The difference between high strength bolt friction type and pressure type connection

1.Physical and chemical tests
The bolt fractured at the shank, and the macroscopic appearance of the fracture is shown in Figure 1. The bolt fracture is perpendicular to the axis, and the entire fracture can be divided into three regions macroscopically: fracture source region, radiation region and shear lip region. The fault source area is located in the center of the fracture, and the area is small; the radiation area is a fast brittle fracture area, showing obvious radial stripe characteristics, and the radial stripes converge to the center of the fracture, which is the source of the fault, and the radiation area is larger. It accounts for 70% to 80% of the total area of the fracture; the shear lip area is located on the peripheral edge of the fracture, which is the final fracture zone, and is about 45° to the fracture surface. These characteristics indicate that the bolt fracture is a typical mixed fracture under tensile load, with brittle fracture as the main one.

2. Chemical composition analysis
The chemical composition of the bolts was analyzed using a Labspark750 spark spectrometer, and the results are shown in Table 1. From Table 1, it can be seen that the chemical composition of the bolts meets the technical requirements of the agreement for B7 steel.

3. Fracture analysis
Using Japan JSM-6480 scanning electron microscope and Oxford INCA-350 energy spectrometer to observe the micro-morphology and micro-area composition of the bolt fracture. The fracture is an abnormal intergranular or intergranular + local transgranular cleavage fracture. The size of this area is 400-550 μm. The morphology of the intergranular fracture in the fracture source area is shown in Figure 2. 3, the local brittle transgranular cleavage fracture morphology in the fracture source area is shown in Figure 4, and the quasi-cleavage fracture characteristics are shown in Figure 5. The fracture area in the radiation zone is relatively large, accounting for 70% to 80% of the entire fracture, and its microscopic characteristics are typical brittle cleavage fractures. Around the fracture is the shear lip region of the last fracture, which is microscopically characterized by a large number of tear dimples (Fig. 6).

4. Metallographic inspection
(1) Non-metallic inclusions After grinding and polishing the fracture, make a metallographic sample and observe it under an optical microscope, and grade the non-metallic inclusions in the steel according to the GB/T10561-2005 standard rating chart. The non-metallic inclusions in the fractured bolt steel are shown in Figure 7. There is no significant difference between the surface layer and the core non-metallic inclusions in the fractured bolt steel. The grade of D-type ring oxide inclusions is about coarse D2-D2.5.
(2) Metallographic structure of bolt surface and core After being etched by a 3% nitric acid alcohol solution, the metallographic structure of the fractured bolts was observed and analyzed with an optical microscope and a scanning electron microscope. After the cross-section sample of the bolt shank is etched, the metallographic structure of the surface layer of the bolt is observed under an optical microscope, as shown in Figure 8, which is a fine and uniform tempered sorbite structure. At about 15mm from the surface, there are more point-like and small-block ferrite structures in the local area, and there are dark black structures distributed along the grain, with traces of incomplete hardening, and the light gray area is small and uniform The tempered sorbite structure that still retains the morphological characteristics of acicular martensite.
The metallographic structure of the bolt core is shown in Figure 9. From Figure 9, it can be seen that the metallographic structure of the bolt core is uneven, and there are many point-like and small-block ferrites, and there are strips or needles. Wei's organizational characteristics. Under the scanning electron microscope, the metallographic structure of the bolt surface is a normal and uniform tempered sorbite structure with needle-like characteristics (Figure 10). The small ferrite in the center of the bolt is black (white under the metallographic microscope), and the metallographic structure is also mostly lath-like, and needle-like features can be seen locally (Figure 11).

5. Hardness test
The HR-150A Rockwell hardness tester was used to conduct hardness tests on the cross-section of the bolt fracture, and the hardness test results from the surface to the core of the bolts are shown in Table 2. It can be seen from Table 2 that the surface hardness is 35.1-38.3 HRC, and the core hardness is 35.4-37.7 HRC.

6. Analysis and discussion

Fracture analysis of the fractured bolt shows that the fracture source is located at the center of the bolt, and the fracture source fracture is intergranular or intergranular + local transgranular cleavage fracture. The size of the fracture source area is 400-550 μm, and dimple characteristics are hardly visible. Abnormal fracture. The radial area of the fracture is relatively large, accounting for 70% to 80% of the entire fracture, with obvious radial striations, which is the rapid expansion area of cracks, and is a typical brittle cleavage fracture.

When the bolt is installed, under the action of tensile load, the brittle fracture caused by the rapid expansion of the crack caused by the brittle fracture of the core part along the grain. The high-strength bolts have been quenched and tempered, and their strength and toughness are well matched. There is a shear lip in the final fracture zone of the bolts, but the intergranular fracture at the center indicates that the grain boundary strength is low. due to organizational defects.

The metallographic structure analysis of the bolt fracture shows that: the surface metallographic structure is uniform tempered sorbite, and the structure is uneven from about 12 to 15 mm away from the surface. There are a small amount of point-like, small block ferrite and dark black structure along the grain (with the characteristics of incomplete hardening). Dealing with insufficient heating. When the bolts are quenched and heated in the mesh belt furnace, due to the large furnace load and the fast movement speed of the mesh belt, the heating of the bolts is insufficient, and the austenitization of the core is insufficient and uneven, resulting in the incomplete hardening of the bolts.  Metallographic analysis shows that point-shaped and small-block ferrite structures appear at 12-15mm from the surface, indicating that the core of the bolt is not completely hardened. High-temperature tempering treatment will cover up the truth of incomplete quenching. The empirical method should measure the hardness of the core of the bolt in the quenched state. 5. In the case of insufficient quenching and heating, the surface hardness of high-strength bolts can reach the technical requirements of 36-38 HRC, and meet the strength requirements of grade 10.9 in GB/T 3098.1-2010 "Mechanical Properties of Fasteners - Bolts, Screws and Studs". When the amount of non-martensite in the bolt structure is large, the strength and toughness of the bolt will be reduced, but it will not cause abnormal intergranular fracture in the fracture source area of the bolt core.

Conclusions and recommendations. The early fracture of the bolt shaft occurred during the installation process of the bolt, which is an abnormal brittle fracture. The intergranular fracture and cleavage fracture in the fracture source area are mainly caused by the structural defects of the bolt hot-rolled raw material. There is a small amount of small ferrite in the center of the bolt and the intergranular distribution is dark black, which indicates that the bolt has insufficient heating during heat treatment, but it is not the main reason for the brittle fracture of the bolt. 2. The reasons for the insufficient quenching and heating of the bolts may be related to factors such as excessive charging capacity of the mesh belt furnace, fast moving speed of the mesh belt, and uneven charging of the bolts. It is recommended that during the heat treatment of each batch of bolts, one quenched bolt should be randomly inspected in each shift, and the cross-sectional sample of the rod should be taken, and the hardness change from the surface to the core should be measured at intervals of 3 to 4 mm. At the same time, the hardening situation should be analyzed with metallographic inspection to ensure that the core structure of high-strength bolts reaches about 90% martensite. 3. The physical metallurgical quality of bolt B7 steel is average, and no serious defects such as non-metallic inclusions have been found. Strengthen the incoming inspection of the hot-rolled raw material structure of the bolt B7 steel, and analyze its banded structure and defective structure of poor thermal processing. Through the process test, determine the reasonable heat treatment process.