Abstract:
A method of reducing stress corrosion cracking in a member made of metal, such as a pipe, comprises applying a load to the member to prevent stress corrosion cracking.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to stress corrosion cracking and, in particular, stress corrosion cracking in steel.  
         BACKGROUND OF THE INVENTION  
         [0002]    Stress corrosion cracking (SCC) deterioration of steel pipelines is a major problem for the oil and gas transmission industry. There are two types of SCC occurring in pipeline steels: high pH SCC and near neutral pH SCC.  
           [0003]    Throughout the years, many efforts have been made to identify the mechanisms responsible for SCC in pipeline steels, in developing viable techniques for inspecting pipelines for these defects, and for managing the failure risks. Little work, however, has been done to develop means of proactively slowing or arresting the growth of existing cracks in pipelines or to reduce crack growth due to daily operation.  
           [0004]    SCC in pipeline steels results from a synergistic interaction of a corrosive medium with susceptible steels under operating stress. For buried steel pipes this synergistic interaction appears to continue despite the fact that the pipelines are nominally protected against environmental deterioration by cathodic protection (CP).  
           [0005]    Only about 5% of near neutral pH SCC cracks have the potential to propagate to failure. The rest become dormant with a depth usually less than 10% of the wall thickness. A problem is to determine what causes this small fraction of cracks to grow and how this growth can be minimized.  
           [0006]    Room temperature creep is often of significant importance in structural materials. Its occurrence, for example, may be an important factor contributing to the crack growth during stress-corrosion cracking. For pipeline steels used in gas transmission or for structural materials for aerospace application, room temperature creep deformation near the crack tip may result in a time dependent crack growth.  
           [0007]    Room temperature creep is a consequence of time-dependent dislocation glide. It normally exhibits features of work hardening. The creep strain-rate has its highest value at the start of the creep, and decreases with time until an eventual exhaustion.  
           [0008]    The creep occurring under cyclic loading behaves differently compared to that under static loading. Cyclic loading can cause either an enhancement or a decrease of creep deformation referred to as cyclic creep acceleration and retardation, respectively. The former is more frequently reported than the latter in the literature. It has been shown that a virtual exhaustion of creep under static loading can be dramatically revived by cyclic loading. As a whole, however, the creep behaviour under a combination of static and cyclic loading may be influenced by many factors. Because of its complicated nature, the current understanding on room temperature creep behaviours is still insufficient, particularly under alternating static and cyclic loading conditions.  
         SUMMARY OF THE INVENTION  
         [0009]    In accordance with the instant invention, it has been determined that dynamic strain is a key factor in influencing the growth of cracks either of high pH type or near neutral pH type. A crack that lacks dynamic strain may cease to propagate or may propagate slowly. Therefore, control of the growth of cracks may be obtained by controlling the dynamic strain of the crack tip.  
           [0010]    A strain in the material is generated due to the movement of “dislocations”. The strain at the crack tip can be produced either by pre-existing mobile dislocations in the steel, which are generated during the fabrication of a pipe, or mobile dislocations which are generated by high stresses at the crack tip. If these dislocations are released in a way that constantly breaks the oxidation film, or maintains an enhanced dislocation rate at the crack tip, a crack will propagate at a high rate. On the other hand, if these mobile dislocations are exhausted or essentially exhausted, a crack may cease to grow, or may grow slowly. Therefore, crack growth may be controlled by essentially exhausting and, preferably by exhausting, the mobile dislocations at the crack tip, preferably in a short time.  
           [0011]    In accordance with the instant invention, the mobile dislocations may be exhausted through a combined cyclic and static loading process, which may be used to not only exhaust the mobile dislocations at the crack tip in a pipe, such as a steel pipe, but to also blunt a sharp crack due to the generation of significant strains at the crack tip in a short time. A blunt crack will reduce the stress concentration at the crack tip and the potential of the crack growing.  
           [0012]    In accordance with another aspect of the instant invention, the mobile dislocations may be exhausted through the application of a static load.  
