A Complete Guide to Bolts


The fastener is a kind of widely used mechanical basic part, which not only plays a fastening role in connection but also can transfer load. Its quality and reliability play an important role in the working performance and structural safety of mechanical products. Fasteners are used in various mechanical and electrical products, automobiles, agricultural machinery, heavy-duty, mining, engineering, transportation machinery, construction, railways, Bridges, vehicles, equipment, ships, tools, instruments, meters, and daily necessities. There are many kinds of fasteners, different performances, and use, and the degree of standardization and serialization is very high.

What Is Fastener

Fasteners, also known as standard parts, standard fasteners, a kind of widely used mechanical base parts, its main role is to connect and fasten.

Fasteners usually include bolts, studs, screws, nuts, washers, pins, tapping screws, retainers, rivets, assemblies components, etc., in more than a dozen categories. Due to the different specifications, geometry, and materials of fasteners, the total number of fasteners has reached more than 500,000 kinds.

How Are Bolts Made?

Bolt processing methods are generally divided into three kinds: cold/hot forging screw head processing; Direct head forming machining without cold or hot forging; impact molding. Machining high-strength bolts generally use cold hot forging to form a head shape.

The specific manufacturing process of the bolt is as follows:

  1. Green rod
  2. Cold drawing
  3. Spheroidizing Annealing(Preheat treatment)
  4. Phosphorus removal
  5. Pickling
  6. Cold drawing
  7. Cold forging forming
  8. Thread processing
  9. Feeding
  10. Phosphorus removal
  11. Quenching
  12. Cleaning
  13. High tempering
  14. Water cooling
  15. Drying
  16. Fuel injection
  17. Surface finish
  18. Quality inspection

Cold-Drawing Technics

Bolts are usually made of straight round steel or wire rods cut to fit the length. The development of cold processing requires(suitable for the bulk of production) continuous feed to the bolt forming machine, and most of the raw materials for bolts are wire rods.

Because straight round steel or wire rod are made by rolling forming, the precision of outer diameter is difficult to meet the specified tolerance of bolts. Before entering the molding machine, the straight round steel or wire rod is generally wiredrawing according to the outer diameter of the bolt specified by the molding, and then cut into appropriate length, and then head molding, thread molding, and heat treatment.

In the process of cold drawing, the deformation of material grain increases the hardness of the material, that is, the phenomenon of cold work hardening occurs. But because the material is in the process of restructuring, sometimes need to reach the design diameter by several cold drawing, so in order to eliminate work hardening and uneven organization phenomenon, and increase the plasticity and toughness of the material, usually use annealing treatment to achieve the goal. The annealing process generally choose recrystallization annealing or spheroidization annealing, this step is between multiple drawing process, so it is also called intermediate heat treatment. The annealing process should pay attention to the control of annealing temperature and protection of the atmosphere if the compression ratio and annealing temperature with improper grain coarseness, which will lead to the mechanical properties of fasteners greatly reduced, but also easy to cause bolt damage. If the protective atmosphere is not properly controlled, the material and oxidation atmosphere contact will produce a decarbonization phenomenon, decarbonization will weaken the fastening function of fasteners, resulting in loosening or tripping.


Pickling is also one of the key drawing processes. Because the material will form a layer of the scale of different thicknesses on the surface of the material after hot rolling or heat treatment, the existence of oxide skin directly affects the lubrication effect of drawing, increases the drawing force, and aggravates the wear of drawing die. Due to the large hardness of the oxide scale, it is easy to scratch the material, so the finished product can not meet the surface roughness requirements after drawing, so the material must be de-scaling by pickling before drawing. The most important problem of pickling is hydrogen embrittlement. Part of hydrogen generated in the pickling process of steel is absorbed by the steel matrix and diffused into the steel matrix, which increases the brittleness of steel and leads to delayed fracture. Therefore, the process of pickling should be strictly in accordance with the concentration and temperature specified in the operation process of pickling technology, for high-strength fasteners after pickling need to be heated within a specified time, so as to make the hydrogen atoms in the steel discharged.

Molding Techniques

The forming process of fasteners can be divided into a cold heading, a warm heading, and a hot heading according to different processing temperatures.

  1. Cold heading technology is at indoor temperature, the blank material is placed between the upper, lower, and mold and a certain pressure is applied to make the blank material produce axial compression and radial expansion into a reserved shape. Its advantages are high precision, reasonable metal streamline can be obtained after upsetting, its structure and mechanical properties can be improved, as well as the material utilization rate. Its disadvantage is that the upsetting force is large, and the material preparation is more demanding.
  2. The temperature upsetting process is a method of pressure machining by heating the material below the recrystallization temperature (the upper limit is about 600℃). Its advantage is that the forging upsetting force is reduced by 25% compared with cold upsetting, the precision of the finished product is similar to that of cold upsetting, there is basically scale freely, and the die life is significantly improved compared with cold upsetting. Its disadvantage is to control the temperature to be more accurate, too high some metal will appear blue shortness, and too low will affect filling, but also to choose a specific lubricant.
  3. Hot upsetting technology is to heat the metal to the solid line 150~250℃, placed in the mold, under pressure to produce plastic deformation and obtain the desired shape and size. The hot upsetting process is carried out at high temperatures. The metal under high temperature has the characteristics of good plasticity, low deformation resistance, and easy forming. It is suitable for processing small batch repair parts, abnormal parts, stainless steel, and other fasteners which with great difficulty in the cold heading. Its disadvantages are high heating temperature processing, easy-to-produce oxide, and decarburization layer, poor precision of finished products and machining allowance, and low material utilization rate.

