ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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14.
E.P. Papadakis and B.W. Petersen, Ultrasonic Velocity As A Predictor of Density in Sintered Powder Metal
Parts, Mater. Eval., April 1979, p 76
15.
A. Gallo and V. Sergi, Orientation of Porosity of P/M Materials Evaluated by Ultrasonic Method, in
Horizons in Powder Metallurgy,
1986 International Powder Metallurgy Conference Proceedings, Part II, p
763
16.
M.P. Jones and G.V. Blessing, Ultrasonic Evaluation of Spray-
Dried Alumina Powder During and After
Compaction, in NDT of High Performance Ceramics,
Proceedings of the 1987 Conference, American
Ceramics Society/American Society for Nondestructive Testing, 1987, p 148
17.
B.R. Patterson and C.E. Bates, Nondestructive Property Prediction in Gray Cast Iron Using Ultrasonic
Techniques, Paper 65, Trans. AFS, 1981, p 369
18.
R.H. Brockelman, Dynamic Elastic Determination of the Properties of Sintered Powder Metals,
Perspect.
Powder Metall., Vol 5, 1970, p 201
19.
E.R. Leheup and J.R. Moon, Yield and Fracture Phenomena in Powder Forged Fe-
0.2C and Their
Prediction by NDT Methods, Powder Metall., Vol 23 (No. 4), 1980, p 177
20.
K. Subramanian and J.L. Rose, C-Scan Testing for Complex Parts, Adv. Mater. Process. inc. Met. Prog.,
Vol 131 (No. 2), 1987, p 40
21.
A. Hecht and E. Neumann, Detection of Small Inclusions in P/M Alloys by Means of Nondestructive
Ultrasonic Testing, in Horizons of Powder Metallurgy, 1986 International P
owder Metallurgy Conference
Proceedings, Part II, P 783
22.
G.Y. Baaklini and P.B. Abel, Flaw Imaging and Ultrasonic Techniques for Characterizing Sintered Silicon
Carbide, in Nondestructive Testing of High Performance Ceramics, Proceedings of the 1987 Co
nference,
American Ceramics Society/American Society for Nondestructive Testing, 1987, p 304
23.
E.R. Generazo and D.J. Roth, Quantitative Flaw Characterization With Scanning Laser Acoustic
Microscopy, Mater. Eval., Vol 44 (No. 7), June 1986, p 864
24.
P
. Cawley, Nondestructive Testing of Mass Produced Components by Natural Frequency Measurements,
Proc. Inst. Mech. Eng., Vol 199 (No. B3), 1985, p 161
25.
P. Cawley, Rapid Production Quality Control by Vibration Measurements, Mater. Eval.,
Vol 45 (No. 5),
May 1987, p 564
26.
R. Phillips and W. Franciscovich, Free-
free Resonant Frequency Testing of Powder Metal Alloys to
Determine Elastic Moduli, Prog. Powder Metall., Vol 39, 1983, p 369
27.
Y. Xu, S.H. Carpenter, and B. Campbell, An Investigation of the A
coustic Emission Generated During the
Deformation of Carbon Steel Fabricated by Powder Metallurgy Techniques, J. Acoust. Emiss.,
Vol 3 (No.
2), 1984, p 81
28.
A. Rosencwaig, Thermal Wave Imaging, Science, Vol 218 (No. 4569), 1982, p 223
29.
L.J. Inglehart, Photothermal Characterization of Ceramics, in
Nondestructive Testing of High Performance
Ceramics,
Proceedings of 1987 Conference, American Ceramics Society/American Society of
Nondestructive Testing, 1987, p 163
30.
A. Lewis, "Nondestructive Inspectio
n of Powder Metallurgy Parts Through the Use of Resistivity
Measurements," Paper presented at the Prevention and Detection of Cracks in Ferrous P/M Parts Seminar,
Metal Powder Industries Federation, 1988
31.
