## 2 Research Method
### Examination of Heat Treatment Process for AM Metal Powder Materials  
Because the material manufactured via AM is melted at a high temperature and then rapidly cooled, it undergoes quenching during the manufacturing process, even without an additional heat treatment. Because of the characteristics of the manufacturing process, certain alloy powder materials can obtain high strength and hardness values via AM. However, these materials tend to have low toughness values because they do not undergo tempering, unlike in forging manufacturing processes, in where general heat treatment is applied. Additionally, for certain materials, there may be deviations in the mechanical properties of the surface and interior of the AM output. Based on the usage conditions, these deviations may cause damage to and shorten the lifespans of the printouts. When a commercial forging material is powdered and used for AM, it is possible to use the same heat treatment process that is normally used for forging material in cases where additional heat treatment is required. However, for metal powder materials used in AM, there is no separate forging material, and thus, further research and development regarding the heat treatment process is necessary. In this study, the quenching and tempering heat treatment processes for AM_A and AM_B after AM of the specimens were examined. The quenching and tempering processes were performed using a heat treatment furnace. In general, for a carbon tool-steel heat treatment, the tempering process is determined by the temperature and time. Furthermore, the appropriate temperature and time are selected according to the C content of the material \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Conditions**} & \multirow{2}{*}{**Laser Power (W)**} & \multirow{2}{*}{**Powder Supply (g/min)**} & \multicolumn{2}{c}{**Average of 6 Measurements (HRC)**} \\ \cline{3-4}  & & & **Surface** & **Section** \\ \hline
1 & 250 & 7.3 & 55.4 & 51.2 \\
2 & 300 & 7.0 & 58.4 & 43.9 \\
3 & 350 & 6.5 & 58.9 & 52.6 \\
4 & 400 & 5.4 & 58.1 & 53.4 \\
5 & 450 & 5.0 & 59.6 & 54.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Hardness measurement results for AM_B with respect to changes in heat input. \begin{table}
\begin{tabular}{l c c} \hline \hline \multirow{2}{*}{**AM Parameter**} & \multicolumn{2}{c}{**Sample Name**} \\ \cline{2-3}  & **AM\_A** & **AM\_B** \\ \hline Supplied powder (g/min) & 5.5 & 5.0 \\ Supplied powder gas (L/min) & 2.50 & 2.00 \\ Supplied shield gas (L/min) & 7.00 & 7.00 \\ Laser power (W) & 350 & 450 \\ Deposition speed (m/min) & 0.85 & 0.85 \\ Deposition height per layer (mm) & 0.25 & 0.25 \\ Deposition width pitch per layer (mm) & 0.50 & 0.50 \\ Laser beam diameter (mm) & 0.80 & 0.80 \\ Laser wavelength (nm) & 1060 & 1060 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Optimal AM process conditions for AM_A and AM_B. and the alloy composition [18]. Figure 3 shows the interdependence of the tempering temperature with the C content of general C steel. This diagram shows the correlation between the C content and hardness after tempering at various temperatures for 1 h.  
[PERSON] and [PERSON] developed an equation for predicting the post-tempering hardness of alloy and C steels, known as the Holloman-Jaffe equation (more commonly referred to as the Larson-Miller equation), which is expressed as Equation (3) [18]:  
\[\mathrm{P_{H}=T_{C}\left(C+\log t\right)\,10^{-3},} \tag{3}\]  
where \(\mathrm{P_{H}}\) is the tempering parameter, t is the time at temperature \(\mathrm{T_{C}}\) (in Kelvin), and C is the material-dependent \"Holloman-Jaffe\" constant. The quenching temperature during the heat treatment of general C tool steel was 1020-1060 \({}^{\circ}\)C, whereas the C content of AM_A and AM_B ranged within 0.34-0.35%.