Metallographic diagram of 45 steel (940℃ powder boronizing for 5.5h)

Microstructure of A335 Steel (940℃ Powder Quenching 5.5h)

The microstructure of A335 steel (940℃ powder quenching 5.5h) was studied by optical microscopy and scanning electron microscope. The optical microscopy shows the microstructure of the steel was composed of martensite and bainite. The martensite was distributed uniformly with different grain sizes ranging from 5 to 15 μm. The canite was mainly located at the grain boundaries of the martensite and its grain size was smaller than that of the martensite with diameter ranging from 3 to 5 μm. The morphology of the microstructure was fine and was possible to observe various morphologies including spheroidization, Widmanstatten and some other grains. After scanning electron microscope observation, it is revealed that the particles of the steel were mainly composed of ferrite, cementite, and pearlite. The ferrite grain size ranged from 1 to 5μm, lozenge shaped interstitial particles distributed in the ferrite, and the cementite was smaller than the ferrite grain size, the cementite was mainly distributed in the boundary of ferrite grain. The pearlite distribution was finer than the ferrite and cementite. It was distributed evenly among the grains and its diameter was about 0.2 to 0.5 μm.

The results of hardness tests show that the hardness of the steel sample quenched at 940℃ for 5.5 hours was 205HBW. With the increasing of tempering temperature, the hardness of the steel increases at the same time. The maximum hardness of 292HBW was obtained under the tempering condition at 850℃ for 45 min.

The microstructure analysis has indicated that the A335 steel quenched and tempered at 940℃ for 5.5h is composed of martensite and bainite, while the pearlite and ferrite were also observed. The hardness of the self-tempered sample was 205HBW. With the increasing of tempering temperature, the hardness of the steel increases at the same time., and the maximum hardness of 292HBW was obtained under the tempering condition at 850℃ for 45 min.

描述

The water-atomised ferritic-bainitic steel S45 examined in this study is a low-carbon steel containing 0.45 wt.% C and 0.15 wt. % Si. This steel is commonly used in automotive products and their components due to its good strength and ductility.

The conclusion was drawn by studying the ferritic-bainitic S45 steel microstructure obtained after quenching from 940℃ and powder boriding for 5.5 hours.

Microstructural examination was performed using optical and scanning electron microscopes (SEM). The boride layer was investigated using energy-dispersive X-ray spectroscopy (EDS) in SEM.

The microstructure of the steel was comprised of ferrite one-dimensional array grains and bainite cell-like structure, giving it an average grain size of 5μm. The boride layer was found to have thickness up to 5μm, depending on the heat treatment process. The presence of this boride layer with thickness is believed to give the steel its excellent strength properties.

The presence of M23C6 carbonitride was also observed on the grain boundary, which enhances the initiation of bainite further. This increasing strength of the steel due to boriding and formation of M23C6 carbonitride is utmost important for automotive components.

In conclusion, ferritic-bainitic S45 steel exhibits excellent strength and ductility after boriding and quenching from 940℃. The microstructure of the steel consists of ferrite one-dimensional array grains and bainite cell-like structure with M23C6 carbonitride. The boride layer can reach up to 5μm of thickness. This study provides useful information for design and production of automotive components.