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    Effects of GMAW conditions on the tensile

    properties of hot rolled Complex Phase 780 steel

    Carlos Cardenas

    Luis HernandezJaime Taha-Tijerina

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    OUTLINE

    1. Introduction

    2. Material Description

    3. Experimental Procedure

    4. Results and Conclusions

    • Phase 1• Phase 2

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    Introduction

    • There have been several efforts to understand different aspects of AHSS(2-

    7). A trend to focus on DP steels is observed, with little work dedicated to

    other steels, particularly CP.

    180

    55 50 45

    14

    0

    50

    100

    150

    200

    DP 22MnB5 TRIP Mart CP

    • CP steels have a comparable higher YS and Hole Expansion Coefficient(HEC), while still offering good formability, better suited for some chassis

    applications where certain ductility is needed to form a structural part,

    which once fabricated, will require it to withstand high service loads.

    Number of presentations

    per steel type along the

    different GDIS editions.

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    An important aspect that deserves to be studied, is the effect of arc weldingon the mechanical properties of AHSS. In that regards, several studies have

    been presented during previous GDIS editions, but none of them has been

    focused on CP steels (vs 28 on DP steels).

    • Notable conclusions that have been obtained from those studies are:

    − Under matching filler material does not affect static performance, as

    failure was generally observed on HAZ. (4-7)

    − Weld tensile strength generally increases as base metal strength

    increases, though with a weld efficiency reduction. (4,7,8)

    • This research focuses on the effects of gas metal arc welding (GMAW) on

    the mechanical properties of hot rolled CP with 780 MPa tensile strength.

    Introduction

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    OUTLINE

    1. Introduction

    2. Material Description

    3. Experimental Procedure

    4. Results and Conclusions

    • Phase 1• Phase 2

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    Material Description

    HR660Y780T-CP supplied by Tata Steel

    • Chemical Composition (Max values wt. ):

    • Thickness: 3.2mm

    •Microstructure:It is made of fine-grained bainitic matrix with pearlite and martensite islands.

    This steel is additionally strengthened by the presence of Ti-carbides. The

    volume fraction of second phase constituents ranges between 5 and 15 vol.%.

    C  Mn  P  S  Si  Nb  Al 

    0.16  2.10  0.040  0.015  0.5  0.01  0.5 

    20 μm

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    Mechanical Properties:

    HR CP780* CR DP780 *

     Yield Strength (MPa) 726 470

    Tensile strength (MPa) 833 806

    Elongation (%) 18 18

    Material Description

    * Properties and curves provided by TATA steel.

    0

    100

    200

    300

    400

    500

    600

    700

    800

    900

    0 2 4 6 8 10 12 14 16 18 20

    E

    S

    e

    M

    P

    Eng. Strain ( )

    CR-DP-780

    HR-CP-780

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    OUTLINE

    1. Introduction

    2. Material Description

    3. Experimental Procedure

    4. Results and Conclusions

    • Phase 1• Phase 2

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    Phase 1*:

    Objective: Evaluate effect of different transfer modes.Using under-matched filler wire (ER70S-6) and 4 different GMAW transfer modes.

    Experimental Procedure

    Phase 2*:

    Objective: Evaluate effect when including stronger filler materials.

    Pulsed GMAW using two additional filler materials:

    1. ER90S-D2

    2. ER100S-G

    * Robotic Welding and Manual Repair for each condition.

    Short CircuitPulsed CMT CMT Twin

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    Cutting Sheets Welding

    sheets

    Cross

    Section

    MachiningTensile TestMicro-hardness

    Cut 2”

    Samples

    Experimental Procedure

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    Welding parameters:

    • Robotic Parameters:

    • Power source: Fronius TPS5000 CMT

    • Gas = 80-20 (Ar-CO2) @ 14 Lpm (30 Lpm for CMT-Twin)

    • CTWD = 15mm

    • Welding position = 2F• All manually repaired samples were pulse-welded with:

    • Voltage= 24.5 V

    • Current=244 A

    • WFS = 8 m/min

    All samples in accordance

    to AWS D8.8M.

    Filler

    Material

    Wire ø(mm)

    Process Voltage (V)Current

    (A)

    Welding Speed

    (cm/min)

    Pulsed Spray 23.6 298 125

    Short Circuit 17.5 207 60

    CMT 18.4 242 100

    CMT Twin 28 & 16.3 350 & 270 300

    ER90S-D2 1.4 22.1 295 125

    ER100S-G 1.2 27.9 311 125

    ER70S-6 1.2Phase 1

    Phase 2 Pulsed Spray

    Experimental Procedure

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    OUTLINE

    1. Introduction

    2. Material Description

    3. Experimental Procedure

    4. Results and Conclusions

    •Phase 1

    • Phase 2

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    * At least 3 Repetitions of each sample were made.

    * Weld Efficiency = tensile strength after welding / tensile strength of the base material. Area assumed for stress

    calculation was the cross section of base metal.

    Different transfer modes with ER70S-6:

    Phase 1 - Lap Joint Tensile Testing

       S   t   r   e   s   s   (   M   P   a   )

    W el   d E f  f  i   c i   en c  y 

    92% 86%79%

    75%69% 68% 69% 66%

    0%

    20%

    40%

    60%

    80%

    100%

    0

    200

    400

    600

    800

    Pulsed CMT CMT-Twin Short

    Circuit

    Pulsed CMT CMT-Twin Short

    CircuitRobotic Pulsed Manual Repair

    Tensile Strength (MPa) Wire Tensile Strentgh (MPa) Weld Efficiency

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    Short CircuitMTulsed CMT Twin

    Robotic

    Repaired

    * Fracture Location at weld metal = Cross sectional area unknown.

