ISSN 1061-8309, Russian Journal of Nondestructive Testing, 2006, Vol. 42, No. 3, pp. 212–216. © Pleiades Publishing, Inc., 2006.Original Russian Text © V.M. Zuev, 2006, published in Defektoskopiya, 2006, Vol. 42, No. 3, pp. 84–90.

212

Testing of Metal Structure during X-Raying of Welded Joints

V. M. Zuev

OAO Izhorskiye Zavody, pr. Lenina 1, Kolpino, St. Petersburg, 196651 Russia

Received August 9, 2005

Abstract

—The possibility of using images of structural origin observed in X-ray pictures of weldedjoints as a source of information about the structure of tested metal is considered. Practical recommen-dations for testing welding flaws and the state of metal structure in welded joints via X-raying are given.

DOI:

10.1134/S1061830906030119

A method for identifying X-ray images of structural origin in the form of light and dark strips and spotsis presented in [1, 2]. Such images, which are observed in X-ray images of austenitic and other welded jointswith a coarse-grain metal structure, hinder interpretation of pictures. The appearance of structural strips andspots is facilitated by diffraction (wave) phenomena that develop when the X-rays belonging to the softlongwave part of the spectrum interact with the granular structure of the X-rayed metal. The method foridentification of images is based on rescanning of ambiguous areas with filtered X-ray radiation or (if thescanned thickness is large) with

γ

-radiation of

192

Ir or

60

Co isotopes. In this procedure, only “hard” quantaare involved, which virtually do not exhibit the wave properties. Owing to this circumstance, the reason forthe appearance of structural images is removed and, correspondingly, the structural background that masksimages of welding flaws disappears. This phenomenon prevents both over-rejection of welded joints on thebasis of structural images and under-rejection, i.e., omission of flaws obscured in the original picture bystructural images.

However, images of structural origin, which are considered in practical applications of X-ray testing asa structural noise that deteriorates detectability of flaws and reliability of testing, may contain informationabout specific features of X-rayed metal. In contrast to data of metallographic analysis conducted on simu-lation specimens, X-ray data are related directly to the tested item. Thus, a possibility appears to concur-rently test (during X-raying of austenitic and other welded joints) both the welding flaws and the state andcharacteristics of the weld and base metal.

The possibility to obtain information about the structure of tested metal via radiographic methods isillustrated in Figs. 1 and 2. These figures schematically display X-ray images of structures belonging to dif-ferent types and the dependence of

∆

D

/

γ

D

(where

∆

D

is the image’s contrast and

γ

D

is the contrast coefficientof the film) for the images of structural origin on X-raying angle

ϕ

, voltage

U

Xrt

applied to the X-ray tube,and thickness

δ

f

of the lead filter on the exit window of the X-ray tube. These images are presented in com-parison with images of standard grooves simulating welding flaws.

During assessment of the state of the metal structure in a tested article using results of X-ray analysis,the factors described below must be taken into account.

The type of structure (transcrystallite, granular volume-distributed, or dendritic) may be identified fromspecific features of the structure’s image (Fig. 1, Ia, b, c) and appearance/disappearance of the structure’simage when the direction of X-rays changes (Fig. 1, IIa, b, c).

According to metallographic research, the shape and orientation of structural grains, crystallites, anddendrites in metal are related to the shape and orientation of the structural images in the X-ray pictures(Fig. 1, I).

The presence of structural inhomogeneities whose density and/or chemical composition significantlydiffer from regular structural components of the tested metal may be detected from the dependence of con-trast

∆

D

/

γ

D

on the voltage at the X-ray tube,

U

Xrt

, (Fig. 2b) and the thickness of the X-ray filter,

δ

f

, (Fig. 2c)that differs from an analogous dependence for a specimen welded and tested under standard conditions. Forexample, in the case of local microporosity in a tested specimen, the plot of the dependence

∆

D

/

γ

D

=

f

(

δ

f

)will deviate from the curve for a specimen with the normal structure (Fig. 2c, curve

2

) toward the curve forstandard grooves simulating welding flaws (Fig. 2c, curve

1

). The respective images that look like dark spotsand strips persist when the thickness of the X-ray filter,

δ

f

, reaches the value at which structural images dis-

RADIATIONAND OTHER METHODS

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2006

TESTING OF METAL STRUCTURE DURING X-RAYING OF WELDED JOINTS 213

appear in X-ray pictures of the rescanned specimen welded under normal conditions, that is, the specimenthat simulates the tested welded joint.