R.A. Ketterer and N. McQuiddy, "Resistivity Mea
surements on P/M Parts: Case Histories," Paper presented
at the Prevention and Detection of Cracks in Ferrous P/M Parts Seminar, Metal Powder Industries
Federation, 1988
32.
E.R. Leheup and J.R. Moon, Electrical Conductivity and Strength Changes in Green
Compacts of Iron
Powder When Heated in Range 50-400 °C in Air, Powder Metall., Vol 23 (No. 4), 1980, p 217
33.
E.R. Leheup and J.R. Moon, Electrical Conductivity Changes During Compaction of Pure Iron Powder,
Powder Metall., Vol 21 (No. 4), 1978, p 195
34.
E.R. Leheup and J.R. Moon, Relationships Between Density, Electrical Conductivity, Young's Modulus,
and Toughness of Porous Iron Samples, Powder Metall., Vol 21 (No. 4), 1978, p 1
35.
P. Neumaier, Computer-Aided Tester for Nondestructive Determination of Material Properties,
Metallurg.
Plant Technol., No. 3, 1987, p 58
36.
R.C. O'Brien, "Analysis of Variance of Sintered Properties of P/M Transmission Parts," Unpublished
Report, Hoeganaes Corporation, 1984
37.
W.B. James, "Quality Assurance Procedures
for Powder Forged Materials," SAE Technical Paper 830364,
Society of Automotive Engineers, Feb 1983
38.
Y.F. Cheu, Automatic Crack Detection With Computer Vision and Pattern Recognition of Magnetic Particle
Indications, Mater. Eval., Vol 42 (No. 11), Nov 1984, p 1506
39.
I. Hawkes and C. Spehrley, Point Density Measurement and Flaw Detection in P/M Green Compacts,
Mod.
Develop. Powder Metall., Vol 5, 1970, p 395
Nondestructive Inspection of Powder Metallurgy Parts
R.C. O'Brien and W.B. James, Hoeganaes Corporation
References
1. J.W. McCauley, Materials Testing in the 21st Century, in Nondestructive Testing of High Perf
ormance
Ceramics,
Conference Proceedings, American Ceramics Society/American Society for Nondestructive
Testing, 1987, p 1
2.
R.W. McClung and D.R. Johnson, Needs Assessment for NDT and Characterization of Ceramics:
Assessment of Inspection Technology for Green State and Sintered Ceramics, in
Nondestructive Testing of
High Performance Ceramics,
Conference Proceedings, American Ceramics Society/American Society for
Nondestructive Testing, 1987, p 33
3. R.C. O'Brien, "Fatigue Properties of P/M Materials," S
AE Technical Paper 880165, Society of Automotive
Engineers, March 1988
4.
G.F. Bocchini, "High Pressure Compaction, High Pressure Coining, and High Pressure Repressing of P/M
Parts," Paper presented at the Prevention and Detection of Cracks in Ferrous P/M
Parts Seminar, Metal
Powder Industries Federation, 1988
5. F.V. Lenel, Powder Metallurgy Principles and Application,
Metal Powder Industries Federation, 1980, p
112
6. C. Rain, Uncovering Hidden Flaws, High Technol., Feb 1984
7. B. Chang et al., Spatial Resolution in Industrial Tomography, IEEE Trans. Nuclear Sci.,
NS30 (No. 2),
April 1983
8. H. Heidt et al., Nondestructive Density Evaluation of P/M Objects by Computer Tomography, in
Horizons
of Powder Metallurgy, 1986 International Powder Metallurgy Conference Proceedings, Part II, p 723
9.
G. Schlieper, W.J. Huppmann, and A. Kozuch, Nondestructive Determination of Sectional Densities by
the Gamma Densomat, Prog. Powder Metall., Vol 43, 1987, p 351
10. C.T. Waldo, Practical Aspects of the Gamma Densomat, in Horizons in Powder Metallurgy,
1986
International Powder Metallurgy Conference Proceedings, Part II, p 739
11.