Thread Processing Technology

The main processing method of the external thread is rolling thread, threading and die tapping thread, etc., and the main processing methods of internal thread are taped thread (cutting tap, squeeze tap), car thread, etc.

The choice of thread machining method depends on the shape, size, batch size, and machining size of the product. Fastener bolts, screws, studs, and other external thread processing, due to large batch, most will use rolling thread for processing. Rolling thread is a scrap-free processing method. It is also a cold extrusion process for rolling screw blank materials at indoor temperatures. The process is essentially a process of material metal redistribution. The rolling thread has a smooth surface and continuous metal fiber flow, and the static tensile strength is increased by 20%~30% compared with the cutting thread. General high-strength bolts need to be tempered, if the thread is processed first and then tempered, proper measures should be taken in the heat treatment process to avoid thread deformation and surface treatment decarbonization after heating. In the case of higher requirements for fatigue mild, it is necessary to adjust and quench the thread before rolling the thread, which can not only avoid the decarburization of the root thread but also make the residual compressive stress at the bottom of the tooth. For example, the high strength double end stud’s fatigue strength can be increased by 200%, but the service life of the roller is reduced and the manufacturing cost is increased.

Tapping thread is generally selected for large-scale processing of internal thread, which is economical and effective. The equipment used is a vertical single-axis threading machine, a horizontal double-axis automatic threading machine, horizontal four-axis automatic threading machine.

There are many types of bolts, up to 500,000 kinds, YIJIN Hardware as the famous custom fastener supplier with 2o years of fastener manufacture experience, provides customers with a wide range of custom fasteners, including screw, rivet, nuts and bolts, bolts, self-tapping screws, anchor bolt, toggle bolts, drywall screws, nut bolt, u bolt, concrete screws, eye bolt, wood screws, lag bolts, carriage bolt, wing nut, grub screw, lock nut, clevis pin, spring washer, decking screws, set screw, pan head screw, brad nails, stud bolt, dowel pin, pop rivet, split pin, tek screws, special rivet, expansion bolt, hex bolt, pocket hole screws, coach bolts, threaded inserts, contact us we can custom the fasteners you need.

Types of Bolt

Hexagon-headed bolt(full thread): Grade A grade B DIN EN ISO 4017-2001.

Full thread hexagon head bolt: Grade A grade B DIN 933 – 1987.

Hexagon-headed screw: Product grades A and B ISO 4017 – 2011.

These three criteria can be applied to the bolt picture below. DIN933 is the German standard bolt, is also a hexagon head bolt (full tooth) mechanical bolt, and is also used for nuts with cylindrical threaded fasteners. A type of fastener consisting of a head and a screw (a cylinder with external threads) is used with a nut to connect two parts with a through-hole. This type of connection is called a bolt connection. If the nut is unscrewed from the bolt, the two parts can be separated, so the bolt connection is a detachable connection.

cuctom hex bolts manufactured by YIJIN Hardware

Hexagon-headed bolt: Product grades A and B ISO 4014 – 2011.

Hexagon-headed bolt: Product grades A and B ISO 4014 – 1999.

Hexagon-headed bolt: Product grades B DIN 931-2 – 2009.

These three criteria can be applied to the following bolt picture. DIN931 is the German standard bolt, hexagon head bolt (half tooth), for the purpose of connection without full thread, this standard bolt screw body is only two-thirds of the thread.

Hexagon socket head cap screws: DIN 912 – 1983.

DIN912 is the German standard bolt, known as hexagon socket head screw, the following two pictures are applicable to the DIN912 standard, one is the bolt head without fancy teeth, the other is the bolt head with teeth pattern (also known as anti-skid teeth).

Hexagon flange bolts: Product grade B DIN 5787 – 1986.

DIN5787 is a kind of bolt of German standard, called hexagon flange face bolt. The following picture applies to the DIN5787 standard. The most important role of hexagonal flange bolts is that hexagonal is more than a flange surface, which can reduce the hexagonal stress area and increase the contact area of bolts during connection. So that it is not easy to break, and reduce the use of gaskets, thus saving the cost and saving the bolt installation connection time!

The above four bolts are the standard number of bolt-standard parts. This is just the tip of the iceberg of the bolt category. For more information about standard bolts and non-standard bolts, please contact Yijin Hardware. Yijin hardware is committed to the production of bolts for more than 20 years and has a rich production of bolt manufacturing ability.

