Nondestructive Testing for Evaluating Surface Integrity

Key-Note-fJapers

Nondestructive Testing for Evaluating Surface Integrity E. Brinksmeier, Hannover; E. Schneider, Saarbrucken; W. A. Theiner, Saarbrucken; H. K. Tonshoff, Hannover The

mechanical

properties

and

the

structural

state

of

machined

surfaces

determine

the

functional

behavior of components. The states of the surface-near zone, especially hardness- and residual stress distributions are influenced by machining parameters. Conventional testing methods such as metallographical inspection, x-ray diffraction, and hardness measurements are time consuming and cannot

be

used

for

techniques.

real

time

testing.

Therefore,

there

is a

considerable need

Beside the well known eddy current methods,

be used for measuring

for

nondestructive

testing

ferromagnetic- and ultrasonic techniques can

microstructure parameters and residual stresses. Whereas electromagnetic- and

ferromagnetic analysing methods are restricted to surface-near regions -especially a few millimetersand also restricted to electrical conductive- or ferromagnetic materials, ultrasonic techniques allow one to evaluate the bulk properties of metallic and ceramic material. In addition, superficial states can

also

be

analysed

by

ultrasonic

methods.

In

this

paper

we

focus

on

state

of

the

art

of

nondestructive procedures and we discuss first application for the evaluation of the surface integrity.

1. Introduction It is well known that physical surface properties t"\rtr(]lt'C

can determine the lifetime and function of highly loaded workpieces and components /1-5/. For this reason, more and more the manufacturing industry

lolifiO tranSit on

.

r ad,

\

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J~

,

,------,

asks for information, how to influence the surface state of workpieces and how to achieve constant properties /6/. The physical surface state of a machined workpiece is a function of its material processing and the machining conditions. This surface state is summed up by the expression "Surface Integrity" /1,7/ which

, I d

includes knowledge about the residual stress conditions, hardness values and the metallurgical structures, especially in the surface and subsurface layers.

Fig. 1: Possibilities for increasing the lifetime of a steering knuckle /8/ An optimized design with reduced notch effects at

the cross-sectional steps yields to a threefold lifeti~e.

Examples of the detrimental influence of residual

increase in

stresses are: deformations of a workpiece, either

of the lifetime, however, was achieved by surface

The most efficient improvement

caused by removing stress-affected layers or by intro-

hardening operations. Work hardening by rolling as

ducing residual stresses during the machining opera-

well as nitriding hardening improved the lifetime

tion, decrease of the static and especially the

by a factor of five.

dynamic strength, and the increase of sensitivity to stress corrosion cracking /3/. On the other hand,

To achieve the desired roughness and dimensional

compressive residual stresses can influence the

quality of a workpiece, in many applications after

dynamic strength /1-3,7,8/ positively.

the hardening process a finiShing operation is

The hardness of a workpiece surface can be influenced

operation can lead to critical structure and hardness

necessary. Nevertheless, the subsequent machining in several ways. The chemical composition, the

changes, as will be demonstrated later. An example

metallographic structure and especially the carbon

how a machining operation can influence the fatigue

content determines the hardness of the base material.

strength of workpieces is given in Fig. 2. The fatigue

An increase of hardness can be achieved by thermal

test was carried out with specimens which were ground

treatment (martensitic transformation, casehardning,

with corundum and boron nitride wheels on a surface

nitriding) as well as by mechanical treatment (work

grinder. The machining conditions had been chosen

hardening). A high hardness of workpieces is often

to obtain equal surface roughness. The result is

desired in order to increase the strength, the wear

that the CBN-ground specimens can stand a 70 , higher

resistance and the lifetime. Figure 1 gives an

loading stress amplitude than the corundum ground,

example, how to improve the lifetime of a steering

if a 50 , propability of fracture is defined. This

knuckle by the steel quality, the design and the

effect can be explained by completely different

surface treatment /8/. By increasing the static

residual machining stresses which lie in the tensile

strength of the workpiece material, only minor

region for the corundum ground and in the compressive

improvement could be achieved.

region for the CBN-ground surfaces. Thus, for the manufacturing of high quality workpieces, the physical surface state and above all the influence of the machining operation, have to be well known to the designer and to the operator.

Annals of the CIRP Vol. 33/2/1984

489

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1000 1200 N/mml1400 stress amplitude ao

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Thermal and mechanical impacts caused by machining

Fatigue strength of ground specimens /5/ The combination of the mechanical and thermal impacts

Up to now, calculations of residual stresses and

together with the given material's properties and

hardness alterations due to machining have not been

structure characteristics produced by heat treatment

sufficient for reliable evaluations. Therefore, the

determines the final results.

measurement of surface properties is the only way to find correlations between the machining conditions and the physical properties. only in this way the desired quality can be assured.

The heat influenced surface state of workpieces which represents a main thermal impact can already lead to a wide range of possible distributions of hardness, structure and residual stresses. If a martensitic

Due to the expensive and time consuming determination

transformation takes place during casshardening

of residual stress and hardness analysis, most of

normally compressive residual stresses with a small

this work has to be done in the laboratory and cannot

penetration depth are observed /4/. If the surface

be realized in the process itself. In order to

layers alone are hardened by interrupted quenching,

overcome these restrictions, new methods of analysis

casehardening or nitride hardening, the hardness and

based on magnetic and ultrasonic principles have

residual stress gradients are dependent on the heat

been developed and optimized in the last years. The

treatment operation. In caeehardening, the residual

state of the art of these methods and their ability to evaluate surface integrity of machined surfaces

stress profile depends on the hardness capacity of the material, the quenching process, the carburi-

will be discussed in this paper. Before explaining

zation temperature, depth of casehardening, the

these methods and their results we will give some

dimension of the workpiece etc. In general, com-

typical examples of residual stresses and hardness

pressive residual stresses are observed /14/, the

profiles caused by machining and a short description

maximum of which is in alignment with the depth of

of current methods to analyse these surface states.

carburization (Fig. 4)

Since the subject of surface topography and surface

501~-------,------~-------;

texture has been studied extensively by various

l

scientists it will not be discussed in this paper.

t-----=.f;....------+-- ---2.

I ............

Demands on Nondestructive Techniques for Analyzing Stress and Structure Changes caused by Machining

I I .... } IIop'h 01

0." 'lUI' (orburlZall(J't

2.1 Influence of the Machining Process

integrity have been studied and published by many

II

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drplh b.n.oll'l SIJ,loc. l

authors /1-5, 9-13/. It is not our intention to give in this paper a complete overview, but we will discuss

/

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-5

The influence of machining processes on surface

: / 1 :

I ndrtdp hardpnlng

I

mal )4(,lI6

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Fig. 4: Residual stresses in casehardening and nitride

some typical results of residual stress and hardness

hardening

profiles, especially with respect to extreme surface impacts, which have to be controlled by different

Employing nitride hardening compressive stresses are

methods.

dominant due to the diffusion of nitrogen into the surface layers. The penetration depth can be in-

The main surface impacts during machining are of

fluenced for example by the nitriding time /14/ as

thermal and mechanical nature. The machining condi-

it i. shown in Fig. 4, too.

tions, the properties of the tool and the environment determine the amount, the induction period and the

Thermal impacts without a structural transformation,

induction direction of mechanical and thermal impacts,

however, lead to tensile residual surface stresses

and thus the formation of the physical properties

/3/ .

(Fig. 3).

490

Returning to machining operations, it must be stated

Machining operations such as turning, milling, shaping

that pure mechanical or thermal impacts from the

and gr ind ing always proJuce an overlapping in fl uence

machining process occur rarely. One example for a

of thermal and mechanical

dominant mechanical influence is the shot peening pro-

paratively high metal removal rates, the turning

impacts. Due to the com-

cess which always results in an increase of hardness

process influences the workpiece surface considerably.

and compressive residual stresses at the surface

Although most of the heat produced in the shearing

Jue to plastic deformations /3, 15/.

