A comprehensive guide to the screw manufacturing process

Although the screw seems to be an inconspicuous small part, it cannot be separated from the national heavy weapon and the daily necessities. Do you know how screws are made? Despite its small size, the manufacturing process alone has seven steps.

• The first step is to select materials. According to actual life requirements, the factory should clarify the factory plate, specification, material, product name, weight, and quantity and buy suitable wire rods. We should pay attention not to choose inferior products only for cheap. For the sake of life, we should choose high-quality products.
• The second step is annealing, which increases the forging capacity of screws, making post-processing production more convenient.
• The third step is pickling. Although the process is relatively simple, it is just to treat the surface of the screw; this process will make the following process more convenient.
• The fourth step is to draw the wire to undertake the above pickling process.
• The fifth step is to start, which is to complete the shaping of teeth.
• The sixth step is heat treatment to change the mechanical properties of the screw.
• The seventh step is electroplating, which is very important to meet customers’ requirements and the beauty of products.

The process of manufacturing screws

The following is the whole process of screw production. I hope it can help you understand screw production.

Selection of wire rod

Before wire drawing, we must have a preliminary understanding and understanding of the required metal materials. There are many kinds of wire, and you can choose the corresponding material for different purposes.
Analysis of standard raw materials and characteristics of fastener screw products
1). 45 # steel – high-quality carbon structural steel is the most commonly used medium carbon quenched and tempered steel.
Main features: the most commonly used medium-carbon quenched and tempered steel has good comprehensive mechanical properties, low hardenability, and is easy to crack during water quenching. Small parts should be satisfied and tempered, and large amounts should be normalized.
Application example: mainly used for manufacturing high-strength moving parts, such as turbine impellers and compressor pistons. Shaft, gear, rack, worm, etc. Pay attention to preheating before welding and stress relief annealing after welding.
The main fastener categories are high-strength bolts.
2). Q235A (A3 steel) – the most commonly used carbon structural steel.
Main features: high plasticity, toughness, weldability, cold stamping performance, a particular strength, and good cold bending performance.
Application example: widely used for parts and welded structures with general requirements. Such as tension rod, connecting rod, pin, shaft, screw, nut, ferrule, bracket, base, building structure, bridge, etc.
The main categories of fasteners are expansion bolts, carriage bolts, stud bolts, butterfly screws, flange bolts, etc.
3). 40Cr – structural alloy steel is one of the most widely used steel types.
Main features: after quenching and tempering treatment, it has good comprehensive mechanical properties, low-temperature impact toughness and low notch sensitivity, good hardenability, high fatigue strength that can be obtained when oil cooling, parts with complex shapes are easy to crack when water cooling, medium cold bending plasticity, good machinability after tempering or quenching and tempering, but poor weldability, easy to break, should be preheated to 100~150 ℃ before welding, and generally used under quenching and tempering conditions, It can also be carbonitriding and high-frequency surface quenching. Application examples: used to manufacture parts with medium speed and medium load after satisfying and tempering treatment, such as machine gear, shaft, worm, spline shaft, thimble sleeve, etc.; After quenching and tempering and high-frequency surface quenching, it is used to manufacture parts with high surface hardness and wear resistance, such as gears, shafts, main shafts, crankshafts, mandrels, sleeves, pins, connecting rods, screws, nuts, inlet valves, etc.; After quenching and medium-temperature tempering, it is used to manufacture parts with heavy load and medium-speed impact, such as oil pump rotor, slide block, gear, main shaft, collar, etc.; After quenching and low-temperature tempering, it is used to manufacture parts with a heavy load, low impact, and wear resistance, such as worm, spindle, shaft, collar, etc.; The carbonitriding site is used to manufacture large-sized transmission parts and high low-temperature impact toughness, such as shafts, gears, etc.
The main types of fasteners are: high-strength large hexagon bolt connection pairs for steel structure
4). HT150 — gray cast iron
Application examples: gearbox, machine bed, box, hydraulic cylinder, pump body, valve body, flywheel, cylinder head, pulley, bearing cover, etc.
5). 35 # steel — common materials for various standard parts and fasteners
Main features: proper strength, good plasticity, high cold plasticity, and good weldability. Local upsetting and drawing can be carried out in the unconscious state. Low hardenability, used after normalizing or tempering.
Application examples: suitable for manufacturing small section parts, parts that can bear large loads (such as crankshaft, lever, connecting rod, shackle, etc.), various standard features, and fasteners.
6). 65Mn – common spring steel
Application examples: various flat and round springs, seat cushion springs, spring springs of small size, spring rings, valve springs, clutch springs, brake springs, cold coil springs, snap springs, etc.
7). 0Cr18Ni9 – the most commonly used stainless steel (American steel grade 304, Japanese steel grade SUS304)
Characteristics and application: As stainless and heat-resistant steel, it is widely used, such as equipment for food, general chemical equipment, and equipment for the energy industry.
