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TECHNICAL BASIS OF MATERIAL TOUGHNESS REQUIREMENTS IN THE ASME B&PV CODE, SECTION VIII, DIVISION 2

FONDEMENTS TECHNIQUES DES NOUVELLES DE TENACITE DANS LA SECTION

VIII DIVISION 2 DU B&PV CODE DE L'ASME

David A. Osage The Equity Engineering Group, Inc.

Shaker Heights, OH USA

Martin Prager The Materials Properties Council, Inc.

New York, NY USA

ABSTRACT The development of new toughness requirements was a major part of the effort to re-write the ASME B&PV Code, Section VIII, Division 2. The new toughness rules in this code were established using the fracture mechanics assessment procedures in API 579-1/ASME FFS-1 Fitness-For-Service, Part 9. The major changes in the toughness rules when compared to older editions of Section VIII, Division 2 and the current edition of Section VIII, Division 1 are for carbon and low alloy steel materials excluding bolting. The new toughness rules in Section VIII, Division 2 are based on a Charpy V-Notch impact requirement of 27 Joules (20 ft-lbs) consistent with European practice and the beneficial effects of post weld heat treatment are included consistent with the procedures in API 579-1/ASME FFS-1. This paper provides a technical background to the new toughness rules including the development of material toughness requirements and the development of impact test exemption rules.

RÉSUMÉ Le développement de nouvelles règles sur la ténacité représenta une part importante de l'effort déployé pour la réécriture de la Division 2 de la Section VIII du B&PV Code de l'ASME. Les nouvelles règles du Code ont été établie en se basant sur les règles d'évaluation de nocivité des fissures données dans la Partie 9 de l'579-1/ASME FFS-1 Fitness-For-Service (Aptitude au Service). Comparativement aux règles concernant la ténacité actuellement dans la Section VIII Division 1 et à celles de l'ancienne Division 2, les changements le plus notables sont pour les aciers au carbone ou faiblement alliés à l'exclusion des aciers de boulonnerie. Les nouvelles règles de ténacité dans la Section VIII Division 2 sont basées sur une énergie de rupture en flexion par choc sur éprouvettes ISO-V de 27 Joules en conformité avec la pratique Européenne, ceci en tenant compte des effets bénéfiques d'un éventuel traitement thermique après soudage évalué conformément aux procédures de l' API 579-1/ASME FFS-1. Ce papier présente les fondements techniques des nouvelles règles de ténacité ainsi que le développement des exigences qui en découlent et le développement des règles d'exemption d'essais de rupture en flexion par choc.

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INTRODUCTION In 2007, a new version of the ASME B&PV Code, Section VIII, Division 2 (VIII-2) that incorporates many new technologies including an increase in allowable stress to be more globally competitive was issued by ASME. A technical background and commentary on this new code is provided by Osage [1]. One of the new technologies incorporated pertained to changes to the toughness rules necessitated by the increased allowable stresses and the need for corrections as discovered by a comprehensive review of the past practices and assumptions. The development of the material toughness rules for VIII-2 is described by Prager and Osage [2] and summarized in this paper. The approach described herein for calculating fracture mechanics driving forces and setting the corresponding requirements for toughness to develop toughness requirements for VIII-2 is provided. The approach utilizes the fracture mechanics methodology in API 579-1/ASME FFS-1 2007, Fitness-for-Service (FFS). Thus a continuum is now provided between design requirements and the long accepted procedures and models for FFS. The foundations for the toughness requirements in the ASME B&PV Code, Section VIII, Division 1 (VIII-1) and VIII-2 were reported by Corten (see WRC 528 [2], Annex A), Barsom and Rolfe [3], Selz [4], and Jacobs [5]. The VIII-2 and FFS applications of these foundations represent updates that include modern fracture mechanics concepts such as Failure Assessment Diagram (FAD), adjustments to calculation procedures for lower shelf energies, estimation of residual stresses in welded equipment and interpolation between lower and upper shelf Charpy energies. Also described herein are the concepts used to establish the exemption curves and the reductions in allowable stresses required to permit lowering of Minimum Design Metal Temperature (MDMT). In this paper, the editions of the ASME B&PV Section VIII Codes are identified as follows:

• VIII-2 – Section VIII, Division 2, 2007 Edition and later • Old VIII-2 – Section VIII, Division 2, 2004 Edition, 2006 Addenda and earlier • VIII-1 – Section VIII, Division 1, 2007 Edition

BACKGROUND Impact testing requirements for materials that are classified into groups are provided in VIII-1, paragraph UCS-66. For each group, identified with the labels A, B, C, and D, there is a thickness dependent curve in UCS-66 showing the MDMT for which the material is exempt from impact testing. Below the curves materials must be tested unless lower than normal allowable stresses or other specified conditions are met. The VIII-1 curves for various material groups are shown in Figure 1 in which temperature of exemption can be seen to decrease with decreasing thickness. Impact testing is required for the specific combinations of design temperature, material classification and thickness below the respective curves. Thin materials have lower allowable design exemption temperatures and the curves become quite steep as the thickness decreases as shown in Figure 1. The curves as shown are truncated at the thickness of a full size Charpy impact specimen, 0.392 in. The procedures herein can be applied for extension of the curves to lower thicknesses. The VIII-1 toughness rules described above were also used in Old VIII-2. However, these rules required significant updating for VIII-2 because the decision was made by the PVRC and ASME committees in charge of the re-write for this new code to adopt a minimum of 27 Joules (20 ft-lbs) for toughness for ferritic materials that is reasonable for modern steel making practices and expectations and close to European standards.

APPROACH In developing the toughness rules for VIII-2 the entire technical and historical basis for the UCS-66 exemption curves was examined, understood, checked, corrected and upgraded to modern

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fracture mechanics standards. The relevant original equations documented by Corten (see WRC 528 [2], Annex A) were obtained and applied to verify and understand the UCS-66 exemption curves. The equations were then modified to develop exemption curves applicable to the higher design allowable stresses and the special needs of VIII-2. The result of that effort was a systematic approach that can be applied to other design codes and criteria and even modified for particular geometries and assumed flaws if desired. Specifically, the method was updated to use the most modern fracture mechanics approach for welds in API 579-1/ASME FFS-1 including stress intensity factor and reference solutions for the crack driving force, estimation schemes for residual stress, and appropriate Failure Assessment Diagrams. Another element worth noting here is the importance of a systematic and reasonable scheme for correlating fracture toughness with Charpy energy. The approach fashioned to smoothly correlate fracture toughness with Charpy impact test values from the lower shelf to the upper shelf was adjusted to compensate for yield strength and nonlinear behavior. Using this approach, it would be possible to create new exemption curves (e.g. E, F, G, etc.) based on the performance of new materials that may have been improved by control of their composition, melting practice etc., or to justify moving a material from one curve to an existing curve if its classification is now deemed to be inappropriate.

