ASTM A 418 Standard Practice for Ultrasonic Examination of Turbine and Generator Steel Rotor Forgings

Scope:

This method covers the Basic procedure for Ultrasonic Examination of the turbine and generator rotor forgings. This practice is intended to provide a way of inspecting the cylinder so the sensitivity at the center line and surface of the bore is constant. This inspection procedure applies to solid cylindrical forgings with an outside diameter that is 100 inches or greater but not less than 2.5 inches.

Test procedure:

In this method, transducer selection is based on the diameter and the frequency. Here, we have used a 0.5-inch diameter, 2.0 or 2.25 MHz transducer to inspect solid cylindrical sections under 15 inches in diameter and bored sections having wall thicknesses of less than 7.5 inches to inspect solid cylindrical sections under 15 inches in diameter and bored sections having wall thicknesses of less than 7.5 in. The scan was performed in an indication-free region with the signal amplitude set to 100% of the full-screen height. The inspection sensitivity was also adjusted depending on the outside diameter of the forgings. Then Adjusted the sweep length control to position the bore or back reflection approximately three-fourths of the distance across the cathode ray tube. Searched each diameter of the forging and recorded the indications 10 % or greater of the sweep-to-peak CRT segment at the required high sensitivity. Marked the position of the search unit on the surface of the forging when indications are observed so that they may be investigated. When the forgings are tested while they are in rotation, the maximum speed was calculated by,360/πd =maximum revolutions per min, d = diameter of forging in inches. 915/πd = maximum revolutions per min. d = diameter of forging in centimetres. With the forging stationery, measure the intensity and breadth of each indication, and conduct a thorough investigation of a single indication. Another supplementary test was also performed like a special radial test and axial test. 

Video 01: [Field Application] Turbine Rotor Automated Ultrasonic Test

Test Specimen:

Forgings are used as specimen and the Machine turn the forging to provide cylindrical surfaces for the radial test. The surface roughness of exterior finishes was not exceeded 250 µin.

Data:

Recordable indications must be expressed as minimal reflecting areas in the manner described below:

Where,

A_F: minimum reflecting area of discontinuity,

A_R: reflecting area of reference (0.012 in)

a: distance to discontinuity, in.

L: distance to bore or centerline of solid forgings, in.

P: discontinuity reflection amplitude (percent of sweep to-peak CRT segment at the prescribed high sensitivity level).

B: bore diameter 

D: forging cylinder diameter

Conclusion:  

ASTM A 418 gives guidelines to test methods for ultrasonic examination of turbine and generator steel forging.