Reflecting the state of the tested metal’s structure, the characteristics (shape and contrast) of the imagesof structural origin depend on the mode and conditions of welding and thermal processing of the specimen.This dependence makes it possible to check whether the established modes of welding and thermal process-ing of articles were observed. Such an inspection is performed by comparing these characteristics to therespective characteristics of the images of the metal structure in the simulation specimen that passed allrequired mechanical and metallographic tests. For example, experiments show that contrast

∆

D

of transcrys-tallite dark strips in X-ray images of austenitic welded joints steeply increases for exceedingly large valuesof the welding current and welding speed.

These observations enable usage of X-ray images for assessing not only parameters of detected weldingflaws but also the specific features of the recorded metal structure of welded joints. Comparative analysis ofthe images of structural origin in the working images of an article and in the standard images of a simulationspecimen of the tested welded joint makes it possible to record changes in the structure of the X-rayed metal,which are related to the failure of observing established welding regimes, and to make timely amendmentsto the welding process. The control over the structure’s state is especially important during repair of weldedjoints where flaws are selected and the defective sites are welded anew.

The issue of structural images may be expressed in analytic terms as described below. It is assumed that,in a general case, separate fragments in the X-rayed metal’s structure may differ to some extent in theirchemical composition and density and that generation of X-ray images of structural origin depends on bothwave (diffraction, interference, and total internal reflection on crystals' boundaries) and corpuscular (relativeattenuation of the flux of quanta) phenomena. Contrast

∆

D

str

of the images of structural origin may be rep-resented then as

(1)

where

∆

is the corpuscular component of contrast

∆

D

str

determined in accordance with the known

expression for the value of

∆

D

of a flaw containing some filling substance [3],

∆

is the wave compo-

∆Dstr ∆Dstrwave ∆Dstr

corp,+=

∆Dstrcorp 0.43γ D µ µstr–( )/B[ ]∆dstr,=

∆Dstrwave ∆Dstr ∆Dstr

corp,–=

Dstrcorp

Dstrwave

I

RR

II15–30°

III

R

f

γ

–

192

Ir,

60

Co

(a)

1 1 1

(b)

12

3 12

312

3

(c)

1 2 1 2

Fig. 1.

Schematic view of X-ray pictures with images of structural origin: (I) standard X-raying; (II) X-rays directed at anangle with respect to a welded joint’s plane; (III) X-raying using filtered X-rays and

γ

-radiation of the isotopes

192

Ir and

60

Co; (a) images of an austenitic welded joint with transcrystallite structure; (b) images of a welded joint with a volume-distributed granular structure; (c) images of a welded joint made using electroslag welding; (

1

), (

2

), and (

3

) welding flawsbelonging to the discontinuity or foreign-inclusion type.

214

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ZUEV

nent of contrast

∆

D

str

,

µ

and

µ

str

are the linear coefficients of radiation attenuation for the base metal of thewelded joint and of the substance contained in the area of structural inhomogeneity,

B

is the coefficient ofaccumulation of scattered radiation, and

∆

d

str

is the dimension of structural inhomogeneity in the X-raydirection.

Expression (1) shows that, for metal with a uniform coarse-grain structure that is characterized by con-

dition

µ

=

µ

str

, we obtain

∆

= 0 and

∆

D

str

=

∆

. This means that images of structural origin aregenerated in this case only owing to the wave properties of X-ray radiation. An array of densely alternating

Dstrcorp Dstr

wave

(

∆

D

/

γ

D

)

ϕ

/(

∆

D

/

γ

D

)

ϕ

= 0

°

040302010

0.4

0.8

U

r.t.

= 150 kV

d

= 10 mm

1

2

040302010

0.4

0.8

U

r.t.

