High Strength Bolt Manufacturing Process
The processing technology of high-strength bolts is: hot-rolled wire rod-(cold drawing)-spheroidizing (softening) annealing-mechanical descaling-pickling-cold drawing-cold forging-thread processing-heat treatment-inspection
1. Steel structure design
In the manufacture of fasteners, the correct selection of fastener materials is an important link, because the performance of fasteners is closely related to its materials. If the material is not selected properly or correctly, the performance may not meet the requirements, the service life may be shortened, accidents may occur or processing is difficult, and the manufacturing cost may be high. Therefore, the selection of fastener materials is a very important link. Cold heading steel is a highly interchangeable fastener steel produced by cold heading forming process. Because the metal is plastically processed and formed at room temperature, each part has a large amount of deformation and the deformation speed is also fast. Therefore, the performance requirements of cold heading steel raw materials are very strict. On the basis of long-term production practice and user research, combined with GB/T6478-2001 "Technical Conditions for Cold Heading and Cold Extrusion Steel", GB/T699-1999 "High Quality Carbon Structural Steel" and the goal of JISG3507-1991 "Cold Heading" The characteristics of carbon steel wire rods for steel, taking the material requirements of grade 8.8 and grade 9.8 bolts and screws as an example, determine various chemical elements. If the C content is too high, the cold forming performance will decrease; if it is too low, the mechanical performance requirements of the parts cannot be met, so it is set at 0.25%-0.55%. Mn can improve the permeability of steel, but adding too much will strengthen the matrix structure and affect the cold forming performance; it tends to promote the growth of austenite grains during the quenching and tempering process of parts, and it should be increased appropriately internationally. 0.45%-0.80%. Si can strengthen ferrite and promote the decline of cold forming performance. The reduction in elongation of the material was determined to be less than or equal to 0.30% Si. SP is an impurity element, their existence will segregate along the grain boundary, cause grain boundary embrittlement, and damage the mechanical properties of steel. It should be reduced as much as possible. P≤0.030%, S≤0.035%. B. The maximum boron content is 0.005%, because although boron can significantly improve the permeability of steel, it will also increase the brittleness of steel.
2. Spheroidizing (softening) annealing
When cold heading produces countersunk head screws and hexagon socket head bolts, the original structure of the steel will directly affect the forming ability during cold heading. The local plastic deformation in the cold heading process can reach 60%-80%. For this, steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor to determine the plasticity. It is generally believed that thick flaky pearlite is not conducive to cold heading, while fine spherical pearlite can significantly improve the plastic deformation ability of steel. For medium-carbon steel and medium-carbon alloy steel with large quantities of high-strength fasteners, spheroidizing (softening) annealing is performed before cold heading to obtain uniform and fine spheroidized pearlite, which can better meet actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is usually selected at the upper and lower critical points of the steel. The heating temperature is generally not too high. Otherwise, the tertiary cementite will precipitate along the grain boundary, causing cold heading cracking. Medium carbon alloy steel wire rod adopts isothermal spheroidizing annealing. After AC1+ (20-30%) is heated, the furnace is cooled to slightly lower than Ar1, the temperature is maintained at 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 sheet to spherical, and the cold heading cracking rate will be greatly reduced. The softening annealing temperature of 3545ML35SWRCH35K steel is generally 715-735 degrees Celsius; while the spheroidizing annealing temperature of SCM43540CrSCR435 steel is generally 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.
3. Peeling and descaling
The process of removing iron scale from cold heading steel wire rod is peeling off scale. There are two methods: mechanical rust removal and chemical pickling. Replacing the chemical pickling process of wire rods with mechanical descaling not only improves productivity, but also reduces environmental pollution. This descaling process includes bending method (generally using a round wheel with a triangular groove to repeatedly bend the wire), spraying nine methods, etc. The descaling effect is better, but the residual iron scale cannot be removed (the removal rate of the oxide scale is 97%)) , especially when the scale adhesion is strong. Therefore, mechanical rust removal is affected by the thickness, structure and stress state of the iron sheet. Carbon steel wire for low-strength fasteners (≤6.8). After the high-strength fasteners (greater than or equal to 8.8) are mechanically derusted, the scale is completely removed with wire rods, and then the rust is compounded by chemical pickling process. For mild steel wire rod, the iron scale left by mechanical descaling is likely to cause uneven wear of grain draft. When the wire steel wire rubs against the external temperature to make the through hole of the grain adhere to the iron sheet, longitudinal grain traces are produced on the surface of the wire steel wire. When the wire rod is a cold heading flange bolt or cylindrical head screw, more than 95% of the reasons for microcracks on the head are caused by scratches on the wire rod surface during the drawing process. so,
4. Drawing
