Flat Header Endcaps

VGB PowerTech 7/2004

Failure incidents on flat header endcaps with stress relief groove and test measures derived from these

Abstract

Damage Results at Flat Header End- caps of Headers and High-pressure

Resulting Testing Measures

In the recent past some spectaculars failures of flat endcaps of high pressure headers have occurred. The failure caused by the conjunc-tion of exceeding stresses and reduced me-chanical properties (creep rupture strength) of the heat affected zone (HAZ) of the flat end to cylindrical shell connection. In some cases it could be verified that an inadequate mi-crostructure of the endcap material caused by irregular heat treatment is responsible for the failure. Also service conditions like tempera-ture or number of cycles has taken into ac-count. The failure investigation shows creep damage in the HAZ as the prior mechanism. The occurred failures are potentially danger-ous due to the fact that the creation of the cracks occured inside the material of the HAZ in ca. 1/3 of the wall thickness and final failure take place as “break before leak” as an abridg-ment of the endcap. Up to now in the VGB-Commitees there were discussed overall five occurrences. In 1998 a VGB Working Group developed a testing concept based on the in-vestigation results and give an advice to al VGB Members. Presently the experience from the testing of all relevant endcaps will be collected by VGB and for further testing the existing concept should be modified.

Preface

Several failures of flat header endcaps with stress relief groove have occurred in the last 25 years which have led to a number of measures within the scope of recurring testing and the maintenance of boiler components subject to creep stress. After a flat endcap had been torn off a high pressure outlet header in the power station Niederaußem in the year 1998 a VGB working group in conjunction with TÜV Rheinland developed a testing concept which was implemented in the member companies in the years that followed. This contribution

Autor Dr.-Ing. G. Lüdenbach VGB PowerTech e.V., Essen/Germany

reports on past failure incidents, describes the failure mechanisms in more detail and introduces the test recommendation made to

the VGB members by the above mentioned working group in the year 1998 and the experiences gained with this testing practice to date.

The failed components are so-called flat endcaps and flat header endcaps with stress relief groove according to TRD 305 [1], both with and without cut-out, which, in the present cases, is located in the centre of the endcap (inspection socket) ( F i g u r e 1 ) .

This type of components is generally made from forged semi-finished product. Under certain conditions rolled feedstock (round plate blanks) can also be employed. The acceptance tests of the feedstock are conducted according to the respective regulatory works (TRD, AD, DIN, EN, VdTÜV material sheets) which under certain conditions are expanded on the part of the ordering party (operating company) through additional requirements.

The components are designed in accordance with TRD 305. The position of the weld seam immediately in the plane of the endcap inner surface is the most favourable design version with regard to manufacturing expenditure and material consumption. The circumferential stress relief groove immediately next to the machined welding edge ensures

— improved weld seam quality, especially in the root area,

— testability of the weld seam, — lower marginal moments in the weld

seam area because of the lower wall thicknesses in the groove base due to the stress relief groove,

— avoidance of excessive stress peaks through the even transition from the endcap to the cylinder

Testing of the finished weld joint is conducted in accordance with AD-HP 5/3 [2] at 100% with the help of ultrasonic testing through both vertical and inclined ultrasound application.

Failure History

Arzberg power station (1991) In the Arzberg power station of the then EVO Bayreuth a welded lap-joint endcap of the superheated steam outlet header of boiler 7 was torn off explosion-like in 1991. The technical component and operating data are

shown in Table 1. The failure investigation was conducted by TÜV Bayern (Bavaria) by way of detailed analysis of the operating data that prevailed at the time the failure occurred and examination of the torn-off endcap material [3].

A deflection (arch) of 12 mm was present on the endcap after the failure incident. Based on the boiler design in this case the possibility of uneven distribution of the fire in the combustion chamber was initially taken into consideration which could result in uneven distribution of the superheated steam temperature in the two superheater 5 outlet headers and consequently the superheat steam outlet header. From extensive temperature measurements it was possible to reconstruct that steam was apparently conducted with the highest temperature from the boiler centre in front of the failed endcap because of the connection pipes from the two superheater 5 outlet headers having been arranged crosswise in each case. The calculated temperature of the torn-off endcap is obtained from the permissible operating temperature of 535 °C plus the temperature allowance of 15 °C according to TRD 300. This corresponds to the highest component temperature measured at low load of 555 °C which was reconstructed from the records.

