CN108181170B - Tensile fracture judgment method for steel material in crack formation period - Google Patents
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Abstract
A tensile fracture judgment method for a crack formation period of a steel material belongs to the technical field of steel material failure analysis. Firstly, preparing a tensile sample containing cracks, carrying out a stretch breaking test, separating two crack surfaces, and judging the crack forming time period according to the characteristics of existence of a dimple, cleavage, quasi-cleavage or crystal edge and the like of fracture micro morphology. Compared with the existing metallographic observation method, the tensile fracture analysis method does not destroy the appearance of the crack and the microstructure nearby the crack, can improve the judgment accuracy of the crack formation time period, and provides important basis for analyzing the formation reason of the crack and making precautionary measures.
Description
Technical Field
The invention belongs to the technical field of steel material failure analysis, and particularly relates to a tensile fracture determination method in a steel material crack formation period. In particular to a method for judging the crack formation time interval of a ferrite pearlite steel material.
Background
The ferritic pearlite type steel material generally means that the steel structure is ferrite + pearlite at about 600 ℃ or lower, undergoes the transformation "austenite → ferrite + pearlite" at about 600 to 800 ℃, and is austenite at about 800 to 1200 ℃. Cracks are a common defect in ferritic pearlitic steel materials, and are most likely to form in two periods: one is in austenite before the start of transformation "austenite → ferrite + pearlite", and the other is in ferrite and/or pearlite after the start of transformation "austenite → ferrite + pearlite". The scientific and reasonable analysis method is used for accurately judging the crack formation time period, which is a key for analyzing the process link of crack formation and then taking precautionary measures and is also a difficult point.
At present, the analysis method aiming at the cracks of the steel material mainly adopts a conventional metallographic observation method to observe whether decarburization and oxidation fog points exist in a structure near the cracks, if decarburization and/or oxidation fog points exist, whether the cracks are formed in a heating process or a previous process in the steel production process is judged, if decarburization and oxidation fog points do not exist, whether the cracks are formed after the heating process in the steel production process is judged, but at present, no good method is provided for defining whether the cracks are formed before the structure transformation or after the structure transformation after the heating process, so that precautionary measures cannot be taken in the corresponding process in a targeted manner.
Moreover, the traditional metallographic observation method is used for analyzing cracks through metallographic sample preparation and microscopic observation, and has the following defects: firstly, quartz sand particles, polishing paste and water of sand paper in the sample preparation process can remain in cracks to damage or corrode the original appearance of the cracks, and important microscopic features of reaction crack forming time periods can be damaged; and secondly, the two-dimensional appearance of the crack on a metallographic grinding surface is observed by conventional metallographic observation, the three-dimensional characteristic of the crack cannot be completely reflected, the possibility that the observed metallographic grinding surface just cuts through a crack source region is very low, and the analysis result is inevitably 'approximate'. The above-mentioned deficiencies of conventional metallographic observation methods result in their inability to accurately ascertain the specific time period for crack formation.
Disclosure of Invention
The invention aims to provide a tensile fracture determination method for a crack formation period of a steel material, which solves the problem that the specific period of crack formation cannot be accurately determined by a traditional metallographic observation method. The method is called as a tensile fracture analysis method, and comprises the steps of preparing a tensile sample containing a crack, carrying out a tensile fracture test, separating two crack surfaces, and judging the crack forming time period through the characteristics of existence of a dimple, cleavage, quasi cleavage or edge crystal and the like of fracture micro morphology.
A method for judging a tensile fracture in a crack formation period of a steel material comprises the following specific steps and parameters:
1. when a crack exists in the steel, performing protection treatment on the crack at the first time to prevent water, dust and oil stain from entering the crack to damage or corrode the original appearance of the crack, dripping clean antirust oil into the crack to isolate air and water, winding and wrapping the antirust oil by using an adhesive tape to prevent the antirust oil from losing, and completing the following steps within 24 hours;
2. processing a tensile sample containing cracks at the cracks of the sample, wherein the size of the tensile sample is determined according to the size of the cracks, the tensile sample is stretched at room temperature on a microcomputer-controlled electronic universal testing machine, and in order to ensure that the sample is broken along the cracks, the stretching speed cannot be too high, the displacement speed of a cross beam of the tensile testing machine is controlled to be 0.005-0.01 mm/min until the sample is broken along the cracks, so that two crack surfaces can be mechanically separated without damaging the original appearance of the crack surfaces.
3. In order to prevent the fracture from being oxidized or corroded due to long-time contact with air, dust and water after the fracture is broken, firstly, the fracture is protected, clean antirust oil is dripped into the fracture to isolate water, dust and oil stains, the fracture is wrapped by absorbent cotton and sealed by a transparent adhesive tape at the outer layer to prevent the fracture from being damaged by external force and prevent a cutting fluid from polluting and corroding the fracture during linear cutting, and a fracture sample with a crack surface is cut into a section with the length of 10-20 mm by utilizing the linear cutting so as to be convenient to clean and place into a scanning electron microscope for observation.
