1 Scope
This section specifies thickness, corrosion protection, and technical requirements about mechanical and physical properties of steel fastener with regular screw threads.
This section is applicable to passivated or non-passivated zinc flake coatings.
Zinc flake coatings specified in this section also may be used for cutting or roll milling matchable screws with internal thread, such as wood screws, tapping screws, drilling screws with tapping screw thread, thread rolling screws; positive thread and rolled thread, as well as unthreaded steel fastener like gasket and pin. This section also may be used for similar twisted steel parts of other types.
Zinc flake coatings specified in this section also can provide self-lubrication and (or) post-added lubrication.
2 Normative references
The following documents contain provision which, through reference in this text of GB/T 5267, constitute provisions of this national standard. For dated reference, subsequent amendments to (excluding correction contents), or revisions of, any of these publications do not apply. However, parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. For undated references, the latest edition of the normative document referred to applies.
GB / T 90.1-2002 Fastener - Acceptance Inspection (idt ISO 3269:2000)
GB /T 1 97-1981 General purpose screw threads - Tolerances and fits (for diameter from 1 to 355mm)
GB /T 1 237-2000 Designation System for Fasteners (eqv ISO 8991:1986)
GB / T 2516-1981 Common Screw Thread Deviation Table (For diameter from 1 to 355mm)
GB / T 3098.1-2000 Mechanical Properties of Fasteners-Bolts, Screws and Studs (idt ISO 898-1:1999)
GB /T 3 098.1 7-2000 Mechanical Properties of Fasteners - Preloading Test for the Detection of Hydrogen Embrittlement - Parallel bearing surface method (idt
ISO 15530: 1999)
GB /T 5 782-2000 Hexagon head bolts (eqv ISO 4014:1999)
GB /T 9 145-1988 General Purpose Screw Threads - Limits of Sizes for Commercial Bolt and nut threads - Medium Quality (eqv ISO 965-2:1980)
GB /T 3 934-1983 Gauge for General Purpose Screw Threads (neq ISO 1502:1978)
GB /T 1 0125-1997 Corrosion Tests in artificial atmospheres - Salt spray tests (eqv ISO 9227:1990)
GB /T 1 1374-1989 Thermal spraying - Nondestructive methods for measurement of coating thickness (neq ISO 2064:1980)
ISO 1 46 3:1982 Test Methods for Thickness of Metal and Oxide Coating - The Metallographic Method
3 Terms and definitions
Terms and definitions specified in this section and those in GB/T 11374 and GB/T 90.1 shall be used together.
3.1 Non-electrolytically applied zinc flake coatings (self-lubrication or post-added lubrication)
Non-electrolytically applied zinc flake coatings refer to inorganic surface coating with great electrical conductivity and cathodic protection function, which are formed through coating zinc flake (aluminium flake can be blended), and then heating in adequate medium in order to make zinc flakes, zinc flake and backing materials bond well.
Note: It is eligible no matter there is or is not chromate passivation film on zinc coating.
4 General characteristics
A major characteristic of non-electrolytically applied zinc flake coatings is: the covered fastener is unable to absorb hydrogen atom during the process of coating zinc flake. Trend of hydrogen embrittlement will not occur if nascent hydrogen doesn't generate when adopt proper pretreatment (like spray treatment). For fastener whose rigidity is larger than 365 HV, if pretreatment (like pickling) is able to make base materials absorb hydrogen, process that may cause hydrogen embrittlement shall be controlled during the coating process. Preloading test according to GB/T 3098.17 can meet the said requirement.
It is to be noted that: owing to high permeability of zinc flake coatings to hydrogen, absorbed hydrogen may run off through zinc layer during heat-agglomerating process before coating zinc flake.
5 Technical requirements and detection of dimension
Zinc coating thickness of general purpose screw threads which comply with GB /T 197,GB/T 9145 and GB/T 2516 are not only related to basic deviation listed in table 1, but also related to internal and external thread as well as position of the following tolerance zones.
—External thread: g, f, e;
—Internal thread: G; or when there is specific requirement: H,
Zinc coating will not exceed zero line (fundamental dimension) when it is on external thread, and also will not be below zero line when it is on internal thread. What is to say that if tolerance zone fails to reach zero line (fundamental dimension), only moderate coating may be coated on internal thread whose tolerance position is H.
After coating, general purpose screw threads shall comply with provisions of GB/T 3934: respectively inspect external thread or internal thread with go-gauge at position h or H of tolerance zone. When inspect coated thread with ring gauge, the permissible maximal torque shall be 0.00 1d3(N ·m), in which d refers to nominal diameter of thread (mm).
