1. Introduction
Non-quenched and tempered steel is an alloy that has been widely used in the manufacture of components for mechanical structures. It is not subjected to quenching and tempering (QT), but has mechanical properties equivalent to those of alloys subjected to QT. In general, it is possible to manufacture non-quenched and tempered steel with high mechanical properties by securing the precipitation strengthening effect through the addition of fine-precipitate-forming elements (e.g., V and Nb) into conventional carbon steel and by controlling the constituent phase through the control of the cooling rate during the manufacturing process1-5).
Non-quenched and tempered (NQT) steel has been applied to components under high loads in mechanical devices, such as hydraulic cylinders, crankshafts, and connecting rods. Therefore, the durability of the base metals and welded-joints of the NQT steels is very important for the performance and safety of mechanical devices1-5).
In general, the welded-joint consists of weld metal (fusion zone) that involves melting and a heat-affected zone (HAZ) without melting, which is located next to the weld metal and affected by high temperature. HAZ was originally a part of the base metal, but it has a microstructure significantly different from that of the base metal due to the phase transformation caused by rapid thermal cycling during welding6-8). HAZ generally exhibits inferior mechanical properties compared to the base metal due to grain growth and the formation of martensite structures6-8). For this reason, research on welding characteristics is essentially required simultaneously with base metal development for materials that are used in structural components.
In this study, research was conducted on the correlation between the microstructure and mechanical properties of the HAZ of the non-quenched and tempered steel designed for hydraulic cylinder manufacturing. To this end, HAZ was simulated using the Gleeble simulator for the non-quenched and tempered steel designed by adding V, and a total of six HAZ samples were prepared by varying the weld heat input and HAZ peak temperature, respectively. The microstructures of the base metal and simulated HAZ were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD) while the mechanical properties of HAZ were evaluated through the Vickers hardness test and the U-notch impact test. Finally, the correlation between the microstructure evolution and the changes of mechanical properties in the HAZ of non-quenched and tempered steel according to changes in weld heat input and peak temperature was investigated.
2. Experimental Method
Table 1 shows the chemical components of the non-quenched and tempered steel used in this study. As can be seen from the table, the microstructure of the HAZ of the non-quenched and tempered steel with the composition of Fe-0.28C-0.97Si-1.61Mn-0.14V was analyzed and its mechanical properties were evaluated in this study.
Table 1
Chemical composition of non-quenched and tempered steel (in wt.%)
| C | Si | Mn | V | Fe |
|---|---|---|---|---|
| 0.28 | 0.97 | 1.61 | 0.14 | Bal. |
Fig. 1 shows the results of calculating the equilibrium phase diagram of the non-quenched and tempered steel with the composition listed in Table 1 using the Thermo-Calc. software. It can be seen that the Ae3 temperature is approximately 800°C, and the 800-1400°C temperature range is identified as the single-phase austenite region. In this study, the peak temperature for HAZ reproduction was set by referring to the equilibrium phase diagram of Fig. 1. In other words, the coarse grained heat-affected zone (CGHAZ) was reproduced by setting 1300 and 1150°C, which are high temperatures within the single-phase austenite region, as peak temperatures while fine grained heat-affected zone (FGHAZ) was reproduced by setting 1000°C, a relatively low temperature within the region, as a peak temperature. Meanwhile, the heat input conditions for HAZ reproduction were changed to 30 and 150 kJ/cm. Fig. 2 shows the thermal cycle of HAZ calculated using Rosenthal’s heat flow equation9). Fig. 2(a) and 2(b) show the HAZ thermal cycles calculated under heat input conditions of 30 and 150 kJ/cm, respectively. Under each heat input condition, the HAZ peak temperatures of 1300, 1150, and 1000°C were classified as CGHAZ 1, CGHAZ 2, and FGHAZ 1, respectively. Meanwhile, it can be seen from Fig. 2 that the heating and cooling rates decreased during the HAZ thermal cycle as the heat input increased and the peak temperature decreased. According to Rosenthal’s heat flow equation analysis results, the cooling time (Δt8-5) in the 800-500°C range, where phase transformation occurs in carbon steel during cooling, increases as the heat input increases9), indicating that the cooling rate decreases within the temperature range.
Fig. 2
HAZ thermal cycles simulated by gleeble simulator. Heat input condition, (a) 30 kJ/cm and (b) 150 kJ/cm
The microstructures of the base metal and simulated HAZ were observed through OM, SEM, and EBSD. The specimens for OM and SEM observations were polished using SiC paper and etched with a 3% Nital solution. The specimens for EBSD analysis were polished using colloidal silica suspension.
Finally, the mechanical properties of the base metal and simulated HAZ were evaluated through hardness measurements and impact testing. The hardness was measured through the Vickers test while the impact properties were measured through the Charpy U-notch impact test at room temperature.
3. Experiment Results and Discussion
3.1 Base metal and HAZ microstructure analysis results
Fig. 3 shows the microstructure of the base metal of non-quenched and tempered steel. It can be seen that the base metal is composed of a typical polygonal ferrite + pearlite structure.
