INVESTIGATION OF THE OBSERVED STRESS CORROSION CRACKING OF T24 MATERIAL

Ullrich, C.1,2, Rademacher, H.-G. 1, Tillmann, W. 3, Zielke, R. 1, Körner, P. 2

1 Institut für Forschung und Transfer (RIF e.V.), Dortmund Germany 2 VGB PowerTech e.V., Essen Germany

3 Technische Universität Dortmund, Lehrstuhl für Werkstofftechnologie, Dortmund Germany

ABSTRACT Starting in 2010 a new generation of coal fired power plants in Europe operating at a steam temperature of up 620°C was commissioned. During that commissioning process many cracks occurred in welds of T24 material which was extensively used as membrane wall material in nearly all of the new boilers. The cracks were caused by stress corrosion cracking (SCC) only occurring in the areas of the wall being in contact to high temperature water during operation. The question which step of the commissioning process really caused the cracking was not answered completely even several years after the damage occurred.

To answer this question and to define parameters which will lead to cracking in high temperature water many tests were conducted. Generally it was found that slow tensile tests in controlled environment are well suited to get information about materials SCC sensitivity in the laboratory. In the present paper, first the influence of the cracking of welded T24 material in acidic environment containing well-defined amounts of H2S is investigated to address the question if a chemical cleaning process prior to the testing might lead to hydrogen induced SCC. As a second step, cracking behaviour in high temperature water is being investigated. Here the influence of the temperature, the oxygen concentration of the water, the deformation speed of the sample, the heat treatment and the condition of the material on the SCC is analysed.

1 Introduction / Motivation T24 is a low alloyed heat resistant steel designed for use in membraned walls, supporting tubes, and superheater tubes in thermal power plants. The development of this steel was intended to achieve higher temperatures and to therefore increase efficiency of power plant. Materials such as T12, were calculated to not possess sufficient mechanical strength to be economically used as membrane wall tube, especially in the hotter (upper) parts of the boiler. Therefore T24 became the new standard material combining excellent creep strength, high temperature strength and good high temperature ductility.

Shortly after the commissioning in the beginning of 2010 first operational problems with the material were reported by various operators. Several hundred cracks have been formed in the evaporator which is in contact to water. All of these cracks were located in the area of welds. This severe damage resulted in an intensive root cause analysis. Different institutes were involved all coming to the same finding that the boiler was affected by stress corrosion cracking (SCC) as the primary cause of damage. Three different root causes for SCC were considered possible:

Cathodic SCC caused by chemical cleaning

Anodic SCC driven by temporary high O2-content in the water

Cathodic SCC driven by the magnetite reaction and H2 production

In the current article especially the influence of the chemical cleaning process as well as the anodic driven SCC is under investigation.

2 Current situation

2.1 Properties of T24 / T23 Material For the new generation of plants materials with higher mechanical strength were needed for the membrane wall. 13CrMo4-5 (T12) which was the former standard material could not fulfil these needs any longer. Therefore a new material had to be developed, having a higher strength but similar good properties in terms of manufacturing. The solution for this needs was T24 (7CrMoVTiB 10-10) and T23. Both of these materials are low-alloyed heat-resistant steels having excellent mechanical behaviour for use in membrane walls, supporting

tubes and superheater tubes. In comparison with conventional steels such as T12, the strength of these alloys has been significantly increased by the addition of elements such as vanadium, niobium and titanium. The increase in strength is caused by forming fine precipitations especially vanadium carbides. An important requirement was to avoid the need for the post-weld heat treatment (PWHT) for typical wall thicknesses relevant for boiler tubes. Significantly reducing the carbon content compared to the alloys T12 and T22 (10CrMo9-10) it was found to not require PWHT for wall thicknesses below 10 mm. The reduction of the carbon concentration was implemented to avoid to high content of martensite and therefore keep the hardness after welding in an acceptable range (<350HV) [15, 16, 17].

In addition to intensive laboratory investigations in the course of normal qualification programs, field tests were carried out with T24 to learn more about the in-service behaviour. The material was therefore installed in membrane wall configuration in at least in four different plants. Already in the mid of the 1990ies T24 was installed in the RWE plant Weisweiler unit G operated at a steam temperature of about 500°C at 215bar for more than 60.000h now. In EnBW’s plant Altbach a T24 panel is installed in the membrane wall tested under water conditions at 440°C and 280 bar. Other installations in steam were realized in EON’s plant Schloven unit F and in DONG’s plant Asnæsværket unit 4. None of the plants, which tested the material under real plant conditions, reported any unusual problems. However it has to be mentioned that the material was installed in plants not being commissioned for the first time. They were installed in a stable running unit in all the cases mostly being in contact with steam or at least with water with temperatures significantly exceeding 300°C under normal operational conditions.

