Process Explanation: The Whole Process of Bolt Production And Processing

Process explanation: The whole process of bolt production and processing

1. Commonly used materials for bolt processing

Different materials are used according to the strength level of the bolt: Currently, the standard parts on the market mainly include three materials: carbon steel, stainless steel, and copper.

▌Carbon steel

We distinguish low carbon steel, medium carbon steel, high carbon steel, and alloy steel based on the carbon content in carbon steel.

A. Low carbon steel C%≤0.25% is usually called A3 steel in China. It is basically called 1008, 1015, 1018, 1022, etc. abroad. It is mainly used for products without hardness requirements such as 4.8 grade bolts and 4 grade nuts, small screws, etc. (Note: Drilling nails are mainly made of 1022 material);

B. Medium carbon steel 0.25%;

C. High carbon steel C%>0.45%. It is basically not used in the market at present;

D. Alloy steel: Add alloy elements to ordinary carbon steel to increase some special properties of steel: such as 35, 40 chromium molybdenum, SCM435, 10B38. Fangsheng screws mainly use SCM435 chromium-molybdenum alloy steel, the main components are C, Si, Mn, P, S, Cr, Mo.

▌Stainless steel

Performance level:

A, 45, 50, 60, 70, 80 Mainly divided into austenite (18%Cr, 8%Ni) with good heat resistance, good corrosion resistance and good weldability;

B, A1, A2, A4 Martensite 13%Cr has poor corrosion resistance, high strength and good wear resistance;

C, C1, C2, C4 Ferritic stainless steel. 18%Cr has good upsetting and forging properties and better corrosion resistance than martensite;

D. At present, the imported materials on the market are mainly Japanese products. According to the level, they are mainly divided into SUS302, SUS304, and SUS316.

▌Copper

Common materials are brass and zinc-copper alloy. H62, H65, and H68 copper are mainly used as standard parts in the market.

2. Spheroidizing (softening) annealing

◆1. When spheroidizing (softening) annealing countersunk screws and hexagon socket head bolts are produced by cold heading, the original structure of the steel will directly affect the forming ability during cold heading.

◆2. The plastic deformation of the local area during cold heading can reach 60%-80%, so the steel must have good plasticity. When the chemical composition of the steel is certain, the metallographic structure is the key factor that determines the plasticity. It is generally believed that coarse lamellar pearlite is not conducive to cold heading, while fine spherical pearlite can significantly improve the plastic deformation ability of the steel.

◆3. For medium carbon steel and medium carbon alloy steel with a large amount of high-strength fasteners, spheroidizing (softening) annealing is performed before cold heading to obtain uniform and fine spheroidized pearlite to better meet actual production needs.

◆4. For the softening annealing of medium carbon steel wire rod, the heating temperature is mostly selected to be kept warm around the critical point of the steel. The heating temperature should not be too high, otherwise tertiary cementite will precipitate along the grain boundary, causing cold heading cracking.

◆5. The wire rod of medium carbon alloy steel adopts isothermal spheroidizing annealing. After heating at AC1+ (20-30%), the furnace is cooled to slightly lower than Ar1, the temperature is about 700 degrees Celsius for a period of time, and then the furnace is cooled to about 500 degrees Celsius and air-cooled. The metallographic structure of the steel changes from coarse to fine, from lamellar to spherical, and the cold heading cracking rate will be greatly reduced. The softening annealing temperature of 35\45\ML35\SWRCH35K steel is generally in the range of 715-735 degrees Celsius.

3. Shelling and descaling

The process of removing iron oxide plate from cold heading steel wire rod is peeling and descaling. There are two methods: mechanical descaling and chemical pickling.

◆1. Mechanical descaling replaces the chemical pickling process of wire rod, which not only improves productivity but also reduces environmental pollution. This descaling process includes bending method (circular wheels with triangular grooves are commonly used to repeatedly bend wire rods), spraying method, etc. The descaling effect is good, but the residual iron scale cannot be completely removed (the removal rate of iron oxide scale is 97%), especially when the iron oxide scale has strong adhesion. Therefore, mechanical descaling is affected by the thickness, structure and stress state of the iron scale, and is used for carbon steel wire rods for low-strength fasteners (less than or equal to grade 6.8).

