**7.4 Plasma nitrided FGSS316 wires at 623 K**

FGSS316 wire is expected to be working as a tension member and component for reinforcement of prestressed concrete and for steel wires in medical equipment. These members often experience the friction and wear in severe surface chemical conditions; they have to be surface treated to improve their wear resistance and corrosion toughness. The low temperature plasma nitriding at 623 K is also effective to improve the surface hardness as well as the electro-chemical properties. FGSS316 wire with the diameter of 2.6 mm was prepared for plasma nitriding at 623 K for 14.4 ks by 70 Pa.

**Figure 25** depicts the SEM image and nitrogen mapping on the cross section of FGSS316 wire. The wire surface is homogeneously processed to form the nitrided layer with high nitrogen concentration. The layer thickness approaches to 35 μm, which is a bit less than that seen in **Figures 22** and **23** for nitrided FGSS316 plates. A hard wire often experiences the bending and wiping as well as the tensile stretching in its usual operation. Owing to the surface hardening of wire by the plasma nitriding at 623 K, the bending and torsion rigidities as well as the tensile rigidity are expected to increase [48].

#### **7.5 Summary**

The plasma nitriding in the lower temperature regime than 700 K provides a means to modify the microstructure of FGSS by nitrogen alloying and to

#### **Figure 24.**

*Efficient production of the multi-head punch by the plasma nitriding with use of the plasma nitriding at 673 K. (A) Meshing pattern on the screen, (B) plasma nitrided pattern on the FGSS316 die, (C) three dimensionally textured FGSS316, (D) microtextured FGSS316 punch, and (E) meshing-texture arrayed punch.*

**21**

industries and medicals.

and medical equipments.

**8. Conclusion**

**Figure 25.**

*mapping on the cross section.*

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

significantly improve the mechanical and electro-chemical properties of FGSS. The nitrided FGSS316 layer consists of homogeneous two-phase microstructure with finer grain size than 0.1 μm. Since the original grain size of FGSS316 is 1.7 μm, the strength of nitrided FGSS316 increases by more than 10 times after the Hall-Petch relationship. This surface treatment by low temperature plasma nitriding is available not only in improvement of the wear resistance of dies for microstamping and forging but also in high strengthening of wires for medical equipment. In the former, the micropunch with multi punch head array is fabricated by blasting the un-nitrided substrate surface areas. This plasma printing is responsible to make microtextured die and punch in much less takt time for microembossing and micropiercing the metallic and polymer sheets. In the latter, the stainless steel bars, wires and fibers are surface treated to have higher nitrogen content layer. This nitrogen supersaturated surface layer works as a hardened and strengthened layer even after warm and hot drawing them to smaller diameters. Effect of supersaturated nitrogen distribution in them on their strength and ductility could be of much importance in application to

*Plasma nitrided FGSS316 wire at 623 K for 14.4 ks. (A) Cross-sectional SEM image, and (B) nitrogen* 

Fine-grained stainless steels (FGSS) have grown up as a key material for mechanical and short-pulse laser machining, for metal forming and diffusion bonding, and for surface treatment to fabricate the miniature automotive parts and fine medical components. Their high strength without loss of ductility by grain-size refinement is not only attractive to improve the work and die material performance but also effective to enhance the workability in their integrated manufacturing. Smooth finish in mechanical machining as well as deep cutting by laser machining are intrinsic to FGSS. Precise shaping by micropiercing as well as low temperature diffusion bonding are also unique to FGSS. In their plasma nitriding, the holding temperature is significantly lowered down to 623 K for homogeneous formation of the hardened layer by 1400 HV with the thickness of 40 μm. Each advancement in this integrated manufacturing is fused and edited to construct the production line toward FGSS-based fine medical tools and precise mechanical parts. In particular, FGSS bars, wires, sheets, and plates transform to high strength, precisely shaped reliable media in mechanical units

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

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

**Figure 25.**

*Engineering Steels and High Entropy-Alloys*

**7.4 Plasma nitrided FGSS316 wires at 623 K**

actual machining.