Fig.1 Flat header endcap with stress

relief groove and inspection socket according to TRD 305 with schematic representation of the deformation conditions (dashed) and incipient crack formations (red) which occur when subjected to internal pressure loading.

Flat Header Endcaps

VGB PowerTech 7/2004

Table 1: Compilation of the failure incidents on flat header endcaps discussed within the VGB technical committees.

Arzberg Hüls AG Nijmegen Weisweiler West Burton

Failure occurrence: 7 October 1991 2 September 1994 14 June 1997 6 October 1998 28 December 1998

Designation: Outlet header HP distributor Outlet header Outlet header Outlet header

Material: Header endcap X20 1.4988 X20/10CrMo X20 P91

Pressure (design) in bar: 215 318,8 212,5 195 160

Temperature (design) in °°C: 535 585 516 530 568

Operating time in h: 83000 7732 115000 h 260000 36526

Temperature in °°C: 555 – 540 — —

Header äØ = 250 x 52 äØ = 290 x 60 äØ = 406 x 36 äØ = 310 x 40 äØ = 242 x 33

Endcap äØ = 366 x 64 äØ = 290 x 40 äØ = 406 x 90 äØ = 310 x 50 äØ = 242 x 60

This exceeding of the permissible operating temperature was also confirmed through an assessment of the steam-side oxide layer thicknesses with the help of Tammann’s scale law. Despite this, excessive temperature of this magnitude does not explain such an early time of failure.

The technological material analysis of the torn-off endcap consisted of a fractographic assessment of the fracture surface across the fracture edge. The fracture surface was characterised by a concentrically running relatively highly oxidised area near the header internal surface and a light-grey few millimetres wide edge area representing the residual force fracture surface. There were no signs of high-frequency fatigue failure . With the help of the metallographic analysis it was possible to clearly trace the oxidised fracture surface area back to earlier creep damage. The course of the fracture is orientated in the fine-grained heat-affected zone (HAZ), merely the residual force fracture runs through the welding stock. With regard to the structure of the endcap it was not possible to verify even the beginnings of the typical martensitic lattice structure otherwise typical for the material X20CrMoV 12-1. The highly spheroidised bainitic-ferritic structure present was not caused through the operating load with a correct structure condition that was present originally, but rather points to irregular heat treatment of the forged part. It is assumed that the forged part was either heat-treated at an inadequate austenising temperature below 950 °C or quenched with an inadequate quenching speed after the austenising heat treatment. The cause of the crack already postulated from the different oxide cover of the fracture surface pointing from the inner surface to the outside was confirmed through a continuous increase of the creep pore concentration from inside to the outside.

Irregular heat treatment of the forged part is responsible for the verified atypical structure condition of the endcap which is the cause of the failure. The instances of locally exceeded permissible operating temperature also discovered during the failure investigation can merely be assessed as having favoured

the failure.

As part of the failure analysis additional flat endcaps of the material X20CrMoV 12-1 were also checked with the help of the surface structure examination (paint impression technology) and the itinerant hardness test. During this process additional components were found where low hardness values or crystal structures differing from the normal condition were discovered.

Following this failure incident tests on flat header endcaps (however only on components made of the material X20CrMoV 12-1) were conducted initially in the area of control of the TÜV Bavaria/Saxony, later on throughout the territory of the Federal Republic. The failure committee of the German Boiler Committee (DDA) recommended the following procedure having thoroughly studied the facts:

— Recalculation of header endcaps affected,

— Establishment of any signs of endcap arching,

— Surface structure analysis for verifying proper structural condition,

— Ultrasonic testing of the connection seam for incipient cracks in the endcap-side HAZ,

— Replacement of the endcaps affected by arching or incipient cracks.

The failure that occurred gave rise to attaching greater importance to the properly conducted heat treatment of the component even as part of the acceptance. As a rule this is performed on the finished component through checking the amount and duration of the heat treatment temperature and the type of quenching. Surface structure analysis (foil impression) on the finished component with appropriate proof can be demanded from product manufacturers through additional semi-finished product requirements. The accompanying circumstance favouring the failure of a possible local occurrence of higher thermal loading of certain components that can be brought about

through various contaminations or unfavourable designs should be monitored through continuous temperature measurements and included in the fatigue calculation.

