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: gloriane.kam@towngas.com

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