Materials of Bolt

Different materials are used according to the strength level of the bolt:

At present, the main standard parts on the market are carbon steel, stainless steel, copper, titanium alloy, and super duplex steel.

For carbon steel, we distinguish low-carbon steel, medium-carbon steel, high-carbon steel, and alloy steel based on the carbon content of carbon steel materials.

  1. Low carbon steel C%≤0.25%, China known as A3 steel, the world is basically known as 1008,1015,1018,1022 and so on, mainly used for grade 4.8 bolts and grade 4 nuts, small screws and other bolts without hardness requirements. (Note: Drilling tail screws are mainly adopted 1022 material).
  2. Medium carbon steel 0.25%<C%≤0.45%, China known as no. 35 steel, no.45 steel, the world is basically known as 1035, CH38F, 1039,40ACR, and so on. Mainly used for grade 8 nuts, grade 8.8 bolts, and grade 8.8 hexagon socket head bolts.
  3. High carbon steel C%>0.45%, so far there is basically no material on the market.
  4. Alloy Steel: Add alloying elements in plain carbon steel, increase some special properties of steel: such as 35 and 40 chromium-molybdenum, SCM435, 10B38,12.9 grade bolts are mainly used SCM435 chromium-molybdenum alloy steel, which main components are C, Si, Mn, P, S, Cr, Mo.

Stainless steel performance grade:

45,50,60,70,80, mainly points austenite (18%Cr, 8%Ni) which not only can be welded but also with good heat resistance and corrosion resistance.

Copper commonly adopts brass, and zinc copper alloy as materials, and H62, H65, and H68 copper are standard parts in the market.

Titanium alloy, titanium alloy refers to a variety of alloys made of titanium with other metals. Titanium is an important structural metal developed in the 1950s. Titanium alloy has high strength, good corrosion resistance, and high heat resistance. In the 1950s and 1960s, it mainly developed high-temperature titanium alloy for aero-engine and structural titanium alloy for the airframe. Including TC4、4、TA1、Gr.6、Gc.2、BT5-1、Gr.3、Gr.7、Gr.4、Ti-811.

Super duplex steel: Super duplex stainless steel refers to PREN>40, containing 25%Cr and high molybdenum (>3.5%), high nitrogen (0.22% ~ 0.30%) duplex steel. Including UNS S32550(UR52N+), S32750 (SAF 2507), and S32760(Zeron 100), which were developed by French CLI, Sweden SANDVIK, and British MATER+PLATT respectively. Their deformed materials came out successively in 1990 and 1991 and entered the market in recent years.

Bolt Size

Bolt thread is divided into many national standards. Including German standards, Japanese standards, American standards, and International standards. American standards and international standards are the most commonly used standards in the world, and each standard is divided into fin thread and coarse thread. The following tables are a brief introduction to mechanical thread specifications, which are the commonly used standard.

The above examples are only part of the commonly used mechanical bolt threads. There are many kinds of bolt sizes and standards. I wonder if you want to know more about bolt thread sizes, please contact Yijin Hardware freely. We would like to tell you more about it.

How To Measure The Bolt Size

ways to Measure The Bolt Size

The measurement of bolt size is divided into the outer diameter and length of the thread, such as M6*50, which refers to the outer diameter of the thread being 6mm, the total length of the thread position is 50mm, and if there is a special note to the half tooth, then the length of the thread is less than 50mm. Then it is to measure the head of the bolt, because the size of the head is different for each customer’s purpose, and the size required is also different, so we will not expand into details. There are several thread length measurements for bolts, for example, cross recessed pan head screws, hexagon headed bolts, hexagon socket head bolts, tumbler bolts, tapping screws, etc., which length measurement method is to directly measure the length of the bolt thread diameter, excluding the bolt head. That is to say, when measuring with calipers, it is not necessary to include the bolt head measurement, only the length of the bolt thread diameter. Like a countersunk head screw, or flat head screws, this kind of bolt length measurement is different, which length size measurement standard is thread diameter including bolt head measurement, using caliper measurement, from countersunk head screw head to screw thread tail directly measured, that is, the length of the whole screw size is its thread length. Commonly used tools for measuring bolt size are a Digital caliper, digimatic micrometer, thread ring gauge, screw pitch ruler, and so on.

Bolts and Nuts

bolts and nuts manufactured by YIJIN Hardware

Bolts are the fasten used to match nuts. Nuts are parts that hold mechanical equipment together. Nuts are parts that tightly connect with mechanical equipment. Bolt: machine parts, cylindrical threaded fasteners with nuts.


Combined with all the above, we have learned the relevant knowledge background of fasteners(nuts and bolts). Fastenersplay a pivotal role in the industrial era, and many industrial household products need bolts to connect and fix.

Yijin Hardware has more than 20 years of experience in the production of bolts, bolts of various sizes have produced more than 100 million pieces, and accumulated rich experience in the production of nuts and bolts, as well as fastener solutions. Please contact Yijin Hardware fastener experts to custom u bolt solutions, we will design fasteners and bolts you want for free and quickly, and tell you the use and choice of bolts, for your product fastening and connection custom-made.


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