◆2. After mechanical descaling of wire rods for high-strength fasteners (greater than or equal to grade 8.8), in order to remove all the iron oxide scale, it is subjected to a chemical pickling process, i.e. composite descaling. For low-carbon steel wire rods, the iron scale remaining after mechanical descaling is easy to cause uneven wear of the particle drafting die. When the die hole is adhered to the iron sheet due to the friction of the wire rod, the wire rod surface will produce longitudinal grain marks. When the wire rod is cold-forged into flange bolts or cylindrical head screws, the head will have micro cracks. More than 95% of the causes are caused by scratches on the wire surface during the drawing process. Therefore, mechanical descaling is not suitable for high-speed drawing.

4. Cold drawing

◆1. The cold drawing process has two purposes:

One is to modify the size of the raw materials;

The other is to make the fasteners obtain basic mechanical properties through deformation strengthening. For medium carbon steel and medium carbon alloy steel, there is another purpose, that is, to make the lamellar cementite obtained after the wire rod is controlled to break as much as possible during the drawing process, so as to prepare for the subsequent spheroidizing (softening) annealing to obtain granular cementite. However, in order to reduce costs, some manufacturers arbitrarily reduce the number of drawing passes. Excessive reduction rate increases the work hardening tendency of the wire rod steel wire, which directly affects the cold heading performance of the wire rod steel wire.

◆2. If the reduction rate of each pass is not properly distributed, the wire rod steel wire will also produce torsional cracks during the drawing process. This crack distributed along the longitudinal direction of the wire and with a certain period is exposed during the cold heading process of the wire.

In addition, if the lubrication is not good during the drawing process, it can also cause regular transverse cracks in the cold drawn wire rod steel wire.

◆3. When the wire rod is rolled up from the die, the tangent direction is not concentric with the wire drawing die, which will cause the wear of the single-side hole of the wire drawing die to increase, causing the inner hole to lose roundness, resulting in uneven drawing deformation in the circumferential direction of the wire, causing the roundness of the wire to exceed the tolerance, and the cross-sectional stress of the wire is uneven during the cold heading process, which affects the cold heading qualification rate.

◆4. During the drawing process of wire rod, excessive partial surface reduction rate deteriorates the surface quality of the wire, while too low surface reduction rate is not conducive to the crushing of flake cementite, and it is difficult to obtain as many granular cementite as possible, that is, the spheroidization rate of cementite is low, which is extremely unfavorable to the cold heading performance of the wire. For bars and wire rods produced by drawing, the partial surface reduction rate is directly controlled within the range of 10%-15%.

5. Cold Forging

Cold forging is usually performed by cold heading plastic processing for the bolt head. Compared with cutting processing, the metal fiber (metal wire) is continuous along the product shape without cutting in the middle, thus improving the product strength, especially the excellent mechanical properties. The cold heading forming process includes cutting and forming, which is divided into single-station single-click, double-click cold heading and multi-station automatic cold heading.

◆1. Cut the blank with a semi-closed cutting tool. The simplest method is to use a sleeve cutting tool;

The angle of the cut should not be greater than 3 degrees;

When an open cutting tool is used, the bevel angle of the cut can reach 5-7 degrees.

◆2. Short-sized blanks should be able to flip 180 degrees during the transfer from the previous station to the next forming station, so that the potential of the automatic cold heading machine can be exerted, fasteners with complex structures can be processed, and the precision of parts can be improved.

◆3. Each forming station should be equipped with a punch return device, and the die should be equipped with a sleeve ejector device.

◆4. The number of forming stations (excluding the cutting station) should generally reach 3-4 stations (more than 5 in special cases).

◆5. During the effective service life, the structure of the main slide rail and the process components can ensure the positioning accuracy of the punch and the die.

◆6. The terminal limit switch must be installed on the baffle that controls the material selection, and attention must be paid to the control of the upsetting force. The out-of-roundness of the cold-dial wire used to manufacture high-strength fasteners on the automatic cold heading machine should be within the diameter tolerance range, while the out-of-roundness of the wire for more precise fasteners should be limited to 1/2 of the diameter tolerance range. If the wire diameter does not reach the specified size, the upsetting part or head of the part will have cracks or burrs. If the diameter is smaller than the size required by the process, the head will be incomplete, and the edges or upsetting parts will be unclear.