TIl~

penetration

depth of residual stresses depends upon the peening conditions and can be quite large - up to 400

~m

and more (Fig. 5).

zone flows into the chips, tensile surface stresses are prevalent. This can be explaineJ by the fact that the machined surface is generated mainly by the secon~ary

cutting edge, producing friction

heat. Typical, however, . !1I0r---------------:~

generatc~

is that the tensile surface

stresses transform to compression some micrometer below the surface due to the plastic deformations in these layers. This result cannot be derived by measuring the surface stresses only.

'Eo E

Figure 7 shows examples with a stress penetration depth of up to 400

~m

/19/.

610..--,...------, I N/mm 4GO

cz=m

800L...-----~--------"

~

. ZOO

Residual stresses in shot peening

If stress gradients over 10 N/mm> are to be measured, the

. 4000':-......,,':-:-~ZO~O-~m~JOO depth z

analysis becomes more difficult and the results

are often questionable.

mol

(arbon stee

Other examples for mechanical surface treatments are

10JI

nard metal

(i.~ I(

2G

the surface rolling process and the abrasive tumbling operation, where the latter process produces an essentially lower degree of plastic deformation /3/. Thermal cutting operations like electro-discharge machining (EDM)

/16/ and laser-beam machining /17,18/

have their surface influences in tensile residual stresses and an annealed microstructure. If the conditions for a martensitic structure change are present, a re-hardening together with compressive stresses within the upper surface layers is possible. The penetration depth of the EDM-process can be up to 500

~m,

whereas the laser cut in general produces

smaller heat-affect-ed zones. Figure 6 shows a comparison of the in-fluences of laser and mechanical cutting on a 75 Cr 1 steel workpiece edge.

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Residual stresses in turning

The residual stress distributions produced during milling are very similar to those produced in turning due to the comparable chip formation. The similar behavior is observed for the penetration depth of residual stresses /1,3,19/. The most relevant machining process for surface

finishing, however, is the grinding process. Because this process is very often the last tOOling operation, it determines the physical surface properties most essentially. On the other hand, the depth of influenced surface layers is usually much less than in other machining proceses. From the size of the influenced layers it is therefore a difficult task for stress and hardness analysing methods to give reliable results. Figure 8 shows residual stress and hardness distributions in grinding bearing steel samples with a corundum grind. wheel. The depth of cut varied from 2.5 ~m up to 10 ~m /4/. The hardness measurements

mm

were made using the slope method /4/, and the stress analysis was done using the X-ray method. The hardness

.Iasercut • shearing cut

alterations are typical for a surface which has been ground with a high amount of induced heat. The basic hardness of about 700 IlV 0,05 was lowered due to

moteflol

7S [r 1

annealing effects. The upper surface layers, however, were re-hardened up to 830 - 860 HV 0,05. This steep rise in hardness occurs within a depth of about 10

~m

which is very difficult to measure with the conven~ Hardness alteration by laser cutting

tional cross-section method. The residual stresses show a quite similar penetration depth to the hardness profiles. High tensile stresses are observed togehter with the most significant hardness

Ch.'Ul11f"S

491

always when abusive grinding conditions have been

detrimental influence of the roughing operation,

applied. For this case the thickness of the influenced

however,

surface layers is about 150

lhe sub-surface residual stress profile, offering

~m.

is still present which can be seen from

still a high amount of tensile stresses. '>I.Jrt,]cp

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Fig. 10: Residual stress profiles in internal grinding ~

Residual stresses and hardness distributions

cycles (corundum wheel)

in grinding This example shows very cLearly how important the Other grinding conditions were chosen in Fig. 9 /4/.

analysis of surface integrity below machined surfaces

The CBN grinding wheel delivered compressive stresses

is. Figure 11 concludes how advantageous it can be to

with a very steep gradient of about 100 N/mm>·~m.

use CBN grinding tools also for this task. In both

The mechanical influence is predominant here, which

cases, roughing and finishing, only compressive

can also be seen from the corresponding hardness.

stresses are produced which do of course remain in

No soft annealing zone occurs, but a rise in hardness

the sub-surface layers, if both operations are applied in succession. Another important fact in grinding is that the

near the surface due to cold hardening is evident. surlace gJlnd,ng 100(r6. 61 HRC, emuls'on 1'1. 1A1·I~O·1O·~ ·117·B64·1 BO· M( v",J80mm/S. a.. 2.6~m. v; ,50w/nvn

distribution of physical properties over the surface is not necessarily constant, Due to unstable cutting

i--

or cooling conditions, the surface stress pattern can change as observed in Fig. 12 /4/. This requires

+

a sufficient local solution of measuring data, especially for the residual stress analysis.

X· roy siress anal YS'S s,ngle pOlnl d,omond dreSSing ~,' 30 m/s. I.. O.4mm. 0,01 ~m (lmul~lon

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Residual stress and hardness distributions

u"

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in grinding with boron nitride (CBN)

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As the grinding process is not only a finishing opera-

combined to grinding cycles. A roughing operation i. followed by a finishing cycle which gives the workpiece the desired dimensions and surface quality. Grinding cycles with little finishfng allowance can be very deceitful and misleading in terms of the residual stress profile. Figure 10 shows the residual stre •• profiles for roughing and finishing in internal grinding with corundum. The finishing operation

.

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tion but is also used for high amounts of metal removal, these two machining conditions are often

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Fig. 11: Residual stress profiles in internal grinding cycles (CBN wheel) Nondestructive methods which are sensitive to hardness and .tres8 fields as outlined above should fulfill the following conditionsl

produces much less cutting heat and thus no tensile .tres.es in contrast to roughing. If finishing is applied after abusive grinding, the initially high tensile stresses are lowered at the surface. The

492

- Ability for the solution ot steep .tress and hardness gradients, which means little penetration depth

in Fig. 15. J.. r oy o:. Iftl 5S analysIs

sUlloce grmdlng

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Fig. 12: Surface profile of residual grinding stresses - High local resolution, which means small measuring areas

Fig. 14: Grinding cracks after etching /20/ One can see that a brittle martensitic white surface layer is followed by an annealed zone. The residual stress investigation on the same workpiece revealed high tensile stresses below the surface and decreasing hardness values determined by the half-width value of the X-ray diffraction peaKs.

- Reliable results within short measuring times In order to avoid the sectioning of specimens and the metallographic preparations, the scanning electron

- Low purchase price and low costs for operation and maintenance.

micr o scope (SEM) has also been used to detect surface

2.2 Present Measuring Techniques

damage . Its use, however, is generally restricted to specimens anu small workpieces. Furthermore, evaluation of the physical properties of the surface topography which are scanned is complex as shown

The classification of techniques for measuring surface integrities into destructive and nondestructive methods is not clear, because most of the present techniques are at least partially destructive. Hence in this article only those methods are considered which damage the workpiece only in small areas or do not influence the material state at all.

Fig. 13: Grinding cracks in a bearing The factory worker, quality controler, are often confronted with surface integrities in the last finishing step. Thermal surface damage, burning, and microcracks can be found by visual inspection or by penetrant inspection teChniques. In this case the workpiece must be rejected in many cases (Fig. 13).

in Fig. 16 /21/. The combination of an SEM with a microprobe analysis can, however, provide some additional information about the martensite and austenite content of the surface.

I

Fig. IS. Hetallographic section of an abusive ground bearing with the correlated stress and hardness distributions /20/

Thermal damage of machining hardened workpieces Can be made visible by etching with hydrochloric acid. The interaction of acid and high tensile stresses leads to a network of cracks as can be seen in Fig. 14.

An accepted method for evaluating the surface state is the well known hardness testing. The influence of various machining processee on the surface conditions of a workpiece can be investigated by measuring the indentation hardness /3,4,7,9,22/.

