Question

 

a) Assume that you have two AISI 1045 cylindrical samples with two different diameters: one 1 in. thick and the other 2 in. thick. You astenitize and quench both samples in water. Then you cut a section and measure the hardness at the center of each sample. Would you expect to get the same results? Explain your answer.

b) Assume that you have two cylindrical steel samples with 1 in. diameter, one made out of AISI 1045 and another one made out of AISI 4140. You austenitize and quench both samples in oil. Then you cut a section and measure the hardness at the center of each sample. Would you expect to get the same results? Explain your answer.

Answer 1

(a)

AISI 1045 steel is a medium tensile steel supplied in the black hot rolled or normalized condition. It has a tensile strength of 570 - 700 MPa and Brinell hardness ranging between 170 and 210.

The term "austenitization" means to heat the steel to a temperature at which it changes crystal structure from ferrite to austenite. The ferrite exists from room temperature to about 1,330 degrees F, at which point, under equilibrium conditions, it begins to change to austenite. A medium carbon steel will be completely transformed to austenite at about 1,550 degrees F.

The steel expands as it is heated (thermal expansion). As it transforms to austenite, the crystals expand so that the net volume of the product increases. If it is a solid part, most of the expansion goes to the outside. However, tubulars can expand to the outside diameter (OD) and the inside diameter (ID). The pipe having less thickness that is 1 in. will be heated uniformly throughout the lenght. But in case of higher thickness pipe, that is 2 in pipe, the heating translate into nonuniform heating, which creates distortion and nonuniform properties.

After the austenization process, as the pipes wete taken into the quenching process, the same result will not be obtained. It is because the pipe having smaller thickness will be cooled uniformly, and tbe formation of grain structure will be homogenous. On the other hand, quenching tne higher thickness pipe , will not give a high strenght pipes. It is because the formation of grain structur during the quenching process is not homogenous. The variation in temperature during tbe quenching process in case of high thickness pipe is not homogenous. It is because in quenching of higher thickness pipe, the time reqrequi for distribution pf temperatire is very high . Thus the grain formation at various centric distances varied.

Therefore, pipes having less thickness, that is 1 in thick pipe will have higher strengrh and hardness property than that of the 2 in thick pipe in their cutted section.

(b),

As the outcome rest fron quenching depends on the material proprty and grain structures. The oil quenching process of AISI 4140 leads to formation of ferrite and martensite .

On the other hand the qunching of AISI1045 in oil leads to formation of martensite and pearlite. As we know that the combination of microstructure having martensite and pearlite, have hardness higher than that of ferrite an martensite.

Therefore, the AISI 1045 will have higher strength and hardness than that of the AISI 4140, upon quenching them by oil.