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Research Papers

Fiber Laser Welding of Direct-Quenched Ultrahigh Strength Steels: Evaluation of Hardness, Tensile Strength, and Toughness Properties at Subzero Temperatures

[+] Author and Article Information
Farhang Farrokhi

Department of Mechanical and
Manufacturing Engineering,
Aalborg University,
Aalborg East DK-9220, Denmark
e-mail: ffk@m-tech.aau.dk

Jukka Siltanen

SSAB Europe Oy.,
Hämeenlinna FI-13300, Finland
e-mail: jukka.siltanen@ruukki.com

Antti Salminen

Professor
Department of Mechanical Engineering,
Lappeenranta University of Technology,
Lappeenranta FI-53851, Finland
e-mail: antti.salminen@lut.fi

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received November 16, 2014; final manuscript received March 19, 2015; published online September 9, 2015. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 137(6), 061012 (Sep 09, 2015) (10 pages) Paper No: MANU-14-1603; doi: 10.1115/1.4030177 History: Received November 16, 2014

The recently developed direct-quenched ultrahigh strength steels (UHSS) possess an appropriate combination of high tensile strength and toughness properties at subzero temperatures down to −80 °C, while simultaneously having low carbon contents, which is beneficial for weldability. In this study, butt joints of Optim 960 QC direct-quenched UHSS with a thickness of 8 mm were welded with a 10 kW fiber laser to evaluate the characteristics of the joints within the range of low to high heat inputs possible for this welding process. The mechanical properties of the joints were studied by subjecting the specimens to a number of destructive tests, namely, hardness and tensile testing, as well as impact toughness testing at temperatures of −40 °C and −60 °C. It was found that high quality butt joints with superior tensile strength and good impact toughness properties at −40 °C could be obtained. However, having a high level of all these properties in the joint narrows the process parameters’ window, and the heat input needs to be strictly controlled.

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Figures

Grahic Jump Location
Fig. 1

Shielding gas and lower cross jet tubes arrangement used in order to protect the weld bead and push the laser induced metal vapor plume away from the laser beam

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Fig. 2

Summary of joint configuration and the prepared edges’ properties. Size of specimens was chosen according to laser welding procedure test EN ISO 15614-11.

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Fig. 3

Top and root concavity (d1 and d2) and average width of weld (wf) measured for each specimen

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Fig. 4

Schematic preview of rows of indentation for hardness measurement according to hardness testing of laser welded joints ISO 22826. BM: base metal.

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Fig. 5

Measurement of the average base metal hardness (hb), average FZ hardness (hf), average minimum hardness in HAZ (hm), and the average width of the soft zone of both sides (w1 and w2) from a schematic average hardness profile

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Fig. 6

Energy absorption at different roughness levels [42]. The vertical dashed line denotes the average edge surface roughness for the current study.

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Fig. 7

The width of weld (wf) increased as the energy input increased

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Fig. 8

Visual comparison of top side (half-length) and cross section of the welds welded with highest and lowest welding speeds

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Fig. 9

Measured concavity in the top and root sides of the macrospecimen of welds. Apart from weld #806, all values were below the specified critical limit for quality level B according to EN ISO 13919-1.

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Fig. 10

Comparison of root sides of the stable fast welds with the unstable slow welds (half-length images)

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Fig. 11

Hardness profile of top side (upper) and root side (lower) of the welds with lowest and highest line energy

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Fig. 12

Soft zone width (ws) and the degree of softening (hb − hm) decreased as the welding speed increased

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Fig. 13

Average hardness in the FZ (hf) with a standard deviation varying between 5 and 10 HV

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Fig. 14

Laser cut softened area overlapping with the narrowest weld (1) and the widest weld (2). Width of laser cut softened area was narrower than the welds.

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Fig. 15

Tensile test results. Linear trend line of the average values predicts the maximum line energy and cooling time before the loss of strength occurs in the joints.

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Fig. 16

Absorbed impact energy at −40 °C of each joint and their corresponding standard deviation (vertical lines). The horizontal dashed line depicts the minimum toughness requirement of 22 J. CL: weld centerline and FL: fusion line.

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Fig. 17

Absorbed impact energy at −60 °C of each joint and their corresponding standard deviation (vertical lines). The horizontal dashed line depicts the minimum toughness requirement of 22 J. CL: weld centerline and FL: fusion line.

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Fig. 18

Width of CGHAZ in the vicinity of the welds was about 0.3 mm at its maximum in the case of the widest weld

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Fig. 19

Summary of the study, showing the experiments within the acceptable thresholds of cooling times (t8/5) for tensile strength and impact toughness properties. Quality threshold was estimated approximately.

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