China Hardware’s Overview of the Process of Thread Fastener Production – Bolts, Screws and Studs

Keywords:fastener,bolts,screws,studs

As an important part of the whole life cycle of fasteners, the production of threaded fasteners is an important process to determine the reliability of joint pairs in the whole life cycle. The main production processes of threaded fasteners include material modification, blank forming, thread forming, machining, heat treatment, surface treatment, surface post-treatment and screening and packaging.

 

For a particular threaded fastener, the production process does not necessarily include all of the above processes, but rather some combination of the above processes.

 

For example: Q1841050TF61 Hexagon flange bolt M10X1.5×50 grade 10.9 silver grey chrome free zn-al coating. The production process is: material reforming → cold heading forming → thread forming → quenching and tempering heat treatment → surface treatment → screening → packaging

 

Material restructuring

Material restructuring as a process, is the production of steel mills hot-rolled wire, after processing into a fine line suitable for cold heading forming. (Red molding can be cut directly from the bar, without restructuring)

 

In the process of material restructuring, wire rod has undergone three changes:

  1. The wire diameter changes from the inlet diameter to the diameter required by cold heading process, and ensures roundness;
  2. The internal structure of the wire is changed from the hot-rolled sheet pearlite structure to the spherical pearlite structure, with lower hardness and better plasticity;
  3. The surface state changes from hot rolled scale state to phosphorus saponified state, suitable for cold heading lubrication.

 

In order to achieve the above three purposes, the material modification is generally composed of three separate processes: acid pickling phosphorus saponification, wire drawing, spheroidizing annealing/recrystallization annealing.

 

Pickling is to clean the oxide scale/rust on the surface of the disk circle with acid; Phosphating is to form a porous phosphate coating on the surface of the disc matrix. Saponification is a layer of saponification lubricant attached to the phosphating disk circle, phosphating and saponification together on the disk circle surface to form a lubrication layer, for drawing or cold heading lubrication. These three processes are carried out on the pickling phosphorus saponification growth line.

 

Drawing is the process of reducing the diameter of the phosphor saponified disc through the drawing die to the wire rod of the required diameter, and processing with the drawing machine.

 

The fire is to put the disk circle at 20-30℃ above the Ac1 temperature of its material, hold for a period of time after slow cooling, to obtain the structure of spherical or granular carbides evenly distributed on the ferrite matrix. The spheroidized annealed material has good plasticity and is easy to cold heading. Recrystallization annealing is to anneal the rough-drawn wire again to eliminate cold hardening in order to fine draw to the appropriate wire diameter again.

 

To complete the reform of a kind of wire, often need the above – mentioned 3 working procedure for many times combination and become. Form such as one ball one stroke, one ball two strokes, two shots two strokes of the process combination.

 

For example, φ25 SCM435 material, after two ball two draw for φ24 fine line, its reform route is: pickling → spheroidization → pickling phosphorus saponification φ→ coarse extraction φ24.49→ recrystallization annealing → pickling phosphorus saponification → fine extraction φ24.

 

1.Cold heading molding

Cold heading molding refers to the use of cold heading machine to cold heading the fine line die into parts blank, in addition to the special size (thin rod, large chamfering, etc.) and thread, all dimensions of the parts are obtained in this process.

 

Cold heading mold is divided into two parts, one part is mainly used to form the screw rod, called the main die; Part of the molded bolt head is called the die. The head of bolt fasteners generally needs to be deformed several times to form, but the threaded rod is not necessarily.

 

According to the complexity of the mouldable parts, the cold heading machine is divided into a variety of models, according to the number of main die die: one die two punch, two die four punch, three die three punch, four die four punch, five die five punch. The above equipment is generally sufficient for bolt and screw parts. More complex equipment is generally used to form non-standard parts.

 

According to the size of the mouldable parts, the equipment is divided into several models. The equipment model usually consists of a code representing the molding capacity and a code representing the number of cavities, such as 10B4S,

 

Cold heading die is the combination of multiple components, such as a set of four four impact molding die, it consists of four steps of molding, each work step into the main mold and die again, of master mold and die and mould, die, die, blunt needle, thimble, spring and so on, some of the parts is common, some parts (involving the size parts) is based on the special parts.

 

Parts cold heading molding processing first needs according to the mold design will need to assemble the mold combination together, and then into the cold heading machine. Then the design of the wire diameter fine line into the equipment has a cutting die, cut the length of the design, and then through each step upsetting molding, each step needs to be adjusted according to the size of the final molding parts, such as increasing or reducing the length of the cutting, adjusting the position of the thimble and so on. Until the qualified parts are cold upset. For complex products, this process often takes several hours or even days (new products, new molds, new processes, initial debugging). This is also the key process that restricts the whole cold heading productivity.

