There are many failure modes of bolts , and most of the failures are related to installation and construction . However , the common failure analysis at present is to send the failed bolts to a third-party testing organization. , only the bolt itself is detected and analyzedMainly , they often do not go to the site to conduct on-site surveys , and ignore on -site installation and construction .
Therefore , this paper lists common bolt failure forms and typical cases , and conducts a comprehensive analysis from the aspects of design , material , bolt manufacturing process , installation and construction process , etc. , hope that the bolt manufacturer , screwBolt users and failure analysts can learn from it .
U-turn
U- turn is a common form of bolt failure , and it is also a relatively serious failure form . According to the most fundamental cause of the bolt U -turn , mainly from the stress concentration at the joint of the head rod and the edge stress of the washer surfaceConcentration , forging cracks , hydrogen embrittlement fractures , design and material selection areanalyzed.
Stress concentration at the joint of the head and rod
( 1 ) Failure cases . Figures 1 and 2 are the most typical cases of bolts turning around due to quenching cracks . The black area of the break in the figure is the obvious quenching crack feature .
( 2 ) Cause analysis . When the high – strength bolt is quenched and cooled , the austenite structure in the steel changes to the martensite structure , the volume increases , and the internal stress expands from the inside to the outside . face existenceWhen the area changes sharply , the stress concentration at the sharp corner will be generated during the stress release process , and when the stress concentration at the sharp corner is greater than that in the surface hardened state When the martensitic strength of _ _cracked . The root cause of the U- turn of the bolts in Fig. 1 and Fig. 2 is that the arc r under the bolt head is too small or irregular , resulting in stress concentration during the quenching process , resulting in the formation of quenching crack .
( 3 ) Preventive measures . For high – strength bolts , the arc r under the head is very important . The 1976 edition of the national standard ( GB30-76 ) stipulates that r is a specific value , but from 19 8.6 edition _ At the beginning , the national standard for bolts stipulates that the arc r under the head is the minimum value .
Corresponding to the arc r under the bolt head , whether it is cold heading or hot forging , the key control in the actual production process of the bolt should be the r of the lower die for bolt upsetting . is to consciously increase the r value , and the second is Ensure that the r arc transition is standardized , and the phenomenon of knife sticking must not occur in the process of mold processing ( as shown in Figure 3 ) .
Stress concentration at the edge of the gasket face
( 1 ) Failure cases . In Figure 4 , the black area is the original quenching crack , and the gray area is the secondary crack after stress .
( 2 ) Cause analysis . The hexagonal head of this kind of problem bolt is longitudinally sectioned , and the cross- section is shown in Figure5. There are obvious cracks at A and B on the edge of the washer surface of the hexagonal head , indicating that the bolt hexagonal head gasket There are stress concentrations at the edge of the face , which form cracks during quenching , which will eventually cause the bolt to turn around .
The metallographic structure of the crack is shown in Figure6 ; the magnetic particle inspection photo is shown in Figure 7 , and there is obvious magnetic powder accumulation on the edge of the washer surface shown in the marking circle .
( 3 ) Preventive measures . As shown in Figure 3 ( same as above ) , the effective control measure for this kind of failure is to appropriately enlarge and standardize the arc transition of the hot forging lower die R 0.5 and the R 0.2 and R 0.2 in the figure . R 0 . slope between 5 . _
This type of failure is prone to occur in the production of large diameter bolts of M 5 6 and above . For small bolts , due to the smaller height of the washer surface ( ie the size in Figure 3 C ) It is not easy to produce stress concentration , so it rarely happens .
In order to avoid the above two types of failure modes , it is essential to conduct 100 % magnetic particle inspection after heat treatment , because in the production process of bolts , cracks cannot be completely removed . to avoid .
forging crack
( 1 ) Failure cases . The failure case is shown in Figure 8. There is no obvious quenching crack feature on the bolt fracture of this case .
( 2 ) Cause analysis . Through comprehensive inspection and analysis of macroscopic , metallographic , mechanical properties , fracture , energy spectrum , etc. , it is believed that the failure reason in this case is : the bolt was not heated through when heated , and the forging process medium deformationThe resistance does not match the deformation rate , and cracks are formed in the core of the hexagonal head , and the cracks are further expanded after quenching and tempering treatment and use , and finally break . The macroscopic morphology of the fracture is shown in Figure9 .