Since abusive machining conditions lead to a rehardened zone at the surface, this inspection does not work if there are low surface tensile stresses. A metallographic investigation at a cross section, however, demonstrates a worse damage for the example

Vickers and Knoop-type indenter can be used, preferably with loads less than 1 N in order to keep the indentation area small enough for the analysing of steep hardness gradients. The data for the two types

493

of indenter are

in Fi g . 17 .

summari2~O

The so- called sl o pe :o.,tho.j represents a special prepft r ~ti on t ec hnique for the test specimen /4/. With the aid of a special polishing arrangement, a small test surface can be produced which i8 inclined abou t 1 : 200 against the machined surface . The preparation of the specimen and the test approach are shown in Fig . 19. For the hardness measurements the indenter i s placed at different points of the slope which correspond with different value s of depth below the surtace. Thus a high resolution of hardness values is aChieved near the surface. Other advantages are the simple preparation technique and the possibility to carry out X-ray residual stress measurements at the aame specimen /23/.

,~~/

hardness

r liS. st r ess l'S

-~--Fig. 16 . SEH views of Rene '95 surfaces produced by grinding /21/ Figure 18 gives an impression of the difficulties involved in setting the indenter correctly when measuring ground surfaces (z • 0 ~m). on the other hand, the application of the alope method. which was also used in Fig. 18 for 50 ~m and 100 ~m below the surface, has the advantage that steep hardness gradients can be detected due to the high sensitivity and good local resolution of this technique (see also Fig. 8 and 9).

--

--

....

".

~

~ '--/

/

--

Pig . 17. Vickers- and Knoop-indenter /22/

-=.- l'll'ctralyt

-= HV, (1

electropol ished slope

1:200

Fig . 19. Slope method for the determination of hardness and residual stress profiles The measurement of residual st r esses is necessary because the residual stre.s pattern is a very sensitive indication of ther~ally and mechanically induced structure changes. The complex variety of interactions between the thermal, the mechanical and the metallurgical states which determine the residual stress state is discussed schematically in Pig. 20. As the residual stresses cover a wide range of microscopic and macroscopic parameters (Fig. 25), several analysing methods had been tested and developed in /24,25/. Among the so-called "Nondestructive Techniques" the X-ray diffraction method is the most developed /24-26/. This is due to the high reliability and sensitivity of this method and to the possibility of doing measu~ements on workpieces and components of nearly all geometries and sizes.

Pig . 18. Vickers indenter (HV 0,05) on ground surfaces and aub-aurfaces

494

Fig. 20. Residual stres. fonaetion ceused by thermal, mechanical and . . tallurgical interactions

A detailed description of this method sha l l not be

close to the tube.

given here, restrictions are: The material must be

crystalline so that Bragg's law is fulfilled and the elastic constants for the material must be known to evaluate macroscopic

stresses~

An advantage of the X-ray method is the relatively

small penetration depth of X-rays, which is important for detecting stress gradients. Th e penetration depth of X-ra ys is dependent o n the material and on the wavelength of the radiation source. By installing tubes with different sources, this depth can be modified as Fig.

21 shows. Thus a rough residual

stress profile can be measured without destruction of the

workpi~ce

surface /26/.

An exact stress - depth a nalysis, however. affords

layer removal techniques as electro-polishing. The slope method (Fig. 19) has proved to be very advanta-

Fig. 22: X-ray diffractometer with PSD

geous for these examinations, because the stress

profile ca n b e measured automatically by computeraided system /4/.

The rotation of the specimen is the only adjustment to set the angle Yof inclination. No inclination of the specimen would be n e cessary if a second PSD could be

The me asuring area is determined by the dimensions

attached to the measuring circle.

of the focal spot, which is adjusted by the slit system of the goniometer. Measuring areas as small as 0.1 mm' are possible.

X-ray tube in equal distances to the focal spot of

If residual stresses are measured by X-rays. this

the anode. The angles between the primary beam and

can only be done if the necessary material parameters are known. Furthermore, recent examinations have

wavelength of radiation used. The difference between

can compensate each other /27/.

'a -

workpl P[ p

the diffracted beams are an index for the Bragg-angle depending on the workpiece material and the

shown that residual stresses in two phase systems

rO Olahon

A po ssible realization is suggested in Fig. 23 /31/. Two position-sensitive detectors are fastened at the

the 1"!.ns ,Iv

molpr lOI

necessary~-angles

results from the radiation

geometry to: ~ '1' = 180· - 29,..

'- I,,,·~ '0.. I

fit) F. AI

'''~ od(lplhZ"bfn(lottl

>

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10

15

','

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18" I -r ot luOP 1

5urt O((I

foa 10 I. on lr .. 0"

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017

.,n'.,

" 10

10

~m

<

'--ilo

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PS 0 2

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Fig . 21 : Influence of X-ray radiation on the depth of penetration in metallic materials A heavy restriction which prevent a wide application of the X-ray stress analysis in practice are the high

Fig. 23: Fast stress measurement with two PSDs

purchasing costs of the equipment and the long analysing times. Due to the precise mechanical and

If a suitable radiation is chosen for a given

electrical requirements the costs cannot be brought

material. residual stresses can be determined with

below a certain limit. Analysis times, however, can

sufficent accuracy in about IS aeconds.

be reduced by automation of the measuring process /4,23/. by optimized measuring conditions, by

The set-up suggested could be used for real time

evaluation algorithms /4,28/. and new detector

quality control during the manufacturing process as

designs as well. In this connection the position

it is sketched in Fig. 23 for an external grinding

sensitive detector (PSD) which allows stress

operation. The X-ray tube and the two position-

measurements in some minutes should be mentioned in

sensitive detectors are installed in a fixed ar-

this connection /29.30/. A PSD set-up is shown in

rangement which avoids any displacement of parts of

Fig. 22. Since the position sensitive detector is

the measuring system while working.

able to receive the diffraction peak intensities at one shot for one adjusted1'-angle, this adVantage can

The lattice planes are measured with a displacement

be used to think about new arrangements of the

of
complete measuring set-up. For a given x-radiation,

are chosen. This information is sufficient for the

the detector is attached to the measuring circle

calculation of a rough stress value. Due to the small

•

when Cr-r"diation and ferrite workpieces

495

numbers of ,,-angles a high reliability cannot be expected. For practical application, however, it should be sufficient. !....!._<.l"" ... j •.• ~I'or_

... ,(·O"C'~

·s . , . . ···Q\:?·

measuring technl Que

• meosurempnt at dltlerpnt mptatturglcat structure componentsfphases) • studlPs lOra mpasurpmPflts of workP'PCPS with lorgp groin-slIPs and tpMturpd materials • detHmlnatlon at thrpe -dimenSion] stress tensors • turthH devetopment Into fast stress anatysls (wpor-free P50S,npw detector priOClptPS) • development of small tubes ond gpneratars • Improvempnt of automatic cantrot and evatuatlon .,nllupnce of phose - rplated rpSI dual stresses on fatigue • stress measurempnt dUring dynaAlc IO'~lng

• In -process stress mposurempnt • measurement of reslduat maChlfl'9 stresses tor quottly assurancp • us@ af mobile measuring equlpmen for big components

Fig. 24. Future developments into X-ray stress analysis Future developments for X-ray stress analysis will be in the field of measuring technique and extended applications. The knowledge about residual stresses in different structural phases is still rather amall. It can be a.surned that the future research activities will concentrate on the relation between second-order strease. and fatigus life /32/. Parallel to these activities much effort will be necessary to reduce the size and the weight of mobile measuring equipments. For fast stress analysis new detectors have to be developed, so that in the future inprocess measurements will be possible Fig. 24 .ummarizes some aspects for future developments as far aa they are of interest for production engineers.