 

2.The machining

 

Machining is used for forming parts of special sizes that cannot be cold headed, such as large chamferes, radial holes, etc. There are many machining equipment for fasteners, both special such as bolt chamfering machine, but also general CNC lathes, drilling machines and other equipment. The following is a brief introduction to common fastener machining.

 

Chamfering: mainly used for bolt tail chamfering, especially thread guide end such as truncated cone end, taper end of the tail chamfering need to be machined for molding. This kind of tail chamfering can be processed quickly by automatic chamfering machine.

 

Turning: refers to the use of CNC lathe to special size of the rod, head processing. The fine middle smooth rod, the groove of the rod, the ball head and the large size shaft shoulder generally need to be processed by turning. The turning process can only be completed with the suitable fixture.

 

Drilling: The hole of the bolt head through the wire, the center hole and radial hole of the hollow bolt (such as the brake caliper through the oil bolt) need to be drilled by the drilling machine

 

Straightening: for bolts with large aspect ratio (generally greater than 10), it is easy to produce bending in the processing process such as heat treatment and surface treatment. It is necessary to straightening parts after heat treatment or before packaging according to product characteristics. Before and after straightening, it is generally necessary to use the straightness checking tool for full selection to ensure qualified.

 

Grinding: for high precision grade required screw diameter and bolt rod with high precision fit requirements, need to use grinder to process to ensure the size progress.

 

Tail cutting: bolts with tail cutting grooves need to be processed by special tail cutting machines. Also useful for cold upsetting directly shaped, but this way of scraping groove part is circular arc, is not conducive to scraping paint and foreign bodies.

 

  1. Thread forming

Thread forming is the process of processing threads. At present, the threads are molded by extrusion. According to the size of the forming force, it is divided into twisting and rolling threads, as shown in Figure 1.

 

Rub the thread is to use two flat rub the thread board, a reciprocating motion, a static, rub the thread board in a process to complete the screw extrusion molding; Then the thread is rubbed back to the starting point, and the feeding mechanism will feed the new blank again, and start the new cycle.

FIG. 1 Schematic diagram of twisting (left) and rolling (right)

Thread rolling extrusion from motor reciprocating force, for large size bolt strength is not enough, the need to use thread rolling, rolling is to use two screw rolling wheel, screw rolling wheel with screw thread, forming, light grey into the first rotating screw rolling wheel, and then use hydraulic extrusion side screw rolling wheel, reaches the set extrusion end returns a thread parts processing is completed. For the large size (M18 above) and part of the parts after heat treatment screw molding using wire rolling. There are some special shapes, with the screw machine is not good feeding parts also useful rolling wire processing. Rolling wire generally uses artificial feeding, processing efficiency is lower than twisting wire.

 

Thread molding is divided into hot before the rub wire and hot after the rub wire, hot before it is to point to thread molding before heat treatment, at this time bolt material hardness is lower, easy to process, the life of the rub wire board is also high. Hot thread after rubbing refers to the thread forming after heat treatment, hot thread forming can retain the compressive stress on the surface of the thread when forming, the fatigue resistance of the thread is better, more used for engine and other parts under alternating load, but hot thread after rubbing the thread of the thread plate life is low, the cost is higher.

4. Heat treatment

Heat treatment is through the combination of heating and cooling to obtain the required bolt material structure, to achieve the required mechanical properties of the process. There are three main heat treatments for fasteners: quenching and tempering, carburizing (which can be carried out simultaneously with quenching and tempering) and induction quenching.

 

Quenching and tempering heat treatment is a heat treatment process of quenching + high temperature tempering to obtain tempered martensite (sotensite), which is completed by heat treatment processing line. See Figure 2 for typical quenching and tempering heat treatment line of fasteners.

 

The bolts are fed, cleaned and degreed (now dephosphorization is usually integrated into this step, called online dephosphorization), and then enter the quenching and heating furnace. After holding for a period of time at the austenization temperature, the bolts fall into the quenching oil tank to complete quenching; After quenching the parts of martensite structure, high hardness, but brittleness is also large, and then after cleaning to remove oil into the tempering furnace high temperature tempering, composition into sostenite, hardness is reduced, plastic toughness is also improved, obtain comprehensive excellent mechanical properties.

FIG. 2 Schematic diagram of heat treatment production line

 

FIG. 3 shows the microstructure changes of fastener material during the whole processing process. (a) is the large lamellar pearlite + ferrite structure of hot rolled plate; After spheroidizing annealing, the carbide becomes (b), which is spherical particles evenly distributed on the ferrite and has good plasticity, which is conducive to cold heading forming. After quenching, it becomes (c) martensite structure; After tempering at high temperature, the tissue becomes (d) sostenite.

FIG. 3 Change process of material internal structure during fastener processing

 

In the process of quenching and heating, in order to ensure that the bolt surface does not decarbonize, it is necessary to control the carbon potential in the furnace to achieve the balance of carbon diffusion on the material surface. Generally, the carbon potential will be equal to or slightly higher than the carbon content of the bolt material.

