**5.3 Process affected zone in micropunched hole**

Why the stability of burnished surface length changed with grain size? **Figure 12** shows the EBSD analysis results of punched samples with three different varying grain sizes of 1.5, 3.0, and 7.5 μm, respectively. The punching conditions were following; the material thickness is 100 μm, the punch diameter, 80 μm, the die diameter, 85 μm,

**11**

*Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals*

the clearance between punch and die, 2.5 μm, and the punching speed, 4.2 mm/s. In **Figure 12**, IQ + phase map shows the grain phase and the IQ + KAM (Kernel average misorientation) map shows the grain misorientation angle. The grain misorientation

along with the punched hole. From the grain size 7.5 μm phase map results (**Figure 12(C)**), the border line between *α*′ phase and austenitic *γ* phase looks like hackly. This trend is caused by grain unit transformation. On the other hand, from the grain size 1.5 μm phase map results (**Figure 12(A)**), the border line between *α*′ phase and *γ* phase looks like stable. From the **Figure 12(D)–(F)**, process-affected areas were existed along with punched hole, respectively. Especially, the highly 5° misorientation angle color red has spread along with the wall of punched hole at each samples.

From **Figure 12(A)**–**(C)**, the strain-induced martensitic *α*′ phase was remained

**5.4 Deformation and transformation mechanism in micropunching of FGSS304**

**Figure 13** shows the total frequency of *α*′*-*phase and total amount of misorientation angle that calculated from the sample shown in **Figure 12** and the other two samples. From **Figure 13(A)**, Z minimizes at the grain size of 3 μm. From **Figure 13(B)**,

angle was known to correspond to equivalent plastic strain [25].

*DOI: http://dx.doi.org/10.5772/intechopen.89754*

**Figure 10.**

**Figure 11.**

*Standard deviation of ratio of sheared surface.*

*Ratio of the sheared surface.*

*Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals DOI: http://dx.doi.org/10.5772/intechopen.89754*

**Figure 11.** *Standard deviation of ratio of sheared surface.*

**Figure 10.**

the clearance between punch and die, 2.5 μm, and the punching speed, 4.2 mm/s. In **Figure 12**, IQ + phase map shows the grain phase and the IQ + KAM (Kernel average misorientation) map shows the grain misorientation angle. The grain misorientation angle was known to correspond to equivalent plastic strain [25].

From **Figure 12(A)**–**(C)**, the strain-induced martensitic *α*′ phase was remained along with the punched hole. From the grain size 7.5 μm phase map results (**Figure 12(C)**), the border line between *α*′ phase and austenitic *γ* phase looks like hackly. This trend is caused by grain unit transformation. On the other hand, from the grain size 1.5 μm phase map results (**Figure 12(A)**), the border line between *α*′ phase and *γ* phase looks like stable. From the **Figure 12(D)–(F)**, process-affected areas were existed along with punched hole, respectively. Especially, the highly 5° misorientation angle color red has spread along with the wall of punched hole at each samples.

## **5.4 Deformation and transformation mechanism in micropunching of FGSS304**

**Figure 13** shows the total frequency of *α*′*-*phase and total amount of misorientation angle that calculated from the sample shown in **Figure 12** and the other two samples. From **Figure 13(A)**, Z minimizes at the grain size of 3 μm. From **Figure 13(B)**,

*Engineering Steels and High Entropy-Alloys*

**10**

**Figure 9.**

**Figure 8.**

**Table 3.**

Tensile strength [N/mm2

0.2% proof stress [N/mm2

from **Figure 11**, the standard deviation of burnished surface ratio took a different trend [24]. The grain size 3.0 μm took a smallest standard deviation. It was found

*Constitution of sheared surface and SEM images of cross section of micropunched hole. (A) Constitution of sheared surface, (B) Grain size 1.0μm, (C) Grain size 1.5μm, (D) Grain size 3.0μm, (E) Grain size 7.5μm.*

Why the stability of burnished surface length changed with grain size? **Figure 12** shows the EBSD analysis results of punched samples with three different varying grain sizes of 1.5, 3.0, and 7.5 μm, respectively. The punching conditions were following; the material thickness is 100 μm, the punch diameter, 80 μm, the die diameter, 85 μm,

that the stability of the sheared surface length changing with grain size.

