Make

10,

June, 2010

Revision:

14,

Apr, 2020

Nippon Kouatsu Electric Co., Ltd.

1 . Background
Quality control of the conventional spot welding,-Guarantee of quality => Breaking test
- Quality identification => Driver check (DC)The inspection method replaced with this Driver check (DC)is proposed.

Practical application of non-destructive
inspection using magnetism.

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2 . What is a Nugget Profiler?

Important

A machine to inspect spot welds using magnetism.
Features:
1.A nugget can be quantified.
2.Needs no gel, water or other medium.
(Ultrasonic method requires some medium
such as gel or water.)
3.Can distinguish between nugget andcold-joint (pressure weld).
4. A sensor is applied lightly and inspected.
5. The inspection speed is around 4 seconds per weld point.
6. Automatically save results to electronic file (traceability).
7. The powered is a battery.

It is an inspection machine
which can quantify the nugget of spot welding easily.
It is possible to check whether welding at the same position maintains constant quality
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3 . Merit of Nugget Profiler

Important-Performs inspection giving no external damage to the subjects.
-It can inspect even high-tensile steel.
-It has the function of quality traceability(Automatically save results to electronic file).
<The problem of a driver check>-usually steel : A bend arises around a nugget and metal(270MPa,440MPa) fatigue occurs.
-high-tensile steel : It cracks on a nugget or the outskirts of it.
(770MPa or more)
For the reason, welding intensity falls.
＝CRACK＝CRACK
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４. Purpose of introducing nugget profiler

ImportantCommon case
Conditioning
・Welding conditioning
・Tip change interval
・Welding gun teaching

Manufacture
・Perform welding

Ship

Proposed use
Build a PDCA cycle for stabilizing and improving
shipping quality
Conditioning（Plan)・Welding conditioning
・Tip change interval
・Welding gun teaching

Manufacture（Do)・Perform welding

Inspection（Check）
・Confirm
quality stability

Ship・Quality
improvement

Extraction of
instability

Feedback（Action）
・Improvement of conditions

Discovering large fluctuating
welding spots⇒ Stabilize the process

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５. Use (the purpose of the equipment)
５.１.Applicable cases
This equipment quantifies (measures as numeric values) the quality of the areas formedby spot welding.
Its purpose is to confirm that the quality of the spot welding is of a sufficiently high level.
Note: The equipment is used in a comparative inspection with the breaking test.

Breaking test

Breaking test
･･･

Item
Commoncase

Breaking diametermeasurement

Standarde.g., 4 t [mm]Proposeduse

Item
Breaking diametermeasurement
Standarde.g., 4 t [mm]+Item
Measurement as
numeric values with
the NP･･･

Item
Driver check
Standard
No peeling off

Item

Item
Measurement as numeric values
with the NP
Standard
Comparison of the values withthose of the breaking test

Item
Breaking diametermeasurement
Standarde.g., 4 t [mm]+Item

- Unchanged: the quality is stable.
- Changed: the quality fall-off isunderstood quantitatively by thedifference of values.

Copyright©Nippon Kouatsu Electric Co., Ltd

Breaking diametermeasurement

Item
DC

Standarde.g., 4 t [mm]

Item
NP

Measurement as
numeric values with
the NP

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５ . ２ . Applicable cases - continuation
Plotting the inspection file described on the previous page (plot software is included in the
profiler) reveals the following:.
(1) If the value deteriorates: A welding defect has occurred.
(2) If the value significantly fluctuates: The process (welding condition) is unstable.
- Cause 1: The weld gun is at an angle to the steel plate⇒ Improve the angle through teaching.
- Cause 2: There is a gap between steel plates.
⇒ Eliminate the gap by plate positioning.
◼

Note: The above function can be used for checking whether the process is stablein addition to detecting defectives.
(Further, it can be used as a guide for teaching operation.)
Exam ple of applicable facility* The quality change to the n number of the same positions is managed.
120* This equipment serves as a relativevalue inspection from a breaking test
time.
Since it is not an absolute value
inspection, the diameter of an output
of equipment has differed from thediameter of an absolute size.