    Phase 1 - Lap Joint Tensile Testing

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    Phase 1 - Microhardness

    Remarks:

    1. Softening/Hardening did not exceed 9% (HAZ Average / BM Average)

    2. In accordance with the failure locations, the welds showed lower average hardness, with

    respect to HAZ, except for the robotic-pulsed combination.

    3. Contrary to the tensile results, robotic pulsed showed the softer HAZ and weld.

       H  v

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    • When ER70 filler material was used Pulsed Transfer mode showed the

    highest weld-efficiency, and failed at the HAZ, while the other methods

    failed at the weld.

    • Contrary to the above observation, Pulsed Transfer mode showed the

    softest weld and HAZ compared to the other methods.

    • Due to uncertain cross sections at weld metal, no full relationship could

    be determined between tensile and microhardness results.

    • Failure location on most of the samples was in the weld metal.

    • To understand better the effect of the HAZ softening on the tensile

    properties, welding trials with stronger filler materials are needed for

    Phase 2. Test is to be performed using same transfer mode (Pulsed).

    Phase 1 – Preliminary Conclusions

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    OUTLINE

    1. Introduction

    2. Material Description

    3. Experimental Procedure

    4. Results and Conclusions

    •Phase 1

    • Phase 2

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    * At least 8 Replicates of each condition were tested.

    Different filler materials on Pulsed Transfer Mode:

    Phase 2 - Lap Joint Tensile Testing

    W el   d E f  f  i   c i   en c  y 

       S   t   r   e   s   s   (   M   P   a   )

    92%

    69%

    87% 88% 93% 90%

    0%

    20%

    40%

    60%

    80%

    100%

    0

    200

    400

    600

    800

    Robotic

    Pulsed

    Manual

    Pulsed

    Repair

    Robotic

    Pulsed

    Manual

    Pulsed

    Repair

    Robotic

    Pulsed

    Manual

    Pulsed

    Repair

    70 Ksi 90 Ksi 100 Ksi

    Tensile Strength (MPa) Wire Tensile Strength (MPa) Weld Efficiency

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    ER70 ER90 ER100

    Robotic

    Repaired

    Phase 2 - Lap Joint Tensile Testing

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    Phase 2 - Micro-hardness

    Remarks:

    1. As expected, for robotic welding, the stronger the filler material, the higher the average

    hardness at the weld.

    2. Only the weld metal from the 100 ksi filler material presented higher hardness than HAZ

    in both conditions.

       H  v

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    General Conclusions

    • No full correlation between tensile failure and microhardness could

    be obtained. Further analysis needed.

    • Samples welded with ER90 and ER100 failed at the HAZ, while most

    of ER70 samples failed in the weld.

    • Repair has no significant impact when welding with ER90 & ER100

    filler material.

    • Therefore, it is recommended to utilize a stronger filler material than

    the more common ER70 when welding CP780.

    • Weld efficiency of HR-CP780 is comparable to other 780 steels.

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    Bibliography

    [1] Anderson, Soren. Resources for the Future. Automobile Fuel Economy Standards. 2010. Web.

    .

    [2] World Auto Steel, (2009). Advanced high strength steel (ahss) application guidelines.

    Retrieved from website: http://www.worldautosteel.org/projects/ahss-guidelines/ 

    [3] Growth of AHSS. (n.d.). Retrieved from http://www.autosteel.org/Research/Growth of AHSS.aspx 

    [4] Feng, Z. (2007, 03). Weldability and performance of gmaw joints of advanced high strength steels (ahss).

    Presentation Delivered at Great designs in steel.

    [5] Kapustka, N., Conrady, C., Babu, S. (2008, 06). Effect of GMAW Process and Material Conditions on DP780 and

    TRIP 780 Welds. The Welding Journal.

    [6] Koganti, R., Jian, C., Karas, C. (2007) GMAW Process Optimization for uncoated Dual Phase 600 Material

    combination with aluminized coated and uncoated Boron Steels for Automotive Body Structural Applications. 

    Proceeding of ASME IMECE 2007.

    [7] Burns, Trevor. “Weldability of a Dual-phase Sheet Steel by the Gas Metal Arc Welding Process,” 2010.

    http://uwspace.uwaterloo.ca/handle/10012/4928 .

    [8] Sperle, J.-O., & Olsson, K. (1996). High strength and ultra high strength steels for weight reduction in structural and

     safety-related applications. In 29 th International Symposium on Automotive Technology and Automation. (Vol. 1, pp.

    115–125). Retrieved from http://sperle.se/referenser/pdf/artiklar/V4_ISATA.pdf  

    http://www.worldautosteel.org/projects/ahss-guidelines/http://www.autosteel.org/Research/Growth%20of%20AHSS.aspxhttp://uwspace.uwaterloo.ca/handle/10012/4928http://sperle.se/referenser/pdf/artiklar/V4_ISATA.pdfhttp://sperle.se/referenser/pdf/artiklar/V4_ISATA.pdfhttp://sperle.se/referenser/pdf/artiklar/V4_ISATA.pdfhttp://uwspace.uwaterloo.ca/handle/10012/4928http://www.autosteel.org/Research/Growth%20of%20AHSS.aspxhttp://www.autosteel.org/Research/Growth%20of%20AHSS.aspxhttp://www.worldautosteel.org/projects/ahss-guidelines/http://www.worldautosteel.org/projects/ahss-guidelines/http://www.worldautosteel.org/projects/ahss-guidelines/http://www.worldautosteel.org/projects/ahss-guidelines/

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    Presentations will be available

    May 18 at www.autosteel.org 

    Great Designs in Steel is Sponsored by:

    http://www.autosteel.org/http://www.autosteel.org/