 

For the parts that need surface carburizing to increase the surface hardness, such as ST tooth tapping screw, it is necessary to increase the carbon potential in the furnace during quenching and heating to achieve the purpose of carburizing, and the rest are the same as quenching and tempering treatment.

 

Some special bolts need to increase the local surface hardness, such as stiffening the head of the track plate bolt and stiffening the penetrating teeth of the triangular tooth bolt. In this case, induction quenching is needed to complete the process. The induction quenching shall be carried out after the quenching and tempering heat treatment is completed, and it is necessary to ensure that the surface of the bolt material is not decarbonized.

 

  1. Surface treatment

The surface treatment of fasteners is primarily for the corrosion protection of the parts, including storage and functional corrosion protection, as well as to obtain certain surface requirements, such as friction coefficient, conductivity, and appearance.

 

Common protective surface treatment is divided into: short-term low grade anticorrosion, blackening oil and phosphating oil; Medium grade anticorrosion, electric galvanizing; High grade anticorrosive, electrogalvanized alloy and chromium-free zn-al coating. Less commonly used are electrophoresis and hot dip zinc. Other functions or surface treatments include zinc – tin alloy, copper plating and chromium plating.

 

Blackening oil: blackening is through strong oxidant on the surface of parts to form a layer of iron oxide film, to achieve the purpose of corrosion prevention, traditional blackening in high temperature strong alkaline solution immersion blackening, pollution, high energy consumption. Now there is also a gas blackening method by injecting special gas into the tempering furnace to blacken, and a liquid blackening method by adding blackening agent in the tempering cooling tank by using the residual temperature of tempering. However, it should be noted that the latter two blackening methods do not form dense ferric oxide film on the surface of the parts, and the liquid blackening is a layer of black resin attached to the surface of the parts. Although can also achieve the effect of improved corrosion, but in the surface friction performance is different.

 

Phosphating oil: phosphating on the surface of the parts through a chemical reaction to form a layer of phosphate deposits, and then fill the gap through the oil, to achieve the purpose of corrosion prevention, and phosphating layer loose easy to slide, can play a good lubrication, surface friction coefficient is stable. Phosphating according to the different potions, zinc, manganese, zinc calcium system, etc., they are different in the structure of phosphate crystals formed, so there are differences in corrosion and friction coefficient.

 

Electrogalvanizing: Electrogalvanizing is the most commonly used anti-corrosion measure for iron and steel parts, and has a good appearance. It is also widely used on fasteners.

 

The process of electrogalvanizing is as follows: oil production → rust removal → galvanizing → passivation → sealing. Firstly, clean surface is obtained by removing oil and rust, then zinc layer is plated on the surface of parts by electrochemical reaction, and then chromate passivation film is formed on zinc surface with passivation agent, and finally passivation film cracks are filled with sealing agent.

 

According to the formula of galvanizing tank liquid acid and alkaline, galvanizing is divided into acid galvanizing and alkaline galvanizing, according to whether the passivation solution contains hexavalent chromium and divided into trivalent galvanizing and hexavalent galvanizing, hexavalent chromium is a highly toxic, plating wastewater treatment is difficult, so now phasing out hexavalent chromium passivation. In order to control the surface friction coefficient of galvanizing, friction control lubricant is generally added to the sealing agent.

 

Electrogalvanizing is generally recommended for fasteners of class 8.8 and below. Electrogalvanizing is not recommended for bolts of class 10.9 because of the risk of hydrogen embrittability. Hydrogen flooding must be added when used. 12.9 Galvanized bolts of important parts are prohibited.

 

Note that:

A.No matter alkaline or acid galvanizing, the plating solution is aqueous solution with hydrogen ions, so there is a risk of hydrogen embrittleness.

B .hydrogen flooding can only reduce the risk of hydrogen embrittlement, but cannot completely eliminate it

C.Passing the test of the parallel support surface for hydrogen embrittlement does not completely rule out the possibility of hydrogen embrittlement.

D .Hydrogen ions enter the steel matrix mainly in the electroplating process, pickling process time is short, low temperature. Contribution to hydrogen embrittlement: electroplating > phosphating > pickling.

 

When the electrogalvanized layer is corroded by the outside world, the outer passivation sealing layer is destroyed first, and the zinc layer reacts with water and oxygen to form white rust; Then the corrosion continues. After the zinc layer penetrates the metal matrix, the red rust of iron oxide is generated. This is the white rust time and the red rust time in the salt spray test. Increasing the sealing is to increase the white rust time, and increasing the thickness of zinc layer can increase the time from white rust to red rust. There is no zinc layer in blackening and phosphating, so there is no white rust time, only red rust time.