The DDA did not issue any recommendation to do without the flat header endcaps with stress relief groove in future.

Hüls AG (1994)

On the 2nd September 1994 the welded lap-joint endcap of an HHD distributor in block 3 of power station II of Hüls AG in the Marl plant was torn off explosion-like. Failure analysis was conducted by the department material technology of Hüls AG [4]. The component data and operating parameters are shown in Table 1.

The live steam line affected was manufactured from highly heat-resistant austenitic steel 1.4988 (X 8CrNiMoVNb 16-13) and at the time the failure occurred roughly 220,000 hours (approximately 28 years) in operation. One year previously, “Brettschneider closures” of the two HHD distributors that had developed leaks were replaced with flat header endcaps with stress relief groove. The failure occurred on one of these two endcaps after only one year in operation.

The ultrasonic test of the connection seams between endcaps and cylinder did not yield any impermissible findings after welding–on. The arching amounted to 1 and 5 mm, which is due to the clearly more pronounced shrinkage of the austenitic welding stock.

To clarify the cause of the failure both the torn-off as well as the prophylactically removed second endcap were subjected to technological material analysis. Measurement of the endcaps revealed an arching of the torn-off endcap of 12 (originally 5 mm) as well as 6 to 7 mm (originally 1 mm) of the intact endcap prior to cutting off. The fracture surface analysis of the failure d component clearly shows that the crack originated from the weld seam root and evenly grew in parallel with the endcap-side fusion line almost over the entire circumference to the external surface of the component.

,

Flat Header Endcaps

VGB PowerTech 7/2004

The ultrasonic test performed before the cutting-off of the endcap still intact yielded impermissible displays next to the weld seam in the area of the internal surface. With the help of metallographic analysis in the area of the US displays incipient cracks with a length of 12 (endcap side) and 2.5 mm (housing side) were discovered on both sides of the weld seam.

The metallographic analysis of the torn-off endcap vertically to the fracture edge revealed an intercrystalline cause of the crack and, in the area of the crack origin, an oxide layer thickness that corresponded to that of the endcap surface in contact with the medium. This points to the formation of an incipient crack at a very early time after commissioning. The structure is characterised by finely dispersed precipitations in the interior of the grain and the formation of inter-metallic phases at the grain boundaries. A two-parameter (bimodal) grain size distribution was discovered which is typical for an incompletely recrystallised structure, which is caused for instance through an inadequate degree of transformation. The additionally established cavities predominantly stretched in one direction with a diameter of approximately 100 µm also point to insufficient forging-through of the semi-finished product. However the cavities have no direct influence on the failure incident. In comparable failure analyses stress conditions had already been established with the help of finite element calculation which yielded the highest stress values for the area of the transition from the cylindrical part to the stress relief groove, while the stress level is clearly higher in the endcap-side HAZ than in the housing-side HAZ situated opposite [6]. These stresses resulting from pure internal pressure loading have been superimposed by the internal welding stresses in the present failure case, which, because of the relatively large welded wall thickness and the high coefficient of expansion of the austenitic feedstock, happen to be significantly higher than with comparable welded designs of ferritic steels.

In the heat-affected zone (HAZ) the heat introduced in the present steel through the welding process has resulted in the dissolution of the niobite and vanadium carbides (or carbonitrides) which resulted in finely dispersed precipitation in the grain interior upon subsequent quenching. Owing to the hardening caused by this, stresses within the grains can be increasingly reduced to a lesser degree by way of plastic deformations. The shift of these processes to the grain boundaries, the deformation capacity of which is also reduced through the precipitation of intermetallic phases,

ultimately led to the established intercrystalline fracture occurrence.

Remedial measures in the concrete failure case: design change (lap-joint end and semi-spherical endcap) as well as utilisation of the largely precipitation-free steel X3CrNiMoN 17-13 (Material No.1.4910).

Power station Nijmegen (1997 )

A header endcap made of material 10 CrMo 9-10 of a superheater outlet header made of material X20 was torn off on 14 June 1997 in the Dutch power station Nijmegen. The component and operating parameters are represented in Table 1.

