 25mm.

[0071] (4) End-hardenability test: a) Place the core and eccentric specimens from (3) into a heating furnace capable of reaching the specified quenching temperature of 930℃, and heat for 40 minutes. After the core is uniformly heated, keep it at the temperature for 35 minutes. During heating, cover it with a special heat-resistant steel sleeve and add fine charcoal inside the sleeve to prevent oxide scale from forming on the quenched surface of the specimen and eliminate the effects of oxidation and decarburization. b) Quickly transfer the heated specimen to the quenching device and place it vertically on the specimen support. The time interval between taking the specimen out of the heating furnace and starting quenching is 4 seconds. c) Immediately perform end-water quenching on the heated specimen until the entire specimen is cooled. Keep the cooling time at 18 minutes, and then put it in water to cool it completely. After the water cooling starts, the water spray height should immediately reach the specified free height of 70 mm and keep the free height unchanged during cooling.

[0072] (5) Hardness testing of end-quenched specimens: a) Grinding of test surfaces: After cooling, the specimen is ground along its entire length at corresponding positions at 90° angles, with a grinding depth of 0.50 mm, completing the grinding of four planes, such as... Figure 7 As shown, there are 8 planes in total, including the center sample (planes A, B, C, and D) and the eccentric sample (planes A, B, C, and D). During grinding, care must be taken to avoid microstructural changes due to grinding heat. b) Along the sample axis, starting at a distance of 1.5 mm or more from the quenched end, select points at 1.5 mm, 3 mm, 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, 15 mm, 20 mm, and 25 mm from the quenched end, and measure the hardness at each point on each of the 8 planes. c) Record the test results and calculate the average hardness of the corresponding points on all four sides of the sample. Record the axial hardness and distance, calculate the hardness value and hardness range of the end-quenched carburized steel sample at each distance, and evaluate the hardenability and hardenability bandwidth of the carburized steel.

[0073] The hardenability test results of 20CrMnTiH3 round steel (specification: Ф120mm) were obtained by using the evaluation method of the present invention. The results are shown in Table 2.

[0074] Table 2. Hardenability test results of 20CrMnTiH3 round steel in Example 2.

[0075]

[0076] As can be seen from the end-quenching test results of the round steel in Table 2, if the test method in the national standard GB / T 225-2006 is strictly used to evaluate the hardenability bandwidth of the carburized steel, its hardenability bandwidth is ≤2.6HRC, which is obviously inconsistent with the actual hardenability bandwidth of the material being ≤6.3HRC. However, by using the hardenability evaluation method of the carburized steel of this invention, the uniformity of the carburized steel round steel can be evaluated in a true and effective manner, thereby scientifically judging the application range of the material and reducing the risk of substandard production quality when using this material in shaft and gear parts.

[0077] Example 3

[0078] This embodiment provides an application of a method for evaluating the uniformity of hardenability of Ti-containing carburized steel to 20MnTiBH2 round steel (specification: Ф50mm), including the following steps:

[0079] (1) For 20MnTiBH2 round steel with a specification of Ф50mm, it is necessary to take samples of the center blank and the eccentric blank respectively, such as Figure 1 As shown, along its rolling direction, two end-quenched sample blanks are wire-cut from two 120mm long round steel bodies: a) center sample blank: the center of the sample coincides with the center of the round steel; b) eccentric sample blank: the distance between the center of the sample and the outer circle of the round steel is 20mm; the dimensions of the end-quenched sample blanks are: diameter 30mm and length 120mm.

[0080] (2) Normalizing treatment of end-quenched blanks: Place the center blank and eccentric blank processed in (1) in a heating furnace and keep them at 960℃ for 30 minutes to fully austenitize them;

[0081] (3) End-quenched sample processing: The normalized center blank and eccentric blank are processed according to... Figure 4 The end-quenched specimen size design drawing was cut and machined into end-quenched specimens. The dimensions of the center specimen and the eccentric specimen are: length 102mm and diameter 25mm.

[0082] (4) End hardenability test: a) Put the central sample and the eccentric sample from (3) into a heating furnace that can reach the specified quenching temperature of 935℃. The heating time is 30min. After the core is uniformly heated, keep it at the temperature for 30min. During heating, put on a special heat-resistant steel sleeve and add fine charcoal inside the sleeve. This will prevent the quenching surface of the sample from developing oxide scale and eliminate the effects of oxidation and decarburization. b) Quickly transfer the red-hot sample after heating and holding to the quenching device and place it vertically on the sample support. The time interval between taking the sample out of the heating furnace and starting quenching is 4s. c) Immediately perform end water spray quenching on the red-hot sample until the entire sample is cooled. Keep the cooling time at 15min. Then put it in water to cool it completely. After the water cooling starts, the water spray height should immediately reach the specified free height of 65mm and keep the free height unchanged during cooling.

[0083] (5) Hardness testing of end-quenched specimens: a) Grinding of test surfaces: After cooling, the central specimen and the eccentric specimen are ground along the entire length of the specimen at corresponding positions at a relative 90° angle. The grinding depth is 0.45 mm, completing the grinding of 4 planes, such as... Figure 6 As shown, there are 8 planes for the co-centered specimen (planes A, B, C, and D) and the eccentric specimen (planes A, B, C, and D). When grinding the specimens, care must be taken to avoid changes in the microstructure caused by grinding heat. b) Take points along the axial direction of the specimen from a distance of 1.5 mm or more from the quenched end, selecting points at 1.5 mm, 3 mm, 5 mm, 7 mm, 9 mm, 11 mm, 13 mm, 15 mm, 20 mm, and 25 mm from the quenched end, and measure the hardness of each point on each of the 8 planes. c) Record the test results and calculate the average hardness of the corresponding points on the four sides of the specimen. Record the axial hardness and distance, calculate the hardness value and hardness range of the end-quenched carburized steel specimens corresponding to each distance, and evaluate the hardenability and hardenability bandwidth of the carburized steel.

