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Heat affected zone COD testing
Theoretical and Applied Fracture Mechanics 6 (1986) 39-44 North-Holland
39
HEAT AFFECTED ZONE COD TESTING G. K O H L E R , R. W H I T F I E L D and C. C H I P P E R F I E L D Slab and Plate Products Division, BHP Steel International Group, Port Kembla, Wollongong, NS W 2500, Australia
The paper focusses on additional COD test validity criteria which are seen as necessary to ensure the accurate assessment of heat affected zone toughness. Theoretical considerations indicate that it is necessary for the COD notch to intersect the heat affected zone for at least 8% of the specimen thickness in order to ensure accurate toughness measurement.
1. Introduction The Crack Opening Displacement ( C O D ) test is being used increasingly to quantify the toughness of structural materials. Indeed, the test is m a n d a t o r y in m a n y oil and gas offshore platform specifications and is used, in these instances, to ensure that the standard weld procedures to be used in construction are consistent with good weld zone toughness. The toughness of the weld and heat affected zone is therefore closely scrutinised to ensure that the structure can ultimately tolerate the presence of relatively large defects. In conducting such a toughness assessment, for example on the Heat Affected Z o n e (HAZ), it is clearly important to ensure that a 'significant' a m o u n t of the specimen notch intersects the weld H A Z . The question is, however, " w h a t constitutes a significant amount?". Unfortunately, existing C O D testing codes of practice (e.g. BS5762: 1979) provide no guidance at all on this matter. In the case of H A Z testing, it is clearly impractical, and indeed unnecessary, to specify that the stress concentrator must intersect a specific area (e.g. the grain coarsened region) of the heat affected zone across 100% of the specimen thickness. On the other hand, and in the other extreme, if the notch intersects this region only at a point, then any localised fracture event that may occur during toughness testing will not necessarily be of sufficient magnitude to become evident as a load drop on the test record. Therefore, in case of a welded joint with, for example, a brittle heat affected zone, the C O D toughness value obtained will not only be dependent on the test validity criteria of BS5762 but also on whether or not the stress
concentrator intersects the H A Z over some m i n i m u m distance. Definitions of notch tip sampiing requirements are therefore required as additions to existing codes in order to ensure that representative toughness values are established. O n the other hand, it is important that these be realistic in order that testing costs be retained at reasonable levels. The following sections serve to outline proposals and supporting data on this important topic. Firstly, a model is developed to assist in the formulation of notch tip sampling requirements. Emphasis in this discussion is placed on testing of heat affected zones but the model and conclusions of the paper are seen as being equally relevant to the C O D testing of any material or composite composed of separate regions of potentially differing toughness.
2. The model
Consider a conventional, Crack O p e n i n g Displacement, bend test piece of width IV, crack size a, thickness B and loading span S. For modelling purposes, we will assume that the test specimen contains material of yield stress Oy and suffers a small pop-in at a low C O D value at or near the general yield load P, given by [1]: p = 1 . 5 o y B ( W - a ) 2.
(1)
The change in load at pop-in ( d P ) is related to the increase in crack length ( d a ) and can be
0167-8442/86/$3.50 © 1986, Elsevier Science Publishers B.V. (North-Holland)
40
G. Kohler et al. / Heat affected zone COD testing
calculated by differentiating eq. (1), leading to
p
2
I+ l (If-a)
'
Hence
(5)
(2)
Noting that, for the standard COD test piece, ( W - a ) is equal to the specimen thickness, B, then eq. (2) is equivalent to:
Therefore, equating eqs. (4) and (5) gives Fp
/~,~ ( r ) 2
2 p-
2
(3)
.
2\B
and, hence:
We shall refer to the "fractional load drop" which occurs at pop-in as Fp and hence: Fp 2
da B "
(4)
Now consider the situation where this load drop is caused by a small localised fracture of semi-elliptical shape measuring 2r in the specimen thickness direction and/3r (where/~ is a constant) in the specimen depth direction. This localised pop-in is equivalent to a uniform across the thickness crack extension of magnitude (da) if (by equating fracture areas): ~r 2 da.B-
2r
(6)
Equation (6) gives the relationship between the fractional load drop Fp and the fraction of the specimen thickness undergoing a pop-in event (2r/B).