4. And (3) sequentially removing the transparent adhesive tape wound on the outer side of the fracture sample and the wrapped absorbent cotton after the wire cutting, carefully placing the sample into a beaker filled with 50-80 ml of acetone solution, enabling the fracture of the sample to face upwards, then placing the beaker into an ultrasonic cleaning machine for ultrasonic vibration cleaning, and repeating the steps for 2-3 times until the cleaned acetone solution is clean, wherein the cleaning temperature is 10-30 ℃ and the cleaning time is 5-10 min. And then putting the sample into a beaker filled with 50-80 ml of alcohol solution, enabling the fracture of the sample to face upwards, then putting the beaker into an ultrasonic cleaning machine for ultrasonic vibration cleaning, wherein the cleaning temperature is 10-30 ℃, the cleaning time is 5-10 min, and repeating the steps for 2-3 times. And finally, taking out the sample, completely drying the sample by using cold air of an electric hair drier, and putting the sample into a glassware and conveying the glassware to a scanning electron microscope for fracture observation.
5. Magnifying 500 ~ 1000 times under scanning electron microscope and observing on the fracture with the corresponding region of original crack face, establish as the micro morphology characteristic in A district, according to steel material fracture science professional common general knowledge, fracture morphology characteristic mainly divide into when intact steel material breaks under the effect of internal and external stress: the fracture is mainly determined by the method according to the crack formation time period. The following three observations were made:
1) and if the area A is one or more of a dimple fracture, a cleavage fracture or a quasi-cleavage fracture, judging that the crack formation period is after the transformation of austenite → ferrite + pearlite is completed.
2) If the area A is a fracture along the crystal, measuring and counting the average diameter d of 30-50 crystal grains on the fracture1Simultaneously measuring and counting the average diameter d of 30-50 ferrite and pearlite grains of the material2、d3E.g. d1＞d2And d is1＞d3It means that the path along which the crack propagates at the time of crystal fracture is not ferrite and pearlite grain boundaries but austenite grain boundaries, and the crack is formed in the austenite phase region; e.g. d2And/or d3And d1In the approach, the crack propagation path along the crystal fracture is ferrite and pearlite grain boundaries, and the crack is formed after the transformation of austenite → ferrite + pearlite is completed; according to the common sense of steel materials, d is unlikely to occur2＞d1Or d3＞d1The case (1).
3) If the microstructure of the region "a" does not have any of the above-described characteristics, it is determined that the crack formation period is in the heating or hot working process before the start of transformation of "austenite → ferrite + pearlite", and the shape of the crack is destroyed after hot working such as rolling or forging, and the crack does not have the above-described fracture characteristics.
Compared with the existing metallographic observation method, the tensile fracture analysis method does not damage the appearance of the crack and the microstructure nearby the crack, can improve the judgment accuracy of the crack formation time period, and provides important basis for analyzing the formation reason of the crack and making precautionary measures.
Drawings
FIG. 1 is a schematic representation of a tensile specimen containing a crack.
FIG. 2 is a micro-morphology characteristic of a fracture, which belongs to a schematic diagram of a cleavage-type fracture.
Detailed Description
Example 1
Taking a crack in a Q345D bar material with the specification of phi 250mm as an example, the crack is found in a macroscopic examination, and in order to protect the appearance of the crack, clean antirust oil is dripped into the crack to isolate air and water, and the antirust oil is wrapped by an adhesive tape to prevent the antirust oil from losing. Processing a tensile sample containing cracks at the cracks, wherein the thickness of the tensile sample is 5mm, the length of the tensile sample is 150mm, stretching the tensile sample at room temperature on a microcomputer-controlled electronic universal tester, and setting the displacement speed of a cross beam of the tensile tester to be 0.01mm/min to ensure that the sample is broken along the cracks until the sample is broken along the cracks, and mechanically separating two crack surfaces without damaging the original appearance of the crack surfaces. In order to prevent the fracture from being oxidized or corroded due to long-time contact with air, dust and water after the fracture is broken, firstly, the fracture is protected, clean antirust oil is dripped into the fracture to isolate water, dust and oil stains, the fracture is wrapped by absorbent cotton and sealed by transparent adhesive tape at the outer layer to prevent the fracture from being damaged by external force and prevent cutting fluid from polluting and corroding the fracture during linear cutting, and a fracture sample with a crack surface is cut into a section with the length of 15mm by utilizing the linear cutting so as to be convenient to clean and place into a scanning electron microscope for observation.