As part of the failure analysis by KEMA [5] it was discovered that the crack was running along the endcap-side HAZ, having the characteristic features of creep failure . It was determined that the cause of the failure was due to excessive operating temperature of around 550 °C so that the stresses resulting from the TRD calculation already correspond to the creep strength (lower scatter band) of the material 10 CrMo 9-10, without the local stress increase due to the design in the area of the welding joint having been taken into account. With regard to the failure that occurred the excessive stress in the area of the connection joint must be seen in conjunction with the operating temperature which was too high for the material 10 CrMo 9-10.

The remedial measure implemented as part of the rehabilitation consisted in the use of material X20CrMoV 12-1 for the new header endcaps while the previous design was retained.

Power station West Burton (1998)

A flat endcap tear-off also occurred on one of a total of four superheater outlet headers in the British power station West Burton [7] on 28 December 1998. The material used here was the modified 9% material F 91 (X 10CrMoVNb 9-10). The component and operating data is reflected in Table 1.

The torn-off endcap had been subjected to an ultrasonic test 8,663 operating hours previously, when no impermissible findings were made. The failure was taken as an opportunity to analyse the three remaining endcaps of the block concerned and the four endcaps of another block with the help of ultrasonic testing, during which additional damages were discovered. Recalculating the flat endcap by means of the finite element method showed that the stresses on the internal surface in the connection area are roughly two to three times greater than the permissible design stresses, which however is not in conflict with the British Standard

(BS 1113).

Through metallographic examination of the failure d component creep damage, occurring in the area of the fine grain zone of the altogether 2.3 mm wide HAZ, was verified as failure mechanism. Remarkable is the low hardness of 150 to 160 HV in the failed material area. In comparison with this, hardness values of approximately 190 HV were measured in the undamaged HAZ on the header side. Comparative hardness measurements on the header and endcap sides show hardness which is up to 35 HV less in the endcap side HAZ and a hardness of the endcap which is 13 HV less compared with the header. The lower strength in the HAZ in conjunction with the design-related excessive stress in the connection area has therefore led to the failure.

As part of the rehabilitation endcaps were employed where the weld seam was placed at an appropriate distance from the “inner endcap surface”.

Power station Weisw ei ler (1998)

A tear-off of a flat endcap on one of the four HP outlet headers of Block E occurred on 6 October 1998. The component and operating data is compiled in Table 1.

In the follow up to the “Arzberg failure” in the years 1992/1993 in accordance with the decree of 27 May 1992 all endcaps were subjected to US testing of the endcap connection seam according to the Test Instruction S 001 (status 1992), surface structure analysis of the weld seam between inspection socket and endcap by means of the magnetic particle testing method (MT) and measurement of the arching. None of the test results yielded any impermissible findings.

Failure analysis was performed by TÜV Rheinland [7] on the failed component itself and on two additional header endcaps which, during the tests performed after the failure incident, yielded impermissible findings. The surface crack test by means of magnetic particle testing (MT) of the welding joint between inspection socket and endcap revealed all-round and interrupted crack indications on the entire circumference in the endcap-side seam transition. During the ultrasonic test of the endcap connection seam roughly 15 and 150 mm long indications, established according to the half-value method, were obtained in the root area. The arching of the two endcaps amounted to approximately 1.5 to 2 mm. The fourth endcap which was also tested had only been renewed in the year 1994 and did not yield any impermissible findings.

Flat Header Endcaps

VGB PowerTech 7/2004

The ultrasonic tests conducted on site were repeated in the laboratory under optimum test conditions to determine whether the previously applied test parameters had to be possibly optimised and improved. The results obtained on site were confirmed both with regard to establishing the indications subject to mandatory registration and with regard to the description of the defect expansion (indication length and expansion in thickness direction). It further transpired that in the areas where no impermissible indications were observed initially, indications could be established in the endcap-side HAZ on the entire circumference with higher sensitivity. Although these were clearly below the registration limit, they nevertheless showed a clear reflected pulse pattern.

The result of the fracture surface analysis of the torn-off endcap largely corresponded to that of the failure incidents already discussed. The concentric area of the old incipient crack on the “inside” of the endcap noticeable through the increased oxide cover is clearly distinguished from the light-grey forced fracture without a clear separating line being ascertainable between these areas (Figure 2).

