What are the defects caused by the forming process of fasteners

During the processing and manufacturing of fasteners, due to unreasonable design, improper material selection, processing technology, heat treatment and surface treatment technology, the quality of fasteners will be poor, resulting in surface or internal defects. The defects of fasteners caused by improper forming, heat treatment and surface treatment processes are introduced in detail below.

There are many kinds of process defects caused by improper forming process. For example, improper forming process leads to uneven coarse grains or grains, improper forming process leads to uneven distribution or flow through the thread streamline, improper thread rolling process leads to defects, improper processing process leads to defects, and improper forming process leads to cracks.

Unevenness of coarse grains or grains caused by improper forming process

When the fastener is handed over to form the head, the deformation of the head shall not be within the critical deformation area. If the deformation ratio of the fastener is improper and the deformation amount is just in the critical deformation zone, the deformed grains of the head will grow after heat treatment. Because the fastener rod is not deformed and the grains of the rod do not grow, the grains at the head and rod are uneven. Another reason is that the uneven deformation of fasteners makes the degree of grain breakage inconsistent, or the local work hardening of the alloy.

The influence of titanium alloy structure on the degree of deformation is very obvious. For example, after titanium alloy bolt buttress the head, different structural characteristics appear in the head and rod. The following figure shows the structural differences between the head and rod of BT16 titanium alloy small flat round head bolt. The microstructure of bolt head is bimodal, α phase is fine granular, as shown in Figure B below. The microstructure of the rod is bimodal, α phase is in fine needle shape, as shown in Figure c below.

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Heat resistant steels and superalloys are particularly sensitive to uneven grains, which significantly reduces the durability and fatigue performance of bolts. Under the action of assembly or working stress, cracks occur and expand at the junction of coarse and fine grains. See the following figure for the characteristics.

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Improper forming process will also lead to coarse grains. It is usually caused by too high initial forging temperature, insufficient deformation, too high final forging temperature, deformation falling into the critical deformation zone, etc. Coarse grains may also be caused when the deformation of aluminum alloy is too large to form texture or the deformation temperature of superalloy is too low to form mixed deformation structure. Coarse grains will reduce the plasticity and toughness of fasteners, and also significantly reduce the fatigue performance.

Unsmooth streamline distribution or cross flow caused by improper forming process

The manufacturing process requirements of fasteners are higher and higher. Turning is no longer used for high-strength bolts (nails), and most of them use cold sensitive and heat sensitive forming heads to improve the connection strength of bolts. The thread is rolled, and the metal streamline of the bolt head and the rod is required to be continuously distributed along the shape of the head. Only qualified metal streamline can improve the fatigue strength of fasteners.

Improper forming process leads to unqualified metal flow line at the head of the fastener. The figure below shows that the metal flow line at the head of the bolt is not distributed along the shape of the bolt head.

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The unsmooth distribution of streamline mainly refers to the disorder of streamline such as streamline cutting, reflux and eddy current. It is due to the uneven flow of metal caused by improper design of forging die, unreasonable selection of forging method and improper manual operation. The unqualified metal streamline results in the reduction of many mechanical properties of fasteners, including fatigue strength.
Cross flow is also a form of improper distribution of flow lines, which is formed by the confluence of flow lines that originally formed a certain angular distribution. It is caused by the confluence of two metals or one metal with the other, but unlike folding, the metal in the through flow part is a whole.

Defects caused by improper thread rolling process

The requirements of fastener manufacturing process are higher and higher. Thread rolling process is more used for threads with small size, and rolling process or direct rolling forming is generally used for threads with large size, especially for materials with high requirements for surface integrity, such as titanium alloy. Previous studies have shown that thread rolling strengthening is one of the most effective ways to improve the fatigue life of threaded fasteners.
The defects often caused by screw rolling or rolling of fasteners include folding cracks, root microcracks, holes in threads and local breakage.

Due to improper selection of process parameters, poor material, improper lubrication and other reasons, small folding cracks are easy to occur on the thread surface in the process of threading and rolling. Therefore, when selecting thread rolling parameters, a certain range should be selected according to various data such as thread pitch, material and hardness, and the final range of process parameters should be determined through tests. The figure below shows the folding crack on the thread surface of a titanium alloy bolt caused by improper technology.

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The root microcrack caused by rolling generally occurs at the transition between bolt head and screw r fillet. Due to the influence of R size, it brings some difficulties to the rolling process control and surface quality control. A BT16 titanium alloy butterfly bolt appeared low stress fracture during assembly. Through analysis, it is found that at the transition between the faulty bolt head and the screw r fillet, the surface integrity after rolling is poor, and there are a large number of circumferential linear microcracks, as shown in the figure below.

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Folding cracks generally have the following characteristics:

  • (1) The position and shape of folding crack in each thread are roughly the same;
  • (2) The folding crack is relatively smooth, the crack is wide, and there are no metallurgical defects on both sides;
  • (3) The direction of folding crack is at an included angle with the thread surface and is related to the direction of streamline.

