CN113960163A

CN113960163A - Inspection and analysis method for heat treatment cracks of 30CrMo valve body

Info

Publication number: CN113960163A
Application number: CN202111127736.1A
Authority: CN
Inventors: 黄智�; 宋雷钧; 李祝茂
Original assignee: Jiangsu Yulong Forging Co ltd
Current assignee: Jiangsu Yulong Forging Co ltd
Priority date: 2021-09-26
Filing date: 2021-09-26
Publication date: 2022-01-21

Abstract

The invention relates to a method for inspecting and analyzing cracks of a 30CrMo valve body, which is characterized by comprising the following steps of carrying out flaw detection on a valve body forging stock by ultrasonic waves twice, wherein one flaw detection is carried out before the heat treatment of the valve body forging stock, and the other flaw detection is carried out after the heat treatment of the valve body forging stock, and the valve body is a cross valve: s1: numbering the partitions; carrying out flaw detection on each area of the valve body forging stock through ultrasonic waves; s2: performing data statistics on the defective area, wherein the statistics comprises the defect size, the defect depth, the bottom wave and the representative waveform; s3: sampling, detecting and analyzing the valve body forging stock, and judging whether the chemical components, the mechanical properties and the metallographic structure of the steel ingot reach the standard or not; s4: and judging which step of the valve body manufacturing process the crack of the valve body is related to according to the analysis results of S2 and S3. The invention avoids the waste of steel ingots, improves the forging efficiency and saves the labor cost and the time cost of forging.

Description

Inspection and analysis method for heat treatment cracks of 30CrMo valve body

Technical Field

The invention also relates to a method for inspecting and analyzing the heat treatment cracks of the 30CrMo valve body, belonging to the technical field of steel forging.

Background

As shown in fig. 1 and 2, the heat treatment method of the 30CrMo valve body comprises the following steps:

1) putting the steel ingot into a furnace and heating to 1250 ℃;

2) uniformly heating the S1 steel ingot at 1250 ℃ for 4 h;

3) forging the S2 steel ingot, wherein the forging process comprises pressing bar chamfering, drain pan upsetting and drawing out; the forging ratio is 4: 1;

4) cooling the forged steel ingot, and then carrying out normalizing heat treatment;

5) performing primary processing on the forged piece subjected to normalizing heat treatment, and performing primary flaw detection on a valve body forged blank basically formed after the primary processing; the valve body is a cross valve;

6) carrying out quenching and tempering heat treatment on the valve body forging stock without obvious flaw in the first flaw detection;

7) carrying out secondary flaw detection on the valve body forging stock subjected to quenching and tempering heat treatment, and if the flaw detection result is normal, determining that the valve body is a qualified valve body; and if the flaw detection result is abnormal, the valve body is unqualified.

At present, a method for detecting and analyzing the defects of the cross valve body does not exist, when unqualified products appear, the problem that the quality of the steel ingot is difficult to judge or the problem that the operation is improper during heat treatment is caused, so that the responsibility cannot be traced, and meanwhile, the processing method or raw materials cannot be timely improved, so that the waste of materials, labor cost and time cost is caused.

Disclosure of Invention

The invention aims to overcome the problems in the prior art and provide a 30CrMo valve body heat treatment method which can ensure the safety of data and ensure the real-time property of the data under the condition of large data volume.

The invention discloses a method for inspecting and analyzing heat treatment cracks of a 30CrMo valve body, which is characterized in that flaw detection is carried out on a valve body forging stock by ultrasonic waves twice, one flaw detection is carried out before the heat treatment of the valve body forging stock, the other flaw detection is carried out after the heat treatment of the valve body forging stock, and the valve body is a cross valve and comprises the following steps:

s1: numbering the partitions; carrying out flaw detection on each area of the valve body forging stock through ultrasonic waves;

s2: performing data statistics on the defective area, wherein the statistics comprises the defect size, the defect depth, the bottom wave and the representative waveform;

s3: sampling, detecting and analyzing the valve body forging stock, and judging whether the chemical components, the mechanical properties and the metallographic structure of the steel ingot reach the standard or not;

s4: and judging which step of the valve body manufacturing process the crack of the valve body is related to according to the analysis results of S2 and S3.

