The fishtail bolt is cracked. What is the problem?

Release time:

2021-09-26

The fishtail bolt is cracked. What is the problem?

A batch of Φ21.66 wire, material brand 20MnTiB, produced M24 grade 10.9 fishtail bolts, and found cracking in the production process. After a series of testing and analysis, not only in the raw materials found material segregation and hair lines, but also, in the series of testing and analysis, the heat treatment after the product crack expansion, extension, while the proportion of cracking products increased; the reason, in addition to the raw material itself, but also closely related to the heat treatment process.

Production status of fishtail bolt

1. Technical conditions of the product: using Φ16.88mm diameter wire and cold heading forming method to produce M24 grade 10.9 fishtail bolts.

2, production process: after the material cold heading forming, the blank for rolling wire, the finished product sent to the heat treatment company for surface carburizing quenching.

3. After heat treatment, check and find that the product is cracked seriously (as shown in the red box in Figure 1); The crack on the left of Figure 1 extends from the head to the neck and runs through the whole head. Figure 1 right, the crack extends from the joint of the hosel to the position of the shank. The total length of the crack reaches more than 3CM, and the thread below the head shows an arbitrary and curved arc state. Figure 1 Right, the crack length is increased compared to the raw material.

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Figure 1 Cracking of fishtail bolt　　

Wire condition

The cracking of fishtail bolts has seriously affected the quality of the company's products and the reputation of the company. It is necessary to trace the root cause of the fracture. According to the production statistics of the quality assurance department, the proportion of cracks found in the latter half of the cold heading process is 5%-6%. After material selection, some products are found to be cracked after heat treatment. The total number of cracks in the two parts of the products exceeds 10% of the total number of products, causing high attention from enterprises.

First of all, check the actual situation of the production of the first line of wire production, as the first basis for finding and judging the root causes of cracks. In the workshop site analysis, there are some cracks in the material (see Figure 2).

1. wire condition

Judging from the actual situation of the wire, there are obvious cracks on the surface of the wire. These cracks belong to the type of tiny cracks-hair lines. Judging from the left of Figure 2, the length and depth of the hair lines are different. On the left of Figure 2, the crack is very obvious and has a certain depth. The length of the hair lines reaches 5CM. On the right of Figure 2, the hair lines are relatively shallow and short, but the length also reaches about 2CM, the appearance of these cracks seriously affects the quality of the product. Cracks of different sizes and shades are found in many product cuts. According to the statistics of the quality department, the proportion of crack blanks reaching more than 5% of the total amount is produced. As can be seen from Figure 2 above without cold heading, these cracks are generated in the raw material.

2. detection condition:

In order to further check the causes and analyze the cracks, we have carried out various inspections on materials, semi-finished products, and heat-treated products.

1. Hardness test:

(1) Hardness of raw materials:

The hardness of raw materials is in the HRB85.6-HRB88.7 range, and the average value is HRB87.1. According to the technical requirements of raw materials, the hardness is controlled within the range of ≦ HRB90. Although the hardness of the material is on the high side, the difficulty of cold heading deformation is increased, but within the required range, the technical requirements are met.

(2) Hardness after heat treatment:

The hardness of the bolt after heat treatment, whether it is a broken bolt or a normal bolt, is in the range of HRC33- HRC36.9, which meets the technical requirements of the 10.9 high-strength bolt HRC32-HRC39. The hardness is the appearance of performance and must be further analyzed.

2. Chemical composition analysis

3. Crack detection:

The cracked raw materials were tested by metallographic sample preparation, as shown in fig. 3, the left and broken raw materials were cracked (opening width: 41.47 μm, opening depth: 25.73 μm); Fig. 3 right, fracture raw material crack (opening: 37.27 μm, depth: 44.52 μm), the width and depth of the crack are different; However, the cracks of these raw materials will increase the possibility of crack expansion for cold heading, it affects product quality and increases the difficulty of heat treatment and the risk of crack propagation.

4. Annealed microstructure:
　
The state of 20MnTiB material entering the company is annealed. We made the material into a metallographic sample and observed its organization (see Figure 4)

According to the metallographic picture of annealed microstructure, the microstructure state is pearlite + ferrite, and the microstructure distribution in most areas is relatively uniform, and there is no more than normal state where large pieces of ferrite are concentrated. Generally speaking, the metallographic structure belongs to a qualified and acceptable microstructure form. However, in some areas (the part in the black circle in the metallographic picture), there is a phenomenon of concentrated segregation of carbides. If these carbides are concentrated on the surface of the product, carbide will affect the deformation of the product to a certain extent, and the area with large deformation is prone to cracking, which is also one of the reasons for the formation of cracks, which must be paid attention.