2.2 Stress Corrosion Cracking (SCC) in steels Generally it is well known that for initiation and propagation of stress corrosion cracking three factors have to act together. First factor that needs to be fulfilled is the material’s condition that has to be sensitive to suffer from SCC in specific environments. This sensitivity is mainly determined by the chemical composition, microstructure and heat treatment of the material. Second parameter which decides if SCC appears is stress which is exposed to a component. For the initiation of stress corrosion cracking an individual threshold of stress has to be exceeded. This level from which stress corrosion cracking appears is specific for the SCC system and will directly be influenced by the other two relevant factors [1, 2, 11]. The level of the threshold can therefore strongly differ. It was reported in [4] that the limit may be in the range of the yield stress whereas SCC in less sensitive cases may also need stresses approaching the ultimate tensile stress [4]. The stress can be applied as external stress e.g. by the inner pressure of the tube or as residual stress out of a welding process. The standard type of stress corrosion cracking works well with a static stress [3]. However, if a component is exposed to a load either cyclic or constantly increasing with a slow strain rate the susceptibility to cracking can be reduced or increased, depending on the system. The last and third factor which has an impact on SCC is the liquid media in which the component is being stressed. It is important to know the ingredients of the media as they can cause a significant change in materials behaviour even in low concentrations significantly change materials behaviour. If a crack is being formed by SCC the resulting fracture normally shows little or nearly no deformation. Macroscopically this can be seen measuring the elongation or the necking of a specimen or microscopically observing the deformation on the cracked surface [3, 11]. The cracks appear to have a branched structure often with lots of secondary cracks forming a network. Corrosion products can normally not be found along the crack path [11]. Depending on the media SCC can be either inter- or transgranular [3].

SCC in high temperature water can appear by a concentrated anodic metal dissolution. This mechanism is called anodic SCC. This mechanism can appear if the oxide is locally fissured by straining the material. A crack can now propagate if the fissure which represents a local anode is not directly re-passivated. In case of fast re-passivation the anodic metal dissolution is stopped or not even started. If metal is now being exposed to a local anodic dissolution at a faster rate than the crack tip is re-passivated, SCC will propagate through the metal [1, 2, 5]. This local dissolution model needs a small anode during the process achieving high local potential. This only works if the forming flanks of the crack also form an oxide, concentrating the anodic dissolution on the tip of the crack which is not yet passivated [5, 6]. In high temperature water oxidising ingredients (e.g. oxygen or copper in water) change the corrosion potential, shifting it to generally more critical values [2].

Beside the anodic SCC hydrogen induced SCC is a well-known mechanism which can lead to cracking. This mechanism is often observed in materials which are exposed to acidic fluids. Due to the higher solubility of iron in such an environment hydrogen production is higher. In the case of hydrogen induced cracking materials strength is of high importance. Once the hydrogen is absorbed it shows the tendency to diffuse to a plastically deformed zone, located at the tip of the crack. The correlation between strength and susceptibility for the cracking is caused by the fact that material of higher strength will form a smaller plastically deformed zone. This effect leads with the same amount of hydrogen being formed to a locally higher hydrogen concentration [6, 7, 8, 11]. Therefore in literature a threshold for the hardness was defined to reliably avoid the cracking even under the

presence of strong promotors such as H2S. This hardness was defined to be 248HV10 [8, 9]. The volume of hydrogen required for cracking is very small. In his investigations, Bäumel observed that even small amounts of hydrogen produced in boiling high-purity water may lead to the failure of a component caused by hydrogen-induced cracking [10].

In power plant operation, hydrogen is produced by the cathodic reaction of metal dissolution (magnetite formation). Intensive hydrogen production is well known for power plant commissioning process. Very often it is hard so strongly distinguish between anodic or cathodic SCC as both mechanisms may appear alternately or even superimposed. Therefore literature defined a mechanism called “anodic hydrogen embrittlement”. For this damage mechanism the first step is controlled by anodic metal dissolution. As consequence of the re-passivation process oxide and therefore hydrogen is being produced. A small amount of the hydrogen is absorbed diffusing to the crack tip. Here the hydrogen reduced the binding forces of the material (hydrogen embrittlement) [3, 5].

2.3 SCC of low-alloyed steels in high-temperature water In literature many reports about SCC of low-alloyed steels in high-temperature water can be found. Some of them try to define sharp limits of crack initiation. In this context it should be mentioned that the given critical parameters are specific to each corrosion system [2]. Seifert defines in his work the critical temperature range between 120-250°C, with high sensitivity in the range between 200 and 250°C. The critical temperature range said to be directly depended on the dissolved oxygen concentration of the water as well as a potential sulphur content of the alloy [2]. In a recent work by Metzger [26] the dependence of temperature on the cracking behaviour of T24 material was also confirmed. It was found that at a testing temperature of 300°C even under critical circumstances cracking could not be initiated. At a testing temperature of 180°C however by unchanged testing conditions lead to cracking of the samples showing a typical fracture for SCC. An identification of the most critical temperature was not done.

