Research on the heat treatment process of Q235A steel

Q235A steel was heat treated by using different heat treatment process parameters. The microstructure after heat treatment was observed microscopically and its hardness was measured. The results indicated that the complete austenitization temperature has not been reached at 850°C and even under water-cooled conditions, only a very small amount of martensite can be obtained. After water cooling at 900°C, 950°C, and 1000°C, lath martensite can be obtained, which made the hardness of water cooling significantly higher than that of oil cooling, air cooling, and furnace cooling. The quenching temperature of 1000°C was too high and the microstructure obtained after water cooling, oil cooling, air cooling, and furnace cooling was relatively coarse, which was not suitable as the heating temperature for heat treatment of Q235A steel.


Introduction
Due to its low price, environmental friendliness, convenient recycling, abundant resources, reliable performance, and ease of processing, steel has become the main choice for engineering structural materials in the 21st century and has the capacity for mass production.Among them, the emergence of carbon steel has opened up a new path for the development of the steel industry.Plain carbon steel is currently one of the most widely used steel grades in industrial production and the national economy.Domestic and foreign scholars have never interrupted their research on steel-strengthening processes.Applying modern heat treatment technology to improve the strength and toughness of low-carbon steel [1][2][3][4][5][6], this paper used different heat treatment process parameters to heat treat low-carbon steel Q235A and analyzed the changes in microstructure and hardness of Q235A steel under different heat treatment processes.The research in this paper had both theoretical and practical significance.

Experimental materials and processes
Q235A steel is an ordinary carbon structural steel with a wide range of uses.The chemical composition and mechanical properties of Q235A steel are shown in Table 1 and Table 2.The initial metallographic structure of Q235A steel is shown in Figure 1.The initial hardness of Q235A steel was 164.73 HV.The initial microstructure of Q235A steel was ferrite and pearlite.Q235A was hot-rolled steel with obvious deformed grains and non-uniform alternating bands of ferrite and pearlite, which mainly formed a banded structure throughout the field of view.
According to the composition of Q235A steel, the phase transition point of Q235A steel was calculated to be around 860℃.This experiment designed four temperature points for heating, namely 850℃, 900℃, 950℃ and 1000℃.The holding time for this heat treatment was 10 minutes.The heating rate was 8.85℃/min.In order to better observe the microstructure of steel under different cooling methods, this experiment ultimately determined four different cooling methods: water cooling, oil cooling, air cooling, and furnace cooling.The final heat treatment scheme is shown in Figure 2 and Table 3.  3. Metallographic structure of Q235A steel after heat treatment (1) The microstructure under different cooling methods at a heating temperature of 850℃ is shown in Figure 3. (a) The microstructure obtained after water cooling at 850℃ was ferrite and a small amount of martensite.Due to the fact that 850℃ had not the austenitizing temperature and belonged to sub-temperature quenching, the amount of martensite after water cooling quenching at 850℃ was relatively small.(b) The microstructure obtained after oil cooling at 850℃ was ferrite and pearlite.Due to the heating temperature not reaching the austenitizing temperature and the cooling rate of oil cooling being lower than that of water cooling, the microstructure after oil quenching was ferrite and pearlite with a higher content of pearlite than the initial state.(c) The microstructure obtained after air cooling at 850℃ was ferrite and pearlite.The microstructure after air cooling at 850℃ was smaller and more uniform than the initial microstructure of Q235A steel with a lower content of pearlite compared to the initial state.(d) The microstructure obtained after furnace cooling at 850℃ was very coarse ferrite and pearlite and the content of pearlite was significantly lower than the initial state.
(2) The microstructure under different cooling methods at a heating temperature of 900℃ is shown in Figure 4. (a) The microstructure obtained after water cooling at 900℃ was lath martensite and a small amount of ferrite.900℃ has reached the austenitizing temperature of Q235A steel, so a lot of lath martensite was generated after water cooling quenching, but a small amount of undissolved ferrite remained possibly because the holding time was too short and the ferrite was not completely dissolved into austenite.(b) The microstructure obtained after oil cooling at 900℃ was pearlite, ferrite and a small amount of bainite.When oil was used as a cooling medium, its cooling capacity was inferior to that of water.The undercooled austenite underwent pearlite transformation, generating a large amount of pearlite.(c) The microstructure obtained after air cooling at 900℃ was ferrite and pearlite.Due to insufficient air cooling capacity, ferrite and pearlite were formed after austenitization.Due to the heating temperature of 900℃, the obtained pearlite and ferrite grains were similar in size to the initial Q235A steel microstructure and no significant grain growth occurred.(d) The microstructure obtained after 900℃ furnace cooling was ferrite and pearlite and the grains were significantly coarser than the initial Q235A steel microstructure before heat treatment.
(3) The microstructure under different cooling methods at 950℃ is shown in Figure 5. (a) The microstructure obtained after water cooling at 950℃ was lath martensite.After austenitizing at 950℃ and water cooling quenching, a typical low-carbon lath martensite structure was formed with obvious lath characteristics.The original austenite grain boundaries were visible.At the grain boundaries of the original austenite, a small amount of fine acicular ferrite can still be seen, indicating that the hardenability of Q235A steel was relatively low.(b) The microstructure obtained after oil cooling at 950℃ was bainite and pearlite.After reaching the austenitizing temperature, oil quenching resulted in bainite and pearlite microstructures due to insufficient cooling rate and low hardenability of the steel.(c) The microstructure of Q235A steel after air cooling at 950℃ was ferrite and pearlite and relatively uniform.(d) The microstructure of Q235A steel after 950℃ furnace cooling was coarse ferrite and pearlite.The grains of the Q235A structure after 950℃ furnace cooling were severely grown.
(4) The microstructure under different cooling methods at a heating temperature of 1000℃ is shown in Figure 6.(a) The microstructure of Q235A steel after water cooling at 1000℃ was coarse lath martensite.Q235A steel had been fully austenitized at 1000℃.Coarse lath martensite was formed after water cooling.(b) The microstructure of Q235A steel after oil cooling at 1000℃ was bainite and pearlite and the grains were very coarse.(c) The microstructure of Q235A steel after air cooling at 1000℃ was ferrite and pearlite and relatively uniform.(d) The microstructure of Q235A steel after 1000℃ furnace cooling was coarse ferrite and pearlite with ferrite grains being equiaxed.Due to the low carbon and manganese content, the number of pearlite was relatively small.