J. Nonde.tructive Techniques (NOT) Ths mechanicaL behavior of the components under con.ideration i • • trongly determined by different microstructural parameters like lattice defects. precipitation •• inclusions as well a. by pr.f.rred grain orientation. (textures). grain .izes. diff.r.nt pha •••• and micro- and macroscopic stress fieLds. It is evident that a technique to anaLyze structure and .tr.ss state. has a. more success a. the m.a.uring quantitie. are .en.itive to aLL structure and .tres. paramet.r. mention.d above.

Fig. 25: Size of structure parameters and corresponding examination methods damping. these techniques have penetration depths of about 1 mm. These methods will be described in section 3.1. In contrast to the micromagnetic techniques whose spatial resolution is less than a few mm' because of a penetration depth and lateral dimensions in the range of mm. the ultrasonic method. are integrating at least over a section of a few 100 mm' because of sound beam diameters and sound paths in a range greater than 10 mm. The most promising application of ultrasonic techniques is the characterization of the volume of components. but by exploiting surface waves it is also possible to characterize surface layers from ~ 0.1 up to '" 5 mm depth. Ultrasonic techniques to analyze structure and stress states in metals are discussed in section 3.2. 3.1 Micromagnetic methods 3.1.1 Physical background and measuring quantitiee The ferromagnetic states of many steel quaLities react very sensitive to changes of the (residuaL) stress state and to changes of microstructural parameters /33.34.35/. If the specimen has a well known geometry (cylindrical. spherical). one can demonstrate these reactions in characteristic changes of the hysteresis loop B(H). For example. increaSing hardness values imply in many cases aLso increasing coercive forces. Changing stress states imply for a given steel quality a typical shearing behaviour of the hysteresis (Fig. 26).

..... .........-._---~"

A. can be .een in Fig. 25. micromagnetic and eddy current mea.uring techniques cover a wide range of .tructural parameters .0 that. together with the fact that •• veral ind.p.ndent measuring quantities can be u ••d. the.e techniques can be applied to anaLyze .tructure and stres. state •• Due to the eddy current

496

'----

._

-

. ....... ..,........ --..,.. " ...... ~

Fig. 26. Magnetization reversaLs and hysteresis shearing In reality. however. the B(H)-loop cannot be mea.ured in the set-up technique and therefore it i. not possible to use thi. quantity in a dir.ct way for nd examination •• Therefore. in the la.t 15 years

different magnetic nd measuring quantities have been

incremental permeability (H

CJJ

) as shown in Fig. 28.

developed (Fig. 27), which also can be used for the The

mag n e t i c

Bar k h a u s e n

n 0 i s e

detection of surface integrities. As outlined in Fig. 27 schematically, nearly all magnetic quantities are

M(II) /36-51/ can be detected during the excitation

influenced by microstructure and (residual) stress

of the ferromagnetic material by an alternating

states. As shown in chapter 1 and 2 and figures 4-11

magnetic field with air coils or tape recorder heads.

and in IS, machining operations such as turning,

The main activity is very often observed at field

milling, shaping and grinding, and heat treatments,

strengths around Hc (Fig. 28), where the 180 Bloch wall has the greatest density. This process is

such as case hardening, laser hardening,

.. , result

in residual stress- and microstructure gradients,

0

irreversible.

implying that several independent measuring quantities have to be used to separate the 'multiparameter

The

influence', which will be superimposed normally in one nd quantity. An independent nd quantity /36,37,38/

J.ld(H) /36,37,54,55/ can be measured with and without a magnetic field excitation. All Bloch wall inter-

means: magnetic reversals which are detected by one

actions can be studied by this measuring quantity.

IMicrostructure I

I(Relldual ) Stress I

~

The

i n c rem e n t a l

d y n ami c

p e r mea b i i i t Y

mag n e t

0

s t ric t ion

E", /36,56,57,58,59/ can be excited by EMUS (!lectro~agnetic ~ltra~onic)

transducers in magnetostrictively

active material. Only magnetostrictively active processes are influenced by this method. Under a practical point of view the nd quantities

nd quantities

M(H) and J.ld(H) are the most important ones. Therefore,

co.relvltw

further discussions are restricted to these two

m.gnetlc e.rkh.u •• n nol ••

methods.

ICOU.tlC ••rkhlua.n nol,.

Iner.mlnta' perm •• blllty

The experimental set-up is shown in Fig. 29. The

d,namlc m••n.toltrlctlon

pick-up coils without and with ferritic core are

Fig. 27: Nondestructive measuring quantities and main interactions, schematically.

mounted between the exciting electromagnet. The Barkhausen noise signals are detected in a frequency range of about 0.5 ~ f [kHZ]

~ 250.

The total

amplification lies between 40 dB and 100 dB. The nd measuring quantity are mainly determined by reversible 0 r irreversible processes or by one d e f i n e d micromagnetic process - 900 Bloch wall motions, 180 0 Bloch wall motions or rotation processes. The regions where the different micromagnetic

incremental permeability JJ A can be measured with normal eddy current equipment in a wide frequency

processes are observed - for iron/steel - are outlined in Fig. 26 /35/.

H

probe (EMAT. eddy current coil •.. 1 Fig. 29: Exciting and transducer system (schematically) •

mag. Barkhau,en nol,e

8 range (e.g. 1 " f [Hz] " 10 ). Both quantities 1' .. and M are measured over the tangential magnetic field strength H which can be detected with Hall probes. Incremental permeability

3.1.2 Stress and microstructural dependence of different magnetic quantities For the steel quality SA 508 Cl.2 the microstructure

Fig. 28. Deduced measuring quantities from the rectified magnetic Barkhausen Signal and from the incremental permeability The

c

0

e r c i v i t y

Hc /36,38/ can be deduced

from the magnetic Barkhausen noise (H

cM

) and from the

and stress dependence of the nd quantities HeM' ~AX and 41J... (Fig. 28) are shown in figs. 30-32. The index indicates the HVIO-hardne.s value •• In the stres. free state (6 - 0 N/mm O ) the coercivity HeM increa.es with increa.ing mechanical hardne •• (Fig. 30). In the ten.ile stre.s region the .tre •• dependence of HeM i. usually les. pronounced than in the compre •• ive region.

497

The maximum MMAX of the magnetic Barkhausen noise

nd quantities and that compressive and tensile stress

amplitude shows a complex behaviour in the tensile

states change the magnetic state in a characteristic

stress region (Fig. 31). For hardness values smaller

manner. Therefore, it is necessary for all practical

than

applications to use several independent measuring

,,300 HVIO MMAX shows a max imum in the

MMAX/ () plot. The shape of the MMAxi C; curve depends

quantities especially to overcome a possibly small

on the transducer system (frequency range, resolution

sensitivity in some of the measuring quantities and

etc.), but one can see that the sensitivity of MMAX

to decide between ambiguous values.

is larger in the tensile region than for compressive states. If the total irreversibility

JMdH is

measured additionally, the ambiguous behaviour in the tensile stress region does not occur. The quanti ty the

<1~.d

~.d(H)-curve

is a measure of the broadening of for a given

~4-value.

One can see

that the stress dependence is quite similar but more I inear than for the quantity HCM (Fig. 32,30). One

3.1.3 Nondestructive stress and hardness measurements

on components Results of a hardness and residual stress test on low pressure turbine blades with conventional methods (HVIO, hardness tester, 6 _ x ray ' X-ray method) and micromagnetic methods (magnetic Barkhausen noise) are shown in Fig. 33,34.