FRACTURE MECHANICS APPROACH TO ESTABLISH MATERIAL TOUGHNESS REQUIREMENTS

Description of FAD-Based Fracture Mechanics

Overview In API 579-1/ASME FFS-1, the Failure Assessment Diagram (FAD) is used for the evaluation of crack-like flaws in components. The FAD approach was adopted because it provides a convenient, technically based method to determine the acceptability of a component with a crack-like flaw. In this method the driving force for failure is measured by two distinct criteria: unstable fracture and limit load. Unstable fracture usually controls failure for small flaws in components fabricated from a brittle material and plastic collapse at a limit load typically controls failure for large flaws if the component is fabricated from a material with high toughness. Mixed mode fracture occurs between these extremes. In the analysis of crack-like flaws, the results from a stress analysis, stress intensity factor, limit load solutions, material strength, and fracture toughness are combined to calculate a toughness ratio, rK , and a load ratio,

rL . These two quantities represent the coordinates of a point that is plotted on a two-dimensional FAD to determine acceptability. If the assessment point is on or below the FAD curve, the component is suitable for continued operation. Additional information on the FAD approach to fracture mechanics is covered in WRC 430 [6], by Anderson [7], and Anderson et al. [8].

Reference Flaw Size To compute the crack driving force, an elliptical surface flaw with the depth, a , and length, 2c , was assumed. The flaw size was established as shown below based on early research work pertaining to the sensitivity and detection capability of radiographic examination as reported in WRC 175 [9].

min , 1.04ta in⎡ ⎤= ⎢ ⎥⎣ ⎦

(1)

2 6 3c a or c a= = (2)

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Primary and Residual Stress To compute the crack driving force, the following membrane stresses are assumed for the applied primary stress, P

mσ , and residual stress, SRmσ . Note that although allowable design stress is based

on the specified minimum yield strength, the specified minimum ultimate tensile strength and time-dependent properties, the primary membrane stress is conservatively approximated using Equation (3), i.e. it is a function of the specified minimum yield strength only.

23

Pm ysσ σ= (3)

( )23

SRm ys not subject to PWHTσ σ= (4)

( )0.20SRm ys subject to PWHTσ σ= (5)

Setting the residual stress to 20% of the yield strength for components subject to post weld heat treatment is consistent with API 579-1/ASME FFS-1, Annex E.

Required Material Fracture Toughness The toughness ratio for the FAD-based fracture mechanics approach discussed above is given by Equation (6). This equation is taken from API 579-1/ASME FFS-1, Part 9, paragraph 9.4.3.2.l).

P SRI I

rmat

K KKK+Φ

= (6)

The FAD described using Equation (7) was used, see API 579-1/ASME FFS-1, Part 9, Figure 9.20. This is a conservative FAD diagram when (max) 1.0r

PL = .

( )0.22.51.0 ( )Pr rK L= − (7)

Combining Equation (6) with Equation (7) and solving for the required material toughness, matK , the following expression is obtained.

( )( )1 1

0.22.5( )

1.0

P SR

matpr

K KK tL

+Φ=

− (8)

In this expression the required material fracture toughness is a function of the thickness of the material. The fracture toughness parameters, 1

PK and 1SRK in Equation (8), are defined by Equations (9),

(10) and (11). In these equations, the parameter CylinderRFK was derived using API 579-1/ASME

FFS-1, Annex C using the KCSCLE2 Solution with a 1 ksi membrane stress and the reference flaw defined in Equations (1) and (2). The membrane stresses in equations (9) and (10) are set in accordance with Equations (3), (4) and (5). The resulting equations for the fracture toughness parameters are a function of the cylinder wall thickness ( )t and radius-to-thickness ratio ( )/R t .

1P P Cylinder

m RFK Kσ= ⋅ (9)

1SR SR Cylinder

m RFK Kσ= ⋅ (10)

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[ ]

[ ]

[ ] [ ]( )2

76.127581 29.28642 ln 190.73697122.7235 12.153937120.62934 ln

0.42094737 ln0.1760457613.33629 exp

CylinderRF

t t t

K ttt

R tt

R t R t

⎛ ⎞⎜ ⎟

+ ⋅ ⋅ − ⋅ +⎜ ⎟⎜ ⎟⎜ ⎟= ⋅ + − +⎜ ⎟⎜ ⎟⋅⎜ ⎟⋅ − − −⎜ ⎟⎝ ⎠

(11)

Equation (11) utilized in the development of the toughness rules herein was developed using the data in API 579, 2000 Edition, Appendix C and is valid for a thickness range of 0.25 4in t in≤ ≤. The plasticity interaction, Φ , defined by Equation (12) was derived by curve fitting the plots shown in Figure 9.19 of API 579-1/ASME FFS-1.

( )( ) ( )( ) ( )( ) ( )( ) ( ) ( )

2

2 3

3 2 2

0.99402985 0.34259558 0.07849594 1.3153525

0.035075224 0.2222982 0.97610564

0.0041367592 0.0062624497 0.16970127

P SR Pr r r

SR P SR Pr r r r

SR P SR P SRr r r r r

L L L

L L L L

L L L L L

⎛ ⎞− ⋅ + ⋅ + ⋅ −⎜ ⎟⎜ ⎟Φ = ⋅ + − ⋅ +⎜ ⎟⎜ ⎟

⋅ − ⋅ − ⋅⎜ ⎟⎝ ⎠

(12)

The load ratio parameters, PrL and SR

rL in Equation (8) and Equation (12), are defined by Equations (13), (14) and (15). In these equations, the parameter Cylinder

RFR was derived using API 579-1/ASME FFS-1, Annex D using the RCSCLE2 solution with a 1 ksi membrane stress and the reference flaw defined in Equations (1) and (2). The membrane stresses in Equations (13) and (14) are set in accordance with Equations (3), (4) and (5). The resulting equations for the load ratio parameters are a function of the cylinder wall thickness and radius to thickness ratio.