( 3 ) Preventive measures . This case mainly occurs on large- diameter bolts . The preventive measures and control methods are to use infrared automatic temperature control technology to ensure that no overheating or overburning occurs before envoy _ _The heating of the bar is sufficient and uniform , and it is especially necessary to strengthen the effective control of the first piece of the heated bar .
hydrogen embrittlement fracture
( 1 ) Failure cases . The failure case is shown in Figure 10 . The bolt diameter specification is M 3 6 , the performance class is 8 . 8 , the material is 4 2 C r M o , the surface treatment is galvanized , and it is broken Happened after installation . _
( 2 ) Cause analysis . Macro and micro morphology . The macro- morphology of the bolt fracture is shown in Figure11 , and the macro- and micro- morphology of the fracture source area ( Area ) is shown in Figure12 .
Decarburization test . According to GB / T 3 0 9 8 . 1 – 2 0 1 0 , the metallographic decarburization test is carried out on the thread part , and the longitudinal section is taken along the axis line 1 d away from the end of the screw Surface sample , polished , polished and corroded , the morphology is shown in Figure 13 , there is an incomplete decarburization layer on the surface , and the height E of the undecarburized layer of the thread is 1.411 mm , which is higher than the standard Minimum required value 1 . 2 2 _ _ 7 m m 。
Hardness test . The hardness test was carried out from the surface to the inside at the section near the source area , the hardness gradually increased , and the hardness was 256HV ( 23.0HRC at 0.1 mm from the surface ) _ ) , 0 from the surface 332HV ( 34.0HRC ) at .3mm , 38.5HRC at 0.8mm from surface , close to 10.9 grade bolts _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ The upper limit of hardness . Near surface hardness Lower is related to incomplete decarburization of the surface .
Chemical composition analysis . The chemical composition of the fractured bolts was analyzed by direct reading spectrometer , and the results met the requirements of 4 2 C r M o in GB / T 3077-2015 . _ _ _ _ Use inert gas The hydrogen content of the fractured bolt was determined by fusion thermal conductivity method and found to be 1.2 ppm .
Artificial fracture observation . Take a longitudinal smooth sample from the fractured bolt shank , and use manual tapping to obtain a simulated fracture for electron microscope observation . The microscopic morphology is characterized by dimples , as shown in Figure 1 4 .
After the above fracture observation , metallographic analysis , hardness test , chemical composition analysis , etc. , this case is considered to be hydrogen embrittlement delayed fracture . The bolt has broken in a short period of service , so the source of hydrogen It should be absorbed in pickling , electroplating and other processes . If the source of hydrogen is slowly entering in the service environment, then the bolt should be in service for a long time . fracture occurs .
( 3 ) Preventive measures . First , high- strength bolts should control the upper limit of hardness as much as possible , and higher hardness increases hydrogen embrittlement susceptibility ; secondly , high- strength bolts ( especially 1 2 . 9 grades ) as much as possible It can avoid the use of electroplating process , thereby avoiding hydrogen embrittlement caused by pickling and electroplating hydrogen absorption .
Design and material selection
( 1 ) Failure cases . This case is GB / T 14-2013 ” Square Neck Bolts with Flat Round Head ” , the performance level is 4.8 , and it turned around during installation and use , see Figure 1 _ _ _ _ 5 .
( 2 ) Cause analysis . First of all , from the macroscopic appearance , the first feeling is that the head deformation problem caused the U – turn , then let’s look at the physical tensile test , as shown in Figure16 .
From the physical tensile test results , the fractures all occurred in the thread part , and the tensile strengths all meet the GB / T 3098.1-2010 standard _ _ _ _ _ _ Regulations , indicating that the bolt is turned around does not It is caused by the quality problem of the bolt itself . Of course , considering the head deformation , 0 8 Al material with better plasticity is used , which is consistent with the lower tensile strength of the physical tensile test .
Later, the 8.8 grade was used , and there was no U – turn event , which means that this case was caused by the bolt design and the selected performance grade . In actual use , the flat round head There may be clipping below .
( 3 ) Preventive measures . In the design and selection of the structural type and performance level of fasteners , the use environment and stress conditions should be comprehensively considered , and the principles of science , economy and rationality should be adhered to .
Of course , for bolts with large head deformation , it is very important to design the initial punch of the cold heading die to ensure a reasonable metal flow line during the cold heading deformation process .
2
fracture
overload fracture _
( 1 ) Failure cases . The bolt specification is M 3 6 × 2 2 5 , the performance level is 1 0.9 , and the material is 4 2 C r M o A . The appearance of the fractured bolt and the macroscopic appearance of the fracture are shown in Fig . 17 and Fig. 18 ; The otherpartbrokenboltisscrewedthe threadedholeinbaseoftheequipment, andisdifficultto takeoutspecialmeansbyprofessionalfitters.
( 2 ) Cause analysis . The fractured bolts have been comprehensively tested for chemical composition , low magnification , grain size , non – metallic inclusions , microstructure , decarburization , mechanical properties , etc. , in line with GB / T 3 0 7 7 — 2015 , GB / T 3098.1-2010 and other standards stipulate that the tensile strength Rm is 1068MPa . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
After looking at the macroscopic appearance of the fracture in Figure 18 , many people will simply judge that this case is due to the existence of cracks in the material , and the bolt is caused by the internal crack defect of the material . cause failure , it is not .