Values of flV/~/4, which we redefine as constant a, depend on the assumed shape of the pop-in fracture: a value of 0.89 represents a semicircular pop-in (eq. (6) with fl = 1), while actual available data (Fig. 1) supports a similar value of 0.73. We require now to consider the minimum magnitude of load drop (i.e., Fp) which will just
2
SQR
(LOAD
DROP F R A C T I O N )
:
~ {2r/B).
o : 9.5mm : B. = 12.7mm = B. SQR LOAD DROP FRACTION. 0.9
a - 4.00
°..........'"'"'"
0.8
a
- 0.88
0.7 °.....0......"'"';~o,~ ~'°~w°
0.6
•, 0 . 7 3 ...'"°"°
o
~
~
0.5 0.4 0.3 0.2 O.i ~r-. . . . . .
0 0
i .........
O.i
i .........
0.2
i .........
0.3
I,~ . . . . . . .
0.4
WIDTH OF CRACK /
Fig. 1. Sqr. (Load Drop Fraction) = a
(2r/B).
i .........
0.5
THICKNESS
i .........
0.6 (2r/B)
i .........
0.7
I
0.8
G. Kohler et al. / Heat affected zone COD testing
permit the visual detection of a pop-in from a C O D test record. This depends principally on the experimental method used (and also on the load accuracy of the test machine) but may typically be taken as 0.5%: (this corresponds, on the standard, load versus displacement, C O D test record, to a 1 m m reduction in load on the graph paper if the general yield load occurs 20 cm above the zero load (start of test) condition). Equation (6) for this condition, and a semicircular pop-in, reduces to ~p-p= 0.89 2 r / B > ~ ,
and gives
2 r / B > 8%.
(7)
3. Model implications
.....
{b]
Notch
The other important conclusion which can be drawn from the above model is one relating to choice of weld preparations. Figure 2a shows the weld profile of the type used in pipe seam welding. H A Z C O D testing of such a weld would require notching such that the stress concentrator traverses the heat affected zone perhaps only once. In such a case and if we assume that, for example, the coarse grained H A Z extends for a distance of 1 m m from the fusion line, then the above 8% intersection criterion would imply that an H A Z C O D test piece of this type is, in principle, acceptable provided that the H A Z constitutes the region of lowest toughness and the plate and test piece thickness, B, complies with the following criterion: 1 mm
The above model and eq. (7) can therefore be used to define the notch tip sampling requirements necessary for a 'valid' H A Z test as follows: in order that any pop-in at low COD values may be detectable on the C O D test record, the fatigue crack tip should intersect the H A Z for at least 8% of the specimen thickness. It is to be noted that this minimum requirement of 8% has some inbuilt conservatism since it ignores the fact that any pop-in will generally propagate beyond the boundaries of any embrittled area (i.e., the crack extension will not generally arrest precisely at the interface between the H A Z and weld metal or base material). A lesser percentage (e.g. 7%) may therefore be acceptable. It is to be noted that the above H A Z intersection requirement is broadly insensitive to the assumed geometry of the pop-in, as indicated: rectangular pop-in ( w i d t h / d e p t h = 2) = 7.1%, semicircular pop-in ( w i d t h / d e p t h = 2) = 8.0%, elliptical pop-in ( a = 0.73, Fig. 1) = 9.7%. (a)
41
location
Fig. 2. Examples of welds which can be used for HAZ COD Testing.
