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Research Papers

Influence of Temperature and Deformation on Phase Transformation and Vickers Hardness in Tailored Tempering Process: Numerical and Experimental Verifications

[+] Author and Article Information
B. T. Tang

Institute of Engineering Mechanics,
Shandong Jianzhu University,
Fengming Rd,
Jinan 250101, China
e-mail: tbtsh@hotmail.com

Q. L. Wang

Institute of Engineering Mechanics,
Shandong Jianzhu University,
Fengming Rd,
Jinan 250101, China
e-mail: 947683166@qq.com

S. Bruschi

DII University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: stefania.bruschi@unipd.it

A. Ghiotti

DII University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: andrea.ghiotti@unipd.it

P. F. Bariani

DII University of Padova,
Via Venezia 1,
Padova 35131, Italy
e-mail: paolo.bariani@unipd.it

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received February 13, 2014; final manuscript received May 31, 2014; published online August 12, 2014. Assoc. Editor: Gracious Ngaile.

J. Manuf. Sci. Eng 136(5), 051018 (Aug 12, 2014) (14 pages) Paper No: MANU-14-1062; doi: 10.1115/1.4027816 History: Received February 13, 2014; Revised May 31, 2014

Hot stamping of quenchenable ultra high strength steels currently represents a promising forming technology for the manufacturing of safety and crash relevant parts. For some applications, such as B-pillars which may undergo impact loading, it may be desirable to create regions of the part with softer and more ductile microstructure. In the article, a laboratory-scale hot stamped U-channel was produced with segmented die, which was heated by cartridge heaters and cooled by chilled water recirculation independently. It can be concluded that in order to satisfy tailored mechanical properties by introducing regions, which have an increased elongation for improved energy absorption, the minimum die temperature should be no less than 450 °C. Optical micrographs were used to verify the microstructure of the as-quenched phases with respect to the heated die temperatures. For the cooled die region, the microstructure was predominantly martensite for all the die temperatures interested. With the increase of heated die temperature, there was a decrease of Vickers hardness in the heated region due to the increasing volume fractions of bainite. The finite element (FE) model was developed to capture the overall hardness trends that were observed in the experiments. The trends between the simulations and experiments were very similar, with acceptable differences in the magnitude of Vickers hardness. The transition widths were measured and simulated and there was a quite good agreement between experiment and simulation with almost the same value of 10 mm by taking heat conduction into account.

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Figures

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Fig. 1

Hot stamped parts in a typical middle class car [3]

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Fig. 2

Schematic of the segmented tooling: (a) perspective view of tooling, (b) structure of the cooled tool section, and (c) structure of the heated tool section

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Fig. 3

Experimental setup and final workpiece: (a) experimental setup, (b) final workpieces, (c) transitions and locations for microstructure observation, and (d) locations for microhardness test

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Fig. 4

Temperature–time history of blank for tool temperatures of 300 °C and 450 °C

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Fig. 5

Experimental results across the transition zone (L1, L2, and L3) for different die temperatures: (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C, and (e) locations of measurements

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Fig. 6

Optical microscopy images of different locations: (a) A, (b) B, (c) C, (d) D, (e) E, and (f) F

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Fig. 7

Optical microscopy images of transition zone L1: from bainite to martensite

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Fig. 8

Optical microscopy images of transition zone L2: from ferrite + pearlite + bainite to martensite

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Fig. 9

Microhardness of the heated region with various temperatures, showing a gradual decrease of hardness with the rise of temperature

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Fig. 10

FE model of the experimental case study

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Fig. 11

Distribution contour of different phases with die heated to 450 °C

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Fig. 12

Predicted microhardness with various die temperature

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Fig. 13

CCT for Usiblor® 1500P provided by Arcelor (2003) [35]

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Fig. 14

Temperature–time history for different locations overlaid CCT diagram supplied by Barcellona and Palmeri [25]: (a) P1, P3 and (b) P4, P6

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Fig. 15

Temperature–time history for different locations overlaid CCT diagram supplied by Riera et al. [36]: (a) P2 and (b) P5

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Fig. 16

Numerical and experimental results across the transition zone (L1, L2, and L3) for various die temperatures. The figures also show locations and width of transition zones of fully hardened and fully soft microstructures.

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Fig. 17

Numerical and experimental results of the heated and cooled elements (elements P1 and P4 on L1, elements P2 and P5 on L2, elements P3, and P6 on L3) for various die temperatures. The figures also show the effect of temperature on Vickers hardness.

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