P CylinderP m RFr

ys

RL σσ⋅

= (13)

SR CylinderSR m RFr

ys

RL σσ⋅

= (14)

The parameter CylinderRFR is evaluated using the following equation.

( )

( ) ( ) ( )

( ) ( ) ( )

2

32

2

3 2

1.30182060.99829577 0.0071541778 0.0019047184

4.3132859 0.042484369 0.00011487158

5.4284626 0.12675001 0.0033013072

CylinderRF

t tR t

tR tR tR t

t tR tR t R t

⎛ ⎞⎜ ⎟+ ⋅ + − ⋅ −⎜ ⎟⎜ ⎟

⋅⎜ ⎟= − + ⋅ +⎜ ⎟⎜ ⎟⎜ ⎟⋅ ⋅

+ −⎜ ⎟⎜ ⎟⎝ ⎠

(15)

Equation (15) utilized in the development of the toughness rules herein was developed based on API 579, 2000 Edition, Appendix D and is valid for a thickness range of 0.25 4in t in≤ ≤ .

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Derivation of Material Charpy V-Notch Impact Test Requirements

Required Material Fracture Toughness The required material toughness, ( )matK t , as a function of thickness is based on the reference flaw and applied stress is given by Equation (8). When ( )matK t is evaluated, the fracture toughness parameter given by Equation (11) and reference stress parameter given by Equation (15) were evaluated at 100R t = . In addition, the primary and residual stress membrane stresses in the fracture toughness equations, Equations (9) and (10), and in the load ratio Equations (13) and (14), are given by Equations (3), (4) and (5).

MPC CVN and Fracture Toughness Model The Materials Properties Council (MPC) has developed a Charpy impact energy correlation and a methodology to relate this energy to both the dynamic and static fracture toughness, see API 579-1/ASME FFS-1, Annex F, paragraph F.4.5.3. This correlation uses a two-step procedure for correlating static fracture toughness with Charpy test results in the transition region using temperature shift of the dynamic fracture toughness correlation.

( ) ( )1 1C s dK T T K T−Δ = (16)

where:

( )1 15 ,dK CVN ksi in ft lb= − (17)

42 (75 )o osT C FΔ = (18)

The dynamic fracture toughness at a temperature, T of an ASME exemption curve material (A, B, C, or D) with a reference temperature, 0T , and a specified minimum yield strength, ysσ may be estimated as shown below, (also see API 579-1/ASME FFS-1, Annex F, paragraph F.4.5.3). This equation provides more reasonable values for the dynamic fracture toughness than the simple equation of Corten in the important region approaching the lower shelf where Corten’s equation gives unrealistically low numbers for steels [2].

( )01

273 3 tanh , , od ys

ys

T TK ksi in ksi FC

σσ

⎧ ⎫⎛ ⎞ −⎪ ⎪⎡ ⎤= + − ⋅⎜ ⎟⎨ ⎬⎢ ⎥⎜ ⎟ ⎣ ⎦⎪ ⎪⎝ ⎠⎩ ⎭ (19)

where:

0 114 oT F for ASME Exemption Curve A= (20)

0 76 oT F for ASME Exemption Curve B= (21)

0 38 oT F for ASME Exemption Curve C= (22)

0 12 oT F for ASME Exemption Curve D= (23)

66 oC F= (24)

The dynamic fracture toughness for a material for a given yield strength, ysσ , temperature, T , and ASME exemption Curve may be estimated by Equation (19). The materials assigned to each of the exemption curves, A, B, C, and D are shown in Table 1. The static fracture toughness is determined by applying the temperature shift indicated by

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Equation (16), or

( ) ( )01

( 75273 3 tanh , , oC ys

ys

T TK ksi in ksi F

Cσ

σ

⎧ ⎫⎛ ⎞ − −⎡ ⎤⎪ ⎪= + − ⋅⎜ ⎟⎨ ⎬⎢ ⎥⎜ ⎟ ⎣ ⎦⎪ ⎪⎝ ⎠⎩ ⎭ (25)

Determination of CVN Requirements To derive the required CVN for a material as a function of thickness and yield strength, the CVN transition curve is divided into four regions: lower shelf, near lower shelf region, transition range, and upper shelf. In addition, a code minimum CVN is used. The CVN requirement is calculated for parts not subject to PWHT and parts subject to PWHT because of the difference in crack-driving force resulting from residual stresses. CVN requirements were calculated for specified minimum yield strengths of less than 260 MPa (38 ksi), 345 MPa (50 ksi), 450 MPa (65 ksi) and greater than or equal to 550 MPa (80 ksi). In the following equations, ysσ is taken as the specified minimum yield strength. The thickness range for parts not subject to PWHT is 0.0 mm to 38 mm (1.5 in) and for parts subject to PWHT is 0.0 to 100 mm (4 in).

Minimum Code CVN Requirement The minimum permissible CVN in accordance with the requirements of VIII-2 is 27 Joules (20 ft-lbs). This value is essentially in agreement with current European practice for higher allowable stresses and is a reasonable expectation for modern steel production practices.

20code minCVN ft lbs− = − (26)

Lower Shelf Region CVN Requirement For low temperatures, 0T T<< , i.e. lower shelf behavior, an estimate of the lower shelf fracture toughness can be inferred directly from Equation (19):

27lsK ksi in= (27)

and 2

15ls

lsKCVN ⎛ ⎞= ⎜ ⎟

⎝ ⎠ (28)

Near Lower Shelf Region CVN Requirement The CVN requirement for the near lower shelf region, or region close to the transition region, as a function of thickness is given by Equation (29). This equation is based on the 1dCVN K− correlation given by Equation (17) and a review 1dCVN K− data that indicates that the magnitude of 0.45 ysσ is a good upper bound approximation to the near transition region value for CVN.

2( )( ) min , 0.4515mat

nls ysK tCVN t σ

⎡ ⎤⎛ ⎞= ⎢ ⎥⎜ ⎟⎝ ⎠⎢ ⎥⎣ ⎦

(29)

An extensive review of fracture toughness data by MPC indicates that Equation (29) applies for the indicated limitation based on the yield strength of the material. The corresponding fracture toughness expected in the near lower shelf region is given by Equation (30).

