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High strength bolts Processing

Processing technology for high strength bolts: hot rolled coil - (cold call) - Ball of (softening) annealing - Mechanical Descaling - pickling - cold call - cold forming - thread processing - heat treatment - Inspection

First, steel design

In the fastener manufacturing, the correct selection of fastener materials are an important part, because the performance of fasteners and their materials have a close relationship. If improper or incorrect material selection, may result in performance is mainly the shorter service life, or even an accident or processing difficulties and higher manufacturing costs, so the choice of fastener material is a very important part. Cold Heading Steel Cold Heading molding process is used interchangeability of the higher production of steel fasteners. Because it is room temperature, the use of metal plastic forming process, each part of the deformation of large deformation to withstand high speed, therefore, cold heading steel raw materials, performance requirements are very stringent. In the long-term production practice and research users, based on the combination of GB/T6478-2001 "cold heading and cold extrusion of steel and technical conditions" GB/T699-1999 "high-quality carbon structural steel" and target JISG3507-1991 "Cold Heading steel with carbon steel wire rod, "the features to 8.8, 9.8 bolt screw material requirements, for example, the determination of various chemical elements. C content is too high, cold forming properties will be reduced; too low will not be able to meet the requirements of mechanical properties of parts, so set at 0.25% -0.55%. Mn can improve the permeability of steel, but add too much to strengthen the matrix will be affected by cold-forming properties; in part quenched and tempered from time to time to promote a tendency to austenite grain growth, so in international basis, as appropriate to improve, will be to 0.45% -0.80%. Si help to strengthen the ferrite to promote cold-forming performance degradation, material elongation decreased as the Si is less than equal to 0.30%. SP for the impurity elements, and their presence would create segregation along the grain boundaries, leading to grain boundary embrittlement and damage the mechanical properties of steel should be reduced as much as possible, set at P less than or equal 0.030%, S less than or equal 0.035%. B. the maximum amount of boron were 0.005%, due to boron, while having significantly increased permeability of steel and other role, but will also lead to increased brittleness of steel. Boron-containing excessive, for bolts, screws and studs is such a need a good comprehensive mechanical properties of the workpiece is very unfavorable.

2, the ball of (soft) annealed

Countersunk head screws, Hexagon head bolt cylinder with cold heading process of production, the steel of the original organization will have a direct impact on when the cold upsetting process shaping ability. Cold-heading the process of local plastic deformation of the region up to 60% -80%, have requested the steel must have a good plasticity. When a certain chemical composition of steel, the microstructure is to determine strengths and weaknesses of the key factors of plastic, usually thick flaky pearlite that is not conducive to cold upsetting, while the small spherical pearlitic can significantly improve the capacity of plastic deformation of steel. Use a greater number of high-strength fasteners in carbon steel and medium carbon alloy steel, in the cold before heading the ball of (softening) annealing, in order to obtain even a ball of fine pearlite in order to better meet actual production needs. Pairs of medium carbon steel wire rod softening annealing, its heating temperature and more choices at the critical point of the upper and lower steel insulation, the heating temperature is not too high, otherwise it will generate three cementite along the grain boundaries, resulting in cracking cold heading, but for carbon-alloy steel wire rod ball isothermal annealing, in the AC1 + (20-30%) when heated, furnace cooling to slightly below the Ar1, isothermal temperature of about 700 degrees Celsius for some time, and then furnace cooling to 500 degrees Celsius baked air-cooled. Steel microstructure changes from coarse thin sheet changes from spherical, cold heading cracking rate will be significantly reduced. 35 \ 45 \ ML35 \ SWRCH35K steel softening annealing temperature for the general area of 715-735 degrees Celsius; and SCM435 \ 40Cr \ SCR435 ball annealing the heating temperature for the general area of 740-770 degrees Celsius, the isothermal temperature of 680-700 degrees Celsius.

Third, decorticating Descaling

Cold Heading Steel Wire Rod for the stripping process to remove iron plate light, in addition to scales, mechanical descaling and chemical pickling of two ways. By mechanical descaling chemical to replace the wire rod pickling process, not only increases productivity but also reduce environmental pollution. This descaling process involves bending method (commonly used in the circular groove with a triangular plate bending wheel over and over again), the spray nine law, apart from better-performing scales, but did not go to the net residual iron scale (iron oxide skin clearance rate was 97% ), especially iron oxide skin adhesion is strong, therefore, mechanical descaling by the metal thickness, structure and stress state effects of fasteners used in low-intensity (less than equal to 6.8) with a carbon steel wire rod. High-strength fasteners (¡Ý 8.8) with the rod after mechanical descaling, for addition to the net all of the iron oxide skin, and then by chemical descaling pickling process that is complex. Of low carbon steel wire rod, the mechanical descaling metal residues likely to cause uneven wear of grain pull-mode. When the particle-hole Taper steel wire rod due to friction and adhesion on the metal when the outside temperature, so that the surface of steel wire rod vertical grain marks, cold heading steel wire rod or the cylinder head flange bolt screw, the head causes micro-cracks occur, 95% above the surface of steel wire drawing generated in the process caused by scratches. Therefore, the mechanical descaling method should not be used for high-speed drawing.

Fourth, drawing

Drawing process has two purposes, first, the size of raw material restructuring; Second, through strengthening the role of deformation so that access to basic mechanical properties of fasteners, for medium carbon steel, medium carbon alloy steel there is a purpose, which is to wire rod obtained by controlled cooling of the flake cementite in the drawing as much as possible during the break, for the subsequent spheroidization (softening) annealed granular cementite to be prepared, however, some manufacturers to reduce costs, any reduction in pull - Road times, too large an increase the rate of reduction in surface work hardening tendency of steel wire rod, steel wire rod directly affect the performance of the cold upsetting. If all pass rate of reduction in surface distribution of inappropriate, but also make steel wire rod produced in the drawing process to reverse the cracks, this vertical distribution along the wire, the cycle a certain crack in the exposed steel cold heading process. In addition, the drawing process, such as poor lubrication can also result in cold drawn steel wire rod horizontal cracks appeared regularly. Steel wire rod out of a silk-mode particles at the same time I roll the tangent direction and drawing die of different heart, drawing die can cause increased wear and tear of unilateral pass, so that loss of a round hole, resulting in circumferential direction of wire drawing deformation uneven, so that roundness wire ultra-poor, in the cold heading steel wire during the non-uniform cross-section of the stress affecting the cold upsetting pass rate. Steel wire rod drawing process, the excessive part of the cut surface so that the rate of deterioration of surface quality of steel wire, while the low rate of reduction in surface is not conducive to flake cementite fragmentation, lack of access to as much as possible of the granular cementite that the cementite spheroidization rate is low, the performance of the wire cold heading extremely unfavorable, using drawing produced the bar and wire rod steel wire, in part by a straightforward control surface at 10% -15% range.
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