casting quality test methods

8/17/2019 Casting Quality Test Methods

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Casting Quality Test MethodsInspection and testing of castings encompasses five main categories: casting finishing, dimensional accuracy,

mechanical properties, chemical composition and casting soundness.

Casting Finish

The surface finish of a metal casting can be influenced by the type of pattern or molding sand, mold coating, andmethod of cleaning. So far, instrumentation for measuring surface roughness has not provided a useful evaluation,

so it is performed largely through simple visual comparison using a series of test panels ith increasing surface

roughness.

!imensional "ccuracy

#ariation in the dimensions of a casting can be the result of mold cavity e$pansion caused by the heat and head

pressure of molten metal, the contraction of the metal as it cools and heat treatment. These e$pansions and

contractions are predicted by the patternma%er ho ill compensate for the variations in the pattern&s design. For

large volumes of castings, casting facilities may measure the critical dimensions of the castings more often to

chec% for possible drift, particularly drift due to pattern ear. If a casting re'uires tight tolerances that are

critical to the part&s application, those tolerances should be specified by the customer.

Casting customers should chec% ho their casting supplier ill verify the dimensions of the parts they produce.

The accuracy of the measuring tools is (ust as important as the dimensional accuracy of the castings. In many

instances, the gauges or fi$tures needed to routinely chec% the dimension are supplied to the casting facility by

the customer.

)$pecting e$act dimensions over the course of a production run ill result in frustration. The dimensions of each

casting ill vary slightly, so castings are specified by setting a range of values that the dimensions can fall ithin.

The range beteen the loer tolerance limit and upper tolerance limit can be set by the supplier, but the

narroer the range, the more difficult to produce and test and therefore more costly the casting ill be.

Mechanical *roperties

Mechanical testing gives an evaluation of the metal and the casting to determine hether the properties are in

compliance ith the specified mechanical re'uirements. Folloing are common mechanical tests used in

metalcasting facilities.

Hardness testing—the most commonly used procedure for mechanical property testing, it provides a numerical

value and is nondestructive. +ardness values generally relate to an alloy&s machinability and ear resistance. The

rinell hardness test uses a -/mm diameter carbide ball to indent a 0,/%g load. The impressions are large

enough to provide a dependable average hardness. 1oc%ell hardness tests ma%e smaller indented impressions,

hich also can be satisfactory if the median of several values is used.

Tensile and impact testing—conducted on test specimens of standardi2ed dimensions, the to most commont

types are tensile and Charpy impact. Tensile testing provides ultimate tensile strength, yield strength, elongation

and reduction of area data. Charpy impact testing determines the amount of energy absorbed during fracture and

is used to gauge ductility and strength.

Service load testing—usually conducted on the entire casting to evaluate its properties, it can be conducted in a

number of ays. Castings that must carry a structural load can have a load applied in a fi$ture hile the deflection

and the load is measured. *ressure/containing parts can be hydraulically tested to a proof load or destruction.

1otating parts can be spin tested. These types of tests chec% the soundness of the casting, as ell as its

properties.


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Chemical Composition

The chemical composition of an alloy has a significant bearing on its performance properties. Chemical

composition can be further affected by minor alloying elements added to the material. Casting alloys are typically

specified accorded to "STM, S") and "MS alloy specifications. !epending on ho susceptible an alloy is to variation

of its chemical composition, chemical analysis may be re'uired to verify the proper composition is present to

achieve a certain set of properties.

Chemical analysis often involves a sample of molten metal poured in to a special mold and evaluated by

spectrographic atomic absorption or $/ray fluorescence analysis. Many metalcasting facilities chec% the chemical

composition of the alloys they are pouring throughout the course of a day, so melt shop personnel can ma%e

re'uired ad(ustments to the alloy composition as needed.

Casting Soundness

The performance of metal components can be notably affected by internal and surface defects that can not be

detected through the regular course of visual inspection. Several nondestructive methods can be employed to

inspect castings for these 3invisible4 flas. 5ondestructive tests determine the integrity of a casting ithout

causing physical damage, so once it passes the tests, it can be used for its intended application. elo is a

detailed list of nondestructive tests.

565/!)ST17CTI#) T)STI58 M)T+6!S

5on/destructive testing gives the metalcasting facility the capability of assuring the 'uality of a casting ithout

destroying it. " metalcasting facility may have internal standards regarding nondestructive testing, but it is up to

the customer to specify specific tests or fre'uency of testing. 9hile various methods of nondestructive testing

e$ist to measure mechanical properties, chemical composition, casting soundness or ma$imum service loads, a

single test that encompasses all these factors does not e$ist. " combination of nondestructive methods may be

re'uired to document the soundness and 'uality of a casting. The most common methods available are described

belo.