-->8% B or
B ~< 12.5 mm. Thus, test in Fig. 2(a) thicknesses. require weld Fig. 2(b).
piece geometries of the type described are appropriate only for small plate H A Z tests on thicker plate would preparations of the type described in
4. Additional testing requirements In addition to the above requirement, there are several other potentially important factors which need also to be considered: (a) Firstly, the heat affected zone of most materials is complex in nature; it varies in microstructure from a grain-coarsened structure at the fusion line to a subcritical or grain refined region at some further distance from the fusion line; it is envisaged that the "8% intersection requirement" will need to refer strictly to regions of the H A Z which, in any particular case, display similar microstructural features and toughness. (b) Secondly, it is recognised that the above model is based on an assessment of specimen behaviour for the situation where the notch tip intersects one, continuous area of potentially low toughness: the question arises whether 8% intersection by the notch is still adequate if this 8% comprises several (rather than one) areas of
42
G. Kohler et al. / Heat affected zone COD testing
H A Z / n o t c h intersection: the modelling of multiple intersections is difficult to treat theoretically but practical results below indicate that this is not the case. (c) Consideration also needs to be given to dimensional restrictions which are likely to be implied by mechanistic aspects of the crack tip fracture process. For example, are notch tip intersections to be considered 'acceptable' if they occur in the plane stress, surface regions of the test piece? Experience with COD tests on ductile material [2] would suggest that intersections closer to the specimen surface than 12.5 times the C O D value (say 1.25 mm for a minimum COD of 0.1 mm) should be ignored. While such a criterion is probably relevant to a pop-in situation involving (strain-controlled) fast, ductile fracture, it is possible that it may be unconservative with regard to fracture events involving stress-controlled cleavage fracture [3]. This situation would appear to require further experimental clarification.
5. Practical use of the 8% criterion
Over the past year, the authors have been involved in the. development of a new, 345 MPa (minimum) yield steel for critical applications such as offshore oil and gas platforms [4]. In support of a detailed steel property and weldability evaluation, an extensive assessment of HAZ toughness was undertaken. The major property attributes of the steel resulted from a small addition of titanium (0.015 wt%) which forms a fine dispersion of thermally-stable, titanium nitride particles in the matrix. During welding, these particles serve to pin grain boundaries, producing a very narrow and finely-structured heat affected zone. The steel, as a consequence, exhibited lower H A Z hardnesses and improved HAZ toughness and resistance to H A Z cold cracking when compared with a conventional non-Ti steel of the same carbon equivalent [4]. H A Z COD testing of this Ti-treated steel was, however, particularly difficult due to the narrowness of the heat affected zone. The H A Z COD testing strategy adopted was to use the type of K-preparation shown in Fig. 2(b) in 55 m m plate thickness. Several welds were produced at a heat input of 5 k J / m m using an extremely adverse procedure which generated large regions of unrefined heat affected zone. This ap-
i !i~! ¸¸¸
Fig. 3. BKS-10, 5 kJ/mn, 73% C o a r s e G r a i n e d H A Z .
44
G. Kohler et al. / Heat affected zone COD testing
(HAZ) Crack Opening Displacement (COD) testing. A model has been developed which suggests that, in order to generate accurate HAZ toughness data, it is necessary for the test piece notch to intersect the grain coarsened or subcritical HAZ for at least 8% of the specimen thickness. While this criterion may be difficult to achieve at low heat inputs in titanium-treated steels, it does appear to be readily applicable to a wide variety of steels at high heat inputs ( > 3 kJ/mm), where toughness is of primary concern. An example has been given where COD tests satisfactorily passed the 8% intersection and minimum HAZ COD requirements. The underlying implications of the paper are that in the case of HAZ testing, COD testing codes should include additional requirements for:
(a) post-test metallographic analysis of COD specimens to assess the notch placement accuracy, (b) notch intersection requirements, (c) weld geometries suitable for COD testing. All these aspects have been quantitatively discussed in the paper although several require further experimental verification. References [1] A.P. Green and B.B. Hundy, Physics of Solids 4, 128-144 (1956). [2] C.G. Chipperfield, Internat. J. Fracture 13, 847-860 (1977). [31 J.F. Knott, Introduction to Fracture Mechanics, Butterworths, London (1973). [4] C.G. Chipperfield and J.G. Williams, "Properties and Weldability of N b / T i Offshore Platform Steels", HSLA 85 Conference, Beijing, China 1985.