In previous investigations the authors of this article found a clear dependence of cracking and the oxygen content of the high temperature water the specimens were exposed to [20]. These tests were all performed at a temperature of 180°C which is reported to be critical in other publications. At this temperature an O2 concentration exceeding 150ppb led to premature cracking. Higher concentrations led to a further reduction of the strain to rupture as well as a reduced necking. The finding of a significant influence of dissolved oxygen concentration was in parallel clearly confirmed by Devrient et al. [21]. In this investigation the testing temperature was also chosen to be 180°C. With static Jones specimens as well as slow tensile tests in oxygenated water this influence was confirmed for T24 material. The statically loaded Jones specimens were tested in “oxygen free” and also in highly oxygenated water. In oxygen free atmosphere none of the samples showed cracks. With O2 saturated water however a probability for cracking of 57% was found [21]. Lots of the tests which are presented for the tensile test in [21] were done on round cross weld specimens. Therefore the results should generally be comparable to the results in [20]. Due to the fact that Devrient et al. decided to not reduce the samples diameter in the area of the weld in the heat affected zone (HAZ) all specimens broke in the base material. However a general tendency of O2 influence on the mechanical behaviour was found but is not comparable to the findings in [20].

Another influence on SCC investigated can be the strain rate which is exposed to a component. Some SCC systems show cracking after reaching a sufficiently low strain rate but they do not crack at higher rates. Further reducing the strain rate will lead to more severe conditions. Other systems, however, only form SCC if a faster deformation is applied. Slower or faster strain rates will in this case lead to less critical testing conditions [3, 12]. Kim et al. utilize the strain rate sensitivity of a material to assess if anodic or cathodic SCC is the dominant mechanism. In case of a mainly anodic SCC a strain rate which is too low will lead to a passivation of the crack tip. This prevents the material to form a small anode with high potential. Purely anodic SCC is therefore found to be no longer active at too low strain rates. A comparable correlation is not known for hydrogen-induced stress corrosion cracking. The hydrogen produced by the cathodic reaction diffuses into the material until it is saturated. Below saturation level, the local concentration in the area of the crack tip increases steadily with time. This should lead to a higher susceptibility for the cracking with increasing time in the critical medium [13]. This idea is supported by Kirschner's studies that, especially with small amounts of hydrogen being formed, very low strain rates must be selected to see the effect of hydrogen-induced cracking. In this case, he suggests that a strain rate in the range of 10-7 1/s should be selected. However, if the material adsorbs a considerable quantity of hydrogen (e.g. in a system with H2S) the strain rate can be significantly increased because slow strain rates and long dwell times overemphasize the influence of hydrogen. It is then difficult to distinguish between susceptible and non-susceptible materials [14].

3 Investigations

3.1 Stress Corrosion cracking by chemical cleaning Process As already described in the previous chapter low alloyed steels can also tend to crack as a caused by hydrogen induced cracking under presence of H2S. One potential root cause is the chemical cleaning process of the boiler before starting the commissioning process. It is believed that a degradation of the inhibitor called ‘CL4’ will create H2S. This H2S which is well known to decelerate the recombination of hydrogen can therefore promote hydrogen induced SCC. This effect was referred to as potential root cause..

3.1.1 General considerations about the inhibitor CL4 Using inhibitors during the chemical cleaning process is a state of the art method protecting the material from a too massive attack. Inhibitors are mixtures out of many different chemicals which protect the material during the chemical cleaning process from to high attack [22]. Also in case of H2S containing fluids some inhibitors are able to protect the material from cracking. In this very specific case the inhibitor reduces the absorption of hydrogen which is promoted by H2S [23]. For the present investigation only the effect of the inhibitor called CL4 which was used in the case of the damaged boilers is investigated. This inhibitor contains Sodiumthiocyanat (NaSCN) up to a maximum of 10 vol.%. This Sodiumthiocyanate is well known to degrade in contact of acids to Thiocyanic acid and Hydrogen sulfide H2S [24].

The chemical cleaning process in a plant was done with hydrochloric acid (1%) mixed with 0.04% of the inhibitor CL4. In case of a complete degradation of the inhibitor, which is chemically very unlikely, 4.25mg (2.32 Vol%) H2S can be formed per 100ml solution.

As a first step the conditions were investigated which will lead to a degradation of the inhibitor resulting in a production of H2S. This test was done in a set-up in accordance with Figure 1. In case of H2S production in the test volume nitrogen gas will transport the reaction gas into a lead acetate solution. Also very small amounts of H2S will directly lead to a formation of lead sulfide resulting in a black color.

To identify the degradation condition the chemical cleaning solution was heated up until its boiling temperature. Formation of H2S was not identified. A thermal degradation is therefore not expected at atmosphere. However, if the chemical cleaning solution gets in contact with metal H2S formation is found at all investigated temperatures (20, 60 and 97°C). This result shows, that the inhibitor used tends to degrade even if it is dosed to a rather mild acid (1% HF). In further tests it has to be assessed if the amount of H2S and also the other conditions are sufficient to promote hydrogen induced SCC.

