Use of Replication and Portable Hardness Testing for High Temperature Plant Integrity and Life Assessment
Seminar Power & Process Plant Issues Lahore, Pakistan: May 2011 Use of Replication and Portable Hardness Testing for HighTemperature Plant Integrity and Life Assessment M Hussain, ETD Pakistan 1 email@example.com, +92 345 812 4575
Overview Material degradation assessment is one of the most important fields of life assessment It allows to identify, localize and quantify the damage mechanism impact in a certain component It is used worldwide specially in the power plants (conventional and CCGTs), petrochemical and process industries European Technology Development Ltd. 2
Introduction Life assessment based on calculational procedures is conservative (use minimum parent material properties, etc) Difficult to account for welds Metallographic methods assess the actual material condition of the component/weld Metallographic replication Hardness assessment European Technology Development Ltd.
Life Limiting Areas Welds are frequently the locations for high temperature plant failures HAZ with differing properties Residual stresses Fabrication defects Welds are often located at stress concentrations Welds need detailed evaluation of defects and properties NDE and possibly materials testing carried out European Technology Development Ltd.
Microstructure & Damage Microstructure degrades & creep cavitation damage develops as the consumed life fraction increases Can see these changes if polish and etch at site Hard to use microscope in power plant Need flat surface - Not normal in plant Hence replication European Technology Development Ltd.
Creep Damage Accumulation Isolated and oriented cavities, linking to form micro-cracks, then macro-cracks European Technology Development Ltd.
Precipitate Coarsening Progressive coarsening of precipitates in a low alloy steel European Technology Development Ltd.
Metallographic Replication Must be targeted at the areas most likely to show creep damage Normally welds and bends in pipework High temperatures; end-loads on pipework; known problems Apply across Weld, HAZ and Parent material Examine replicas at site optically before re- lagging Further examination in laboratory European Technology Development Ltd.
Replication Procedure1. Area is grinded to high metallographic standard2. Then polished and etched several times to remove all traces of cold work from polishing3. Finally surface wetted with acetone and acetate foil applied4. Acetone softens foil and capillary action forces the foil to conform to the etched structure5. After drying, the foil is removed with image of the etched surface impressed on foil surface European Technology Development Ltd.
Replication Procedure 1) Rough component surface 2) Component surface polished 3) Component etched to reveal microstructure (Steps 2 and 3 repeated ~4 times) Film 4) Film applied to surface Film 5) Film conforms to surface as solvent dries Film 6) Film stripped from surface with record of microstructure European Technology Development Ltd.
Replication ProcedurePolishing of a P91 reducer Replication of etched crosswelded to a CrMoV HPHT weld surface steam turbine valve (the arrows show the acetate foil) European Technology Development Ltd. 12
Replica Positions Pipework 4 points round welds Intrados, extrados of bends Intersections - saddles and crotch positions Tubing Hottest tubes - superheater, reheater outlets Swollen tubes to quantify remaining life Headers Antler/stub tubes – minimum grinding Plain barrel if any signs of distress Nozzles Turbines Rotor bores European Technology Development Ltd.
Portable grinding and polishing equipment Grinding and polishing headControl unit European Technology Development Ltd. 14
Sentencing of Replicas –Quantifying the Damage Several means of quantifying damage: ‘A’ parameter depends on counting the number of cavitated grain boundaries Gives numerical answer but time-consuming Normal use life based on damage classification schemes Action may be advised but depends on History Future outage schedules Operating practice Best assessed individually European Technology Development Ltd.