5. Examination of tempered tissue:

According to the national standard, GB/T13320 "steel die forgings gold phase tissue rating chart" rating standard, the gap tissue is: tempering sothebody ferrite, the tissue form is relatively small, rated as the quenching and tempering tissue level 2, belongs to the normal qualified gold phase tissue. The left picture is the metallographic picture of the cracked product, the microstructure is slightly worse than the normal bolt on the right, and the metallographic distribution of the normal bolt on the right is more uniform and the microstructure is more excellent.

6. Condition of crack after heat treatment

Heat treatment process due to heating, cooling process, thermal stress, tissue stress increased significantly, increasing the risk of cracking products, especially in the material cracks will be extended, and even fracture, whether it is product cracking, or bolt fracture, is not allowed to exist. The crack opening in fig. 6 is the condition after heat treatment. the crack after heat treatment is larger than that in the material. the opening width of the crack is 129.51 μ m, which is more than 3 times of the original crack width of 41.47μm. The depth of the crack reaches 109.47 μ m, which is about 4 times of the original crack depth of 25.73μm. Moreover, new cracks appear at the lower end of the notch and below the original crack, which is evidence of thread expansion caused by heat treatment. Compared with the initial crack, the heat-treated thread has relatively large changes in length, width and shape. These changes are the thermal strain caused by thermal stress after heat treatment, which promotes crack expansion and growth. This is due to improper selection of heat treatment process, there are significant connections.

Analysis of the causes of cracks

1. Initial crack:

There are initial cracks in the material. As can be seen from the macroscopic photos in Figure 2, both the left and right images show cracks in the raw materials. The size and depth of the cracks are different (Figure 3 metallographs crack photos), and they all show straight lines.

2. After testing, the hardness of the material is within the required range, the chemical composition of the material meets the technical standard, and the quality adjustment organization and annealing organization of the material meet the standard requirements.

3. Crack propagation:

The initial crack is expanded during the heat treatment process (Figure 6); From Figure 1, it can be seen that the surface of the head flange ring is straight, which is the shape of the original thread. The shape of the neck crack below begins to change, which is an irregular crack. The shape of the crack is an irregular arc. The length of the thread increases and cracks until the shank of the bolt. It can be seen that the crack is the original crack of the hair began to grow and develop, after heat treatment to form the head to the rod of the relatively long crack, the length and width of the crack compared with the original hair, in the continuous increase, expansion.

4. Improper carburizing heat treatment process:

20MnTiB material production of 10.9 grade high strength bolts to be carburized quenching. However, in the process of carburizing and quenching, the quality of quenching and tempering is not in place due to the selection or improper arrangement of the process, which in turn causes cracking.

1) Improper carburizing process: as can be seen from fig. 8 below, the thickness of carburizing layer is uneven, with deep carburizing layer in some areas and shallow carburizing layer in some areas; In the surface range of less than 150μm in the picture, the carburized layer on the upper left of fig. 8 is 70.91 μm, while the carburized layer close to the lower part reaches 106.70 μm, which shows that the carburized layer is extremely uneven, this is the cause of improper selection of carburizing process parameters.

2) Large quenching stress: In Figure 7, the head of the fishtail bolt is cracked, and the shape of the crack is irregular arc. The arc is arbitrary and irregular. The shape is similar to the description of fastener surface defects and bolt quenching crack (see GB/T5779 standard for details).

3), uneven seepage layer, reflected in the carburizing process, to increase the temperature instead of carburizing time, resulting in thick and thin uneven seepage layer, increasing the internal thermal stress; At the same time, after carburizing, quenching is carried out when the precooling time and precooling temperature are not in place, thus irregular crack arcs are generated by thermal stress → thermal strain → cracking, which is caused by improper selection of heat treatment process parameters, it is the main cause of quenching crack.

Cracks are always initiated in the weakest part of the material, and cracks are easy to occur on the surface or defects of the bolt. Inclusions, segregation or second phase particles in the interior of the material tend to pile up at the defect, resulting in stress concentration, which may develop and transform into strain, thus leading to crack initiation. Once the crack occurs, it will continue to develop and expand in various processes. If the fastener heat treatment quenching and tempering process properly, can improve the microstructure; On the contrary, improper heat treatment quenching and tempering, will promote the continuous development of small defects.

Conclusion:

There are two reasons for the cracking of fishtail bolts:

Internal cause: material segregation and hairline of raw materials are the primary causes of cracks; external cause: carburizing heat treatment process is not in place, which is the driving force to increase cracks.

(Source: Golden Spider Fastener Web)