An increasing need for energy worldwide has caused a huge rise in demand for high-efficiency power stations, which have to produce electricity both in an economical and an environmentally sustainable manner.
These can be achieved by reducing the specific fuel and heat consumption required to generate 1 kWh, so in the case of fossil fuel facilities the pressue is on to further increase their efficiency. An efficiency increase can be achieved by raising two steam parameters – pressure and temperature. Steam parameters ranging from 605°C and 300 bar in the case of live steam to 625°C and 80 bar for hot reheat steam have become an important issue when seeking to build and operate new fossil fuel-fired power plants.
These higher steam temperatures and pressures thus require the use of new base materials, such as bainitic steel T24 and the martnsitic steels P92 and VN12-SHC. These steels are being used in new modern fossil-fired power plants in which the steam temperature can be up to 620°C. A prerequisite for the acceptance of such new materials is evidence of adequate creep rupture strength.
But such novel steels need also appropriate welding fillers with adequate creep characteristics that fully meet the strength and corrosion requirements of the base materials, while ensuring good weldability and sufficient toughness.
In response, Böhler Welding has developed matching filler metals for steel T24 (7CrMoVTiB10-10; 1.7378; ASME SA 213) and martensitic steels P92 (1.4901; X10CrWMoVNb9-2; ASME A 355) and VM12-SHC, all approved by Germany’s VdTÜV, which means they are suitable for use in pressure-bearing components. The developments were conducted in close co-operation with Vallourec & Mannesmann Deutschland.
So what do the metallurgical properties of these materials after gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW) and submerged arc welding (SAW) show? And what are the peculiarities of their welding and heat treatment, and what conclusions can be drawn about their proper handling?
Bainitic steel T24
For steel T24, welding fillers have been developed for GTAW, SMAW and SAW. A comparison of the all-weld metal compared to the base metal (Table 1) found that because of the titanium’s high oxygen affinity a more or less visible titanium burnout occurs during the welding, compared with the initial values of the wire. This applies for GTAW, where the arc should be optimally protected by inert gas, and for welding with the slag forming SMAW and SAW. This caused the welding filler metals to alloy with niobium instead of titanium, two elements that are carbide-forming and help provide a creep strength like that of martensitic steels P91 and P92.
In general a post-weld heat treatment (PWHT) is not necessary if the hardness values are below 350 HV10. According to a continuous cooling temperature (CCT) diagram of the base metal from Vallourec & Mannesmann, the hardness level is between 310 and 360 HV10. A CCT diagram of the all-weld metal shows hardness levels between 340 and 360 HV10 for normal cooling rates after welding. It also shows that the weld metal has a bainitic/martensitic structure. The martensitic part can be 15–30 per cent of the weld metal.
GTAW of thin-walled tubes of T24 requires no PWHT if the welding parameters are optimised so that the preheating is at 100–150°C, the interpass temperature is 200–250°C and the welding involves thin beads and a narrow scatter band of current at welding speed. A PWHT at 740°C/2 hours reduces the hardness of the all-weld metal to less than 250 HV10.
In the as-welded condition, the strain test Charpy V-notch (CVN) values for GTA weldments are high and will not increase significantly after a PWHT because during the welding of the multiple layer joint of the all-weld metal specimen every layer has an annealing effect. Nearly the same situation arises with welding tubes of wall thicknesses above 10 mm. Here CVN values can be achieved that are always high. However, with a 6 mm wall thickness low CVN values of less than 15 J will result if the welder is not properly trained.
A multiple-layer technique is also required for welding thin tubes. The welding parameter has to be optimised so that the layer beneath experiences an annealing effect.
The GTA weldments are sensitive to end crater cracking in the root. This can be avoided by training the welder and optimising the welding parameters. The down slope amperage after closing the root should be reduced by no more than 50 per cent to prevent the critical area of the root cooling too quickly. Purging gas is highly recommended on welding the root and during the hot pass.
In the case of SMAW of T24, the stick electrode mentioned in Table 1 has a carbon content of 0.08 per cent. It should undergo PWHT otherwise deformability and toughness are low. For welding on side in particular, a carbon-reduced electrode (0.06 %) has been developed for use without PWHT (Böhler Fox P24 WW or Thermanit P24 WW). In the as-welded condition, the all-weld metal of this electrode also shows low CVN values, of less than 27 J, but good deformability is guaranteed. In any case, a preheating at 100°C is highly recommended and the interpass temperature should be 200–250°C.
The SAW of T24 waterwalls (tube to fin) requires the use of special flux BB 305 or UV 305. The waterwall will not undergo a PWHT. The hardness distribution over the cross-section of such joints shows high values in the weld and the heat affected zone (HAZ), up to 380 HV10. This cannot be avoided totally. The carbon contents of the base and metals have a significant influence but the carbon content is also important to guarantee the creep properties for parts that will be under pressure. Therefore a carbon content of 0.07–0.09 per cent is necessary in the steel and in the weld.