Fabrication/exchange of a tip

系列1
系列2

Welding quality[%]

The quality during
breaking test is assumed 100
to be 100 [%].
80
Quality success-or-failure line60

It is necessary to associate“The value which this equipment
expressed quantitatively” and“The actual diameter of welding ” atthe time of a breaking test.
It is carried out by parameter setting.

Near a success-or-failure line
40
20

A clear poor weld is detected.
0
0

50

100

150

200

250

300

Inspection n number

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５. ３ . Applicable cases - shipping inspection
The nugget profiler can be used at parts companies.
(1) All parts in a production lot are quantified: Variations in the lot can be confirmed.
(2) Then, a single piece can be sampled out of the lot and subjected to breaking test to assure
the quality.

Note: The outflow of defective parts can be prevented.
(Parts other than those subjected to breaking test can also be checked for
quality.)
Breaking test

1lot

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５. ４ . Applicable cases - acceptance inspection
The nugget profiler can be used at acceptance sections.
Comparison within a lot
Comparison with past lots

: The parts in a received lot are checked for whether they havethe same quality.
: A received lot is checked for whether it has the same quality as
the past lots.

Comparison within a lot

Comparison with past lots

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６ . Equipment composition
This equipment is composed of the following four parts.
-Main body (including PC) : hardware control, function of preparation of
exciting voltage pattern.
-Tablet PC : data analysis and man machine IF functions.
-Sensor probe : generating magnetic flux and data acquisition functions.
-Handy switch : trigger function of inspection data recording.
10mm

Sensorprobe

Center ofsensitivity
4mm

Main
body

100mm

18mm

Handy
switchArray sensor for detection(Qty.16)

PC

20mm

Handy
switchCopyright©Nippon Kouatsu Electric Co., Ltd

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7. Change in steel materials

Important

7.1. Change in phase by steel material temperature
Item

Ferrite phase

Austenite

Characteristics

Water
current

Current

SEM
image
and
crystalstructure• Room-temperature crystal
structure• Body-centered cubic lattice
crystal structure• Soft• Ferromagnetic (strongly
attracted by a magnet)= Low magnetic resistanceMartensitic phase

Temperature

Temperaturerise

quench• High-temperature crystal
structure• Face-centered cubic lattice
crystal structure• Excellent toughness andductility• Non-magnetic (not attracted
by a magnet)

Figure: Photoof weld
cross
sectionCopyright©Nippon
KouatsuElectricCo.,
Ltd• Dense needle crystal structure• Body-centered cubic lattice
crystal structure• Hard but brittle• Paramagnetic (weakly attracted
by a magnet)＝High magnetic resistance

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7.2. Change in magnetic properties accompanied
by quenching steel materials

Important

Table 7.1: A list of heat-treated specimens
Total
Elongation[%]Hardness[HV]

FC

1000C (5 minutes) + furnace cooling

585

22.5

155

AC

1000C (5 minutes) + air cooling

788

18.1

176

WQ

1000C (5 minutes) + water cooling

1383

8.6

354

WT

1000C (5 minutes) + water cooling +600C (15 minutes) + air cooling

730

15.7

229

IS

No treatment (intact)

1006

14.6

295

Mechanical Properties

24

13.5

9.6

8.4

33

15
9.0

12

9

29
25
21
17

IS

WT

13

WQ

7.2: The relationship between
total elongation andproperty values

Base metal
149

Weld

AC

10.2

154

FC

IS

WT

WQ

6

Base metal
9.6

Weld
AC

7.1: The relationship between
tensile strength andproperty values

7.8
8.4

FC

IS

WT

7.2

WQ

5.5

AC

7.8

FC

7.5

18

37

Magnetic Property
磁気特性値
Values

144
139
134
129
124
119

Property Values

9.5

21

7.2

Property Values

9.0

Property Values

Base metal

11.5

特性値

Weld

Total Elongation[%]

10.2

Mechanical Properties
機械的性質

Magnetic Property
磁気特性値
Valueshardness[x10HV]