Further, if the representative waveform described in S2 at the time of the first flaw detection is a grass-like wave, it indicates that the internal structure of the forging is loose.

Further, the crack described in S3 is a stress crack if the gap is fine and the tail is sharp.

Further, the sampling comprises low-power sample sampling and metallographic structure sampling.

Further, the method for detecting the macroscopic sample comprises the following steps: corroding the macroscopic sample in a 1:1 hydrochloric acid aqueous solution at 70 ℃ for 25 minutes, judging the macroscopic structure form except the original cracks, and if no white band, slag and white spot appear, indicating that the cracks and the macroscopic sample acid-washed surface have no obvious loose, macroscopic visible inclusion and holes and have no direct correlation.

Further, if the metallographic structure detection result shows that dendrite segregation is serious, the stress is structural stress, and the occurrence of cracks in the valve body is related to the quality of the steel ingot and the forging quality.

Further, if the metallographic structure of S3: the metallographic structure on the surface of the valve body is quenched martensite and tempered, and when the structure from the inside of the valve body is a pearlite structure directly converted from austenite, the valve body is not quenched completely in the quenching treatment, and the stress crack is a brittle crack.

The invention has the beneficial effects that: (1) the method has the advantages that the reasons of forge piece scrapping caused by cracks generated during forging can be detected and classified in time, an improved square needle is provided for subsequent forging, waste of steel ingots is avoided, forging efficiency is improved, and labor cost and time cost of forging are saved;

(2) because the raw material cost and the manufacturing cost of the forge piece are very expensive, and the requirement of an application scene on the forge piece is high, the invention can not only avoid scrapping of a large amount of forge pieces in the subsequent forging process, but also accurately explore the responsibility of scrapping the forge pieces.

Drawings

FIG. 1 is a temperature profile of a valve body normalizing heat treatment of the present invention;

FIG. 2 is a temperature profile of a modulating heat treatment of a valve body according to the present invention;

FIG. 3 is a waveform diagram of ultrasonic flaw detection in Point No. 3 according to the present invention;

FIG. 4 is a waveform diagram of ultrasonic flaw detection in Point No. 5 according to the present invention;

FIG. 5 is a waveform diagram of ultrasonic flaw detection in point No. 10 according to the present invention;

FIG. 6 is a schematic view of an internal fracture surface of the valve body of the present invention;

FIG. 7a is a schematic view of the macro-topography of the valve body and the sampling of the macroscopic sample according to the present invention;

FIG. 7b is a graph of the surface crack topography of the A and C surfaces of FIG. 7 a;

FIG. 7c shows the surface crack shapes of the surfaces B and D in FIG. 7 a;

FIG. 8a is a schematic view of the No. 1 macroscopic specimen tissue;

FIG. 8b is an enlarged view of the rectangular box of FIG. 8 a;

FIG. 8c is a schematic view of the No. 2 macroscopic specimen tissue;

FIG. 8d is a schematic view of the tissue of No. 3 macroscopic sample;

FIG. 8e is a schematic view of the No. 4 macroscopic specimen tissue;

FIG. 9 is a schematic view of the appearance of the fracture side surface of sample No. 1;

FIG. 10 is a schematic diagram of the structure of the fracture-side surface of sample No. 1;

FIG. 11 is a schematic view of the matrix structure of sample No. 1;

FIG. 12 is a schematic view of the appearance of the main crack of sample No. 2;

FIG. 13 is a crack morphology schematic diagram of sample No. 2;

FIG. 14 is a schematic diagram of the crack morphology of sample No. 2;

FIG. 15 is a schematic view of the main crack structure of sample No. 2;

FIG. 16 is a schematic view of the crack structure of sample No. 2;

