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Technical Details : |
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Head |
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The head is the enlarged shape preformed on one end of a headed
fastener to provide a bearing surface. Types of Head
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Points |
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The point of a
fastener is the configuration of the end of the shank of a
headed fastener or of each end of a headless fastener.
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Standard |
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JMT (JIS TRUSS HEAD MACHINE & TAPPING SCREW)
JMO (JIS OVAL HEAD MACHINE & TAPPING SCREW)
JMB (JIS BINDING HEAD MACHINE & TAPPING SCREW)
AMT (ANSI TRUSS HEAD MACHINE & TAPPING SCREW)
AMF (ANSI FLAT HEAD TAPPING SCREW)
AMO (ANSI OVAL HEAD TAPPING SCREW)
AMR (ANSI ROUND HEAD TAPPING SCREW)
AMP (ANSI PAN HEAD MACHINE & TAPPING SCREW) |
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Mechanical Properties |
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Scope and field of application : The
property classes and their mechanical properties apply to bolts,
screw and studs, with metric (ISO) thread, with nominal thread
diameter d <= 39 mm, made of carbon steel of alloy steel and
when tested at room temperature. The do not apply to set screws
and similar or to specific requirements such as weldability,
corrosion resistance, ability to withstand temperature above +
3000 C or below -500 C. The designation system may be used for
sizes, provided that all mechanical requirements of the property
classes are met |
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Designation system of property classes :
The property class symbols, indicating the most important
properties, consist of two figures, one on either side of a dot. |
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Mechanical Properties |
Property Class
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3.6
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4.6
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4.8
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5.6
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5.8
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6.8
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8.8
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9.8
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10.9
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12.9
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d<16mm
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d>16mm2
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| 1 |
Tensile Strength |
nom. |
300
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400
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500
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600
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800
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800
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900
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1000
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1200
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| 2 |
RM N/mm2 |
min. |
330
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400
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420
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500
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520
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600
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800
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820
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900
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1040
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1220
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| 3 |
Vickers Hardness HV F>=98N |
min. |
95
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120
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130
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155
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160
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190
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250
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255
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290
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320
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385
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| max. |
250
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320
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335
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360
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380
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435
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| 4 |
Brinell Hardness HB F=30 D2 |
min. |
90
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114
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124
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147
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152
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181
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238
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242
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276
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304
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366
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| max. |
238
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304
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318
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342
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361
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414
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| 5 |
Rockwell Hardness HR |
min. |
HRB |
52
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67
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71
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79
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82
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89
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-
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-
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-
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-
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-
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HRC |
-
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-
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-
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-
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-
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-
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22
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23
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26
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32
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39
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| max. |
HRB |
99.5
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-
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-
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-
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-
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-
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HRC |
-
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32
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34
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37
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39
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44
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| 6 |
Surface Hardness HV 0.3 |
max. |
-
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5
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| 7 |
Lower Yield Stress N/mm2 |
nom. |
180
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240
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320
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300
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400
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480
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-
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-
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-
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-
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-
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| min. |
190
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240
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340
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300
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420
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480
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-
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-
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-
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-
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-
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| 8 |
Proof stress RP
0.2 N/mm2 |
nom. |
-
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640
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640
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720
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900