           [0013]    The total treatment time may be from about to 1 to about 48 preferably from about 2 to about 24 and most preferably from about 4 to about 20 hours. The actual time required to essentially exhaust the dislocations will vary depending upon a number of factors including the strain rate, the number of cycles the metal is exposed to during the cyclic loading, the severity of the strain, and the time for which the static load is held. While the actual amount of time may be longer, a shorter time period is preferred since it accelerates the rate at which the material may be treated.  
           [0014]    Accordingly, in accordance with one aspect of the instant invention, there is provided a process for treating a metal member comprising:  
           [0015]    a) applying a series of cyclic loads to the member; and,  
           [0016]    b) subsequently apply a static load to the member.  
           [0017]    In one embodiment, from 1 to 1000 cyclic loads are applied at a stress level R of from 0.1-1 wherein 
           R=minimum stress/maximum stress for each cycle. 
           [0018]    In another embodiment, the cyclic loads are applied at a strain rate of from 10 −3  per second to 10 −6  per second.  
           [0019]    In another embodiment, the maximum stress per cyclic may be from 5% higher than the daily operating stress to 130% of the design hoop stress of the metal.  
           [0020]    In another embodiment the static load is applied for 1 to 24 hours.  
           [0021]    In another embodiment, from 40 to 1000 cyclic loads are applied at a stress level R of from 0.1-1 and the static load is applied at from 5% higher than the daily operating hoop stress to 130% of the design hoop stress of the metal. Preferably, the cyclic loads are applied at a stress level R of from 0.5-1 and more preferably at 0.7-1. Alternately, or in addition, the static load are preferably applied at from 5% higher than the daily operating hoop stress to 110% of the design hoop stress of the metal.  
           [0022]    In accordance with another aspect of the instant invention, there is also provided a process for treating a metal member comprising applying a sufficient series of cyclic loads to the member at a severity and frequency which, together with a subsequent a static load which is also applied to the member reduce stress corrosion cracking in metal.  
           [0023]    In accordance with another aspect of the instant invention, there is also provided a metal member which has been treated by applying a sufficient series of cyclic loads to the member at a severity and frequency which, together with a subsequent a static load which is also applied to the member, reduces stress corrosion cracking in the metal member.  
           [0024]    In one embodiment, the metal member has been treated by applying from 0 to 1000 cyclic loads at a stress level R of from 0.1-1 wherein 
           R=minimum stress/maximum stress for each cycle. 
           [0025]    In another embodiment, the metal member has been treated by applying the cyclic loads at a strain rate of from 10 −3  per second to 10 −6  per second.  
           [0026]    In another embodiment, the metal member has been treated by applying cyclic loads which have a maximum stress per cyclic from 5% higher than the daily operating stress to 130% of the design hoop stress of the metal member.  
           [0027]    In another embodiment, the metal member has been treated by applying a static load for 1 to 24 hours.  
           [0028]    In another embodiment, the metal member has been treated by applying the static load at from 5% higher than the daily operating stress to 130% of the design hoop stress of the metal member.  
           [0029]    In another embodiment, the metal member is a pipe.  
           [0030]    In accordance with another aspect of the instant invention, there is also provided a process for treating a metal member comprising applying a static load which is in the range of from 5% higher than the daily operating stress to 110% of the design hoop stress of the metal to the member for 1 to 24 hours.  
           [0031]    In one embodiment, the static load is applied at from 5% higher than the daily operating hoop stress to 110% of the design hoop stress of the metal.  
           [0032]    The loading pattern to control strain in accordance with the instant invention may be implemented using any technique known in the art. Generally, the loading may be applied by pressurizing the interior of the pipe, such as by sealing one end of a section of pipe and applying a gas compressor at the other, or, alternately, attaching gas compressors at the ends of a pipe. The process may be applied not only to pipes which have been used (e.g. buried pipes) with existing SCC cracks, but also to newly installed pipes for crack initiation mitigation. Further, the process may be used in conjunction with hydrostatic testing for crack detection so as to operate the process in such a way that a pipe line will not experience an increased crack growth during its subsequent use after passing the hydrostatic testing.  