The drawing process serves two purposes. One is to change the size of the raw material; the other is to obtain the basic mechanical properties of the fastener through deformation strengthening. For medium-carbon steel and medium-carbon alloy steel, there is another purpose, which is to crack the flaky cementite obtained after controlled cooling of the wire as much as possible during the drawing process, so as to prepare for the subsequent spheroidizing (softening) annealing. granular cementite. However, in order to reduce costs, some manufacturers reduce drawings without authorization. For the pass, the excessive reduction of the surface area increases the work hardening tendency of the wire rod, which directly affects the cold heading performance of the wire rod. If the distribution of the reduction ratio of each pass is not appropriate, it will also cause the wire rod to be twisted during the drawing process. The cracks distributed longitudinally along the wire rod with a certain period are exposed during the cold heading process of the wire rod. In addition, if the lubrication is not good during the drawing process, it will also cause regular transverse cracks in the cold-drawn wire rod. The tangent direction of the wire outlet winding die is not concentric with the wire drawing die, which will increase the wear of the unilateral hole of the wire drawing die, make the inner hole not round, and cause the wire drawing deformation to be uneven in the circumferential direction of the wire. The roundness of the steel wire is too poor, and the force on the cross section of the steel wire is uneven during the cold heading process, which affects the qualified rate of cold heading. During the wire drawing process, too large surface reduction rate will make the surface quality of steel wire worse, while too low surface reduction rate is not conducive to the crushing of flaky cementite, and it is difficult to obtain as much granular cementite as possible. possible. That is to say, the spheroidization rate of cementite is low, which is extremely unfavorable to the cold heading performance of steel wire. For rods and wire rods produced by drawing, the local surface reduction rate is directly controlled within the range of 10%-15%. However, too low surface reduction rate is not conducive to the crushing of flaky cementite, and it is difficult to obtain as much granular cementite as possible. That is to say, the spheroidization rate of cementite is low, which is extremely unfavorable to the cold heading performance of steel wire. For rods and wire rods produced by drawing, the local surface reduction rate is directly controlled within the range of 10%-15%. However, too low surface reduction rate is not conducive to the crushing of flaky cementite, and it is difficult to obtain as much granular cementite as possible. That is to say, the spheroidization rate of cementite is low, which is extremely unfavorable to the cold heading performance of steel wire. For rods and wire rods produced by drawing, the local surface reduction rate is directly controlled within the range of 10%-15%.
5. Cold forging
Usually, the bolt head is machined from cold heading plastic. Compared with the cutting process, the metal fiber (wire) is continuous along the shape of the product without cutting in the middle, which improves the strength of the product, especially the mechanical properties. The cold heading forming process includes cutting forming, single-station single-click cold heading, double-click cold heading and multi-task automatic cold heading. Automatic cold heading machines perform multitasking processes such as stamping, upsetting, extrusion and reducing in multiple forming dies. The processing characteristics of raw materials used in single-station or multi-station automatic cold heading machines are determined by the size of bars with a length of 5-6 meters or wires with a weight of 5-6 meters. 1900-2000KG, that is, the characteristics of the processing technology. Cold heading does not use pre-cut single-piece blanks, but uses the automatic cold heading machine itself to cut and upset the blanks of the bar (if necessary), and wire rods. Before the cavity is extruded, the blank must be shaped. Through shaping, a blank that meets the process requirements can be obtained. The billet does not need to be shaped before upsetting, diameter reduction and forward extrusion. After the blank is cut, it is sent to the upsetting station. This station can improve the quality of the blank, reduce the forming force of the next station by 15-17%, and prolong the life of the mold. Bolts can be manufactured in several diameter reductions. 1. Use a semi-closed cutter to cut the blank. The easiest way is to use a socket-type cutter; the angle of the cut should not be greater than 3 degrees; when using an open cutter, the bevel angle of the cut can reach 5-7 degrees. 2. When the short material is transferred from the previous station to the next forming station, it should be able to rotate 180 degrees, so as to exert the potential of the automatic cold heading machine, process fasteners with complex structures, and improve the precision of parts. 3. Each forming station should be equipped with a punch ejector, and the mold should be equipped with a sleeve ejector. 4. Forming stations (not including cutting stations) should generally reach 3-4 stations (more than 5 stations in special cases). 5. During the effective use period, the structure of the guide rail of the main slider and the process components can ensure the positioning accuracy of the punch and die. 6. Terminal limit switches must be installed on the baffle for controlling material selection, and attention must be paid to the control of upsetting force. The out-of-roundness of cold-drawn wire used for high-strength fasteners on automatic cold heading machines should be within the diameter tolerance range, while the out-of-roundness of wire rods used for more precise fasteners should be within the diameter tolerance range. It should be limited within the 1/2 diameter tolerance range, if the wire diameter does not reach the specified size, cracks or burrs will appear on the upset or part head. If the diameter is smaller than the size required by the process, the head will be incomplete, and the edges and corners or swollen parts will not be clear. The precision that cold heading can achieve is also related to the choice of forming method and the process used. In addition, it also depends on the structural characteristics, process characteristics and state of the equipment used, the precision, life and wear degree of the mold. For high alloy steel for cold heading and extrusion, the roughness of the working surface of the cemented carbide mold should not be greater than Ra=0.2um. When the working surface roughness of this type of mold reaches Ra=0.025-0.050um, the service life is the highest.
