Metallographic diagram of T12A

A microscopic examination is carried out on a specimen of the steel grade AISI 4340 which was heat treated in the lab. This steel grade is a low alloy, heat treatable steel grade containing chromium, nickel, molybdenum, and other alloy elements. A polishing of the specimen was completed in the lab and the microscope was used to analyze the material grain. In order to better understand the grain structure, a microphotograph was also taken of the polished specimen.

The given microphotograph of the specimen is a bright field view of the grain structure of AISI 4340 steel. The background is black due to the use of a dark field illumination. The picture shows a multi directional grain structure which can be observed as a creamy-white color in the microphoto. The grain size is hard to determine without a grain size calibration, but generally speaking, the higher the magnification, the finer the grain size.

The light areas in the microphoto indicate an area of low alloy elements and the darker areas indicate an area of high alloy elements. The different shapes and sizes of the areas are a result of the formation of various types of grains during the heat treatment process. Some of the grains appear to be round in shape whereas others are sharper due to the presence of chromium and other alloy elements.

The microphotograph shows that the steel has been uniformly heat treated. This is evidenced by the presence of some elongated grains which may have formed due to the repeated heating and cooling cycles imposed during the heat treatment process. The uniformity of the grain structure suggests the steel has been heated for an appropriate time and at the correct temperature.

It can also be observed that some martensite and bainite have formed in the steel during the heat treatment process. The martensite can be seen as small plates forming at the grain boundaries while bainite appears as larger areas having brighter colors. The presence of martensite and bainite indicates that the steel has been heat treated to attain strength and toughness.

Overall, the microphotograph shows a uniform grain structure in the AISI 4340 steel specimen. The grains appear to be well-formed and of a medium size, indicating that the steel has been uniformly heat treated. The light and dark areas in the microphotograph suggest the presence of high and low alloy elements. The presence of martensite and bainite in the steel suggests that the steel has been heat treated to attain strength and toughness.

Figure 1 shows a metallographic photograph of the alloy sample AISI 4340. AISI 4340 (American Iron and Steel Institute designation 4340) is a heat-treatable, low alloy steel consisting of chromium, molybdenum, and nickel. The microstructure observed in the as-received 4340 sample consists of ferrite grains and a small fraction of pearlite. It is seen in the magnified area of the microstructure that the ferrite grains are oriented ferrite grains are substructurally bands in fibre shape. The remaining microstructure appears to be the formation of pearlite. It can be seen in the figure that several spots of martensite are evident in the microstructure which indicate that the alloy sample has not been heat treated.

The size of the ferrite grains is observed to be in the range of 1.2-2.0 µm which is consistent with the ferrite grain size found of the as-received and heat-treatment 4340 specimen. It is also seen that the fraction of pearlite is quite small indicating that the 4340 sample has been austenitized but not sufficiently enough to form large pearlite grains. The ferrite and pearlite grains both have a slightly random orientation in 4340 as-received state.

In conclusion, the metallographic photograph of AISI 4340 sample showed a microstructure with ferrite and small fractions of pearlite, martensite being present in the microstructure. The ferrite grains had a substructural banding which affected their orientation on a macroscale. The pearlite grain size was quite small which indicated that the4340 alloy had been only partially processed. The microstructure of the 4340 sample is thus indicative of an un-heat treated state.