Nowadays, advanced manufacturing represented by large aircraft, large power generation equipment, automobiles, high-speed trains, large ships, and large complete sets of equipment has entered an important development direction. As a result, fasteners will enter an important stage of development. High strength bolts are used for the connection of important machinery. Repeated disassembly or various installation torque methods require high strength bolts. Therefore, the quality of its surface condition and thread accuracy will directly affect the service life and safety of the host machine. In order to improve the friction coefficient and avoid rusting, seizing, or seizing during use, the technical requirements stipulate that its surface should be treated with nickel phosphorus plating. The coating thickness shall be ensured to be within the range of 0.02 to 0.03 mm, and the coating shall be uniform, dense, and free of pinholes.
1. Steel design
In fastener manufacturing, the correct selection of fastener materials is an important step, because the performance of fasteners is closely related to their materials. Improper or incorrect material selection may result in unsatisfactory performance, shortened service life, or even accidents or machining difficulties, resulting in high manufacturing costs. Therefore, the selection of fastener materials is a very important step. Cold heading steel is a fastener steel with high interchangeability produced by cold heading forming process. Because it is formed by metal plastic processing at room temperature, each part has a large amount of deformation and a high deformation rate, the performance requirements for cold heading steel raw materials are very strict. Based on long-term production practice and user use research, combined with the characteristics of GB/T6478-2001 "Technical Conditions for Steel for Cold Heading and Cold Extrusion" GB/T699-1999 "High Quality Carbon Structural Steel" and the target JISG3507-1991 "Carbon Steel Wire Rods for Cold Heading Steel", various chemical elements are determined using the material requirements for Grade 8.8 and Grade 9.8 bolts and screws as examples. If the content of C is too high, the cold formability will decrease; If it is too low, it cannot meet the mechanical performance requirements of the part, so it is set at 0.25% - 0.55%. Mn can improve the permeability of steel, but excessive addition of Mn will strengthen the matrix structure and affect the cold formability; There is a tendency to promote austenite grain growth during part quenching and tempering, so it should be appropriately increased on an international basis, and set at 0.45% - 0.80%. Si can strengthen ferrite and reduce cold formability. The reduction in material elongation is determined to be less than or equal to 0.30% for Si. S.P. is an impurity element that can cause segregation along the grain boundary, leading to grain boundary embrittlement, and damaging the mechanical properties of the steel. It should be reduced as much as possible, with P less than or equal to 0.030% and S less than or equal to 0.035%. B. The maximum boron content is 0.005%, because although boron can significantly improve the permeability of steel, it can also lead to increased brittleness of steel. High boron content is very detrimental to workpieces that require good comprehensive mechanical properties such as bolts, screws, and studs.
2. Spheroidizing annealing
When countersunk head screws and hexagon socket cylindrical head bolts are produced by cold heading, the original structure of the steel directly affects the forming ability during cold heading. During cold heading, the plastic deformation in local areas can reach 60% - 80%, which requires that the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor determining the plasticity. It is generally believed that large pieces of pearlite are not conducive to cold heading, while small spherical pearlite can significantly improve the ability of the steel to plastic deform. For medium carbon steel and medium carbon alloy steel with a large amount of high-strength bolts, spheroidizing (softening) annealing is performed before cold heading to obtain uniform and fine spheroidized pearlite to better meet the actual production needs. For softening and annealing of medium carbon steel wire rods, the heating temperature is usually chosen to be above or below the critical point of the steel. The heating temperature should generally not be too high, otherwise three times of cementite precipitation along the grain boundary will occur, resulting in cold heading cracking. For medium carbon alloy steel wire rods, isothermal spheroidization annealing is used. After heating with AC1+(20-30%), the furnace will be cooled to slightly lower than Ar1, and the temperature will be about 700 degrees Celsius for a period of time, Then, the furnace is cooled to about 500 degrees Celsius and discharged for air cooling. The metallographic structure of steel changes from coarse to fine, from sheet to spherical, and the cold heading cracking rate will be greatly reduced. The general softening annealing temperature range for 35 45 ML35 SWRCH35K steel is 715-735 degrees Celsius; The heating temperature range for spheroidizing annealing of SCM435 40Cr SCR435 steel is generally 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.