Figure 1: Experimental set-up for detection of H2S forming in the solution [25]

3.1.2 Experimental set-up and procedure Material behavior in chemical cleaning solution To investigate materials behavior under various chemical cleaning solutions tests in an autoclave were carried out. The autoclave used provides the possibility to uniaxial stress a specimen. At the same time the specimen can be exposed to an aggressive environment. A schematic cut through the used autoclave can be found in Figure 2.

For the test welded and hardened T24 material was used. Due to fact that the carbon content might have an impact on the cracking behavior a commercial heat with a quite high concentration of 0.09% was used. The

analysis provided by the tube manufacturer can be found in table 1. The welds which were tested in this work were produced by experienced welder under excellent workshop conditions. All of the welds were produced as three-pass welding in accordance with a weld valid welding procedure specification. Due to the experience from the plant the interpass temperature was limited to 100°C. The hardening of the material was done at a temperature of 1050°C for 15min followed by water quenching.

Figure 2: Schematical cut through the autoclave used [25]

All the tests in chemically cleaning solution were done at a temperature of 60°C as this was the relevant temperature used in the plants. The chemical cleaning solution was led into the test chamber and was subsequently heated up to 60°C. Reaching this temperature the samples were stressed until they reached the yield point. Temperature and strain were kept constant for 6h. To find out about T24’s susceptibility for hydrogen induced SCC in chemical cleaning conditions, a welded and a hardened specimen were exposed to regular chemical cleaning solution. Other specimens were exposed to specific elevated concentrations of H2S which was dosed to the autoclave in gaseous form. The amount of gas dosed to the autoclave was varied. The highest concentration which was dosed in this investigation corresponds to 10 times the amount of H2S that could theoretically be formed if the inhibitor is completely degraded. Beside that the inhibitor concentration was changes between 0 and 0.04 Vol. % to simulate different degradation cased.

3.1.3 Material behavior in chemical cleaning solution with elevated H2S concentrations As first step hardened and welded samples were exposed to a normal chemical cleaning solution with its regular composition (1% HF + 0.04% CL4). The stressed sample did not show any cracking under this exposure. Stepwise the H2S concentration was increased up to the 23.4 Vol % representing 10 times the amount which can be formed due to the complete degradation of NaSCN. The inhibitor concentration was kept constant at 0.04%. Also in this case cracking was neither observed for the hardened nor for the welded specimen (Figure 3). After that test sequence the material was tested in the chemical cleaning solution without any inhibitor. With these tests the complete degradation of the inhibitor should be investigated. A concentration 5 times higher than the theoretical maximum concentration of H2S cracking was not observed independent on the materials condition investigated (welded / hardened) (figure 4). Exposing the samples to a concentration 10 times higher than possible in the chemical cleaning process, cracking was observed for both the hardened and also the welded specimen (Figure 5). At this test 23.4 Vol. % of H2S was led into the autoclave without the presence of any inhibitor. Finally the effect of minor concentrations of inhibitor in the solution was analysed. This investigation represents a degradation of the inhibitor which from technical point of view might be possible. In this test an inhibitor concentration of 0.004% was present in the solution. However already at this very low inhibitor concentration (10% of the normal) cracking is not observed independent on the material condition and the H2S concentration which was 23.4 Vol % at the maximum. This result shows that already very small amounts of the inhibitor which will normally not degrade completely protect the material from H-induced SCC during the chemical cleaning. The inhibitor seems to prevent hydrogen absorption effectively [25].

a)

b)

Figure 3: T24 material tested in 1%HF + 0.04% CL4 + 23.42 vol.-% H2S, 6h at 60°C, a) welded specimen,

b) hardened specimen

a)

b)

Figure 4: T24 material tested in 1%HF + 0.0% CL4 + 11.7 vol.-% H2S, 6h at 60°C, a) welded specimen,

b) hardened specimen

a)

b)

Figure 5: T24 material tested in 1%HF + 0.0% CL4 + 23.4 vol.-% H2S, 6h at 60°C, a) welded specimen, b)

hardened specimen

3.2 Specimen production for uniaxial tests in high temperature water All the tests presented in this paper were carried out on a commercial heat of T24 with the chemical composition given in table 1.

