Nick den UijlCorus RD&T

nick.den-uijl@corusgroup.com

Using Sorpas to support weldability research at Corus

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VerificationVerification of resistance spot welding (RSW) models against a single weld(e.g. through micrographs) can be misleading because of the wide band of scatter.Therefore: Verification using weld growth curves

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Sorpasexperimental5SQRTt

5 root tI = 11,1 kA

1.5mm TRIP700 GI

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10 10,5 11 11,5 12 12,5

I [kA]

d [m

m]

5√t = 6.14mm

I [kA]

d [mm]

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Weldability

Weldability:1. process

Can welds actually be made?2. quality

How do the welds perform?

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Weldability

Process: Can welds actually be made?

1. process windowWhen the material parameters have been verified (against experimental results), it is found that Sorpas is a very useful tool to examine process window and parameter settings.

– optimisation (5√t)– weld growth curves– welding lobes

2. process parametersSorpas can also be used to establish the influence of process parameters.

– surface contamination– electrode geometry

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Process windowWhen the material parameters have been verified (against experimental

results), it is found that Sorpas is a very useful tool to examine process window and parameter settings.

• optimisation (5√t)• weld growth curves• welding lobes

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Weld growth curveWeld growth curves provide information on the welding range, i.e. the process

settings that will ensure a sufficently large weld nugget size.To make a weld growth curve experimentally is complicated as a lot of welds

need top be made (ISO resulated). Interpretation can be cumbersome as experimental scatter is usually high in resistance spot welding of steels.

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d [mm]

DP 800

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Welding lobeWelding lobes provide a schematic on the robustness of the process.As they basically consist of a range of weld growth curves grouped together

they are correspondingly expensive to produce experimentally:• man-machine hours• process settings

– experimental scatter– electrode wear

• material– trial material is expensive &

scarce

5√tI = 11,1t = 360 ms

5√tI = 11,1t = 360 ms

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Representation of results

resistance

powervoltage

current

process parameters

report

Sorpas can be used to provide “added bonusses”

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Process parameters

Various electrode geometries:• influence nugget growth mode• influence weld nugget size• influence weld nugget volume• comparison experimental resulst• application of literature

ISO 5821-1979 (E)Resistance spot welding electrode caps

TRIP 700 GIt = 1,65 mmtweld= 16 cyclesI = 10,5 kAFelec = 4,5 N

type G d=6, R=6type B d=6

little difference in electrode geometry?

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Electrode geometry

TRIP 700 GIt = 1,65 mmtweld= 16 cyclesI = 10,5 kAFelec = 4,5 N

type G d=6, R=6type B d=6

TRIP 700 GIt = 1,65 mmtweld= 16 cyclesI = 10,5 kAFelec = 4,5 N

type G d=8, R=8type B d=8

type G more stable than type B?

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Electrode geometry

TRIP 700 GIt = 1,65 mmtweld= 16 cyclesI = 10,5 kAFelec = 4,5 N

type G d=8, R=8type B d=8

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Electrode geometry

Same electrode type, variable dimensions.Weld growth curve displacement with changing contact surface area

(important in case of wear of electrode)

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Type B R=8Type B R=8Type B R=6Type B R=6

TRIP 700 GIt = 1,65 mmtweld= 16 cyclesFelec = 4,5 N

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Example TRIP800

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ExperimentalSplashSimulationSimulated splash

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Type B, D=6

Type G, D=6

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Type B, D=5

Type B, D=6

Type B, D=8

thickness sheet 1,65 mm5√t = 6,4

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Chemistry & weldabilityTraditionally textbook weldability is described as the inverse of hardenability.

I.e. the post weld hardness determines the quality of the weld.

The weldability of a material is represented by a Carbon Equivalence (CE) number, describing the ability of a material to form martensite after welding.

The CE relates chemical composition of a material to post weld hardness.The CE can be used to predict post weld hardness.Post weld hardness is dependent upon chemical composition and cooling rate

In resistance spot welding the weld becomes fully martensitic due to the very high cooling rates.

If post weld hardness exceeds 450 HV, crack formation is likely, leading to undesirable failure modes (i.e. plug failure).

Excess hardness can be removed by tempering of the martensite (PWHT)

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Chemistry & weldabilityCarbon Equivalence numbers:

• IIWCE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 + Si/6

• Japanese alternativeCE = C + Si/24 + Mn/6 +Ni/40 +Cr/5 + Mo/4 + V/14

• Ito-BesseyCE = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

• WirtzCE = C + Mn/6 + Ni/15 + Mo/4 + Cr/5 + Cu/13 + Si/4 + P/2 + V/3

• NissanCE = C + Si/30 + Mn/20 + 2P + 4S

• Pearlite freeCE = C + Si/25 + (Mn+Cr)/16 + Cr/20 + Ni/20 +Mo/20 + V/15

• SMICE = C + Si/90 + (Mn+Cr)/100

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Chemistry & weldabilityCarbon Equivalence numbers are dependent on the chemical

composition of materials.