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1080
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| min. |
-
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640
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660
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720
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940
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1100
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| 9 |
Stress Under Proofing load,
SP |
SP/RP
0.2 |
0.94
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0.94
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0.91
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0.93
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0.90
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0.92
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0.91
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0.91
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0.90
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0.88
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0.88
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| N/mm2 |
180
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225
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310
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280
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380
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440
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580
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600
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650
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830
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970
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| 10 |
Elongation After Fracture A
in % min. |
25
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22
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14
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20
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10
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8
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12
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12
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10
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9
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8
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| 11 |
Strength Under Wedge
Loading |
The values for full size bolts and screws (not studs)
shall not be smaller than the minimum values for tensile
strength
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| 12 |
Impact min. Strength, J |
-
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25
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-
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30
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30
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25
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20
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15
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| 13 |
Head Soundness
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no fracture
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| 14 |
Minimum Height of
non-decarburized thread zone, E |
-
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1/2H1
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2/3H1
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3/4H1
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| Maximum Depth of Complete
Decarburization, G |
-
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0.015
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Minimum Breaking Torques |
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Thread size
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Thread pitch
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Property Class
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4.6
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4.8
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5.6
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5.8
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8.8
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10.9
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12.9
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Minimum breaking torque, in mm
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M1
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0,25
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0,020
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0,020
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0,024
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0,024
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0,033
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0,040
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0,045
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| M1,2 |
0,25 |
0,045 |
0,046 |
0,054 |
0,055 |
0,075 |
0,092 |
0,10 |
| M1,4 |
0,3 |
0,070 |
0,073 |
0,084 |
0,087 |
0,12 |
0,14 |
0,16 |
| M1,6 |
0,35 |
0,098 |
0,10 |
0,12 |
0,12 |
0,16 |
0,20 |
0,22 |
| M2 |
0,4 |
0,22 |
0,23 |
0,26 |
0,27 |
0,37 |
0,45 |
0,50 |
| M2,5 |
0,45 |
0,49 |
0,51 |
0,59 |
0,60 |
0,82 |
1,0 |
1,1 |
| M3 |
0,5 |
0,92 |
0,96 |
1,1 |
1,1 |
1,5 |
1,9 |
2,1 |
| M3,5 |
0,6 |
1,4 |
1,5 |
1,7 |
1,8 |
2,4 |
3,0 |
3,3 |
| M4 |
0,7 |
2,1 |
2,2 |
2,5 |
2,6 |
3,6 |
4,4 |
4,9 |
| M5 |
0,8 |
4,5 |
4,7 |
5,5 |
5,6 |
7,6 |
9,3 |
10 |
| M6 |
1 |
7,6 |
7,9 |
9,1 |
9,4 |
13 |
16 |
17 |
| M7 |
1 |
14 |
14 |
16 |
17 |
23 |
28 |
31 |
| M8 |
1,25 |
19 |
20 |
23 |
24 |
33 |
40 |
44 |
| M8 x 1 |
1 |
23 |
23 |
27 |
28 |
38 |
46 |
52 |
| M10 |
2,5 |
39 |
41 |
47 |
49 |
66 |
81 |
90 |
| M10 x 1 |
1 |
50 |
52 |
60 |
62 |
84 |
103 |
114 |
| M10 x 1,25 |
1,25 |
44 |
46 |
53 |
54 |
74 |
90 |
100 |
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Overview and Definitions of Steels for
Fasteners |
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The word steel is understood to mean a
deformable iron (Fe)-carbon (C) alloy with a maximum carbon
content of 1,5%. So it is not correct to speak, for example
about iron bolts or rivets. The word "iron" should only be used
to indicate the chemical element Fe, 100% pure iron and in the
combination of the word malleable iron as distinct from
malleable steel.
Unalloyed, low carbon steel as per DIN 17111 with a C% <=0,22%
is used for the lower property classes of bolts, screws and
nuts. this steel group is indicated with the letters St followed
by a number corresponding with 1/10 of the minimum tensile
strength in N/mm2
Depending on the steel processing method, (desoxydation method)
a distinction is made between:
- rimmed steel, indicated with U before St. In this process
gases continue to evolve as the steel solidifies
- killed steel, indicated with R before St,that gradually
changes from a liquid to a solid when silicon or aluminium is
added, resulting in a better quality of structure
Sometimes an extra quality number 1 or 2 is added. Quality
number 2 requires maximum phosphorus (P) and sulphur (S) content
limits whereas quality number 1 does not.
Carbon steel as per DIN 1654 cold heading steels, DIN 17200
steels for quenching and tempering and DIN 17210 case hardening
steels. The carbon steels can be divided into 3 types:
- quality steel, indicated with the letter C followed by the C%
multiplied by 100
- high quality steel, indicated with the letters Ck with a lower
P and S content
- cold heading steel, indicated with the letters Cq having
special cold forming characteristics
Alloy steel as per DIN 1654 cold heading steels, DIN 17200
steels for quenching and tempering and DIN 17210 case hardening
steels. In this steel group the percentage of elements which
normally only occur as traces or impurities has been increased
and/or other elements have been added to achieve or improve
special characteristics, such as higher mechanical properties,
better resistance against corrosion, low or high temperatures,
etc.
The designation starts with a number indicating 100 x the
C-content, followed by the symbols of the relevant alloying
elements in sequence of their quantity, starting with the
largest, and finally another number indicating a certain ratio
of the percentage of the alloying element(s).
- 4 for the elements : Cr-Co-Mn-Ni-Si-W
- 10 for the elements : Al-Cu-Mo-Ti-V
- 100 for the elements : C-P-S-N
- 1000 for the elements : B (boron)
The most common elements used with fasteners have the
following influence :
- Carbon (C) is the most important element and influences the
mechanical properties considerably. For fasteners the percentage
varies up to 0,5% maximum. with increasing C content the
strength increases, but the cold formability is reduced. From
about 0,3%C the steel can be heat treated.
- Nickel (Ni) improves the through hardening, toughness at low
temperatures and the non-magnetic properties. The combination of
at least 8%Ni with about 18% Cr results in the important
austenitic stainless steel quality A2.
- Chromium (Cr) also increases harden ability and strength. A
minimum content of about 12.5% is necessary for a steel to be
qualified as stainless.
- Molybdenum (Mo) increases harden ability and reduces temper
brittleness. High temperature strength is improved. When 2 - 3%
Mo is added to an alloy with about 18% Cr and about 12% Ni
corrosion resistance increases considerably. This quality of
austenitic stainless steel is used frequently for fasteners and
is designated with A4.