Cavitation Damage Classification A: Clear B: Isolated cavitation C: Orientated cavitation D: Microcracking <2mm NDE non-detectable E: Macrocrack >2mm NDE detectable European Technology Development Ltd.After Neubauer
Introduction to Hardness Hardness is used in power, petrochemical and process industry for: quality control life assessment It is defined as the ability of a material to resist permanent indentation or deformation when in contact with an indenter under load European Technology Development Ltd. 18
Introduction to Hardness Basically a hardness test consists of pressing an indenter of known geometry and mechanical properties into the test material European Technology Development Ltd. 19
Hardness – Life relationship Hardness indentation technique is one of the oldest testing methods applied to analyse the materials properties It gives the hardenability of a certain component Conversion charts can convert hardness values to tensile strain and consequent probability to type IV damage. European Technology Development Ltd. 20
Hardness changes and testing Several investigators have developed hardness models to calculate the remaining life of piping and tubing components. Models based on hardness have been developed for low alloy steels and modern steels based on creep data. European Technology Development Ltd. 21
Portable hardness testing equipment MIC 10 control unit Vickers5kgf Probe European Technology Development Ltd. 22
Hardness test for superheater header Shape of indent of on-site hardness measurementRobertson D. et al; ETD Lifing Procedure European Technology Development Ltd. 23
Main steam lineMaterial PropertiesMaterial of pipe A 335 P11Material of fittings A 234 WP11Pipe GeometryOuter diameter, mm 450Wall thickness, mm 40Operating ConditionsPressure, Bar 95Temperature, C 530 Case Study from Recent ETD Work in EuropeService time, hours* 180,071 European Technology Development Ltd. 25
Main steam line (contd.) Base Metal microstructures consisted of ferrite and bainite (or ferrite and pearlite). HAZ microstructure was bainitic and/or martensitic, the Weld Metal microstructure was martensitic. Bainite and pearlite microstructure showed some degradation due to long-term exposure at elevated temperature (i.e. spheroidisation) European Technology Development Ltd. 26
Main steam line (contd.) BM HAZ WM Pipe thermally Moderately Martensite; 166 HB degraded, partially degraded bainite, (400x)spheroidized ferrite partly spheroidized;/ bainite and 130HB 151 HB (400x) (400x) European Technology Development Ltd. 27
Main steam line (contd.) Using optical microscopy, one of the valves, showed intergranular crack of ~2mm length in the coarse-grained region of the HAZ (CGHAZ) on the forging side of the joint (see next slide) The morphology and the crack location were indicative of stress relief damage European Technology Development Ltd. 28
Main steam line (contd.)Stress relief cracking at the forging side (100X) European Technology Development Ltd. 29
Main steam line (contd.) Hardness testing was carried out at-site for each of the replica locations. The hardness values of Base Metal, Weld Metal and HAZ were within the expected ranges for 1¼Cr-½Mo steel after long-term, high-temperature exposure. Some welds exhibited hardness differentials between the weld metal/HAZ and the base metal of ~60-80HB. European Technology Development Ltd. 30
Main steam line (contd.) Hardness (HB) Position Weld HAZ BM 12 O’Clock 203 204 126 4 O’Clock 202 195 128 Valve Pipe 8 O’Clock 204 198 132 Avg. Hardness 203 199 129 12 O’Clock 208 198 135 4 O’Clock 204 201 136 Valve Forging Side 8 O’Clock 206 195 140 Avg. Hardness 206 198 137The hardness test results for the base materials, HAZ andweld metals were within the expected range for P11 steelafter long-term operation at elevated temperature. European Technology Development Ltd. 31
Main steam line (contd.) The creep life fraction consumed for the components exhibiting isolated creep cavities is estimated to be 50% - as the worst case scenario. This means that the remaining creep life of these components is at least 190,000 hours. The components examined showed limited microstructural degradation which was consistent with the plant operating conditions and service time, and the hardness levels at the examined locations were within the expected range for P11 steel after long- term operation at elevated temperature. European Technology Development Ltd. 32
Main steam line (contd.) Due to the presence of the micro crack, it was recommended that the valve should be re-inspected using metallographic replication and appropriate NDE (MT and UT flaw detection) after a further 10,000 hours service. European Technology Development Ltd. 33
Advantages of replication & hardness The technique can be used easily on-site and it is non- destructive. Good resolution of microstructural constituents if surface is well prepared. This technique has a good adaptation on flat and curved surfaces Can be used to monitor the evolution of microstructural changes and it is useful for assessing creep, fatigue, corrosion damage in elevated-temperature components. Can be applied to conventional materials and also to the steel alloys used in turbines and boilers. Can be used to complement other non-destructive techniques such as ultrasonic testing. European Technology Development Ltd. 34
Limitations of replication & hardness The technique only analyzes the microstructure of the outer surface of a material/component. In many cases the surface microstructure can be different from the microstructure found in the interior of the component The replica only reveals the topographic features at the surface; therefore it is impossible to analyse the chemical composition of the elements Contamination may be a problem in harsh or dusty environments Precision is required to operate the hardness probe European Technology Development Ltd. 35
Conclusion It has been proved that replication and hardness can be efficiently used to perform life assessment Sampling removal and analysis are relatively simply and not time consumable Cost of performing these tests is lower in comparison with other non- destructive techniques European Technology Development Ltd. 36
Thank You very much for your attention. Questions? European Technology Development Ltd. 37