In a first step the unknown microstructure states, correlated in this case with the mechanical hardness values. are determined by the coercivity HeM' After a calibration of the HVIO- over II -values CM one can determine the nondestructive HVnd-values (Fig. 33). In a second step the stress sensitive quantity MMAX and the structure sensitive quantity HcM are used. in conjunction with the X-ray

meth~i

in order to

calibrate the MMAX' HCM-values over the 0x-rayvalues. After these two calibration steps a quantitative residual stress measurement is possible. Deviations from the exact hardness and stress values Fig. 30. Stress and microstructure dependence of the nd quantity HcM /37/

are observed if in the interacting volume of the sensor microstructure gradients normal or parallel to the surface are obtained. The measuring time for both values HV nd and ~nd with the EMAG prototype

[mY]

A

650

/38/ is less than 1 second. The surface resolution

HV10

~256 ""303 .. 353-

of the sensor is smaller than 8 mm in diameter.

HV1OxiliHCM IA/CM)

420-100

50

o

Fig. 31. Stress and microstructure dependence of the nd quantity ~I\X /37/

HV10

~~""""HVnd

can also see from the results of Fig. 30-32 that the microstructure /60/ determines the magnitude of the

[A/em]

!lilt.

o

HV10

420_

5

em

Fig. 33, Conventional hardness test (HVIO) and nondestructively determined hardness values HV

353_

along the same trace of a low pressure nd turbine blade. Material. Super 12 , Cr. One can see from the results (fig. 33.34) that new

T

o

-100

Fig. 32, Stress and microstructure dependence of the nd quantity

498

~~4

/37/

developments in magnetic testing will influence the quality- and production control in future.

By changing this variable fA one can detect charac-

I

teristic Barkhausen noise signals from the surface

MMU

:~ o

5 +

layer (fA») provided this layer has different magnetic properties as the bulk material. If the OX-ray

analyzing frequency fA is lowered, Barkhausen noise signals of the bulk material state will also contribute to the detected signal at the surface, if

.100

em

the surface layer is not too thick.

~ Or----+.5~--cm--

By the magnitude of the exciting magnetic field Hi the H-field range in which Barkhausen noise signals are detected can also be changed. If the incremental permeability

~a(lI)

is used (Fig.

37), the main variables are the exciting frequency fA of the probe, the working range IIi on the hysteresis Fig. 34: Residual stress determination along the same trace as in Fig. 33.

loop, and the alternating field amplitude All of the e.g. eddy current probe. A signal can only be deduced from the

~A(II)

curve, if the ferromagnetic properties

of surface and bulk material states are different. 3.1.4 Determination of hardened surface states by micromagnetic methods The main variable which can be used for getting different penetration depths or interacting volumina is the frequency of electromagnetic waves. For a plane incident electromagnetic wave the decay of their amplitude is shown in Fig. 35.

In this example

the ferromagnetic material has a conductivity of (3 = 5 cm/.nmm 2 and an incremental permeability of ~4

= 50.

depth

One can see from Fig. 35 that the penetration

d covers

a range between

~ 1. 6 mm up to N5 I'm,

for the frequencies between 0.4 kHz and 40000 kllz.

't. -

ac exciting frequency Hi - working range on hysteresis loop t.H -alternating field amplitude fE -exciting frequency of hysteresis loop Fig. 37: Determination of surface near microstructure

10

Ii

states by the incremental permeability A further variable for both methods (M(II),

ImmJ

~A(II»

is the exciting frequency of the electromagnet f

. E In this case the magnetic flux density in the surface

near region can be changed by the penetration depth as outlined in Fig. 35. The result of a 0.001 L - _ - ' -_ _~_",,:,,:!::----:=_~ 0.4 40 400 4000 '1kHz J

g u d 9 eon

cas e

pin

h a r den e d

is shown in Fig. 39 /93/.

Fig. 35: Penetration depth of an incident plane electromagnetic wave.

fA =1,6·103 Hz

If the magnetic Barkhausen noise M(II) is used (Fig. 36), the main variables are the analyzing frequency fA and the H-field range IIi (see also /51/). The frequency fA determines the mean penetration depth.

o

]\]j

H

H fA :i.·10

4

Hz

H fA - analysing frequency Hi- working range on hysteresis loop fE - exciting frequency of hysteresis loop Fig. 36: Determination of surface near microstructure states by the magnetic Barkhausen noise

Fig. 39. Case hardened gudgeon pins. Magnetic Barkhausen noise profile for different analyzing frequencies fA' For different analyzing frequencies fA the magnetic Barkhausen noise curve M(II) is changed in a charac3 1.6 10 Hz ( d ~0.9 mm) the

teristic way. For fA -

the bulk material (Q magnetic soft material) contributes to the M(II) signal at the coercivity H ' c1

499

If the frequency fA increases, the soft material

If one <1nalyzes only the M(H) profile at position

signal at the position IIcl decreases. At " frequency fA = 4 104 Hz ( 6 ~ 0.16 mm) any contribution of

and 2 and defines that the hardening depth should be

the bulk material can be detected. The noise signal at position Hc2 is generated by the case hardened sur-

determined at the frequency fA whenever MMAX(l) = ~AX(2), one gets with p~ = 50 anJ G = 5 cm/nmm' a laser hardening depth of -0.2 mm.

face layer whereas the signal at position IIc3 can be correlated with carbides in this surface layer. For

G r

the determination of the case hardening depth one can

typical residual stress and hardness (microstructure)

use this behaviour of the M(II,f )-curves for quantiA tative measurements.

distributions (see Chapter 2.1; Fig. 8,9) in the

i

surface near region. If the grinding conditions are abusive,

For many applications it is not necessary to change the analyzing frequency fA'

If the Barkhausen noise is

detected in the 'broadband mode'

(0.5 kHz-50 kllz) , one

can also deduce the case hardening depth (Fig. 39) /83/.

(mV

N""'

(Alcmi

~"

100

H

~.<-.-

50

• tensll.

50

..

".

o

0.5

30

it? .,

1.0 (mml

0,5

will be rehardened (M) and the adjacent zone annealed (Ma) during the grinJing process. Because of the (Fig. 20) the hardness profil of Fig. 41 will be

~ .. ..,

o

1.0 Imml

it is possible to produce hardness profiles

as outlined in Fig. 41 schematically. The surface near ( <100 pm) case-hardened microstructure zone (e)

thermal, mechanical and metallurgical interactions

coerclvlty

~

states are always correlated with

n din 9

superimposed by a typical residual stress field .(G). Up to now it was not possible to separate microstructure from residual stress gradients. The magnetic hardness profile Hcp (6), Fig. 42, is therefore only correlated to the microstructure (hardness) influence. One can see from Fig. 42 that the rehardened surface near region shows higher Hcp-values than the adjacent annealed zone and that two different grinding states

Fig. 39: Case hardened gudgeon pins

can be characterized by this method. The results have shown that the metallographically determined

The left side of Fig. 39 shows that with increasing

rehardeneJ zones correlate in a linear manner with

hardening depths the magnetic Darkhausen noise ampli-

the magnetically determined rehardening depths.

tude M MAX decreases, whereas the coercivity HCM increases (right side). If there are tensile stresses superimposed, the measured value of ~ is considerably larger, so that it can be distinguised from its values without stress as indicated by the stray band. In contrast, the HCM-value shows the opposite behaviour. This is correspondingly reversed if compressive stresses are applied. A

las e r

h a r den e d

V

III III C!J

· · , c ,... · ·· ~

C "0

o

CK45), Fig. 40 /84/, which was also examined with the magnetic Barkhausen noise method, shows quite similar results as the case-hardened samples. At position 1 the soft bulk material is detected; at position 2 the

..

.

;

..

··

I-

specimen (material:

~.

.L:.

o

100 1000

Fig. 41: Possible hardness distribution normal to

laser hardened surface state. The metallographically

the surface after grinding (schematically).

determined martensitic surface layer is

M

~

0.28 mm

z

e •

thick.

martensite, Ma • annealed martensite, case hardened zone,

G. residual stress

grinding states 2Hc~

M

lA/em)

80 100

-100

-100

rl'

-1&

r

fA =2.S.10'Hz

t

a

Fig. 40: Laser hardened specimen

500

A/em

100

A/em

fA = 2.10J HZ e

0-

100

t

6r

A/em

60

LO 20

o

50

100

150 6 (Il m)

Fig. 42. Hardne •• profile of two different grinding state. /85/.

