past paper - example 2
TRANSCRIPT
The 14th IGEM Young Person Paper
Competition
Simple Bolts Are Not Simple
Author name: Kam Chung Ken, Gloriane
Title: Engineer
Organization: The Hong Kong and China Gas Co Ltd
Address: 363 Java Road, North Point, Hong Kong
Email: [email protected]
Office Phone: +852 2963 2219
Date: 12th
June 2015
2/18
Table of content
Section Page
1. Introduction 3
2. Existing standards and published information 3
3. Recommended boundary for safe steel bolting materials 5
4. Use of the recommended boundary to select appropriate bolting
materials
6
4.1 Property class 12.9 steel bolt 7
4.2 The appropriate bolting materials 7
5. Quality assurance (QA) measures on bolts 8
6. Conclusion 10
Figure 1 Failure of high tensile steel bolt due to stress corrosion
cracking
3
Figure 2 Failure of high tensile steel bolt due to hydrogen
embrittlement
3
Appendix A: Different steels for different pressure level application 11
Appendix B: Hydrogen embrittlement 12
Appendix C: Stress corrosion cracking 13
Appendix D: The identification of EN 10269 material for the
starting material of ISO 898-1:2013 property classes 5.6 and 8.8
14
Reference 16
Table 1 Different warning levels on various mechanical properties of
steels suggested by the existing standards and related information to
deal with brittle failure
4
Table 2 Comparison between the key selection indicators and the
mechanical properties of different property classes of steel in ISO
898-1:2013
7
Table 3 The starting materials of EN 10269 for property classes 5.6
and 8.8
8
Table 4 Recommended standards for the testing methods according
to the key selection indicators
9
Table 5 Dimensional limitations on testing bolt in the elongation
percentage at break and impact strength tests
9
Table a: The chemical composition of ISO 898-1:2013 property
classes 5.6 and 8.8
14
Table b: The matching of EN 10269 materials to property classes 5.6
and 8.8 according to the chemical composition
14
Table c: The matching of EN 10269 materials to property classes 5.6
and 8.8 according to the tensile strength:
15
3/18
1. Introduction:
Bolt is tiny in terms of size when comparing with the entire gas infrastructure but they
are used extensively in various pressure equipment (e.g. valves, pipework assembly
etc.) of gas network to be the primary means of transferring loads. Thus, bolt failure
in gas network could lead to serious consequences. For this reason, correct use of
bolts is important.
But Are Higher Strength and Harder Bolts Better? This is a common myth. In fact,
they are much susceptible to brittle failure. The common brittle failure modes are
stress corrosion cracking and hydrogen embrittlement. For example, the Hendrix
Group reported that the hard low alloy steel bolts (HRC 44) failed due to hydrogen
embrittlement which caused complete separation of the 1 inch gas ball valve without
warning leading to natural gas leakage [1]. Also, the US General Motor “A” ear rear
suspensions began to fail after just 2 years of service which led to the recall of 6.4
million cars as the carbon steel property class 12.8 bolts failed due to stress corrosion
cracking [2].
Although there are some existing standards of particular applications, national
guidelines and studies providing the information about preventing brittle failure in
steels, there is challenge on how to gather and use the important information for
pressure applications in the gas industry. This paper discussed the appropriate
selection and the quality assurance measures of the carbon steel and low alloy steel
bolts for gas distribution network in order to maintain a safe gas system.
2. Existing standards and published information:
Since hydrogen embrittlement (HE) and stress corrosion cracking (SCC) [Appendix B
and C] can be serious problems in some carbon and low alloy steels as shown in
Figures 1 & 2, it is important to choose the right bolting material. Standards and
published information now exist for steels which give different advices to one or two
mechanical properties to prevent brittle failure from happening. This paper studied
and listed them in Table 1 below.
