All bolts with an M-head are bolts, not specifically high-strength bolts. The calibration of general hexagon head bolts is generally in the following format: M30*300-10.9-tZn, where M30 indicates the thread specification, that is, the outer diameter of the thread is 30mm, 300 is the available length of the bolt, that is, the nominal length, and grade 10.9 marks the bolt The use performance level, tZn is the surface treatment method. Other forms of bolts have their own expressions, but generally threaded ones start with "M", and the same is true for nuts.
Focus on performance levels. The performance grades of bolts are divided into 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 10.9, 12.9 and other grades from small to large. The meaning of this marking method is to take 10.9 as an example: 10 means 1/100 of the tensile strength of the bolt, which means that the tensile strength of the bolt of this grade must reach the 1000MPa level, which may actually be higher than 1000MPa.
GB/T3098.1 stipulates that the minimum tensile strength of this grade of bolts must be greater than 1040MPa. The number after the decimal point indicates the yield-to-strength ratio, that is, the bolt's (yield strength/tensile strength)=0.9, that is, the yield strength=1000*0.9=900MPa. The yield strength of this grade of bolts must be higher than 900MPa to be considered qualified. Usually we refer to the bolts of the three grades of 8.8, 10.9, and 12.9 as high-strength bolts.
It should be noted that GB/T3098.1 does not give specific values of yield strength for these three grades of bolts, because for high-strength bolts 0.8 and 0.9 are the minimum yield strength ratios specified by the grade. , Since high-strength bolts are used for very important links, there is no spot on the market, and they must be ordered from the bolt manufacturer. Therefore, the specific yield ratio is generally produced by the buyer putting forward specific values.
Bolt strength grade marking code consists of two parts of numbers separated by ".". The meaning of the number part before the "." in the marking code represents the nominal tensile strength, such as "4" in grade 4.8 represents 1/100 of the nominal tensile strength of 400N/MM2. The meaning of the "." and the number after the point in the marking code represents the yield-to-strength ratio, that is, the ratio of the nominal yield point or the nominal yield strength to the nominal tensile strength.
For example, the yield point of grade 4.8 products is 320 N/mm2. Carbon steel: The mechanical properties of metric bolts can be divided into: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8. The meaning of the first 8 of the 8.8 bolts is that the tensile strength per square millimeter is 800 Newtons. The tensile force of 80 kg, the last eight means that the yield point of the 8.8 grade product is 6400N/mm2. As for 8.8s high-strength steel bolts, the difference from ordinary 8.8-grade bolts is that a certain preload must be loaded during installation and cannot be reused.
Bolt concept, bolt: a type of fastener consisting of a head and a screw (a cylinder with an external thread), which needs to be matched with a nut to fasten and connect two parts with through holes. This form of connection is called a bolted connection.
