A high-strength bolt machining process

Abstract

This invention relates to the field of bolt processing technology, and in particular to a high-strength bolt processing process, comprising the following steps: S1: cutting; S2: forging; S3: preliminary inspection; S4: heat treatment; S5: machining, using a CNC machine tool to grind and cut the bolt part, followed by cold work hardening of the bolt head and shank; S6: thread treatment; S7: surface treatment; S8: final inspection, performing dimensional inspection, metallographic inspection, completion inspection, and non-invasive NDT inspection on the bolt part. This invention, by forging the bolt head, ensures a seamless connection between the bolt head and shank. Simultaneously, cold work hardening of the connection between the head and shank enhances the strength of the connection. Furthermore, when the bolt shank is segmented, the segmented arcs in the middle of the shank are also cold work hardened to improve the mechanical properties of the connection, thereby improving the overall strength and mechanical properties of the bolt.

Description

Technical Field

[0001] This invention relates to the field of bolt processing technology, and in particular to a high-strength bolt processing technology. Background Technology

[0002] A bolt is a mechanical part, a cylindrical threaded fastener used with a nut. It consists of a head and a shank (a cylinder with external threads). It requires a nut to fasten two parts with through holes; this type of connection is called a bolted connection. The two parts can be separated by unscrewing the nut, making it a detachable connection. Bolts are classified by the type of force applied during connection: ordinary bolts and bolts for reamed holes. They are also classified by head shape: dodecagonal head, hexagonal head, round head, square head, countersunk head, etc. Dodecagonal heads are used in aerospace applications, hexagonal heads are the most common, and countersunk heads are generally used where a connection is required. Bolts are indispensable in daily life and industrial manufacturing. Bolts are often referred to as the "rice of industry," highlighting their widespread use. Their applications include electronic products, mechanical products, digital products, power equipment, electromechanical products, ships, vehicles, water conservancy projects, and even chemical experiments. In short, bolts are used in countless places. For example, precision bolts are used in digital products, miniature bolts are used in DVDs, cameras, eyeglasses, watches, and electronics, and general bolts are used in televisions, electrical appliances, musical instruments, and furniture. Large bolts and nuts are used in engineering, construction, and bridges, while transportation equipment, airplanes, trams, and automobiles utilize a combination of bolts of various sizes. Bolts play a vital role in industry, and as long as industry exists on Earth, the function of bolts will remain crucial.

[0003] Currently, the maximum tensile strength of bolts on the market is only 1200MPa. In high-precision fields or harsh working environments, bolts may sometimes break or turn over during use, posing a significant threat to industrial production and safety. Therefore, the strength performance of bolts must be further enhanced. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a high-strength bolt processing technology.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a high-strength bolt processing technology, comprising the following steps:

[0006] S1: Cutting, using a CNC cutting machine to cut the raw material, and then using a grinding machine to grind the ends of the cut raw material;

[0007] S2: Forging, which includes both cold heading and hot heading;

[0008] S3: Preliminary inspection, check the basic dimensions of the parts after rough machining, perform metallographic and hardness tests on the parts, and observe the surface of the parts for cracks using a microscope.

[0009] S4: Heat treatment, placing the bolted parts in a heating furnace for heat treatment;

[0010] S5: Machining, using CNC machine tools to grind and cut the bolt parts, and then cold working the connection between the head and the shank of the bolt parts;

[0011] S6: Threading, using threading equipment to thread the shank of bolted parts;

[0012] S7: Surface treatment: First, clean the surface of the bolts, then plate them with chromium, aluminum, or chrome, spray with salt and apply anti-corrosion treatment. Finally, color the bottom, threads and head of the bolts.

[0013] S8: Final inspection, including dimensional inspection, metallographic inspection, tensile strength inspection, shear strength inspection, fatigue inspection, and non-invasive NDT inspection of bolted parts.

[0014] As a further implementation of the above technical solution:

[0015] The cutting error in step S1 is less than 0.003thou.

[0016] As a further implementation of the above technical solution:

[0017] In step S2, forging and cold heading are performed using a cold heading machine. Hot heading requires heating the cut blank first using an electric heating coil. After heating, the blank is placed in the cold heading machine for pressing. After pressing, the blank is cooled in the mold.

[0018] As a further implementation of the above technical solution:

[0019] In step S4, after heat treatment, the surface of the bolt needs to be sandblasted to clean the residue on the surface of the bolt.

[0020] As a further implementation of the above technical solution:

[0021] In step S5, the machining process involves grinding and cutting the bolt shank, head, and bottom.

[0022] As a further implementation of the above technical solution:

[0023] In step S6, after the threading is completed, the inspector performs a destructive test, cuts open the threaded part, and checks the metallographic grain flow, whether there are cracks, and whether the folding is within the standard range.