Fig. 4 shows the results of observing the microstructure of simulated HAZ using SEM. It can be seen that HAZ is composed of a large amount of martensite and a small amount of bainite and grain boundary ferrite unlike the base metal. This is because a small amount of ferrite is generated first along the prior austenite grain boundary (PAGB) below the Ar3 temperature during rapid cooling in the HAZ thermal cycle, and a small amount of intragranular bainite is generated and then the remaining austenite regions are transformed into martensite as the temperature decreases. It can be seen from Fig. 4 that the ferrite fraction increased as the heat input increased from 30 to 150 kJ/cm. The ferrite fraction also increased in FGHAZ, which had the lowest HAZ peak temperature of 1000°C. This results from the difference in HAZ cooling rate depending on the heat input and HAZ peak temperature. In other words, as described in Fig. 2, the cooling rate of HAZ decreases as the heat input increases and the HAZ peak temperature decreases, thereby increasing ferrite transformation at relatively high temperatures during cooling10).
Fig. 5 shows the EBSD inverse pole figure (IPF) map of HAZ. It can be seen that HAZ is mostly composed of fine lath structures, but a large amount of ferrite was generated in the sample with high heat input and a low peak temperature (150 kJ/cm-1000°C) as described earlier in Fig. 4. In this study, the peak temperature for HAZ reproduction was set was measured through the analysis of the EBSD IPF map in Fig. 5. The measurement results are summarized in Fig. 6. It can be seen from the figure that the PAGS of HAZ significantly increased as the heat input and peak temperature increased, which is related to changes in HAZ thermal cycle due to changes in heat input and peak temperature. First, as can be seen from Fig. 2, the heating and cooling rates in HAZ decreased as the heat input increased, which can cause grain growth in HAZ by increasing the retention time at high temperatures. Next, the grain growth rate generally increases alongside the increase in temperature11). Therefore, an increase in peak temperature appears to have promoted grain growth in HAZ as shown in Fig. 6.
3.2 Mechanical property evaluation results for HAZ
Fig. 7 shows the results of measuring the Vickers hardness of each HAZ sample. First, as can be seen from Fig. 7, low hardness values were observed from cases with high heat input under the same peak temperature condition (CG 1-1, CG 2-1, and FG 1-1). This appears to be due to the grain growth and the increase in ferrite fraction caused by the increase in heat input as explained earlier. It can be seen from Fig. 7 that CGHAZ (CG 2, CG 2-1) with a peak temperature of 1150°C under the same heat input condition exhibits a higher hardness value than CGHAZ (CG 1, CG 1-1) with a peak temperature of 1300°C and FGHAZ (FG 1, FG 1-1) with a peak temperature of 1000°C. This appears to be because excessive grain growth occurred when the peak temperature was too high (CG 1, CG 1-1) and the ferrite fraction significantly increased in FGHAZ with the lowest peak temperature as shown in Fig. 4.
Fig. 8 shows the results of conducting the impact test at room temperature for each HAZ sample. First, it can be seen that the impact properties significantly decreased as the heat input increased as in the hardness measurement results above. This is because the propagation of cracks during the impact test becomes easier as PAGS increases. Next, CGHAZ 2, 2-1 with a peak temperature of 1150°C exhibited the highest impact properties. This appears to be because CGHAZ 2, 2-1 has a lower ferrite fraction than FGHAZ and a finer grain size than CGHAZ 1, 1-1 as in the hardness results above. Finally, Fig. 9 shows the results of observing the fracture surface after the impact test for the FGHAZ 1-1 sample. It can be seen that cracks propagated along PAGB through ferrite. In other words, the propagation of cracks is easy during the impact test when PAGS is coarse or when the ferrite fraction is high as explained earlier, resulting in the degradation of impact properties.
4. Conclusions
In this study, research was conducted on the correlation between the microstructure and mechanical properties of the heat-affected zone (HAZ) of non-quenched and tempered steel during welding. To this end, six HAZ samples with different heat input and peak temperature conditions were prepared using the Gleeble simulator, and microstructure analysis and mechanical property evaluation were performed for simulated HAZ. The main results of this study are as follows.
1) While the base metal had a typical polygonal ferrite + pearlite structure, HAZ was composed of a large amount of martensite and a small amount of bainite and grain boundary ferrite. The ferrite fraction in HAZ increased as the heat input increased and the peak temperature decreased. It was also confirmed that the grain size of HAZ increased as the heat input and peak temperature increased.
2) The HAZ hardness measurement results revealed low hardness values when the heat input was high under the same peak temperature condition, which results from the grain growth behavior caused by the increase in heat input. Under the same heat input condition, the coarse grained heat-affected zone (CGHAZ) sample with a peak temperature of 1150°C exhibited the highest hardness value. This is because excessive grain growth occurred in CGHAZ with a higher peak temperature of 1300°C and the ferrite fraction significantly increased in fine grained heat-affected zone (FGHAZ) with the lowest peak temperature.
3) In the HAZ impact test results, the propagation of cracks was easy during the impact test when the grain size of HAZ was coarse or when the ferrite fraction was high, resulting in the degradation of impact properties. In other words, as in the hardness test results, the CGHAZ sample with a peak temperature of 1150°C exhibited the highest impact properties.
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