J.L. Rose, M.J. Koczak, and J.W. Raisch, Ultrasonic Determination of Density Variations in Green and
Sintered Powder Metallurgy Components, Prog. Powder Metall., Vol 30, 1974, p 131
12. B. Patterson, C. Bates, and W. Knopp, Nondestructive Evaluation of P/M Materials, Prog. Powder Metall.,
Vol 37, 1981, p 67
13. M.F. Termine, "Ultrasonic Velocity Measurements on Green and Sintered P/M Com
pacts," Unpublished
Report, Hoeganaes Corporation, 1985
14.
E.P. Papadakis and B.W. Petersen, Ultrasonic Velocity As A Predictor of Density in Sintered Powder
Metal Parts, Mater. Eval., April 1979, p 76
15. A. Gallo and V. Sergi, Orientation of Porosity
of P/M Materials Evaluated by Ultrasonic Method, in
Horizons in Powder Metallurgy,
1986 International Powder Metallurgy Conference Proceedings, Part II, p
763
16. M.P. Jones and G.V. Blessing, Ultrasonic Evaluation of Spray-Dried Alumina Powder During and
After
Compaction, in NDT of High Performance Ceramics,
Proceedings of the 1987 Conference, American
Ceramics Society/American Society for Nondestructive Testing, 1987, p 148
17. B.R. Patterson and C.E. Bates, Nondestructive Property Prediction in Gray Ca
st Iron Using Ultrasonic
Techniques, Paper 65, Trans. AFS, 1981, p 369
18. R.H. Brockelman, Dynamic Elastic Determination of the Properties of Sintered Powder Metals,
Perspect.
Powder Metall., Vol 5, 1970, p 201
19. E.R. Leheup and J.R. Moon, Yield and Fracture Phenomena in Powder Forged Fe-
0.2C and Their
Prediction by NDT Methods, Powder Metall., Vol 23 (No. 4), 1980, p 177
20. K. Subramanian and J.L. Rose, C-Scan Testing for Complex Parts, Adv. Mater. Process. inc. Met. Prog.,
Vol 131 (No. 2), 1987, p 40
21.
A. Hecht and E. Neumann, Detection of Small Inclusions in P/M Alloys by Means of Nondestructive
Ultrasonic Testing, in Horizons of Powder Metallurgy,
1986 International Powder Metallurgy Conference
Proceedings, Part II, P 783
22. G.Y. Baaklini and
P.B. Abel, Flaw Imaging and Ultrasonic Techniques for Characterizing Sintered Silicon
Carbide, in Nondestructive Testing of High Performance Ceramics,
Proceedings of the 1987 Conference,
American Ceramics Society/American Society for Nondestructive Testing, 1987, p 304
23.
E.R. Generazo and D.J. Roth, Quantitative Flaw Characterization With Scanning Laser Acoustic
Microscopy, Mater. Eval., Vol 44 (No. 7), June 1986, p 864
24. P. Cawley, Nondestructive Testing of Mass Produced Components by Natural Freque
ncy Measurements,
Proc. Inst. Mech. Eng., Vol 199 (No. B3), 1985, p 161
25. P. Cawley, Rapid Production Quality Control by Vibration Measurements, Mater. Eval.,
Vol 45 (No. 5),
May 1987, p 564
26. R. Phillips and W. Franciscovich, Free-free Resonant Freq
uency Testing of Powder Metal Alloys to
Determine Elastic Moduli, Prog. Powder Metall., Vol 39, 1983, p 369
27.
Y. Xu, S.H. Carpenter, and B. Campbell, An Investigation of the Acoustic Emission Generated During the
Deformation of Carbon Steel Fabricated by Powder Metallurgy Techniques, J. Acoust. Emiss.,
Vol 3 (No.
2), 1984, p 81
28. A. Rosencwaig, Thermal Wave Imaging, Science, Vol 218 (No. 4569), 1982, p 223
29. L.J. Inglehart, Photothermal Characterization of Ceramics, in Nondestructive Testing of Hig
h Performance
Ceramics,
Proceedings of 1987 Conference, American Ceramics Society/American Society of
Nondestructive Testing, 1987, p 163
30.