           [0033]    In accordance with the broad aspect of this invention, the process is applicable to any metal to prevent stress corrosion cracking. In an alternate embodiment, it is applicable to metal, which has been fabricated into cylindrical sections for the production of, e.g., a pipe line. In accordance with another aspect of the invention, the process is applicable to a pipeline constructed from steel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    These and other advantages of the instant invention will be more fully and completely understood in connection with the following description of the preferred embodiment in which:  
         [0035]    [0035]FIG. 1( a ) is a stress-strain curve of X-52 pipeline steel;  
         [0036]    [0036]FIG. 1( b ) is a schematic illustration of the loading procedure for creep testing;  
         [0037]    [0037]FIG. 2 is a comparison of creep curves obtained under static and cyclic loading with various R-ratios;  
         [0038]    [0038]FIG. 3( a ) is a comparison of creep curves obtained under various loading conditions (a) (curve  1 , starting with cyclic loading with t h =2 s and N c =2 cycles; curve  2 , starting with static hold with t h =2 s and N c =2 cycles; curve  3 , starting with cyclic loading with t h =1200 s and N c =100 cycles.) and curve  4 ;  
         [0039]    [0039]FIG. 3( b ) is an enlarged scale version of curve  3  of FIG. 3( a );  
         [0040]    [0040]FIG. 4( a ) is a graph showing creep deformation under subsequent static loading following 1-60 cycles of cyclic loading;  
         [0041]    [0041]FIG. 4( b ) is a graph showing creep deformation under subsequent static loading following 80-7200 cycles of cyclic loading;  
         [0042]    [0042]FIG. 5 is a graph showing the dependence of cumulative creep strain (the sum of cyclic and static creep strain) on the number of cycles of the prior cyclic loading;  
         [0043]    [0043]FIG. 6 is a creep curve under static loading following initial 7200 cycles of cyclic load, showing the incubation time for the burst of creep strain;  
         [0044]    [0044]FIG. 7( a ) is a graph showing the influence of the number of cycles on the incubation time;  
         [0045]    [0045]FIG. 7( b ) is a graph showing the creep strain accumulated within 300 min under subsequent static loading;  
         [0046]    [0046]FIG. 8( a ) is a graph of creep curves produced during repeated cyclic and static loading;  
         [0047]    [0047]FIG. 8( b ) is a graph showing partial curves of FIG. 8( a ) on an enlarged scale;  
         [0048]    [0048]FIG. 9( a ) is a graph of creep curves under alternating static and cyclic loading;  
         [0049]    FIGS.  9 ( b ) and ( c ) are graphs, each of which shows a portion of the curve of FIG. 9( a ) on an enlarged scale;  
         [0050]    [0050]FIG. 10 is a graph showing cyclic stress-strain hysteresis loops for the initial few cycles of cyclic loading immediately following initial loading;  
         [0051]    [0051]FIG. 11 is a graph showing variation of cyclic plastic strain range and cyclic creep strain per cycle with the number of cycles following initial loading;  
         [0052]    [0052]FIG. 12 is a graph showing two duplicate crack growth tests showing the reduced crack growth rate as a result of creep deformation produced by a 10% over load and hold for 24 hour; and,  
         [0053]    [0053]FIG. 13 is a scanning electron microscope image showing a stress corrosion crack that became blunt and branched due to the loading process producing immediate creep deformation at the crack tip.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0054]    In accordance with the instant invention, a metal member, e.g. a pipe, is treated by applying a static load for an extended period of time and, optionally, applying a cyclic load prior to the static load.  