*EBSD inverse pole figure of four grain size austenitic stainless steel AISI304 including FGSS304.*

*Mechanical characteristics of four grain size austenitic stainless steel AISI304 including FGSS304.*

**Grain size [μm] 1.0 1.5 3.0 7.5**

Elongation [%] 6.0 45.8 49.5 56.6 Hardening exponent [−] — 0.31 0.37 0.43 Vickers hardness [HV] 350 261 227 191

] 1181 875 845 803

] 1000 599 504 433

**5.3 Process affected zone in micropunched hole**

**Figure 12.** *EBSD analysis results of cross section of punched hole.*

#### **Figure 13.**

*Total frequency of α*′*-phase and total amount of misorientation angle in the cross section of punched hole. (A) Total frequency of α*′ *phase and (B) total amount of misorientation angle.*

Θ maximizes at the grain size of 1.5 μm; Θ1.5 μm is more than Θ3.0 μm by 24 %. When choosing the grain size 1.5 μm, small grain size has limited work hardening ability. So that, strain-induced transformation works actively and the frequency of

**13**

**Figure 14.**

*Developed full-martensitic stainless steel AISI304 (WC).*

*Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals*

*α*′ phase more increase than the other large grains as shown in **Figure 13(A)**. In our experiment, full-*α*′ phase material has only 1.2% elongation and 1877 MPa tensile strength. This smallest elongation and higher tensile strength make fracture surface longer than the other large grains as shown in **Figure 10**. When choosing the grain size of 7.5 μm, the stability of sheared surface length become unstable as shown in **Figure 11**. These characteristics is related to the effect of distribution variation in strain-induced martensitic transformation [26, 27]. If choice the grain size 3.0 μm, the work hardening ability is maintained during micropunching and effect of distribution variation in strain-induced martensitic transformation decrease with the grain size decreasing. Therefore, the deformation and the transformation characteristics are optimized and the stability of sheared surface length is minimized as shown in **Figure 11**. For the stable punching, selecting a grain size 3.0 μm is considered to be

Fine-grained microstructure in FGSS changes the deformation and phase transformation characteristics in the punching process. In particular, the sheared surface length is more stabilized to improve the punched-out product quality. The punching

In the solid-phase diffusion bonding process, the holding temperature (TH) plays a role to govern the microstructure. When TH is higher than the recrystallization temperature, the crystal grain easily grows to be coarse one within the bonding time and reduces the strength. Hence, TH must be lowered as possible to maintain high strength in the mechanical characteristics. In particular, this task in the diffusion bonding of AISI304 must also solve an issue to remove the passive film at low temperature. In the conventional process, TH becomes higher than 1100 K [28] to eliminate the oxide film on the stainless steel and to accelerate the bonding.

In recent years, new materials have been developed. **Figure 14** shows the newly developed full-martensitic stainless steel (hereafter called WC). This material can accelerate the diffusion bonding at low temperature by introducing a large amount of strain into austenitic stainless steel before bonding. The process of introducing distortion into the material was carried out as follows and as shown in **Figure 14** [29]. First, blocks of AISI304 were cut into 40 × 40 × 20 mm samples (**Figure 15(A)**). The samples were then compressed, cut, and rolled into a 10-mm-thick sheet, so that the equivalent strain in the compression direction was 90% at 573 K (less than

*DOI: http://dx.doi.org/10.5772/intechopen.89754*

effective at the AISI304 micropunching.

**6. Diffusion bonding of FGSS**

process can be advanced by using FGSS in practice.

**5.5 Summary**

*Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals DOI: http://dx.doi.org/10.5772/intechopen.89754*

*α*′ phase more increase than the other large grains as shown in **Figure 13(A)**. In our experiment, full-*α*′ phase material has only 1.2% elongation and 1877 MPa tensile strength. This smallest elongation and higher tensile strength make fracture surface longer than the other large grains as shown in **Figure 10**. When choosing the grain size of 7.5 μm, the stability of sheared surface length become unstable as shown in **Figure 11**. These characteristics is related to the effect of distribution variation in strain-induced martensitic transformation [26, 27]. If choice the grain size 3.0 μm, the work hardening ability is maintained during micropunching and effect of distribution variation in strain-induced martensitic transformation decrease with the grain size decreasing. Therefore, the deformation and the transformation characteristics are optimized and the stability of sheared surface length is minimized as shown in **Figure 11**. For the stable punching, selecting a grain size 3.0 μm is considered to be effective at the AISI304 micropunching.

#### **5.5 Summary**

*Engineering Steels and High Entropy-Alloys*

**12**

**Figure 13.**

**Figure 12.**

*EBSD analysis results of cross section of punched hole.*

Θ maximizes at the grain size of 1.5 μm; Θ1.5 μm is more than Θ3.0 μm by 24 %. When choosing the grain size 1.5 μm, small grain size has limited work hardening ability. So that, strain-induced transformation works actively and the frequency of

*(A) Total frequency of α*′ *phase and (B) total amount of misorientation angle.*

*Total frequency of α*′*-phase and total amount of misorientation angle in the cross section of punched hole.* 

Q

Fine-grained microstructure in FGSS changes the deformation and phase transformation characteristics in the punching process. In particular, the sheared surface length is more stabilized to improve the punched-out product quality. The punching process can be advanced by using FGSS in practice.