[0084] The hardenability test results of 20MnTiBH2 round steel (specification: Ф50mm) were obtained by using the evaluation method of the present invention. The results are shown in Table 3.

[0085] Table 3. Hardenability test results of 20MnTiBH2 round steel in Example 3.

[0086]

[0087] As can be seen from the end-quenching test results of the round steel in Table 3, if the test method in the national standard GB / T 225-2006 is strictly used to evaluate the hardenability bandwidth of the carburized steel, its bandwidth is ≤2.1 HRC, which is obviously inconsistent with the actual situation of the material. However, using the hardenability evaluation method of the carburized steel of this invention, the bandwidth is ≤5.1 HRC, which can truly and effectively evaluate the uniformity of the carburized steel round steel, thereby scientifically judging the application range of the material and reducing the risk of substandard production quality when using this material in shaft and gear parts.

[0088] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for evaluating the homogenization of hardenability of Ti-containing carburized steel, characterized in that, Includes the following steps: S1. Take samples of Ti-containing carburized steel round bars to obtain end-quenched sample blanks; S2. The above-mentioned end-quenched blank is normalized and machined to obtain the sample; S3. Place the processed sample into a heating furnace, heat and hold it at that temperature, then transfer it to a quenching device for end-hardenability test. S4. Grind the cooled sample to obtain a flat surface. On the flat surface, take a preset point from the quenched end to the far end along the sample axis and measure the hardness of the preset point. S5. Record the average hardness of the corresponding points on the four sides of the sample, calculate the hardness value and hardness range of the end-quenched carburized steel sample corresponding to each distance, and evaluate the hardenability and hardenability bandwidth of the carburized steel. in, In step S1, the sampling method specifically includes: when the diameter of the Ti-containing carburized steel round bar is less than 30mm, it is directly forged into an end-quenched sample blank; when the diameter of the Ti-containing carburized steel round bar is 30-32mm, it is directly used as an end-quenched sample blank; when the diameter of the Ti-containing carburized steel round bar is 32-40mm, it is directly processed into an end-quenched sample blank, the axis of the end-quenched sample blank coincides with the axis of the Ti-containing carburized steel round bar; when the diameter of the Ti-containing carburized steel round bar is 40-60mm, sampling is performed on a central sample blank and an eccentric sample blank respectively, the axis of the central sample blank coincides with the axis of the Ti-containing carburized steel round bar, and the distance between the axis of the eccentric sample blank and the surface of the Ti-containing carburized steel round bar is 20-25mm. In step S4, the grinding method specifically includes grinding four mutually perpendicular planes along the entire length of the sample at positions parallel to the sample axis and at 90° to each other.

2. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, In step S1, the sampling method further includes: when the diameter of the Ti-containing carburized steel round bar exceeds 60mm, and it meets the requirements of center porosity ≤ 0.5 grade, general porosity ≤ 0.5 grade, ingot segregation ≤ 0.5 grade, and center segregation ≤ 0.5 grade, sampling is performed on the center sample blank and the eccentric sample blank respectively. The axis of the center sample blank coincides with the axis of the Ti-containing carburized steel round bar, and the distance between the axis of the eccentric sample blank and the surface of the Ti-containing carburized steel round bar is 20-25mm. When the diameter of the Ti-containing carburized steel round bar exceeds 60mm, and it meets any one of the following conditions: center porosity ≥ 1.0 grade, general porosity ≥ 1.0 grade, ingot segregation ≥ 1.0 grade, or center segregation ≥ 1.0 grade, sampling is performed on both the center sample and the eccentric sample. The distance between the axis of the center sample and the axis of the Ti-containing carburized steel round bar is 12.5–15mm, and the distance between the axis of the eccentric sample and the surface of the Ti-containing carburized steel round bar is 20–25mm.

3. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, The end-quenched blank in step S1 is a cylinder with a diameter of 30-32 mm and a length of 120 mm.

4. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, In step S2, the length of the sample is 101.5 to 102.5 mm; the sample includes a main body segment and an annular flange located at one end of the main body segment, the diameter of the main body segment is 24.5 to 25.5 mm; the thickness of the annular flange is 5 mm, the outer diameter of the annular flange is larger than the diameter of the main body segment and is coaxially arranged with the main body segment, and the outer diameter of the annular flange is 30 to 32 mm.

5. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, In step S2, the normalizing temperature is 940–960°C, and the holding time is 30–35 min.

6. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, In step S3, the heating temperature is 925-935℃, the heating time is not less than 20 minutes, and the holding time is 30-35 minutes.

7. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, Step S3 specifically includes: transferring the heated and heat-preserved sample to the quenching device and placing it vertically on the sample support. The time from when the sample is taken out of the heating furnace to when the quenching begins shall not exceed 5 seconds.

8. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, Step S3 further includes: performing end-spray water quenching on the sample in the quenching device, with a spraying time of ≥10 min, a free height of 55-75 mm, and the free height remaining constant during the spraying process.

9. The method for evaluating the homogenization of hardenability of Ti-containing carburized steel according to claim 1, characterized in that, In step S4, the preset points are points on the plane along the axial direction of the sample, ranging from 1.5mm, 3mm, 5mm, 7mm, 9mm, 11mm, 13mm, 15mm, 20mm, and 25mm from the quenching end; the grinding depth is 0.4 to 0.5mm.

10. The application of the homogenization evaluation method as described in any one of claims 1 to 9 in evaluating the hardenability of Ti-containing carburizing steel.

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