Non-destructive testing

Raw material, raw castings and completely finished compo­nents can be examined physically to determine the quality of certain aspects of the material. This type of examination falls into two categories:

• surface inspection,

• interior inspection.

Surface inspection looks for discontinuities in the surface which could be detrimental to the service life of the component. Cracks in the surface create stress raisers which can lead to fa­tigue failures. Pinholes in the surface may indicate porosity.

Internal examinations can show the integrity of the material and identify any impurities, inclusions or voids in critical locations. Impurities, inclusions and voids detract from the cross-sec­tional area available for stressing and create stress raisers. Po­rosity can lead to problems of leakage.

When flaws are detected it has to be decided whetherthe flaw is serious, if it can be repaired or whether it should be repaired. Some national standards, particularly pressure vessel stan­dards, have categories for defects. The manufacturer’s re­quirements may be more or less stringent than published stan­dards. If the flaw is in raw material, a casting or piece of plate, it may be more cost-effective to scrap it rather than expend more time and money on repairs. Iftheflawis in a semi-finished piece there may be more incentive to repair. If the flaw is in a finished component there may be compelling financial reasons for a repair.

Visual inspection

Sand cast axial impeller blades and hubs for all types of fan may be made by pouring molten metal into a prepared mould and al­lowing it to solidify. Following shake-out from the mould and clean-up, many such casting will be heat treated and machined. During this process certain surface and subsurface imperfec­tions may become evident.

Surface imperfections in sand castings can vary in the level of importance from significant to superficial. Surface imperfec­tions in their approximate order of importance based on the im­perfection type and its effect on casting serviceability are now discussed:

A) Cracks in castings appear as tight, linear separations in the material that are continuous or intermittent. Cracks may be jagged or straight. Cracks are not acceptable.

B) Surface hot tears are likely to be found at tight curvatures in the casting or where there is an abrupt change in casting thickness. Hot tears are not acceptable.

C) Surface shrinkage is occasionally visible on the cast sur­face where a riser has been removed or on a machined surface.

D) Surface and subsurface porosity or pin-holes in castings are formed as a result of gas formation during solidifica­tion. Sub-surface gas inclusions or porosity are evaluated by the radiographer if the casting is radiographic quality. Surface porosity is often the result of moisture in a sand mould which has not been pre-heated properly.

Generally, surface porosity in castings is not considered harmful if it is 0.8 mm diameter or less and not concen­trated. In such cases, it is customary to explore grind 10% of the indications and accept the condition if the porosity is shallow and no subsurface pockets are opened. Porosity in castings is considered unacceptable when it is concen­trated in a specific area.

E) The surface of a casting may show evidence of trapped sand which was dislodged from the sand mould during a pour and which floated to the upper part of the casting (cope side). Rough surface indentations are evidence of sand inclusions where the sand has been removed during abrasive clean-up. It should be kept in mind that such cri­teria are for guidance.

F) In order to accelerate the solidification process in specific areas of a casting, the foundry mould designer places metal chills in those areas of the mould. Such chills are ex­pected to be melted by the molten metal and fused com­pletely into the solidified casting. Occasionally, the chills do not completely fuse with the cast metal.

G) Veins are irregular, linear ridges on the surfaces of cast­ings which are produced by cracks in the sand mould. For most applications, moderate veining is considered ac­ceptable.

H) Rat tails are irregular, linear depressions on the surface of castings which result from ridges in the mould surface. Such depressions are relatively shallow. For most applica­tions rat tails are considered acceptable.

I) Wrinkles, laps and coldshuts are surface irregularities caused by incomplete fusion or by folding of molten metal surfaces. Where such imperfections are of negligible depth the condition is considered acceptable. Where a lack of fusion exists, the imperfection should be explored to sound metal.

J) Scabs are raised imperfections adhering to the cast sur­face. They are usually sand crusted over a thin porous layer of metal and are often considered unacceptable. Re­moval by grinding usually verifies that scabs are surface imperfections only.

Radiographic inspection

Radiography, X-ray, is accepted as the highest grade of internal inspection and the most costly. Radiography is good because a permanent record of the inspection is available at any time for review. It is used mostly for welds and also for critical areas in castings. Components can be taken to fixed X-ray equipment. Large components or assemblies are radiographed using ra-

Non-destructive testing

Figure 17.1 Room based real-time radiographic system

Dioactive isotopes. Strict safety precautions must be enforced. In view of its importance for the integrity of high performance fans, this method is described in the greatest detail.