3. 2 Ultrasonic techniques

elastic constants.

3.2.1 Physical background

As described elsewhere /67/, the relative difference of the velocities of the two shear waves polarized perpendicular to each other (2b,c) can be developed to the following equations after expressing strains Ei by stresses 0i' and velocity by pathlength divided by time of flight t.

3.2.1.1 Elastic parameters of ultrasound propagation i) Sound velocity in general The propagation velocities of ultrasonic waves in an isotropic material are given by the density! and the elastic constants A and ~, or E, G and the POISSON ratio ~ which are more common in engineering /63/: 2 jVL

+ 2 I'

,\.

2 fV T = I' v

R

a

E(l-~)/

(1-2 v)( l+~)

Da)

G

K vT

«O.87+1.12~)/(l+v»

vT

VSH = v T

(3a) Cyclic permutation of the indices in the equations 2b and 2c yields to

(lb)

(3b)

(tc)

(3c)

(ld)

Those equations are independent of the ultrasonic pathlength or the thickness of the component. Substituting the appropriate strains or stresses in the equat i ons (2) and (3) the influences of different stress states on the sound velocities can be calculated:

v L ' v T and v R are the velocities of longitudinal, shear and Rayleigh waves respectively. A and ~ are the LAME-constants, E is the YOUNG-modulus and G represents the shear modulus of the material. In the case of longitudinal waves the mass particles vibrate along the propagation direction, whereas the vibration d i rection of shear waves is perpendicular to the direction of propagation. In contrast to these free waves, Rayleigh-waves propagate along a surface. The displacement vectors describe an ellipse with the big axis perpendicular to the propagation direction and the surface. The Bound energy of the surface waves decreases exponentially with the depth; a reasonable value for the penetration depth is one wavelength A /65/ . Changing the frequency f - viA the penetration depth can be varied between about 0.1 up to 5 mm using Rayleigh waves. Also there is the possibility of grazing incidence of SH-waves. The polarization direction of SH-waves lies in the surface and perpendicular to the propagation /64/.

~v23

Exploiting the equations (1) alterations of the density and/or the elastic properties can be detected and correlated to mechanical properties like hardness, - depth, cold working, porosity and purity. Based on the elastic anisotropy of crystals, measurements of the sound velocities as function of the propagation or polarization direction enable one to characterize preferred grain orientations (textures).

Accurate velocity determinations, this means accurate pathlength and time-of-flight measurements enable one to evaluate the strain! and. using HOOKE's law, the stress in I-direction. The relations (3b,c) yield the stress determination exploiting only time-of-flight measurements /67/. The equations (4) are mainly us.d to evaluate the clastic constants of higher order 1, m and n /68/.

ii) Stress dependence of the sound velocity

TwO-dimensional stress state. For a two-dimensional stress state with 52'~3 ~ 0 the equation (2a) becomes

In order to determine macroscopic stress states, the nonlinear elasticity theory has to be applied. An originally isotropic solid lossss the isotropy when stress is applied. The sound velocities are dependent on the values and the directions of the strains originated by applied or residual stresses /66/.

~Vll fVl2 !'vI 3

2

2 2

-

-

~

+ 21' + (2l+~)e + (4111+4.\+101l)€1

(2a)

II + (A+m)e +41lE 1+211 £2- 0 • 5 nE:3

(2b)

II + (A+m)e +4Ilfl+211!)-0.5 n( 2

(2c)

Here (i (i • 1,2,3) are the strains parallel to the a~es of a cartesian coordinate system. The first inde~ of v represents the direction of sound propagation, the second the direction of polarization. eis the sum of Ei and l,m,n are the third-order

One-dimensional stress state. For a tensile stress in l-direction, the strain !l in (2) ~ill be E and £2 - E3 - -v£. The following equations are obtained. l'v ll 2 • fV12

~v22

2

2

\'11 21 2 2

~+2..,+(4('\+2..,)+2(I'+2m)+21',,(l+21/'\ȣ

• I'v13

2

= 1'+(411+0.5nv+m(1-2v»[

(4a) (4b)

• \"v33

2 = A+21'+(2l(1-2v)-4v(m+.lI+2..,»t:

(4c)

• ~v3l

2 • 1I+«'\+211+m) (1-2'1)+0.5 nv)£

(4d)

2

(4e)

• \i'v 32

2

11+( (Mm) (1-2-.1)-611'1-0.5 n)f

~l

• 0:

when £1 is rsplaced by (-Y/E)(5 +6'3) and e by 2 «1-2-.1)/E)(~2+~3)' The ralativs difference of the longitudinal wave velocity is proportional to the sum of the principle stress acting in the plan. perpendicular to the sound propagation diretion. TOgether with the equation (3a) the absolute values of 3' 2 and ~3 are determinable. If the geometry of the component under consideration enables on. to incite shear waves in the 1 and 2 or land 3 direction, the equations (3a) and (3b) or (3a) and (3b) can be used to evaluate ~2 and ~3 also /68/.

501

surface stress states. In order to determine surface stress states with

<\,G 2 ~ 0; 6"3« <3"1 oro 2 , the stress influenced dependence of the velocity change of surface waves must be utilized. As mentioned above the Rayleighwave velocity is proportional to the shear wave velocity, the SII-wave velocity is equal to the shear wave velocity, so that the influence of surface stresses on each kind of surface waves can be calculated using (2b) and (2c). The velocities vcr l and v62 of a SI\-wave propagating in the direction of G and are given by: l 2

°

(6a) The relative velocity differences of Rayleigh waves propagating parallel to 0 or ~2 are evaluated to . 1

(6b) Vo is the SH- or Rayleigh wave velocity in the atressfree sample. The constants "" Band C are only functions of the elastic constants. A ~

i) Ultrasonic absorption

(1J(41'+m)+~(41'+O.25n»/1'(3.\+21');

B -

(1'(21'+m)+~(1' +0.25n»/1'(3.\+21'):

C -

(l'(m-0.5n)-A(21'+0.5n»/1'(3o\+21') /68/.

It should be emphasized that especially the combination of SH- and Rayleigh-waves (6a-6b) propagating in the same direction are very suitable for the stress determination. Three-dimensional stress states. The absolute values of ~l'~2'~l can only be evaluated by measurements of the absolute sound velocites corresponding to the equations (2). For sane fracture mechanical calculations it is sufficient to know the differences of the principle stressee, which can be deter~ined using the equations (3) /68/. iii) Frequency dependence of the sound velocity The frequency dependence of the longitudinal and shear wave velocities is caused by the elastic anisotropy of the crystals. The value of this effect (until a few percent) is mainly determined by the factor of anisotropy. Theoretical descriptions using the backscattering theory /69/ and experimental results indicate that the dispersion effect offers the possibility to characterize textures and alloye with two phases /70/. This dispersion effect should not be confounded with the change of the surface wave velocities as function of the penetration depth which is adjusted by the change of the wavelength or frequency. ",lao the frequency dependence of the eurface waves propagating along curved surfac.s has other physical reasons /65/.

3.2.1.2 Anelastic parameters of ultrasound propagation Scettering and absorption effects result in sn exponential decay of the ultra.ound amplitude A with

"'0

"'0

increasing sound path x: '" exp(-_x). is the initial amplitude and ~ • «", + ~S is the attenuation coefficient, ... A the absorption and O(s the scatterir\9 coefficient.