= 230 kV

d

= 30 mm

1

2

ϕ°

(∆D

/γD

)U/(

∆D/γ

D)U

= 1

00 k

V

0400200

0.4

0.8d = 10 mm

2

1(∆

D/γ

D) ϕ

/(∆D

/γD

) ϕ =

0°

(∆D

/γD

)U/(

∆D/γ

D)U

= 1

50 k

V

0400200

0.4

0.8d = 30 mm

2

1

ϕ

d

Ur.t., kV

(∆D

/γD

) δf/

(∆D

/γD

) δf

= 0

°

0 2.01.0

0.4

0.8d = 12 mm

2

1

0.5 1.5

Ur.t. = 300 kV

(∆D

/γD

) δf/

(∆D

/γD

) δf

= 0

°

0 2.01.0

0.4

0.8 d = 12 mm

2

1

0.5 1.5

Ur.t. = 400 kV

δf, mm

δϕ

(a)

(b)

(c)

Fig. 2. (1) Contrast of images for grooves in a simulation calibration block and (2) images of structural origin belonging tostrip type as a function of (a) tilt angle of X-rays, (b) voltage on the X-ray tube, and (c) thickness of a lead filter installedon the exit window of an X-ray tube ((a, b) austenitic specimens with thicknesses d = 10 and 30 mm and the transcrystallitestructure of the welded joint, (c) welded specimen with thickness d = 12 mm made of steel 09X17H with a crystallite elon-gated structure of metal in the area surrounding the welded joint).

RUSSIAN JOURNAL OF NONDESTRUCTIVE TESTING Vol. 42 No. 3 2006

TESTING OF METAL STRUCTURE DURING X-RAYING OF WELDED JOINTS 215

dark and light spots is an example of such images. They are a consequence of the diffraction of soft X-rayson a uniform volume-distributed structure of X-rayed metal (Fig. 1, Ib).

If a structure exhibits strongly pronounced nonuniform density ρ and chemical composition (atomicnumber of substance Z) in separate local areas of tested metal, inequality µstr � µ may hold (since µ =f(ρ, Z)) and, if the wave (diffraction) effects of X-ray radiation are exhibited in an image comparatively

weakly, ∆ � ∆ and ∆Dstr ≅ ∆ . Such a situation may be observed, for example, if metal con-tains a large number of microporosity areas that look like a set of dark spots in the image. If only regularwelding flaws are detected in an image (high-energy radiation and carbon steel), this circumstance may beconsidered a partial case of the general mechanism for generation of X-ray images described by expression(1). Namely, this case corresponds to the absence of significant manifestation of the wave properties of radi-

ation (∆ < ∆Dmin) and, correspondingly, to the presence of virtually only the corpuscular component

that generates the image of a flaw: (∆ = ∆Dflaw).

Situations intermediate with respect to the considered two extreme cases are possible, where the typicalstructure of metal is characterized by the presence of inhomogeneities that differ, to some extent, in the den-sity and chemical composition from the average characteristics for this metal. The value of ∆Dmin then con-

tains both ∆ and ∆ . The relation between these two components depends on the structure’sdegree of homogeneity. It should be understood in this case that the physical processes generating the cor-puscular and wave components of a structural inhomogeneity’s contrast are different and that overall con-

trast ∆Dstr is determined by a random combination of its components ∆ and ∆ in the picture. The

images (local variations of optical density) corresponding to ∆ and ∆ differ to some extent intheir shape and dimensions.

The sign of contrast ∆D (dark or light spots or strips) for the wave and corpuscular components may beeither the same or opposite. If the sign is the same, the wave component may mask and obscure the corpus-cular component of structural inhomogeneity and a flaw’s image. If the signs of the components are differ-ent, the wave component is usually perceived as a background for the corpuscular component that belongsto a structural inhomogeneity’s contrast and that is observed in an image.

Using expression (1), one can enhance the assessment of the tested welded joint’s structure with anapproximate quantitative estimate for certain characteristics of detected structural inhomogeneities: µstr =f(ρstr, Zstr), ρstr, and ∆dstr. Such an estimate can be obtained when images are identified by re-X-raying of an

article and an X-ray filter is used whose thickness equals that of filter that allows removal of theimages of structural origin in the pictures that are made anew for a simulation specimen of the tested weldedjoint which passed the required metallographic tests. It is then assumed that the images observed in the arti-

cle’s images that are made anew are generated by the corpuscular component of contrast alone, i.e., ∆

( = ) = ∆ and ∆ < ∆Dmin, where ∆Dmin is the minimum contrast of the observed imagewhose shape and dimensions (length and width) are similar to those of the image identified in the originalpicture.

During second X-raying of an article, identification standard groove ∆ is detected in the image.

