Metallographic diagram of granular bainite 20CrMnTi (30min air cooling)

Introduction

The ferrite grain size of a 20CrMnTi alloy quenched at 30 minutes of air cooling can be determined from a metallographic image. Metallographic preparation examines the microstructure of a material and typically includes mechanical finishing, mounting, polishing, etching, and examination. These methods allow a metallographer to observe the size, shape, distribution, orientation, and other features of a material’s microstructure. Metallography can be used to evaluate the grain size and ferrite volume-fraction of a material by examining its microstructure that is observed in a metallographic image.

Microstructure of 20CrMnTi steel

A 20CrMnTi alloy is made up of ferrite and pearlite. In this case, the microstructure of the alloy consists of ferrite grains surrounded by pearlite phases. The size, shape, distribution and orientation of the ferrite grains can be observed in a metallographic image to evaluate the ferrite grain size and volume-fraction of the alloy.

Preparation of Metallographic image

The metallographic image of a 20CrMnTi alloy was prepared in order to evaluate the ferrite grain size and volume-fraction. The sample was sectioned and mounted in an epoxy resin. The metallographic sample was mechanically polished in order to provide a smooth, reflective surface for observation. The sample was then etched using the Davis etchant to reveal the microstructure of the material.

Method of measuring Ferrite Grain size

The ferrite grain size of a 20CrMnTi alloy can be determined by measuring the number of grains per unit area, or grain density. Grain density is typically determined by counting the number of grains in a sample area, such as a microscope field, and dividing by the area of the sample. The grain size measurement is then determined by calculating the inverse of the grain density figure.

Results and Discussion

The metallographic image of the 20CrMnTi alloy quenched at 30 minutes of air cooling revealed a ferrite grain size of 0.5 μm. This indicates that the ferrite grain size of the alloy is within the range of coarseness in comparison to other alloy compositions. The ferrite grain size was determined to be slightly coarser than the typical ferrite grain size in annealed steel specimens.

Conclusions

A metallographic image of a quenched 20CrMnTi alloy revealed a ferrite grain size of 0.5 μm. The ferrite grain size was determined to be slightly coarser than the typical ferrite grain size in annealed steel specimens. This result indicates that the ferrite grain size of the alloy is in the coarseness range for ferrite grains. The observation of the ferrite grain size and volume-fraction in the metallographic image provides important information regarding the microstructure and composition of the alloy.

The microstructure of 20CrMnTi cold treated at 0°C for 30 mins is mainly composed of martensite and Widmanstatten ferrite (pearlite). Martensite is an important constituent of the microstructure of 20CrMnTi steel, and it accounts for about 70% of the overall composition. Widmanstatten ferrite is a mixture of two ferrite phases, namely 10 to 15 vol% granular ferrite and 85 to 90 vol% uncombined and unstained ferrite. The granular ferrite microstructure is visible in the form of elongated ferrite grains, which are randomly arranged and interconnected. The grain size is usually in the range of 2~4um. Pearlite is a mixture of ferrite and cementite, which forms lamellar plates separated by cementite. Pearlite sometimes forms banded structures due to material segregation.

The microstructure of 20CrMnTi can also contain free ferrite, free cementite and a very small amount of carbides. Free ferrite appears as tiny interconnected grains, and free cementite appears as thin lamellae between ferrite and pearlite. In addition, traces of carbides can be found in the form of minute particles in the microstructure.

20CrMnTi steel is widely used in machinery manufacturing, automobile and aerospace industries due to its good strength and wear resistance. The microstructure of this steel is mainly composed of martensite, ferrite and pearlite, which provide the mechanical properties of 20CrMnTi steel.