39
HEAT AFFECTED ZONE COD TESTING G. K O H L E R , R. W H I T F I E L D and C. C H I P P E R F I E L D Slab and Plate Products Division, BHP Steel International Group, Port Kembla, Wollongong, NS W 2500, Australia
The paper focusses on additional COD test validity criteria which are seen as necessary to ensure the accurate assessment of heat affected zone toughness. Theoretical considerations indicate that it is necessary for the COD notch to intersect the heat affected zone for at least 8% of the specimen thickness in order to ensure accurate toughness measurement.
1. Introduction The Crack Opening Displacement ( C O D ) test is being used increasingly to quantify the toughness of structural materials. Indeed, the test is m a n d a t o r y in m a n y oil and gas offshore platform specifications and is used, in these instances, to ensure that the standard weld procedures to be used in construction are consistent with good weld zone toughness. The toughness of the weld and heat affected zone is therefore closely scrutinised to ensure that the structure can ultimately tolerate the presence of relatively large defects. In conducting such a toughness assessment, for example on the Heat Affected Z o n e (HAZ), it is clearly important to ensure that a 'significant' a m o u n t of the specimen notch intersects the weld H A Z . The question is, however, " w h a t constitutes a significant amount?". Unfortunately, existing C O D testing codes of practice (e.g. BS5762: 1979) provide no guidance at all on this matter. In the case of H A Z testing, it is clearly impractical, and indeed unnecessary, to specify that the stress concentrator must intersect a specific area (e.g. the grain coarsened region) of the heat affected zone across 100% of the specimen thickness. On the other hand, and in the other extreme, if the notch intersects this region only at a point, then any localised fracture event that may occur during toughness testing will not necessarily be of sufficient magnitude to become evident as a load drop on the test record. Therefore, in case of a welded joint with, for example, a brittle heat affected zone, the C O D toughness value obtained will not only be dependent on the test validity criteria of BS5762 but also on whether or not the stress
concentrator intersects the H A Z over some m i n i m u m distance. Definitions of notch tip sampiing requirements are therefore required as additions to existing codes in order to ensure that representative toughness values are established. O n the other hand, it is important that these be realistic in order that testing costs be retained at reasonable levels. The following sections serve to outline proposals and supporting data on this important topic. Firstly, a model is developed to assist in the formulation of notch tip sampling requirements. Emphasis in this discussion is placed on testing of heat affected zones but the model and conclusions of the paper are seen as being equally relevant to the C O D testing of any material or composite composed of separate regions of potentially differing toughness.
2. The model
Consider a conventional, Crack O p e n i n g Displacement, bend test piece of width IV, crack size a, thickness B and loading span S. For modelling purposes, we will assume that the test specimen contains material of yield stress Oy and suffers a small pop-in at a low C O D value at or near the general yield load P, given by [1]: p = 1 . 5 o y B ( W - a ) 2.
(1)
The change in load at pop-in ( d P ) is related to the increase in crack length ( d a ) and can be
0167-8442/86/$3.50 © 1986, Elsevier Science Publishers B.V. (North-Holland)
40
G. Kohler et al. / Heat affected zone COD testing
calculated by differentiating eq. (1), leading to
p
2
I+ l (If-a)
'
Hence
(5)
(2)
Noting that, for the standard COD test piece, ( W - a ) is equal to the specimen thickness, B, then eq. (2) is equivalent to:
Therefore, equating eqs. (4) and (5) gives Fp
/~,~ ( r ) 2
2 p-
2
(3)
.
2\B
and, hence:
We shall refer to the "fractional load drop" which occurs at pop-in as Fp and hence: Fp 2
da B "
(4)
Now consider the situation where this load drop is caused by a small localised fracture of semi-elliptical shape measuring 2r in the specimen thickness direction and/3r (where/~ is a constant) in the specimen depth direction. This localised pop-in is equivalent to a uniform across the thickness crack extension of magnitude (da) if (by equating fracture areas): ~r 2 da.B-
2r
(6)
Equation (6) gives the relationship between the fractional load drop Fp and the fraction of the specimen thickness undergoing a pop-in event (2r/B).