Table 1: Chemical composition of materials used

Material C (%)

Si (%)

Mn (%)

P (%)

S (%)

Cr (%)

Mo (%)

Al (%)

Ti (%)

V (%)

N (%)

B (%)

min - 0.05 0.15 0.3 - - 2.2 0.9 - 0.05 0.2 - 0.002

max - 0.1 0.45 0.7 0.02 0.01 2.6 1.1 0.02 0.1 0.3 0.01 0.007

parent 0.09 0.24 0.53 0.17 0.002 2.44 0.99 0.014 0.08 0.23 0.006 0.005

The specimens were removed out the tube as shown in Figure 6 by a wire cut EDM process. This process was chosen to reduce the degree of deformation to a minimum. After removing the specimen they were machined and finally grinded with grit 320. In case of weld connection the weld and the HAZ were reduced in thickness. This was done due to the different strength of the weld and the base material. Compared with the ultimate tensile strength of the base material which is around 450 MPa at 20°C, the weld has a much higher strength of 600 MPa at the same temperature [18, 19]. This clearly indicates that strain will concentrate in the base material and the specimen will crack in the base material if tested in air. During the tensile test in high-temperature water the weld will not be significantly deformed. If the stress level will exceed the threshold to generate SCC is not clear. It was therefore decided to weaken the weld area by reducing its cross section. The final geometry of the welded specimen after machining is shown in Fig. 7 b.

Figure 6: Specimen taken from a weld, a) specimen as removed from tube, b) specimen after constricting in area of weld and HAZ

4 Results of lab tests in controlled high temperature water The tests which are reported in the following were all conducted in the test set-up described in detail in [20]. This set-up allows controlling the mechanical loading as well as the water chemistry in a wide range over a long period of time. In the following several operational influences on the material behavior were investigated.

4.1 Effect of Temperature To investigate the impact of temperature on the cracking behaviour of T24 material in high temperature water, tests were conducted in the range between 120°C and 215°C. All other parameters were kept constant. The oxygen concentration was kept at a high value of 1000ppb. This value was identified to be critical in previous investigations of the author [20]. As crosshead speed 0.25μm/min was chosen. The overview of the specimens tested is given in table 2.

Table 2: Specimens tested to investigate effect of temperature

Specimen Temperature O2- concentration Deformation speed

Strain at rupture in % Material

1 120°C 1000ppb 0.25μm/min > 10 T24 weld

2 150°C 1000ppb 0.25μm/min 2,7 T24 weld

3 180°C 1000ppb 0.25μm/min 1,7 T24 weld

4 195°C 1000ppb 0.25μm/min 1,5 T24 weld

5 210°C 1000ppb 0.25μm/min 1,8 T24 weld

6 215°C 1000ppb 0.25μm/min 2,2 T24 weld

The investigation clearly showed a significant influence of the temperature. At 120°C ductile fracture was observed (Figure 7). Starting with 150°C the strain to rupture is significantly reduced. Therefore the damage mechanism clearly changed in that range of the temperatures. Increasing the temperature to 180°C the strain to rupture is further decreased, reaching its minimum at a temperature of 195°C. On the fracture surface intergranular fracture showing nearly no deformation can be seen. Also many secondary intergranular cracks can be observed (Figure 8). The crack is located in the coarse grained HAZ. Further increasing the temperature to 215°C the strain to rupture slightly increases again. However the fracture surface still shows partly intergranular/brittle cracking, showing that at this temperature the cracking mechanism can still be initiated or propagated.

a) b)

Figure 7: Fracture surface after cracking of specimen 1, tested at highly oxygenated water at 120°C, transgranular cracking with significant degree of deformation

Figure 8: Fracture surface after cracking of specimen 4, tested at highly oxygenated water at 195°C, intergranular cracking portion with lots of secondary intergranular cracks

4.2 Effect of strain rate The investigation of the influence of the strain rate was tested on different specimens. The majority of the test were done at an oxygen concentration of 1000ppb and a temperature of 195°C. The crosshead speed of the machine was varied between 0.05-0.45μm/min. All the experiments conducted to investigate the influence of deformation speed are summed up in table 3.

The investigation clearly showed a significant influence of the deformation speed on the cracking behaviour. At a high oxygen concentration a tendency to reduced strain to fracture with a reduced deformation speed can be observed (Figure 9). These specimens show significant portions of intrgranular/brittle fracture. At a deformation speed of 0.45μm/min a ductile fracture in the base material was observed with a strain to fracture of more than 9% compared to 0.7% at a deformation speed of 0.05μm/min.

The influence of deformation speed however seems to change with reduced oxygen concentration. At an oxygen concentration of 450ppb specimens deformed with a slow deformation speed show a higher strain to rupture whereas the sample deformed faster showed a lower strain to rupture. This behaviour was confirmed also at oyxgen concentrations of 250ppb.

Table 3: Specimens tested to investigate effect of strain rate

Specimen Temperature O2- concentration Deformation speed Strain at rupture in % Material

7 195°C 1000ppb 0.05μm/min 0.7 T24 weld

8 195°C 1000ppb 0.1μm/min 0.85 T24 weld

9 195°C 1000ppb 0.15μm/min 1.2 T24 weld

10 195°C 1000ppb 0.25μm/min 1.6 T24 weld

11 195°C 1000ppb 0.35μm/min 1.8 T24 weld

12 195°C 1000ppb 0.45μm/min 9.5 T24 weld

13 180°C 450 ppb 0.1 μm/min 2.8 T24 weld

14 180°C 450 ppb 0.25 μm/min 1.6 T24 weld

15 180°C 250 ppb 0.1 μm/min 2.8 T24 weld

16 180°C 250 ppb 0.25 μm/min 1.8 T24 weld

Figure 9: Influence of deformation speed on the strain to rupture at an O2 level of 1000ppb

4.3 Effect of heat treatment To investigate the effect of heat treatment on stress corrosion cracking sensitivity, an untreated specimen and specimens heat treated under different conditions were tested. All these tests were conducted in water at significantly increased oxygen content. An overview of the specimens tested is given in Table 4.