         [0055]    It has been determined that cyclic loading has a significant effect on subsequent static creep (i.e. when the static load is applied). The nature of the cyclic loading, as well as the nature of the subsequent static loading may vary depending on a number of factors including the type of metal which is to be treated, whether the material forms a protective passivating layer during corrosion exposure and the geometric dimension of the cracks. If the pipe has a protective passivating layer, it is preferred to perform the plastic deformation quickly so as to minimize the time that the layer is ruptured also to allow the protective layer to reform.  
         [0056]    The cyclic loading which may be used in accordance with the instant invention is defined by the number of cycles as well as the severity of the cycles. With respect to the number of cycles, the process may be conducted with from about 0 to about 1000, and preferably from about 40 to about 1000 cycles. The cycling may be achieved in less than about two days and may use a strain rate of from about 10 −3  per second up to about 10 −6  per second. Preferably, the cycling is conducted in from about 10 −4  per second to about 10 −5  per second. It will be appreciated that the treatment time will be significantly longer at a strain rate of 10 −6  compared to a strain rate of 10 −3 .  
         [0057]    The severity or the stress level of each cycle may be measured by R wherein 
         R=minimum stress/maximum stress for the cycle 
         [0058]    While a large variation in R may be utilized, (e.g. it may vary from about 0.1 to about 1), preferably R is from 0.5 to about 1 and, more preferably, greater than about 0.7.  
         [0059]    As the process is to control the strain at the crack tips, the loading process may be conducted with very mild changes to the operating pressure. An advantage of this process is that the cyclic loading would be unlikely to cause any mechanical damage to the part of the pipe free of cracks. Further, the cyclic loading will not cause the crack to propagate by a fatigue mechanism.  
         [0060]    The maximum stress per cycle is preferably higher than the daily operating hoop stress for the system. The daily operating hoop stress is the hoop stress that the metal will be exposed to during normal use. For example, in the case of a pipe, the daily operating hoop stress is the pressure that the pipe would be exposed to during daily as it is used to transport a fluid and is based on the pressure of the fluid traveling in the pipe. Preferably, the maximum stress per cycle could be from about 5% higher than the daily operating hoop stress up to about 130% of the design hoop stress, more preferably, from about 5% higher than the daily operating hoop stress up to about 110% design hoop stress and most preferably from about 5% higher than the daily operating hoop stress up to about 105% design hoop stress. The design hoop stress may be determined based on the specified minimum yield stress of the material and, typically, is set at about 80% of the minimum yield stress of the material and must be consistent with the codes and standards such as ASTM 1003-95, or CSA 2662-99 (Canadian Standard), or CFR 49 part 192 (Code/Standard of the United States)  
         [0061]    Subsequent to the cyclic loading, the pipe is subjected to static loading for an extended period of time. The static loading may be held for from about 1 to about 24 hours, preferably from about 3 to about 20 hours, more preferably from about 3 to about 10 hours and most preferably from about 3 to about 6 hours. The static load should not exceed the maximum cyclic stress, and is preferably equal to the maximum cyclic stress. It will be appreciated that about 80% of the creep deformation occurs in about the first 1-2 hours. The static loading is preferably conducted for a sufficient period of time to essentially blunt the crack tip.  
         [0062]    The static loading may be conducted from about 5% higher than the daily operating hoop stress up to about 130% of the design hoop stress, preferably from about 5% higher than the daily operating hoop stress up to about 110% of the design hoop stress and, more preferably, from about 5% higher than the daily operating hoop stress up to about the design hoop stress.  
         [0063]    If the process utilizes only static loading, then the static load is conducted from about 5% higher than the daily operating hoop stress up to about 110% the design hoop stress and, preferably, from about 5% higher than the daily operating hoop stress up to about the design hoop stress. Accordingly, in accordance with this aspect of the invention, the static loading is applied at a pressure less than that used for hydrostatic testing and may be applied for from about 1 to about 24 hours, preferably from about 3 to about 20 hours, more preferably from about 3 to about 10 hours and most preferably from about 3 to about 6 hours.  