Values of flV/~/4, which we redefine as constant a, depend on the assumed shape of the pop-in fracture: a value of 0.89 represents a semicircular pop-in (eq. (6) with fl = 1), while actual available data (Fig. 1) supports a similar value of 0.73. We require now to consider the minimum magnitude of load drop (i.e., Fp) which will just
2
SQR
(LOAD
DROP F R A C T I O N )
:
~ {2r/B).
o : 9.5mm : B. = 12.7mm = B. SQR LOAD DROP FRACTION. 0.9
a - 4.00
°..........'"'"'"
0.8
a
- 0.88
0.7 °.....0......"'"';~o,~ ~'°~w°
0.6
•, 0 . 7 3 ...'"°"°
o
~
~
0.5 0.4 0.3 0.2 O.i ~r-. . . . . .
0 0
i .........
O.i
i .........
0.2
i .........
0.3
I,~ . . . . . . .
0.4
WIDTH OF CRACK /
Fig. 1. Sqr. (Load Drop Fraction) = a
(2r/B).
i .........
0.5
THICKNESS
i .........
0.6 (2r/B)
i .........
0.7
I
0.8
G. Kohler et al. / Heat affected zone COD testing
permit the visual detection of a pop-in from a C O D test record. This depends principally on the experimental method used (and also on the load accuracy of the test machine) but may typically be taken as 0.5%: (this corresponds, on the standard, load versus displacement, C O D test record, to a 1 m m reduction in load on the graph paper if the general yield load occurs 20 cm above the zero load (start of test) condition). Equation (6) for this condition, and a semicircular pop-in, reduces to ~p-p= 0.89 2 r / B > ~ ,
and gives
2 r / B > 8%.
(7)
3. Model implications
.....
{b]
Notch
The other important conclusion which can be drawn from the above model is one relating to choice of weld preparations. Figure 2a shows the weld profile of the type used in pipe seam welding. H A Z C O D testing of such a weld would require notching such that the stress concentrator traverses the heat affected zone perhaps only once. In such a case and if we assume that, for example, the coarse grained H A Z extends for a distance of 1 m m from the fusion line, then the above 8% intersection criterion would imply that an H A Z C O D test piece of this type is, in principle, acceptable provided that the H A Z constitutes the region of lowest toughness and the plate and test piece thickness, B, complies with the following criterion: 1 mm
The above model and eq. (7) can therefore be used to define the notch tip sampling requirements necessary for a 'valid' H A Z test as follows: in order that any pop-in at low COD values may be detectable on the C O D test record, the fatigue crack tip should intersect the H A Z for at least 8% of the specimen thickness. It is to be noted that this minimum requirement of 8% has some inbuilt conservatism since it ignores the fact that any pop-in will generally propagate beyond the boundaries of any embrittled area (i.e., the crack extension will not generally arrest precisely at the interface between the H A Z and weld metal or base material). A lesser percentage (e.g. 7%) may therefore be acceptable. It is to be noted that the above H A Z intersection requirement is broadly insensitive to the assumed geometry of the pop-in, as indicated: rectangular pop-in ( w i d t h / d e p t h = 2) = 7.1%, semicircular pop-in ( w i d t h / d e p t h = 2) = 8.0%, elliptical pop-in ( a = 0.73, Fig. 1) = 9.7%. (a)
41
location
Fig. 2. Examples of welds which can be used for HAZ COD Testing.