A. Lewis, "Nondestructive Inspection of Powder Metallurgy Parts Through the Use of Resistivity
Measurements," Pap
er presented at the Prevention and Detection of Cracks in Ferrous P/M Parts Seminar,
Metal Powder Industries Federation, 1988
31.
R.A. Ketterer and N. McQuiddy, "Resistivity Measurements on P/M Parts: Case Histories," Paper
presented at the Prevention and
Detection of Cracks in Ferrous P/M Parts Seminar, Metal Powder
Industries Federation, 1988
32.
E.R. Leheup and J.R. Moon, Electrical Conductivity and Strength Changes in Green Compacts of Iron
Powder When Heated in Range 50-400 °C in Air, Powder Metall., Vol 23 (No. 4), 1980, p 217
33.
E.R. Leheup and J.R. Moon, Electrical Conductivity Changes During Compaction of Pure Iron Powder,
Powder Metall., Vol 21 (No. 4), 1978, p 195
34. E.R. Leheup and J.R. Moon, Relationships Between Density, Electrical Conduc
tivity, Young's Modulus,
and Toughness of Porous Iron Samples, Powder Metall., Vol 21 (No. 4), 1978, p 1
35. P. Neumaier, Computer-Aided Tester for Nondestructive Determination of Material Properties,
Metallurg.
Plant Technol., No. 3, 1987, p 58
36. R.C.
O'Brien, "Analysis of Variance of Sintered Properties of P/M Transmission Parts," Unpublished
Report, Hoeganaes Corporation, 1984
37.
W.B. James, "Quality Assurance Procedures for Powder Forged Materials," SAE Technical Paper 830364,
Society of Automotive Engineers, Feb 1983
38.
Y.F. Cheu, Automatic Crack Detection With Computer Vision and Pattern Recognition of Magnetic
Particle Indications, Mater. Eval., Vol 42 (No. 11), Nov 1984, p 1506
39. I. Hawkes and C. Spehrley, Point Density Measurement and Fla
w Detection in P/M Green Compacts,
Mod. Develop. Powder Metall., Vol 5, 1970, p 395
Nondestructive Inspection of Steel Bar, Wire, and Billets
Introduction
THE INSPECTION OF STEEL BARS will be the focus of this article, but the principles involved also apply for the most
part to steel wire. In many cases, as far as nondestructive inspection is concerned, steel bars and wire are the same.
Steel billets, mainly because their irregularities differ significantly from the irregularities found in bars, require special
inspection techniques. Billet inspection is covered in the section "Nondestructive Inspection of Steel Billets" in this
article.
The primary objective in the nondestructive inspection of steel bars and wire is generally the same as for the inspection of
other products, that is, to detect conditions in the material that may be detrimental to the satisfactory end use of the
product. There is, however, an additional objective in attempting to detect undesirable conditions in semifinished products
such as bars, namely, to eliminate unacceptable material before spending time, money, and energy in manufacturing
products that will later be rejected.
The nondestructive inspection of bars and other semifinished products does not impair the product, provides rapid
feedback of information, and can be utilized as either an in-line or off-line system. It makes use of several devices, such
as visual, audio, and electromagnetic, for the detection of flaws and of variations in composition, hardness, and grain
structure. A wide range of selectivity is provided in each device, permitting acceptance or rejection at various
specification levels. The most common function of nondestructive inspection in the steel industry is the detection and
evaluation of flaws. It is also used for the detection of variations in composition and physical properties. No amount of
nondestructive inspection can ensure an absolutely flawless bar, but it does provide a consistent specified degree of
quality during everyday operation.
