Power-plant piping, boiler heating surfaces, turbine blades and wind-turbine flanges and bolts serve for years under high temperature, high pressure, steam oxidation and alternating loads; pearlite spheroidization, graphitization, creep cavities and decarburization are the main forms of aging and failure. Metallographic examination, including field replication, is a core tool of metal supervision in thermal power plants, while wind-turbine tower flanges and high-strength bolts rely on metallography and hardness testing to verify quenching-and-tempering quality and surface decarburization control.
Typical test objects
- 20G, 15CrMoG and 12Cr1MoV boiler heating-surface tubes and main steam piping
- P91/P92 high-parameter piping welded joints
- 1Cr13, 2Cr13 and 17-4PH turbine blades
- 42CrMoA and 40CrNiMo wind-turbine flanges and high-strength bolts
- QT400-18 wind-turbine hubs and frame nodular iron castings
- ZG230-450 power-station valve steel castings
Inspection challenges
- Heating-surface tubes have thin walls and large curvature and deform easily during preparation; pearlite spheroidization rating demands clear structure display without scratch artifacts;
- The fine structure of P91 weld metal and HAZ requires high magnification, and the etching window for tempered martensite laths is narrow;
- The blade airfoil is curved and flexible, making sampling and clamping difficult, with high edge-retention requirements;
- Wind-turbine bolts are high-strength quenched-and-tempered parts, and decarburization at the thread root must be measured on a section cut precisely normal to the thread axis;
- Field sampling of large flanges and piping is restricted, so field metallography or replication often has to be combined with laboratory examination.
Recommended preparation and inspection route
Cut transverse sections from the pipe circumference; mount small-diameter tubes upright in cold resin. Grind with SiC papers from P240 to P2000, then polish with 3 μm and 1 μm diamond. Etch ferrite-pearlite steels with 4% nital and rate pearlite spheroidization according to DL/T 884; lightly etch P91 welded joints with Vilella's reagent or 4% nital and observe martensite laths and carbide precipitation at 400–500×; reveal grain boundaries and carbides of martensitic stainless blades by 10% oxalic acid electrolytic etching or an aqua regia-glycerol etchant. Measure the decarburized layer by the microscopic method of GB/T 224, arbitrated by the microhardness profile method (HV0.3) when disputed; verify the quenching-and-tempering level by macro Vickers (GB/T 4340.1) or Rockwell hardness.
Common standards
- DL/T 884 Technical guideline for metallographic inspection and evaluation of thermal power plants
- GB/T 13298 Metallographic examination methods for metals
- GB/T 13299 Steel — Determination of microstructure
- GB/T 6394 Determination of average grain size of metals
- GB/T 224 Determination of depth of decarburization of steels
The above methods and parameters are application examples for technical communication rather than fixed process recipes; the actual preparation route, etchants and acceptance criteria should be confirmed against the material, dimensions and inspection purpose of each specific specimen.
Typical inspection scenarios
Pearlite spheroidization and aging assessment of 12Cr1MoV boiler heating-surface tubes
- Challenge
- After long-term high-temperature service the carbides in pearlite gradually spheroidize, and the degree of spheroidization directly correlates with remaining tube life, so rating demands a true and clear structure display; the thin, highly curved wall deforms easily during sampling and grinding, and scratches or relief may be mistaken for carbide morphology; oxidation and decarburized layers exist on the service tube surfaces, so rating must avoid the surface-affected zones.
- Preparation & method
- Cut a tube ring and section it axially on a low-speed precision saw with a resin-bonded diamond wheel and ample cooling to avoid structural change; cold mount upright to hold the transverse section. Grind with SiC papers from P240 to P2000 and polish with 3 μm and 1 μm diamond to a mirror finish. Etch with 4% nital and observe pearlite morphology and carbide distribution in bright field at 400–500×, rate the pearlite spheroidization level against the standard charts of DL/T 884, and check for creep cavities and graphitization tendency; measure micro-Vickers hardness at HV0.5 at mid-wall and judge the tube aging degree and fitness for further service by combining the spheroidization level with the hardness drop.
