Grade 8 bolts refer to grade 8 high strength bolts, which are generally defined as grade 8.8 bolts. The first number represents 1 / 100 of the tensile strength, and the second number represents 10 times the ratio of yield to tensile strength. The tensile strength of Grade 8 bolts or grade 8.8 bolts is 800MPa, and the yield strength is 800 x 0.8 = 640mpa. The bearing capacity of the bolt can be calculated by selecting the cross-sectional area of the bolt.
Grade 8 bolt are high-strength bolt
Bolts made of high-strength steel or requiring large preload can be called high-strength bolts. High strength bolts are mostly used in the connection of bridges, rails, high pressure, and ultra-high pressure equipment. The fracture of this kind of bolts is a brittle fracture.
High-strength bolts are made of high-strength materials. The screw, nut, and washer of the high-strength bolt are made of high-strength steel, commonly used 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Common bolts are usually made of Q235 steel.
From the strength level: high strength bolts are widely used. 8 s and 10.9 s are commonly used, of which 10.9 is the most. The strength grade of common bolts is lower, generally 4.4, 4.8, 5.6, and 8.8.
Types of Grade 8 bolt
DIN 931 8.8 bolt, hex head with semi-thread bolt
DIN 912 8.8 bolt, Hexagon socket bolt
DIN 6921 8.8 bolt, Hexagon flange bolt
DIN933 8.8 bolt, hexagon head with full thread bolt
Surface treatment of Grade 8 bolt
8.8 Class bolts are high-strength bolts, and the main surface treatment is zinc plating, yellow plating, black, or hot-dip galvanizing.
In order to enhance the hardness and corrosion resistance of blackened bolts, blackening treatment (also called bluing treatment) is to use a chemical reaction to react a dense Fe3O4 oxide film on the surface of steel
DIN 931 8.8 black bolt size:
DIN 912 8.8 black bolt size:
DIN 6921 8.8 bolt black
DIN933 8.8 black bolt
Grade 8 bolt hot-dip galvanized finish
There is a layer of zinc on the surface of the hot-dip galvanized bolt with a thickness of 50um. The thick zinc layer forms a protective layer on the bolt, which can prevent the air from oxidizing the bolt. Moreover, the zinc layer with enough thickness can slow down the atmospheric corrosion of the zinc layer. It won’t rust for at least 20 years.
DIN 931 8.8 Hot Dip Galvanizing ( HDG) bolt size:
DIN933 8.8 Hot Dip Galvanizing ( HDG) bolts
Grade 8 bolts with zinc or yellew plating
DIN 931 8.8 zinc plating bolts size:
DIN 912 8.8 zinc plating bolts size:
DIN 6921 8.8 bolts zinc plating
DIN933 8.8 zinc plating bolts
What material made for grade 8 bolts
8.8 bolts do not specify materials, only the chemical composition of the materials: low carbon alloy steel (such as boron, manganese, or chromium) is quenched and tempered, or medium carbon steel is quenched and tempered. As long as the material is within the specified scope, it can be done according to the specified heat treatment requirements.
Class 8.8 screws are generally c1035 for tempering and heat treatment, with the hardness of 22-32hrc; for nuts below M5, we use the products of plain carbon steel as grade 8; for M6 to M10, we use c1035 cold heading products; if they are not heat-treated, they are used as grade 8; M12 to M16 are made of c1035 material, and the heat treatment can reach grade 8; if the hardness exceeds M16, we use c1045 or better materials to make them, and add the heat place Only by reason can we guarantee level 8.
So according to our usage habits, the performance grade of Grade 8 nut is wider than that of grade 8.8 screw, and the lower hardness down (i.e. lower limit of 8.8) and upward hardness is slightly higher (upper limit of grade 8.8).
In most cases, the strength of the screw will be higher than that of the nut; this can ensure that the life of the screw is longer than that of the nut when used together. This way, when the fasteners are removed and replaced, the cost will be lower and easier.
Just as the hardness of the spanner than the screw must be higher so that in the process of repeated use, the wrench is worn first instead of the spanner so that the spanner life is protected.
What is the quenching temperature and tempering temperature of grade 8 bolts?
Generally, 35 or 45 steel is used, 35 steel is quenched at 840 ~ 870 ℃ and 45 steel is quenched at 830 ~ 860 ℃. Of course, the temperature of specific products and materials fluctuates to a certain extent. Attention should be paid to prevent deformation when quenching water. 45 steel bolts should be especially prevented from cracking. If they are quenched with oil, it is better to use oil quenching to avoid cracking
Of course, the tempering temperature should also be adjusted according to the actual situation. The materials from different manufacturers, the shapes of products from different batches of the same manufacturer, and the cooling performance of quenching oil may be different. The heat treatment process will have certain changes. These can only be adjusted according to the actual situation by the operator.