This is a typical case of over – screwing and overload fracture . In Figure 17 , there are obvious twisting and extrusion marks at three places A , B , and C. According to the introduction of the on- site broken bolt removal personnel , the light holes of the equipment base and the bolts The holes are seriously misaligned , and the axis lines of the two holes are deviated by at least 2 mm . In addition , when the tensile strength of the bolt is 1068 MPa , the necking of the fractured part is obvious ( as shown in Figure 19 ) , which fully shows that the severity of the overload . _
Based on the above analysis , when the screw hole and the smooth hole are seriously different from the axis , the screw is forced to be tightened when the unthreaded rod and the smooth hole are seriously interfered with and cannot be tightened . The bolt is subjected to overload stretching and wrenching The combined effect of torsion and interference shearing will eventually lead to bolt fracture .
( 3 ) Preventive measures . There are many cases of bolt overload and fracture , and preventive measures should be determined according to the specific reasons . As far as this case is concerned , the bolts used for flange connection should be crossed in multiple steps . The tightening method minimizes the cumulative error of the coaxiality between the screw hole and the optical hole .
fatigue fracture
( 1 ) Failure cases . This case is a collapse accident caused by a broken tower connecting bolt of a wind turbine . The broken bolt is : diameter specification M 3 6 , performance level 1 0 . 9 _ Material 4 2 C r M o A .
( 2 ) Cause analysis . During the on-site investigation , it was found that there were four fracture surfaces of the bolts that were roughly perpendicular to the axial direction of the bolts , which were the first fractured parts ( as shown in Figure 20 ) ; other fractured bolts Fracture average It is relatively rough , with obvious bending deformation and plastic deformation characteristics , and it is a post – fractured part.
Observing the fracture of the bolt in Fig. 20 , the features are basically similar . The fracture contains two areas with obvious features , and some of the fractures are roughly perpendicular to the axis of the bolt . , relatively flat and delicate , _ It is the starting area of fracture ; the other part of the fracture is relatively rough , and the section is roughly at an angle of 45 ° to the axis of the bolt , which has shear characteristics and is the last one – time instantaneous fracture . area .
Scanning electron microscope morphology observation results show that fatigue striations exist in relatively flat areas on the four fractures , and the fracture morphology of the area at an angle of 45 ° to the bolt axis For the fossa . See the case bolts It belongs to fatigue fracture .
( 3 ) Preventive measures . Fatigue fracture of bolts is generally related to preload or bolt loosening . _ There are many reasons for bolt loosening , such as : vibration during equipment operation , changes in high and low loads , shock , and the preload is too low during installation without taking appropriate anti – loosening measures , improper assembly , etc. Therefore , the key to preventing bolt fatigue fracture is to strictly implement the installation specifications .
reduction crack _
( 1 ) Failure cases . The bolts in this case are : diameter specification M 3 6 , performance class 1 0. 9 , material 4 2 C r M o A . The production process is : raw material annealing → sawing → flat head Chamfering → lubrication → diameter reduction → hot forging → hexagonal head chamfering → quenching and tempering treatment → straightening → thread rolling → surface treatment .
The macroscopic appearance of the fractured bolt is shown in Figure 21 , and the macroscopic appearance of the fracture is shown in Figure 22 .
( 2 ) Cause analysis . The chemical composition of the material and the mechanical properties of the bolts are comprehensively tested , all in line with GB / T 3077-2015 and GB / T 3098.1-2 _ _ _ _ _ _ _ _ _ _ 0 1 0 and other standards .
The macroscopic appearance of the fracture is shown in Figure 22. The fracture is divided into four regions A , B , C , and D for analysis . It can be seen that the three regions of A , B , and C are flush in section , and there are obvious gaps . ray , converges in area A ; area D The shear lip area is roughly at an angle of 45 ° to the axis of the bolt . The origin and direction of crack propagation are shown by the arrows in the figure .
The low magnification morphologies of the fractured metallographic samples are shown in Figure 23 and Figure 24 .
Figure 25 shows the sampling position of the longitudinal section at the end of the broken bolt and the macroscopic appearance of the longitudinal section . It can be seen that there is a transverse crack in the corresponding position of the third thread from the end .
The bolt fracture occurs in the thread part , and the diameter of the bolt thread blank is completed by reducing the diameter . From the metallographic analysis of the fracture , this is a typical bamboo – shaped diameter reduction crack . grain features , which may cause reduction cracks The direct cause of the grain is insufficient and uneven annealing of the material .