FIG. 17 is a schematic view of the branched crack structure of sample No. 2;

FIG. 18 is a schematic view of the matrix structure of sample No. 2;

FIG. 19 is a first schematic view of the crack morphology of sample No. 3;

FIG. 20 is a second schematic view of the crack morphology of sample No. 3;

FIG. 21 is a first schematic view of the crack structure of sample No. 3;

FIG. 22 is a second schematic view of the crack structure of sample No. 3;

FIG. 23 is a third schematic view of the crack structure of sample No. 3;

FIG. 24 is a first schematic view of the crack morphology of sample No. 5;

FIG. 25 is a second schematic view of the crack morphology of sample No. 5;

FIG. 26 is a third schematic view of the crack structure of sample No. 5;

FIG. 27 is a schematic view of the crack structure of sample No. 5;

FIG. 28 is a first schematic view of the matrix structure of sample No. 5;

FIG. 29 is a second schematic view of the matrix structure of sample No. 5;

FIG. 30 is a flow chart of the present invention.

Detailed Description

The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic views illustrating only the basic structure of the present invention in a schematic manner, and thus show only the constitution related to the present invention.

As shown in fig. 30, the method for inspecting and analyzing heat treatment cracks of a 30CrMo valve body according to the present invention detects flaws of a valve body forged blank by ultrasonic waves twice, wherein one flaw detection is performed before the heat treatment of the valve body forged blank, and the other flaw detection is performed after the heat treatment of the valve body forged blank, and the valve body is a cross valve, and the method comprises the following steps:

the statistical results are shown in table 1:

representative waveforms are shown in fig. 3-5. If the representative waveform is a grass-shaped wave in the first flaw detection, the internal structure of the forging is loose.

According to the table 1 and the distribution condition of the cracks on the surface of the valve body, the internal fracture surface of the valve body is preliminarily determined as shown in fig. 6.

the crack in S3 is a stress crack if the gap is small and the tail is sharp. As shown in FIGS. 7a to 7c, the appearance of the A-plane crack is shown in the 'A-plane partial discharge' in FIG. 7b, and a longitudinal straight crack is arranged at a position about 5mm away from the top of the cross valve and meets an arc-shaped crack formed at the transition arc of the cross step. The tail parts of two ends of the arc-shaped crack at the transition arc of the cross step are tapered; the appearance of the C-face crack is shown in figure 7b as "C-face partial discharge" and is located in the top circular part of the cross valve. C surface cracks are expanded to positions about 30mm away from two sides of the top circle and are stopped, the cracks almost penetrate through the diameter, and tail parts of two ends of the cracks are sharp; the appearance of the crack on the surface B is shown in the partial discharge on the surface B in fig. 7c, and is a cross valve transverse crack which is equivalent to a longitudinal crack on a transverse cross part of the cross valve; the distribution state of the surface D crack appearance is similar to that of the surface A arc crack, and is shown as the 'surface D partial discharge' in fig. 7 c. The cracks are rigid and slender and belong to stress cracks.

If the metallographic structure detection result shows that the dendrite segregation is serious, the stress is the structural stress, and the crack of the valve body is related to the steel ingot quality and the forging quality.

The sampling comprises low-power sample sampling and metallographic structure sampling.

The method for detecting the macroscopic sample comprises the following steps: corroding the macroscopic sample in a 1:1 hydrochloric acid aqueous solution at 70 ℃ for 25 minutes, judging the macroscopic structure form except the original cracks, and if no white band, slag and white spot appear, indicating that the cracks and the macroscopic sample acid-washed surface have no obvious loose, macroscopic visible inclusion and holes and have no direct correlation. As shown in fig. 8a to 8e, the macroscopic samples had dense structures and remarkable dendrites, and had no defects such as white band, slag, white spots, and the like.