The most technically advanced companies in the fan industry have facilities which by using real time techniques cut the time defects by about two-thirds, enabling production and delivery improved. The system (Figure 17.1) is more sensitive, more t also provide a far more comprehensive and easily accessible s: previous X-ray units. Each moving part is stamped with a all X-ray images are automatically archived onto 50 mm laser. 30 years, providing full component traceability.

Inspection by X-rays is carried out by irradiating one surface of the specimen with X-rays whilst a radiation sensitive electronic imaging sensor is held against the opposite surface. The radia­tion, in passing through the specimen, is differentially absorbed by discontinuities caused by flaws, voids, changes in thickness or material density, and an image of the variations integrated throughout the sample thickness, is produced on the surface of the electronic sensing screen.

After the electronic image has been noise reduced, it is dis­played on screen where variations within the specimen appear as shadow objects of differing half tones, from which informa­tion may be obtained about the presence of flaws. The record produced in this way is known as a real time radiograph. Real time because the image display is live and if the specimen is moved the X-ray radiograph changes to show the correspond­ing incident shadow on the image display. The use of X-rays to produce a radiograph is called X-Radiography. Figures 17.2 and 17.3 show two examples of impeller blade radiographs.

X-rays are a form of electromagnetic radiation which may be generated by causing a stream of fast-moving high energy electrons to strike a metal target. The sudden deceleration of electrons gives rise to radiation of photons (X-rays) with a con­tinuous energy spectrum.

X-rays possess great penetrating power which increases with increasing energy of the waves (increasing frequency or shorter wavelengths). X-ray equipment is defined by the energising voltage, which can typically range from 25 kV to 15 m V. X-rays can be used to examine items varying from layers of paper to steel of thickness up to 0.5 metre. All materials are penetrated by X-rays, but the greater the density, the less the penetration.

Radiation of short wavelength produced by high target potential is said to be of high energy and is described as a hard X-ray, having greater penetrating power. Longer wave radiation pro­duced by lower target potential is said to be low energy and is described as soft X-ray, having lower penetrating power.

The penetration ability of X-radiation may be expressed in terms of a given material thickness (e. g. steel or aluminium) that can be adequately inspected.

For low energy, constant potential X-ray generators, the beam intensity produced by the X-ray tube is determined mainly by the magnitude of the filament current, and to a lesser extent by the target potential. A near linear relationship exists between fil­ament current and beam current so it is customary to express the output capability of such a tube in terms of filament current.

Figure 17.2 Example of acceptable blade radiography

подпись: 
figure 17.2 example of acceptable blade radiography
The quality of a real time X-ray radiograph is nearly always quoted in terms of the amount of detail discernible on the image of an image quality indicator (IQI) of the same material as the specimen placed on the surface of the specimen. This IQI sen­sitivity depends upon the radiographic technique used, the type of IQI and specimen thickness. When radiographing other ma­terials other than steel it is customary to use conversion tables related to the material and radiation energy to obtain approximate equivalent thickness factors.

In the UK two different patterns of IQI are recommended, known as the “wire” type and the “step hole” type and one or the other is commonly used in most European countries. In the USA an ASTM-plaque is generally used.

IQI sensitivities are expressed as percentage values, i. e. the size of the minimum discernible IQI details is expressed as a percentage of the specimen thickness, thus a smaller numeri­cal value implies a better sensitivity. Typical radiographic sensi­tivities range between 0.5 and 2.5 percent depending upon inspection variables.

A recommended procedure for reporting weld and castings de­fects in a radiograph is to use a three-part code:

(1) A number to denote the horizontal or vertical distance in inches between “the reference mark or the lowest number on the radiograph and the start of the defect.

(2) A code letter or letters to denote the type of defect (see ab­breviations in Table 17.1).

(3)

Figure 17.3 Example of unacceptable blade radiography

подпись: 
figure 17.3 example of unacceptable blade radiography
A number to denote the approximate length in inches over which the particular defect extends.