3. Shelling and phosphorus removal
The process for removing iron oxide plates from cold heading steel wire rods includes polishing and phosphorus removal. There are two methods: mechanical phosphorus removal and chemical acid cleaning. Replacing the chemical pickling process of wire rods with mechanical phosphorus removal not only improves productivity, but also reduces environmental pollution. This phosphorus removal process includes the bending method (generally using circular wheels with triangular grooves to repeatedly bend wire rods), the spray nine method, and other methods. The phosphorus removal effect is good, but it cannot remove residual iron and phosphorus (the iron oxide scale removal rate is 97%), especially when the iron oxide scale has a strong adhesion. Therefore, mechanical phosphorus removal is affected by the thickness, structure, and stress state of the iron sheet, and is used for carbon steel wire rods for low strength fasteners (less than or equal to Grade 6.8). After mechanical phosphorus removal, the wire rod used for high-strength bolts (grade 8.8 or higher) is used to remove all iron oxide scales, and then subjected to a chemical pickling process, which is a composite phosphorus removal process. For low carbon steel wire rods, the residual iron sheet from mechanical phosphorus removal is prone to uneven wear during grain drawing. When the grain drawing hole adheres to the iron sheet due to the friction of the wire rod and the external temperature, longitudinal grain marks are generated on the surface of the wire rod and the wire rod. When the wire rod and the flange bolt or cylindrical head screw are cold upset, the cause of microcracks on the head is more than 95% caused by scratches on the wire surface during the drawing process. Therefore, mechanical phosphorus removal is not suitable for high-speed drawing.
4. Pulling
The drawing process has two purposes: one is to modify the size of raw materials; The second is to achieve basic mechanical properties of fasteners through deformation strengthening. For medium carbon steel and medium carbon alloy steel, there is also a goal, that is, to make the flake cementite obtained after controlled cooling of the wire rod break as much as possible during the drawing process, and prepare for the subsequent spheroidization (softening) annealing to obtain granular cementite. However, some manufacturers arbitrarily reduce the drawing passes to reduce costs, Excessive reduction in surface area increases the work-hardening tendency of wire rods, which directly affects the cold heading performance of wire rods. If the reduction ratio distribution of each pass is not appropriate, it can also cause torsional cracks in the wire rod during the drawing process. Such cracks, which are distributed along the longitudinal direction of the wire and have a certain period of time, are exposed during the cold heading process of the wire. In addition, poor lubrication during the drawing process can also cause regular transverse cracks in the cold drawn wire rod. The tangential direction of the wire rod when it comes out of the grain die and rolls up is not concentric with the drawing die, which can cause increased wear of the single side pass of the drawing die, causing the inner hole to be out of circle, causing uneven drawing deformation in the circumferential direction of the wire, causing the roundness of the wire to exceed the tolerance, and uneven stress on the cross section of the wire during cold heading, which affects the cold heading qualification rate. During the drawing process of wire rod steel wires, excessive partial reduction in surface area worsens the surface quality of the wire, while low reduction in surface area is not conducive to the crushing of lamellar cementite, making it difficult to obtain as much granular cementite as possible, that is, the low nodularization rate of cementite, which is extremely detrimental to the cold heading performance of the wire. For rod and wire rod produced by drawing, the partial reduction in surface area is controlled within the range of 10% to 15%.
5. Cold forging forming
Generally, the forming of bolt heads adopts cold heading plastic processing. Compared to cutting processing, metal fibers (metal strands) are continuous along the product shape without cutting in the middle, thereby improving the strength of the product, especially with excellent mechanical properties. The cold heading forming process includes cutting and forming, single station click, double click cold heading, and multi station automatic cold heading. An automatic cold heading machine performs multi-station processes such as stamping, upsetting, extrusion, and diameter reduction in several forming dies. The processing characteristics of the original blank used by a single station or multi-station automatic cold heading machine are determined by the size of a bar with a material size of 5-6 meters long or a wire rod with a weight of 1900-2000 KG. That is, the processing process is characterized by using a blank that is not cut in advance but is cut and upset (if necessary) by the automatic cold heading machine itself from the bar and wire rod. Before pressing the cavity, the blank must be reshaped. Through shaping, a blank that meets the process requirements can be obtained. Before upsetting, reducing, and forward extrusion, the blank does not need to be shaped. After cutting the blank, it is sent to the upsetting and shaping station. This station can improve the quality of the blank, reduce the molding force of the next station by 15-17%, and extend the life of the mold. Multiple diameter reductions can be used for manufacturing bolts. The accuracy achieved by cold upsetting is also related to the selection of the forming method and the process used. In addition, it also depends on the structural characteristics of the equipment used, the technological characteristics and their status, the accuracy of the tooling, the life span, and the degree of wear. For high alloy steels used in cold heading and extrusion, the working surface roughness of cemented carbide molds should not be greater than Ra=0.2um. When the working surface roughness of such molds reaches Ra=0.025-0.050um, they have the highest service life.