Other dimension requirements are only applicable before coating.
In order to meet specified requirements of corrosion protection property (refer to Chapter 6), when it is required specifying minimum coating thickness (tmin), scope of zinc coating thickness shall be specified based on considering the dimension shall be fundamentally equal to the minimum coating thickness. Therefore, maximal coating thickness according to estimation is twice of the required minimum coating thickness. Refer to table 2. For coating that mandatory minimum thickness of 4tmax (or 8tmin), its minimum basic deviation of proper thread are also given in table 2.
For a specified pitch, if its basic deviation listed in table 1 can't satisfy the required minimum coating thickness, then:
— Change tolerance zone position of the thread (like replace g with f); or
— Tolerance shall be confined to specified tolerance range so as to produce thread meeting the following requirements: internal thread shall be upper deviation value of its tolerance; external thread shall be lower deviation value of its tolerance.
In order to achieve the specified anti-corrosive property, minimum partial zinc coating thickness is given in table 3.
Note: Refer to Informative A for examples of selecting zinc coating thickness according to requirement of anti-corrosive property.
If minimum thickness of partial zinc coating is specified (refer to table 3), it shall be measured with magnetic thickness tester or X-ray thickness gauge. When it is in dispute, it is required to adopting metallographical microscope method specified in IS0 1463 as arbitration method. Refer to figure 10 for surface and position for thickness measurement.
If pitch is less than 1mm (CM6) or with a small internal driving space or when fastener of groove requires coating, both supply and demand sides shall make particular agreement.
1-Measuring points
Figure 1 Measuring point for thickness of partial zinc coating of threaded fastener
Table 1 Theoretical deviation value of non-electrolytically applied zinc flake coatings thickness of general purpose screw thread Unit is micrometer
Pitch P/mm Nominal diameter of coarse thread d/mm Internal thread External thread
Position G of tolerance zone Position g of tolerance zone Position f of tolerance zone Position e of tolerance zone
Basic deviation Thickness of zinc flake coating max Basic deviation Thickness of zinc flake coating max Basic deviation Thickness of zinc flake coating max Basic deviation Thickness of zinc flake coating max
0.2
0.25
003
1; 1.2
1.4 +17
+18
+18 4
4
4 -17
-18
-18 4
4
4
0.35
0.4
0.45 1.5; 1.8
2
2.5; 2.2 +19
+19
+20 4
4
5 -19
-19
-20 4
4
5 -34
-34
-35 8
8
8
0.5
0.6
0.7 3
3.5
4 +20
+21
+ 22 5
5
5 -20
-24
-26 5
6
6 38
38
40 9
9
10 56
60
60 14
15
15
1.25
1.5
1.75 8
10
12 +28
+32
+34 7
8
8 -28
-32
-34 7
8
8 -42
-45
-48 10
11
12 -63
-67
-71 15
16
17
2
2.5
3 16;14
20;18;22
24;27 +38
+42
+48 9
10
12 -38
-42
-48 9
10
12 -52
-58
-63 13
14
15 -71
-80
-85 17
20
21
3.5
4
4.5 30;33
36;39
42;45 +53
+60
+63 13
15
15 -53
-60
-63 13
15
15 -70
-75
-80 17
18
20 -90
-95
-100 22
23
25
5
5.5
6 48;52
56;60
64 +71
+75
+80 17
18
20 -71
-75
-80 17
18
20 -85
-90
-95 21
22
23 -106
-112
-118 26
28
29
Note: Theoretical deviation value of zinc flake coatings thickness is calculated according to lower deviation value (for internal thread) or upper deviation value (for external thread) of the specified thread tolerance.
a Data about nominal diameter of coarse thread are only provided for convenience. The one which determine characteristics is pitch.
Table 2 Non-electrolytically applied zinc flake coatings thickness and required
Unit is micrometer
Zinc flake coatings thickness a Required minimum basic deviation
min
(Refer to table 3 when it is required) max
(Expected value)
4
5
6 8
10
12 32
40
48
8
9
10 16
18
20 64
72
80
12 24 96
a The significant factor shall be considered is that clamping load may reduce because zinc flake coatings thickness on supporting surface of fastener may cause looseness.