Table 4: Specimens tested to investigate effect of heat treatment

Specimen Temperature O2- concentration Heat treatment

Deformation speed

Strain at rupture in %

Material

17 180°C Saturated None 0.1 μm/min 1.5 % T24 weld 18 180°C 900 ppb 520°C/24 h 0.25μm/min 2.0 % T24 weld 19 180°C 900 ppb 550°C/48 h 0.25μm/min 2.1 % T24 weld 20 180°C Saturated 600°C/2 h 0.1 μm/min 4.6 % T24 weld

The specimen without heat treatment was deformed at a cross head speed of 0.1 μm/min. This specimen failed after 140 h at a total elongation of 1.5 % in the HAZ. On the fracture surface clear portion of intercristalline cracking was found. Beside that a network of intercristalline cracks is observed looking on the fracture surface. The outer surface of specimen is in the neighboured region of the main crack (HAZ) affected by a network of small fissures (Figure 10 a)).

The specimens subjected to heat treatment at 520°C/24 h and 550°C/48 h were loaded at a faster cross head speed (0.25 mm / min) and failed also after a short time (<100h) at about 2 % total strain. The elongation was

slightly above the value for the non heat treated specimen. Similar to the not-heat treated specimen, the specimens fail suddenly without a previous load drop. The main cracks occurred also in the HAZ.

The specimen heat treated at 600°C/2 h demonstrated a very different material behaviour. The crosshead speed was lower than for the other heat treated specimens, but the same as for the not heat treated specimen. SCC did not occur in this case. The stress-strain curve shows a very different pattern to the not heat treated and 520/550°C specimens (Figure 11). While the other specimens fractured rather abruptly in the transition to plastic deformation, the test with the specimen heat treated at 600°C was interrupted at about 4.5 % total elongation and 430 h. Compared to the not heat treated specimen the damage of the outer surface is quite different. In contrast to the not heat treated specimen, fissures on the outer surface cannot be observed. Small defects in the oxide which are normal at such a strain were subject to passivation not to formation of cracks (Figure 10 b)).

4.4 Effect of oxygen content For the investigation of the oxygen content different cross weld specimens were tested in high temperature water with varied oxygen content. Except the specimen tested in water saturated with oxygen all samples were deformed with a crosshead speed of 0.25μm/min.

a) b) Figure 10: Comparison of surface condition of the specimen not heat treated (a) and heat treated at 600°C (b) after the test in highly oxygenated water

Figure 11: Stress-strain curves of specimens with various types of heat treatment

17

18

19

20

Table 5: Specimens used to study the effect of oxygen content on stress corrosion cracking

Both specimens tested at 450 ppb and 250 ppb oxygen fractured at a relatively low elongation of 1.6 and 1.8% respectively. Necking of the specimen was not observed. Both fractured surfaces are located in the HAZ. The specimen tested in water with an oxygen concentration of 150 ppb (specimen 23) shows a significantly higher elongation of 3.1% and fractured after 118h also in the HAZ. Nevertheless necking in the base material of this specimen was observed. In addition to that a secondary crack in the base material can macroscopically be seen on specimen’s surface (Figure 13). A specimen tested in hot temperature water with a very low controlled oxygen content of <10 ppb showed a completely different behaviour. This sample showed significant necking with a total strain of 8.9%. Finally the specimen cracked in the base material. The stress strain curves of these experiments is illustrated in figure 12.

Figure 12: Stress-strain curves of specimens loaded in high temperature water of various oxygen contents

Figure 13: Macroscopic assessment of the samples cracked in oxygenated water

4.5 Effect of hydrogen versus oxygen Based on the preceeding results, it is unclear if the observed damage mechanism is driven by an anodic or cathodic process. To answer that question an investigation in the autoclave presented in chapter 3.1 of this paper on T24 cross weld samples were made. Different to the other tests in chapter 4 the load was exposed stepwise. All tests were done at a temperature of 180°C and a starting pH of 9.5 dosed with ammonia. Due to the different set-up a first experiment with similar parameters except from the loading process was carried out. Therefore, a cross weld sample was exposed to highly oxygenated water and stressed stepwise until fracture. The specimen