The crack at the root of thread caused by thread rolling or rolling is mainly due to the improper selection of process parameters caused by too hard or too soft materials. Because the crack is located at the root of the thread and the stress concentration is serious, it is easy to lead to the propagation of micro cracks in the process of use, and eventually lead to the early failure of fasteners, which is more harmful. The reason why the material is too hard and too soft can easily lead to the root micro crack is that when the material is too hard, the thread root is easy to produce cold work hardening; Too soft material is generally related to structural factors, such as surface ferrite is also prone to root microcracks.
Holes are serious defects in screw rolling and rolling of fasteners. Hole types include central opening hole, central surface hole and central closed hole.
The effects of rolling process and wire rolling blank diameter on the formation of hole defects have been systematically studied. The researchers carried out process tests with different diameter of wire rolling blank, wire rolling pressure and speed. They are divided into three groups according to the diameter, with 40 pieces in each group and 20 pieces in each of the two processes. The grouping test results are as follows:
(1) Normal blank diameter
The test results of normal wire rolling blank diameter, normal wire rolling pressure and speed and increasing wire rolling pressure and speed.
(2) Increase the blank diameter by 0.02-0.04mm
The blank diameter of the test piece is increased by 0.02-0.04mm.
From the research results, it can be concluded that in practical engineering application, the main cause of hole defects is that the blank size of bolt and screw thread rolling is too large, resulting in the increase of thread rolling and rolling stress. Under the action of large rolling stress, there are open holes in the center of bolt and screw thread end; If the materials of bolts and screws are plastic and the holes are not open, a central closed hole will be formed inside the bolts and butterfly nails.
The thread rolling wheel is a pair of rigid bodies rotating at constant speed, and the distance between them is the bottom diameter of the thread, and the diameter of the thread rolling blank is about the pitch diameter of the thread (greater than the distance between the thread rolling wheels). In this way, the stop pin must be deformed by a pair of concentrated compressive stresses with equal size and opposite direction in the thread rolling, and the thread is formed with the rotation of the thread rolling wheel. Under the action of a pair of concentrated compressive stresses, the stress on the center surface is the largest. The diameter of wire rolling blank is the key to directly affect the stress and deformation. The larger the diameter of the wire rolling blank, the greater the compressive stress and deformation. When this deformation is greater than the plastic index of the material, failure and cracking will inevitably occur. The direction of deformation and failure changes continuously with the rotation of the wire rolling, forming radial cracks on the axis. When the part is separated from the wire rolling wheel after the wire rolling process, due to the residual stress of cold deformation, it rebounds and cracks into holes.

The screw is rolled to cause an open hole in the center of the thread end section, as shown in Fig. 5-16. Irregular hole in the center of screw thread end surface, as shown in Figure 5-17. See Fig. 5-18 for the internal center closed hole of the screw with good material plasticity.

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There is also a kind of local “broken” defect caused by raw material defects during thread rolling or rolling. This kind of defect is mainly caused by serious inclusions or residual shrinkage cavities in the raw materials.

Defects caused by improper processing technology

The closing size of self-locking nut is large. Under the action of closing stress, micro cracks are easy to occur at the closing end. Under the action of assembly or use stress, micro cracks expand into macro cracks. The crack at the closing end may be related to the excessive stress caused by closing. There are also closing cracks caused by excessive residual stress due to local coarse grain of material or improper heat treatment process. The crack at the closing end of a superalloy self-locking nut is shown in the figure below.

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Cracks are common at the closing of self-locking nuts. In addition to the large stress caused by closing, many cases are related to the poor plasticity of materials. This is because the closing crack is essentially that the plastic deformation of the closing exceeds the deformation capacity of the material. For example, cracks appear many times at the closing of a certain type of Superalloy self-locking nut. The reason is that the closing is carried out after aging strengthening. At this time, the cracking occurs due to the high strength and low plasticity of the material due to aging strengthening. Then, the cracking phenomenon is solved by closing first and then aging strengthening.

Improper forming process leads to cracks or over burning defects

Bolts (nails), nuts and other fasteners are forged, and the pier forging temperature is improper. If the pier forging heating temperature is low, cracks are easy to occur, and overburning is easy to occur if the temperature is high. When the forging temperature of the pier is low, the cracks generally appear in the places with large deformation or the largest stress and the thinnest thickness. This is mainly because such cracks are usually caused by large tensile stress, shear stress or additional tensile stress formed during pier forging. Excessive deformation of cold lock will also produce cracks. For example, the 30CrMnSiA cross recessed flat round head screw breaks from the head during assembly, as shown in the figure below. The failure analysis results show that the screw cold pier cross slot is too deep, resulting in microcracks at the root of the cross slot, which has a certain impact on the strength of the screw, which is the root cause of the screw turning.

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Improper pier forging process or unclean oxide scale on the blank surface will also lead to folding cracks. After high temperature, decarburization will occur on both sides of this kind of folding crack for steel, while oxygen enriched layer will appear for titanium alloy. Figure below shows the folding crack caused by hot forging of 30CrMnSi bolt head and the decarburization morphology on both sides of the crack.

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If the heating temperature of pier forging is too high, it is easy to overheat in some parts. When overheated, it will lead to coarse grain of material and excessive brightness in serious cases. Overheating refers to the phenomenon of coarse grains of metal materials caused by too high heating temperature, too long residence time within the specified forging and heat treatment temperature range, or too high temperature due to thermal effect. Overburning refers to that the heating temperature is too high or the residence time in the high-temperature heating zone is too long, resulting in the melting of low melting point materials in the material, or the oxidizing gas in the environment penetrates into the grain boundary to form fusible oxide eutectic. During overburning, local grain boundaries are melted and small holes appear along the grain boundaries. There is no strict temperature limit between overheating and overburning. Overburning is generally judged by the appearance of oxidation or low melting point phase and melting of grains. Whether overheated or overburned, the performance of fasteners is reduced, and early failure is easy to occur in use. In general, fastener materials can be recovered by heat treatment when they are overheated, while over burning can only be scrapped. Figure below shows the remelting characteristics of low melting point eutectic phase of Superalloy due to overburning.

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For titanium alloy, when the pier forging temperature is high, it is also easy to have uneven structure, even coarse widmanstatten structure and thermal shear phenomenon, which is related to the lower thermal conductivity of titanium alloy than other alloys.

Source: China Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading flange manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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