Table 3 Period of neutral salt spray test
Test period/h Minimum thickness of partial zinc flake coatings (it shall be determined by demand side when required)
Zinc coated layer with chromate passivation/ (flZnyc) Zinc coated layer without chromate passivation/ (flZnnc)
240
480
720
960 4
5
8
9 6
8
10
12
Note: If average weight (g/m2) of zinc flake coatings is specified by demand side, thickness may be converted as follows:
— Zinc coating with chromate passivation: 4.5g/m2, which is equivalent to 1μm thickness;
— Zinc coating without chromate passivation: 3.8g/m2, which is equivalent to 1μm thickness.
a Demand side may specify whether its required zinc coating is chromate passivated (flZnyc) or non-chromate passivated (flZnnc); If it doesn't propose such requirement, the code name flZn shall be used. Refer to Chapter 9.
6 Test for anti-corrosive property
Assess quality of zinc coating with neutral salt spray test specified in GB/T 10125. This test shall be carried out for parts under delivery. Properties of parts found through such test are independent of anti-corrosive property under specific service environment.
Note: Under normal conditions, period for neutral salt spray test of zinc coating shall be chosen from table 3. Refer to example 1 of Chapter 9. After conducting neutral salt spray test according to test period in table 3, megascopic rust (presents in red) shall not exist on metal body.
7 Mechanical and physical properties and tests
7.1 General provisions
Zinc coating technology shall not bring injurious influence to fasteners' mechanical and physical properties specified in national standards.
For uncoated fastener in special pattern, in order to determine whether heating temperature and period of coating is proper (if necessary), manufacturer shall provide research condition verified through tests.
7.2 Appearance
Zinc coating shall be silver-gray, without defects like bubbles, partial thicker zinc layer or non-zinc layer because these defects may cause harmful effect to fasteners' anti-corrosive property and exchangeability.
Professional technology may be required so as to protect parts (such as gasket, nut and notched screw) from thicker zinc layer or partial non-zinc layer.
7.3 Heat-resistance property
Zinc coated fastener shall still meet requirements of anti-corrosive property as specified in Chapter 6 after being heated to 1500℃ and keeping for 3h.
7.4 Toughness
After load test according to GB/T 3098.1, anti-corrosive property of zinc coated fastener, except the section subjects to thread engagement during the test, shall still comply with those specified in Chapter 6. This requirement is only applicable to bolt, screw and stud.
7.5 Adhesive strength
Prepare adhesive tapes of 25mm wide and adhesive strength is (7+1-1), then solidly presses the tape to zinc coated part surface by hand, finally pull apart the tape rapidly along the direction perpendicular to the surface. The zinc coating shall not shed from metal base. But small amount of zinc coating materials sticking to tape is allowed.
7.6 Cathodic protection
Cathodic protection ability of zinc coating also may be tested, namely prepare specimen whose zinc coating is scratched to metal base by instrument whose maximal scratch width is 0.5mm, and then carry out salt spray test according to requirements of Chapter 6. After 72 h test, red rust shall not appear at the scratched position.
7.7 Torque-tension relationship of zinc coating with self-lubrication or post-added (external) lubrication
Technical requirements for torque-tension relationship of zinc coating with self-lubrication or post-added (external) lubrication shall be made through mutual agreement.
8 Applicability of tests
8.1 General provisions
All technical requirements given from Chapter 5 to Chapter 7 are general requirements of zinc coating's characteristics. These requirements and those solely proposed by demand side shall put on trial. Each batch of fastener shall be tested according to 8.2 (refer to GB/T 90.1). Test given in 8.3 is only used in process control because it is not applicable to every batch.
8.2 Compulsive tests for every batch of product
— Gauge inspection for thread (refer to Chapter 5);
— Appearance (refer to 7.2)
— Adhesive strength (refer to 7.5)
8.3 Tests for process control and management
—Neutral salt spray test (refer to Chapter 6)
—Heat-resistance property (refer to 7.3)
—Toughness (refer to 7.4)
—Cathodic protection (refer to 7.6)
8.4 Tests implemented when customer requires
— Thickness of zinc coating (refer to Chapter 5);
— Torque-tension test of zinc coating with self-lubrication or post-added (external) lubrication (refer to 7.7)
9 Designations
According to those specified in GB/T 1237, designation of non-electrolytically applied zinc flake coatings shall be added in products designation: non-electrolytically applied zinc flake coating is expressed with code name flZn; if indoor salt spray test is required and the test period is specified, express the required test period with number; those with chromate passivation film shall be expressed with yc, and those without chromate passivation film shall adopt nc.