…………………. II

Surface imperfections

Code

Description

SXP

Excessive penetration

SRC

Root concavity

SGI

Incompletely-filled groove

SGS

Shrinkage groove

Sue

Undercut

SSP

Weld spatter

SED

Underflushing (excessive dressing)

SMG

Grinding mark

SMC

Chipping mark

SMH

Hammer mark

STS

Torn surface

SPT

Surface pitting

Internal defects

Code

Description

K

Crack

KL

Longitudinal crack

KT

Transverse crack

KE

Edge crack

KC

Crater crack

L

Lack of fusion

LS

Lack of side fusion

LR

Lack of root fusion

LI

Lack of inter-run fusion

RP

Incomplete root penetration

I

Inclusion

IL

Linear inclusion

IT

Tungsten inclusion

IC

Copper inclusion

PG

Gas pore

P

Porosity

PU

Uniform porosity

PL

Localised porosity

PP

Linear porosity

EC

Elongated cavities

WH

Worm hole (pipe)

CP

Crater pipe

BT

Burn-through

DM

Diffraction mottling

For example, an X-ray radiograph image showing the existence of lack of fusion commencing 50 mm (2 inches) from the refer­ence mark over a length of 25 mm (1 inch) and the defect re­peated 150 mm (6 inches from) the reference mark over a length of 25 mm (1 inch), and also localized porosity for 19 mm (0.75 inch) at a distance of 150 mm (6 inches) from the refer­ence mark, the code would be 2-L-1: 6-PL-0.75: 8.5-L-0.5.

Realtime radiographic facilities are used in any of the following modes:

A) Intermediate stage product inspection or intermediate ra­diography. As a general rule when the items are cast, an inspection at that stage segregates the good castings and rejects before any value is added to the casting. This mini­mises wastage of time before the casting is handled and cleaned for burrs and excessive materials.

Radiographic technique is usually defined for different products. This inspection stage can be carried out any number of times before the finished product stage. In this mode product inspection records are not normally re­quired. Good castings are transferred to the next produc­tion stage and rejects dealt with as appropriate.

B) Intermediate radiography with image storage. Quality de­mands on products may stipulate a minimum acceptance quality for defect sizes and type. Proof of acceptance based on records may be required by independent inspec­tors. Once the products are accepted for the next stage, radiographic records may be required for short term stor­age requirements, possibly 6 to 24 months.

C) Real time radiography with record keeping and digital or analogue long term image storage. Safety critical and sensitive application products normally require inspection records to be held in archives for the duration of the prod­uct life. A complete history of the product has to be main­tained. Stringent quality control inspection specifications are stipulated and adherence to the specification is man­datory.

Recommended procedures demand that:

1. Each product item to be inspected is identifiable with a unique numbering system.

2. Each product type will adhere to radiographic inspection techniques. This setup will ensure repeatable and reliable inspection of castings.

3. Each radiographic image will be identified by the unique product reference number that is stamped on the cast­ings.

4. Multiple views of a product may require different set up and recorded sequentially on video.

5. The operator manually logs on the inspection records the verdict of the image and total acceptability of the items.

It should be noted that subsurface imperfections can only be determined by methods such as radiography and ultrasonic. There have been many instances where an apparently good casting has failed, only to reveal quite massive internal faults.

Subsurface imperfections include shrinkage, hot tears and

Inclusions as follows:

A) Shrinkage — subsurface: this is often referred to as centreline shrinkage since it occurs near the mid point of the casting wall which is the last to solidify. Since shrink­age is a subsurface condition, it should be evaluated by ra­diography.

B) Hot tears: Casting tears usually appear at points of thick­ness transition and are attributable to contraction stresses during cooling and the low hoop strength of the casting.

C) Inclusions: Subsurface non-metallic inclusions such as sand, slag and trapped gas pockets or porosity are readily identifiable during radiographic inspection.

Acceptance criteria for X-ray examination

It will be apparent that strength and integrity are closely related to the quality of a component as cast. The criteria for accep­tance are generally those described in ASTM Standard E155 together with its reference radiographs. A procedure should be adopted which defines the inspection process. Before X-ray or fluoroscopic examinations are carried out, the following checks should be made:

• In aluminium die castings, there should be no visible signs of surface porosity or cracks.

• In aluminium sand castings, there should be no visible signs of blowholes or mould misalignment

• Aluminium heat treated castings should be checked for hardness.

• Blade carriers in malleable cast iron should have no blow­holes or surface scabs.

Additional criteria are given for blades and hubs according to the stresses imposed on them during operation.