8). Cr12 – Common cold work die steel (American steel grade D3, Japanese steel grade SKD1)
Characteristics and application: Cr12 steel is a widely used cold work die steel, which belongs to high carbon and high chromium type ledeburite steel. The steel has good hardenability and wear resistance; Because the carbon content of Cr12 steel is as high as 2.3%, its impact toughness is poor, it is easy to crack, and it is easy to form uneven eutectic carbides; Due to its good wear resistance, Cr12 steel is mainly used to manufacture cold stamping dies, punches, blanking dies, cold heading dies, punch and concave dies of cold extrusion dies, drill sleeves, gauges, drawing dies, embossing dies, wire take-up plates, drawing dies and cold pressing dies for powder metallurgy with low impact load and high wear resistance.
9). DC53 — Common cold working die steel imported from Japan
Characteristics and application: high-strength toughness cold-working die steel, Datong Special Steel (Zhuzhou) steel grade, Japan. After high-temperature tempering, it has high hardness, toughness, and good wire-cutting properties. For precision cold stamping dies, drawing dies, wire rolling dies, cold blanking dies, punches, etc. 10, SM45 — ordinary carbon plastic mold steel (Japanese steel grade S45C).
10). DCCr12MoV-wear-resistant chrome steel
Domestic. Compared with Cr12 steel, the carbon content is lower, and the inhomogeneity of carbides is improved by adding Mo and V. MO can reduce the segregation of carbides and improve the hardenability. V can refine the grains and increase their toughness. This steel has high hardenability. The section below 400mm can be fully quenched. It can still maintain good hardness and wear resistance at 300~400 ℃. It has high toughness compared with Cr12. It has small volume change during quenching, high wear resistance, and good comprehensive mechanical properties. Therefore, it is possible to manufacture various molds with large sections, complex shapes, and large impact, such as ordinary drawing dies, punching dies, blanking dies, trimming dies, flanging dies, wire drawing dies, cold extrusion dies, cold cutting scissors, circular saws, standard tools, measuring tools, etc.
11). SKD11 – Tough chrome steel
Produced by Hitachi in Japan. Technically, the casting structure in the steel is improved, the grain is refined, the toughness and wear resistance are improved compared with Cr12mov, and the service life of the die is extended.
12). D2 — High carbon and high chromium cold-worked steel
Made in America. It has high hardenability, hardenability, wear resistance, high-temperature oxidation resistance, good corrosion resistance after quenching and polishing, and small heat treatment deformation. It is suitable to manufacture various cold working dies and measuring tools that require high precision and long life, such as drawing dies, cold extrusion dies, rigid shear knives, etc.
13). SKD11 (SLD) — high chromium steel with no deformation toughness
Produced by Hitachi in Japan. As the content of MO and V in the steel increases, the casting structure in the steel is improved, the grain size is refined, and the carbide morphology is improved. Therefore, the strength and toughness (bending strength, deflection, impact toughness, etc.) of this steel are higher than that of SKD1 and D2, and the wear resistance is also increased, and it has a higher resistance to backfire. The practice has proved that the service life of this steel die is higher than that of Cr12mov. It is often used to manufacture molds with high requirements, such as drawing molds for impacting abrasive discs, etc.
14). DC53 — High toughness, high chromium steel
Japanese Datong plant type production. The hardness of heat treatment is higher than that of SKD11. After tempering at a high temperature (520-530), it can reach 62-63HRC high hardness. DC53 exceeds SKD11 in strength and wear resistance, and its toughness is twice that of SKD11. The toughness of DC53 is rarely cracked and cracked in cold working die manufacturing, which significantly improves the service life and reduces the residual stress; The residual stress is reduced after high temperature turning back because the cracks and deformation after wire cutting are suppressed. The machinability and readability exceed SKD11 and are used for precision stamping dies, cold forging, deep drawing dies, etc.
15). SKH-9 – General high-speed steel with high wear resistance and toughness
Produced by Hitachi in Japan. Used for cold forging dies, cutting machines, drills, reamers, punches, etc.
16. ASP-23 — powder metallurgy high-speed steel
Made in Sweden. Carbides are evenly distributed, wear-resistant, high toughness, easy to process, and heat treatment size is stable. Used for punch, deep drawing die, drill die, milling cutter, shear blade, and other long-life cutting tools.
17). P20 – General size plastic mold
Made in America. It can be operated by electric etching, pre-hardened HB270-300 in ex-factory condition, and quenched hardness HRC52.
18). 718 – High demand plastic mold
Made in Sweden. Especially for electric corrosion operation, HB290-330 is pre-hardened in ex-factory condition, and the quenching hardness is HRC52.
19). Nak80 — high mirror, high precision plastic mold
Produced in Datong, Japan. Pre-hardening HB370-400 in ex-factory condition, quenching hardness HRC52.
20). S136 — Anti-corrosion and mirror polishing plastic mold
Made in Sweden. Pre-hardening HB ＜ 215 in ex-factory condition, quenching hardness HRC52.
21). H13 – standard die-casting die.
Used for aluminum, zinc, magnesium, alloy die casting, hot stamping die, and aluminum extrusion die.
22). SKD61 — Advanced die-casting die
Hitachi, Japan, produces it. The service life is significantly longer than that of H13, including the hot stamping die and aluminum extrusion passes through the electro-slag remelting technology.
23). 8407 — Advanced die-casting die
It was made in Sweden. Hot stamping dies, aluminum extrusion die.
24). FDAC — added sulfur to enhance its workability

