Integration of advanced high strength steels (AHSS) into the automotive architecture has brought renewed challenges for achieving acceptable welds. Resistance spot welding (RSW) is the primary method used in welding automotive structures, which has resulted in a demand to better understand RSW of AHSS. The varying alloy contents and processing techniques used in their production has further complicated this initiative. The current study examines resistance spot welding of AHSS including 590R, DP600, DP780 and TRIP780. HSLA material is also included to represent conventional high strength steels and benchmark AHSS performance. The mechanical properties and microstructure of these resistance welded steel alloys are detailed. Furthermore, a relationship between chemistries and fusion zone hardness is produced.

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Microstructure and Mechanical Properties of Resistance Spot Welded Advanced High Strength Steels

Using the compression tests on a Gleeble-3800 thermo-mechanical simulator, the hot deformation behavior of ultrahigh strength Aermet100 steel was studied in the temperature ranges of 800–1200 °C and the strain rate ranges of 0.01–50 s−1. Dependence of the peak stress on temperature and strain rate for Aermet100 steel is described by means of the conventional hyperbolic sine equation; By regression analysis, the activation energy in the whole range of deformation temperature was determined to be Q = 489.10 kJ/mol. The dynamic recrystallization (DRX) kinetics of Aermet100 steel is established by further analysis of true stress–true strain curves. The complete DRX grain size is dependent only on Zener–Hollomon parameter (Z) and is independent of the initial grain size and accumulated strain, because Z controls the stored energy. The complete DRX grain size is a power law function of Z with an exponent of −0.24.

Research on the dynamic recrystallization kinetics of Aermet100 steel

First-principles prediction of partitioning of alloying elements between cementite and ferrite

Designing nanoprecipitation strengthened UHS stainless steels combining genetic algorithms and thermodynamics

The secondary hardening and fracture behavior in P/M high speed steels of W–Mo–Cr–V(–Co) system has been investigated in terms of Co addition and austenitizing temperature. Austenitizing was conducted at 1100 and 1175 °C of relatively low and high temperatures, respectively. Tempering was performed in the range of 500–600 °C. Coarse primary carbides retained after heat treatment were V-rich MC and W–Mo-rich M6C types. Since the dissolution of W–Mo-rich M6C with relatively low stability was more promoted with increasing austenitizing temperature, the alloy content of the W and Mo in the matrix was enhanced. In turn, it gives a significant influence on the precipitation of fine secondary alloy carbides, that is, secondary hardening during tempering. The major secondary carbides were W–Mo–Cr-rich M2C type. The peak hardness was observed in the tempering range of 500–540 °C, depending on Co addition and austenitizing temperature. With an increase in austenitizing temperature, the aging deceleration was observed. This phenomenon may be attributed to the increased content of W and Mo in the matrix, both diffusing slowly in the matrix and inhibiting growth of M2C carbide, as compared to Cr. In addition, the aging acceleration occurred in the Co bearing alloy, promoting the precipitation of M2C carbides, as well as the overall increase in hardness. In general, the impact toughness was decreased with an increase in hardness. In addition, the impact toughness to hardness balance was lowered in the Co bearing alloy.

Influences of Co addition and austenitizing temperature on secondary hardening and impact fracture behavior in P/M high speed steels of W–Mo–Cr–V(–Co) system

The effect of alloying additions on secondary hardening behavior in Fe-Mo-C steels has been investigated by means of the successive alloying additions of Cr, Co, and Ni The Cr additions promote M3C cementite formation The Ni additions destabilize the cementite formation, while the Co additions retard dislocation recovery and present the necessary sites for M2C formation which provides the secondary hardening

Effect of alloying additions on secondary hardening behavior of Mo-containing steels

The purpose of this study was to analyze the effect of Co and Ni additions on secondary hardening and fracture behavior of the martensitic steels bearing W and Cr, where W exhibits a much weaker effect on the secondary hardening compared with Mo and Cr can contribute to an increase in toughness even though the Cr additions weaken the secondary hardening in Mo and W steels. The chemical compositions of the alloys used in this study are presented in Table 1. Impact specimens were austenitized in a flowing argon atmosphere at 1,200 C for 1 hour and then water- or oil-quenched. Austenitized specimens were aged in a neutral salt bath at 400 C and 650 C for 1 hour and then water-quenched. Fractography was conducted on fracture surfaces cut off the impact-tested samples, while samples for hardness measurements were obtained from a second cut 5 to 10 mm below the fracture surface. Hardness was measured using the Rockwell C scale (HRc), and average values of five readings are reported. In order to observe martensitic substructures (M{sub 2}C and M{sub 3}C carbides), thin foils were examined in a JEOL transmission electron microscope (TEM), operated at 120 kV. The variations in hardnessmore » for 6W, 3W-3Cr, 3W-3Cr-14Co, and 3W-3Cr-14Co-10Ni steels are shown.« less

Effects of Co and Ni on secondary hardening and fracture behavior of martensitic steels bearing W and Cr

The effect of Cr, Co, and Ni additions on the secondary hardening behavior in Fe-C-Mo steels was systematically analyzed in terms of their effect on M{sub 2}C carbide formation and reported elsewhere in this journal. In this study, the fracture behavior has been analyzed for the same alloy systems. Impact toughness was determined as a function of aging temperature.

C. M. Kim

Papers

Effect of alloying additions on secondary hardening behavior of Mo-containing steels

Effects of Co and Ni on secondary hardening and fracture behavior of martensitic steels bearing W and Cr

Effect of alloying additions on fracture behavior of Mo-containing secondary hardening steels