Example:IIW C > 0,18 wt%Ito-Besseyo C < 0,18 wt%

Carbon Equivalence numbers cannot be used to compare the effect of all heat treatment and welding processes. This is because the cooling rate plays an important part in the resultant hardness too.

Cooling rate:• welding process (flame, arc, laser, resistance, etc).• process settings (welding times, energy input)• material thickness (heat sink)• additional cooling effects (e.g. gasses & clamping)

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Chemistry & weldabilityCooling rate

higher coolingrates lead toformation ofharder phases

But:Increased coolingrates also leadto increased hardnessof phases

martensite,bainite & ferrite/perlitemostly martensite ferrite/perlite

Tem

pera

ture

Time

Chemical Composition in %

Typical surface treatment cycle, Tmax=1350°C

Curves of cons tant

cooling rate

Vickers Hardness [HV30]

F: ferrite Zw: bainite M: martensite

Percentage of transferred phase

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Chemistry & weldabilityIncreased cooling rates also lead to increased hardness of phasesThis is caused by the introduction of gridfaults (dislocations) in the metallic grid.The occurrence of faults increases with increasing cooling rates.

Martensite:127 + 949 C + 27 Si + 11 Mn 8 Ni + 16 Cr + 21logtr

Bainite:-232 + 185 C + 330 Si +153 Mn + 65 Ni + 144 Cr + 191 Mo + logtr(89 + 53 C + 55 Si – 22 Mn – 10 Ni – 20 Cr – 33 Mo)

Ferrite:42 + 223 C + 53 Si + 30 Mn + 12,6 Ni + 7 Cr + 1 Mo+ logtr(10 – 19 Si + 4 Ni + 8 Cr + 130 V)

composition in %tr = cooling rate at 700°C (in °C/hr)

• 0,1% < C < 0,5%• Si < 1%• Mn < 2%• Ni ≤ 4%• Cr ≤ 3%• Mo ≤ 1%• V <0,2%• Cu < 0,5%• 0,01% < Al < 0,05%

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Chemistry & weldability

cooling rate:1275 K in 265 msdT/dt = 4800 K/s

TRIP 800 GI30 cycles (50 Hz) forced cooling

log tr = 7,2321 log tr = 152

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Post Weld Hardness

HV = 127 + 949 C + 27 Si + 11 Mn + 8 Ni + 16 Cr + 21logtr

949 * CE RSW:tr = 3750 °C/s = 13,5 106 °C/hr21logtr = 150

HV = 277 + 949 CECE = C + (28,5Si+11,6Mn+8,4Ni+16,9Cr)/1000

chemistry and cooling rate dependent:HV = P + Q*CE + R(t) → HV = (P + R(t)) + Q*CE

These figures are from anearlier simulation, spotwelding mild steel.Different steel, similarthickness.

mild steel:cooling rate = 3750 K/s21 log tr = 150

TRIP:cooling rate = 4800 K/s21log tr = 152

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Post Weld HardnessHV = 127 + 949 C + 27 Si + 11 Mn + 8 Ni + 16 Cr + 21logtr

949 * CE RSW:tr = 3750 °C/s = 13,5 106 °C/hr21logtr = 150

HV = 277 + 949 CECE = C + (28,5Si+11,6Mn+8,4Ni+16,9Cr)/1000

21 log tr

double cooling rate: 21log2tr = 21(logtr+log2) = 21(logtr+0,3) = 21logtr+6,3halve cooling rate21logtr/2 = 21(logtr-log2) = 21(logtr-0,3) = 21logtr-6,3

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Blondeau hardness [HV]

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Post Weld Hardness

Example (SMI):Yurioka (1987) HV = 257 + 756 * CECE = C + Si/90 + (Mn+Cr)/100

(cooling) time & chemistry dependent models

(HV = f(Hi(Xj),t8/5))

e.g. Blondeau

chemistry dependent models

(HV = Σ Hi(Xj)·Vi)

chemistry dependent models incorporate the cooling rate in a constant.

Therefore they are always process and material thickness specific.