The pre-delivery hardness is 338-42HRC, which can be directly carved without quenching and tempering. Used for small batch mold, simple mold, various resin products, sliding parts, mold parts with short delivery time, zipper mold, and glasses frame mold.

1. Screw production process – annealing

1.1 Purpose:

Heat the wire rod to the appropriate temperature, keep it for a specific time, and then slowly cool it to adjust the crystal structure, reduce the hardness, and improve the normal temperature processability of the wire rod.
1.2 Operation process:
Feeding: Hang the products to be treated into the furnace, and pay attention to the furnace cover should be tightly covered. Generally, one furnace can simultaneously process seven rolls (about 1.2 tons/registration).
Temperature rise: slowly raise the temperature in the furnace (about 3-4 hours) to the specified temperature.
Thermal insulation: 1018 and 1022 wires are kept for 4-6 h at 680 ℃ – 715 ℃, and 10B211039 and CH38F wires are held for 5.5-7.5 h at 740 ℃ – 760 ℃.
Cooling: slowly reduce the temperature in the furnace (about 3-4 hours) to below 550 ℃, then cool it to the average temperature with the furnace.
1.3 Quality control:
1) Hardness: the hardness of 1018 and 1022 wire rods after annealing is HV120-170, and the hardness of medium carbon wire rods after annealing is HV120-180.
2) Appearance: the surface shall be free of oxide film and decarburization.

2. Screw production process — pickling

2.1 Purpose:

In addition to the oxide film on the wire surface, a layer of phosphate film is formed on the metal surface to reduce the scratch on the tool and die during wire drawing and cold pier or forming.
2.2 Operation process:
Pickling: To remove the oxide film on the wire surface, immerse the entire disk element in three hydrochloric acid tanks with a concentration of 20-25% at room temperature.
Water purification: remove the hydrochloric acid corrosion products on the wire surface.
Oxalic acid: increase mental activity to make the following process’s natural skin film dense.
Film treatment: immerse the disc element into phosphate, and the steel surface contacts with the formation treatment solution. The steel dissolves the naturally insoluble compound (such as Zn2Fe (Po4) 2 · 4H2O) and attaches to the steel surface to form a film.
Water purification: remove the residue on the surface of the skin film.
Lubricant: Because the friction coefficient of phosphate film is not very low, it cannot give sufficient lubrication during processing, but it reacts with metal soap (such as sodium soap) to form a hard metal soap layer, which can increase its lubrication performance.