( ) 15 ( )nls nlsK t CVN t= (30)

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Upper Shelf Region CVN Requirement For extreme values of temperature, 0T T>> , i.e. upper shelf behavior, an estimate of the upper shelf fracture toughness may be inferred directly from Equation (19), or:

( )1 2 3 27us d ysK K σ= = ⋅ − (31)

The Rollfe-Novak-Barsom correlation given by Equation (32) and API 579-1/ASME FFS-1, Annex F, paragraph F.4.5.2 provides a correlation between the upper shelf CVN and the upper shelf fracture toughness and the yield strength.

2

5 20ysus

usys

KCVNσ

σ= + (32)

Substituting Equation (31) into Equation (32), and expression for the upper shelf Charpy energy is obtained.

( )22 3 27

5 20ys ys

usys

CVNσ σσ

⋅ −= + (33)

Intermediate Transition Region CVN Requirement Equation (34) was used to model the intermediate portion of the transition region between 0.45 ysσ and the upper shelf. This equation maintains proportionality between the increases in

CVN and fracture toughness

( )2

( ) ( )( ) ( ) ( )( )

mat nlstrans us nls nls

us nls

K t K tCVN t CVN CVN t CVN tK K t

⎛ ⎞⎛ ⎞−= ⋅ − +⎜ ⎟⎜ ⎟⎜ ⎟−⎝ ⎠⎝ ⎠

(34)

The parameters in Equation (34) are defined above.

Final CVN Requirement The required CVN for a material as a function of thickness is determined by Equation (35).

[ ]( ) max , , ( ), ( ),code min ls nls trans usCVN t CVN CVN CVN t CVN t CVN−= (35)

Note that the required CVN for a material is a function of the yield strength and nominal thickness as shown in Figures 2 and 3 for components not subject to PWHT and components subject to PWHT, respectively.

Derivation of Impact Test Exemption Curves The impact test exemption curves in Figures 4 and 5 provide an exemption temperature based on a nominal component thickness for components not subject to PWHT and components subject to PWHT, respectively, and can be derived starting from Equation (19) and the subsequent relations shown. Solving for the temperature in this equation directly provides the expression where the exemption temperature is a function of the wall thickness, or:

( )1

0

( ) 3( ) tanh

3 27mat ys

ys

K tT t C T

σ

σ−⎡ ⎤− ⋅⎢ ⎥= ⋅ +⎢ ⎥−⎣ ⎦

(36)

The required material toughness, ( )matK t , based on the reference flaw and applied stress is given

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by Equation (8). When ( )matK t is evaluated, the fracture toughness parameter given by Equation (11) and reference stress parameter given by Equation (15) are evaluated at / 100R t = . In addition, the primary and residual stress membrane stresses in fracture toughness equations, Equations (9) and (10), and the load ratio equations, (13) and (14), are given by Equations (3), (4) and (5). For all materials covered by the four exemption curves labeled A, B, C, and D, the yield stress in equation (36) is conservatively set as 80ys ksiσ = . In addition, the cut-off limit for the lower bound of the curve is taken as -58°F or the temperature at which the thickness is equal to 0.4 in, ( )0.4t in= , whichever is less.

It should be noted that the new ASME exemption curves for Section VIII, Division 2 are independent of the strength of the steel because the yield strength in Equation (36) is set at

80ys ksiσ = . This assumption is made to simplify use of the exemption curves. If the yield dependence of the exemption were honored, then multiple set of exemption curves labeled A, B, C, and D would need to be generated for different values of the yield strength. In earlier editions of Section VIII, Division 2 and the current edition of Section VIII, Division 1, the strength independence of the exemption curves lies in assumptions regarding the material property model used when the exemption curves were developed, see WRC 528 [2], Annex A, Equations (1), (2), and (3). The main assumption that was used in the development of the exemption curves was that the fracture toughness at the indexing temperature should be proportional to the yield strength of the material and vary with temperature in a strength independent manner. This assumption can be assured through application of yield strength dependent Charpy test requirements.

Derivation of Curves for Reduction in the MDMT Without Impact Testing The reduction in the MDMT for reduction in applied stress without impact testing shown in Figures 6 and 7 for components not subject to PWHT and components subject to PWHT, respectively, is derived using Equation (36) with the following modifications. The required material toughness, ( )matK t , based on the reference flaw and applied stress is given by Equation (8). To simply address temperature reduction and remove thickness dependence when ( )matK t is evaluated, the fracture toughness parameter given by Equation (11) and reference stress parameter given by Equation (15) are evaluated at 2t in= and / 100R t = . The residual stress membrane stress in the fracture toughness Equation (10) and the load ratio Equation (14) are given by Equations (3), (4) and (5). Then, the primary membrane stress in the fracture toughness Equation(9), and the load ratio Equation (13), are given by Equation (37) where 0.24 1.0tsR≤ ≤ .

23

Pm ts ysRσ σ⎛ ⎞= ⋅⎜ ⎟

⎝ ⎠ (37)

With these assumptions, Equation (36) may be written as follows:

10

( ) 3( ) tanh

273

mat ts ysR ts

ys

K RT R C T

σ

σ

−

⎡ ⎤⎢ ⎥

− ⋅⎢ ⎥= ⋅ +⎢ ⎥⎛ ⎞⎢ ⎥−⎜ ⎟⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

(38)

Note that in Equation (38), the temperature is now a function of the stress reduction ratio, tsR ,

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rather than the wall thickness. The final equation for the temperature reduction, TΔ , in Figures 6 and 7 is given by:

( ) (1) ( )ts R R tsT R T T RΔ = − (39)

This equation is evaluated for both the non PWHT and PWHT condition, and for the two yield strengths shown in the figures. In ASME Section VIII-2, if the computed value of the tsR ratio is less than or equal to the 0.24, see Figure 6, then the MDMT may be set to -155°F and impact testing is not required unless a lower MDMT is required. This requirement essentially stipulates that if the operating stresses are equal to or less than 10% of the ultimate tensile strength, then operation for ferritic materials is permitted on the lower shelf. This rule is approximately consistent with older versions of ASME Section VIII-2 where the limit for the tsR ratio is 0.3 and for ASME Section VIII-1 where the limit for the tsR ratio is 0.35. The justification for lower shelf operation is that the stress is low enough such that brittle fracture is not likely. However, lower shelf operation may not be justified for welded components as explained in WRC 528 [2].

