With this article, the authors present a number of technological characteristics determined for a dual-phase steel with 0.09% C and 1.90% Mn. This steel was obtained through intercritical quenching: heating at 740, 780 and 820 °C, maintaining for 30 minutes and then cooling in water, oil, oil in magnetic field of direct current (DC) and oil in ultrasonic field. It was determined the degree of cold upsetting, the ultimate tensile strength of the resistance butt welded joints and the cutting property based on the study of cutting forces and surface roughness; it was analyzed the influence of heat treatment parameters (heating temperature, quenching medium) and ferrite-martensite structure on these characteristics.
In recent years, to reduce cars costs, research has been conducted on dual-phase steels with low manganese content (below 1.0%). This study investigated the influence of technological parameters of heat treatment (heating temperature and cooling medium) on such steels’ structures and mechanical properties. The ferrite-martensitic structures, specific for dual-phase steels, were obtained by intercritical quenching: heating of samples (made of alloys with 0.511% Mn, respectively 0.529% Mn) to temperatures located between critical points Ac1 and Ac3, followed by cooling in water without mechanical agitation and in water activated with ultrasounds at the frequency of 59 kHz. Through metallographic analyses and tensile tests, it was possible to determine the volume fraction of martensite, the ferrite microhardness, the ultimate tensile strength, the total elongation, and with the obtained data, their variations with the heating temperature and the cooling medium were established. Raising the heating temperature (between 760 °C and 820 °C) and using ultrasounds at cooling increased the volume fraction of martensite and the ferrite microhardness. This fact has increased the mechanical strength and reduced the deformability of the studied dual-phase steels. Intercritical quenching in water activated with ultrasounds provided values of structural characteristics and mechanical properties very close to those obtained by quenching in water without mechanical agitation, but was accomplished using a higher-temperature heating. The results obtained were compared with those determined in previous research, performed on dual-phase steel with 1.90% Mn.
The hardness of the coated surfaces by nickel-iron electroplating process is closely linked to the characteristics of deposited layer. These characteristics depend on various process variables, such as current density, temperature, pH and stirring. This work presents the modelling and the optimization of nickel-iron electroplating process variables to maximize the surface hardness. To study the combined effect of current densityJ(A/dm2), temperature T (°C) and pH were used a 23orthogonal central composite experimental design for experiments design and Response Surface Methodology for analysis of experimental results. The modelling was performed using the following intervals for process variables: 1.75 - 3.51 A/dm2for current density, 25-35 °C for temperature and 2.5-3.5 for pH. The empirical model was further used in the optimization process using the Gradient method. The optimum values of these electroplating process variables were found to beJ= 2.23 A/dm2,T= 30.80°C andpH= 2.81; in this point the surface hardness is 136.89 HV given by empirical model and 137.52 confirmed experimentally.
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