Технические науки /1. Металлургия

Bogomolov A.V., Latyshev I.V.

Pavlodar State University named after S. Toraigyrov, Kazakhstan

Innovative University of Eurasia, Kazakhstan

Employment of high-tensile fasteners

By the current international classification high-tensile fasteners include fasteners (bolts, nuts, studs), which temporary resistance is greater than or equal to 800 MPa. Based on this parameter, classes of rurability for high-tensile fasteners starts from class 8.8 for bolts and class 8 for nuts.

Low-carbon and alloy steels are used as a raw material (carbon content is less than 0.40%), which grades are 20, 35, 20Mn2B, 65Mn, 40Cr. Mechanical properties of high-tensile fasteners are also determined by the properties of used steel, with subsequent heat treatment in electric furnaces with a protective environment, which prevents decarburization of products.

We should keep in mind tensile strength and yield strength for any kind of fasteners. Tensile strength is the amount of load on fastener, above which the destruction occur, which breaks up the tension. Yield strength - is the amount of mechanical stress that effects on the fasteners, above which plastic deformation occurs. As a symbols of strength class there are written two numbers separated by dot. The first number represents a percentage of the nominal value of tensile strength, measured in megapascals. For example, in the case of 4. 8 the first 4 is the 4 x 100 = 400 MPa = 400 N/mm2 = 40 kgf/mm2.The second digit is the value of the yield stress, referred to the yield, divided by 10. The pair of numbers lets calculat the material yield 8 × 8 × 10 = 640 N/mm2. The yield stress has a great practical importance, because it is the maximum working load of fastening products. Fasteners (bolts, nuts, studs) comes with a strength classes like 4.8, 5.8, 8.8, 10.9, 12.9.

Fasteners are used in various industries, also in different environments and operating conditions:

• High-strength fasteners in chemical plants with corrosive environments.

• Fasteners for use in high temperature conditions.

• High-strength fasteners in structures, machinery and other facilities operating at low temperatures.

• High-strength fasteners with high demands in storage of large explosive effort placed on the fasteners which can work with higher dynamic and static loads.

Fasteners combines high-strength properties of high strength with good ductility and toughness. Strength characteristics of fasteners are determined by the choice of a suitable type of steel and its production technology. The most used steel brands are 35, 40Cr, calibrated with special handle surface, diameter: M20, M22, M24, M27, M30, M33, M36, M42, M48, M56, M64, M72, M110.

Recently, widespread replacement of high-and medium-low-alloy steels for boron. Boron steel is used instead of hard-deformable steel, which reduces consumption of scarce alloying elements. Feature of the boron-containing steels (> 0.001%) for the production of high-strength fasteners is their high technology plasticity and high (to a depth of 50 mm) hardenability. It is established that the boron steel whith content of carbon consists 0.20 -0.35% , has high hardenability and ensure the desired high strength of finished product after curing (σv = 800 -1 300 MPa) in the initial state have a higher ductility than conventional steel, used to achieve of the same strength in the products[1,2].

The durability of connection and fastener material depends on the quality of hardware. There is a wide range of fastening products today. The most famouse product for connecting items is bolt. Bolts are different in form and use (The foundation bolt, anchor bolt with nut, bolts and high strength, etc.). The foundation bolt is a high quality Hardware product. It is used for connecting structures and equipment. It is a rod, at one end of which the device located, which holds the bolt in the foundation, and another end is a thread. The existing anchor bolt with nut is a special rod, which creat anchor clips in a concrete or brick foundation. Ancher bolts with nuts are made of galvanized steel, or stainless steel (stainless steel fasteners), and they have threaded rod with spacer element on one end, and with another nut and washer on other end. For the connection of steel (and bridge as well) and metal constructions used in heavy machinery people use high-strength bolts with hexahedral head. And above all high-tensil bolts are tasted on every stage of production, that’s why all standarts observance are guaranteed.

Important advantages of bolted connections are: simplicity, reliability, interchangeability of the ability to create large compressive forces in the compound (up to 106H and above).

Nuts wind on the bolts with the torque wrenches of various designs to ensure required effort tightening bolts and even distribution of effort by group connection bolts (steel flanges).

The main criterion of the bolted connections efficiency is the strength of the rod bolt. Bolts with thickened purely treated (polished) rod for openings from the scan are calculated for shear and crushing the lateral surface by the action of high shear stress. These bolts are not tightened. Most of the bolt connections are installed in the holes of details with a gap. That’s why such bolts are calculated by taking into account the twisting tension rod by point of friction in the thread and doesn't depend on the direction of external loads. In these compounds, the bolts are tightened by pre-tightening force, which under the action of shear stress is a part of the conditions of mutual immobility of parts and which determines the diameter of the bolt.

A general calculation of the strength of the bolted connection, especially for pipe fittings, tanks, boilers, chemical plant machines, is the calculation of the flange connection of lid to the cylinder, which is under the internal pressure. The calculation is made in case of tightness of the connection. The sealing element is a ring-shaped gasket. It must be compressed by the pressure, that far superior to the pressure in the cylinder. The gasket is deformed by this pressure, so it fills the irregularities of the contacting surfaces and prevents the leakage of fluid or gas from cylinder.

The fatigue strength of bolts in stock are checked at variable external loads.

Normal-strength and high-strength bolts are used, when the bolt-on construction mounting.

The usual strength bolts are made of three types - coarse, normal and high accuracy. The differences are characterized by a quality of manufacturing, material strength remains unchanged. This allows you to use all the usual strength bolts subspecies interchangeably.

High-strength bolts are used in compounds, which characterized by the resistant to shear, as well as in compounds with carrier bolts.

Calculation of the bolted connection is to select the optimal number of bolts of a given diameter, or to determ the diameter of the bolt and the thickness of contracted by bolt items. In compounds, which are resistant to shear, bolts are calculated on the tensile force, required for drawing package, and ensure sufficient quantities of the friction force between the plates of the package. This friction force prevents the displacement of the plate, so bolt works only in tension.

Calculation of the bolt connection with bolt involves the calculation of stresses, generated in the bolts rod for a cut. Based on the calculation of the shear bolt is determined by the number and diameter of bolts. Relying on the collapse of the plate pack is determined by the thickness of the plate package.

Fastened details are preparing, before bolting device. Polished edge of the plate holes and burrs to the plate tightly adhered to the entire area. After checking and cleaning all parts of bolted connection, the bolt holes combine and fix. For a more accurate alignment of holes we use checkpoint mandrel. This is a special rod with a diameter, which is smaller for 0.2 mm than the diameter of bolt hole, it’s made with one hand in the form of a cone. Checkpoint mandrel is inserted into the hole and hammered in. After that, combined hole is fixed by cap - smooth steel rod.

Tighten bolts should ensure tightnessof the connection. This is achieved by compliance with the bolt tightening sequence for the project. In most cases, the bolts in the connection delay from center to edge. In this case, the plate can be transferred to the edges of each other in process. So it turns thick plate pack.

When the factory construction is installing the displacement of the holes in adjacent plates is inevitably. That’s why in such constructions holes have a smaller diameter. After combining the holes they drill out under the desired diameter of bolts.

Bolts perform special calibration keys. They allow you to control the torque of bolted connections. Keys to the regulated puff so arranged that when they the efforts of torque gets a certain value, mechanism is triggered and the neck is impossible. At the same time, the installer knows that the bolt is tightened as needed. The other type is the key with LEDs. They have an instrument with a scale showing the magnitude of torque bolt or allowing to calculate this value based on his testimony.

Reliability of compounds, which are resistant to shear, depends on the quality of the bolts. That’s why bolted connection structure is under control. The torque of 1 / 4 of all the bolts in the node are checked. If the number of bolts in the connection at least 6 pieces, the nessesary is to check all bolts. If at least one bolt is tightened to 80% of the required effort - control applies to all bolts connections. These bolts are tightened. After tightening and control, bolt heads are painted.

So, the feasibility of using high-strength bolted connections in many designs of mounting of technological equipment is confirmed.

References:

1.Zharmenov AA, Mykanov D.M., Akberdin AA and others M.ZH. Bor Process of preparation and metallurgical processing of iron ore. B Vol.: Complex processing of mineral raw materials in Kazakhstan (condition, problems, ways of determination). B 10 min. Tom 3., Astana: Foliant, 2007 .- sec.,6-87.

2. Batinev, GP, AV Bogomolov Prospects for production of high-strength fasteners. Proceedings of International Conference «IV Chokinskie Reading" - Pavlodar: PSU, 2010, sec. 47-51.