A hardness of 350 HV10 means that the material has a yield strength of more than 800 MPa and a tensile strength above 1000 MPa. That means there is a high risk of hydrogen-induced cracks forming. It is vital to eliminate hydrogen sources during SAW of the water walls. Preheating at 80–100°C and a reheating via a soaking treatment directly behind the welding station is highly recommended to reduce the cooling rate. The flux must be rebaked before use.
Table 1 does not show SAW all-weld metal properties of the wire-flux-combination because flux BB 24/UV 305 is designed only for single layer applications and high welding speeds. Using the multiple-layer technique with this flux means that due to the high level of oxygen a burnout of some elements cannot be avoided – and very low CVN values will be the consequence. The all-weld metal test has nothing to do with the real application of single bead welds.
Figure 1 shows the results of a bending test in the as-welded condition, and no cracks were detected. This test shows that the deformability of these high-strength weldments is acceptable.
|Figure 1: Bending test of a SA-welded T24 tube to T24 fins: no PWHT; no cracks; mandrel Ø = 50 mm|
T24 is a high-strength material, in particular when it is in weldments without PWHT, so much more attention has to be paid to the preheating and interpass temperatures and the welding parameter than is the case for lower alloyed steels.
Martensitic steel P92
Almost parallel to the development of the base material P92 came the development of welding fillers of the same composition for GTAW, SMAW, SAW and flux cored arc welding. (FCW) Numerous publications describe the latter’s properties, so it is suffice to mention that what requires close attention besides the precisely controlled additions of the creep-relevant alloying elements, such as carbon, vanadium, niobium, nitrogen, boron and tungsten, is heat control during welding.
Martensitic steel P92 is welded in the martensitic temperature range of 250–350°C. Because of the microstructure of this material, temperature control during welding and during the PWHT requires the utmost care.
After welding, the weld metal hardness lies at around 400 HV10. The PWHT, recommended at 760°C, reduces this to about 250 HV10. The holding time depends on the wall thickness and the welding process.
The relatively low hardness reduces the risk of cold cracking in the welded condition, so components with low residual stresses may be cooled to room temperature after welding. However, they must be stored dry and free from external loads, even during transport. Under this condition there is no time limit between welding and PWHT from a metallurgical point of view.
Power plant operators and supervisory agencies attach great importance to the maximum toughness properties of welding filler metals. Metallurgically, however, there is not much scope with martensitic grades for raising the impact energy of SMA and SA weld metals to a level significantly above 47 J. Their toughness can be influenced somewhat via the selection of the welding and heat treatment parameters. Here it is important to allow the welded joint to cool to below the martensite finish (Mf) temperature before heat treatment. This ensures complete tempering of the martensite. The Mf temperature for the welding fillers of the same composition as P92 is about 150°C, so the welded joint must be cooled to at least 100°C. As an additional safeguard against hydrogen-induced cold cracking, the material can be soaked at 250–300°C immediately after welding, for 2–3 hours, to allow the hydrogen to diffuse. This is not necessary with thin walls and GTAW or GMAW.
Figure 2 shows the heat control during welding and PWHT. Keeping the welding passes thin will improve the joint toughness. The thinner each pass, the greater the tempering. This must be taken into account, especially in SAW.
|Figure 2: P92, heat control during welding and PWHT condition|
Several pipe joints were welded during the development of the welding material and the qualification measures. Table 2 provides composition figures for elements in welding fillers of the same composition as P92 and for welded joints in P92 pipes. A PWHT with an extended hold time of at least 4 hours at a temperature of 760°C is recommended for SAW.
|Figure 3: Influence of the PWHT condition to the toughness of the all weld metal for P92|
Figure 3 shows the dependency of the notched bar impact value on the conditions of the PWHT for the weld metal that matches the P92 base material, while Figure 4 shows the notch bar impact values of all-weld metals aged at 600°C. After a holding time of 1000 hours at 600°C the CVN values drop from above 41 J to 25 J. This behaviour is well known and should be taken into consideration during the pressure test that follows the repair welds of used pipes.
|Figure 4: Toughness of the all-weld metal for P92 after aging treatment at 600 °C; SMAW-all weld metal|
The technical data sheet for P92 specifies a temperature range of 730–780°C for the PWHT after the welding procedure. However, a temperature of 760°C is required for the welding fillers to restrict the required hold time to an economically reasonable 2–4 hours (see Figure 2). Low temperatures can be selected but they reduce the toughness in the weld metal or require substantially longer hold times to regain the required level. Temperatures significantly higher than 760°C can cause the Ac1b temperature to be exceeded. However, it is not critical to exceed the temperature by up to 15 °C and for a short time.
Fifteen years after service of P91 in power plants the Mn+Ni values of filler metals are under discussion. It is well known that increasing the nickel content lowers the creep strength. But this test has been carried out on base metal, not on weldments. It is also well known that the HAZ is the weakest point, where type IV cracking occurs. If the weld metal creep strength is inside the scatter band of the base metal, the creep fracture always occurs in the HAZ.