3. Screw production process — wire drawing

3.1 Purpose:
Cold pull the coil element to the required wire diameter. Some products can be divided into uneven extraction (shelling) and delicate extraction.
3.2 Operation process
After pickling, the coil element is cold-drawn to the required wire diameter through the wire drawing machine. It applies to the wire used for large screws, nuts, and dies.

4. Screw production process — molding

4.1 Purpose:

The wire rod is cold cast (or hot cast) to achieve the semi-finished product’s shape and length (or thickness).
4.2 Operation process:
(1) Hexagon bolt (four die four punch or the third mock examination three punch)

• 1) Cutting: cut the wire stuck in the cutting die into the required blank by moving the movable scissors in one direction.
• 2) First punch: the back punch presses the blank against the blank effectiveness, and the blank is initially formed, and then the back punch pushes the blank out.
• 3) Second punch: the embryo enters the second punch, the second punch extrudes, the source is oblate, and the later energy pushes the seed out.
• 4) Three-punching: the embryo enters the third punch, and the hexagon of the source is initially formed through the cutting of the hexagon punch core. After that, the rear energy pushes the head into the third punch, and the cutting is cut from the hexagon, and the hexagon is formed.

(2) Hexagon bolt (the third mock examination and three punches)
(3) Screw (general head type the first mock examination and two punches)

• 1) Cutting: cut the wire stuck in the cutting die into the required blank by moving the movable scissors in one direction.
• 2) One punch: fix the die, and the first punch will initially shape the product head so that the next stroke can be fully formed. When the product is slotted, one point is concave and elliptical groove; when the product is a cross groove, one punch is open and square groove.
• 3) Second punch: after the first punch, the punching tool runs as a whole, the second punch moves to the front of the mold, and the double energy moves forward to form the final product. After that, the blank is pushed out by the back punch.

4.3 Hot stamping
Hot beating is also called red beating.
1. Heating: heat one end of the blank to be formed in the heating equipment to the white heat state, and set the heating temperature and time according to the product specification. Generally, heating for less than 3/4 is 7-10 seconds, and heating for 7/8-1 “is about 15 seconds.
2. Forming: move the heated blank quickly to the forming machine, fix it through the back seat, clamp the mold, and impact the blank with the head mold to form. The spacing of the backseat can be adjusted according to the length of the blank.
3. Tie rod: Extrusion to shrink the product on the tie rod machine.
4.4 Nut forming:
(1) Operation process:
Cutting: the wire rod is cut into the required blank by combining the internal knife die (410) and the shear knife (301).
First punch: the deformed cut blank is shaped by the front punch die (111), the stroke die (411), and the back punch bar (211), and the blank is pushed out by the back punch bar (211).
Second punch: the operation clamp (611) clamps the blank from the first punch to the double point, which is matched by the front effectiveness die (112), the stroke die (412), and the back punch bar (412) to further shape the blank, and strengthen the flattening and saturation effect of the first punch, and then the blank is pushed out by the back punch bar (212).
Three-punching: the operation clamp (612) clamps the blank from the second punch to the third punch, and the blank is squeezed again by the front effectiveness die (113), the stroke die (413), and the back punch bar (213), so that the lower energy can be fully formed, and then the blank is pushed out by the back punch bar (213).
Four punches: the operation clamp (613) clamps the blank from three points to four holes, and the nut is entirely formed by the cooperation of the front punch die (114), the stroke die (414), and the back punch bar (214). The thickness of the nut is adjusted by controlling the thickness of the iron filings, and then the blank is pushed out by the back punch bar (214).
Five-punching: the operation clamp (614) clamps the blank from four to five punches, and the front punch die (119), and the stripper plate (507) cooperate to punch the formed blank entirely and make the broken iron filings enter the lower part of the punching die, and finally complete the forming of the nut. The head mark of the nut is formed in this process.

