Fastener

 
  Machine Screws  
 
 

 

 

 

   
  Technical Details :
   
Head
  The head is the enlarged shape preformed on one end of a headed fastener to provide a bearing surface. Types of Head
   
Points
  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.
Standard
  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)
   
Mechanical Properties
   
 

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

   
 

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.

   
 
  Mechanical Properties
Property Class
3.6
4.6
4.8
5.6
5.8
6.8
8.8
9.8
10.9
12.9
d<16mm
d>16mm2
1 Tensile Strength nom.
300
400
500
600
800
800
900
1000
1200
2 RM N/mm2 min.
330
400
420
500
520
600
800
820
900
1040
1220
3 Vickers Hardness HV F>=98N min.
95
120
130
155
160
190
250
255
290
320
385
max.
250
320
335
360
380
435
4 Brinell Hardness HB F=30 D2 min.
90
114
124
147
152
181
238
242
276
304
366
max.
238
304
318
342
361
414
5 Rockwell Hardness HR min. HRB
52
67
71
79
82
89
-
-
-
-
-
  HRC
-
-
-
-
-
-
22
23
26
32
39
max. HRB
99.5
-
-
-
-
-
  HRC
-
32
34
37
39
44
6 Surface Hardness HV 0.3 max.
-
5
7 Lower Yield Stress N/mm2 nom.
180
240
320
300
400
480
-
-
-
-
-
min.
190
240
340
300
420
480
-
-
-
-
-
8 Proof stress RP 0.2 N/mm2 nom.
-
640
640
720
900
1080
min.
-
640
660
720
940
1100
9 Stress Under Proofing load, SP SP/RP 0.2
0.94
0.94
0.91
0.93
0.90
0.92
0.91
0.91
0.90
0.88
0.88
N/mm2
180
225
310
280
380
440
580
600
650
830
970
10 Elongation After Fracture A in % min.
25
22
14
20
10
8
12
12
10
9
8
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
12 Impact min. Strength, J
-
25
-
30
30
25
20
15
13 Head Soundness
 
no fracture
14 Minimum Height of non-decarburized thread zone, E
-
1/2H1
2/3H1
3/4H1
Maximum Depth of Complete Decarburization, G
-
0.015
   
Minimum Breaking Torques
   
 
Thread size
Thread pitch
 
 
Property Class
4.6
4.8
5.6
5.8
8.8
10.9
12.9
Minimum breaking torque, in mm
M1
0,25
0,020
0,020
0,024
0,024
0,033
0,040
0,045
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
   
Overview and Definitions of Steels for Fasteners
   
 

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

   
Overview and Definitions of heat treatments for fasteners
   
 

Annealing :

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.
 
Normalizing (Recrystallization) :
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.
 
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
 
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.
 
Tempering :
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.
 
Quenching and tempering :
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.
 
Decarburizing :
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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