[0024] The present invention has the following beneficial effects:

[0025] Compared with existing technologies, the high-strength bolt processing technology of this invention forges the head of the bolt, resulting in a seamless connection between the shank and the head. Furthermore, cold-working the connection between the head and shank enhances the strength of this connection. When the bolt shank is segmented, the segmented arc in the middle is also cold-worked to improve the mechanical properties of the connection, thereby increasing the overall strength and mechanical properties of the bolt. Detailed Implementation

[0026] This invention provides a high-strength bolt processing technology, comprising the following steps:

[0027] S1: Cutting, using a CNC cutting machine to cut the raw material, and then using a grinding machine to grind the ends of the cut raw material;

[0028] After the raw materials pass inspection, they are first cut. The size varies depending on the machine, and the typical cutting error is less than 0.003 mm. Cutting is done using a CNC cutting machine, with specialized grinding wheels as the blades. Titanium alloys require different types of grinding wheels. The cutting method and efficiency also vary depending on the diameter of the material. Smaller diameter bars can be cut in multiple pieces simultaneously, while larger diameter bars (over three-quarters of an inch) are best cut individually to ensure quality and precision. After cutting, the cut surface is machined with sandpaper or a grinding wheel to clean the edges and remove burrs, preventing marks after forging. In rare cases, improper handling in this step can cause cracks after high-temperature forging. Finally, it is crucial to note that cutting precision is very important, as it directly affects the precision and efficiency of all subsequent processes.

[0029] S2: Forging, which includes both cold heading and hot heading;

[0030] After cutting, the parts proceed to the forging stage. Forging is divided into cold heading and hot heading, with hot heading being the most common method used in the aerospace fastener industry. This process is suitable for various high-strength, high-temperature resistant special alloy raw material blanks used in aerospace applications, including nickel-chromium alloys 718, A286, MP35n / MP159, titanium alloys, H11 carbon steel, 4340 high-strength stainless steel, and PH13-8 high-strength stainless steel. This process is suitable for processing bolts and nuts of various shapes. Using different dies, it can process bolts with round heads, flat heads, 12-corner heads, hexagonal heads, square heads, D-type heads, and spline heads. After adjusting and setting up the machine, the first part to be forged needs to undergo preliminary inspection, and only after the results are approved can the next part be processed. Production continues. Most forged parts are formed in one step, while some complex processes require two forming steps; some cold heading processes require three forming steps. First, according to the requirements of the raw materials and the processing parts, the cut blanks need to be heated. The heating process is detected by an inductor and completed by an electric heating coil. After heating is completed, the parts are placed on the forging press for pressing. After pressing, the blanks are left in the mold to be removed. The formed parts need to be cooled, usually by water cooling or air cooling depending on the raw materials (common nickel-chromium alloys are water-cooled, while stainless steel and titanium alloys are usually air-cooled). This process usually lasts 18-30 seconds, but for blanks with larger diameters, a longer heating time is required.

[0031] S3: Preliminary inspection, check the basic dimensions of the parts after rough machining, perform metallographic and hardness tests on the parts, and observe the surface of the parts for cracks using a microscope.

[0032] After rough machining, check the basic dimensions of the parts. Usually, the head is formed in one step. Cut the first part to be forged, grind the surface, polish it, and then etch the surface to prepare for metallographic inspection. Parts that have undergone hot forging need to be subjected to metallographic analysis and hardness testing. Inspectors also need to perform destructive testing, mainly checking the metallographic structure (whether the grains flow normally, whether there are cracks or folds), and the hardness of the head or heated parts. Qualified products enter the heat treatment stage. After all parts have been forged, the dimensions need to be checked again and the surface needs to be observed under a microscope for cracks. Finally, the inspection results of this step are recorded in the file.

[0033] S4: Heat treatment, placing the bolted parts in a heating furnace for heat treatment;

[0034] Different materials require different heat treatment methods. Even the same material can achieve different mechanical properties through different heat treatment methods. For example, nickel alloys: structural parts are generally aged; engine parts: solution treatment plus aging; titanium alloys: solution treatment plus aging. After heat treatment, hardness testing is also required. At the same time, the surface of the heat-treated product needs to be sandblasted to clean the residue.

[0035] S5: Machining, using CNC machine tools to grind and cut the bolt parts, and then cold working the connection between the head and the shank of the bolt parts;

[0036] Centerless grinding: This process is completed by automatic or manual CNC centerless grinding machine, with the grinding wheel as the main tool; it is used to process the diameter of the shank of bolt parts, and can also be processed in sections, as well as the flat or conical surface of the head base, and the arc at the connection between the head and the shank.

[0037] Cutting: Machining the shank and head of the bolt and chamfering the bottom; Drilling: Drilling holes at the tail of the shank and the threaded part; The head is usually formed in one step during the forging process and does not need to be machined. Instead, the head needs to be cut and chamfered. Sometimes the head of the bolt needs to be drilled (round hole, hexagonal hole, spline hole, slotted hole, arc hole, etc.).

[0038] Cold work hardening: This is accomplished using a cold work hardening machine. Typically, three rolling rollers are used to press the arc at the head and shank joint of the workpiece. This process is mostly automated, but sometimes robots are used for large-diameter parts. The strength of the head and shank joint can be increased by pressing the arc. For some bolt parts with segmented shanks, the segmented arcs in the middle of the shank also need to be rolled by a fillet roller to improve the mechanical properties of the joint.