Toughness Requirements for Materials with a UTS Greater Than 95 ksi

Overview In the 2007 edition of VIII-2, the toughness requirements for high strength steels with an Ultimate Tensile Strength greater than 95 ksi (UTS > 95 ksi) were kept the same as in Old VIII-2. However, because of the higher allowable design stress permitted in VIII-2, in some cases the allowable design stress can be higher by a factor of 1.25 when compared to Old VIII-2, an increase in the fracture toughness is required to maintain an equivalent fracture resistance between Old VIII-2 and VIII-2. Therefore, a fracture mechanics procedure described below was performed by Rana [10] to evaluate toughness requirements for the materials with a UTS > 95 ksi with VIII-2 allowable stresses. The results of this work are documented here and indicate that the toughness requirement for VIII-2 should be increased. This increase was subsequently included in the 2008 Addenda of VIII-2.

Analysis Procedure In VIII-2, the Charpy V-Notch Lateral Expansion (CVN LE) requirement at the MDMT for materials with a UTS > 95 ksi, is a function of thickness as shown in Figure 8 from Old VIII-2 and Figure 9 for VIII-2. It should be noted that in earlier editions prior to 1987, the CVN LE requirement was set at a constant value of 15 mils at the MDMT. After 1987, the CVN LE requirement was modified to be a function of thickness in both Old VIII-2 and VIII-1. The basis for 15 mils requirement is given by Gross [11]. Two approaches were followed to establish the CVN LE required for VIII-2. Approach 1 is based on establishing the toughness in terms of CVN LE that is required to maintain the equivalent fracture resistance between Old VIII-2 and VIII-2. The second approach is based on establishing the toughness required to resist the reference flaw that is a semi-elliptical surface crack with a 0.25t depth and 1.5t length. This is the same reference flaw that has been used in establishing the toughness rules for ferritic materials with UTS ≤ 95 ksi in VIII-2.

Approach 1 for Determining CVN LE Requirements Approach 1 is based on establishing the toughness in terms of CVN LE that is required to maintain the equivalent fracture resistance between Old VIII-2 and VIII-2. For a given semi elliptical surface crack with a depth equal to a and a length equal to 2c , the applied stress intensity factor may be written as shown below for Old VIII-2 and VIII-2.

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2 2VIII VIIIaK M

Qπσ− −= (40)

2 2Old VIII Old VIIIaK M

Qπσ− −= (41)

where 1.65

1 1.464 aQc

⎛ ⎞= + ⎜ ⎟⎝ ⎠

(42)

For equal fracture resistance or an equal reference flaw size,

2 2VIII Old VIIIK K− −= (43)

From Equations (41) and (40) it follows that:

2 2

2 2

VIII VIII

Old VIII Old VIII

KK

σσ

− −

− −

= (44)

The value of 2VIIIσ − may be represented as follows.

2 2 2P SR

VIII VIII VIIIσ σ σ− − −= + (45)

where

2 min ,2.4 1.5

ysP utsVIII

σσσ −

⎡ ⎤= ⎢ ⎥

⎣ ⎦ (46)

( )2 0.15SRVIII ys subject to PWHTσ σ− = ⋅ (47)

( )2SRVIII ys not subject to PWHTσ σ− = (48)

Likewise, for Old VIII-2,

2 2 2P SR

Old VIII Old VIII Old VIIIσ σ σ− − −= + (49)

2 min ,3.0 1.5

ysP utsOld VIII

σσσ −

⎡ ⎤= ⎢ ⎥

⎣ ⎦ (50)

( )2 0.15SROld VIII ys subject to PWHTσ σ− = ⋅ (51)

( )2SROld VIII ys not subject to PWHTσ σ− = (52)

From API 579-1/ASME FFS-1, the following equation may be used to determine the relationship between fracture toughness and Charpy V-Notch impact energy.

( ) ( )0.631 9.35 ,cK CVN ksi in ft lbs= − (53)

In terms of VIII-2 and Old VIII-2:

( ) ( )1

0.632

2 ,9.35

VIIIVIII

KCVN ksi in ft lbs−

− = − (54)

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( ) ( )1

0.632

2 ,9.35

Old VIIIOld VIII

KCVN ksi in ft lbs−

− = − (55)

The Lateral Expansion (LE) in a Charpy V-Notch test may be related to the corresponding Charpy V-Notch impact energy (CVN) using the Equation (56). This equation was obtained from actual test data on SA 353, SA 517 and SA 645.

( )1.84 1.56 ,CVN LE ft lbs mils= ⋅ − − (56)

In terms of VIII-2 and Old VIII-2:

( )22

1.56,

1.84VIII

VIII

CVNLE ft lbs mils−

−

+= − (57)

( )22

1.56,

1.84Old VIII

Old VIII

CVNLE ft lbs mils−

−

+= − (58)

Combining Equations (44), (54), (55), (57) and (58), an expression for LE for VIII-2 can be derived as:

[ ] ( )2

1.56, ,

1.84VIII

BLE ksi ft lbs mils−

+= − (59)

where

( )1

0.632

22

1.84VIIIOld VIII

Old VIIIB LEσ

σ−

−−

⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

= ⋅ ⋅ (60)

Calculations were performed for SA 517, grade F and SA 353 for vessels with a wall thickness up to 3 in. The results are presented in Table 2.

Approach 2 for Determining CVN LE Requirements The second approach is based on establishing the toughness required to resist the reference flaw that is a semi-elliptical surface crack with a 0.25t depth and 1.5t length. TWI software program CW4, based on BS7910:1999 was used in the analysis. The use of the failure assessment diagram (FAD) has become the standard FFS method to evaluate crack-like flaws found in in-service components. The FAD provides a methodology to evaluate the interaction between a pure fracture mechanics approach and a pure limit load failure. The limit load condition in the FAD approach addresses the condition flow-stress controlled failure of a component. Both the API 579-1/ASME FFS-1 and BS 7910 Standards have adopted the FAD methodology for the evaluation of crack like flaws. The failure criterion for both standards is given by Equation (61).

