Whereas 4140 being an alloy steel the alloy content

This preview shows page 19 - 21 out of 24 pages.

Whereas 4140 being an alloy steel, the alloy content in the steel was responsible for higher hardenability of the steel (i.e. martensite can be obtained even at lower cooling rates). It was detected from figure- 3.3 & 3.11, that 4140 comprised entirely of martensite whereas, 1045 had bainite as well as martensite in its microstructure. Also 4140 did not consist any quench cracks, which was mainly due to the lower cooling rate, causing lower shear in the microstructure and offering a uniform martensite structure. The microstructure obtained for 1045 and 4140 through annealing were almost identical to each other. They both formed lamellar pearlite and proeutectoid ferrite in the microstructure. But when observed Kanishk Rathod 44888136 Page 15 of 26
under low magnification as shown in figure- 3.1 & 3.8, it was observed that the plain carbon steel (1045) had a higher concentration of ferrite (white region) and the alloy steel (4140) had a higher concentration of the lamellar pearlite. This was due the presence of alloy elements in 4140 which shifted the transformation curve to the right, forming more pearlite. The two microstructures of 1045 and 4140 after normalization appeared to be very different. Even though they had the same phases formed in the microstructure, the orientation for each phase was very different. As observed from figure-3.2 & 3.9, 1045 had defined globular dark patches of lamellar pearlite forming along with thin boundary like structure of proeutectoid ferrite in the microstructure, whereas in 4140 both pearlite and ferrite were observed, but in a different fashion. The grain size of pearlite was comparatively very small and a lot more proeutectoid ferrite was observed in the structure. This showed that normalizing the alloy steel produced much finer grains than the plain carbon steel. When a comparison was carried out between the microstructures obtained from both annealing and normalizing, it was found that both 1045 and 4140 formed distinct structures after normalizing. The annealed microstructure contained proeutectoid ferrite and lamellar pearlite whereas, the normalized 1045 consisted bainite and ferrite and 4140 showed the presence of bainite, ferrite and pearlite. The reasoning behind this was the alloy content which changed the transformation curve in the CCT diagram and formed different microstructure for 4140. From the results obtained for both 1045 and 4140 through tempering, it was observed that as the temperature increased, the amount of carbide precipitating also increased. This was because at higher temperatures the diffusion between phases increased which resulted into more precipitation of cementite in the matrix. The difference between the microstructure obtained for 1045 and 4140 after 250 of tempering was minimal, whereas tempering at higher temperatures such as 450 and 650 led to a much finer grain size in 4140 when compared with 1045. 4.2 Hardness Analysis As discussed above, martensite can only be formed by high cooling rate from austenite. Martensite is a carbon supersaturated ferrite with Body-centred tetragonal structure (BCT). This unusual structure formed for FCC is because of the high cooling rate. So, as the cooling rate is so fast, neither Fe nor the carbon diffuse immediately, which causes a martensitic transformation without diffusion. So, the

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture