3 Results and discussion
3.1 Macroscopic
Figure 3 shows the macrographs of 4Cr13 linkage and the other components. In Fig. 3a and b, the tripping
plate was connected with the rod through a square opening with the size of 1.05 mm×1.8 mm. The size of lateral
(X) and longitudinal (Y) directions was 1.0 mm and 1.6 mm, respectively. Therefore, the rod could solidly
connect with the tripping plate. During loading condition, there was an elastic force at the position of 90o angle.
In Fig. 3c, two solder joints were present on the surface. The spot welding had an influence on the
microstructure and mechanical properties of local region near the solder joint on the surface of the component.
The component had to be annealed, subsequently. Fig. 3d displays the macrograph of the fractured rod. From the
optical micrograph, the fracture surface shows the metallic silver, and a 45o angular along the fracture surface
was observed. After the observation and analysis, the fracture failure of the component was brittle fracture in an
overloading condition.
3.2 Microscopic
Figure 4 shows the SEM images of the fracture surface of the component. Fig. 4a illustrates the low
magnification image of the fracture surface. The nickel phosphorus layer was plated on the surface of the
component. The inner and outer zones of the fracture surface are shown in Fig. 4b and c, respectively. It can be
observed that the fracture surface was composed of cleavage terrace, dimple and river pattern. It can be inferred
that the fracture failure mode was quasi-cleavage fracture with tore edge.
Figure 5 displays the optical image of the fracture surface of the component. It can be observed that the
grains were fine, and no cracks were present on the fracture surface. After high-temperature tempering, the
microstructure of 4Cr13 component was mainly composed of tempered sorbite, which was a dispersed variety of
pearlite with a eutectic mixture of ferrite and cementite. These structures were in line with those of 4Cr13
stainless steel after quenching and tempering.
3.3 Mechanical properties
The hardness values of the substandard and standard products are shown in Table 1. The result shows that
the hardness of the substandard product was higher than that of the standard product. The high hardness in
crystalline materials could cause the strong rigidity, but resulting in the lack of enough toughness. Therefore, the
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substandard product was easily fractured under overloading condition. After quenching and tempering, the
strength and hardness of 4Cr13 stainless steel increased, but it had severe crack sensitivity. During heat
treatment or spot welding, the crack was easily formed. For substandard products, there would be quenched
residual stress and tempered martensite embrittlement in 4Cr13 stainless steel after heat treatment. The
nonuniform distribution of the stress at the position of 90o angle might result in the fracture failure of the rod
component in an overloading condition.
The mechanical properties of the 4Cr13 samples are displayed in Table 2. The tempering temperature
influenced the mechanical properties of the samples, when the quenching temperature was kept stable. With
increasing the tempering temperature, the hardness was decreased. For the sample with a highest hardness of
50.5 HV0.3, both fracture position and failure mode were similar to the substandard products. The tensile strength
and strain were 1555 MPa and 3.4%, respectively. When the hardness was decreased to 47.1 HV0.3, the fracture
position and failure mode were unchanged. However, the plastic deformation occurred before the sample started
to be fractured. The strain was increased to be 4.2 %. When the hardness of the sample was decreased to 40.2
HV0.3, the sample was not fractured. These data indicated that the high hardness for metal materials could cause
the strong rigidity, but resulting in the decrease of the toughness. While the strength was the controlling property
if a component must withstand a specific load, toughness was the limiting property if a component must be
capable of absorbing a special quantity of mechanical energy without fracturing[2]. Generally, increasing strength
usually leads to decreased toughness.
The SEM images of the fracture surface of the component are shown in Figure 6. Fig. 6a shows the SEM
image of the fracture surface of the standard sample. Fig. 6b displays the SEM image of the fracture surface of
the substandard sample. Compared with the fracture surface of the standard sample, the clearage terrace and
dimple on the fracture surface of the substandard sample were obvious. It can be inferred that the heat treatment
influenced the microstructure of the component. The effect of manufacturing process on the microstructure and
mechanical properties of linkage component was further discussed.
The manufacturing process is a key factor to make sure the quality of linkage components. In this stud