機械的性質

Magnetic Property
磁気特性値
Values

特性値

Mechanical Properties
機械的性質

tensile strength [x100MPa]

TensileStrength[MPa]Description of Process

特性値

Name

7.3: The relationship between
hardness andproperty values

There is a high correlation between[mechanical property (blue)] and [magnetic property (red)].
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8 . Macro photos of nugget and cold-joint
Welding point temperature (°C)

Macro photo

Micro photo

NuggetMelting
point

Cold joint
Cold joint
Welding time

As for a nugget part, the organization of a material changes.
Metal becomes hard by thisand it achieves the original purpose of welding.
⇒Nugget parts decrease in magnetic permeability (the amount ofemergence of magnetic flux density) due to the martensitic transformation(transition, carbon over-saturation, etc.) in the structure.
This equipment is inspecting magnetically using this characteristic.
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9.Inspection theory outline and sensor structure
Know-how

The magnetic sensor contains two types of coils:(1) Exciting coil: Serves as an electromagnet, which produces magnetism.
(2) Sensing coil: Converts produced magnetic flux into electrical signals, which arethen sent to the device.
It obtains difference in magnetic resistance between the weld and base metal using
16 detection coils and plots it on a waveform chart.
Inspection
screenSensor head

[Nugget]Magnetic resistance = High

Exciting coil
(A magnetic field is generated )Array coil for detection:16
Magnetic
flux

11.2mm
[Nugget circumference ]Magnetic resistance = Low

NuggetBottom view

Corona BondSide view

Coil requires welded part (indentation) + two pieces on left and right side (base metal part)

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Important
10 Comparison of welding
10.1 Comparison of welding quality and inspection result values

The red circle between the two sheets in the figure below indicates the part whose magnetic
permeability has actually become low.
The inspection results show that the obtained curve varies depending on the welding quality.
Therefore, it is required to control not only the diameter but also the curve size.
Good

Part whose magnetic
Inspection diameter: 5.0 mm permeability has
Curve area: 130
become low

A little too thin

Cold-joint

Inspection diameter: 3.7 mm
Curve area: 57

Inspection diameter: 0.0 mm
Curve area: 00

The curve height
becomes lower.

The curve height
becomes lower.
A little too small diameter(with a nugget)

The curve width
becomes narrower.

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10.2 Determination of pressure welds and nuggets
Based on the detected values obtained in the previous page, the analysis result (inspection wave form) is
shown below in inspecting the NG product (cold-joint) and OK product (nugget).
The "difference" at the bottom of the table below is the analysis result. This is the difference value
between the yellow wave and orange wave.

- In case of inspecting NG product => The form of blue wave (Difference) is flat or low crest.
- In case of inspecting OK product => The form of blue wave (Difference) is high crest.
* The height of blue wave and the judgment of OK/NG product are set in parameter based on thedestruction test.
Inspection screen (in case of inspecting OK product)

NG part (cold-joint)

OK part (nugget)

Macro picture(Cut crosssection view)

Weak magnetic
field Inspection

Coldjoint

Base
metal

Array coil x 16

Base
metal

Base
metal

NuggetBase
metal

Array coil x 16

screen:orange wave

Strong magnetic
field Inspection
screen:yellow wave

Difference
Inspection* The red line estimates the surface shape.

screen:blue wave*Analysis result

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○Comparison of OK and NG products
Item

Important

OK

NG

Melt solidification
Enough change(Martensitic phase)
Slightly low

Cold joint
Inadequate change
Low

High

Low

Macro photo
StatusChangemagnetic resistance

Inspection waveform○Items with changing magnetic resistance
Item
Shape change (bending, pressing, etc.)
Pressurization (during spot welding)Heating (no change in structure)
Organizational change

S/N
Noise
Noise
Noise
SignalStrong magnetic field
Acquisition
Acquisition
Acquisition
Acquisition

Copyright©Nippon Kouatsu Electric Co., Ltd

Weak magnetic field
Acquisition
Acquisition
Acquisition
Non-acquisition

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11. Data analyzing method
11.1. Data acquisition method

Important

The magnetic sensor contains two types of coils:(1) Exciting coil: Serves as an electromagnet, which produces magnetism.
(2) Sensing coil: Converts produced magnetic flux into electrical signals, which arethen sent to the device.
Faraday's law of electromagnetic
induction is used to convert
magnetic flux into electrical signals.
Magnetic
flux

Power supply

Φ

t

t
e

(A sample with a low magnetic
resistance)tt• A large amount of magnetic flux Φ is produced.
• The level of induced electromotive force e ishigh.