6. Thread treatment
Bolt threads are generally cold processed, so that the thread blank within a certain diameter range is rolled (rolled) through the wire plate (die), and the thread is formed by the pressure of the wire plate (rolling die). The plastic streamline of the threaded part is not cut, which increases the strength, high precision and uniform quality, so it is widely used. In order to make the thread outer diameter of the final product, the required thread blank diameter is different because it is limited by factors such as thread accuracy and whether the material is coated or not. Rolling (rolling) thread refers to a processing method that uses plastic deformation to form thread teeth. It uses rolling dies with the same pitch and tooth shape as the thread to be processed. It rotates the screw blank while extruding the cylindrical screw blank, and finally transfers the tooth profile on the rolling die to the screw blank to form threads. The common point of rolling (rubbing) thread processing is that the number of rolling revolutions does not need too much. If there are too many, the efficiency will be low, and the thread surface is easy to separate or buckle randomly. On the contrary, if the number of revolutions is too small, the diameter of the thread is likely to be out of round, and the initial pressure of rolling is abnormally increased, which will shorten the life of the die. Common defects of rolled threads: cracks or scratches on the thread surface; random buckling; thread out-of-roundness. If these defects occur in large numbers, they will be caught during the processing phase. If the number of occurrences is small, these defects will be passed on to users inadvertently during the production process, causing trouble. Therefore, the key issues of processing conditions should be summarized, and these key factors should be controlled during the production process.
7. Heat treatment
High-strength fasteners must be quenched and tempered according to technical requirements. Heat treatment and tempering are to improve the comprehensive mechanical properties of fasteners to meet the specified tensile strength value and yield ratio of the product. The heat treatment process has a crucial influence on high-strength fasteners, especially their internal quality. Therefore, to produce high-quality high-strength fasteners, advanced heat treatment technology and equipment are required. Due to the large production volume and low price of high-strength bolts, the structure of the threaded part is relatively fine and precise, so the heat treatment equipment must have a large production capacity, a high degree of automation, and good quality heat treatment. Since the 1990s, protective atmosphere continuous heat treatment lines have dominated. Shaking bottom mesh belt furnace is especially suitable for heat treatment and tempering of small and medium-sized fasteners. In addition to the good sealing performance of the furnace, the quenching and tempering line also has advanced microcomputer control of atmosphere, temperature and process parameters, and equipment failure alarm and display functions. High-strength fasteners are fully automatically controlled from loading-cleaning-heating-quenching-cleaning-tempering-coloring to off-line, which effectively guarantees the quality of heat treatment. Thread decarburization can cause the fastener to jump out first when the resistance required by the mechanical properties is not met, which will cause the threaded fastener to fail and reduce service life. Due to the decarburization of the raw material, if the annealing is not proper, the decarburization layer of the raw material will deepen. During the quenching and tempering heat treatment, some oxidizing gases are usually brought in from outside the furnace. The rust of the rod steel wire or the residue on the surface of the wire rod after cold drawing will also decompose after heating in the furnace, and the reaction will generate some oxidizing gases. For example, the rust on the surface of steel wire is composed of iron carbonate and iron hydroxide, which will be decomposed into CO2 and H2O after heating, which will intensify 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. Since the attachment on the surface of the steel wire will quickly decompose and synthesize CO2 and H2O under certain conditions, if the continuous mesh belt furnace gas is not properly controlled, it will also cause excessive decarburization of the screw. When high-strength fasteners are cold-headed, the decarburized layer of the raw material and annealing not only remains, but is also extruded to the top of the thread. For the surface of the fastener that needs to be quenched, the required hardness cannot be achieved. Mechanical properties (especially strength and wear resistance) are reduced. In addition, the surface of the steel wire has been decarburized, and the expansion coefficients of the surface layer and the internal structure are different, and surface cracks may appear during quenching. For this reason, it is necessary to protect the top of the thread from decarburization during quenching and heating, and to properly carbonize the fasteners whose raw materials have been decarburized, so as to adjust the advantages of the protective atmosphere in the mesh belt furnace to the original level. Carbon coated parts. The carbon content is basically the same, so that the decarburized fasteners slowly return to the original carbon content. The carbon potential was set at 0.42%-0.48%. The carbon coating temperature is the same as that of quenching and cannot be carried out at high temperature. , so as not to affect the mechanical properties due to coarse grains. The quality problems that may occur during the quenching and tempering process of fasteners mainly include: insufficient hardness in the quenched state; uneven hardness in the quenched state; excessive quenching deformation; quenching cracking. Such problems on site are often related to raw materials, quenching heating and quenching cooling. Correctly formulate the heat treatment process and standardize the production operation process, often can avoid the occurrence of such quality accidents.
8. Conclusion
To sum up, the process factors affecting the quality of high-strength fasteners include steel design, spheroidizing annealing, peeling and rust removal, drawing, cold heading, thread processing, heat treatment, etc., and sometimes it is the superposition of various factors. . We know that fastener defects are caused by fluctuations in product quality characteristics. Only by accurately grasping the technological factors in the product manufacturing process and generating a huge driving force for continuous quality improvement, can we obtain more profits and stronger competitiveness through continuous quality improvement!
