Tempering of Martensite in Dual-Phase Steels and Its Effects on Softening Behavior
- PDF / 1,627,388 Bytes
- 15 Pages / 593.972 x 792 pts Page_size
- 41 Downloads / 210 Views
UCTION
DUAL-PHASE (DP) steel comprises a soft ferrite (a) matrix dispersed with hard martensite (a¢) phase to offer favorable combinations of high strength and good deformability, which has recently attracted an increasing application in automobile industries.[1] However, softening, reduction in hardness with respect to the base metal, of DP steel that occurs in a number of manufacturing processes, such as welding and joining[2,3] and laser heat treatment,[4] becomes an important issue that hinders the progress of implementing DP steel in many practical applications. For example, it is reported that softening occurring in the heataffected zone (HAZ) caused by welding adversely affects the formability of tailor-welded blanks of DP steel due to high strain concentration in the softened region, leading to premature failure of the blanks.[5,6] HAZ softening was clearly linked to the tempering of a¢ phase in DP steel.[7–10] V.H. BALTAZAR HERNANDEZ, formerly Graduate Student with the Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada, is now Professor with the MpyM-EPMM Academic Unit of Engineering, Autonomous University of Zacatecas, C.P. 98000 Zacatecas, Mexico. S.S. NAYAK, Postdoctoral Fellow, is with the Centre for Advanced Materials Joining, University of Waterloo, is also with the Mechanical and Mechatronics Engineering, University of Waterloo. Contact e-mail: [email protected] Y. ZHOU, Director, is with the Centre for Advanced Materials Joining, University of Waterloo, and is also a Professor with the Mechanical and Mechatronics Engineering, University of Waterloo. Manuscript submitted December 23, 2010. Article published online May 28, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
In theory, tempering of martensite in DP steel occurs, just as tempering of fully martensitic steels,[11–13] when it is heated to temperatures close to or below lower critical transformation temperatures (Ac1 line) irrespective of the manufacturing processes. The extent or degree of tempering (softening) fundamentally depends on diffusion of carbon and is controlled by two parameters, i.e., temperature and time; accordingly, the tempering (softening) process can be divided into two categories: (1) isothermal tempering and (2) nonisothermal tempering. Isothermal tempering includes the conventional heat treatment process (slow heating, long holding time at peak temperature, and slow cooling) implemented in order to improve the ductility of steel (mostly martensitic steel),[11] whereas nonisothermal tempering occurs generally during manufacturing processes such as welding and joining involving rapid heating, negligible holding time at tempering temperature, and rapid cooling. Tempering of martensite is well documented in the literature but has been mostly addressed to fully martensitic steels subjected to isothermal tempering treatment.[11–13] It is generally agreed that the tempering process is developed in a series of overlapped stages: the first stage (up to 523 K [250 C]) involves clustering
Data Loading...