Figure 1. Failure of high tensile steel bolt due to stress corrosion cracking [3]
4/18
Figure 2. Failure of high tensile steel bolt due to hydrogen embrittlement [4]
Mechanical
properties:
Existing standards and
published information:
Concerned
failure
mode:
Warning level:
Tensile
strength σT /
Yield
strength σY
Scott MacKenzie, Houghton
International, Inc, Overview of
the Mechanisms of Failure in
Heat Treated Steel Components,
ASM International [5]
SCC σT ≥ 1380 MPa
The Hendrix Group, Inc. [6]
HE σT ≥ 1200 MPa
Health and Safety Executive of
the UK Government [7]
HE σY ≥ 725 MPa
Hardness API 6D:2008 [9] / ISO
14313:2007 [10]
HE ≥ HRC 35
Journal of The Australian Steel
Institute [11]
HE & SCC ≥ HRC 37
Industrial Fasteners Institute
(North America) [12]
HE ≥ HRC 37
Distributor’s Link Magazine,
Spring 2005 [4]
HE ≥ HRC 37
ASTM A490M-12 [13]
Brittle failure ≥ HRC 38
Elongation at
break
European Pressure Equipment
Directive (PED) 97/23/EC
Brittle failure < 14 %
Impact
strength
European Pressure Equipment
Directive (PED) 97/23/EC
Brittle failure < 27 J at ≤ 20 °C
but not > the
lowest operating
temp.
Table 1 Different warning levels on various mechanical properties of steels suggested
by the existing standards and related information to deal with brittle failure
5/18
However, the warning levels to different mechanical properties of steel to prevent the
occurrence of brittle failure shown in Table 1 are dispersed in different sources which
include national guidelines of particular areas, standards of particular applications,
professional journals and local technical institutes, and each source does not provide
comprehensive requirements on steels. Thus, there is a challenge on how to gather all
the useful information and make use of them when selecting the appropriate steel
bolts for pressure application in the gas industry.
3. Recommended boundary for safe steel bolting materials
Since the challenge was identified, this paper studied different relevant sources and
formed a safe boundary for steel bolting materials suitable for gas distribution system
after the study. The safe boundary was composed by 4 key selection indicators (KSIs)
recommended by this paper related to the mechanical properties after the study in a
conservative point of view. Any steel bolting materials which can fall into this safe
boundary should be the appropriate bolts to be used in the gas distribution system for
pressure application.
The 4 KSIs were shown below:
i. If tensile strength ≥ 1200 MPa or yield strength ≥ 725 MPa, extra caution
should be needed in ensuring the toughness and ductility of the bolt.
ii. Elongation percentage at break ≥ 14 %
iii. ISO V-notch impact strength ≥ 27 J at - 20 oC
iv. Hardness < HRC 35.
Figure 3 Recommended boundary for appropriate bolts in gas distribution system
6/18
4. Use of the recommended boundary to select appropriate bolting materials
To select the appropriate carbon steel bolting materials for the gas distribution system,
this paper compared the safe boundary with ISO 898-1:2013 which offers various
property classes of carbon steel and low alloy steel bolting materials and are widely
suggested by different ISO bolt and screw standards[14],[15],[16],[17]. The bolt
grades in ISO 898-1 which fall into the boundary should be the appropriate bolt
grades.
As shown in Figure 4, classes 8.8 and lower bolts of ISO 898-1:2013 could fall into
the boundary. Thus, they should be suitable to be used. On the other hand, property
class 12.9 was totally out of the boundary. Therefore, property class 12.9 bolts should
be forbidden to be used. The detail comparison of the boundary and all bolt grades in
ISO 898-1:2013 was also shown in Table 2.
Figure 4 Comparison of the boundary with bolt grades in ISO 898-1
7/18
P.C. Tensile
strength, MPa,
min
Yield
strength,
MPa, min
Elongation
percentage at
break, %, min
Impact
strength, J,
min
Hardness
KSIs
≥ 1200
* take extra
caution
≥ 725
* take extra
caution
14
27 at the
specified
temp.
< HRC 35
Different property classes (P.C.) of ISO 898-1:
4.6 400 240 22 N.A.
< HRC 22
4.8 420 N.A. N.A. < HRC 22
5.6 500 300 20 27, At -20 oC < HRC 22
5.8 520
N.A.