( )( )2 61 0.14 0.3 0.7exp 0.65r r rK L L⎡ ⎤= − + −⎣ ⎦ (61)

For all practical purposes, the API 579-1/ASME FFS-1 Level 2 procedure is identical to that of BS 7910 Level 2. The two standards differ in the solutions provided for the applied stress intensity factor that is used in the rK

calculation and the reference stress used in the rL

calculation. A comparison by Rana [12] of the 579-1/ASME FFS-1 Level 2 procedure and of BS 7910 indicates that BS 7910 would result in a conservative fracture strength prediction.

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The following assumptions were used in the analysis. a) The API 579-1/ASME FFS-1 Level 2 FAD is used. b) The ratio of the vessel radius to thickness is set equal to 50. c) A surface elliptical crack on the outside surface is assumed. The crack depth is set equal to

0.25t and the crack length is set equal to 1.5t where t is the thickness of the vessel. d) The crack is located in the center of a longitudinal weld. e) The material data is:

1) SA 517, Grade F: UTS=115 ksi, YS=100 ksi, the allowable stress S=115/2.4=47.9 ksi, the stress in the hydrostatic condition is 1.43(47.9 ksi)=68.5 ksi, and

2) SA 353: UTS=100 ksi, YS=85 ksi, the allowable stress S=100/2.4=41.7 ksi, the stress in the hydrostatic condition is 1.43(41.7 ksi)=59.6 ksi.

f) The residual stresses from welding are computed by the TWI CW4 software based on the PWHT and non-PWHT conditions for a given thickness and material considering the hydrotest conditions.

Using the above assumptions and input data, the required toughness was computed using the TWI CW4 software. The resulting thickness was adjusted for plane stress conditions using Equation (62).

0.5421

11.01.0 1.4 c

c cys

KK Kt σ

⎡ ⎤⎛ ⎞⎛ ⎞⎢ ⎥= + ⎜ ⎟⎜ ⎟ ⎜ ⎟⎢ ⎥⎝ ⎠ ⎝ ⎠⎣ ⎦ (62)

The CVN is computed using Equation (53) setting 1c cK K= . The value of CVN LE is finally determine using the resulting value of CVN in Equation (56). A summary of calculations performed for SA 517, Grade F and SA 353 are shown in Table 3.

Comparison of Approaches 2 Determining CVN LE Requirements A comparison of the CVN LE determined using Approaches 1 and 2 are shown in Figure 10. The CVN LE for OLD VIII-2 and the proposed CVN LE for VIII-2 is also shown in the figure. The proposed CVN LE for VIII-2 is determined using a conservative upper bound estimate from the CVN LE values determined using Approaches 1 and 2 described above. A comparison between the CVN LE of Old VIII-2 and VIII-2 is shown in Figure 11. As anticipated, a higher fracture toughness value is required when comparing VIII-2 to Old VIII-2 because of the increase in the allowable stress now permitted in VIII-2.

CONCLUSIONS The development of the material toughness rules for carbon and low alloy steels, i.e. required CVN, impact test exemption curves, and the additional reduction in the impact test temperature based on loading condition incorporated into the new ASME Section VIII Division 2, 2007 Edition is covered in this paper. Changes to the toughness rules in earlier version of this code were necessitated by increased allowable stresses under that ASME Code and the need for corrections as discovered by a comprehensive review of the past practices and assumptions. The new toughness rules in this code were established using the fracture mechanics assessment procedures in API 579-1/ASME FFS-1 Fitness-For-Service, Part 9 and a new correlation that determines the required Charpy V-Notch impact energy (CVN) from the fracture toughness. A methodology to relate this energy to both the dynamic and static fracture toughness using the model is also described.

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REFERENCES 1. Osage, D.A., ASME Section VIII – Division 2 Criteria and Commentary, ASME PTB-1

2007, ASME, New York, N.Y. 2. Prager, M. and Osage, D.A., Development of Material Fracture Toughness Rules for the

ASME B&PV Code, Section VIII, Division 2, WRC Bulletin 528, The Welding Research Council, New York, N.Y., August, 2010.

3. Barsom, J.M. and Rolf, S.T., Fracture and Fatigue Control in Structures, Applications of Fracture Mechanics, Third Edition, ASTM, West Conshohocken, PA.

4. Selz, A., “New Toughness Rules in Section VIII, Division 1 of the ASME Boiler and Pressure Code,” 88-PVP-8, ASME, New York, N.Y., 1988.

5. Jacobs, W.S., “ASME Code Material Toughness Requirements for Low Temperature Operation, Section VIII, Division 1 and Division 2, 1998, 1999 Addenda,” PVP Vol. 407 Pressure Vessel and Piping Codes and Standards – 2000, ASME 2000, pg 23-38.

6. Scott, P.M., Anderson, T.L., Osage, D.A., and Wilkowski, G.M., A Review Of Existing Fitness-For-Service Criteria For Crack-Like Flaws, WRC Bulletin 430, The Welding Research Council, New York, N.Y., April, 1998.

7. Anderson, T.L., Fracture Mechanics – Fundamentals and Applications, 3nd Edition, CRC Press, Boca Raton, Florida, 2005.

8. Anderson, T.L. and Osage, D.A., “API 579: A Compressive Fitness-for-Service Guide”, International Journal of Pressure Vessels and Piping, 77 (2000) 953-963.

9. WRC, PVRC Recommendations on Toughness Requirements for Ferritic Steels, PVRC Ad Hoc Group on Toughness Requirements, WRC Bulletin 175, The Welding Research Council, New York, N.Y., August, 1972.

10. Rana, M.D., Private Communication 11. Gross, J.H., “Effect of Strength and Thickness on Notch Ductility,” Welding Research

Supplement, The Welding Research Council, New York, N.Y., October, 1969. 12. Rana, M.D. and Rawls, G.B., “Prediction of Fracture Stresses of High Pressure Gas

Cylinders Containing Crack-Like Flaws,” ASME, Journal of Pressure Vessel Technology, November 2007, Volume 129, Issue 4,639 (5 pages).

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NOMENCLATURE a reference flaw depth. 2c reference flaw length. C 1dK parameter. CVN Charpy V-Notch impact energy

code minCVN − minimum CVN requirement of the code.

lsCVN CVN of lower shelf.

usCVN CVN requirement for the upper shelf.