502

In the ranges of room temperature and ultrasonic frequencies of about 1 to 30 MHz the absorption is mainly influenced by the interactions of the ultrasonic waves with dislocations and magnetic domain walls, whereas the scattering is caused by changes of the acoustic impedance ~v at grain and phase boundaries. The direct interactions with dislocations and domain walls, which are aleo influenced by other lattice defects, inClusions, precipitations and stress fields as well as with grain and phase boundaries give rise to assume that the.e quantities are qualified to characterize microstructure states. But, due to the different kinds of influencing parameters, there existe no physical description of the influence of a concrete heat treatment parameter on the ultrasonic attenuation. Only the stress influenced changes of the attenuation and the backscattering effects are theoretically described. Nevertheless, there exist some promiSing correlations between the experimentally found attenuation and the hardne •• or a specific heat treatment parameter.

ACcording to the vibrating string model, the dislocation loop length between the pinning points increases with increasing static stress /71/. These diSlocations interact with the superimposed alternating stre.s field of an elastic wave and therefore affect the change of absorption as a function of stress. The absorption coefficient ~A increasea linearly with the dislocation density and with the forth power of the effective loop length. Both parameters are affected by stress but, due to influencing microscopiC parameters which usually are not known for technical materials like steel, it is not possible to deteraine the stress state exploiting absorption measurements. In addition to that, it is very difficult to obtain absolute absorption values with the necessary accuracy. The measurement of the relative change of the absorption, e.g. a. function of the polarization direction, yields a helpful additional information about the stress state /72/. ii) Ultrasonic backscattering Scattering, occurring on each grain and phase boundary, enables one to determine the grain size in metals as well as to indicate inhomogeneities in materials. Assuming an isotropic, monophase material and neglecting multiple Bcattering, theory yields for the scattering coefficient ~S • 5 4 3 f4 provided that the grain diameters d are small compared with the ultrasonic wavelength A /70/. The scat.tering parameter 5 depends on the wave type, the ultrasonic velocities a. well as on the density and the anisotropy factor. The frequency dependence of the absorption coefficient ~A in steel. is generally well described by ~A • C f. The constant C aainly contents microscopical material parameters. Keeping in mind the Itmitation conditions, it is obvious frca the following equations that grain sizes can be det.erained by measuring the ultrasoniC attenuation at two frequencies fl and fl'

cx;rxA+CX

~l

C fl + S d

0(2 ;

relative velocity values.

S

C f2 + S d

J

J

f~

(7)

Caused by these measuring errors,

the calculated

inaccuracy of the elastic constants is 2 to 5 %.

fi

The accuracies of the determined stress values are or~er

III

~l'

~2

to determine the attenuation coefficients

ultrasonic backscattering measurements arc very

appropriate. The backscattered amplitude AS tlecays as function of the sound path x like AS '" from which

0{

v-;;;"

exp(-"',,),

is easily tl"rived. This methotl offers

of the same range; the lowest detectable stress value is about

] 0 MPa.

The most common method

to determine

the ultrasonic attenuation is based on tIle measure-

ment of the amplitudes of at least two successive ultrasonic pulse echoes. Because the additional sound

important odvantages compared with the usual way

energy losses, due to the coupling medium between

to determine the attenuation: Neither special geo-

probe and sample and due to the tlivergence of the

metries, like plane-parallel specimens, nor complex

sound field, are difficult to be taken into account,

sound field corrections are required /70,73/.

the accuracy of the attenuation values are typically

Based on the fact that the scattering parameter is

the backscattering technique mentioned above,

strongly influenced by material characteristics,

possible to evaluate the attenuation coefficients

backscattering measurements are very appropriate to

with an error of a

detect inhomogeneities like segregations and

advantage of this teChnique lies in the possibility

microcracks in materials.

to separate the absorption and the scattering part of

in the range of 10% - 20%.

Due to the advantages of it is

few percent only. An additional

It should be emphasized that these backscattering

the attenuation coefficient. Using backscattering

measurements can be done with free waves /73/ as well

measurements with frequencies between 2 and 30 MHz,

as with surface waves /74/.

grain sizes in the range of ASTM 1 to ASTM 10 can be determined with a typical error of + half an ASTM class.

3.2.2 Measuring techniques and resolutions

An equipment (Fig. 44) which enables fast back-

In order to determine sound velocities or velocity

evaluation of the attenuation coefficient and the

changes, measurements of the time-of-flight and

grain size is also available on the market /75/.

scattering measurements including automatic

pathlength measurements are necessary. There are a lot of di fferent methods to measure times-of- fl ight; one of the simplest ultrasonic set-ups for time-offlight measurements consists of a commercial ultra-

sonic apparatus to transmit and receive the electric signals converted by an ultrasonic probe and a twochannel oscilloscope with a crystal reference for time interval measurements (Fig. 43).

Fig. 43: Ultrasonic equipment for time-of-flight measurements

Fig. 44: Prototype for ultrasonic backscattering measurements

Ultrasonic transducers for each kind of waves are also available as set-ups for time interval measurements with an accuracy better than 1 in 10 4 • This

3.2.3 Experimental results

means, that the accuracy of velocity measurements

The measurement of ultrasonic velocities or velocity

is mainly determined by the accuracy of the path-

changes are qualified for hardness and hardness-depth

length measurement. The usual accuracies of velocity

determinations, provided that calibration curves as

measurements in technical components are 0.1 % - 1 %

they are shown in the foliowing can be used. There

for absolute velocity values and 0.01 % - 0.1 % for

503

are a lot of publications treating this subject. The few chosen figures show typical results. More experimental results concerning the influences of cold working, texture, porosity and purity are reviewed in

/79/.

11 ~--J

U

-l -l ILl

K-~'--~l

50

~

~

0

40

VI VI ILl

30

U

Il::

"',

u'

Z

(.

30001

I 10

0

'-5 MHz


:r 20

COMPRESSIONAL BULl( WAVE

10 2220

""~ __

42C

/. ') ~r.

'j, -u2C

• "1!5

....~-s or,

I I

29001

Il::

.:

31'

3100,-

" HS'.',

60

.

I 30'

32

30

21 ' \

I~'5~

-I

i

50

HARDNESS HRC

32' 33' 70

Fig. 47: Rayleigh-wave velocities versus hardness HRC in steel samples after different heat treatlnents /78/. I: as delivered 2: tempered at 600 °c 3: tempered at 540°C 4: tempered at 400 °c 5: hardened at 1200 °c

""L-" "---1-

2240 2230 2250 VELOCITY OF SOUND (10610 / sec)

Fig. 45. Sound velocity of 5 MHz longitudinal waves versus the hardness HRC of steel D6ac /76/ In Fig. 45 the influence of different hardness values on the velocity of a longitudinal wave propagating through the sample is shown /76/. The change of the Rayleigh wave velocity in an oil-hardened steel sample is shown in Fig. 46 as a function of wavelength. this means penetration depth /77/. Figure 47 shows also the change of the Rayleigh wave velocity as function of the hardness. The points at I are the measuring results in the steel sample with the asdelivered hardness. The results 2 to 5 are measured after different hardening treatments /78/.

Fig. 48: Relative change of Rayleigh-wave velocity as function of the hardness depth /78/

QUlIIOIU STIli.

2.96

Fig. 46: Rayleigh-wave velocity as a function of the wavelength in an oil-hardened steel sample /77/

18(r

Fig. 49. Relative change of the velocities of the .hear wave components of a Lamb-wave aa a function of the propagation direction in

The relative changes of the Rayleigh wave velocity as

a .lightly textured Al sample /80/

function of the hardness depth shown in Fig. 48 give also an impre •• ion of the sensitivity. The 10 MHz Rayleigh wave has a penetration depth of about 0.3 mrn

/78/.

The relative VelOcity change of the .hear waves polarized perpendicular to each other i . exploited to characterize a rOlling texture in an aluminum plate. This value changes trom 0. 4 ,

it the wave propagate.

parallel to the rolling direction to about 0.05 ,

504

~hen

the propagation direction is perpendicular to

o

the former one (Fig. 49) /80/. It is found that the anisotropy values determined ~ith ultrasonic techniques agree very well with X-ray

a 200

[~]

-----

tt rae

---

(Jrad

60

results.