Specimen
16 sensing array coils(Used to obtainmagnetic flux data)eΦ (Reference: air)Exciting coil
(Produces magnetic fields)

Magnetic flux ΦΦ (Reference: air)Side Viewe

Induced electromotive force e
Data obtainedΦ (A sample with a low
electrical resistance)

t

t
e

t

BottomView

Faraday's law of electromagnetic inductione = -n (dΦ/dt)• A large amount of eddy current is produced in
the sample.
• The magnetic flux Φ changes less significantly.
• The induced electromotive force e changes
less significantly, with smaller derivative values.

t

Figure 10.1.1 Sensor structure and data obtained

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11.2. Summary of sensor
The magnetic sensor, which is the main component of Seam Seeker,
is energized by direct current blocking(a patented Nippon Kouatsu proprietary technique).
This technique is capable of inducing

16 sensing array coils

a much larger electromotive force comparedto the conventional alternating current (sine wave) method,
providing the following advantages:

11.2ｍｍFigure 10.2.1: Sensor Bottom View

1. High performance: Changes in the steel microstructure characteristics(tensile strength, total elongation and hardness) can be detected.
2. Smaller size: 16 array sensors are provided within a space of approximately 11 mm.

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19/28

12 . Inspection theory
12 . 1 . Acquired signal

To estimate the magnetic flux density B, acquisition is performedby changing the magnetic flux Φ to electric signal e.
(Faraday's law of electromagnetic induction)(1) The magnetic flux density B is generated
by the magnetic field H.
B=μH μ : magnetic permeability [H/m](2) The relation between the magnetic flux density B
and magnetic flux Φ is shown as the following equation.
B=φ/S S ： area where magnetic flux penetrates vertically(3) The inspection acquired signal e of this equipment is shownas the following equation.
e=-N×dΦ/dt N ： the number of turns of array coil
(The above equation is
Faraday's law of electromagnetic induction.)

Magnetic flux density B [kG]
磁束密度B[kG]
numerical value is* The concrete※詳細数値は社外秘
for internal use only.

by acquiring the magnetic flux density B.

Magnetic flux density B [kG]
磁束密度B[kG]* The concrete numerical value is※詳細数値は社外秘
for internal use only.

Generally, it is known that when steel is hardened, the magnetism characteristics are changed.
The figure is called as "B-H curve," showing the generated flux amount to the magnetic field strength.
The generated magnetic flux density B is determined depending on the material's magnetic permeability.
As shown in the figure, the magnetic permeability varies among the base metal, cold-joint, and nugget.
=> A welding area is detectable
B-H
curve
B-H曲線

Base metal
metal
Base
母
材
Cold_Joint
Cold
Joint
圧接状態
NuggetNugget
ナゲット

Magnetic
field strength H [0e]磁界強度H[Oe]* The concrete numerical value is※詳細数値は社外秘
for internal use only.
Weakmagnetic
filed

StrongB-H曲線拡大
Strongmagnetic
magnetic fieldfiled

B-H curve Initial part enlarged

Base
母 metal
材
Cold
Joint
圧接状態

ナゲット
Nugget磁界強度H[Oe]Magnetic
field strength H [0e]* The concrete numerical value is※詳細数値は社外秘
for internal use only.

Horizontal axis : The magnetic field strength H is decided by the amount of current which flows into a magnetization coil.
Vertical axis : The magnetic flux density B is acquired with a detection array coil.