2Old VIIICVN− representative CVN for Old VIII-2

2VIIICVN− representative CVN for VIII-2

( )CVN t CVN requirement as a function of thickness. ( )nlsCVN t CVN requirement for the near lower shelf as a function of thickness.

( )transCVN t CVN requirement for the transition region as a function of thickness. ( )R tsT RΔ temperature reduction as a function of tsR .

STΔ temperature shift from dynamic to static toughness. MDMT Minimum Design Metal Temperature.

cK fracture toughness

lsK fracture toughness estimate for the lower shelf.

matK value of the material fracture toughness. ( )matK t value of the material fracture toughness as a function of thickness.

rK toughness ratio.

usK fracture toughness estimate for the upper shelf.

2Old VIIIK− representative fracture toughness for Old VIII-2

2VIIIK− representative fracture toughness for VIII-2

1cK plane strain static fracture toughness.

1dK dynamic fracture toughness. PIK stress intensity factor based on primary stresses. SRIK stress intensity factor based on secondary and residual stresses. CylinderRFK stress intensity factor

( )nlsK t fracture toughness requirement for the near lower shelf region as a function of thickness.

rL load ratio. PrL load ratio based on primary stress. SRrL load ratio based on secondary and residual stresses.

LE linear expansion from a Charpy impact test 2Old VIIILE

− representative LE for Old VIII-2

2VIIILE− representative LE for VIII-2

M stress intensity factor coefficient MDMT acronym for Minimum Design Metal Temperature

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Φ plasticity correction factor R radius of the cylinder

tsR stress ratio defined as the stress for the operating condition under consideration divided by the stress at the design minimum temperature. The stress ratio may also be defined in terms of required and actual thicknesses, and for components with pressure temperature ratings, the stress ratio is computed as the applied pressure for the condition under consideration divided by the pressure rating at the MDMT .

CylinderRFR reference stress factor.

ysσ engineering yield stress evaluated at the temperature of interest.

utsσ engineering ultimate tensile stress evaluated at the temperature of interest.

, 2m Old VIIIσ− representative membrane stress for Old VIII-2

, 2m VIIIσ− representative membrane stress for VIII-2

P

mσ primary membrane stress

, 2

P

m Old VIIIσ− primary membrane stress for Old VIII-2

, 2

P

m VIIIσ− primary membrane stress for VIII-2

SR

mσ secondary-residual membrane stress , 2

SR

m Old VIIIσ−

secondary-residual membrane stress for Old VIII-2

, 2

SR

m VIIIσ− secondary-residual membrane stress for VIII-2

t thickness of the component. T temperature.

0T 1dK parameter. ( )T t temperature or MDMT as a function of thickness

RT temperature reduction. ( )R tsT R temperature reduction as a function of tsR . (1)RT ( )R tsT R evaluated at 1tsR = .

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TABLES

Table 1 (VIII-2 Table) – Material Assignment Table Based on Exemption Curves and Notes for Figure 4 and 5

Curve Material Assignment A a) All carbon and all low alloy steel plates, structural shapes and bars not listed in Curves B, C, and D below.

b) SA-216 Grades WCB and WCC if normalized and tempered or water-quenched and tempered; SA -217 Grade WC6 if normalized and tempered or water-quenched and tempered

B a) SA-216 Grades WCA if normalized and tempered or water-quenched and tempered; Grades WCB and WCC for thicknesses not exceeding 50 mm (2 in.) if produced to a fine grain practice and water-quenched and tempered

b) SA-217 Grade WC9 if normalized and tempered c) SA-285 Grades A and B d) SA-414 Grade A e) SA-515 Grades 60 f) SA-516 Grades 65 and 70 if not normalized g) SA-662 Grade B if not normalized h) SA/EN 10028-2 Grade P355GH as-rolled i) Except for cast steels, all materials of Curve A if produced to fine grain practice and normalized which are

not listed for Curve C and D below; j) Pipe, fittings, forgings, and tubing not listed for Curves C and D below; k) Parts permitted from paragraph 3.2.8, shall be included in Curve B even when fabricated from plate that

otherwise would be assigned to a different curve. C a) SA-182 Grades F21 and F22 if normalized and tempered.

b) SA-302 Grades C and D c) SA-336 Grades F21 and F22 if normalized and tempered, or liquid quenched and tempered. d) SA-387 Grades 21 and 22 if normalized and tempered, or liquid quenched and tempered. e) SA-516 Grades 55 and 60 if not normalized f) SA-533 Grades B and C g) SA-662 Grade A h) All materials listed in (a) through (g) and in (i) for Curve B if produced to fine grain practice and normalized,

normalized and tempered, or liquid quenched and tempered as permitted in the material specification, and not listed for Curve D below

D a) SA-203 b) SA-508 Class 1 c) SA-516 if normalized d) SA-524 Classes 1 and 2 e) SA-537 Classes 1, 2, and 3 f) SA-612 if normalized; except that the increased Cb limit in the footnote of Table 1 of SA-20 is not permitted g) SA-662 if normalized h) SA-738 Grade A i) SA-738 Grade A with Cb and V deliberately added in accordance with the provisions of the material

specification, not colder than -29°C (-20°F) j) SA-738 Grade B not colder than -29°C (-20°F) k) SA/EN 10028-2 Grade P355GH if normalized [See Note d)3)]

Notes a) Castings not listed as Curve A and B shall be impact tested b) For bolting see p VIII-2, Part 3, paragraph 3.11.6. c) When a class or grade is not shown in a material assignment, all classes and grades are indicated. d) The following apply to all material assignment notes.

1) Cooling rates faster than those obtained in air, followed by tempering, as permitted by the material specification, are considered equivalent to normalizing and tempering heat treatments.

2) Fine grain practice is defined as the procedures necessary to obtain a fine austenitic grain size as described in SA-20.

3) Normalized rolling condition is not considered as being equivalent to normalizing. e) Data of Figure 4 are shown in VIII-2, Part 3, Table 3.14. f) Data of Figure 5 are shown in VIII-2, Part 3, Table 3.15. g) See VIII-2, Part 3, paragraph 3.11.2.5.a.5.ii for yield strength greater than 450 MPa (65 ksi).