Blades may be divided into 3 basic categories as shown in Fig­ure 1 7.4 where areas required to be of high integrity are shown cross-hatched, whilst areas having a lower integrity require­ment are shown plain.

Areas of high integrity should be substantially free of any de­fects, the maximum allowable being:

• An area of porosity not greater than 5 mm diameter as sam­ple micrograph No. 1, as defined in ASTM E155.

• A single isolated defect not greater than 2 mm diameter.

Non-destructive testing

Non-destructive testing

Non-destructive testing

Non-destructive testing

(ASdmwwons m mДmelms)

Figure 17.5 Acceptance criteria for hubs

In areas of low integrity the maximum defect allowable should be:

• An area of porosity not greater than 10 mm diameter.

• A single isolated defect not greater than 5 mm diameter.

The defects should not be within 5 mm of the boundary of the casting and only one defect should be allowed in each compo­nent.

Hubs for axial flow impellers are as shown in Figure17.5.

The acceptance criteria for cast aluminium hubs and clamp plates should be:

1. There shall be no surface porosity visible on the casting.

2. There shall be no porosity breaking through into cored holes.

3. There shall be no porosity within 10 mm of any boundary surface looking in the axial direction.

4. There shall be no inclusions greater than 1 mm in diame­ter.

5. There shall be no group of inclusions (each less than I mm diameter) greater than 10 mm diameter in total and shall comply with 3. above.

6. There shall not be more than two such groups in note 5. and they shall not be adjacent to one another.

7. Gas holes or porosity shall be acceptable so long as they comply with 1. to 6. above. Shrinkage cavities and poros­ity, foreign materials, micro shrinkage etc. shall not be ac­cepted.

8. The level of porosity shall be no greater than plate 4 alu­minium — gas porosity (round) as per ASTM E155.

9. There shall be no continuous defect line during X-ray be­tween the insert and aluminium casting. The defects shall not be longer than 3 mm. The total defects shall not be greater than 6 mm and shall not be adjacent to one an­other.

10. On the machining of the casting, a fine “Witness" line will be tolerated as long as a fine pointer scriber will not pene­trate more than 0.5 mm deep.

11. Flaking or pitting of the area between the insert and alu­minium is not acceptable.

The techniques described will act as a powerful tool to identify areas for improvement or for process variables to be tailored to improve overall quality of product. It is essential that a design and testing procedure is adopted which recognizes that a major cause of failure especially in axial flow impellers is due to insuf­ficient knowledge of the fatigue criteria and how they are af­fected by casting quality.

Close co-operation between design and production depart­ments is necessary to ensure that the stated operating life is achieved. Constant vigilance is, nevertheless, indicated with continual research to improve knowledge. By this vigilance, product integrity can be assured.

Ultrasonic inspection

Ultrasonic inspection is popular because it can be conducted with portable equipment. The display on a cathode ray tube in­dicates the position of flaws in respect to the thickness of the material. The size of the flaw must be assessed by an experi­enced operator. This technique is very good for inspecting large flat plates prior to fabrication or forgings. It is also of great value in assessing the porosity of cast aluminium motor rotors.

Dye penetrant inspection

Dye penetration, or “dye pen” is a surface inspection method which is ideal for finding pinholes and hairline cracks.

The surface is first sprayed with dye which is allowed to soak into any surface defects. After a specified time, the dye is washed off and the component cleaned. Chalk is finally sprayed onto the surface. If surface defects exist, the dye trapped in the defect is drawn into the chalk by capillary action. Defects are outlined by dye indications in the chalk. The tech­nique is generally used on finished machined surfaces. A skilled operator can judge the depth of the defect by the size of the “bleed-out”.

“Dye pen” is particularly useful on welds and surface coatings. Weld integrity is degraded significantly by the presence of sur­face imperfections. “Dye pen” can indicate if a surface coating has achieved complete coverage without porosity, pinholes or cracks. It works on non-metallic coatings as well as metallic.

Magnetic particle inspection

Magnetic particle, or “mag particle” is a surface inspection method which is popular for cast materials. This inspection can only be conducted on materials which can be magnetised by an electric current. The surface is coated with a liquid bearing small magnetic particles. If the surface contains flaws, the mag­netic flux is concentrated around them, drawing the magnetic particles towards the flaw.

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