N.A. N.A. < HRC 22
6.8 600 < HRC 22
8.8
d ≤ 16mm 800
12
27, At -20 oC
HRC 22 - 32
8.8
d > 16mm 830 HRC 23 - 34
9.8
d ≤ 16mm 900 10 HRC 28 - 37
10.9 1040 9 HRC 32 - 39
12.9/12.9 1220 8 Under
investigation HRC 39 - 44
Table 2 Comparison between the KSIs and the mechanical properties of different bolt
grades in ISO 898-1:2013 [18]
4.1 Property class 12.9 steel bolt:
From Table 2, the P.C. 12.9 steel bolts could not meet the key requirements which
implied that their susceptibility to HE and SCC. Furthermore, ISO 898-1:2013 has a
warning on the use of P.C. 12.9 bolts. It states that “Caution is advised when the use
of property class 12.9/12.9 is considered. The capability of the fastener manufacturer,
the service conditions and the wrenching methods should be considered.
Environments may cause stress corrosion cracking of fasteners as processed as well as
those coated.” [18] This warning was only added since 2009 version and not in the
older versions, year 1999 and before. On top of it, the standard of The Society of
Automotive Engineers, SAE J1199, no longer allows the use of high-hardness P.C.
12.9 bolt [19] because of its susceptibility to SCC which was in response to the US
General Motor case mentioned above [2]. Thus, it was suggested that P.C. 12.9 should
not be suitable to be the pressure bearing bolt in gas network.
4.2 The appropriate bolting materials
From Table 2, the P.C. 8.8 and lower could meet all key requirements except for the
min. elongation percentage at break (12 %) of P.C. 8.8. But the 12 % is just the
minimum value and there is still high possibility that P.C. 8.8 can still meet this 14 %
8/18
requirement with careful selection of materials. Moreover, EN 1515-4:2009, which
provides a mean to conform to the European Pressure Equipment Directive PED
97/32/EC , states that P.C. 5.6 and 8.8 can meet PED 97/32/EC if the starting
materials conformed to EN 10269. Thus, it was suggested that bolts of P.C. 8.8 and
below were the appropriate bolting materials.
As for the P.C. 9.8 and 10.9 in which not all their properties can meet the KSIs, there
is no warning on the use of them in any standard. If the use of them is necessary, it
was recommended to select the bolts carefully so that the tested mechanical properties
can meet the KSIs as far as possible, and ensure the coating process to be carried out
as per the national or international standards in order to minimize the effect of
hydrogen generated in the coating process which causes HE [20]. Also, the service
condition should be free of corroding elements such as chloride and sulphur.
5. Quality assurance (QA) measures on bolts:
The appropriate selection of bolts should work with the appropriate QA measures in
order to safeguard the gas network. This section suggested the recommended QA
measures.
i. The selected bolts should meet the international / national standards listed
below. These standards do not only regulate the mechanical properties, but
also the dimensions and the coating process.
Bolt: ISO 4014:2011
Screw: ISO 4017:2014
Bolt, Screw, Nut: BS 4190:2001
Cap screw: ISO 4762:2004
ii. If possible, select the starting materials of bolts according to EN 10269 [21]
which is a harmonized standard of the European Pressure Equipment Directive
and states good quality starting materials. This paper identified the appropriate
starting materials for P.C. 5.6 and 8.8 as below by comparing the chemical
compositions and mechanical properties. The comparisons were shown in
Appendix D.
P.C. Starting materials of EN 10269
5.6 19MnB4; C35E; C43E; 35B2; 20Mn5
8.8 19MnB4; 42CrMo4; 42CrMo5-6; 40CrMoV4-6
Table 3 The starting materials of EN 10269 for P.C. 5.6 and 8.8
9/18
iii. Request the inspection certificate of the bolts meeting EN 10204:2004 Type
3.1 from manufacturers with sufficient traceability information, results of
composition test and the tests related to the KSIs (if bolt dimensions are
allowed).
iv. Conduct new sample evaluation test and test the purchased bolts regularly
according to the KSIs. The recommended test methods were shown in Table 4.
Test items Standards of test method
Tensile strength ISO 898-1:2013 clause 9.1 or 9.7
Elongation percentage at break ISO 898-1:2013 clause 9.3
Hardness Brinell hardness: ISO 6506-1
Rockwell hardness: ISO 6508-1.
