Ultra-high-strength fasteners can reduce weight and increase installation space by reducing their own size under the same clamping force, so the function and volume of the connected parts can be optimized, so that the equipment can achieve the purpose of overall weight reduction and performance optimization.
So what are high-strength bolts? What are the strengths of high-strength bolts? Let me show you today.
On November 28, 2021, the high-performance steel material team led by Professor Dong Han from the School of Materials Science and Engineering of Shanghai University, Hebei Longfengshan Casting Co., Ltd., Qifeng Precision Technology Co., Ltd., Zhoushan 7412 Factory, Jiangsu Metallurgical Technology Research Institute, Shanghai University (Zhejiang) High-end Equipment Basic Parts Materials Research Institute, Shanghai University New Materials (Taizhou) Research Institute and other seven units, after more than a year of joint research, through the "material production-fastener manufacturing-service evaluation" full industry chain collaboration, based on the high-performance theory of steel materials, using the high-purity iron raw materials produced by Longfengshan Casting, successfully developed B17.8 and B19.8 steels for ultra-high-strength fasteners, forming 16.8 and 19.8 grade fastener manufacturing technology.
16.8 and 19.8 grade fasteners
1
What is a high-strength bolt?
High-strength bolt (High-Strength Friction Grip Bolt), English literal translation: high-strength friction preloaded bolt, English abbreviation: HSFG. It can be seen that the high-strength bolts we refer to in Chinese construction are short for high-strength friction pre-tightening bolts. In daily communication, the two words "friction" and "grip" are simply abbreviated, but many engineering and technical personnel have a misunderstanding of the basic definition of high-strength bolts.
Misunderstanding 1:
Bolts with a material grade exceeding 8.8 are "high-strength bolts"?
The core difference between high-strength bolts and ordinary bolts is not the strength of the material used, but the form of force. The essence is whether to apply pre-tightening force and use static friction to resist shear.
In fact, in the British standard and the American standard, the high-strength bolts (HSFG BOLT) mentioned in the specification only have two types: 8.8 and 10.9 (BS EN 14399 / ASTM-A325 & ASTM-490), while ordinary bolts include 4.6, 5.6, 8.8, 10.9, 12.9, etc. (BS 3692 11 Table 2); it can be seen that the strength of the material is not the key to distinguish high-strength bolts from ordinary bolts.
2
Where is the strength of high-strength bolts?
According to GB50017, calculate the tensile and shear strength of a single ordinary bolt (Class B) 8.8 and a high-strength bolt 8.8.
Through calculation, we can see that under the same grade, the design values of tensile strength and shear strength of ordinary bolts are higher than those of high-strength bolts.
So where is the "strength" of high-strength bolts?
To answer this question, we must start from the design working state of the two bolts, study the law of their elastic-plastic deformation, and understand the limit state when the design is destroyed.
Stress-strain curves of ordinary bolts and high-strength bolts under working conditions
Limit state when the design is destroyed
Ordinary bolts: The screw rod itself undergoes plastic deformation that exceeds the design allowance, and the screw rod is sheared.
In ordinary bolt connections, relative slippage will occur between the connecting plates before the shear force begins to be borne, and then the bolt rod and the connecting plate will contact, elastic-plastic deformation will occur, and shear force will be borne.
High-strength bolts: The static friction between the effective friction surfaces is overcome, and the two steel plates undergo relative displacement, which is considered to be destroyed in design considerations.
In high-strength bolt connections, friction first bears shear force. When the load increases to the point where the friction force is not enough to resist the shear force, the static friction force is overcome, and the connecting plate undergoes relative slippage (limit state). However, although it is destroyed at this time, the bolt rod is in contact with the connecting plate, and it can still use its own elastic-plastic deformation to withstand shear force.
Misconception 2:
High bearing capacity means high-strength bolts?
From the calculation of a single bolt, it can be seen that the design strength of high-strength bolts in tension and shear is lower than that of ordinary bolts. Its high strength is essentially: during normal operation, no relative slip is allowed at the node, that is, small elastic-plastic deformation and large node stiffness.
It can be seen that under the given design node load, nodes designed with high-strength bolts do not necessarily save the number of bolts used, but they have small deformation, high stiffness and high safety reserve. High-strength bolts are suitable for main beams and other locations that require large node stiffness, which conforms to the basic seismic design principle of "strong nodes, weak rods".
The strength of high-strength bolts does not lie in their own design value of bearing capacity, but in the large stiffness of their design nodes, high safety performance and strong anti-destruction ability.