50

The figures 50 and 51 show typical results of stress determina tions using ultrasonic techniques. To determine the stresses acting parallel and perpendicular to the fatigue crack propagation direction. the velocities of longitudinal and shear waves propagating through the thickness of the sample are measured /81/. The surface stresses in tangential (~t) and axial (6 ) direction in the austenitic tube 1 are determined using Rayleigh waves /82/.

40

of Fi~

30 .

50

100

• 52: Change of radial stress and attenuation coefficient over the radius of a saw blade

.1

IOtanl

.,

'"

.J I •• ) OI,tonc.ltv_ (NKk ] Ij

.,.,

I

I

I

300 H (altula!oon

.. I

~"

,-

I

I

I

. 1IIO

Sk.J,h of the sample

Fig. 50: Change of principal stress e s 0.1. and <>;1 as a function of distance from crack end in a steel sample

... -- a'an

I

.. -",,,,

""

200

. ".

60

, ,

l~m21

.,

'0]

a

I'"'I\I I

500

.00

--

50

_

,,

,,

100

a'an

40

, .... --,,'~,

30

RadiUS FiS' 53, Change of tangential stress and attenuation coefficient over the radius of a saw blade

400

-. _... .. -- .. --'.

HPQ

.,.

.'

200

:1\

,

60 ~

. ... • 200 t

20

...

· 20 '40

so

100

mm

X

Fig. 51, Change of the surface stresses in tangential

(6 t )

and axial direction (G ) as function of l the distance from a weld seam in an austenitic tube /82/

· 60 - 10 -100

'\ ,

,,

:

,

I

150

....

Aodfu.

!,

.

,...

: : :,

·120

,

.1<0

''':

-160

----0-- UII rasonic

" 110

-

·200

In the figures 52 and 53 the stress affected change of the ultrasonic attenuation. mainly affected by absorption effects. is clearly to be seen /72/. Exploiting this information additionally to the time-

.

,100

X·roy

Fig. 54. Difference of the principal stresses in a textured saw blade versus the radius

of-flight measurements (Eqs. 3). the residual stress state in a textured saw blade is characterized and shown in Fig. 54 /72/. Results of backscattering measurements are shown in the figures 55. 56 and 57. Exploiting the backscatter_ ing curves of Rayleigh waves (Fig. 55) grain sizes in surface layers are determined and compared with the sizes. evaluated by metallographic methods (Fig. 56). The effect of three different kinds of inhomogeneities on the backacattering amplitude are to be seen in Fig. 57 /74/.

.

'.tlft. .

Fig. 55. Backscattering curv.s of surface waves propagating in a f.rritic ste.l sample with grain size 4S ~~

505

properties which can be evaluated by ultrasonic techniques are listed in Table 1. 100

!J 1,

r a soniC

.valuQtll. struclurol

m(losur.ng

:,juontl!'f~

I

ooromtll.r

n a c curory

OOSolulP 01· til.

~Plc(d'PS at surlecE' wavp':.

+

10

IrPIOI". 001 . 01 ".

o

I

hardn",s!.

ro

hardn.s!. lI.pth

~ordn~s -g;ad',irlI rcorr'IOItOn

I

1

,pi otl'~i"

'00 ~ 11'",1

10

obsolulO 01

vploc II IfS of t r" .. a'l~s

001

·1-'_ 01-'_

Fig. 56. Comparison between grain size determination by ultrasonic backscattering measurements

~ ~.~ zoi"

-

400

hordnpss -

para Sit Y

1>--.-----.

I I,. p.rtfnl. Gfp.ncs on IfChnlQu'

absorption

ttHi""

~lm.nSlonol

str~s

statf'\

,rto'·

(hll.r.r.t hfOt Infnl poranwt.rs

aDsolulf VOlutS wllh,n 2· 5'/_ .rro, aDsol ul. valu,s wllh,n 2· 5-'_ (lrro, eorrflotlon -c~ alisolul. vaI~ POSSIIIII! bul cOlTl'ioilOn curvos 011

pur,ly tfl. p.,tlnt

bockscotlflrlng

grOin Sil'

InhQmcgfnflllP<

I

2·5-'_ .rror absolutt-;a'11WS tar It'It dilltr.nus 01 Iwa p"nclpi' slrfSWS wlIh,n 1 . S·~ eorr.lallon (urv,s

musl D. flplOll.d plastiC dftormotlon dflarm.d 10ntS Cltlt(· lobi ....Ioli .. volu"

r--"lfSS

...

.......

,

!o~:I~~tr~~~ :;~s

I

,!

._~_-,-

k'~

.Ioshe conslanls

lot. C" ...... 1fCDIIMIIf!II

,.c amtnf"dfd l - - - 'lib.Olul, ,0lutS wllh

(dusl and by metallographic methods (dMl

lli t\ =r

-------

amount of pr.l.rr.d .,0IuaDI. Dul com· on,nlpd gTol ns.

AS TM -12 11 10 9 e 7 6 5 4 J 2 1

CUrYH ~IOllon~

DflnClpl, d.T.ctlons .volueDI. WlttH" 1 Z II• • rror 01 lulur.s

sur tat. strtss.s 1

r .mor ks (orr,IOllon CUryn fI'lIS b. flplQI,.d

rtlotlv' valu.s collbratlon curv's 01'1

Itcomm.ndtcl oDSOlul' vol~s tor SI!!'l b.lw •• n A5TM 1and A5TM 10.IIh,n tIolt on ASIM closs localion oDsalul.dI'itr mlnabl •. no posSibility

10 dlSI mguish D.lw.,n

dIU.r.nl kinds

......

Fig. 57. Detection of inhomogeneities using ultrasonic backscattering

Table I. Mechanical properties which can be evaluated by ultrasonic techniques

4. Hints for Practical Applications and Further possibilities 4.1 Magnetic techniques Magnetic. magnetoelastic and eddy current quantities can be used to detect surface integrities. Because most of these quantities are sensitive to changes of the residual stress and microstructure state and because the penetration depth can be changed by eddy current damping. surface integrities up to about 1 mm (acoustic Barkhausen noise ~ 10 mm) can be investigated. First applications have demonstrated that the magnetic Barkhausen noise and the incremental perm.abilities can be used with success for detecting microatructure (hardnesa) gradients. The local resolution (a) ia given by the sensor geometry. In the caae of the magnetic Barkhausen noise and the incremental permeability. s ~ 0.5 mm. for the acoustic Barkhausen noiae and the dynamic magnetostriction. s ) 5 rom.

4.2 Ultrasonic techniques The elastic and anelastic interactions between the ultrasonic waves and the material can be exploited to characterize structure and stress states in the volume and also in the surface near regions of metallic components. The minimum of the interaction volume determined by the diam.ter of the ultrasonic beam and the pathlength has to b. a few 100 mm>. otherwise the effects are to small to be accurately measured. Mechanical

506

The major subject of the current and the future research activities is the separation between two overlapped influencing parameters. TWo kinds of possibilities are treated in order to determine stress states in textured materials. One of them exploits the additional information of absorption measurements or the additional measurement of the frequency dependence of the ultrasonic velocities. The other way is the combination of the velocities of waves propagating and vibrating in different directions related to the principle stress and texture directions. so that texture-independent equations can be derived and used. In order to distinguish between the changes of microstructure and stresses in the hardened layers. the additional measurement of the frequency dependence of the attenuation seems to be helpful. The development (and the use) of phase-insensitive transducers and the measurement of the frequency spectrum in layers with a defined structur. indicat. the next step.

Acknowledg ....nt This contribution is based on work performed with the support of the C.naan Res.arch Foundation (oro). the VW-Foundation. the German Ministry for Research and Technology and the European community for Carbon and Steel.

6.

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~

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509