Impact strength ISO 148-1
Table 4 Recommended standards for the testing methods for the key selection
indicators
Test items Dimensional limitation on testing bolt
Elongation percentage at
break
For P.C. 4.6 and 5.6
3 ≤ d < 4.5 mm Bolt length ≥ 6.5d
d ≥ 4.5 mm Bolt length ≥ d + 26 mm
For P.C. 8.8, 9.8 and 10.9
3 ≤ d < 4.5 mm Bolt length ≥ 6.5d
4.5 ≤ d ≤16 mm Bolt length ≥ d + 26 mm
d > 16 mm Bolt length ≥ 5.5d + 8 mm
Impact strength
For P.C. 5.6, 8.8, 9.8 and 10.9
d ≥ 16 mm
Bolt length ≥ 55 mm
Table 5 Dimensional limitations on testing bolt in the tests[18]
Since some tests have sample size limitation as shown in Table 5, if the testing
bolts cannot meet the size limitation, this paper recommended the following
actions.
- Take the tensile strength σT and hardness results as the indexes to indicate the
toughness and ductility of the bolt.
- If the bolt is not long enough as per the standard elongation test method, it
was recommended conducting the test despite it is not long enough and take
the result as a reference. Bolt with shorter gauge length would obtain a higher
elongation % than those with longer gauge length due to the fact that
localized deformation becomes the principal portion of measured elongation
and leads to a higher elongation % [22].Thus, if the elongation % of a shorter
bolt cannot meet the requirement, it is unlikely for the bolt to meet the
requirement even its length is long enough.
10/18
6. Conclusion:
This paper formed a safe boundary for steel bolting materials suitable for gas
distribution system after various study. The safe boundary was composed by 4 Key
Selection Indicators (KSIs) recommended by this paper in a conservative point of
view as shown below:
i. If tensile strength ≥ 1200 MPa or yield strength ≥ 725 MPa, extra caution
should be needed in ensuring the toughness and ductility of the bolt.
ii. Elongation % at break ≥ 14 %
iii. Impact strength ≥ 27 J at the – 20 °C
iv. Hardness < HRC 35.
Also, this paper pointed out that the use of P.C. 12.9 bolt of ISO 898-1 should be
forbidden in the gas distribution network system while bolts of P.C. ≤ 8.8 were
suggested as the appropriate bolting materials. Moreover, some appropriate quality
assurance measures on bolts were recommended to work with the bolt selection
boundary in order to ensure the safety of the gas distribution system.
11/18
Appendix A
Different steels for different pressure level application:
According to EN 1515-1:2000, low to medium carbon steel and low alloy carbon steel
bolts are suitable for the application under pressure level at and below 40 bar while
high alloy steel bolts are needed for the application under a higher pressure level [23].
It is because that high alloy steels possess less stress relaxation at constant strain at
elevated temperatures as shown in BS 4882:1990 Appendix A and Table 24 which is
suitable for high pressure application and better ductility as well as toughness [24].
BS 4882:1990 points out that relaxation of stress at constant strain occurs in all bolts
at operating temperature > 300oC and the initial elastic stress applied to the bolt
would be reduced. Thus, the tightening force of the bolt reduces. Since high alloy
steels possess less stress relaxation at elevated temperature, they are much suitable to
be used for high pressure application.
12/18
Appendix B
Hydrogen embrittlement:
ASTM F1624 points out that the cause of hydrogen embrittlement is the introduction
of hydrogen into steel that can initiate fracture when stress, including residual stress
or external stress applied during service, is present [25]. ASM Handbook states that
even small amounts of hydrogen can have a deleterious effect, particularly for high-
strength steels with tensile strengths of 1240 MPa or more. A few parts per million of
hydrogen dissolved in steel can cause hairline cracking and loss of tensile ductility.
Even when the quantity of gas in solution is too small to reduce tension-test ductility,
hydrogen -induced delayed fracture may occur [26].
Source of hydrogen:
The source of hydrogen can be come from cleaning or plating processes or the
exposure of cathodically protected steel [25].
How to eliminate hydrogen:
During the acid pickling cleaning process before plating, the addition of suitable
inhibitors to the pickling solution eliminates or minimizes attack on the metal and the
consequent generation of nascent hydrogen.