         [0064]    If the crack tip has a protective passivating layer, then it is preferred to conduct the plastic deformation relatively quickly so that the protective layer may reform as soon as possible. In this way, the protective layer will be disrupted for the shortest possible time thereby permitting the protective layer to protect the pipe from other forms of corrosion.  
       EXAMPLES  
       [0065]    The material used in these examples was X-52 pipeline steel. The chemical composition (in wt. %) is 0.07 C, 0.8 Mn, 0.016 P, 0.27 Si, 0.28 Cu, 0.09 Ni, 0.05 Cr, 0.012 Cb, 0.019 Ti, 0.031 Al, 0.0015 Ca, balance Fe. The steel was annealed at 600° C. for 1.5 h followed by furnace cooling. The steel exhibits a microstructure with prevailing equiaxed ferrite grains (≈12 m). Standard round tension specimens, with a diameter of 6 mm and a length of 36 mm in the reduced section, were used for mechanical testing. The long dimension of the samples coincided with pipe longitudinal direction. Specimens were polished to 600 grit sand paper before mechanical testing, except that several samples were mirror-polished for slip observation on the surface.  
         [0066]    The stress-strain curve of the steel is shown in FIG. 1( a ), which exhibits significant yield point. The upper yield strength and the lower yield strength are ≈435 and 388 MPa, respectively. The yield point elongation is ≈2%. All the tests were carried out in a temperature-controlled laboratory (22° C.) on a servo-hydraulic testing machine (INSTRON 8516), which was computer-controlled with the Instron WaveMaker-Runtime software. The strain was measured using a strain gauge extensometer with a gauge length of 25 mm, which was attached to the specimen with rubber bands. All the specimens were initially loaded following the path, as described in FIG. 1( b ). For all tests, the loading strain rate was controlled to be ≈1×10−2s−1. The loading was first under displacement-control up to the end of discontinuous yielding to avoid a sudden increase in strain, and then under load-control up to the end of discontinuous yielding to avoid a sudden increase in strain, and then under load-control to reach 440 MPa. The creep stress for static creep tests and the peak stress for cyclic creep tests were equal to the flow stress reached in initial loading. This loading process (referred to as ‘initial loading’) produced an initial strain (‘pre-strain’) of ≈2.8%, which indicates that the creep tests were conducted under a stress in the work-hardening region (beyond the discontinuous yielding region).  
         [0067]    After loading to 440 MPa, further tests were conducted under one of the following three conditions: (1) under constant stress (pure static creep); (2) under cyclic stress (pure cyclic creep); and (3) under a combination of static and cyclic stress. One example of the creep process is shown in FIG. 1( b ) by the dashed line, in which the creep load is cyclic at first and static next. The constant stress for static creep and the maximum stress in cyclic creep were kept equal to 440 MPa. For cyclic creep, while the maximum stress remained constant, the stress ratio (R-ratio=minimum stress/maximum stress) varied from 0.1 to 0.90. The cyclic creep was performed using a triangle waveform and at a frequency which varied with R-ratio, keeping the loading rate constant (105 MPa s−1). The frequency was 0.13 for R=0.1, 0.24 for R=0.5, 1.2 for R=0.9, respectively. If not specified otherwise, the R-ratio used was 0.1. All the data were recorded automatically using a computer.  
         [0068]    When a crack is present in the pipe, the crack tip in the pipe is usually subjected to a stress higher than the yield strength of the material because the crack tip tends to magnify the stress. This is true even if the pipe material is exposed to a hoop stress well below its yield strength. Accordingly, in order to simulate conditions at a crack tip, Examples 1-4 were conducted using a smooth metal specimen so as to simulate the conditions at a crack tip. Accordingly, the test stress was above the yield strength of the material (the stress at which a material starts to deform plastically).  
       Example 1  
     Pure Static and Pure Cyclic Creep  
       [0069]    The pure cyclic and pure static creep curves obtained are compared in FIG. 2, where the cumulative creep deformation as measured by creep strain is plotted against creep time (the frequency was 0.13 for R=0.1, 0.24 for R=0.5, 1.2 for R=0.9, respectively). For the cyclic loading, the strain rate was 5×10 −4  and the time for cyclic loading was 500 minutes. The number of cycles was about 3900 cycles when R=0.1, 7200 cycles when R=0.5, and 36000 cycles when R=0.9. For the static loading, the loading stress was equal to the peak cyclic stress.  
         [0070]    With the maximum stress equal to the static creep stress, the cyclic creep deformation is much smaller compared to that of static creep for the steel. With identical maximum stress, the cumulative cyclic creep deformation decreases with decreasing R-ratio (i.e. increasing stress range). As the initial loading processes (independent of cyclic or static creep tests) were identical, the monotonic pre-strain for all creep tests is almost identical, excluding the influence on pre-strain of the results.  
         [0071]    It should be noted that, for cyclic creep deformation, only the maximum strain of each cycle is taken into account (i.e. is plotted in FIG. 2). (As a load cycle begins and ends at maximum stress, the maximum strain in a cycle is the strain at the end of the cycle, i.e. at the second maximum stress of the cycle.) The use of the maximum strain in each cycle makes it possible to directly compare the level of cumulative creep strain generated under pure cyclic and pure static loading, since the creep stress in static loading is equal to the maximum stress in cyclic loading. Moreover, it also allows to present the cumulative creep strain data without interruption for the cyclic loading with peak stress hold.  
       Example 2  
     Cyclic Creep with Peak-stress Hold  
       [0072]    With an inclusion of periodical hold at the maximum stress into the cyclic load scheme, the amount of creep deformation depends on the length of holding time (t h ), the number of cycles in a unit (N c ) and the sequence of static holding and cyclic load (FIG. 3 a ). The creep load for curves  1  and  2  in FIG. 3( a ) is nearly identical, both with t h =10 s and N c =2 cycles, except that the first block in creep load is cyclic for curve  1  and static for curve  2 .  
         [0073]    Without being limited by theory, based on the difference in creep deformation for these two curves, it is believed that the nature of first loading block (cyclic or static) of creep load is very important for cumulative creep deformation as measured by creep strain. In these two cases, the creep deformation generated is larger than that under pure cyclic loading, but smaller than that under pure static loading (FIG. 3 a ).  
         [0074]    An extraordinary loading case (curve)  3  is shown in FIG. 3( a ), see also FIG. 3( b ) For curve  3 , about 100 loading cycles were performed at a strain rate of 5×10 −4 . Subsequently, a static load was applied at stress equal to the peak cyclic stress for about 2 hours. A remarkable jump in creep deformation is observed in the first static hold (20 min) following the first block of cyclic loading with 100 cycles. In contrast to curves  1  and  2 , the creep deformation for curve  3  is much larger than that produced in the pure static creep (FIG. 3 a ).  
       Example 3  
     Cyclic-load-induced Burst of Creep Information Under Subsequent Static Loading  
       [0075]    For specimens that experience first cyclic loading and then static loading (see the dashed line in FIG. 1( b ) for loading procedure), independent of the number of cycles of prior cyclic loading, there is a sharp increase in creep strain in static loading following the initial cyclic loading (FIGS. 4 a  and  4   b ).  
         [0076]    The burst of creep deformation in subsequent static loading occurs normally after an incubation time (FIG. 6). The creep strain in subsequent static loading and the incubation time are dependant on the number of cycles of prior cyclic loading. As shown in FIG. 7( a ), the incubation time increases with the number of cycles performed prior to the static creep. The creep strain in subsequent static loading increases at first rapidly with the number of prior cyclic loading and reaches the maximum at ≈1000 cycles (FIG. 7 b ). It then decreases with the number of prior cyclic loading to a level that is no longer sensitive to the number of prior cyclic loading.  