The bright finely crystalline fracture surface in 6 o’clock position is the forced fracture surface created during the tear-off. The dark-grey area in 12 o’clock (up to 6 o’clock) position is the fracture surface created through the operating stress.

The metallographic analysis of the three endcaps by way of a large number of polished samples taken from throughout the circumference yielded the following results (Figure 3):

— The cracks are clearly due to creep damage.

— Creep pores are exclusively located in the heat-affected zone (HAZ) and not in the base material or the welding stock.

— The cracks run along the endcap-side HAZ from “inside” to “outside”.

—The most severe damage in form of micropores, chains of pores and microcracks is present at a distance of approximately 1/3 of the weld flank width from the inner edge from where it spreads to the “inside” and to the “outside”.

— The structure of the header as well as that of the endcap consists of annealed martensite, showing the lattice structure typical for the material X20CrMoV 12-1.

— The structure in the area of the HAZ subject to creep damage was clearly spheroidised, no longer showing any lattice structure.

From the structure it is possible to conclude proper execution of the heat treatment of the forged part. The extent to which the heat re-treatment performed after welding corresponded to the specifications can no

longer be reconstructed based on the light-microscopic structural examination.

Compared with the previously presented failures neither material-related nor operational factors such as defective heat treatment or excessive temperatures were mentioned as having triggered the failure in the analysis report of TÜV Rheinland. One cause of this failure incident may be found in special operational cyclic loads for instance condensate incurred not optimally discharged.

In all failures presented however the unfavourable design of the endcap with the resultant increased stresses is responsible for the extent of the damage (fracture prior to leak) and the restricted testability. In the cases, which can never be entirely excluded, where inadequate material properties due to the manufacture are present or operational loads occur which are not in keeping with the design, the type of flat endcaps with stress relief groove constitutes a particular hazard potential.

Figure 2: Fracture surface of the torn-

off endcap [7].

Recommendations for recurring test measures

As a reaction to the failures which occurred in the power station Weisweiler a test concept for the recurring testing of flat header endcaps in operation was developed by a VGB working group with the cooperation of TÜV Rheinland.

Although flat endcap failures that cannot be traced back to creep damage of the type and manner explained here but which are largely due for instance to expansion-induced crack corrosion (DRK) and thermal shock cracks, have been known from the past, the group initially concentrated on the problems of components subject to creep stress. Moreover, the recommended measures were not restricted to the material X20CrMoV 12-1 as was the case after the “Arzberg failure”, but affected all heat-resistant steels in the temperature range above 450 °C. In the year 1998 a procedure for the measures to be performed as part of the recurring tests was brought to the attention of all VGB members [9].

Figure 3: Metallographic cross-sectional polished sample through the failed endcap-side

connection seam [7]. Field b) = red marked details from field a).

Flat Header Endcaps

VGB PowerTech 7/2004

Calculation

Owing to the sometimes high number of components to be examined within a plant it was recommended to initially perform a service life calculation so that the components with the lowest theoretical residual service life could be tested first. Successful calculations with the help of the finite element method (FEM) were already available at ELSAM at that time [8]. The calculations are based on the Von-Mises flow criterion assuming elastic-plastic material behaviour. The approach in the present case starts from a comparison of the FEM results with those of the TRD calculation and calculates service life consumption based on purely geometrical data (Figure 4) taking into account the operating parameters, pressure, temperature and the previous operating time which allows it to determine a test sequence for the components to be examined. A total of 34 endcaps were examined by ELSAM-PROJEKT.

Tests

For testing the components different methods have been proposed which complement each other in their respective test statements (Figure 5). The time of the test depends on the material and the theoretical residual service life resulting from the FEM calculation. Accordingly, components from the 9 to 12% chromium steels were to be tested for the first time at a residual service life of less than 100,000 operating hours and all other components made of the low-alloyed steels from a residual service life of 50,000 operating hours.

Further measures are consequently to be derived from the test results of all individual tests and additionally depend on additional peripheral conditions such as the plant mode of operation or the planned overhaul intervals and the plant age.