Bolt diameter reduction is a cold deformation process , except for alloy structural steel ( 35V B , because 35V B is a non – annealing material and has good cold deformation performance _ _ _ ) and carbon content at 0.30 % _ _ _ For the above carbon structural steels , the raw materials should be spheroidized annealed before the diameter reduction , so as to obtain granular pearlescent with lower strength , hardness and better plasticity body .
If the annealing of the raw material is insufficient and uneven , the strength and hardness of the raw material are high , and the diameter is reduced in the state of the flaky pearlite structure with a band-like distribution , the produce a large deformation stress , which Under the action of internal stress , microcracks are generated inside the material . When the quenching treatment is carried out in the later stage , due to the transformation stress from the supercooled austenite to martensite , the micro – cracks in the central area are bound to expand to the surrounding areas.At the same time , the internal crack is squeezed and opened , forming a ” crescent – shaped ” internal crack as shown in Figure 25 .
( 3 ) Preventive measures . The most direct preventive measure for this type of failure is to change the diameter to turning . Of course , considering the factors of production efficiency , the diameter reduction process should be adopted , as well as the use of cold drawing and cold heading . During operation , the annealing process control of raw materials must be strengthened , and the adequacy and uniformity of annealing must be ensured .
Thread slide _
( 1 ) Failure cases . Sliding buckle is a common failure form of bolts . There are many cases , so I will not list them one by one here .
( 2 ) Cause analysis . As far as the quality of the bolt itself is concerned , thread decarburization is the main cause of bolt slippage , but with the advancement of fastener technology in the past 20 years , high- strength bolts have commonly usedThe heat treatment production line of the roller mesh belt furnace with atmosphere protection is used , and the thread processing is all after quenching and tempering heat treatment , which effectively solves the problem of thread decarburization .
Therefore , most of the bolt slide fasteners seen at present are related to the installation and construction ; the specific analysis should be carried out according to the actual failure mode and the installation and construction process . In September 2010 , the author carried the fasteners _Common failure problems , it took a month to visit nearly 10 wind turbine installation sites in 5 provinces in China . _ _ _ _ _ _ _ _ The way to wipe molybdenum disulfide is : _All the bolts are erected and placed on the ground around the tower , and then molybdenum disulfide is smeared . However , the bolts are all over the ground . There is a lot of sand on it , and the sand is sticky On the molybdenum disulfide attached to the threaded part of the bolt , you can imagine what will happen during the installation process .
( 3 ) Preventive measures . The bolt slide fastener fails . For the bolt manufacturer , the key is to adopt process methods such as atmosphere protection during the heat treatment process , first heat treatment and then rolling the thread to prevent the thread from falling off . carbon ; and For bolt users , the key is to operate in strict accordance with the installation and construction specifications and strengthen the protection of bolts on site .
Thread bite
For carbon steel fasteners , the main cause of thread seizure is defects such as thread burrs . The following focuses on the analysis of stainless steel thread seizure .
( 1 ) Failure cases . Figure 26 shows the profile of the cut after the thread is bitten .
( 2 ) Cause analysis . In order to find out the cause of the seizure of stainless steel bolts and nut threads , wire cutting is used to cut the seized sample along the axis of the bolt , and the damage to the screw teeth after the seizure is eliminated. Injury appearanceObservation , the appearance of the internal and external thread joints under the laser confocal microscope is shown in Figure 27. It can be seen that the internal and external threads are severely worn , and some threads are Grind off about 70 % in the height direction , and the wear debris accumulates between the mating threads to form a whole .
The bolt part and the nut part of the cut specimen are separated , and the typical damage morphology of the bolt and nut is shown in Figure 28 . It can be seen that the bolts and nuts are severely worn , and the threads are torn ., the wear debris accumulates between the threads , which has a blocking effect on the screwing in and out of the screw , making the screw completely dead .
The bolt part and the nut part of the cut specimen are separated , and the typical damage morphology of the bolt and nut is shown in Figure 28 . It can be seen that the bolts and nuts are severely worn , and the threads are torn . , the wear debris accumulates between the threads , which has a blocking effect on the screwing in and out of the screw , making the screw completely dead .
The high temperature between the threads caused by quick preloading and quick disassembly leads to severe adhesive wear on the surface of the stainless steel thread ; the wear debris generated by adhesive wear accumulates between the threads , hindering the threading process _ _ _and eventually lead to death .
( 3 ) Preventive measures . Reducing thepreloadandremovalspeedcanreducetemperaturebetweenthreads,therebyreducingadhesivewearandreducingtheprobabilityofstainless steelthreadseizing. GluingcanbeachievedbyreducingtheThe coefficient of friction reduces the temperature between the threads , thereby preventing the seizure of stainless steel threaded connections .