This groove ensures fulfillment in the second image of condition ∆ ≅ ∆ and, correspondingly,

∆ = ∆ . (The same calibration block with grooves of a different depth may be used for the

first and second X-raying procedures.) For a hollow groove, ∆ = 0.43 (µ/B)∆ [3]; for a struc-

tural inhomogeneity, according to expression (1), when ∆ 0, contrast ∆ ≅ ∆ =

0.43 [(µ – µstr)/B]∆dstr. The X-ray images of the calibration block and the structure are obtained in thesame picture, thus ensuring that B = const. It is also possible to ensure that Db.str = Db.gr, where Db.str and Db.str

are the optical densities, respectively, of the picture’s background in the area of the structural image and of

Dstrwave Dstr

corp Dstrcorp

Dstrwave

Dstrcorp

Dstrwave Dstr

corp

Dstrwave Dstr

corp

Dstrwave Dstr

corp

δfart δf

image

Dstrart

δfart δf

image Dstrcorp Dstr

wave

dgrid sec( )

Dgrid sec( ) Dstr

art

Dgrid sec( ) Dstr

corp

Dgrid γ D

gr dgrid

Dstrwave Dstr

art Dstrcorp

γ Dstr

216

RUSSIAN JOURNAL OF NONDESTRUCTIVE TESTING Vol. 42 No. 3 2006

ZUEV

the image of the identification groove in the calibration block and = . Equalities ∆ = ∆ =

∆ and then yield

Let us denote µ – µstr = ∆µstr with the consideration that µ and µstr correspond to irradiation during thesecond X-raying procedure. Coefficients µ, µstr = f(ρ, Z), where ρ and Z are the density and atomic numberof the absorber’s material. Thus, we arrive at the following expression for the generalized (complex) char-acteristic of a structure’s inhomogeneity, (∆µstr/µ)∆dstr, that reflects the extent to which the density andchemical compositions of an inhomogeneity and its dimensions in the cross section of a welded joint aredifferent:

(2)

Expression (2) corresponds to a structural inhomogeneity detected during the second X-raying proce-dure where filtered X-ray radiation is used. At the same time, this expression may be used to estimate theupper limit for the complex characteristic of the structure’s inhomogeneity removed with an X-ray filter. It

is then assumed that ∆ = 0, ∆ = ∆Dmin, and the depth of the identification groove is equal to its

minimum identifiable depth ∆ .

By setting a certain value of ∆dstr that corresponds to the data of metallographic studies of specimens,one can calculate the relative difference between coefficients µ and µstr for the tested metal from a formulathat may be deduced from expression (2):

(3)

If the energy of radiation used for the second X-raying procedure for iron and lighter metals is highenough (E > 200 keV), one can set µ ~ ρ [3]. Correspondingly, when identifying images of the structurewhose density is smaller than that of the base metal, where these images look like dark spots or strips, onecan set µstr/µ ≈ ρstr/ρ. This (approximate) equality enables assessment of the relative difference in averagedensity ρ of a tested metal and density ρstr of a structural inhomogeneity.

Quantitative characteristics of the metal structure in the welded joints tested via X-ray methods can beassessed with the proposed technique in only an approximate way. To obtain more detailed informationabout characteristics of the structure, it seems promising to apply other methods of nondestructive testingand, in particular, the ultrasonic method to the areas of the welded joints whose X-ray exposures clearlyexhibit images of structural origin. For example, experiments detect characteristic ultrasonic echo signalsfrom structural inhomogeneities observed in X-ray images as transcrystallite dark strips (Fig. 1, Ia).

Strength characteristics of welded joints depend on the state of their structure not to a weaker degree butsometimes to an even stronger degree than on the presence of welding flaws. Additional information on thestate of the tested metal’s structure increases the informational content and reliability of radiography andenables implementation of active and regulatory tests. This information can be used for engineering diag-nostics of the operational reliability of tested welded joints.

REFERENCES

1. Gromov, Yu.V. and Zuev, V.M., Ghost Flaw Images and the Technique of Their Identification, Defektoskopiya,1984, no. 1, pp. 26–31.

2. Zuev, V.M., Identification of X-ray Diffraction Images of Structural Origin, Defektoskopiya, 1993, no. 6, pp. 46–51.

3. Rumyantsev, S.V., Radiatsionnaya Defektoskopiya (Radiation Flaw Detection), Moscow: Atomizdat, 1974.

γ Dstr γ D

gr Dstrart Dstr

corp

Dgrid sec( )

µ µstr–( )∆dstr µ∆dgrid sec( ).=

∆µstr/µ( )∆dstr ∆dgrid sec( ).=

Dstrwave Dstr

corp

dgr minid sec( )

µstr/µ 1 ∆dgrid sec( )/∆dstr.–=