Effect of Prior Cold Deformation and Nitriding Conditions on Microstructure and Mechanical Properties of Plasma Nitrided
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INTRODUCTION
INTERSTITIAL-FREE (IF) steel is an established class of the high-formable variety of steel, which is widely used for manufacturing automotive bodies. Though IF steel meets the stringent formability requirements of the automotive industry, it possesses inherently low yield (< 200 MPa) and tensile strength ( 15 randomly chosen locations to determine the nitrogen content of selected spots. To confirm the microstructural evolution in terms of phase identity and volume fraction, nitrided samples were subjected to X-ray diffraction (XRD) analysis using Co-Ka radiation (k = 0.179 nm). Residual macro-stress built up on the surfaces of nitrided samples was carefully assessed by X-ray diffraction analysis using a stress Goniometer (PW 3040/ 60, Panalytical X’pert Pro, Netherland). Vickers hardness measurements were performed on specimens using Microhardness tester (UHL VMHT, Germany) to evaluate the variation of surface hardness as a function of prior cold deformation and plasma-nitriding temperature and time. Wear behavior of the samples was evaluated by a fretting wear tester (Ducom, India) using a 10-mm steel ball indenter as the counter-body against the flat specimens of investigated material at ambient temperature (22 to 25 C), humidity of 50 to 55 pct, applied load of 20 N, oscillating frequency of 10 Hz and constant stroke length of 1 mm. With the help of Winducom 2006 software, the cumulating loss of depth with time was calculated and reported. The electrochemical study was carried out at room temperature by potentiodynamic polarization test using a PS6 Meinsberger Potentiostat/Galvanostat connected to a standard three-electrode cell comprising the sample as the working electrode, a saturated calomel reference electrode and a platinum counter electrode. The electrolyte used in the present work was 3.56 wt pct NaCl solution, and the scanning rate was kept at 1 mV/s
III.
RESULTS AND DISCUSSION
A. Microstructural Evaluation In the present work, a detailed study of the influence of prior cold deformation and plasma-nitriding parameters (temperature and time) on the size, morphology METALLURGICAL AND MATERIALS TRANSACTIONS A
Table I.
Chemical Composition of the Investigated IF Steels (Weight Percent)
C
Mn
S
P
Si
Al
Cu
Ni
Nb
Ti
N
Fe
0.003
0.35
0.008
0.042
0.011
0.043
0.028
0.013
0.024
0.023
0.0026
balance
Table II. Voltage (V) 540–710
Plasma-Nitriding Conditions
Current (A)
Duty Cycle (Pct)
Pulse Frequency (kHz)
Pressure (mbar)
Gas Composition (Pct)
0.03–0.06
66
10
~5
80 pct H2 + 20 pct N2
Fig. 1—Scanning electron micrographs of the IF steel sheet used in the present study in (a) as-received and as-deformed state with (b) 10 pct, (c) 50 pct and (d) 80 pct prior cold deformation.
and distribution of the nitrides has been carried out. Nitriding is predominantly a diffusion-controlled process; hence, the nitriding time and temperature play a crucial role in determining the kinetics of nitriding and properties of the nitrided surface. Moreover, prior cold work may induce defec
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