Relationship Between Design and Materials for Thermal Power Plants. S. C. Chetal

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1 Relationship Between Design and Materials for Thermal Power Plants S. C. Chetal

2 Contents *Introduction to design codes *Operational life vs design basis life *Material selection basics *Materials for boiler tubes and piping *Relatively new boiler tube materials *Manufacturing aspects *Summary

3 Introduction to Design Codes *For boiler tubes, headers, piping and valves, the design codes provide the formulas for establishing the minimum thickness. *Every design code provides the basis for establishing allowable stresses. Most engineers using ASME codes are not familiar with this aspect. *Codes are asset to designers, manufacturers and users. Codes are not handbooks and are not safe in the hands of beginners *ASME codes provide the tabulated data for the allowable stresses. *IBR and European codes do not provide the allowable stress in tabulated manner and one need codified material design data. *The design codes do not exist for turbines.

4 Basis for Establishing Allowable Stresses *Lowest of the following: 1. The specified minimum tensile strength at room temperature divided by The tensile strength at temperature divided by two-third of the specified minimum yield strength at room temperature 4. Two-third of yield strength at temperature( allowed up to 90% of the minimum yield strength for austenitic stainless steels except for flanges. Two sets of allowable stresses in the codes) % of average stress to cause creep rate of.01%/1000 hr 6. 67% of average stress to cause rupture at end of hr 7. 80% of minimum stress to cause rupture at end of hr

5 What is Component Life When Designed as Per Codes *Design of components as per ASME codes based on creep data of hr by no means mean component life of hours or design as per European code of hr has life of hr. *Operating life of a boiler tube as per ASME with factor of safety of minimum 1.25 on minimum stress to rupture is not much different from European code with factor of safety of 1.25 on average stress to rupture for hr. *The real life is linked to how the plant had operated. All things remaining same, thicker parts because of higher fatigue damage will have shorter life. If the operation is with in design conditions, boiler tubes will be able to operate for longer period than main steam header. *A unit designed as base load will be impacted by two shift plant operation due to increased fatigue damage allowing less admissible creep damage. *A boiler tube designed as per ASME codes is assured of life in excess of 230,000 hr due to margins on minimum stress to rupture. It could be further more because of operating pressure lower than design pressure, material stronger than code and tube thicker than design thickness. It will be far less if overheating takes place leading to metal temp higher than design temp.

6 Material Properties and Design Codes *Inclusion of a material in design codes is a long process; new materials normally included initially as code case in ASME. *The tensile ductility and creep ductility do not get directly in the design but codes include materials with adequate ductility. *The fracture mechanics does not enter directly into design but codes specify minimum impact properties at the lowest design temperature and puts upper limit on tensile strength at room temperature for a number of steels as materials with high strength invariably have poor toughness. * Codes do not provide the background of creep data extrapolation methods. European codes demand minimum data generation for hr while ASME demand data for just over hr with values also for shorter time. Material data on least 3 heats required.

7 Material Selection Basics *Adequate mechanical strength at design temperature( high tensile and creep properties, high low cycle and high cycle fatigue, high impact strength at minimum temperature and high ductility) *Corrosion resistance (scaling resistance, also water chemistry important) *Excellent weldability and manufacturability *Physical properties (high thermal conductivity, high thermal diffusivity, low thermal expansion coefficient) *Operating experience for similar application and design conditions *Availability of materials in required sizes *Inclusion in design codes and allowed by regulatory body *Overall economics

8 Development of T91/P91 Steel One of the most popular steel for superheater and reheater. Developed in USA for application in sodium cooled fast reactors which use austenitic stainless steel with view to reduce chromium and nickel. The steel was developed by ORNL after a number of melts were tested with attention paid to mechanical properties, corrosion, welding, fabricability and code requirements apart from economics The strongest material in Cr-Mo family at that time with allowable stresses comparable to SS304. Fatigue resistance much higher than SS 304 due to better physical properties. Chemical composition: 9Cr-1Mo-.2 V-.08 Nb-.05 N. Allowable stresses in MPa Temperature C Gr Gr Gr SS

9 Best Practice Guidelines for Usage of Grade 91 Steel Components *Codes do revise the allowable stresses as more data is available on creep. *A number of users upgrade the material specifications and manufacturing requirements to realise high reliable component. *ASME specifications for nitrogen and aluminium are % and.02% respectively, and hardness is max 250HB. The operating experience of premature failures has led to specify N/Al ratio of minimum 2 and preferably minimum 4. The minimum hardness of base material should be 200HB and limited to 230HB for tubes with severe cold forming operation. *The postweld heat treatment should be performed within 8hours of completion of the joint. PWHT temperature is raised to 750C to improve toughness. The hardness of weld and HAZ is between 190 to 300 HB

10 How to Handle Improper Heat Treatment Example of Gr 91. A few failures reported due to overheating. Recommended PWHT C * If a portion of the component is heated above the heat treatment temperature stated above, one of the following choices: (1)The component in its entirety be normalised and tempered. (2) If the temperature exceedance is not beyond 800C,the weld metal be removed and replaced. (3) The portion of the component heated above 800C and 75mm on either side of overheated zone be replaced. (4) The allowable stress to be taken as that of Grade 9 material and affected portion be heat treated within specified values stated above.