- Reference standards
- DL/T 884 Technical guideline for metallographic inspection and evaluation of thermal power plants; GB/T 13298 Metallographic examination methods for metals; GB/T 13299 Steel — Determination of microstructure
Quenched-and-tempered structure and grain size examination of 1Cr13 turbine blades
- Challenge
- The blade airfoil is a thin-walled curved surface with poor rigidity, making sampling and clamping difficult, and edge rounding during grinding loses surface carbide and grain-boundary information; the quenched-and-tempered sorbite structure is fine, and both over- and under-etching prevent clear display; excessive austenitizing temperature produces coarse grains and delta ferrite, so grain size and phase constitution must be rated together.
- Preparation & method
- Sample at the blade root tenon or a specified airfoil location on a low-speed precision diamond saw with a resin-bonded wheel and ample cooling; cold mount with supporting filler to protect thin edges. Grind with SiC papers from P320 to P2000 and polish with 3 μm and 1 μm diamond. Reveal grain boundaries and carbides by 10% oxalic acid electrolytic etching (about 6 V, 10–20 s) or an aqua regia-glycerol etchant, observe the tempered sorbite and carbide distribution in bright field at 100–500×, rate grain size by the intercept or comparison method of GB/T 6394, and rate delta ferrite per GB/T 13299; measure micro-Vickers hardness at HV0.5 across the airfoil section and judge the acceptability of the quenching-and-tempering process from structure, grain size and hardness.
- Reference standards
- GB/T 13298 Metallographic examination methods for metals; GB/T 6394 Determination of average grain size of metals; GB/T 13299 Steel — Determination of microstructure
12Cr1MoV main steam piping, aged service section
- Challenge
- Thin walls and high curvature make the small cut sections prone to warping during grinding and polishing, degrading flatness and spheroidization ratings; the pearlite spheroidization rating is highly sensitive to surface quality, and scratches or uneven etching create artefacts; field sampling is restricted, so on-site replication metallography and laboratory work must corroborate each other.
- Preparation & method
- Cut small transverse blocks containing both the inner and outer surfaces around the circumference and cold mount them upright to prevent warping; grind with SiC papers from P240 to P2000 and polish with 3 μm then 1 μm diamond to a scratch- and relief-free finish. Etch with 4% nital (picral if pearlite contrast is poor) and rate the pearlite spheroidization level at 400–500× per DL/T 884, examining carbide coarsening and the pearlite morphology within the ferritic matrix; where necessary, confirm softening with an HV0.3 microhardness profile compared against an unused section.
- Reference standards
- DL/T 884 Technical guide for metallographic inspection and evaluation in fossil power plants; GB/T 13298 Metallographic method for microstructure inspection of metals; GB/T 4340.1 Metallic materials — Vickers hardness test
P91 high-grade steel piping weld joint
- Challenge
- The fine tempered martensite laths tolerate only a narrow etching window — over-etching obscures the lath morphology and carbide precipitation; the heat-affected zone contains a softening band prone to type-IV cracking and must be located precisely by a hardness line scan; the zones differ in composition and structure, so a single etch rarely shows all of them clearly at once.
- Preparation & method
- Cut a small transverse section across the weld at low speed with ample coolant; cold mount to protect the weld edges. Grind from P240 to P2000 and polish with 3 μm then 1 μm diamond. Lightly etch with Vilella's reagent (picric acid + HCl + ethanol) and examine the tempered martensite laths, delta ferrite and carbide distribution in the weld, HAZ and base metal at 400–500×; run an HV0.5 micro-Vickers line scan across the joint (0.5 mm steps, refined to 0.1 mm across the softening band) to determine its width and hardness minimum; macro-etch to cross-check the weld contour and root quality.
- Reference standards
- DL/T 884 Technical guide for metallographic inspection and evaluation in fossil power plants; ISO 17639 Destructive tests on welds in metallic materials — Macroscopic and microscopic examination of welds; GB/T 4340.1 Metallic materials — Vickers hardness test
Scenarios are method examples, not customer test reports; confirm the process per specimen.
MICROCRE learning notes · confirm the method for the specimen.