5. Screw production process — tooth rolling

Purpose: Roll or tap the formed semi-finished products to achieve the required thread. In practice, the bolt (screw) is the rolling tooth, the tooth bar is the rolling tooth, and the nut is the tapping tooth.
Tooth rolling: Tooth rolling is to fix one tooth plate, and the other movable tooth plate drives the product to move and uses extrusion to produce plastic deformation of the product to form the required thread.
Tapping: Tapping is to tap the formed nut to form the required thread.
Tooth rolling: Tooth rolling uses two corresponding screw rollers to rotate in a positive direction and extrusion to make the product plastic deformation, forming the required thread. Moving teeth are usually used for tooth strips.

6. Screw production process – heat treatment

6.1 Heat treatment method: Different methods can be selected according to different objects and purposes.
Quenched and tempered steel: high-temperature tempering after quenching (500-650 ℃)
Spring steel: medium temperature tempering after quenching (420-520 ℃)
Carburizing steel: quenched and tempered at low temperature after carburizing (150-250 ℃)
After low and medium carbon (alloy) steels are quenched into martensite, the general rule is that the strength decreases while the plasticity and toughness increase with the increase of tempering temperature. However, due to the different carbon content in low and medium-carbon steels, the tempering temperature affects them differently. Therefore, to obtain good comprehensive mechanical properties, the following ways can be taken:
1) Low carbon (alloy) steel is selected and tempered at a low temperature below 250 ℃ after quenching to obtain low carbon martensite. To improve the surface wear resistance of this kind of steel, only the carbon content of each surface layer can be improved, that is, surface carburization, which is generally called carburized structural steel.
2) Medium carbon steel with high carbon content is adopted. After quenching, it is tempered at a high temperature (500-650 ℃) (quenching and tempering treatment) to maintain sufficient strength under high plasticity. Generally, this type of steel is called quenched and tempered steel. If you want to obtain high strength but prefer to reduce plasticity and toughness, you can adopt low-temperature tempering for gold-bearing quenched and tempered steel with low carbon content. You can get the so-called “ultra-high-strength steel.”
3) If the steel with a carbon content between medium and high carbon (such as 60, 70 steel) and some high carbon steel (such as 80, 90 steel) are used to manufacture springs, to ensure a high elastic limit, yield limit, and fatigue limit, the medium temperature tempering after quenching is adopted.
4) Decarburization: refers to the carbon loss on the surface of black metal materials (steel). Decarburization will occur after heat treatment. A slight decarburization is allowed. The depth of the decarburization layer affects the surface hardness. The deeper the decarburization layer is, the smaller the surface hardness value is.
The specific test is based on GB3098.1.
6.2 Operation process:
Annealed (pearlitic steel)
(1) Preheating treatment: normalizing
High-temperature tempering (martensitic steel)
1) Normalizing is to refine grains, reduce the degree of banding in the structure, and adjust the hardness to facilitate mechanical processing. After normalizing, the steel has equiaxed, refined grains.
(2) Quenching: heat the steel body to about 850 ℃ for quenching. The quenching medium can be selected according to the steel piece’s size and the steel’s hardenability. Generally, water or oil, or even air quenching, can be chosen. Steel is quenched state and has low plasticity and high internal stress.
(3) Tempering:

• 1) To make the steel have high plasticity, toughness, and appropriate strength, the steel shall be tempered at a high temperature of about 400-500 ℃. The steel with increased sensitivity to temper brittleness must be cooled rapidly after tempering to inhibit the occurrence of temper brittleness.
• 2) If the parts are required to have exceptionally high strength, temper at about 200 ℃ to obtain medium-carbon tempered martensite.

6.3 Spring steel:
Quenching: oil quenching at 830-870 ℃.
Tempering: Temper at about 420-520 ℃ to obtain tempered troostite structure.
6.4 Carburizing steel:
Chemical heat treatment refers to the infiltration of C elements into the surface of steel parts in the active medium containing certain chemical elements at a certain temperature. Preheating (850 ℃) carburizing (890 ℃) diffusion (840 ℃) process
Carbon and low alloy carburized steels are generally quenched directly or once.
Temper at low temperature to eliminate internal stress and improve the strength and toughness of carburized layer.