Hardness analysis of Q235A steel after heat treatment
The hardness test results of Q235A steel after heat treatment are shown in Figures 7 to 10. From Figures 7 to 10, it can be seen that ( 1) the hardness after water cooling, oil cooling, air cooling and furnace cooling is directly proportional to the cooling capacity of the cooling medium, that is, the better the cooling capacity is, the higher the hardness is.On the contrary, the poorer the cooling capacity is, the lower the hardness is.Therefore, the hardness of water cooling, oil cooling, air cooling and furnace cooling decreases sequentially at different heating temperatures.Water quenching has the highest hardness.The hardness after air cooling and furnace cooling decreases significantly.(2) When the heating temperature increases, the hardness after water quenching also increases.( 3) From the perspective of metallographic structure and hardness, 950℃ should be the optimal temperature for Q235A heat treatment.

Conclusion
Different heat treatment processes were used to heat treat Q235A steel.Metallographic observation and hardness testing were conducted.The following conclusions were obtained: (1) At different heating temperatures, lath martensite was obtained by water cooling, but the size and distribution of grains and the proportion of lath martensite were different.Pearlite and ferrite were obtained by oil cooling at 850℃.The microstructure obtained by oil cooling at 900℃, 950℃ and 1000℃ contained pearlite and bainite.Both air cooling and furnace cooling resulted in pearlite and ferrite, but there were certain differences in grain size, distribution and proportion of different microstructures.(2) The water-quenched hardness increased with the increase of heating temperature, but if the temperature was too high at 1000℃, the material may lose its original plasticity and toughness.

MATMA-2023
(3) The hardness of water cooling, oil cooling, air cooling and furnace cooling decreased sequentially at different heating temperatures.
(4) 950℃ was the optimal temperature for heat treatment of Q235A steel.At this temperature, the hardness of water quenching and oil quenching had been significantly improved.The plasticity and toughness of the material were not too much lost.

Table 2 .
Mechanical properties of Q235A steel.

Table 3 .
Q235A sample number and heat treatment process parameters.