Ramesh and Prabhu Nanoscale Research Letters 2011, 6:334 http://www.nanoscalereslett.com/content/6/1/334 section thickness of the part to be heat treated and the desired microstructure. Hence there is a need for devel- opment of nanofluids having (i) high quench severity for enhancement of heat transfer for thick sections with low quench sensitivity and (ii) low cooling severity for thin sections with high quench sensitivity [127]. Effect of addition of nanoparticles on microstructure and mechanical properties of components The application of nanofluid in nuclear, rocket, transport and transformer industry is presently well known. It should be noted that there is no metallurgical and mechanical properties change in these applications. How- ever, in quenching heat treatment there is a microstruc- tural change in the component. When steel is quenched from the austentic phase, austenite may transform to fer- rite, pearlite, bainite or martensite depending on the cooling rate. Phase transformations in solid state are accompanied by volume variation and transformation plasticity. Large thermal stresses and residual stresses are Page 12 of 15 developed during quenching because of non-uniform cooling of parts and associated heat of metal parts released during the phase transformation. AISI 1070 spe- cimens quenched into the water and water-Al2O3 nano- fluid showed a martensitic structure (Figure 6). Finer martensitic structure was observed in 0.01% nanofluid with higher hardness [128]. Chakraborty observed the microstructure of the top surface of the steel after spray quenching with water and Water-TiO2 nanofluids. The cooling rate of the nanofluid was much faster than that of water resulting in ferrite-bainite structure whereas only ferrite was obtained for water quenching (Figure 7) [129]. Recent experiments with Al2O3 nanofluid by Gestwa and Przylecka observed that hardening in nano- fluid results in higher impact strength in comparison to the impact strength of the samples hardened in the media without nanoparticles for both the C10 and the 16MnCr5 carburized steel samples. They also observed lowest values of the dimension changes for samples har- dened and carburized in 10% polymer water solution (a) (b) Figure 6 Microstructure of AISI 1070 steel specimen (a) quenched in water (b) quenched in 0.01% nanofluid. (a) (b) Figure 7 SEM micrographs of (a) top surface of steel after cooling with water, (b) top surface of steel after cooling with nanofluid.