Nondestructive Inspection of Steel Bar, Wire, and Billets
Types of Flaws Encountered
The terms used for the various types of flaws discussed in this article may not be the same in various geographic areas. In
many cases, different terms are applied to the same type of flaw. Therefore, this section contains a description and an
illustration of each condition. The term flaw is applied to blemishes, imperfections, faults, or other conditions that may
nullify acceptability of the material. The term also encompasses such terms as pipe, porosity, laminations, slivers, scabs,
pits, embedded scale, cracks, seams, laps, and chevrons, as well as blisters and slag inclusions in hot-rolled products. For
products that are cold drawn, die scratches may be added.
Most flaws in steel bars can be traced back to the pouring of the hot metal into molds. Factors that work against obtaining
a perfect homogeneous product include:
• The fast shrinkage of steel as it cools (roughly 5% in volume)
•
The gaseous products that are trapped by the solidifying metal as they try to escape from the liquid and
semisolid metal
• Small crevices in the mold walls, which cause the metal to tear during the stripping operation
•
Spatter during pouring, which produces globs of metal frozen on the mold walls because of the great
difference in temperature of the mold surfaces and the liquid metal
Pipe is a condition that develops in the nominal top centerline of the ingot as the result of solidification of the molten
metal from the top down and from the mold walls to the center of the ingot (Fig. 1). Because of the metal shrinkage and
lack of available liquid metal, a cavity develops from the top down and, if not completely cropped before subsequent
rolling, becomes elongated and will be found in the center of the final product, as shown in ingot B in Fig. 1.
Fig. 1
Longitudinal sections of two types of ingots showing typical pipe and porosity. When the ingots are rolled
into bars, these flaws become elongated throughout the center of the bars.
Porosity is the result of trapped gaseous bubbles in the solidifying metal causing porous structures in the interior of the
ingot (Fig. 1). Upon rolling, these structures are elongated and interspersed throughout the cross section of the bar
product, as illustrated in Fig. 1.
Inclusions may be the products of deoxidation in the ingot, or they may occur from additives for improving
machinability, such as lead or sulfur. Inclusions and their typical location in a steel bar are shown in Fig. 2(a).
Fig. 2 Ten different types of flaws that may be found in rolled bars. See text for discussion.
Laminations may occur from spatter (entrapped splashes) during the pouring of the steel into the mold. They are
elongated during rolling and are usually subsurface in the bar. Figure 2(b) illustrates a lamellar structure opened up by a
chipping tool.
Slivers are most often caused by a rough mold surface, overheating prior to rolling, or abrasion during rolling. Very
often, slivers are found with seams. Slivers usually have raised edges, as shown in Fig. 2(c).
Scabs are caused by splashing liquid metal in the mold. The metal first freezes to the wall of the mold, then becomes
attached to the ingot, and finally becomes embedded in the surface of the rolled bar (Fig. 2d). Scabs thus bear some
similarity to laminations.
Pits and Blisters. Gaseous pockets in the ingot often become, during subsequent rolling, pits or blisters on the surface
or slightly below the surface of bar products. Other pits may be caused by overpickling to remove scale or rust. Pits and
blisters are both illustrated in Fig. 2(e).
Embedded scale may result from the rolling or drawing of bars that have become excessively scaled during prior
heating operations. The pattern illustrated in Fig. 2(f) is typical.
Cracks and seams are often confused with each other. Cracks with little or no oxide present on their edges may occur
when the metal cools in the mold, setting up highly stressed areas. Seams develop from these cracks during rolling as the
reheated outer skin of the billet becomes heavily oxidized, transforms into scale, and flakes off the part during further
rolling operations. Cracks also result from highly stressed planes in cold-drawn bars or from improper quenching during
heat treatment. Cracks created from these latter two causes show no evidence of oxidized surfaces. A typical crack in a
bar is shown in Fig. 2(g).
Seams result from elongated trapped-gas pockets or from cracks. The surfaces are generally heavily oxidized and
decarburized. Depth varies widely, and surface areas sometimes may be welded together in spots. Seams may be
continuous or intermittent, as indicated in Fig. 2(h). A micrograph of a typical seam is shown in Fig. 3.