**7.5 Summary**

total machining time is estimated to be 1, 280 ks, or 356 h. This micromachining provides no solution to fabrication of the multi-arrayed punch with regular-square meshing heads. After [46, 47], the plasma printing method with use of the low temperature plasma nitriding provides a solution to fabricate this die as shown in **Figure 24**. A meshing pattern is first printed by using the screen with a unit pattern in **Figure 24(A)**. The unprinted cross-meshing surfaces are selectively nitrided at 673 K for 14.4 ks ("C" in **Figure 18**) in **Figure 24(B)**. After sand-blasting, this unit pattern transforms to **Figure 24(C)**. The microtexture arrayed punch is fabricated as shown in **Figure 24(D)** to have regular meshing texture in **Figure 24(E)**.

The takt time of this processing is only 18 ks, 70 times shorter than micromilling even excluding the cutting tool life as well as the preparation for CAM data before

FGSS316 wire is expected to be working as a tension member and component for reinforcement of prestressed concrete and for steel wires in medical equipment. These members often experience the friction and wear in severe surface chemical conditions; they have to be surface treated to improve their wear resistance and corrosion toughness. The low temperature plasma nitriding at 623 K is also effective to improve the surface hardness as well as the electro-chemical properties. FGSS316 wire with the diameter of 2.6 mm was prepared for plasma nitriding at 623 K for 14.4 ks by 70 Pa. **Figure 25** depicts the SEM image and nitrogen mapping on the cross section of FGSS316 wire. The wire surface is homogeneously processed to form the nitrided layer with high nitrogen concentration. The layer thickness approaches to 35 μm, which is a bit less than that seen in **Figures 22** and **23** for nitrided FGSS316 plates. A hard wire often experiences the bending and wiping as well as the tensile stretching in its usual operation. Owing to the surface hardening of wire by the plasma nitriding at 623 K, the bending and torsion rigidities as well as the tensile rigidity are expected to increase [48].

The plasma nitriding in the lower temperature regime than 700 K provides a means to modify the microstructure of FGSS by nitrogen alloying and to

*Efficient production of the multi-head punch by the plasma nitriding with use of the plasma nitriding at 673 K. (A) Meshing pattern on the screen, (B) plasma nitrided pattern on the FGSS316 die, (C) three dimensionally textured FGSS316, (D) microtextured FGSS316 punch, and (E) meshing-texture arrayed punch.*

**20**

**Figure 24.**

*Plasma nitrided FGSS316 wire at 623 K for 14.4 ks. (A) Cross-sectional SEM image, and (B) nitrogen mapping on the cross section.*

significantly improve the mechanical and electro-chemical properties of FGSS. The nitrided FGSS316 layer consists of homogeneous two-phase microstructure with finer grain size than 0.1 μm. Since the original grain size of FGSS316 is 1.7 μm, the strength of nitrided FGSS316 increases by more than 10 times after the Hall-Petch relationship. This surface treatment by low temperature plasma nitriding is available not only in improvement of the wear resistance of dies for microstamping and forging but also in high strengthening of wires for medical equipment. In the former, the micropunch with multi punch head array is fabricated by blasting the un-nitrided substrate surface areas. This plasma printing is responsible to make microtextured die and punch in much less takt time for microembossing and micropiercing the metallic and polymer sheets. In the latter, the stainless steel bars, wires and fibers are surface treated to have higher nitrogen content layer. This nitrogen supersaturated surface layer works as a hardened and strengthened layer even after warm and hot drawing them to smaller diameters. Effect of supersaturated nitrogen distribution in them on their strength and ductility could be of much importance in application to industries and medicals.

### **8. Conclusion**

Fine-grained stainless steels (FGSS) have grown up as a key material for mechanical and short-pulse laser machining, for metal forming and diffusion bonding, and for surface treatment to fabricate the miniature automotive parts and fine medical components. Their high strength without loss of ductility by grain-size refinement is not only attractive to improve the work and die material performance but also effective to enhance the workability in their integrated manufacturing. Smooth finish in mechanical machining as well as deep cutting by laser machining are intrinsic to FGSS. Precise shaping by micropiercing as well as low temperature diffusion bonding are also unique to FGSS. In their plasma nitriding, the holding temperature is significantly lowered down to 623 K for homogeneous formation of the hardened layer by 1400 HV with the thickness of 40 μm. Each advancement in this integrated manufacturing is fused and edited to construct the production line toward FGSS-based fine medical tools and precise mechanical parts. In particular, FGSS bars, wires, sheets, and plates transform to high strength, precisely shaped reliable media in mechanical units and medical equipments.