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Carbon Equivalence & Post Weld Hardness

• Investigate relation chemistry and (post weld) hardness

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Peak hardness RSW (5RT)SMI hardnessBlondeau hardnessSMI(IIW)SMI(Ito-Bessey)SMI(Wirtz)SMI(Nissan)SMI(Blondeau)Blondeau(IIW)Blondeau(Ito-Bessey)Blondeau(Wirtz)Blondeau(Nissan)Blondeau(SMI)SMI(Japan alt)Blondeau(Japan alt)Blondeau(pearlite free)SMI(pearlite free)

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Carbon Equivalence & Post Weld Hardness

• Investigate relation chemistry and (post weld) hardness

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Peak hardness RSW (5RT)Blondeau hardnessSMI(Ito-Bessey)Blondeau(SMI)SMI(pearlite free)

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Carbon Equivalence & Post Weld Hardness

Investigate relation chemistry and (post weld) hardness

• HV = 227 + 949C + 27Si + 11Mn + 8Ni + 16Cr• HV = 257 + 756C + 25Si + 38Mn + 38Cu + 38Cr + 13Ni + 50Mo + 76V +

3780 B• HV = 227 + 949C + 11Si + 95Mn + 95Cr• HV = 257 + 756C + 30Si + 47Mn + 47Cr + 38Cr + 38Ni + 38Mo + 50V

HV = 240 + 855 (C + Si/40 + Mn/20)

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Carbon Equivalence & Post Weld Hardness

HV = 240 + 855 (C + Si/40 + Mn/20)

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experimentalCE

HSLA

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experimentalCE

HSLA

The equation is valid for: resistance spot welding of advanced high strength steel sheetfor automotive applications

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Post Weld Heat Treatment• If the post weld hardness of the weld nugget exceeds 450 HV (rule

of thumb) onset of cracks becomes a possibility. This can lead to undesirable weld failure modes.

• Therefore the hardness of the weld nugget should be decreased bytempering to ensure full plug failure.

• To temper, the weld should be heated to a temperature not exceeding Ac1. If the temperature does exceed Ac1, martensite will transform into austenite. Upon cooling this austenite will transform into hard untempered martensite again.

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Post Weld Heat Treatment

Ac1 [°C] = 723-10,7Mn-16,9Ni+29,1Si+16,9Cr+6,38W+290As

Ac3 [°C] = 910-203√C-30Mn-15,2Ni+44,7Si+104V+13,1W+31,5Mo+20Cu-700P-400Al-120As-400Ti

Ms [°C] = 539-423C-30,4Mn-17,7Ni-12,1Cr-11Si-7Mo

Ac1 [°C] = 723-10,7Mn-16,9Ni+29,1Si+16,9Cr+6,38W+290As

Ac3 [°C] = 910-203√C-30Mn-15,2Ni+44,7Si+104V+13,1W+31,5Mo+20Cu-700P-400Al-120As-400Ti

Ms [°C] = 539-423C-30,4Mn-17,7Ni-12,1Cr-11Si-7Mo

Transformation temperatures

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Martensiteformed [%]

Temperature below Ms

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Growth of martensite

To transform 90% of the austenite to martensite takes 103 K of undercoolingTo transform the last 10% takes an additional 112K; not taking into account carbon enrichment.

undercooling RSW:ca 50 K~ 0,01 s

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PWHTWeld & PWHT

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pulse 1: 10,5 kA

pulse 2: 8 kA

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Other welding challenges

1. Welding of high carbon and phospherous TRIP steels (see yesterdays presentation)

2. Geometry related mechanical characteristics (e.g. residual stresses)

1. Hot Cracking SusceptibilityHCS = (C(S+P+Si/25+Ni/100)*1000)/(3Mn+Cr+Mo+V)If HCS > 4, hot cracking can occur (value may be different

for RSW)2. Temper embrittlement: J = Mn + Si + 104 (P + Sn)

grade HCS DP800 A 0,894 DP800 B 0,553 TRIP 780 A 3,270 TRIP 700 A 3,991 TRIP 700 B 2,468 TRIP 800 A 5,599 TRIP 800 B 2,266

grade JDP800 A 3,87DP800 B 3,55TRIP 780 A 4,16TRIP 700 A 11,54TRIP 700 B 5,74TRIP 800 A 11,90TRIP 800 B 5,13

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Conclusions

1. Verification of simulations is most reliable using weld growth curves2. SORPAS is very suited to support weldability research on process

parameters3. SORPAS can be used as a tool to support weldability research on

metallurgy4. Chemistry and cooling rates determine post weld hardness5. Cooling rate is determined by process, thickness of materials and

chemical composition (in that order).6. Simple equations can be derived to determine post weld hardness of

classes of resistance welded steel sheet7. SORPAS can be used to design PWHT schemes

But: Always take into account the full metallurgical background.