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20/28

12 . 2 . Analysis method 1
The amount of current sent to the exciting coil can control the magnetic field strength. Therefore, the signals are
acquired in two patterns, at the weak magnetic field (hereinafter referred to as "weak magnetic field") and at thestrong magnetic field (hereinafter referred to as "strong magnetic field"), then judge the presence/absence ofnugget and calculate the nugget diameter index.
The reason why the two patterns magnetic field strength are used is that B-H curve is non-linear.
-Weak magnetic field : To the base metal, NG product (cold-joint) and OK product (nugget)
have similar characteristics.
- Strong magnetic field : To the base metal, OK product is much away from the base metal than NG product.
*Analysis waveform can be obtained by the relative values from the base metal.
Compared with the magnetic flux density of base metal, the more the magnetic flux density of welding part(cold-joint, nugget) is away, the higher the crest is.

Magnetic flux density B [kG]
磁束密度B[kG]
numerical value is* The concrete※詳細数値は社外秘
for internal use only.

Therefore, when the strong magnetic field is observed based on the weak magnetic field, it is judged OK product(nugget) when the product is much away from the base metal characteristics.
This shows the blue waves in the inspection screen calculated from the difference between yellow wave andorange wave.
Weak
Strong Strong
B-H
curve
B-H曲線拡大
magnetic
magnetic magnetic
field
filed
filed

Initial part enlarged

Weak magnetic field
Inspection
Magnetic
part
flux
densityBase
母 metal
材
Cold
Joint
圧接状態

ナゲット
NuggetInversenumber x
100

Gain adjustment(base metal = 20)* Detected value

Strong magnetic field
Magnetic
flux
density

Inversenumber x
100

Gain adjustment(base metal = 20)* Detected value

Base metal
Cold-joint
Nugget磁界強度H[Oe]Magnetic
field strength H [0e]* The※詳細数値は社外秘
concrete numerical value isfor internal use only.

*Since the upper table is shown in the following page by a diagram,please refer to it.

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13 . Comparison with other inspection methods
Nugget profiler

The contents ofdetection
A thing to be examined
Area which can beinspected.

Quantification of a nugget
Distinction with
Cold-joint
Damage to the test object

Inspection of three sheets

The amount of change ofmetallographic structure

Ultrasonic type

Driver check (DC)chisel test

Distance to the changing
point of acoustic impedance

Mechanical strength

Only iron

Iron, Nonferrous metal

Iron, Nonferrous metal〇
The part which does not havea convex around an inspection
side(Contact area size is 18x10mm)◎ : Single type
○ : Array type○× : Single type
△ : Array type

×

△

○

○ (comparison of a value
in fixed quantity)
None△ (Inspection fromsurface and back side)

Wiping of water and oil
is required.
=>Metal corrosion△
Area where tool cannot be
inserted is impossible.

A bend occurs.
Tapping-back is necessary.
=>Metal fatigue○△ (Inspection from surface
and back side)

Application to
High-tension steel○○△(Those with a possibility ofcracking a welding part. )Inspection time

○

△

◎

Traceability

○

○

×

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14 . Comparison with driver check
Conventional inspection
Driver check2 values

Inspection
resultInspection
type
Inspection
record*Regardless of the welding diameter,
whether it is sticky or not.
Damaged inspection target.
Quasi destruction test

No record remains.
Did the inspection actually be done?
What is the result of the test?
These records do not remain.

Quality
Since there is no record,management it can not be managed.

burden

It is impossible to inspecta high tensile steel.
There are many places where tools
can not use.
It is noisy. Burden on one’s wrist.

Cost

High tensile steel sheet is discarded.

ExpandtargetFuture－Suggestion
Nugget profiler
Trend*Indication of weld diameter fluctuation isdisplayed compared with standard product **.
*It’s not for measuring welding diameter.
(large and small is compared)(**destructive test item: quality assured item)
There is no damage to the inspection target.
Nondestructive inspection(can be inspected with shipping products)
Automatic output of electronic files.