#isual Inspection#isual inspection is based on the use of the human eye to identify surface defects, improper filling and molding

errors. Casting defects that can be detected via visual inspection include sand holes, e$cessively rough surface,

surface shrin%age, bloholes, misruns, cold shuts, and surface dross or slag.

!imensional Inspection

To ensure a part meets dimensional re'uirements, such as tolerances, a metalcasting facility can chec% the

dimensional accuracy of a part manually or ith a coordinate measuring machine CMM;. Chec%ing the dimensional

accuracy of a part helps guarantee the customer on&t have to perform further costly machining on a part to meet

the specified dimensions.

CMM has improved the speed and accuracy of measuring casting dimensions, and computeri2ation has made it

repetitive and able to be used as a statistical tool.!ye *enetrant and Fluorescent *oder Testing

For tiny crac%s, pores or other surface glitches that are hard to detect by the human eye, dye penetrant testing is

used for both ferrous and nonferrous materials. In this method, a colored dye solution is applied to the surface of

the casting. The dye, hich is suspended in penetrating oil, ill find its ay into the surface defects. 9hen a

special developer is applied, the defects are clearly indicated.


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" similar method involves fluorescent poder suspended in penetrating oil. "gain, the solution penetrates the

defects, so hen the casting is dusted or sprayed drying poder, the solution is dran from the defect and glos

under an ultraviolet light here defects have occurred. Fluorescent poder testing only detects surface crac%s and

flas but is more effective and economical than radiographic testing.

In general, dye/penetrant techni'ues identify defects on the surface of the casting and do not detect internal

porosity or shrin%age that is not open to the surface. ut it can detect rounded indications for porosity or gas on

the casting surface.

Magnetic *article Inspection

Magnetic particle inspection is 'uic%, ine$pensive and sensitive to defects, particularly shallo .0 in.; surface

crac%s and other lineal indications.

It detects small crac%s on or near the surface of ferrous alloys that can be magneti2ed basically any ferrous alloy

e$cept austenitic material;. " high/amperage, lo/voltage current is passed through the casting, hich establishes

a magnetic field.

Crac%s and defects have magnetic properties different than those of the surrounding material, so their presence

ill interrupt the magnetic field, causing distortion. Small magnetic particles sho the path of the flu$ line that

spreads out in order to detour around the distortion, thereby indicating the shape and position of the crac% or

void.

7ltrasonic Testing

Internal defects that are detected by radiography may also be detected by sound. Sound aves have been used by

fisherman to locate hot fishing spots and depth of ater and by the 7.S. 5avy to identify approaching ob(ects. In

casting inspection, ultrasonic testing uses high fre'uency acoustic energy that is transmitted into a casting.

ecause ultrasonic testing allos investigation of the cross/sectional area of a casting, it is considered to be a

volumetric inspection method.

The high fre'uency acoustic energy travels through the casting until it hits the opposite surface or an interface or

defect. The interface or defect reflects portions of the energy, hich are collected in a receiving unit anddisplayed for the analyst to vie. The pattern of the energy deflection can indicate the location and si2e of an

internal defect, as ell as all thic%ness and the nodule count of ductile iron.

7ltrasonic testing re'uires a high does of %noledge and e$perience for an accurate interpretation of the results,

hich ill affect the cost added to the part for the inspection.

1adiographic Inspection

"nother method used to detect internal defects is radiographic inspection. 9hen done correctly, radiographic

inspection is the best nondestructive method for detecting internal defects, such as shrin%age and inclusions.

In this method, a casting is e$posed to radiation from an $/ray tube. The casting absorbs part of the radiation, and

the remaining portion of the radiation e$poses the radiographic film. !ense material ithstands the radiation

penetration, so the film is e$posed to a lesser degree in those areas, giving the film a lighter appearance.


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1adiographic inspection also can be performed ithout film. Instead, the $/ray image is vieed on a video screen.

Computeri2ed a$ial tomography C"T scanning; also is being used to develop 0/! computer imagery to inspect a

casting&s soundness.

)ddy Current Inspection

The eddy current inspection method is applied to the detection of crac%s at or near the surface. "n electrically

charged coil carrying an alternate current causes an eddy current to flo in any nearby metal. The eddy current

may react on the coil to produce substantial changes in its reactivity and resistance, and that reaction is used to

pinpoint small crac%s or defects.

)ddy current inspection is accurate for the detection of small flas or material changes that may not be detected

ith other inspection methods, and the discontinuities in the casting ill give an immediate response on the

monitoring e'uipment. +oever, it re'uires a vast amount of %noledge and e$perience to properly interpret the

results, hich ill affect the added cost to the part. The test only can be used ith electrically conductive

materials.

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casting quality test methods

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