3
Comparison between high-strength bolts and ordinary bolts
Ordinary bolts and high-strength bolts have great differences in construction inspection methods due to their different design force principles.
The mechanical performance requirements of ordinary bolts of the same grade are slightly higher than those of high-strength bolts, but high-strength bolts have one more impact energy acceptance requirement than ordinary bolts.
The marking of ordinary bolts and high-strength bolts is the basic method for on-site identification of bolts of the same grade. Since the values for calculating the torque value of high-strength bolts in British and American standards are different, it is also necessary to identify bolts of two standards.
High-strength bolts: (M24, L60, 8.8 grade)
Ordinary bolts: (M24, L60, 8.8 grade)
It can be seen that ordinary bolts are about 70% of the price of high-strength bolts. Combined with the comparison of their acceptance requirements, it can be concluded that the premium part should be to ensure the impact energy (toughness) performance of the material.
4
How to improve the fatigue strength of bolts
No matter what complex loads are borne, the common failure form of high-strength bolts is fatigue failure. As early as 1980, experts studied 200 cases of bolt connection failure, of which more than 50% were fatigue failures. Improving the fatigue resistance of high-strength bolts is crucial.
Bolt fatigue fracture has the following characteristics:
1. The maximum stress of fatigue fracture is much lower than the strength limit of the material under static stress, and even lower than the yield limit.
2. Fatigue fractures are all brittle fractures without obvious plastic deformation.
3. Fatigue fracture is the result of microscopic damage accumulation to a certain extent.
For bolts, the failure form is mainly plastic deformation of the thread part and fatigue fracture of the screw, among which:
65% of the damage occurs at the first thread connected to the nut;
20% of the damage occurs at the transition between the thread and the bare rod;
15% of the damage occurs at the transition radius between the bolt head and the screw.
01
Optimize design to reduce stress concentration
Strictly control the end size of the bolt to eliminate stress concentration:
a. Use a larger transition radius;
b. Cut a relief groove;
c. Cut a back cutter groove at the end of the thread;
d. Optimizing the head tilt angle of the bolt can also effectively reduce stress concentration;
e. Use reinforced threads.
The main difference between reinforced thread and ordinary thread is the minor diameter d1 and root transition fillet R of the external thread.
The main feature of reinforced thread is that the minor diameter d1 is larger than that of ordinary thread, the root transition fillet radius is increased, R is increased, the stress concentration of the bolt is reduced, and there are specific requirements for R: R+=0.18042P, Rmin=0.15011P, where P is the pitch, while ordinary thread has no such requirement and can even be a straight section.
02
Improve manufacturing process
Strengthening the control of heat treatment and surface treatment processes during the manufacturing process of bolts can effectively improve the fatigue of bolts.
a. Heat treatment The bolts are first heat treated and then rolled, which produces a large residual compressive stress inside the bolts, thereby slowing down the formation and development of cracks, thereby improving the fatigue strength of the bolts.
During heat treatment, decarburization should also be prevented, and the fatigue strength of bolts with and without surface decarburization should be compared.
Since carbon is oxidized in the decarburized layer, the amount of cementite in the metallographic structure is less than that in the normal structure, so its strength or hardness in mechanical properties is lower than that in the normal structure.
Usually, the fatigue strength of the bolt decreases by 19.8% when there is surface decarburization.
b. Phosphating The phosphating treatment of the bolt surface is to prevent rust and stabilize the friction during assembly, but the phosphating treatment can also reduce wear.
Reducing the friction between the thread rolling wheel thread and the screw thread during the thread rolling process will have a positive effect on the stress distribution on the bolt thread after thread rolling and reducing the surface roughness of the thread.
03
Setting appropriate preload
The tensile force of the screw of the ordinary bolt connection is mainly borne by the three-tooth stress-bearing thread at the front.
When the initial preload is large enough, part of the thread root will enter plastic deformation locally, and residual stress will be generated at these thread roots. The residual compressive stress generated at the thread root can improve the fatigue strength of the thread.
At the same time, the thread after plastic deformation can also improve the force distribution of the thread and reduce the contact pressure on the thread teeth.
This also improves the fatigue strength of the thread.
The greater the preload, the greater the bolt connection's ability to resist connection separation and the greater its ability to resist preload relaxation. At the same time, the actual effective fatigue strength of the bolt connection is also greater.
Therefore, increasing the preload of the bolt connection is conducive to improving the bolt connection's ability to resist fatigue failure under cyclic external loads, making the risk of fatigue failure of the bolt connection under vibration impact force and limited overload smaller.
