The materials parameter i.e. microstructure as well as the processing factors i.e. aging time and temperature, the amount of plastic deformation, and residual stresses affecting the kinetic of strain ageing in the S275JRC steel were investigated in detail. For this purpose, 5 % pre-strained steel was aged at 160ºC for different intervals. The ageing behavior was determined for the as-received (AR) and homogenization & normalizing heat treated (H+N heat treated) conditions. The yield stresses before and after aging process and, the hardness values of the aged steels were considered to calculate strain aging progress. It was shown that a noticeable increase in the yield strength, tensile strength and hardness values was observed for aged AR and H+N heat treated specimens. Moreover, the repetitive increase in the yield and ultimate tensile strength was believed to be a result of strain ageing.

In this study, N80-42CrMo4 dissimilar drill pipes were welded by a roto-friction welding machine. A set of welded samples were tested as-welded condition. Another set of welded samples were post weld heat treated (stress relief annealing) at the temperature of 600 °C, for 20 min. and then tested. The microstructures of the as-welded and stress relief annealed samples were examined by optical microscope and SEM. The properties of the all welded samples were determined by hardness measurement, tension and V notch-impact tests. The high hardness especially in the weld interface resulted in low toughness. As a conclusion, the stress relief annealing treatment caused a little decrease in yield strength and ultimate tensile strength, but increase in the elongation. The ductility of samples also increased after annealing treatment which is very important for static and dynamic loads.

The mechanical properties of aluminum alloys are strongly dependent on the thermo-mechanical process so, the strain ageing behavior of 7075 Al-alloy was investigated in this study. A set of test pieces was solution heat treated at 480 °C for 2 h, water quenched (SHTWQ) then pre-strained for 8% in tension. The test samples were aged at 140 °C for 0.5, 1, 2, 3, 4, 6, 8, 10, 12, 24, 48, 72 and 96 h in a furnace. The other set of test samples were solution heat treated at 480 °C for 2 h, quenched in sand (SHTSQ) then pre-strained for 8% in tension. They were also aged at 140 °C for same intervals. The effect of strain ageing, on the mechanical properties of Al-alloy, was investigated by mean of hardness, and tensile tests. The results shown that, the quenching rate after solution heat treatment, ageing time and temperature, as well as pre-strain, play a very important role in the precipitation-hardening associated with ageing process of the 7075 Al-alloy.