         [0077]    The application of up to 40 cycles, prior to the application of pure static creep, reduces the cumulative creep deformation (sum of the cyclic and static creep strain in this case), compared to that of the pure static creep. This retardation effect becomes more significant with an increasing number of cycles of the prior cyclic load from 1 to 20 cycles, and less significant with increasing cycles up to 40 cycles (FIG. 5). When the number of cycles is more than 40 cycles, the cumulative creep strain is larger than the pure static creep strain (FIG. 5).  
       Example 4  
     Creep Behavior After Static Creep Exhaustion  
       [0078]    After the specimen has crept to exhaustion under static creep, further cyclic loading with the maximum stress equal to the static creep stress does not produce a noticeable increase in creep deformation (FIG. 8( a ) and FIG. 9( a )). For example, as shown in FIGS.  8 ( b ) and  9 ( b ), only an insignificant increase in creep deformation occurs in the initial few cycles, which can be seen on an enlarged scale in FIG. 8( b ) and FIG. 9( b ). In contrast to the cyclic loading directly following the initial loading, the cyclic loading after static creep exhaustion does not cause any increase in creep deformation during subsequent static load (FIG. 8( b ) and FIG. 9( c )).  
         [0079]    The steel in this study experiences cyclic creep retardation (i.e. the creep strain was limited). This cyclic creep retardation is, most of all, due to the cyclic hardening effect. In the present ‘pull-pull’ condition, the cyclic loading caused cyclic stress-strain hysteresis loops (FIG. 10). The change of the cyclic plastic strain range (AB in FIG. 10 for the first cycle) or the cyclic strain amplitude (half of AB), can serve as a measure of whether a material experiences cyclic hardening or softening. It can be seen from FIG. 11 that the cyclic plastic strain range decreases with increasing number of cycles for the initial few cycles, indicating a cyclic hardening effect. The hardening effect diminishes at ≈40 cycles, beyond which the change of the cyclic plasticity range is negligible.  
         [0080]    The cyclic creep strain per cycle (CD for the first cycle and DE for the second cycle as shown in FIG. 10) decreases with increasing number of cycles, particularly in the early stage of cyclic loading up to 40 cycles (FIGS. 10 and 11). It is reduced to a relatively low level as the cyclic creep approaches its exhaustion.  
         [0081]    The change in cyclic plastic strain range and in cyclic creep deformation can be traced back to the change in substructure, which is, in turn, attributed to cyclic plastic and creep deformation. The monotonic pre-strain has provided relatively large number of mobile dislocations, resulting in a significant static creep. The dislocations due to pre-strain may exist in the form of dislocation bundles, braids, incipient cell structures or their mixture. From the slip observation on the specimen surface, the pre-strain has induced parallel slip lines, indicating the occurrence of planar slip. Parallel slip lines are also formed on the surface during the static creep following the initial loading. Without being limited by theory, it is believed that the dislocation slip during the static creep following the initial loading exhibit similar characteristics as those during initial loading.  
         [0082]    The dislocation cells are effective barriers to dislocation movement. Without being limited by theory, it appears that with increasing number of cycles, the initial mobile dislocations add into the dislocation cells, resulting in a significant reduction in the number of mobile dislocations, reducing the creep rate. On the other hand, the unloading portion of the cyclic loading, especially in the first few cycles, strongly reduces the dislocation velocity (which is relatively high due to initial loading). This will also cause a smaller cumulative cyclic creep deformation if compared to the pure static creep. The two aforementioned effects should be more significant for smaller R-ratio (larger extent of unloading), but becomes less pronounced by introducing a peak stress hold to the cyclic loading scheme.  
         [0083]    Based on the results obtained in the present study, it is believed that the initial cyclic loading may affect the subsequent static creep through two competitive mechanisms. Firstly, the cyclic loading may cause hardening of the steel, reducing the dislocation mobility as mentioned above. Secondly, dislocation rearrangement will occur during subsequent static loading, releasing dislocation cells formed during prior cyclic creep deformation. The dislocation cells may be in a quasi-stable state under cyclic load, but become destabilized/collapsed under the subsequent static loading, which releases a large number of mobile dislocations available for plastic deformation.  
         [0084]    If the number of cycles is small (up to 40 cycles), the former factor may be predominant, i.e. the dislocation mobility is reduced due to cyclic hardening. The contribution from the latter factor is minor, as the number of cycles is still insufficient for the development of cell structure that will cause a significant increase in creep strain during the subsequent static loading. As a whole, the entire creep strain will be reduced.  
         [0085]    With increasing number of cycles (more than 40 cycles), cell structures may be better defined. Despite the reduction of dislocation mobility due to cyclic loading, dislocation rearrangement and the cell collapse under subsequent static load may result in a relatively large number of mobile dislocations, leading to a burst of creep deformation. Associated with the process, multiple or cross-slip will be the nature of dislocation movement. Under the circumstances, the net increase of creep strain in subsequent static loading is comparable to or much larger than the pure static creep strain. The dislocation configuration incurred by cyclic loading is dependent on the number of cycles, so is creep deformation under subsequent static load.  
         [0086]    This cyclic-load induced burst of strain under subsequent static loading needs certain incubation time. Like strain accumulated during the burst, the incubation time is dependent on the number of cycles prior to static creep loading. With increasing number of cycles of the prior cyclic loading, the dislocation cells are getting more stabilized, resulting in increased incubation time. Compared to the incubation time, the dependence of accumulated burst strain on the number of prior load cycles is more complicated. This is because the latter depends not only on the stability of the dislocation cells, but also on the characteristics of the cells, such as the size, the shape of the cells and the dislocation density of cell wall, which may change continuously during cyclic loading.  
         [0087]    No matter the static creep is performed as the first loading event (FIG. 9) or as the second on following cyclic creep deformation (FIG. 8), upon the exhaustion of static creep, further deformation cannot be produced by alternating the loading conditions, as long as the peak stress is not increased. It seems that the substructure arising from the exhausted static creep may be stable enough to prohibit further change in the subsequent loading.  
       Example 5  
     Reduced Crack Growth Rate Due to a Loading Process Producing Creep Deformation  
       [0088]    [0088]FIG. 12 is a graph of two duplicate crack growth tests showing the reduced crack growth rate as a result of creep deformation produced by an over load and hold for 24 hour.  
         [0089]    In the first portion of the graph, i.e. for the first 170 hours, the pipe was subjected to accelerated loading (at R=0.6 and cyclic frequency f=0.005 Hz) to determine the crack growth rate. Thereafter, the pipe was treated in accordance with one method of this invention. Thereafter, the pipe was again subjected to accelerated loading to determine the crack growth rate after the treatment according to the instant invention.  
         [0090]    In accordance with the method of the instant invention, the pipe specimen was subjected to an overload (an increase in load) about 10% higher than the peak cyclic stress prior to the overloading and the subsequent hold (static load) at the overload stress for 24 hours. After overload and hold, the test was resumed with the cyclic loading conditions identical to those prior to overload and hold.  
         [0091]    As shown in FIG. 12, process of the instant invention itself did not cause noticeable crack growth, while the crack growth rate after the overload and hold is more than one order of magnitude smaller than that before the overload and hold. It was observed that, when an overload of about 10% was applied without subsequent hold, the growth rate during the resumed cyclic loading after overloading was not noticeably reduced.  
         [0092]    [0092]FIG. 13 shows the crack morphology for a specimen with a 30% overload and static hold at the overload stress for 24 hours and subsequent accelerated crack growth testing at cyclic loading at R=0.6 and cyclic frequency f=0.005 Hz for 40 days. The growth rate was reduced by two orders of magnitude after the overload and the static hold. The crack appeared blunt at the position of overload and hold, and became branched during propagation under the accelerated cyclic loading after the overload and hold. This is produced by claimed loading process producing creep deformation at the crack tip.