- Manganese (Mn) usually occurs like the elements silicon (Si),
phosphorus (P) and sulphur (S) only as impurities. By adding Mn,
strength, hardenability and wear resistance are increased.
However the steel becomes more sensitive to overheating and
temper brittleness.
- Titanium (Ti) is used as carbide former for stabilization
against intercrystalline corrosion in e.g. stainless steel. The
elements Niobium (Nb) and Tantalium (Ta) is used as cause the
dame effect.
- Boron (B) is a relatively new alloying element in fasteners
steel. Very small amounts of 0.002-0.003% already improves the
through hardening considerably. Because of this, C% can be kept
lower, improving the cold workability. The application of boron
treated steels has become a very popular alternative in
manufacturing cold formed, heat-treated fasteners.
Case hardening steel as per DIN 17210 and DIN 1654 Part 3. Case
hardening steel has a relatively low carbon content and is used
to get a very hard, wear resistant surface by adding carbon
during the heat treatment. This type of steel is used for
tapping screws, thread cutting and self-drilling screws,
chipboard screws, etc.
Free cutting steel as per DIN 1651. This special type of steel
is characterized by a good metal removal and short chip
breaking. This is achieved by increasing the sulphur content to
c0.34% max. sometimes with an extra addition of lead. A very
popular type for fasteners is 9S20K with C% <=0.13 and 0.18-0.25
S which is machined in the cold-drawn condition.
The manufacturing method of machining on automatic lathes is no
longer used very much for commercial fasteners but it is still
applied for small quantities or for a product configuration,
which is difficult to cold form.
Free cutting steel has restricted properties.
High and low temperature steel as per DIN 267 Part 13, DIN
17240, AD-Merkblatter W7 and W10, SEW680.
Stainless steel as per DIN 267 Part 11, DIN 1654 Part 5, DIN
17440, and ISO 3506 |
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Overview and Definitions of heat treatments
for fasteners |
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Annealing :
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The steel is held at temperature of just below 7210C for
several hours and is then cooled down slowly to make it soft.
The structure changes from hard, lamellar perlite into soft,
globular perlite resulting in an optimal of the raw material for
cold heading.
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Normalizing (Recrystallization) : |
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By heating at 800-9200C for not too a long time and then
cooling slowly, a coarse and thus brittle grain structure due
to, for instance, hot rolling or not forging, especially of
thicker pieces, is brought back again in the original fine grain
structure. Through this refining, yield point and impact
strength are increased without the tensile strength being
reduced too much. |
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Stress-relieving : |
By cold deformation internal stresses are induced in the
material, increasing the tensile strength but decreasing the
elongation. By heating at between 500 and 6000C for a long time
and cooling slowly, most of the cold hardening effect
disappears. This heat treatment is applied to cold headed bolts
and screws of property classes 4.6 and 5.6
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Hardening : |
When steel with a minimum C-content of about 0.3% is heated
at a temperature above 8000C (depending on the type of steel)
and is quenched in water, oil, air or in a salt bath, the very
hard but brittle martensite structure is formed.
The achieved hardness depends on the C% (the higher the carbon,
the harder the steel) and the percentage of martensite, which at
a certain cooling speed, is formed in the core of the material.
So with thinner bolts from unalloyed carbon steel the critical
cooling speed will be reached to the core. However with thicker
sizes the heat from the core cannot be transmitted to the
outside quickly enough and it will be necessary to add alloying
elements like boron, manganese, chromium, nickel and molybdenum,
which the through-hardening i.e. decrease the critical cooling
speed.
In general, when a type of steel with such through-hardening is
chosen, about 90% martensite is present in the core after
quenching. The choice of cooling medium also influences the
cooling speed. Bolts are mainly quenched in oil, because water,
which is otherwise more effective, causes too much risk of
hardening cracks and warpage. |
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Tempering : |
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With increasing hardness, however, the hardening stresses
will rise, and therefore the brittleness of the material will
also increase. Mostly a second heat treatment, called tempering,
must follow as quickly as possible after quenching. For
temperatures of up to 2000C only the brittleness will decreases,
the hardness diminishes and the toughness is improved. |
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Quenching and tempering :
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This is a combined heat treatment of quenching with
high-tempering, at between 3400 and 6500C immediately following.
This is the most important and most commonly practised heat
treatment for fasteners. An optimal compromise is reached
between a rather high tensile strength, particularly a high
yield/tensile strength ratio and sufficient toughness, which is
necessary for a fastener carrying all kind of external forces to
function effectively. The higher property classes 8.8, 10.9 and
12.9 are therefore quenched and tempered.
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Decarburizing : |
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By heat treating carbon and alloy steels the danger exists
that carbon from the outside of the product is removed by the
surrounding atmosphere. The skin then gets a carbon content that
is too low; it is not hardenable and will stay soft. This means
that the screw thread under loading could be slid off. To
prevent this, the quenching and tempering fasteners is always
done when the furnace is supplied with a protective gas, which
keeps the carbon percentage at the level of the steel type. |
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