Fig. 3 Micrograph of a seam in a cross section of a 19 mm ( in.) diam medium-
carbon steel bar showing
oxide and decarburization in the seam. 350×
Laps are most often caused by excessive material in a given hot roll pass being squeezed out into the area of the roll
collar. When turned for the following pass, the material is rolled back into the bar and appears as a lap on the surface.
Chevrons are internal flaws named for their shape (Fig. 2k). They often result from excessively severe cold drawing and
are even more likely to occur during extrusion operations. The severe stresses that build up internally cause transverse
subsurface cracks.
Nondestructive Inspection of Steel Bar, Wire, and Billets
Methods Used for Inspection of Steel Bars
Almost all inspection of steel bars (other than plain visual inspection) is performed by means of the following four
methods, used either singly or in combination:
• Magnetic particle inspection
• Liquid penetrant inspection
• Ultrasonic inspection
• Electromagnetic inspection
Magnetic Particle Inspection
Magnetic particle inspection offers the same visual aid in the nondestructive inspection of bars as it does for castings,
forgings, or machined products. The method is used for detecting seams, cracks, and other surface flaws, and, to a limited
extent, subsurface flaws. As a rule, the method is not capable of detecting flaws that are more than 2.5 mm (0.1 in.)
beneath the surface.
The magnetic particle method utilizes a magnetic field set up in the bar. Flaws cause a leakage of flux if they are at an
angle to the flux flow. This flux is due to the lower magnetic permeability of the material in the flaw (air, oxide, or dirt)
compared with that of the metal. Because the flux leakage forms magnetic poles, fine iron powder sprinkled on the
surface will adhere, indicating the extent of the flaw.
Longitudinal Flaws. Optimum indications are obtained when the magnetic field is perpendicular to the flaws. A
similar result is obtained for flaws slightly below the surface, but the surface leakage is less and, consequently, fewer iron
particles are attracted to the area, producing a less definite indication.
Various colors of iron powders are commercially available to permit the choice of a color that provides maximum
contrast between the powder and the material being inspected. Fluorescent coatings on powders and ultraviolet light can
be used to make the indication more vivid. The powders can be applied in dry form, or can be suspended in oil or a
distillate and flowed over the workpiece during or after the magnetizing cycle.
Transverse Flaws. To detect flaws transverse to the long axis of the bar being inspected, a solenoid winding or
encircling coil is used. For longitudinal-type flaws, circumferential magnetization is utilized; an electric current flowing
through the bar sets up a magnetic field at right angles to the long axis of the bar. To protect the bar from arc burns when
the current is turned on, electrical contact is usually made by soft metallic pads held firmly against the bar ends.
Power Requirements. The power used can be direct current or alternating current. Direct current may be from
batteries or rectified alternating current. Alternating current travels near the surface and should not be used for detecting
subsurface flaws. In most cases, the continuous-magnetization system is used for bars because most bars have low
retentivity for magnetism; therefore, the residual-magnetism system is not suitable. Finished bars must be demagnetized;
otherwise, during manufacturing operations such as machining, steel chips will adhere and possibly cause trouble.
Quantity Requirements. As a rule, the magnetic particle inspection of bars is confined to the inspection of a small
quantity of bars, as in a fabricating shop. The method is, in its present state of development, considered too slow and too
costly for mass-production inspection, as in a mill. Detailed information on magnetic particle inspection as it is applied to
various ferromagnetic products is available in the article "Magnetic Particle Inspection" in this Volume.
Liquid Penetrant Inspection
Liquid penetrant inspection (another visual aid), for several practical reasons, is not extensively used for detecting flaws
in steel bars. These reasons include the following:
• Its use is restricted to the detection of flaws that are open to the bar surface
• Adaptation to automation is limited compared with certain other inspection methods
• Time cycles are too long for the inspection of bars on a mass-production basis
There are exceptions, however, and there are cases where liquid penetrant inspection has been used advantageously for
inspecting from one to a few bars, as in a fabricating shop. Specific advantages are:
• Liquid penetrant in
spection is extremely sensitive and can sometimes detect surface flaws missed by
other methods
• The solvent-
removable system (one of the several liquid penetrant systems) in particular is extremely
flexible and can be used for inspecting bars or portions o
f bars in virtually any location, including in the
field
Detailed information on liquid penetrant inspection is available in the article "Liquid Penetrant Inspection" in this
Volume.