Furthermore, appropriate plating solutions and plating conditions can be selected to
produce a high-cathode efficiency which minimizes the amount of hydrogen
generated on the metal surface. Because the metallic coatings plated on metal often
prevent the hydrogen from leaving the base metal, elevated-temperature baking right
after plating is generally required to allow the hydrogen to move to microstructural
positions in the part interior that are less damaging to the atomic bonds of the iron
matrix. [26]
13/18
Appendix C
Stress corrosion cracking SCC:
Stress-corrosion cracking is a generic term describing the initiation and propagation
of cracks in a metal or alloy under the combined action of tensile stresses (applied
and/or residual) and a corrosive environment [27].
The condition for SCC [28]:
1) The use of susceptible material to SCC
2) Tensile stress, either from structural loading or present as residual stresses from
forming or welding operations during manufacture and installation; and
3) The presence of a specific aggressive environment, e.g. Chloride
The mechanism of SCC [27]:
1. The coating becomes degraded and corroded
2. An electrolyte comes into contact with the surface.
3. The corrosive environment (e.g. chloride and sulphur) and tensile stress cause
SCC, including transgranular stress corrosion cracking (TCSCC) and
intergranular stress corrosion cracking (ICSCC), to develop
4. The initiation and growth of multiple cracks
5. Dominant crack reaches grow a critical size for rapid growth to failure,
producing either a leak or a rupture
14/18
Appendix D
The identification of EN 10269 material for the starting material of ISO 898-1:2013
property classes 5.6 and 8.8:
Table a: The chemical composition of ISO 898-1:2013 property classes 5.6 and 8.8:
Property
class
Material and heat treatment Chemical composition limits (cast analysis, %) Others
C
(min)
C
(max)
P
(max)
S
(max)
B
(max)
5.6 Carbon steel or carbon steel
with additives
0.13 0.55 0.050 0.060 Not
specifi-
ed
8.8 Carbon steel with additives
(e.g. Boron or Mn or Cr)
quenched and tempered
0.15 0.40 0.025 0.025 0.003 min. Mn 0.6 % if
C < 0.25 %
Carbon steel quenched and
tempered
0.25 0.55 0.025 0.025 0.003
Alloy steel quenched and
tempered
0.20 0.55 0.025 0.025 0.003 Also contains Cr
or Ni or Mo or V
Table b: The matching of EN 10269 materials to property classes 5.6 and 8.8
according to the chemical composition:
EN 10269
material:
Suitable for
the property
class of steel:
C % Si % Mn % P %
max
S %
max.
Al % B% Cr% Mo% Ni % V% Others
19MnB4 5.6 / 8.8 0.17 -
0.24
≤ 0.4 0.8 –
1.15
0.03 0.035 ≥0.02 0.0008
–
0.005
C35E 5.6 0.32 -
0.39
≤ 0.4 0.50 –
0.80
0.030 0.035 ≤ 0.4 ≤ 0.10 ≤ 0.4 Cr+Mn
+Ni≤
0.63
C43E 5.6 0.42 –
0.50
≤ 0.4 0.50 –
0.80
0.030 0.035 ≤ 0.4 ≤ 0.10 ≤ 0.4 Cr+Mn
+Ni≤
0.63
35B2 5.6 0.32 –
0.39
≤ 0.4 0.50 –
0.80
0.030 0.035 ≥0.02 0.0008
–
0.005
20Mn5 5.6 0.17 –
0.23
≤ 0.4 1.00 –
1.50
0.030 0.035 ≥0.02 ≤ 0.4 ≤ 0.10 ≤ 0.4 Cr+Mn
+Ni≤
0.63
42CrMo4 8.8 0.38 –
0.45
≤ 0.4 0.60 –
0.90
0.025 0.035 0.90 –
1.20
0.15 –
0.30
42CrMo5-6 8.8 0.39 –
0.45
≤ 0.4 0.40 –
0.70
0.025 0.035 1.20 –
1.50
0.50 –
0.70
40CrMoV4-6 8.8 0.36 –
0.45
≤ 0.4 0.45 –
0.85
0.025 0.030 ≤
0.015
0.90 –
1.20
0.50 –
0.65
0.25 –
0.35
15/18
Table c: The matching of EN 10269 materials to property classes 5.6 and 8.8
according to the tensile strength:
Tensile strength MPa
ISO 898-1:2013 property class 5.6 min. 500
ISO 898-1:2013 property class 8.8 min. 800
EN 10269 material: Tensile strength
match with the
property class of
steel:
19MnB4 5.6 / 8.8 800 - 950
C35E 5.6 500 - 650
C43E 5.6 560 - 710
35B2 5.6 500 - 650
20Mn5 5.6 500 - 650
42CrMo4 8.8 860 - 1060
42CrMo5-6 8.8 860 - 1060
40CrMoV4-6 8.8 850 - 1000
16/18
Reference:
[1] The Hendrix Group, Failure Analysis Case Histroy No. 001,
http://hghouston.com/resources/failure-case-histories/case-history-001.aspx
[2] American Society for Metals (ASM) Handbooks Vol 1, 2002
[3] Hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC)
of 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener, Abhay K. Jha *, Sushant Manwatkar, K.