The metallographic structure sampling part is shown in figure 7b, the fracture surface of the No. 1 metallographic specimen is split into two halves during sampling, the surface appearance of the fracture side surface of one half of metallographic specimen is shown in figure 9, the fracture surface is in a sawtooth shape and is filled with iron oxide, and no impurities are nearby. And (5) observing corrosion, and ensuring that the fracture surface is not decarburized. The zonal segregation structure is obvious, the structure is tempered sorbite and bainite, as shown in figure 10, and the matrix structure is the same as the fracture side structure. Both tempered sorbite and bainite are shown in figure 11.

The appearance of the main crack of the No. 2 metallographic specimen is shown in the figure 12 and the figure 13, branch cracks are arranged on two sides of the main crack, shown in the figure 13, and the branch cracks are distributed in a sawtooth shape, shown in the figure 14. Iron oxide is filled in the main cracks and the branch cracks, and no inclusion exists in the cracks and the periphery of the cracks. The corrosion was observed that the main cracks were not decarburized, see FIG. 15, and the branch cracks were decarburized and distributed along the crystal, see FIGS. 16 and 17. Dendritic crystal segregation is obvious in matrix structure, and the structure is tempered sorbite and bainite as shown in figure 18.

The No. 3 metallographic specimen has a large number of cracks and branches, the branches are distributed in a zigzag manner, iron oxide is filled in the cracks, and the dendritic oxidation phenomenon exists near the tail parts of the cracks. No inclusions are formed in and around the main crack and the branch crack, as shown in FIGS. 19 and 20, the branch crack is decarburized, the decarburized layer has a depth of about 0.04mm and is distributed along the crystal, as shown in FIGS. 21, 22 and 23, and the vicinity of the crack and the matrix structure are the same as those of sample No. 2. The metallographic examination result of No. 4 was similar to that of No. 2.

The sampling position of the No. 5 metallographic sample is about 2/5 distance from the surface of the forging piece, see fig. 8d, the appearance of the sample surface crack is shown in fig. 24, the crack is zigzag and serrated, holes are formed near the crack, and no inclusions are formed in and near the crack, see fig. 25. The cracks are not decarburized and are distributed along the branches of the dendrite structure, see fig. 26. The dendritic crystal of the microstructure is more obvious, the structure among branches is tempered sorbite and bainite, and the structure of branches is troostite, reticular ferrite, tempered sorbite and bainite, as shown in figure 29.

The structure mainly comprising tempered sorbite and bainite belongs to a structure of quenched martensite tempering, Torsite and network ferrite belong to a pearlite structure which is directly transformed from austenite in a pearlite transformation region after being cooled, the hardness and the strength of the two structures are greatly different, the stress difference generated by the adjacent part structures is large, and the stress difference is the structure stress. The more the microscopic component segregation, the greater the stress difference and the greater the structural stress.

The forging has larger dendritic crystal and microsegregation degree, and is mainly related to two factors of steel ingot quality and forging quality. If the control of the casting process of the steel ingot is poor in the casting process, the dendritic structure of the steel ingot is inevitably thick, and the corresponding macro-composition segregation is serious, thereby affecting the forging quality. If the as-cast structure of the steel ingot is normal, the forging ratio is smaller or the forging deformation is insufficient, the as-cast dendritic crystal structure cannot be effectively crushed, and the dendritic crystal structure of the forging is obvious.

When the metallographic structure on the surface of the valve body is quenched martensite and tempered, and the structure of the valve body at about 2/5 away from the surface is a pearlite structure directly transformed from austenite, the valve body is not quenched completely or quenched and fired, and the stress crack is a brittle crack.

For work pieces that cannot be through hardened, the heat treatment stress is mainly type ii or type iii. Type ii stress is the tensile stress resulting from the superposition of structural and thermal stresses, with the tensile stress peaks at the through-hardened and through-hardened interfaces. The type III stress is a tensile stress caused by a pure thermal stress, and a tensile stress peak is positioned in the center of a workpiece. The two stresses, particularly the III-type stress, are easy to cause the workpiece to generate longitudinal splitting and transverse splitting cracks, and the crack source is positioned in the workpiece. Type II and type III stresses generally act on the interior of a material in a three-dimensional tensile stress mode, and the theory of metal mechanical properties shows that metal is in a plane strain state under the action of the three-dimensional tensile stress, the plastic deformation capacity is greatly restricted, and low-stress brittle fracture occurs. That is, brittle fracture occurs below the material yield strength. This analysis better explains that the majority of crack tails are distributed along the crystal by metallographic examination, and conversely, also confirms the correctness of the theoretical analysis.

