65Mn (1100℃×20min+450℃×1s water cooling) metallographic diagram

Abstract

In this experiment, the microstructure of 65Mn steel after heat treatment with 1100°C 20 minutes and 450°C 1 second water cooling (1100°C×20min+450°C×1s water cooling) was observed. The results showed that large columnar crystal grains, Widmanstatten ferrite, untempered martensite, pearlite, and areas of small grains were clearly visible in the morphological map. On the basis of the analysis results, the heat treatment process could make the large grains become finer and more uniform, and the microstructure changed from non-homogeneous to homogeneous. In addition, the shape and size of the grains were greatly changed, which greatly improved the microstructure of the steel and enhanced its metallurgical properties.

Introduction

Steel is a metal alloy composed primarily of iron, typically with carbon as the main alloying material. Steel is a key material in many industrial fields due to its low cost, high strength and good corrosion resistance. 65Mn is a type of low carbon steel with good comprehensive properties and high wear resistance. It has been widely used in various fields such as the manufacture of springs, tools, environmental protection and energy. Heat treatment is an important process in the manufacture of steel, which is to change the appearance, structure and properties of steel by heating and cooling of steel in a definite temperature range. In this experiment, the microstructure of 65Mn steel after heat treatment with 1100°C 20 minutes and 450°C 1 second water cooling (1100°C×20min+450°C×1s water cooling) was observed by metallographic microscope, and the results were analyzed.

Experimental Method

The experiment was carried out using a 65Mn steel sample. The sample was cut from the middle section of the workpiece using a common grinder, polished to 100# oil stone, and then polished and polished to obtain a smooth surface. The sample was then heat-treated using a resistance furnace. The heat treatment process was 1100°C 20min+450°C 1s water cooling (1100°C×20min+450°C×1s water cooling). After heat treatment, the sample was taken out, the surface was observed by naked eye and light microscope, and the microstructure was analyzed by metallographic microscope.

Results and Analysis

The results showed that the surface of the 65Mn steel sample was grey after heat treatment. Under the microscope, the sample morphological map showed large columnar crystal grains, Widmanstatten ferrite, untempered martensite and pearlite, as well as areas of small grains. The area of large grain structure was much larger than that of small grain structure, and the size of the grains was larger than that before heat treatment. It can be seen that the grain size of the steel has changed greatly after heat treatment. The main phases in the sample were Widmanstatten ferrite, untempered martensite and pearlite, which indicated that the heat treatment process could make the large grains become finer and more uniform, and reduce the inclusions between the grains, so that the microstructure changed from non-homogeneous to homogeneous. In addition, the Widmanstatten ferrite and untempered martensite of the sample were more than that of before heat treatment, which could improve the strength and hardness of the steel and enhance its metallurgical properties.

Conclusions

The morphological analysis results of 65Mn steel after heat treatment with 1100°C 20 minutes and 450°C 1 second water cooling (1100°C×20min+450°C×1s water cooling) showed that the grains became larger, the grain size became finer and more uniform, the microstructure changed from non-homogeneous to homogeneous, and the amount of Widmanstatten ferrite and untempered martensite increased. The heat treatment process could effectively improve the microstructure of the steel, increase the strength and hardness of the steel, and enhance its metallurgical properties.

In this study, a 0.5 mm thick sample of 65Mn steel was heated for 20 minutes at 1100°C, followed by a quenching in water at 450°C for 1 sec. The metallographic study of the sample was conducted using a light optical microscope.

The microstructure present in the sample exhibited a martensitic structure labelled as the α’ phase. It was found that austenite had not formed mainly since the curing temperature was below 1000°C. In addition, the incomplete transformation of ferrite to austenite due to insufficient holding time also hastened the formation of martensite. The α’ phase appeared as elongated and misoriented formations with well-defined areas with a needle-like shape.

In the between-grains areas, an interface was observed where the lighter-coloured zones of the ferrite grains and the darker-coloured zones of the martensite α’ laths were depicted. It was also noted that in the end of the austenite grain from which the α’ laths originated, particles of carbides were observed.

Minor defects such as voids, concavities and cavities were also observed in the ferrite grains. On the other hand, the martensite α’ laths were a little weathered due to their transformation from the austenite grains. It has been suggested that the severity of the weathering can be related to the hardness of the steel.

Therefore, the results of the study revealed that the austenization for 20 minutes at 1100°C, followed by cooling in water at 450°C for 1 sec, was insufficient for causing a full transformation of the ferrite to austenite. The martensite α’laths and the ferrite grains had their typical grain boundaries, whereas the undefined austenite was present as small particles of carbides.