Specimen Temperature O2- concentration

Deformation speed

Strain at rupture in %

Material

17 180°C Saturated 0.1 μm/min 1.5 % T24 weld

21 180°C 450 ppb 0.25 μm/min 1.6 % T24 weld

22 180°C 250 ppb 0.25 μm/min 1.8 % T24 weld

23 180°C 150 ppb 0.25 μm/min 3.1 % T24 weld

24 180°C <10 ppb 0.25 μm/min 8.9% T24 weld

250ppb 0,25μm/min Max. Stress: 804MPa

450ppb 0,25μm/min Max. Stress: 802MPa

<10ppb 0,25μm/min Max. Stress: 959MPa

150ppb 0,25μm/min Max. Stress: 845Mpa

17 23

24

21 22

broke in the HAZ without any macroscopically remarkable necking. Similar to the specimen investigated in 4.4 the sample showed many fissures in the HAZ. Looking on the fracture surface macroscopically a crystalline cracking could be observed partly (Figure 14). Even if the load situation of the set-ups is not comparable a fracture the mechanism leading to failure was the same in highly oxygenated water.

As in chapter 4.4 cracking in the HAZ was not observed if the water was free of oxygen, now tests were done also in oxygen free atmosphere but with a different amounts of H2S (2.3 -23 Vol. %) dosed to the water. This was done to produce a hydrogen induced cracking and compare the picture of the damage. The dosing of H2S to the water should increase the absorption of the hydrogen which is being formed as part of the Schikkor reaction. If the amount of hydrogen which is produced as part of the magnetite formation was sufficient cracking is expected. However in all three tests which were done the specimen showed a significant necking and much higher strain to rupture. Typical characteristics of hydrogen induced SCC were not observed. The cracking was in any case located in the weld and not in the HAZ of the weld (Figure 15).

a)

b)

Figure 14: Specimen tested in alternative autoclave at 180°C in highly oxygenated water, a) view on the broken sample with fracture in HAZ b) view on the fracture surface with partly crystalline appearance

Figure 15: Specimen tested at 180°C in deaerated water with an addition of 23 Vol. % of H2S, fracture in the middle of the weld with clear necking to be observed

5 Discussion For the chemical cleaning process it was clearly found that the production of H2S is inevitable if the inhibitor CL4 is used. This statement is also valid if the cleaning solution is produced correctly without any mistake. The amount of H2S being produced could not be quantified. Therefore for the first tests the very conservative approach of total degradation was followed. With the tests partly presented in this paper and more in detail presented in [25] it was proven that even under presence of many times more than the theoretically possible amount of H2S, cracking in 1% HF at 60°C was not found. In the case of only 90% degradation of the inhibitor cracking could not be produced even with very high (unrealistic) amounts of additional H2S. The laboratory investigation showed that the cross weld as well as hardened specimens produced out of T24 material do not tend to fail according to hydrogen induced SCC in the temperature range of 60°C during chemical cleaning conditions. On the basis of the current findings the authors of this paper do not believe in a hydrogen induced cracking being promoted by the chemical cleaning process.

The tests conducted in the autoclave to investigate the materials behaviour in high temperature water show that the methods used are very well suited for the investigation of stress corrosion cracking in T24. The investigation

showed a clear dependency of the cracking behaviour to the temperature. At a temperature of 120°C a ductile material behaviour is observed. Starting at 150°C intergranular/brittle parts are identified on the fracture surface. With increasing temperature the intergranular/brittle portion of the cracking increases. This correlates well with the reduced strain to rupture having a minimum at 195°C. Unfortunately, the upper temperature limit could not be identified with the set-up as 215°C represents the maximum temperature possible to test. At 215°C still significant intergranular/brittle cracking is observed. Other investigations indicate that at a temperature of 300°C the mechanism is not present any more. However a clear limit was not identified in this work [26].

The influence of the strain rate on the cracking seems to have also a correlation with the oxygen concentration. At a high O2 level of 1000ppb it was found that with reduced strain rate the strain to rupture is reduced. This effect is not mainly based on the specimen’s time in the autoclave. This is proven comparing the specimen tested at a deformation speed of 0.05μm/min to the specimen deformed with 0.45μm/min reaching a degree of deformation more than 9 times higher. The influence of deformation speed at lower O2 concentrations (450ppb, 200ppb and 100ppb) is different to the finding at 1000ppb. At the lower concentration the strain to fracture was lower at the higher speed of 0.25μm/min compared to 0.1μm/min. To assess this finding more in detail, different deformation speeds should be tested at the lower concentrations also.

Previous studies presented by the authors already demonstrated a correlation between oxygen concentration and SCC sensitivity. The results presented in this paper confirmed this correlation also for T24 material. At oxygen content of 450 ppb it was found that cracks occurred at very low elongations irrespective of the strain rate. The specimen tested at 150 ppb dissolved oxygen ruptured at a total strain of 3.1% which is significantly more (nearly doubled) compared to the specimens tested with 250 and 450 ppb. In this context it has to be mentioned that at 150 ppb oxygen content the sample showed necking in the not reduced base material. The secondary crack which was found in the base material shows the trend of damage concentration in the base material with reduced oxygen content. At the current status of investigation it cannot finally be stated that an oxygen content of 150 ppb will avoid cracking in any case. To finally assess these results a comparison with the material T12 is desired.

