Heat treatment method for displaying 38CrMoAl nitrided steel banded structure

Abstract

The invention relates to a heat treatment method for displaying a 38CrMoAl nitrided steel banded structure, and belongs to the technical field of metal detection. According to the method, a sample is rapidly cooled to about 660 DEG C after being subjected to heat preservation at the temperature of 925 DEG C, then heat preservation treatment is conducted at the temperature of 655 DEG C, so that non-equilibrium structures such as bainite in the steel are converted into a strip structure with alternately arranged ferrite and pearlite, and then the strip structure can pass GB / T 34474.1-2017 standard detection. According to the method, the problem that the 38CrMoAl hot-rolled steel banded structure cannot be directly evaluated is solved, operation is easy and convenient, and support is provided for material quality control.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal material detection, and in particular to a heat treatment method for displaying banded structure of 38CrMoAl nitriding steel. BACKGROUND

[0002] The banded structure of steel material is a common metallographic structure defect caused by the unevenness of material composition, and usually presents the morphology of alternating parallel arrangement of ferrite and pearlite strips along the rolling direction. This defect can cause the mechanical properties of steel to decrease, so the evaluation and detection of its severity are very important.

[0003] 38CrMoAl steel is an important alloy structural steel (nitriding steel), which is widely used for manufacturing shaft parts with high precision, high wear resistance and high fatigue strength due to its excellent nitriding performance. Currently, the detection of banded structure in steel mainly relies on GB / T34474.1-2017 "Evaluation of Banded Structure in Steel Part 1: Standard Grading Diagram Method", which evaluates the grade of banded structure by the number of ferrite strips, the degree of penetration of the field of view, the continuity and the content of deformed ferrite.

[0004] However, the hot-rolled state of 38CrMoAl steel is mainly composed of non-equilibrium structures such as bainite, with only a small amount of ferrite, pearlite and martensite, and the ferrite is not distributed in strips, which makes the above national standard unable to be directly applied to the detection of banded structure. Therefore, there is an urgent need for a method that can clearly display the banded structure of 38CrMoAl steel. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a heat treatment method for displaying the banded structure of 38CrMoAl nitriding steel, which can convert the non-equilibrium structure of 38CrMoAl hot-rolled steel into detectable balanced structure strips in the existing technology.

[0006] The technical solution adopted by the present application to solve the technical problem is: a heat treatment method for displaying the banded structure of 38CrMoAl nitriding steel, comprising the following steps,

[0007] S1, cutting a hot-rolled 38CrMoAl steel sample with a thickness of 25±2mm;

[0008] S2, placing the sample in a first heating furnace at 925±10℃ for 50±5 minutes;

[0009] S3, immediately after taking out the sample, forced air cooling is performed, and an infrared temperature gun is used to monitor the temperature in real time, and when the temperature drops to 660±10℃, the sample is transferred to a second heating furnace;

[0010] S4, holding at 655±5℃ for 60±5 minutes in the second heating furnace, and then air cooling to room temperature;

[0011] S5, preparing metallographic sample for the heat-treated sample;

[0012] S6, using etchant to corrode, and evaluating banded structure according to GB / T 34474.1 standard.

[0013] Further, in the step S3, the rapid cooling is performed by blowing with a high-power exhaust fan, and the final cooling temperature is 650-670℃, and the wind speed is ≥5m / s.

[0014] Further, the first heating furnace and the second heating furnace are resistance heating furnaces, and the furnace temperature uniformity meets the requirement of GB / T 9452.

[0015] Further, in the step S5, the metallographic sample of 20mm×10mm is cut from the position of 1 / 2 radius of the sample, and the detection surface is parallel to the rolling direction.

[0016] Further, in the step S6, the etchant is 4% nitric acid alcohol solution.

[0017] The present application has the advantages that the defects in the background art are solved,

[0018] 1. The method is simple to operate, and can convert the non-equilibrium structure (bainite, martensite, etc.) of the 38CrMoAl steel into the balanced structure strip of alternating arrangement of ferrite and pearlite;

[0019] 2. The banded structure of the 38CrMoAl steel which cannot be directly detected can be evaluated according to the GB / T34474.1-2017 standard, and a reliable basis is provided for material quality control. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a schematic diagram of the banded structure of the 38CrMoAl steel sample (A sample) subjected to heat treatment according to the present application;

[0021] Figure 2 is a schematic diagram of the banded structure of the hot-rolled 38CrMoAl steel sample (B sample) without heat treatment. DETAILED DESCRIPTION

[0022] The present application will now be further described in detail in conjunction with the drawings and preferred embodiments. These drawings are simplified schematic diagrams, and only illustrate the basic structure of the present application in a schematic manner, and therefore only show the components related to the present application.