11 Introduction to Grade 92 *Continuous development of creep resistant steels with consideration for oxidation resistance.9to 12% Cr steels with different alloying elements by steel producers. *Grade 92,9Cr-2W, an improved creep resistant steel over Gr 91,was introduced initially for steam piping and headers and its usage for superheater and reheater tubing is recent development. The oxidation resistance is comparable to Gr 91 as content of Cr is same. Material C Cr Mo V W Nb others N N.05+ B Allowable stresses in MPa Temp C

12 Economics in Choice of Materials Choice between T91 and T92 for boiler tubing as an a example Benefit of higher admissible stress leading to lower tube thickness in case of T92 to be weighted against higher cost of T92. USC plant with tube design mid-wall temperature of 600C T92 will be around 13% lower in thickness. Cost of T92 tubing is typically 20% higher than T91 mainly due to less usage and still an ASME code case. Overall economics in such a case of boiler tubing in favour of T91. The situation in case of header is different with Gr 92 better from transient thermal stresses leading to relatively shorter start up time

13 Introduction to T23/T24Steels *Need for increased steam parameters to improve efficiency led to development materials with enhanced creep properties and weldability to eliminate postweld heat treatment for water walls. *Improvements in T22 grade. Low carbon from weldability and replacing Mo by W in T23. Addition of carbide forming elements V, Nb, Ti and small addition of B to improve creep properties. *Grade C Cr Mo W V Nb Ti B ppm I ppm T23and T24 tubes heat treatment is normalised and tempered. T23 is developed in Japan And T24 in Germany.

14 T23/T24 Steels *T23 andt24 grades have significantly higher allowable stresses over grade T 22 and thus leading to lower thickness for tubes and headers. *Relative thickness for design temperature of 545C 191bar Grade Thickness comparison in comparison to 22 grade *Limits of temperature from steam oxidation of 580C rather creep *PWHT: Mix reaction to no need for PWHT up to 10mm thickness. Further concern on increase in hardness for welds without PWHT due to ageing. Better to perform postweld heat treatment

15 Austenitic Stainless Steels for Superheater *Adequate mechanical strength (Creep) *Steam oxidation resistance *Weldability Allowable stress in MPa Temperature C H H HFG fine grain ASTM7-10 with better creep and oxidation resistance in comparison to 347H. Code case H Cu(18Cr-9Ni-3Cu-Nb-N. Shot peened for better oxidation resistance) code case

16 VM12- SHC VM12-SHC( super high corrosion resistant) steel developed by Vallourec 12Cr-1.6 Co-1.5 W-.4Si-B. High temperature steam oxidation resistance better than T91 &T92. Allowable stresses better than T91 but lower than T92. Covered in ASME as code case 2781 and allowed up to 620C Maximum allowable stress in ksi Temp F(C) VM12 T92 T (565) (593) (621)

17 Sanicro 25 *Heat resistant austenitic stainless steel 22Cr 25Ni 3W 1.5Co, 3Cu.45 Nb.25 N *Very high elevated temperature strength, precipitation hardening *High corrosion resistance * Manufacturing similar to 304H Cu. No PWHT *yield strength/average stress to rupture in Mpa Temp. C 304H Cu Sanicro 25 Inconel 617M / / / / /

18 Inconel 740H Nickel base precipitation hardenable superalloy that offers unique combination of creep resistance at elevated temperature along with resistance to coal ash corrosion. Specially developed for advanced ultra supercritical power plant by US for steam conditions of 35MPa/760C main steam condition. 24Cr 18Co 1Al 1Ti 1Nb +B Included in ASME as code case Need postweld heat treatment f unlike Inconel 617 for boiler tubes. Suffers embrittlement on elevated temperature exposure with room temperature impact strength falling from 110 to 40J/cm2 Comparison of allowable stress in MPa Temp. C Inconel 617M Inconel 740H About 35% reduction in thickness

19 Maximum Recommended Metal Temperature Maximum recommended design metal temperature is governed by creep properties and corrosion behaviour( both steam side and fire side) Corrosion behaviour: loss in metal thickness, scale formation leading to increased metal temperature and concern for scale exfoliation. Maximum recommended design temperature for a few grades Grade Maximum temperature C T22,T23,T T T VM SS Super 304H 670

20 Welding Dissimilar Materials *Operating experience of dissimilar weldments not as satisfactory as welds in similar materials. *Coefficient of thermal expansion of two materials with austenitic stainless steels having coefficient around 50% higher than Cr- Mo steels *Location of weld important and should be away from discontinuities like shell to nozzles. Weld nozzles to header in same material and then execute dissimilar weld in nozzle to piping. *Execution of dissimilar welds in shop rather at site. *Metallurgical considerations Risk of cracks during welding, carbon transfer during PWHT and service, corrosion resistance and PWHT temperature of two base materials

21 Damage Potential of Overheated Tube Case of T22 as an example Temperature C Average Rupture Strength MPa 10, , , degree rise from 520 to 530 or 530 to 540 C reduces life by half 45 degree rise fro 520 to 565C reduces life to just 5%

22 Creep-Fatigue Interaction for some Materials

23 Summary *Wide choice available for materials for a given component. Economics should be given importance in material selection. *R&D for new materials with improved creep properties and corrosion resistance is a continuous process. *Weldability is crucial for success of any material. *Be in touch with operating experience of power plants and new materials being included in ASME to arrive at optimum material selection and associated fabrication requirements. *Make best use of optimum material specifications to facilitate plant life extension.

24 Thank you