In contrast to all the other specimens the specimen with very low oxygen content of <10 ppb did not show cracking in the weld/HAZ. In this condition the specimen reached a total strain of about 9 %. The ultimate tensile strength is with 959 MPa more than 100 MPa higher than observed for the other not heat treated specimens. On the surface of the not heat treated specimen tested in water with a high oxygen concentration, damage of the oxide layer was identified (cracks). The occurrence of this oxide layer damage may be explained by a multiaxial stress state. The micro-cracks (fissures) observed are probably the initial stage of SCC which finally led to the cracking of the specimens.

The fact that the specimen in nearly O2-free water shows no SCC demonstrates the significant influence of O2 in the crack initiation process in welded specimens. Even if further experiments with different material conditions (such as simulated parent material), and different mechanical loading condition are needed to finally confirm this result a general influence of the oxygen content was demonstrated by this work. Due to the presented literature review this finding is an indication for anodic SCC. Tests in deaerated high temperature water containing H2S provide additional indication that the hydrogen being produced by the Schikkor reaction does not seem to be sufficient in its amount even if absorbed by the material to a higher degree. Due to the presented relation of cracking to the oxygen concentration in the high temperature water as well as the finding that even under presence of H2S in the deaerated water cracking due to SCC is not observed, the authors strongly believe in anodic SCC to be the dominant failure mechanism of T24 in high temperature water in a temperature range between 150-215°C.

As a general point, it should be noted that the number of specimens tested to date is still relatively small and that more tests should be carried out for statistical reasons. In future, the tests will also be carried out on specimens of simulated parent metal in addition to the tests on welds. These specimens have the advantage of having a well-defined, homogeneous structure over the entire test length. To correlate with field experience, T12 should also be investigated

6 Summary Slow tensile tests at different strain rates are outstandingly well-suited for investigating the stress corrosion cracking of T24 steel in high-temperature water as well as the material parameters and the limits of the fluid parameters. The test equipment developed for this investigation can accurately control and measure the water chemistry parameters during the test. This allows test results to be obtained which are reproducible and transferable to power plant operation.

In this paper the two main hypotheses for the observed cracking in the plant on T24 material were investigated. First of all was the chemical cleaning process. Here it was shown that the inhibitor CL4 tends to slightly degrade in 1%HF even if correctly used. As long as a concentration of 0.004% (starting concentration 0.04%) is present, the inhibitor reliably protects the material from cracking even at unrealistically high H2S concentrations (10 times more than can theoretically be formed). Even in the case of total degradation it was shown that the H2S concentration which is present in this case is not sufficient to initiate cracking. Even an unrealistically high concentration (5 times more than can theoretically be formed) does not lead to cracking. As a consequence of these tests the authors do not believe in a cracking caused by the chemical cleaning process.

The investigations on the influence of the heat treatment showed that with temperatures chosen to be 600°C or higher the cracking mechanism can be eliminated. 550°C for 48 h was found not to be sufficient if the water is highly oxygenated. In this context it has to be stated that the positive effect of reducing residual stresses which occur from the heat treatment at 550°C of the entire boiler is not payed any attention to in this context.

The testing temperature was investigated in the range between 120-215°C. A clear influence could be identified. At a temperature of 120°C the damage mechanism is not present. At a temperature of 150°C intergranular/brittle portion of the cracking is observed the first time. The temperature resulting to the lowest strain to rupture was identified to be 195°C. At 215°C however higher strain to rupture was measured but the damage mechanism is still present. Up to now the temperature not causing SCC any more was not identified in this paper. Other author’s reported 300°C to be non-critical again [26].

Strain rate (deformation speed) was also identified to have a strong relation to the damage mechanism. Tested in highly oxygenated water (1000ppb) too fast deformation speeds - starting with 0.45μm/min - did not result in SCC. At the high oxygen level the strain to rupture is reduced the lower the deformation speed is. This finding could not be confirmed for lower oxygen concentrations.

The results of the investigations in high temperature water show that welds in T24 are susceptible to SCC in high temperature water with an increased oxygen concentration. For values up to 450 ppb, it was found that SCC susceptibility was not dependent on the applied strain rates. The fact that the specimen tested in water with an extremely low oxygen concentration (<10 ppb) did not crack at considerably higher elongation shows that oxygen has a strong influence on the crack initiation mechanism. Due to the presented literature review this finding is an indication for anodic SCC. Experiments in deaerated high temperature water containing significant amounts of H2S also indicate the anodic character of the damage mechanism as typical cracking could not be generated under these conditions. This finding could be an indication that the hydrogen being produced by the Schikkor reaction is not sufficient in its amount even if it is absorbed by the material to a higher degree. On the basis of the present findings the authors strongly believe in anodic SCC to be the dominant failure mechanism of T24 in high temperature water in a temperature range between 150-215°C.

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