[0039] S6: Threading, using threading equipment to thread the shank of bolted parts;

[0040] The thread pitch tolerance is designed to be larger near the bolt head and smaller near the bolt base, with the overall thread pitch decreasing proportionally. This design utilizes the principle of average stress to extend the bolt's service life. Threading is the core process for bolts, and production is completed using CNC threading equipment. The threading process is divided into cold rolling and warm rolling, both primarily using roller extrusion forming. Small-sized threads are usually cold rolled vertically, while larger threads require heating before extrusion forming. Threads processed by rollers need to be ground again to remove burrs, have their diameter adjusted, and then undergo surface cleaning. After threading is completed, inspection personnel perform a destructive test, cutting open the threaded portion to check the metallographic grain flowability, for cracks, and whether folds are within standard ranges.

[0041] S7: Surface treatment: First, clean the surface of the bolts, then plate them with chromium, aluminum, or chrome, spray with salt and apply anti-corrosion treatment. Finally, color the bottom, threads and head of the bolts.

[0042] S8: Final inspection, including dimensional inspection, metallographic inspection, tensile strength inspection, shear strength inspection, fatigue inspection, and non-invasive NDT inspection of bolted parts.

[0043] As a further implementation of the above technical solution:

[0044] The cutting error in step S1 is less than 0.003thou.

[0045] As a further implementation of the above technical solution:

[0046] In step S2, forging and cold heading are performed using a cold heading machine. Hot heading requires heating the cut blank first using an electric heating coil. After heating, the blank is placed in the cold heading machine for pressing. After pressing, the blank is cooled in the mold.

[0047] As a further implementation of the above technical solution:

[0048] In step S4, after heat treatment, the surface of the bolt needs to be sandblasted to clean the residue on the surface of the bolt.

[0049] As a further implementation of the above technical solution:

[0050] In step S5, the machining process involves grinding and cutting the bolt shank, head, and bottom.

[0051] As a further implementation of the above technical solution:

[0052] In step S6, after the thread treatment is completed, the inspector will conduct a destructive test, cut open the threaded part, and check the metallographic grain flow, whether there are cracks, and whether the folding is within the standard range.

[0053] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-strength bolt processing technology, characterized in that, Includes the following steps: S1: Cutting, using a CNC cutting machine to cut the raw material, and then using a grinding machine to grind the ends of the cut raw material; the cutting error is less than 0.003 mm. S2: Forging, which is divided into cold forging and hot forging. The hot forging process is used to process aerospace fasteners. Before hot forging, the cut blank is heated. The heating process is detected by an inductor and completed by an electric heating coil. After the blank is heated, it is placed in a forging press for forging. After the forging is completed, the blank stays in the mold to be removed. The formed parts are cooled. Nickel-chromium alloys are water-cooled, and stainless steel and titanium alloys are air-cooled. The cooling process lasts for 18 to 30 seconds. S3: Preliminary inspection, check the basic dimensions of the parts after rough machining, perform metallographic and hardness tests on the parts, and use a microscope to observe whether cracks and folds appear on the surface of the parts; S4: Heat treatment. The bolts are placed in a heating furnace for heat treatment. Different heat treatment methods are selected according to the different materials of the bolts: nickel-chromium alloy structural parts are subjected to aging treatment, nickel-chromium alloy engine parts are subjected to solution treatment and aging treatment, and titanium alloys are subjected to solution treatment and aging treatment. After heat treatment, hardness testing is performed. The heat-treated products are then sandblasted to clean the residue on the product surface. S5: Machining, using CNC machine tools to grind and cut the bolt parts, and then cold working the connection between the head and the shank of the bolt parts; The cold work hardening is completed by a cold work hardening machine, which uses three rolling rollers to roll the arc at the head of the bolt and the joint of the shank. For bolts with segmented shanks, the segmented arcs in the middle of the shank are also rolled by a fillet rolling machine to improve the mechanical properties of the segmented joints. During the cold work hardening process, most parts are completed automatically by the equipment, while large-diameter parts are completed by robots. S6: Threading, using extrusion threading equipment to thread the shank of bolt parts; In the thread processing, the thread pitch tolerance is larger near the bolt head and smaller near the bolt bottom, with the overall thread pitch being proportionally reduced. This design utilizes the principle of average stress to ensure even load distribution across all thread turns, extending the bolt's service life. Thread processing is performed using CNC threading equipment and a roller extrusion forming method. Small-sized threads are cold-rolled vertically, while large-sized threads are heated and then extruded. After roller processing, the threads are cleaned of burrs, the thread diameter is machined, and then the surface is cleaned. After the thread is machined, the inspectors conduct destructive testing, cutting open the threaded part to check the flowability of the metallographic grains, check for cracks, and check whether the folds are within the standard range. S7: Surface treatment: First, clean the surface of the bolts, then plate them with chromium, aluminum, or chrome, spray with salt and apply anti-corrosion treatment. Finally, color the bottom, threads and head of the bolts. S8: Final inspection, including dimensional inspection, metallographic inspection, tensile strength inspection, shear strength inspection, fatigue inspection, and non-invasive NDT inspection of bolted parts.