Effects of Holding Time on Haz-Softening in Resistance Spot Welded DP980 Steels
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EFFECTS OF HOLDING TIME ON HAZ-SOFTENING IN RESISTANCE SPOT WELDED DP980 STEELS C.J. Martínez-González1,2, A. López-Ibarra1, S. Haro-Rodriguez1, V.H. Baltazar-Hernandez1, S.S. Nayak2, Y. Zhou2 1 2
Unidad Académica de Ingeniería, Universidad Autónoma de Zacatecas, México Mechanical and Mechatronics Engineering, University of Waterloo, Canada
ABSTRACT Resistance spot welding (RSW) of dual-phase (DP) steel subjected to various conditions of cooling rate (holding time) is studied in this work. Lap-shear tensile testing is used in order to evaluate the mechanical performance of the weldments. The microstructure is analyzed through optical and electron microscopy and the hardness is obtained through Vickers method. Results indicate an effective tempered region along the sub-critical heat affected zone in all the samples. A broken morphology accompanied with presence of small carbides within tempered martensite phase is consistently observed. Variations in the cooling rate (holding time) indicate minimal effect on the degree of softening and on the mechanical performance of the welds. INTRODUCTION The demand for advanced high strength steel (AHSS) sheets having a combination of strength and ductility has increased in the automotive industry. Dual-phase (DP) steel is part of the family of AHSS and is composed of a ferritic matrix with varied volume fractions of martensite phase. In particular, DP has become popular in auto-body constructions due to their excellent properties and microstructure [1]. Resistance spot welding (RSW) is the dominant process in the automobile maker facilities. RSW is well known being a fast welding process i.e. it involves rapid heating followed by fast cooling rates. An important parameter in the RSW practice is the holding time. The holding time parameter is associated to the time in which both electrodes remain in contact with the specimen just after welding current has been switched off; in fact, the holding time parameter is directly related to the cooling rate of the specimen [2, 3]. Furthermore, RSW is widely utilized on joining DP steels. An interesting issue in RSW of DP steel is a phenomenon known as heat affected zone (HAZ) softening that involves reduction of hardness at the subcritical-HAZ (SCHAZ) with respect to base metal (BM) [4, 5]. HAZ-softening actually promotes localized strain at SCHAZ along with earlier failures [6]. The reason for the HAZ-softening in RSW of DP steel welds has been attributed in fact to tempering of the martensite phase [7, 8]. Tempering of martensite, which occurs at temperature at or below the lower critical (Ac1) line, has been reported to be the reason for HAZ-softening in DP steels [9-12]. The degree or severity of tempering is highly dependent on the maximum (peak) temperature and the time at peak temperature during isothermal heat treatments [13, 14]. Even though martensite tempering has been extensively investigated on martensitic steels subjected to isothermal heat treatments, little work has been addressed on investigating the tempering of marten
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