Matching fillers for P92 are not yet standardised. The manganese-plus-nickel (Mn+Ni) content of P92-filler is a maximum of 1.5 per cent. But specifications in the market require a maximum of 1.2 per cent. Mn+Ni affects the Ac1 temperature. The recommended PWHT temperature of 760°C is 16°C below the Ac1 temperature for a Mn+Ni content of 1.46 per cent. The influence of a lower content of Mn+Ni on the mechanical properties of the weld metal has been analysed. It appears that a low manganese content is more detrimental to toughness than a low nickel content.
The influence of the PWHT temperature on the hardness of the all-weld metal with a Mn+Ni content of 1.43 per cent, which means the Ac1 temperature is 776°C, was investigated There is no negative effect on a small overlap of Ac1 at the PWHT. Above 780–790°C the hardness increases, which means the PWHT at a temperature higher than 10°C above Ac1 creates austenite, which transforms to untempered martensite after the PWHT. Ongoing creep tests shows that there is no negative effect to the creep strength if there is only a small overlap of 10–15°C above Ac1. There is no need to limit the Mn+Ni content from a maximum of 1.5 per cent to 1.2 per cent. But it is very important that the PWHT temperature is measured exactly at the pipe and that the scatter band of the temperature range is small. The PWHT temperature of 760°C ± 10°C can be recommended if the Mn+Ni content is a maximum of 1.5 per cent.
In the tests on P92 joints, all the rupture points at 600°C were within the scatter band of the base material. At higher temperatures, however, some of the rupture points were located outside the scatter band, and at high stresses and test periods of less than 20,000 hours a few isolated ruptures were found in the weld metal. Specimens subjected to extended testing of greater than 20,000 hours suffered creep ruptures in the HAZ At 650°C all the ruptures were located in the HAZ.
P92 is only used up to 600°C for reheat steam tubes because the scaling resistance is insufficient at higher temperatures. Here materials with higher chromium content have to be used. The European research programme COST 536 has optimised 12 per cent chrome steel VM12-SHC, developed by Vallourec & Mannesmann, for applications at up to 625°C. Böhler Schweisstechnik has developed welding fillers matching VM12-SHC for the GTAW and SMAW processes. The wire for GTAW can also be used for gas metal arc welding (GMAW).
The existing alloying concept means the weld metal has high mechanical strength. At the same time, however, there is a toughness level in the welding filler metal showing values below 40 J, lower than the values of the welding filler metal for the 9 per cent chromium steel P92. Nevertheless the toughness level is sufficient because the required minimum values are currently at greater than 27 J for the base metal. However, it is necessary to exercise reasonable care during the welding process. The PWHT should be performed at a temperature of 770°C since sufficiently high toughness values cannot be guaranteed at a temperature of 760°C (Figure 2). Table 3 shows the results for the all-weld metal for GTAW and SMAW.
Compared with P92, this alloy has a higher content of chromium. Since this causes a formation of ferrite, this must be balanced with an austenite-forming element. For this, the chemical element cobalt will be used because it does not have any influence on the Ac1b point compared with nickel.
VM12-SHC was qualified by Germany’s VdTÜV for wall sizes up to 10 mm and is welded using GTA and SMA. The time for the PWHT in the GTA procedure was only 30 minutes, which can be regarded as the lowest limit. The PWHT for welding with SMA and GMA should be specified with a holding time of at least 2 hours.
Creep rupture tests are now being conducted on the welded joints. Test results after 20,000 hours show no cracks in the weld. The Ac1 temperature of the all-weld metal is 756°C, or 14°C below the recommended PWHT. The mechanical properties and creep test results show no detrimental influence of this small overlap of 14°C with the PWHT temperature. VM12-SHC has been used in Europe in nearly all new coal- and gas-fired power plants.
In summary, strict observation of stringent welding parameters is essential, especially for welded joints in T24 tubes without PWHT. Preliminary tests under operating conditions are indispensable here if hardness values of greater than 350 HV10 in the weld metal are to be ensured and cracks in the welded joints avoided.
The development of welding filler metals must progress almost simultaneously with the development of base materials. Design engineers need the strength values of the welded joint as determined in creep tests so that they can reliably design components that are subject to high loads of pressure and temperature.
Finally, developing filler metals that fully meet the strength and corrosion requirements of the base materials while at the same time ensuring good weldability and sufficient toughness is a real challenge and and will remain so.
Dr. Herbert Heuser and Dr. Kwan-Gyu Tak are from Böhler Schweisstechnik Deutschland in Germany. For more information, visit www.boehler-welding.com.
The article is based on a winner at this year’s POWER-GEN Europe Best Paper Awards.
To support the drive towards ever-higher efficiency in power stations, the development of novel welding fillers must go hand-in-hand with that of new advanced base materials
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