Example 1:
Hexagon head bolts: GB/T5782 M12×80 10.9, non-electrolytically applied zinc flake coatings (flZn), Designation for those require salt spray test for 480h:
Hexagon head bolts: GB/T 5782 M12×80 10.9 f1Zn 480 h
If zinc flake coating is required conducting self-lubrication, the letter L shall be put after designation of zinc flake coatings:
Hexagon head bolts GB/T 5782 M12×80 10.9 flZnL 480 h
If zinc flake coating is required adding lubrication (external lubrication) after coating, the letter L shall be put at end of the designation:
Hexagon head bolts GB/T 5782 M12×80 10.9 flZn 480 h L
Example 2:
Hexagon head bolts GB/T5782 M12×80 10.9, Non-electrolytically applied zinc flake coatings without chromate passivation (flZ nnc), Designation for those require salt spray test for 480:
Hexagon head bolts GB/T 5782 M12×8 0 10.9 flZnnc 480 h
Example 3:
Hexagon head bolts GB/T 5782 M12×80 10.9, Non-electrolytically applied zinc flake coatings with chromate passivation (flZnyc), Designation for those require salt spray test for 480:
Hexagon head bolts G B/ T 5782 M12×80 10 .9 flZnyc 480h
10 Technical requirements of ordering non-electrolytically applied zinc flake coatings
When order thread parts with non-electrolytically applied zinc flake coatings according to this section, the following information shall be prepared for worker who is in charge of coating:
a) Reference Designation of non-electrolytically applied zinc flake coatings in this section (refer to Chapter 9);
b) Parts materials and state such as heat treatment, rigidity or other properties may be affected in process of coating zinc flake;
c) When thread precision is different from what specified in product standard;
d) Working performance (such as torque-tension and friction coefficient) and test method for zinc flake coatings with self-lubrication or post-added lubrication based on agreement between both sides
e) Tests require implementing (refer to Chapter 8)
f) Sampling inspection
Appendix A
(Informative)
Example of selecting non-electrolytically applied zinc flake coatings thickness according to requirement of anti-corrosive property
One user plans to order bolts of general purpose screw threads: with M10 coarse thread (1.5 mm pitch) and non-electrolytically applied zinc flake coating.
According to his experience, minimum period for salt spray test is 480h to meet requirements for anti-corrosive property in service condition.
According to table 3, the user determines to select zinc with non-electrolytically applied zinc flake coatings after chromate passivation (flZnyc). Minimum thickness of such zinc layer is 5μm; correspondingly, its minimum test period is 480 h.
According to table 2, the user found that when minimum thickness is 5μm, corresponding maximal thickness is 10μm, so that the minimum basic deviation shall be 40μm.
According to table 1, the user found that when pitch is 1.5 mm, minimum basic deviation can be 40μm if only selecting position f as tolerance zone, which means that the user can't order bolts of 6g before coating zinc flakes.
The user may make the following three determinations:
1) Ratify that thread is 6f before coating zinc flakes for bolt;
It can be gotten from this part: thickness of zinc coating layer can't exceed 11μm so as to avoid problems during installation. The user will order:
Hexagon head bolts (GB/T 5782 M10×60 10.9 flZnyc 480h
Thread is 6f before coating zinc flakes.
2) The user has determined to maintain precision of 6g and reduce requirement of anti-corrosive property.
Table 1 show that basic deviation is 32μm when pitch is 1.5mm, and specifies that permissible maximal thickness of zinc coating is 8μm. Reference table 2, the user found that when minimum thickness of zinc coating is 4μm, period of salt spray test is 240h which is corresponding to zinc coated and chromate passivated (flZnyc) layer.
The user will order:
Hexagon head bolts GB/T 5782 M10×60 10.9 flZnyc 240h
Note: In this instance, thread precision shall be free from special regulations for it meets requirements of product standard.
3) The user wants to keep position g of tolerance zone, but also demands the least period for salt spray test shall be 480h.
In this instance, user will decrease thread's diameter tolerance (it is 132μm for thread whose tolerance grade is 6), namely adopt gap width required to satisfying thickness of coating zinc layer.
480h, Zinc coating with chromate passivation (flZnyc) tmin=5μm (Refer to table 3)
tmax=10μm
Gap width required to coating zinc flakes layer: 40μm
Basic deviation at position g of tolerance zone: 32μm
Thread tolerance demands compression: 8μm
The user will order:
Hexagon head bolts GB/T 5782 MI0×60 10.9 flZnyc 480h
Maximal dimension compression for 6g-level thread is 8μm
Note: Refer to figure A.1 for the situation of level-6 thread tolerance demanding compression.
Unit is micrometer
1- Zero line
a - Basic deviation of tolerance zone position g;
b - Level 6 tolerances.
c - Restricted level 6 tolerance.
Figure A.1 Compress thread tolerance to achieve specified zinc coating