 

Electric galvanizing alloy (zn-ni alloy, zn-fe alloy) : usually, even after the sealing of electric galvanizing, its anti-corrosion salt spray level is only two or three hundred hours. Adding alloy to zinc layer can improve the corrosion resistance of zinc layer, commonly used zinc alloy zinc nickel alloy and zinc iron alloy. The salt spray lasts seven to eight hundred hours. (Now, there are galvanizing methods that can greatly improve corrosion resistance by adding resin sealants, usually black zinc. The principle is to increase the resin protective layer outside the traditional coating, and use the corrosion resistance of the resin to extend the white rust time, so as to improve the corrosion resistance)

 

Similar to electrogalvanizing, electrogalvanizing alloy also has the risk of hydrogen embrittlement, and the higher the strength level of the part, the greater the risk.

 

Chromium-free zn-al coating: chromium-free zn-al coating is also known as environmental protection dacro, and dacro’s relationship can be seen in xiaobian’s previous article “Dacro coating three life three life”. It has the advantages of environmental protection (no waste water residue containing heavy metals), no risk of hydrogen embrittlement and strong corrosion resistance. It is widely used in fasteners with high strength and high corrosion protection requirements. Chrom-free zinc aluminum coating is divided into bottom coating and surface coating, bottom coating is mainly composed of zinc sheet and aluminum sheet and adhesive, to the purpose of corrosion prevention, surface coating generally contains friction coefficient stabilizer, can control the friction coefficient, can also improve the corrosion resistance and get a special appearance.

 

Compared with electroplating, the only disadvantage of chrome-free zn-al coating is that the binding force between the coating and the substrate is weaker than that of the coating, and local shedding may occur under certain conditions, which may affect the corrosion resistance.

 

From the coefficient of friction stability and adaptability to different surfaces, chrome-free zn-Al coating due to electroplating.

 

Electrophoresis: Electrophoresis is the deposition of a high molecular resin on the surface of metal by electrochemical method to prevent corrosion. Hydrogen embrittleness risk is small, but the utilization rate is not high, only in some interior fasteners have application.

 

Hot dip zinc: hot dip zinc fasteners are soaked in molten zinc solution, zinc layer thickness is very thick, up to more than 50um, automotive fasteners are rarely used. Thread engineering needs to choose 6AZ grade, thread strength will be reduced, and high energy consumption, not environmental protection.

 

Zinc – tin alloy: Fasteners used in electrical systems are electroplated with zinc – tin alloy for electrical conductivity.

 

Copper plating: for welding bolts, in order to improve electrical conductivity, copper plating will be used; For some bolts and nuts used in high temperature environment, copper plating is also used. The overall application of copper plating fasteners is less.

 

Chrome: The exposed wheel nuts will be chrome plated to ensure appea

 

Table post-processing

The surface treatment is mainly for the fastener corrosion protection requirements, taking into account the appearance and friction coefficient control requirements. For other special requirements, need to be processed after surface treatment, fasteners commonly used table after treatment with adhesive, smear-wax and anti-stick agent.

 

Gluing: One of the main purposes of gluing is to increase the bolt’s ability to prevent loosening by rotation. According to its principle, it is divided into chemical glue and ninong glue. The other kind of glue is for the purpose of sealing.

 

Chemical glue is adhesive, pre-coated in the bolt meshing thread, glue reaction after installation, the bolt nuts glued to prevent loose. Chemical adhesive is subdivided into AB adhesive and anaerobic adhesive. There are two components of microcapsule in AB adhesive. The microcapsule is crushed by bolt installation, and the two components are solidified and bonded by reaction. The microcapsules of the components of anaerobic adhesive were solidified and bonded in the absence of oxygen after breaking. After the chemical glue is cured, the disassembly will destroy the bonding, and the repair needs to replace the new bolt.

 

Nylon glue is coated with a layer of hard nylon on the surface of the thread. After installation, it depends on the embedding pressure of nylon to prevent loosening. During processing, the bolt is first heated by induction, and then the nylon powder is sprayed on the heating surface of the bolt, and the nylon powder is melted and bonded on it to complete the coating.

Bolts assembled at certain yield points are also smeared with wax for maximum axial preload.

 

Anti-stick agent: typical representative such as teflon, welding bolts, welding nuts and other fasteners with body electrophoresis is used. It can prevent bolts from sticking to welding slag and prevent fasteners from accumulating electrophoretic paint during body electrophoresis.

 

Screening 

The finished bolts need to be screened to pick out the defective parts or mixtures before packaging. The current screening has optical screening, laser screening and eddy current screening. Screening is generally the use of comparison method, optical screening and laser screening can be screened mixing and size defects (such as no threads or incomplete threads, leakage gaskets, etc.), optical screening can also be screened in the groove of liquid; Eddy current screening can screen for large hardness deviation, material error mixing and heat treatment parts abnormity.

 

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