Much more steel must be produced from scrap to meet emissions targets, and utilizing this growing resource is a sound economic strategy. However, the presence of contaminating elements restricts the applications in which end-of-life scrap can replace primary steel. The use of low alloyed quenching and tempering steel grade such as 39MnCrB6-2 to reach high mechanical characteristics (around 1000 MPa) obliges often to apply low tempering temperatures for which tempering embrittlement may be observed. In this paper, it is proposed to reduce the hold time and to increase the temperature during conventional tempering to (1) reduce the embrittlement because of segregation of elements like copper, (2) to change the fracture mechanism with finer martensite sub-grains and (3) to promote θ particles with smaller dimensions but higher density.

Copper is expected to be increasingly used in electric vehicle components because of its high electrical and thermal conductivity. On the other hand, copper has the disadvantage of low fatigue strength compared to structural members such as steel and aluminum alloys. Therefore, the peening treatment is used in this study to increase the strength of copper. However, the projectile used in conventional peening treatments is much harder than copper, which may lead to deterioration of surface properties. Therefore, we decided to use a resin particle peening treatment that uses soft resin particles. For the projectile material, we used particles made from crushed walnut, apricot, and peach, which are natural material particles. Ceramic particles were used for comparison. Hardness measurements revealed that the near-surface hardness increased even when resin particles were used. In addition, compressive residual stresses were observed on the surface. Fatigue tests revealed that the fatigue strength improvement effect was higher than that of nontreated materials or hard particles. These results indicate that the resin particle peening treatment is an effective method for strengthening copper.

Ductile cast iron can be heat-treated to obtain a significant property improvement austempering, resulting in Austempered Ductile Iron (ADI). Performance can be further improved by using boronized surface layers which are capable of reaching high hardnesses (2100 HV). In this work, samples of nodular cast iron alloyed with copper, copper-nickel and copper-nickel molybdenum were borided in a salt bath (borax + aluminum) at temperatures 850, 900 and 950 °C for 2 and 4 hours. After these treatments, the samples were directly austempered from the boriding temperature in salt baths at temperatures of 240, 300 and 360°C (boroaustempering) which avoided the need for a subsequent reheating for such processing. The boriding treatment produced uniform layers with thicknesses in the range 35-130 micrometers and hardness in the range from 1300 to 1700 HV.

Boriding thermochemical treatment produces layers with high hardness which improves the tribological performance of ductile cast iron while the austempering treatment improves the mechanical performance of the substrate. In this work, samples of the ductile cast iron alloyed with copper, copper-nickel and copper-nickel-molybdenum were borided in a salt bath (borax + aluminum) at temperatures of 850, 900 and 950°C during 2 and 4 hours. The data for the layers obtained were used to determine the diffusion coefficients and activation energies of this process. The results of the calculated diffusion coefficients were similar to those obtained by the direct measurements of the layer thicknesses. For the sample alloyed with Cu or Cu-Ni the activation energy obtained was 141.27 kJ/mol, and for the sample alloyed with Cu-Ni-Mo the value was 212.98 kJ/mol. The statistical parameters and the correlation coefficients (R) showed satisfactory agreement.

Transient liquid phase diffusion bonding was used to join stainless steel 304 with pure copper and aluminum foils as interlayers. The bonding process was conducted in a vacuum furnace at various temperatures and diffusion times. The joints were analyzed using optical and scanning electron microscopy, energy dispersive spectrometry, and microhardness measurements. Results indicated that the hardness of the bonds formed with the copper interlayer in a vacuum was higher than those formed with the aluminum interlayer. The poor mechanical properties of the bonds were attributed to the formation of intermetallic compounds within the bond region. Prolonged holding of the parent alloy at the bonding temperature likely led to complete isothermal solidification. The diffusion of the main elements from the interlayers into the base metal at bonding temperatures was the primary factor influencing the microstructural evolution of the joint interface. Selecting an appropriate bonding temperature to achieve the maximum concentration of melting point depressants depended on the duration of isothermal solidification. To assess the corrosion resistance of the joints, Tafel tests were conducted in a 3.5% NaCl solution. The presence of eutectoid γFe + eutectic Cu + Cr and Fe-Al intermetallics was detected at the interface of the joints bonded with copper and aluminum interlayers, respectively. The highest microhardness was observed in the diffusion zone, with hardness values gradually decreasing as the distance from the joint increased. The joints involving stainless steel and copper exhibited crevice corrosion due to the galvanic couple between the stainless steel and copper. Additionally, pitting occurred due to intergranular stress corrosion cracking on the copper surface.