Metallurgical optimization of engineering alloys is traditionally addressed to improve the overall performance from a mechanical point of view. Grain size is one of the most influential and critical parameters to be controlled in nickel alloys, especially in the high-temperature shaping process and final solution treatment, since it can irremediably damage the alloy performance. For this reason, grain coarsening of alloy 625 was investigated in the temperature interval from 980 to 1150 °C ranging from 0.5 to 6 h. The grain-coarsening data were fitted via regression analysis as a function of time and temperature to develop a predictive model. Grain boundary strengthening was studied by hardness and tensile tests, and the relationships between the grain size and the mechanical properties were finally determined by regression analysis. Such equations were included in a thermo-metallurgical model able to predict the mechanical properties after annealing treatment. This predictive model was validated on a forged tube subjected to solution annealing at 1150 °C for 90 min. Then, it was finally used to compare different microstructural conditions in terms of the alloy impact on the environment.

The mechanical properties of steel components are influenced by the microstructure that is determined by the heat treatment cycle. In the quenching of the steel: water, oil, aqueous polymer solutions and aqueous salt solutions (brine) can be used as quenchants that exhibit different characteristic on cooling mechanisms. For example: when water is used as the cooling media, a stable vapor film is formed around the hot component resulting in non-uniformity of surface heat transfer during the cooling process which is often responsible for distortion and cracking. Salt addition can reduce or inhibit vapor film formation enhancing the uniformity of heat transfer during the cooling. This work will investigate the cooling performance of different ionic solutions (NaCl, NaOH, NaNO2, Na2SO4, NaHCO3) varying concentration, temperature (25, 35 and 45°C) and agitation of the quenchants (0, 500 and 800 rpm) and their use on quenching a low hardenable steel. The description of these medium and the influence of external factors were made using cooling curves, cooling rates, heat transfer coefficients and heat flux. Samples of SAE 1045 steel were quenched and evaluated by hardness measures and metallography. The use of ionic solutions, in general, has proved to be effective in to inhibit the vapor blanket, to increase heat extraction and keeping a high heat flux during the most part of the drop of the temperature that results in a homogeneous cooling able to avoid distortions. These solutions were abele to quench SAE 1045 steel.