6. Threading
Bolt threads are generally cold processed, allowing thread blanks within a certain diameter range to pass through a rubbing (rolling) screw plate (mold), and the thread is formed by the pressure of the screw plate (mold). It is widely used to obtain products that do not cut the plastic flow line of the threaded portion, increase strength, have high accuracy, and have uniform quality. In order to produce the outer diameter of the thread of the final product, the required thread blank diameter is different, because it is limited by factors such as thread accuracy, whether the material has plating, and so on. Rolling (rubbing) screw threads refers to a processing method that uses plastic deformation to form the threads. It uses a rolling (thread rolling plate) mold with the same pitch and tooth shape as the processed thread, while pressing the cylindrical screw blank, while rotating the screw blank. Finally, the tooth shape on the rolling mold is transferred to the screw blank to form the thread. The common feature of thread rolling (rubbing) and pressing is that it is not necessary to have too many rolling revolutions. If there are too many, the efficiency is low, and the surface of the thread is prone to separation or disorderly threading. On the contrary, if the number of revolutions is too small, the thread diameter is easy to lose its roundness, and the initial pressure of rolling increases abnormally, resulting in a shortened mold life. Common defects of rolled threads: surface cracks or scratches on the threaded portion; Disorderly deduction; The threaded portion is out of round. If these defects occur in large quantities, they will be discovered during the processing stage. If a small number of defects occur and the production process does not notice them, they will circulate to users, causing trouble. Therefore, the key issues of processing conditions should be summarized and controlled during the production process.
7. Processing
High strength fasteners shall undergo quenching and tempering treatment according to technical requirements. The purpose of heat treatment and tempering is to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength and yield ratio of products. Heat treatment process has a crucial impact on high-strength fasteners, especially their internal quality. Therefore, in order to produce high-quality high-strength fasteners, it is necessary to have advanced heat treatment technology and equipment. Due to the large production volume and low price of high-strength bolts, as well as the relatively fine and relatively precise structure of the threaded part, it is required that the heat treatment equipment must have the ability to have large production capacity, high degree of automation, and good heat treatment quality. Since the 1990s, the continuous heat treatment production line with protective atmosphere has occupied a dominant position. The shock bottom type and mesh belt furnace are especially suitable for heat treatment and tempering of small and medium-sized fasteners. In addition to good sealing performance of the furnace, the tempering line also has advanced computer control of atmosphere, temperature, and process parameters, as well as equipment fault alarm and display functions. High strength fasteners are automatically controlled from loading, cleaning, heating, quenching, cleaning, tempering, coloring to offline, effectively ensuring the quality of heat treatment. Decarburization of threads can cause fasteners to trip before meeting the resistance required for mechanical performance, leading to failure of threaded fasteners and shortening their service life. Due to the decarburization of raw materials, improper annealing will further deepen the decarburization layer of raw materials. During the quenching and tempering heat treatment process, some oxidation gases are generally brought in from outside the furnace. The rust on the rod steel wire or the residue on the surface of the wire rod after cold drawing will also decompose after being heated in the furnace and react to generate some oxidizing gases. For example, the surface of steel wire is rusty, and its components are iron carbonate and hydroxide. After heating, it will decompose into CO ₂ and H ₂ O, thereby aggravating decarburization. The research shows that the decarburization degree of medium carbon alloy steel is more severe than that of carbon steel, and the fastest decarburization temperature is between 700-800 degrees Celsius. Due to the rapid decomposition and synthesis of carbon dioxide and water from attachments on the surface of steel wires under certain conditions, improper control of the furnace gas in a continuous mesh belt furnace can also cause excessive decarburization of screws. When high-strength bolts are formed by cold upsetting, the raw material and annealed decarburized layer not only still exist, but also are pressed to the top of the thread. For fasteners requiring quenching, the required hardness cannot be obtained, and their mechanical properties (especially strength and wear resistance) are reduced. In addition, due to decarburization of the steel wire surface, the surface layer and internal structure have different expansion coefficients, and surface cracks may occur during quenching. To this end, during quenching and heating, it is necessary to protect the top of the thread from decarburization, and to appropriately coat the fasteners whose raw materials have been decarburized. The advantage of the protective atmosphere in the mesh belt furnace should be adjusted to basically equal the original carbon content of the parts covered with carbon, so that the decarburized fasteners can slowly recover to the original carbon content. The carbon potential should be set at 0.42% - 0.48%, and the carbon coating temperature should be the same as that of quenching and heating, and should not be carried out at high temperatures, Avoid coarse grains that affect mechanical properties. The main quality problems that may occur during the quenching and tempering process of fasteners are: insufficient hardness in the quenched state; Uneven hardness in the quenched state; Quenching deformation out of tolerance; Quench cracking. This type of problem often occurs on site related to raw materials, quenching heating, and quenching cooling. Correctly formulating heat treatment processes, and standardizing production operations can often avoid such quality accidents.
8. Inspection
In summary, the process factors that affect the quality of high-strength fasteners include steel design, spheroidizing annealing, shelling and dephosphorization, pulling, cold upsetting, thread processing, and heat treatment, sometimes a combination of various factors.
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