And (3) sequentially removing the transparent adhesive tape wound on the outer side of the fracture sample and the wrapped absorbent cotton after the wire cutting, carefully placing the sample into a beaker filled with 50ml of acetone solution, enabling the fracture of the sample to face upwards, then placing the beaker into an ultrasonic cleaning machine for ultrasonic vibration cleaning, wherein the cleaning temperature is 25 ℃, the cleaning time is 5min, repeating the steps for 3 times, and completely cleaning the cleaned acetone solution. And then putting the sample into a beaker filled with 50ml of alcohol solution, enabling the fracture of the sample to be upward, then putting the beaker into an ultrasonic cleaning machine for ultrasonic vibration cleaning, wherein the cleaning temperature is 25 ℃, the cleaning time is 5min, and the steps are repeated for 3 times. And finally, taking out the sample, completely drying the sample by using cold air of an electric hair drier, and putting the sample into a glassware and conveying the glassware to a scanning electron microscope for fracture observation.
The microstructure characteristics of the area corresponding to the original crack surface on the fracture are observed under a scanning electron microscope at 836 times of magnification, and the fracture is mainly a cleavage-type fracture in a river-like pattern, so that the crack is determined to be formed after the transformation of the structure of austenite → ferrite + pearlite is finished, and corresponding measures are taken in the cooling process after the transformation of the structure in order to prevent the crack.
Claims (1)
1. A method for judging a tensile fracture in a crack formation period of a steel material is characterized by comprising the following specific steps and parameters:
1) when a crack exists in the steel, performing protection treatment on the crack at the first time to prevent water, dust and oil stain from entering the crack to damage or corrode the original appearance of the crack, dripping clean antirust oil into the crack to isolate air and water, winding and wrapping the antirust oil by using an adhesive tape to prevent the antirust oil from losing, and completing the following steps within 24 hours;
2) processing a tensile sample containing cracks at the cracks of the sample, wherein the size of the tensile sample is determined according to the size of the cracks, the tensile sample is stretched at room temperature on a microcomputer-controlled electronic universal testing machine, and in order to ensure that the sample is broken along the cracks, the displacement speed of a cross beam of the tensile testing machine is controlled to be 0.005-0.01 mm/min until the sample is broken along the cracks, so that two crack surfaces can be mechanically separated without destroying the original appearance of the crack surfaces;
3) in order to prevent the fracture from being oxidized or corroded due to long-time contact with air, dust and water after the fracture is broken, firstly, protecting the fracture, fully dripping clean antirust oil into the fracture to isolate water, dust and oil stains, wrapping the fracture by absorbent cotton, sealing the fracture by using a transparent adhesive tape at the outer layer to prevent the fracture from being damaged by external force and prevent a cutting fluid from polluting and corroding the fracture during linear cutting, and cutting a section with the length of 10-20 mm from a fracture sample with a crack surface by using linear cutting so as to be convenient for cleaning and putting into a scanning electron microscope for observation;
4) sequentially removing transparent adhesive tapes and wrapped absorbent cotton wound on the outer side of a fracture sample after linear cutting, putting the sample into a beaker filled with 50-80 ml of acetone solution, enabling the fracture of the sample to face upwards, then putting the beaker into an ultrasonic cleaning machine for ultrasonic vibration cleaning, and repeating the steps for 2-3 times until the cleaned acetone solution is clean, wherein the cleaning temperature is 10-30 ℃ and the cleaning time is 5-10 min; putting the sample into a beaker filled with 50-80 ml of alcohol solution, enabling the fracture of the sample to face upwards, putting the beaker into an ultrasonic cleaning machine for ultrasonic vibration cleaning, wherein the cleaning temperature is 10-30 ℃, the cleaning time is 5-10 min, and repeating the steps for 2-3 times; finally, taking out the sample, completely drying the sample by using cold air of an electric hair drier, putting the sample into a glassware, and sending the glassware to a scanning electron microscope for fracture observation;
5) observing the microscopic morphology characteristics of the area corresponding to the original crack surface on the fracture under a scanning electron microscope at an amplification of 500-1000 times, and setting the microscopic morphology characteristics as an 'A area'; the following three observations were made:
① if the A area is one or more of a dimple fracture, a cleavage fracture or a quasi-cleavage fracture, judging that the crack formation period is after the transformation of austenite → ferrite + pearlite is completed;
② if the region A is along the fracture, measuring and counting the average diameter d of 30-50 crystal grains on the fracture1Simultaneously measuring and counting the average diameter d of 30-50 ferrite and pearlite grains of the material2、d3E.g. d1＞d2And d is1＞d3It means that the path along which the crack propagates at the time of crystal fracture is not ferrite and pearlite grain boundaries but austenite grain boundaries, and the crack is formed in the austenite phase region; e.g. d2And/or d3And d1In the approach, the crack propagation path along the crystal fracture is ferrite and pearlite grain boundaries, and the crack is formed after the transformation of austenite → ferrite + pearlite is completed; according to the common sense of steel materials, d is unlikely to occur2＞d1Or d3＞d1The case (1);
③ if the microstructure of the A region does not have any of the above characteristics, it is judged that the crack formation period is in the heating or hot working process before the transformation of austenite → ferrite + pearlite begins, and the microstructure of the crack is destroyed after the hot working of rolling and forging, and the crack does not have the above fracture characteristics.
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