# Research on Microstructure and Mechanical Properties of Laser Welding of BR1500 HS Joints
## 3 Results and Discussion  
### Effect of Heat Input on Tensile Strength  
The tensile test results of all the samples before and after quenching are shown in Table 3. Laser power and welding speed are two important process parameters that have a significant impact on the tensile strength of the welded joint. The ratio of laser power  
Figure 4: Distribution of microhardness test points: (**a**) schematic illustration; (**b**) test illustration. Figure 3: Dimensions of tensile samples. to welding speed is defined as heat input, which takes into account the effects of the two parameters. The expression for heat input is as follows:  
\[q=\frac{p}{v} \tag{1}\]  
where \(q\) is the input, \(P\) is the laser power, and \(v\) is the welding speed. From Table 3, before quenching, the fracture position of all the samples is on the base material (BM). For all the samples, the tensile strength of the BM is about 630 MPa. It's obvious that the welded joints have good mechanical properties and excellent formation. After quenching, the tensile strength of BM can reach 1800 MPa, while the maximum tensile strength of the welded joint is 1644 MPa. The fracture position of all the samples is on the joint. It indicates that the joint is the weakest area. The tensile strength of all the joints is lower than that of the BM because the BM is in the quenched state, which is the highest strength obtained by the optimal quenching process of BR1500 HS. However, the thermal process of laser welding will affect the quenching process, resulting in a decrease in the tensile strength of the welded joint [27]. The tensile strength of all the samples at different heat inputs after quenching is shown in Figure 5. We can see when the heat input is low, the tensile strength of the samples is also low. And when the heat input increases, the tensile strength gradually increases. When the value of heat input is 1440 J/cm, the tensile strength of the joint reaches a maximum value and then decreases. When the value of heat input is below 1440 J/cm, the BM is not completely fused because the heat input is low. The tensile strength of the joint is insufficient, which is less than 1500 MPa. Therefore, to achieve complete penetration, the value of heat input must be greater than 1440 J/cm during the laser welding process. As the heat input increases, the tensile strength of the joint decreases when the value of heat input is greater than 1440 J/cm. The reason for the above trend is that, as the heat input increases, the range of the HAZ becomes larger, also the range of the BM affected by the welding thermal cycle becomes larger. Therefore, then the grain size increases. At the same time, as the heat input increases, the volume of martensite (M) also decreases. Due to the above two reasons, the tensile strength of the joint will decrease. For BR1500 HS ultra-high-strength steel after quenching, the value of heat input should be kept above 1440 J/cm to obtain the optimal tensile strength. Therefore, according to the experimental results, it can be found that heat input is a very important process parameter in the process of autogenous laser welding, which can reflect the mechanical properties of the joint more specifically.