-->8% B or
B ~< 12.5 mm. Thus, test in Fig. 2(a) thicknesses. require weld Fig. 2(b).
piece geometries of the type described are appropriate only for small plate H A Z tests on thicker plate would preparations of the type described in
4. Additional testing requirements In addition to the above requirement, there are several other potentially important factors which need also to be considered: (a) Firstly, the heat affected zone of most materials is complex in nature; it varies in microstructure from a grain-coarsened structure at the fusion line to a subcritical or grain refined region at some further distance from the fusion line; it is envisaged that the "8% intersection requirement" will need to refer strictly to regions of the H A Z which, in any particular case, display similar microstructural features and toughness. (b) Secondly, it is recognised that the above model is based on an assessment of specimen behaviour for the situation where the notch tip intersects one, continuous area of potentially low toughness: the question arises whether 8% intersection by the notch is still adequate if this 8% comprises several (rather than one) areas of
42
G. Kohler et al. / Heat affected zone COD testing
H A Z / n o t c h intersection: the modelling of multiple intersections is difficult to treat theoretically but practical results below indicate that this is not the case. (c) Consideration also needs to be given to dimensional restrictions which are likely to be implied by mechanistic aspects of the crack tip fracture process. For example, are notch tip intersections to be considered 'acceptable' if they occur in the plane stress, surface regions of the test piece? Experience with COD tests on ductile material [2] would suggest that intersections closer to the specimen surface than 12.5 times the C O D value (say 1.25 mm for a minimum COD of 0.1 mm) should be ignored. While such a criterion is probably relevant to a pop-in situation involving (strain-controlled) fast, ductile fracture, it is possible that it may be unconservative with regard to fracture events involving stress-controlled cleavage fracture [3]. This situation would appear to require further experimental clarification.
5. Practical use of the 8% criterion
Over the past year, the authors have been involved in the. development of a new, 345 MPa (minimum) yield steel for critical applications such as offshore oil and gas platforms [4]. In support of a detailed steel property and weldability evaluation, an extensive assessment of HAZ toughness was undertaken. The major property attributes of the steel resulted from a small addition of titanium (0.015 wt%) which forms a fine dispersion of thermally-stable, titanium nitride particles in the matrix. During welding, these particles serve to pin grain boundaries, producing a very narrow and finely-structured heat affected zone. The steel, as a consequence, exhibited lower H A Z hardnesses and improved HAZ toughness and resistance to H A Z cold cracking when compared with a conventional non-Ti steel of the same carbon equivalent [4]. H A Z COD testing of this Ti-treated steel was, however, particularly difficult due to the narrowness of the heat affected zone. The H A Z COD testing strategy adopted was to use the type of K-preparation shown in Fig. 2(b) in 55 m m plate thickness. Several welds were produced at a heat input of 5 k J / m m using an extremely adverse procedure which generated large regions of unrefined heat affected zone. This ap-
i !i~! ¸¸¸
Fig. 3. BKS-10, 5 kJ/mn, 73% C o a r s e G r a i n e d H A Z .
44
G. Kohler et al. / Heat affected zone COD testing
(HAZ) Crack Opening Displacement (COD) testing. A model has been developed which suggests that, in order to generate accurate HAZ toughness data, it is necessary for the test piece notch to intersect the grain coarsened or subcritical HAZ for at least 8% of the specimen thickness. While this criterion may be difficult to achieve at low heat inputs in titanium-treated steels, it does appear to be readily applicable to a wide variety of steels at high heat inputs ( > 3 kJ/mm), where toughness is of primary concern. An example has been given where COD tests satisfactorily passed the 8% intersection and minimum HAZ COD requirements. The underlying implications of the paper are that in the case of HAZ testing, COD testing codes should include additional requirements for:
(a) post-test metallographic analysis of COD specimens to assess the notch placement accuracy, (b) notch intersection requirements, (c) weld geometries suitable for COD testing. All these aspects have been quantitatively discussed in the paper although several require further experimental verification. References [1] A.P. Green and B.B. Hundy, Physics of Solids 4, 128-144 (1956). [2] C.G. Chipperfield, Internat. J. Fracture 13, 847-860 (1977). [31 J.F. Knott, Introduction to Fracture Mechanics, Butterworths, London (1973). [4] C.G. Chipperfield and J.G. Williams, "Properties and Weldability of N b / T i Offshore Platform Steels", HSLA 85 Conference, Beijing, China 1985.