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Table 2 – CVN LE Requirements Using Approach 1, Equal Fracture Resistance

Material Thickness

(in) LEOld VIII-2

(mils) LEVIII-2

(mils) SA 517, Grade F 0.31 15 17

SA 517, Grade F 1.0 15 19

SA 517, Grade F 1.25 15 19

SA 517, Grade F 2.0 19 23

SA 517, Grade F 2.2 20 26

SA 517, Grade F 3.0 25 32

SA 353 0.31 15 17

SA 353 1 15 17

SA 353 1.25 15 17

SA 353 2 19 20

SA 353 2.2 20 26

SA 353 3.0 25 32

Table 3 – CVN LE Requirements Using Approach 2, Reference Flaw Size

Material PWHT Thick (in)

Crack Depth

(in)

Crack Length

(in) KIc,VIII-2 Kc,VIII-2

CVNVIII-2

(Ft-lbs)

CVN LEVIII-2

(mils)

CVN LEold VIII-

2 (mils)

SA 517, Grade F No 0.31 0.08 0.48 51 73 15 9 15

SA 517, Grade F No 0.58 0.15 0.9 65 85 22 14 15

SA 517, Grade F Yes 1.0 0.25 1.5 63 70 21 12 15

SA 517, Grade F Yes 1.25 0.31 1.86 71 78 25 15 15

SA 517, Grade F Yes 2.2 0.55 3.3 94 104 39 22 20

SA 517, Grade F Yes 3.0 0.75 4.5 110 122 50 28 25

SA 353 No 0.31 0.08 0.48 43 59 11 7 15

SA 353 No 1.0 0.15 0.9 76 103 28 16 15

SA 353 No 1.25 0.25 1.5 84 114 33 19 15

SA 353 No 2.0 0.31 1.86 107 146 48 27 19

SA 353 Yes 2.2 0.55 3.3 85 97 33 18 20

SA 353 Yes 3.0 0.75 4.5 95 106 40 22 25

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FIGURES

Figure 1 – VIII-1, Paragraph UCS 66 Exemption Curves Are Shown. Indicated Notes Define the Materials Covered

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Maximum Nominal Thickness of Material or Weld, in

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Cv,

ft-lb

(ave

rage

of t

hree

spe

cim

ens)

10

20

30

40

50

60

70

(See Note g))

80 ksi≥

38 ksi≤

65 ksi

50 ksi

Figure 2 (VIII-2 Figure 3.3) – Charpy V-Notch Impact Test Requirements for Full-Size Specimens for Carbon and Low Alloy Steels As a Function of the Specified Minimum Yield Strength – Parts Not Subject to PWHT

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Figure 3 (VIII-2 Figure 3.4) – Charpy V-Notch Impact Test Requirements for Full-Size Specimens for Carbon and Low Alloy Steels For Selected Specified Minimum Yield Strength – Parts Subject to PWHT

Notes for Figures 2 and 3

1. Interpolation between yield strength values is permitted. 2. The minimum impact energy for one specimen shall not be less than two-thirds of the average

impact energy required for three specimens. 3. Materials produced and impact tested in accordance with SA-320, SA-333, SA-334, SA-350, SA-

352, SA-420, SA-437, SA-508 Grade 5 Class 2, SA-540 (except for materials produced under Table 2, Note 4 in the specification), SA-723, and SA-765 do not have to satisfy these energy values. Materials produced to these specifications are acceptable for use at a minimum design metal temperature not colder than the test temperature when the energy values required by the applicable specification are satisfied.

4. If the material specified minimum tensile strength is greater than or equal to 655 MPa (95 ksi), then the material toughness requirements shall be in accordance with VIII-2, Part 3, paragraph 3.11.2.1.b.2.

5. Data of Figure 8 is shown in VIII-2, Part 3, Table 3.12. 6. Data of Figure 9 is shown in VIII-2, Part 3, Table 3.13. 7. See VIII-2, Part 3, paragraph 3.11.2.1.b.1 for Charpy V-notch specimen thicknesses less than 10

mm (0.394 in.)

 

Maximum Nominal Thickness of Material or Weld, in

0 1 2 3 4 5 6 7

Cv,

ft-lb

(ave

rage

of t

hree

spe

cim

ens)

10

20

30

40

50

60

70

80 ksi≥

50 ksi

65 ksi

38 ksi≤

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Nominal Governing Thickness, in.

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Min

imum

Des

ign

Met

al T

empe

ratu

re, °

F

-60

-40

-20

0

20

40

60

80

100

120

Impact Testing Required

A

B

D

C

Figure 4 (VIII-2 Figure 3.7) – Impact Test Exemption Curves – Parts Not Subject to PWHT

Nominal Governing Thickness, in.

0 1 2 3 4

Min

imum

Des

ign

Met

al T

empe

ratu

re, °

F

-80

-60

-40

-20

0

20

40

60

80

100

120

Impact Testing Required

A

D

B

C

Figure 5 (VIII-2 Figure 3.8) – Impact Test Exemption Curves - Parts Subject to PWHT and Non-welded Part

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Temperature Reduction - TR, °F

0 20 40 60 80 100

Stre

ss R

atio

- R

ts

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

50 ksi≤

50 , 65ksi ksi> ≤

See paragraph 3.11.2.5.a.5.i when Rts is less than or equal to 0.24

Figure 6 (VIII-2 Figure 3.12) – Reduction in the MDMT without Impact Testing – Parts Not Subject to PWHT

Temperature Reduction - TR, °F

0 20 40 60 80 100

Stre

ss R

atio

- R

ts

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

See paragraph 3.11.2.5.a.5.i when Rts is less than or equal to 0.24

50 , 65ksi ksi> ≤

50 ksi≤

Figure 7 (VIII-2 Figure 3.13) – Reduction in the MDMT without Impact Testing - Parts Subject to PWHT and Non-welded Parts

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Figure 8 – Charpy V-Notch Lateral Expansion Requirements (CVN LE) in Old VIII-2 and the 2007 Edition of VIII-2.

Figure 9 (VIII-2, Figure 3.6)– Charpy V-Notch Lateral Expansion Requirements (CVN LE) in VIII-2, 2008 Addenda.

 

Maximum Nominal Thickness of Material or Weld, in.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

CV

Late

ral E

xpan

sion

, mils

10

12

14

16

18

20

22

24

26

28

30

32

34

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Figure 10 – Required CVN LE for Materials with a UTS> 95 ksi Using Approaches 1 and 2

Figure 11 – Final Required CVN LE for Materials with a UTS> 95 ksi

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ed.