7. Screw production process – surface treatment

7.1 Type of surface treatment:
Surface treatment is the process of forming a covering layer on the surface of the workpiece through specific methods. Its purpose is to give the product a beautiful texture and an anti-corrosion effect. The surface treatment methods are all based on the following ways:
Electroplating: immerse the parts to be plated in an aqueous solution containing the deposited metal compound. Pass the current through the plating solution to make the plated metal precipitate and deposit on the parts. Generally, electroplating includes zinc plating, copper plating, nickel plating, chromium plating, copper-nickel alloy plating, and sometimes boiling black (bluing) plating and phosphating.
Hot-dip galvanizing is completed by immersing carbon steel parts in a molten zinc plating bath at about 510 ℃. As a result, the iron-zinc alloy on the surface of steel parts gradually becomes the passive zinc on the external surface of products. Hot dip aluminizing is a similar process.
Mechanical plating: impact the product’s surface with coated metal particles and cold weld the coating onto the product’s surface.
7.2 Quality control:
Electroplating quality is mainly measured by its corrosion resistance, followed by its appearance. Corrosion resistance is to simulate the working environment of the product, set it as the test condition, and conduct a corrosion test on it. The quality of electroplating products shall be controlled from the following aspects:
Appearance:
The product’s surface cannot be partially free of coating, scorched, rough, dim, peeling, peeling, and apparent stripes. It is not permitted to have pinhole pitting, black plating slag, a loose passivation film, cracking, peeling, and serious passivation marks.
Coating thickness:
The working life of the fastener in a corrosive atmosphere is proportional to its coating thickness. Generally, the recommended economic plating thickness is 0.00015in-0.0005in (4-12um)
Hot-dip galvanizing: the standard uniform thickness is 54um (43um for diameter ≤ 3/8), and the minimum thickness is 43um (37um for diameter ≤ 3/8).
Coating distribution:
With different deposition methods, the accumulation mode of the coating on the fastener surface is also different. During electroplating, the metal coating is not uniformly deposited on the outer edge, and the thicker layer is obtained at the corner. In the threaded part of the fastener, the most viscous coating is located at the top of the thread, gradually thinning along the side of the line, and the thinnest layer is deposited at the bottom of the thread. On the contrary, the thicker coating is deposited at the inner corner and the bottom of the line. The metal deposition tendency of the mechanically plated layer is the same as that of the hot-dip coating, but it is smoother, and the thickness is much more uniform on the whole surface.
Hydrogen embrittlement:
During the processing and treatment of fasteners, especially during the acid and alkali washing before plating and subsequent electroplating, the surface absorbs hydrogen atoms, and the deposited metal coating then captures hydrogen. When the fastener is tightened, the hydrogen turns enough towards the part with the most concentrated stress, causing the pressure to increase to more than the strength of the base metal and causing small surface cracks. Hydrogen is particularly active and quickly penetrates newly formed fractures. This pressure-break-penetration cycle continues until the fastener breaks. It usually occurs within a few hours after the first stress application.
To eliminate the threat of hydrogen embrittlement, fasteners should be heated and baked as soon as possible after plating to make hydrogen seep from the coating. Baking is usually conducted at 375-4000°F (176-190 ℃) for 3-24 hours.
Since mechanical galvanizing is a non-electrolyte, it eliminates the threat of hydrogen embrittlement. In addition, the engineering standard prohibits hot-dip galvanizing of fasteners with hardness higher than HRC35 (British Gr8, metric grade 10.9 or above). Therefore, hydrogen embrittlement rarely occurs in hot-dip fasteners.
Adhesion:
Cut or pry off with a solid blade and considerable pressure. If the coating peels off as a flake or skin in front of the tooltip so that the base metal is exposed, it should be considered that the adhesion is insufficient.
Source: China Screw Manufacturer – Yaang Pipe Industry (www.epowermetals.com)