Metallographic diagram of 40Cr (pit cold after forging)

Metallographic Image Analysis of 40Cr Alloy After Forging

The 40Cr steel is a medium carbon alloy steel that is commonly used for many forge applications. This material is heat treatable and when it is toughened it can be easily machined. The aim of this study was to analyse the metallographic image of 40Cr after forging.

The sample of 40Cr was forged under a large hammer at a temperature of 800°C., the forging process lasted for 3 minutes. After forging, the material was annealed at 500°C for 30 minutes and the sample was then cooled naturally.

For metallographic analysis, the sample was polished using coarse and fine grinding paper before etching. The conditions were optimized to provide high image magnification. The sample was observed using an optical microscope.

The polished sample of 40Cr appeared smooth and uniform in colour. Cross-sections of the sample revealed a microstructure that was composed of ferrite grains, along with a continuous phase of pearlite. The ferrite phase had a shape that was either elongated or rounded, depending on the nature of the forging.

The pearlite phase was found to be more uniform throughout the sample and was in the form of an even mesh of needle-like laths. The laths had a near random arrangement, with a mean diameter of approximately 0.5 μm.

Observation of the sample revealed an intermixed ferrite/pearlite structure, with the pearlite being much more dominant than the ferrite. The pearlite was found to be in the form of laths that were distributed evenly throughout the structure.

In addition to the two main phases, small percentages of bainite and martensite were observed in the microstructure. These phases were much fewer in number than the pearlite and ferrite and they were mostly observed at the grain boundaries.

In summary, the metallographic image of 40Cr after forging revealed a microstructure that consisted of two main phases, ferrite and pearlite, along with small percentages of bainite and martensite. This sample had excellent properties and was suitable for a variety of forging applications.

The microstructure of 40Cr hot forged steel is mainly ferrite-pearlite structure. The micrograph shows that the ferrite matrix occupied most area, which mainly distributed in small finely divided grains. There were also some evenly distributed and large pearlite, while the amount of pearlite varies in different areas.

In general, the iron grains are relatively refined, but some relatively coarse grains can be seen in the picture. Most of them are abundant {111} family of octahedral shape. The second phase is mainly pearlite, with a lamellar structure, and there are still a small amount of ferrite and cementite grains. This microstructure is suitable for manufacturing parts with good comprehensive mechanical properties and sufficient toughness.

The hardness of 40Cr forging steel is relatively high, which is mainly related to the microstructure. Small grains, fine pearlite and appropriate amount of cementite can make its hardness reach or exceed HB200. The hardness can be increased by proper heat treatment, and it is suitable for making high strength parts. So 40Cr forging steel can be used to manufacture parts with high strength and machining performance.