Endcap arching

Establishing endcap arching must be understood as an initial assessment of excessive creep strain that may have taken place. The dimensions must be established by applying a straight edge and documented specifying the respective measuring planes.

Figure 4: Schematic representation of the geometrical data required for the FEM calculation [8].

Geometrical data: di = Internal diameter of header dm = di + Wall thickness of header (S2) S = Thickness of endcap S2 = Residual wall thickness in the base of the groove S2 = Wall thickness of endcap

Operating data: Pressure, temperature and operating time It is possible to have the FEM calculations performed at ELSAM by specifying the mentioned geometrical and operating data. However no conclusions can be drawn from the results of the initial measurement since arching of up to 1 mm may be present even prior to commissioning for instance through the effect of internal welding stresses.

Surface crack test of the (inspection) socket seam

As was shown by the various failure analyses creep damage in the endcap-side HAZ of the weld seam between inspection socket and endcap also occurs in addition to the damage in the endcap connection seam in the event of high endcap loading.

Figure 5: Schematic representation of the procedure for non-destructive testing.

If such damage is present this can be initially verified with the help of the surface crack test using the magnetic particle process and structure analysis if required in several impression positions.

Surface structure analysis

In the case of the failure incident in the power station Arzberg an irregular heat treatment condition with correspondingly low creep strength values of the endcap was present in addition to the stress peaks brought about by the design. A correlation between heat treatment condition and creep strength exists in all high-temperature materials. Less than optimal heat treatment parameters generally result in lower creep strength. A clear relationship between structure and heat treatment and consequently creep strength can however not be equally established for all high-temperature steels. This is clearly shown with the material X20CrMoV 12-1 as was demonstrated by the failure incident in the Arzberg power station. An obviously serious deviation from the specified heat treatment parameters resulted also in a deviation from the otherwise typical lancet-type structural formation and consequently in reduced creep strength. In addition to this the failure analyses have shown that the extent of the creep damage in form of micropores and microcracks is greatest in the material volume and not on the material surface which is consequently not accessible to structural analysis. For this reason surface structure analysis is primarily aimed at verifying a proper structural condition (heat treatment condition). However since the weld seam between inspection socket and endcap also showed creep damage in the examined failure incidents, this area should be selected for the structural analysis so that verification with regard to possible creep damage in form of micropores is also possible in this case. Should no inspection socket be present the centre of the endcap is the test position to be selected for surface structure analysis.

Figure 6. Thermal shock cracks in the groove base of a flat endcap.

Flat Header Endcaps

VGB PowerTech 7/2004

Ultrasonic test

The investigations of TÜV Rheinland have shown that the creep damage could be verified in the removed endcaps from the power station Weisweiler by increasing the test sensitivity compared with the test parameters of the AD-HP/03. As a result the Test Instruction S 001 drawn up after the Arzberg failure was modified in the year 1998. The recommended procedure is schematically shown in Figure 5.

Experiences within the scope of recurring test measures

As part of the presented test measures a large number of components with impermissible findings were discovered in the last five years resulting in the replacement of the components in most cases. Only few of these removed components were subjected to detailed technological material analysis to establish the causes of the established ZfP findings. According to statements of those concerned the findings which resulted in the replacement were crack indications in the weld seam between inspection socket and endcap as well as impermissible ultrasonic indications in the area of the endcap connection seam. The experiences are currently being collected and evaluated in order to modify the previous approach if required.

Defects (lamination/forging defects) due to the manufacture were occasionally detected within the scope of testing the installed endcaps with the help of ultrasonic testing which, had these been recognised, would have resulted in a rejection of the semi-finished product during acceptance testing of the source material (forged part or plate). In the event of such types of findings the established defect is generally assessed and the further procedure agreed between expert and operating company as a function of the operational situation. Thermal shock cracks (Figure 6) have been individually verified through defects (lamination/forging defects) due to the manufacture were occasionally detected within the scope of testing the installed endcaps with the help of ultrasonic testing which, ultrasonic testing as well as through internal inspection using an endoscope some of which have already reached the basic body of the header.

Figure 7: Strain-induced crack corrosion (DRK) in the groove base of a flat endcap from a vertically positioned injection cooler. (The incipient crack is situated in the area of a deposit weld which presumably was carried out after the removal of incipient cracks already detected earlier in this area.

