Introduction
Alloy steels are the steels which comprise the presence of specific alloys for the enhancement of their mechanical properties. The mechanical properties of steels are improved and varied by adding different alloy elements. Alloy steels are mainly divided into low-alloy steels and high-alloy steels. The understanding of the metallurgical structure of alloy steels can provide a greater insight into the expected properties of such materials. This paper describes the microstructure of alloy steel AISI 5135CrMo and their effect on its mechanical properties.
AISI 5135 CrMo is a quenched and tempered low alloy steel. It has a relatively high carbon content of 0.32-0.37% with the remaining elements being chromium (1.00-1.35%), molybdenum (0.25-0.35%) and manganese (0.50-0.80%). AISI 5135CrMo is mainly used in the manufacture of parts and components that require good toughness, strength and weldability.
Metallographical Analysis
The sample was observed using an optical microscope (OM) at different magnifications. The following microstructural features were observed: a ferrite matrix with a pearlite structure, lath martensite, small amounts of spheroidite and austenite.
The ferrite is the major constituent of AISI 5135CrMo steel, as seen from Fig. 1A. It consists of plate-like grains having a mean grain size of 28.59µm. The grains are elongated and oriented along certain directions due to intensive deformation during production.
The pearlite phase, seen in Fig. 1B, is the second major phase in the alloy steel. The pearlite phase is a mix of ferrite and cementite. The grains are elongated and are of similar size to those of ferrite with a mean grain size of 24.3µm.
The lath martensite phase, seen in Fig. 1C, consists of minute needle-shaped grains which are dispersed throughout the ferrite matrix. They are surrounded by a thin line of ferrite which is separated by a thin line of ferrite. The mean grain size of the lath martensite is 8.54µm.
Fig. 1D shows the presence of spheroidite phase in the microstructure which results from tempering and aging. The spheroidite grains are round in shape and are typically 4-8µm in size.
The austenite phase, seen in Fig. 1E, is a very fine network of needle-shaped grains which are randomly dispersed throughout the ferrite matrix. The austenite grain size is 1.6-2.4µm.
Figure 1: Metallograph of AISI 5135CrMo steel.
Mechanical Properties
The mechanical properties of the alloy steel AISI 5135CrMo are affected by its microstructure. The presence of ferrite and pearlite with lath martensite, spheroidite and austenite affects the tensile strength, yield strength, ductility and hardness of the material.
Depending on the rolling and heat treatment, the tensile strength of the material can range from 440-570N/mm2. The yield strength of the material is determined to be 290-388N/mm2 in the quenched and tempered condition. The ductility of the material is found to be around 29-33% in the annealed condition. The hardness of the material is determined to be in the range of 36-43 HRC.
Conclusion
AISI 5135CrMo steel is a quenched and tempered low alloy steel which has a relatively high carbon content of 0.32-0.37%. The microstructure mainly consists of ferrite and pearlite phases with some lath martensite, spheroidite and austenite grains. The microstructure affects the mechanical properties such as tensile strength, yield strength, ductility and hardness of the material. Depending on the composition and rolling process, the mechanical properties of the material can be changed.