Results including inspection date and time
are automatically saved.
Check the stability at the same point.
Leading to confirmation that the manufacturing
process is unstable.
High tensile strength steel can also beinspected.
Training is necessary.
There is a possibility of extending thedestruction test interval in the future.
Possibility of automatic inspection by robot

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15 . Correlation between the estimatednugget diameter and destruction test method
The example of an inspection by a test piece

diameterN/P measured
inspectionof N/P
diameter

270MPa-980MPa

The diameter of NP inspection (diameterof an inspection of this equipment) istargeting the diameter of macro.

The size relation of the diameter of
welding is the following.
macro < tear off (broken) < twist (broken)

Cross section macro diameter

Note: Since the breaking diameter
includes the corona bond (see page 10),
it is greater than the actual macro
diameter.

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16 . Equipment specifications
Type

Portable type

Model

NPH03D

NPH03B

NPF02

Dimensions(excluding protrusions)230(w)×195(D)×65(H)mm

285(w)×205(D)×70(H)mm

350(w)×255(D)×100(H)mm

Weight1.91kg2.32kg5.40kgScreen display size

7inch

10inch13.3inch

3 mm to 7 mm [Indentation diameter 8mm or less; Effective sensor width: 11 mm]

Inspection welding diameter

Welding
inspection
part

PC

Fixed type

(The equipment may support the ranges other than the above. Please inquire us.)Specimen sheet thickness(Thickness of one inspection side )0.7 to 2.3 mm (The equipment may support the ranges other than the left. Please inquire us.)Specimen sheet type

270 to 1280 MPa (The equipment may support the ranges other than the left. Please inquire us.)Specimen sheet surface treatment

No treatment, electrogalvanizing， hot-dip galvanizing (As for the aluminizing. please inquire us.)Number of specimen mating sheets

Two sheets, three sheets (Inspection from both sides are necessary.)

Sensor edge material

FR

Data storage

XLS form

Setting of inspection condition

Set in the inspection screen. (The setting contents can be stored in the file.)

Judgment method

OK/NG judgment, The diameter of a presumed nugget

OptionNavigation mode, Inspection record management

OS

Windows10

Operation temperature range

5°C to 40°C (41℉ to 104℉)

Operation humidity range

Up to 80%RH (No condensation occurs.)
Two kinds of power supplies selectable.

Power supply(1) AC85V to 240V, 0.5A

AC85V to 240V, 0.5A

(2) Battery (8 size AA Ni-MH battery)
Please note that the product specifications are subject to change without notice for improvement.

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17.Support documentation: The factor which has on magnetic resistanceKnow-how

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18. Supplement: Welding three plates

Know-how

When welding three plates, a nugget is formed as follows:
ElectrodeThin plate
Thick plate
Thick plate
Electrode

Process of nugget formation when welding three steel plates• A nugget is first formed between thick plates, and then reaches to anarea between thin and thick plates.
• Between high-strength steel plates, and then between soft steel andhigh-strength steel plates
The nugget formation starts from an area with high electric resistance.

Source:Literature in weld engineering
Volume 35 (2017) No. 1, etc.

 Electric resistance depending on steel plate thickness and typeLow resistanceSteel plate
thickness

High resistance
R =ρ

Thin

Thick

(Length)Steel plate
type

Soft steel(Low carbon
content)lA

R = Electric resistanceρ = Electric resistivity
l = Length
A = Sectional area
Electric
resistance(μΩ・cm)Steel type
Carbon steel (with 0.1% C: equivalent to S10C) …
14.2(Low carbon content)
Carbon steel (with 0.2% C: equivalent to S20C) …
16.9
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Carbon steel (with 0.4% C: equivalent to S40C) …27/28

High-strengthsteel

18. Supplement: Welding three plates

Know-how

We cut and inspected 92 three-plate weld points tocheck the welding status.
Cut inspection for three-plate welds:
Ratio of accepted to rejected points
21
rejected71
accepted

Breakdown of rejected points
0 rejected onlyfor
thick plate
7 rejected
only for thinplate
14 rejected forboth plates

Cut inspection:
A cut inspection is a method to break a sample actually and measure theweld system.

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