Abstract
The martensitic transformation behavior of a type ofFriction Stir Welding C-Mn-Si-Cr-Mo-V-Ti-B hot rolled (HR) steel (grade T10 NW) was studied. The samples were heat treated at 920°C for 4 hours followed by air cooling. Microstructural characterization was performed by optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray diffraction. The results revealed that the HR T10 NW microstructure is composed of polygonal ferrite and tempered martensite. Depending on prior structure, in the retempered structure, round lath martensite was observed in the substitution of polygonal ferrite. During recrystallization, some of the polygonal ferrites decomposed in to tampered martensite and round lath martensite in ferrite. With increasing tempering temperature, tempering precipitation and carbide particles were scattered uniformly in the ferrite matrix and the main carbide formed is M2C. With increasing tempering temperature, the microstructural development was characterized by continual refinement of the ferrite grain size.
Introduction
Steel is an internationally versatile engineering material, used in various applications such as building constructions, automotive, shipbuilding, and energy production. Hot rolled steels are extensively used due to its ductility, formability, high strength to weight ratio and lower alloy cost. The transformation of hot rolled steel to a final product requires a careful selection of the heat treatment process and is crucial for the development of the desired properties.
Materials and Experimental Procedure
This study focuses on the effect of different heat treatments on a hot rolled steel grade Aisi T10 NW, which is a C-Mn-Si-Cr-Mo-V-Ti-B alloy. The steel material was provided by the company Inoxfer. Hot rolled steel is characterized by being ferritic and bainitic after cooling from the rolling process. It requires low cooling rate to produce retain austenite and softening can be achieved with reheating and slow cooling to produce a soft material with desired mechanical properties.
The samples were cut to size and then annealed in a vacuum annealing furnace at 920°C for 4 hours followed by air cooling. An optical microscope (LeicaMEFY) equipped with a computer was employed to obtain microstructural images of the as-received, retempered and recrystallized samples. The microstructures of the samples were compared using a scanning electron microscope (JCSEM) and a transmission electron microscope (JEOL TEM 2200) operating at an accelerating voltage of 200 kV. The microstructure was further validated by X-ray diffraction (WDXD) reveals preferential and structural orientation.
Results and Discussions
The as-received steel microstructure shown in Figure 1 is composed of polygonal and plate ferrite grains that are randomly oriented. The microstructure also contains small amounts of bainite, martensite and intergranular phase. No carbides were observed in the as received samples, as only minor amounts of alloying elements were added.
Figure 2 shows the microstructure of the sample heat treated at 920°C for 4 hours followed by air cooling. The samples revealed tempered martensite in the substitution of polygonal ferrite. The analysis of the microstructure showed that tempering increased the volume fraction of martensite and spheroidal particles. There are also some bainite and intergranular phase occurring between the ferrite and martensite.
Figures 3 shows the microstructure of the samples recrystallized at 920°C for 4 hours. During recrystallization, some of the polygonal ferrite decomposed into tempered martensite and round lath martensite in ferrite. The microstructural characterization using a scanning electron microscope and optical microscope revealed that the recrystallized structure consisted of lath martensite in the circumference of the ferrite. In addition, the ferrite grain size decreased with the recrystallization in comparison to the tempered samples. It is also noticed that large amount of M2C carbides were formed during the recrystallization process.
The X-ray diffraction results showed that the ferrite and martenite crystal structures were well developed in the recrystallized samples, as high intensity peaks of ferrite were observed. The crystallite size in the recrystallized structure was estimated to be 8.3 nm and the lattie parameter of the ferrite was determined to be 0.247 nm.
Conclusion
The T10 NW steel during the heat treatment process of 920°C for 4 hours has presented a tempered martensite structure in substitution of polygonal ferrite. The recrystallization process has developed lath martensite in the circumference of the ferrite and a uniform distribution of M2C carbides in the ferrite matrix. The crystal size in the recrystallized structure was estimated to be 8.3 nm, with the lattice of 0.247 nm. The results from this study can be used to better understand the transformation behavior of the T10 NW steel.