Sreekumar, Engineering Failure Analysis 17 (2010) 777–786
[4] Joe Greenslade, “Here Is What A Hydrogen Embrittlement Failure Really Looks
Like” Extracted from Distributor’s Link Magazine, Spring 2005
[5] Scott MacKenzie, Houghton International, Inc, Overview of the Mechanisms of
Failure in Heat Treated Steel Components, ASM International
[6] David E. Hendrix, the President of The Hendrix Group, Inc., Hydrogen
Embittlement of High Strength Fasteners in Atmospheric Service
[7] Review of the performance of high strength steels used offshore , HSE BOOKS,
UK Government, 2003
[8] European Pressure Equipment Directive 97/23/EC
[9] API 6D:2008, Specification For Pipeline Valves 23rd
Edition, April 2008
[10] ISO 14313:2007, Petroleum and natural gas industries – Pipeline transportation
systems – Pipeline valves
[11] “Are You Getting The Bolts You Specified? A Discussion Paper”, Steel
Construction, Journal of The Australian Steel Institute, Volume 39 No. 2 Dec 2005
[12] “Zinc-Nickel Alloy Plating Provides a Practical Alternative to Zinc Plating on
Socket Products and Other High-Hardness Fasteners.” Industrial Fasteners Institute of
Independence, OH; Sept 2009
17/18
[13] ASTM A490M-12 Standard Specification for High-Strength Steel Bolts, Classes
10.9 and 10.9.3, for Structural Steel Joints (Metric) , alloy steel, bolts, metric
[14] ISO 4014:2011 Hexagon head bolts. Product grades A and B
[15] ISO 4017:2014 Hexagon head screws. Product grades A and B
[16] BS 4190:2001 Metric black hexagon bolts, screws and nuts — Specification
[17] ISO 4762:2004 Hexagon socket head cap screws
[18] ISO 898-1:2013 Mechanical properties of fasteners made of carbon steel and
alloy steel Part 1: Bolts, screws and studs with specified property classes — Coarse
thread and fine pitch thread
[19] SAE J1199, Mechanical and Material Requirements for Metric
Externally. Threaded Fasteners
[20] South African National Standard: SANS 110094 “The use of high-
strength friction-grip bolts”
[21] EN 1515-4:2009. Flanges and their joints — Bolting Part 4: Selection
of bolting for equipment subject to the Pressure Equipment Directive
97/23/EC
[22] Davis, J.R. “Tensile Testing – Second Edition”, ASM International, 2004
[23] EN 1515-1: 2000, Flanges and their joints - Bolting - Part 1: Selection of bolting
[24] BS 4882: 1990, Specification for Bolting for flanges and pressure containing
purposes
[25] ASTM F1624 - 09 Standard Test Method for Measurement of Hydrogen
Embrittlement Threshold in Steel by the Incremental Step Loading Technique
[26] Hydrogen Damage and Embrittlement, Failure Analysis and Prevention, Vol 11,
ASM Handbook, ASM International, 2002, p 809–822
18/18
[27] G.F. Vander Voort, Embrittlement of Steels, Properties and Selection: Irons,
Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International,
1990, p 689–736
[28] Stress corrosion cracking of stainless steel in swimming pool building, Food and
Entertainment Sector, Commercial and Consumer Service, Transportation and
Utilities Sector, Health and Safety Executive, FOD Scotland, 2 August 2002