Therefore, the straight cracks in the longitudinal direction and the transverse direction of the cross valve are brittle cracks formed under the action of type II and type III stress.

As can be seen from FIG. 2, the quenching cooling medium of the cross valve forging is water. The cross valve has large section size, the effective thickness of heat treatment reaches 457mm, and the heat storage amount in the quenching heating process is large, so that the heat stress is increased and the risk of quenching cracking is increased by directly cooling the cross valve to room temperature by using water. Secondly, the dendrite of the cross valve forging is obviously thick, the segregation of microcosmic components is serious, and the structural stress is large, so that the stress bearing capacity of the cross valve forging is greatly reduced. For large forgings with internal defects, water quenching is undoubtedly snow frosting. In addition, two pieces of cross valve forgings without defects in the ultrasonic flaw detection process form quenching cracks at corner transition arcs, and the forgings are scrapped after all although the crack forming mechanisms are different. Therefore, according to the section size and the geometric shape of the cross valve forging, no matter whether the interior of the material is flawless, a corresponding quenching crack prevention measure is adopted in the heat treatment quenching process.

In summary, the cause of the quench cracking is related to factors such as the quench cooling rate, the segregation of the dendrites, the cross-sectional size and the geometry of the cross valve forging.

In light of the foregoing description of the preferred embodiment of the present invention, many modifications and variations will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. The utility model provides a 30CrMo valve body heat treatment crackle's inspection analysis method, detects a flaw to valve body forging stock through the ultrasonic wave, detects a flaw and carries out twice, and one time detects a flaw before valve body forging stock heat treatment, and another detects a flaw after valve body forging stock heat treatment, the valve body is the cross valve, its characterized in that includes the following step:

2. The method for inspecting and analyzing the heat treatment cracks of the 30CrMo valve body according to claim 1, wherein the representative waveform in S2 is a grass-like wave at the first flaw detection, which indicates that the internal structure of the forging is loose.

3. The method for inspecting and analyzing 30CrMo valve body heat treatment cracks as claimed in claim 1, wherein the cracks described in S3 are stress cracks if the gap is fine and the tail part is sharp.

4. The inspection and analysis method for the heat treatment cracks of the 30CrMo valve body according to claim 1, characterized in that: the sampling comprises low-power sample sampling and metallographic structure sampling.

5. The inspection and analysis method for the heat treatment cracks of the 30CrMo valve body according to claim 3, characterized in that: the method for detecting the macroscopic sample comprises the following steps: corroding the macroscopic sample in a 1:1 hydrochloric acid aqueous solution at 70 ℃ for 25 minutes, judging the macroscopic structure form except the original cracks, and if no white band, slag and white spot appear, indicating that the cracks and the macroscopic sample acid-washed surface have no obvious loose, macroscopic visible inclusion and holes and have no direct correlation.

6. The inspection and analysis method for the heat treatment cracks of the 30CrMo valve body according to claim 3, characterized in that: if the metallographic structure detection result shows that the dendrite segregation is serious, the stress is the structural stress, and the crack of the valve body is related to the steel ingot quality and the forging quality.

7. The inspection and analysis method for the heat treatment cracks of the 30CrMo valve body according to claim 3, characterized in that: if the metallographic structure of S3: when the metallographic structure on the surface of the valve body is quenched martensite and tempered, and the structure inside the valve body is a pearlite structure directly converted from austenite, the valve body is not completely quenched inside during quenching treatment, and the stress crack is a brittle crack.