Figure 8: Indications next to the socket weld-in seam after the surface crack test ( Field a). Result of the surface structure analysis: Evaluation class 4 (Field: b)

Figure 9: Metallographic cross-sectional polished sample through the endcap-side connection seam of a flat endcap. Flank binding defects as cause for impermissible ultrasonic indications according to the Test Instruction S 001.

Flat

Header Endcaps

identified as creep damage with the help of surface structure analysis (Figure 8). However no damage in the area of the connection seam has been established yet with the help of ultrasonic testing.

The modification of the ultrasonic Test Instruction S 001 from the year 1992 performed in 1998 (after the Arzberg failure) has resulted in an increase of the test sensitivity so that the indication findings determined in the area of the endcap connection seam with the metallographically analysed components so far could be traced to welding defects such as flank and layer binding defects (Figure 9) due to the manufacture and non-metallic inclusions/slag bands (Figure 10) in the area next to the weld seam.

Prospects

The problems of flat header endcaps with stress relief groove affect mainly two topical aspects:

In these cases the failure areas were rehabilitated and additional measures for avoiding future thermal shock loading carried out.

Crack findings have also been observed in the base of the stress relief groove both through ultrasonic testing and also through internal inspection which grow from the internal surface in contact with the medium into the material according to the mechanism of strain-induced crack

corrosion (DRK) (Figure 7).

Findings have so far been established in many locations through surface crack testing of the weld-in seams of inspection sockets which in individual cases were

—

— the recurring testing of already

operationally loaded components and — the design and configuration within the

scope of the required replacement measures or new establishment.

Recurring testing

The difference between the test according to AD-HP 5/3 and the modified Test Instruction S 001 is shown on the example of the UT testing head K4N as an example in the data sheet (Figure 11). Because of the very high test sensitivity of the ultrasonic test according to the Test Instruction “S 001” flaws due to the manufacture (flank binding defects and slag bands) can not be distinguished from (creep) damage (“pore clusters” and microcracks) brought about by the operation. Attribution would only be possible by verifying crack advance due to operation with the help of the ultrasonic test within the scope of several recurring tests. However, no sustainable results with regard to expected crack growth rate are currently available so that no safe inspection intervals can be recommended. Moreover, extremely accurate test performance and documentation of the recurring test is required to actually verify any crack growth. Inadequate intervals between the tests (< 1 year) result in that crack advance that may have taken place is lost in the range of measurement error. Periods (> 3 years) which are too long in turn pose the risk of premature component failure. In order to ensure that working with test sensitivities appropriate for practical use is possible (e.g. KSR 3 according to AB-HP 5/3) it appears practical to perform a fracture-mechanical assessment with regard to a critical defect quantity for possible material characteristics

Figure 10. Metallographic cross-sectional polished sample through the endcap-side

connection seam of a flat endcap (fusion line is marked red). Non metallic inclusions as cause of impermissible ultrasonic indications according to Test Instruction S 001

Figure 11. Schematic representation between the test according to AD-HP 5/3 (KSR 3) and the

modified test instruction “S 001” (KSR 1.5)01“ (KSR 1,5). KSR = circular disc reflector

Flat Header Endcaps

VGB PowerTech 7/2004

(taking into account the lower creep

strengths of the HAZ and operating conditions (excessive temperatures, increased load cycles). By means of the test results (result of recalculation, arch measurement, surface crack test, surface structure analysis and ultrasonic test) as well as the operational situation (accessibility, mode of operation/overhaul interval etc.) it must be decided in each individual case between operator and expert whether the endcap will be replaced or subjected to recurring tests in future. Components which do not reveal any impermissible findings during the first test will still have to be subjected to recurring testing in future. The recommendations for practical test intervals must be derived from the results of past experiences with practical testing performed.

Replacement measures and new establishment Information concerning the TRD 305 (8.96) having the design change of flat endcaps as its content (Figure 7) was announced On the occasion of the 43rd DDA meeting on 2 November 2002. Owing to the locally excessive stresses and the multi-axial stress condition in the transition area between stress relief groove and the cylindrical part the position of the weld seam is relocated from this area to the cylindrical part.

Literature

Flat Header Endcaps

VGB PowerTech 7/2004