Ultrasonic Inspection
Ultrasonic inspection is done with high-frequency (about 1 to 25 MHz) sound waves and can successfully detect internal
flaws in steel bars. Most often, the ultrasonic inspection of steel bars is restricted to large-diameter bars and to
applications where high integrity is specified. Also, because of the limitations of ultrasonic inspection for detecting
surface flaws, it is ordinarily used in conjunction with some other method that is more suitable for inspecting bar surfaces.
An ultrasonic beam has the valuable property that it will travel for long distances practically unaltered in a homogeneous
liquid or solid, but when it reaches an interface with air (for example, at a crack or at the surface of a metal body), it is
almost completely reflected. The ultrasonic beam is generated by applying a high-frequency voltage to a piezoelectric
crystal, which is thus brought into mechanical oscillation. This energy in turn is fed to the workpiece by a liquid couplant.
The technique most commonly used in the nondestructive inspection of bars or barlike workpieces is the pulse-echo
technique. Short pulses of ultrasonic energy are passed through the bar. The sweep voltage of the time base is coordinated
with the pulse-repetition frequency so that the reflections are indicated on an oscilloscope screen. A certain amount of
energy is reflected at the interface between probe and specimen, giving the first transmission signal. The probe can either
have two separate crystals, a transmitter and a receiver, or have only one, which is used alternately as transmitter and
receiver.
The ultrasonic method is characterized by high sensitivity and very deep penetration, but in addition to its surface
limitations, its production speed is relatively low. A liquid couplant is necessary and can be a source of interference. This
method is suitable for testing ingots, billets, plate, and tubes in addition to bars or barlike workpieces.
In certain cases, ultrasonic inspection has been automated. Typical products that are ultrasonically inspected using
automated equipment are forged axle shafts (which are, in effect, extruded bars) and rolled bars.
Cold-Drawn Bars (Ref 1). The most effective method for the inside flaw inspection of cold-drawn bars is ultrasonic
flaw detection. However, it is becoming more and more necessary to detect the smaller defects in the near-surface area in
accordance with changing the production process. The conventional normal beam method (Fig. 4a) is not satisfactory,
because of untested area near the surface.
Fig. 4 Schematic s
howing position of probe relative to flaw inside of bar and resulting wave display obtained for
two methods of ultrasonic flaw detection. (a) Normal-beam method. (b) Angle-
beam method. Wave display
nomenclature: T, transmit pulse; S, surface reflection echo; F
1
, flaw echo; B
1
, back wall echo. Source: Ref 1
A testing method for detecting smaller flaws immediately under the surface of cold-drawn bars is the angle-beam method
(Fig. 4b), which conveys ultrasonic waves into the material with an angle beam. It can detect the flaws immediately under
the surface that are in the dead zone for the conventional normal-beam method. Entire cross-sectional area testing
becomes possible with the angle-beam method and the conventional normal-beam method in combination. The testing
method to feed the material spirally and to make the probes follow the deflection of the material feeding has already been
adopted in practical use for as-rolled steel bars. It is difficult to obtain higher testing speed for cold-drawn bars because of
smaller dimensions. Therefore, the following method has been developed in which the material is fed straight and the
probes are simultaneously rotated at high speed. Table 1 lists the main specifications of the system, and Fig. 5 shows a
schematic of the setup. For bars with smaller dimensions, guide sleeves and tripplet rollers are used to prevent the
ultrasonic incident angle to the material from changing because of excessive vibration and/or bending of the material. The
water circulation system also incorporates a device that stabilizes the coupling water.