外文文献原文及译文

附录1 外文文献原文及译文

原文：

An evaluation of NDT methods for the location

and sizing of forging discontinuities

In selecting an NDT method for flaw detection in forgings a number of variables must be

considered:

a) the type of discontinuity to be assessed;

b) the method to be used for detection and evaluation, and

c) the variables associated with the forging itself

The variables in item a) will govern the location within the forging and its orientation

with respect to a particular surface Item b) could include a considerable array of NDT

methods, but for the purpose of this paper only the six most widely used are considered一visual testing (VT), penetrant inspection(PI), magnetic particle inspection(MI), eddy current

testing (ET), radiographic inspection (RT) and ultrasonic inspection (UI). In the last item c)

the component race include such things as condition, geometry access for inspection.

a) Forging discontinuities

The location of the discontinuity will have a significant influence on the selection of the

NDT method to be used and they are therefore grouped into three categories, to aid this

selection:

1. open to the surface: laps, seam, burst, slugs, cracks and inclusions

2. slightly subsurface: seam, stringers, inclusions and grain structure variations

3. internal: stringers, burst, lamination, grain structure, inclusions and piping

A brief review of these terms may be helpful:

Lap: folded metal, flattened into the surface but not fusing with it

Seam: linear flaws due to oxidized blow holes or ingot splashes, which are elongated by hot

working

Burst: ruptures caused by failure of plastic deformation by processing at too low a

temperature or excessive working of metal

Stringers: a bar stock defect, due to non metallic inclusions being squeezed out into long

and thin strings

Lamination: planar defect aligned parallel to surface, originating in the original ingot from

rolled out piping

Cracks: transgranular failure, due to localized stresses resulting from non-uniform heating

or cooling and non-plastic deformation

Inclusions: impurities, such as slag, oxide and sulphides, often from the original molten

stage in forming the billet used for forging

Grain structure: depending upon the extent of working, (deformation and recrystllisation)

can be as small as 0.5mm or as large as 10mm

Piping: a cavity at the centre of the ingot or billet, caused by shrinkage during solidification

Slug: a piece of foreign matter that has been pressed or rolled into the surface of the material

b) The NDT Method

VT—visual testing is the oldest of the NDT methods but still valid and widely used today

The system is based upon observation, usually by a human observer, but now increasingly

by digital/video cameras which use pattern recognition to locate dissimilar areas in a surface.

The sensitivity will depend upon the method but typically a good observer with simple visual aids can resolve 0.5mm differences aids will include magnifying glasses (up to x10),

microscopes(up to x100) and fibred-optic bores copes and endoscopes for viewing

internal details in hollow or complex sections. The system is used for surface inspection only

with costs in the range $4 to $4000.

PT一the surface is covered with brightly covered oil (typically red or fluorescent), which

will penetrate any surface openings. After removal of excess, an absorbent, white powder is

applied, which draws any trapped oil to the surface. This creates an indication of the

presence of the surface opening. This process, like visual inspection, also requires visual

acuity, but the indications are ‘enhanced’ by the process, since ‘bleed-out’ spreads the visual

image. Costs can range from as little as $4 for a couple of cans, to $8000 for a process ‘line’.

Both VT and PT are surface inspection systems only arid will therefore detect only those

discontinuities that have a definite surface opening Surface cleanliness is very important,

particularly with PT.

MT一ferromagnetic materials carrying a large flux density; retain the presence internally,

with little external evidence other than at the poles. Any discontinuity in the material will

disturb this uniform flux and create a small ‘leakage’ at the site of the discontinuity. This

leakage can be detected by the fact that finely divided; ferromagnetic particles collect at

the-site, creating an indication. As with PT, the particles can be colored, to increase contrast,

which when viewed under suitable lighting, create a clear visual image of the discontinuity.

However, unlike PT the leakage can pass through thin layers of paint or plating materials, so