**7. Conclusion**

In the metal forming like intense rolling and fine piercing, the microstructure of work materials is changed by the applied plastic distortion with less influence to tool materials. In the rolling process, the original austenitic phase of stainless steels changes to be nearly full martensitic and to have textured microstructure with the preferred orientation to the rolling direction. This crystallographic structuring is intrinsic to the microstructure change by shearing with the reduction of thickness. In the piercing process by shear localization, the austenitic work material after piercing has new sheared and fractured surfaces including the affected zones. These zones consist of the phase-transformed martensite, the work-hardened austenite, and the elastically recovered zones. This crystallographic structure change is precisely described by EBSD on the cross-section of pierced work materials. In addition, various factors influence on this structure change including the grain size of work materials and the shear localization control as well as the chemical components in stainless steels.

In the low temperature plasma nitriding, no plastic strains are externally applied to work materials but nitrogen interstitial atoms are distributed from their surface to their depth with high concentration. Owing to the synergetic process in this inner nitriding process, the plastic distortion is concurrently induced by nitrogen diffusion and supersaturation. Since the nitrogen solute is homogeneously distributed in the nitrided layer with high content, the plastic distortion tensor also uniformly distributes in this layer. This homogeneous plastic distortion changes the normal crystallographic structure of AISI316 plates and wires; e.g., fine-grained AISI316 (FGSS316) microstructure of wires with the average grain size of 2 μm changes to the super-fine grained, two phase structure with the average grain size less than 0.1 μm. During this homogeneous nitriding, the retained austenitic zones distribute in the nitrided layer.

A priori nitriding to cold metal forming is a way to significantly control the microstructure and mechanical properties. The nitrided FGSS316 wire is elastoplastically strained in the uniaxial direction so that the whole nitrided layers have fine-grained two-phase structure without retained austenite. This microstructure evaluation in local reflects on the homogeneous increase of hardness in the nitrided layer. This local interaction between nitrogen solute mobility and externally applied plastic strains at room temperature reveals that the microstructure and mechanical properties of nitrided work materials could be modified and improved by the metal forming posterior to the nitriding. In particular, the warm and hot post-treatment

**73**

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing*

supersaturation also plays a role to control the crystallographic structure.

by drawing, rolling, forging, and stamping has capability to control the crystallo-

Posterior nitriding to metal forming is another way to refine the microstructure and to improve the mechanical properties. In case when the rolled AISI304 plate is further nitrided at 673 K, its textured microstructure completely changes to super-fine grained, two phase structure. The intense plastic straining by nitrogen

The authors would like to express their gratitude to Mr. T. Inohara (LPS-Works, Co., Ltd.), Mr. T. Yoshino, and Y. Suzuki (Komatsu-Seiki Kosakusho, Co., Ltd.) for their help in experiments. This study was financially supported by the METI-

*DOI: http://dx.doi.org/10.5772/intechopen.91281*

**Acknowledgements**

**Conflict of interest**

**Author details**

Tatsuhiko Aizawa1

graphic structure of nitrided parts and components.

Program on the Supporting Industries at 2019.

The authors declare no conflict of interest.

\*, Tomomi Shiratori<sup>2</sup>

1 Surface Engineering Design Laboratory, SIT, Tokyo, Japan

3 Komatsu Seiki Kosakusho, Co., Ltd., Suwa, Nagano, Japan

\*Address all correspondence to: taizawa@sic.shibaura-it.ac.jp

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 University of Toyama, Toyama, Japan

provided the original work is properly cited.

and Takafumi Komatsu3

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing DOI: http://dx.doi.org/10.5772/intechopen.91281*

by drawing, rolling, forging, and stamping has capability to control the crystallographic structure of nitrided parts and components.

Posterior nitriding to metal forming is another way to refine the microstructure and to improve the mechanical properties. In case when the rolled AISI304 plate is further nitrided at 673 K, its textured microstructure completely changes to super-fine grained, two phase structure. The intense plastic straining by nitrogen supersaturation also plays a role to control the crystallographic structure.