[0023] A heat treatment method for displaying banded structure of 38CrMoAl nitriding steel, which comprises the following steps of:

[0024] 1. Sample and equipment preparation

[0025] Take one 38CrMoAl hot-rolled round steel bar as sample, and cut two samples with thickness of about 25mm from the bar (the two samples are respectively coded as A sample and B sample).

[0026] Prepare two laboratory general resistance heating furnaces (the two furnaces are respectively coded as 1# furnace and 2# furnace), and the furnace temperature precision and uniformity can meet the national standard GB / T 9452.

[0027] Prepare one portable infrared temperature measuring gun and one high-power exhaust fan.

[0028] 2. Heat treatment process

[0029] Put the prepared 38CrMoAl steel A sample into the 1# furnace, and set the furnace temperature at 925℃. After the furnace temperature is increased to 925℃, keep the temperature for 50 minutes. Another 2# furnace is powered to increase the temperature to the set 655℃ and keep constant.

[0030] After the furnace holding time, take out the A sample from the furnace chamber, and immediately use the exhaust fan to blow and cool the sample. During the blowing and cooling process, use the infrared temperature measuring gun to measure the temperature of the sample in real time. When the temperature of the sample is about 660℃, immediately put it into the 2# furnace chamber. After the temperature of the 2# furnace is stabilized at 655℃, keep the temperature for 1 hour.

[0031] After the furnace holding time, take out the A sample from the furnace chamber and cool it to room temperature in air.

[0032] 3. Sample preparation

[0033] Use a band saw or other cutting equipment with water cooling function to cut one metallographic sample with a detection area of 20mm×10mm from the radius 1 / 2 position of the A sample after heat treatment and the B sample in hot-rolled state (referred to as metallographic sample).

[0034] Grind and polish the detection surface of the two metallographic samples parallel to the rolling direction of the round steel, and corrode them with 4% nitric acid alcohol solution.

[0035] 4. Banded structure detection

[0036] The two corroded metallographic samples are respectively detected for banded structure under a metallographic microscope according to the requirements of GB / T34474.1 standard.

[0037] The banded structure of the 38CrMoAl steel sample (A sample) after heat treatment is as followsFigure 1 As shown in the figure, the metallographic structure of the A sample is clear ferrite band and pearlite band, and is alternately arranged in parallel, which is a typical metallographic equilibrium structure, and the band structure level is 3.0.

[0038] The band structure of the hot-rolled 38CrMoAl steel sample without heat treatment (B sample) is shown in the figure Figure 2 As shown in the figure, the metallographic structure of the B sample is mainly bainite and a small amount of non-equilibrium structure such as martensite, plus a small amount of blocky ferrite and pearlite structure, and no ferrite band is found, so the band structure level cannot be detected.

[0039] The above description is only a specific embodiment of the present application, and various examples do not limit the essential content of the present application, and those skilled in the art can modify or deform the previously described specific embodiments without departing from the essence and scope of the present application after reading the specification.

Claims

1. A heat treatment method for displaying banded structure of 38CrMoAl nitriding steel strip, characterized in that: The method comprises the following steps, S1, cutting a hot-rolled 38CrMoAl steel sample with a thickness of 25±2 mm; S2, placing the sample in a first heating furnace and keeping at 925±10℃ for 50±5 minutes; S3, immediately after taking out the sample, forced air cooling is performed, and an infrared temperature gun is used to monitor the temperature in real time, and when the temperature drops to 660±10℃, the second heating furnace is entered; S4, keeping at 655±5℃ for 60±5 minutes in the second heating furnace, and then air cooling to room temperature; S5, preparing a metallographic sample of the heat-treated sample; S6, using an etchant to corrode, and performing band structure rating according to GB / T 34474.1 standard.

2. The heat treatment method for displaying banded structure of 38CrMoAl nitriding steel strip according to claim 1, characterized in that: In the step S3, high-power exhaust fans are used for blowing to rapidly cool, and the cooling end temperature is 650-670℃, and the wind speed is ≥5m / s.

3. The heat treatment method for displaying banded structure of 38CrMoAl nitriding steel strip according to claim 1, characterized in that: The first heating furnace and the second heating furnace are resistance heating furnaces, and the furnace temperature uniformity meets the requirements of GB / T9452.

4. The heat treatment method for displaying banded structure of 38CrMoAl nitriding steel strip according to claim 1, characterized in that: In the step S5, a metallographic sample of 20mm×10mm is cut from the position of 1 / 2 of the radius of the sample, and the detection surface is parallel to the rolling direction.

5. The heat treatment method for displaying banded structure of 38CrMoAl nitriding steel strip according to claim 1, characterized in that: In the step S6, the etchant is a 4% nitric acid alcohol solution.

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