◆7. The accuracy that can be achieved by cold heading is also related to the choice of forming method and the process used. In addition, it also depends on the structural characteristics of the equipment used, the process characteristics and its state, the precision of the tooling, the life and the degree of wear. The working surface roughness of high alloy steel and carbide molds used for cold heading and extrusion should not be greater than Ra=0.2um. When the roughness of the working surface of such molds reaches Ra=0.025-0.050um, they have the highest life.

6. Thread processing

◆1. Bolt threads are generally cold processed, so that the thread blanks within a certain diameter range pass through the thread rolling plate (die), and the thread is formed by the pressure of the thread plate (rolling die). The plastic flow lines of the threaded part are not cut off, the strength is increased, the precision is high, and the quality is uniform, so it is widely used.

◆2. 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 precision and whether the material is coated.

◆3. Rolling (rubbing) thread refers to a processing method that uses plastic deformation to form the thread teeth. It uses a rolling (thread rolling plate) die with the same pitch and tooth shape as the processed thread, squeezes the cylindrical screw blank while rotating the screw blank, and finally transfers the tooth shape on the rolling die to the screw blank to form the thread.

◆4. The common point of rolling (rubbing) thread processing is that the rolling revolutions do not need to be too many. If too many, the efficiency is low, and the thread surface is prone to separation or random buckling. On the contrary, if the revolutions are too few, the thread diameter is easy to lose roundness, and the initial rolling pressure increases abnormally, resulting in a shortened die life.

◆5. Common defects of rolling threads: cracks or scratches on the surface of the threaded part; random buckling; threaded part out of roundness. If these defects occur in large quantities, they will be discovered in the processing stage. If the number of occurrences is small, these defects will not be noticed during the production process and will circulate to users, causing trouble. Therefore, the key issues of processing conditions should be summarized and these key factors should be controlled in the production process.

7. Heat treatment

1) Heat treatment High-strength fasteners must be tempered according to technical requirements.

2) Heat treatment and tempering are to improve the comprehensive mechanical properties of fasteners to meet the tensile strength value and yield strength ratio specified by the product.

3) Heat treatment process has a vital impact on high-strength fasteners, especially its internal quality. Therefore, in order to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are necessary.

4) Due to the large production volume and low price of high-strength bolts, the threaded part is a relatively fine and relatively precise structure. Therefore, heat treatment equipment must have large production capacity, high degree of automation, and good heat treatment quality.

5) Decarburization of threads will cause fasteners to be disengaged before the resistance required by mechanical properties is reached, causing threaded fasteners to fail and shorten their service life. Due to the decarburization of raw materials, improper annealing will deepen the decarburization layer of raw materials. During the tempering heat treatment process, some oxidizing gases are generally brought in from outside the furnace.

6) The rust of bar steel wire or the residue on the surface of wire rod steel wire after cold drawing will also decompose after heating in the furnace, and react to generate some oxidizing gases. For example, the rust on the surface of the steel wire is composed of iron carbonate and hydroxide, which will decompose into CO2 and H2O after heating, thereby aggravating the decarburization. Studies have shown that the degree of decarburization of medium-carbon alloy steel is more serious than that of carbon steel, and the fastest decarburization temperature is between 700-800 degrees Celsius.

7) Since the attachments on the surface of the steel wire decompose and synthesize CO2 and H2O very quickly under certain conditions, if the continuous mesh belt furnace gas is not properly controlled, it will also cause screw decarburization to exceed the tolerance.

8) When high-strength fasteners are formed by cold heading, the raw material and annealing decarburization layer not only still exist, but are also squeezed to the top of the thread. For the surface of the fastener that needs to be quenched, the required hardness cannot be obtained, and its mechanical properties (especially strength and wear resistance) are reduced. In addition, the surface of the steel wire is decarburized, and the surface layer has a different expansion coefficient than the internal structure, and surface cracks may occur during quenching.

9) The quality problems that may occur in the fasteners during the quenching and tempering process are mainly: insufficient hardness in the quenched state; uneven hardness in the quenched state; excessive quenching deformation; quenching cracking. 10) Such problems that occur on site are often related to raw materials, quenching heating and quenching cooling. Correctly formulating heat treatment processes and standardizing production operation processes can often avoid such quality accidents.

Summary

The bolt production process involves multiple steps, from material selection to heat treatment, each tailored to ensure the final product meets stringent quality and performance standards. Proper control and optimization of each step are essential to produce high-quality, reliable fasteners.