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High-definition observation of the metal structure of materials for thermal power plant piping


Micron-sized shape measurement using a laser microscope

Thermal power plant
Thermal power plant

Application

The efficiency of thermal power generation has been increased by advances in material technology. Thermal power plants generate high-pressure, high-temperature steam, and the power of the steam drives a turbine. The higher the pressure and temperature of the steam, the stronger the power. One notable example is the Ultra Super Critical (USC) power plant where the temperature at the inlet of the steam turbine is close to 600 °C (1112 °F), providing a higher generation efficiency than a conventional power plant. The piping used at the USC power plant is made from high chromium steel (9Cr steel, etc.) designed to endure the severe pressure and temperature environment of the plant. Steam occasionally leaks out of the piping near the welds, so these welds must be routinely examined. Because power plants prefer nondestructive testing for their equipment, ultrasonic phased array testing is the preferred technology to check for defects inside the piping. However, at a certain distance from the pipe’s outer surface is a dead zone, where defect detection is difficult due to ultrasonic attenuation. It has become clear that the formation of voids and the growth/coarsening of minute metal structures (subgrains) progresses simultaneously both on the outer surface and inside the pipe, as the damage progresses near the welds. To address this problem, a nondestructive testing technique called the SUMP method was developed. In the SUMP method, a transparent film is placed on the pipe’s outer surface to create an impression of the surface’s minute metal structure. This film is then observed using a microscope. One issue with this method is that many microscopes don’t provide enough magnification to view small grain boundaries that may have a diameter φ as small as a few μm, and images of uneven samples cannot be fully focused.

The Olympus solution

Olympus' LEXT 3D laser scanning microscope captures only focused reflected light when shifting the focus position and builds an extended focal image, eliminating unfocused, unclear images. Since the resolving power of an optical microscope is generally defined by the wavelength of the observed light and the numerical aperture of the objective lens, the LEXT microscope uses 405nm wavelength light. Using this wavelength achieves a resolving power much higher than that of a standard optical microscope. Combined with the confocal optical system, the microscope’s resolving power is further increased by approximately 20%. Equipped with a high-sensitivity photomultiplier, the LEXT microscope can accurately capture the reflection for a sample with weak reflection from its surface.

These functions enable users to acquire clear, fully-focused images even for samples with undulation and low reflectivity, such as the transparent film discussed above. The microscope’s 20, 50, and 100X objective lenses have high numerical apertures and are optimized to make the best use of the 405 nm laser’s performance. Combined with optical zooming, the microscope can achieve a magnification of up to 17000x, enabling acquisition of clear, high-resolution images, even for small metal grain boundaries. These images are effective for checking whether or not voids are present near the welds and when observing the change in subgrain size.

< Samples provided by >
Electric Power Development Co., Ltd.

< References >
* "Materials for Ultra Super Critical Thermal Power Plants and the Problem" by Kazuo Nanba, Electric Power Development Co., Ltd. IPEJ Journal Vol.26 No.6
* "Non-destructive Creep Damage Evaluation through Subgrain Size Measurement of High Chromium Steel Weldment" by Kazuo Nanba, Electric Power Development Co., Ltd. The Thermal and Nuclear Power Journal Vol. 66 No. 8

Images

(1) Standard microscopic image (2) Elimination of defocused image using the extended focal image function (3) Image using the 405nm light source and confocal optical system

Thermal power generation_ob100_3D_standard

Thermal power generation_ob100_3D_extended focal

Thermal power generation_ob100_3D_confocal

Olympus IMS

Products used for this application


LEXT OLS5000NEW

With the Olympus LEXT OLS5000 laser scanning confocal microscope, noncontact, nondestructive 3D observations and measurements are easy to produce. Simply by pushing the Start button, users can measure fine shapes at the submicron level. Ease of use is combined with leading-edge features to deliver an acquisition speed four times faster than our previous model. For customers with larger samples, LEXT long working distance objectives and an extended frame option allow the system to accommodate samples as large as 210 mm.
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