Hiroyuki Ogiyama

Surface Roughness and Initial Pressure Effect on Superplastically Carburized Duplex Stainless Steel

Superplastic carburizing (SPC) is a carburizing process that combines carburizing with superplastic deformation. Since SPC involves direct interaction between the superplastically deformed surface and the solid carbon medium, the effect of surface roughness on the process cannot be disregarded. This paper presents the study of surface roughness and initial pressure effects on superplastic carburizing of duplex stainless steel (DSS). SPC was conducted under four different surface roughness (Ra) conditions of 0.9, 0.3, 0.1 and 0.03 μm. The microstructure, surface hardness, and carburized layer thickness were studied. Comparisons were also done on non-superplastic material which has a coarse microstructure. The results showed that the surface roughness strongly affected the properties of the superplastically carburized duplex stainless steel while its effects on the non-superplastic material were not that obvious.

Effects of Superplasticity in Boronizing of Duplex Stainless Steel

In this research, conventional boronizing process (CB) and a new method of boronizing process under compression load condition (LB) were conducted and compared in order to study the effect of superplasticity on boronized substrate. Both processes were conducted on duplex stainless steel (DSS) with two different microstructures; as-received DSS with coarse grain microstructure (CDSS); and thermo-mechanically treated DSS with fine grain microstructure (FDSS) which can show superplastic behavior at high temperatures. Both processes were conducted at duration of 6 hours and temperatures between 1123 and 1223 K. All of boronized specimens demonstrated thin, smooth and compact morphology of boride layer. For CDSS, both CB and LB processes produced about similar surface hardness values within the range of 1425 – 2330 HV. For FDSS, CB process produced surface hardness between 1522 and 2601 HV, while under LB, the highest surface hardness values in the range of 1659 - 2914 HV were obtained. The result shows that introduction of load during boronizing has initiated superplastic deformation on FDSS thus accelerated diffusion of boron atoms into surface which finally lead to significantly higher surface hardness.

Compression Method for Superplastic Boronizing of DSS

Superplastic boronizing (SPB) is a new surface hardening technique utilizing the ultra high plasticity phenomenon in metals in carrying out boronizing process. In boronizing, boron atoms are diffused into the metal substrate to form a hard boride layer. In this research, a new compression method for the SPB process was introduced. A clamp with an initial compressive load of about 1960 N was used. Thermo-mechanical treated duplex stainless steel (DSS) with fine grain microstructure which can show superplastic behavior at high temperatures was used as the superplastic material. SPB experiments were conducted at temperatures between 1123 and 1223 K for durations of 1 - 6 hours. The boronized specimens demonstrated thin, smooth and compact morphology of boride layer. The boride layer thickness was within ±10 0m - ±46.2 0m. On the boride layer, only the favorable single phase of Fe2B was detected. High value of surface hardness was observed in the range of ±847 HV - ±2914 HV. The overall results from the study show that the SPB process can significantly improve the surface properties of DSS.

Enhancing Surface Properties of Fine Grain Duplex Stainless Steel via Superplastic Carburizing

In this research, a new type of surface carburizing method which combines superplastic phenomenon and carburizing process called superplastic carburizing (SPC) was introduced. Thermo-mechanically treated duplex stainless steel (DSS) with fine grain microstructure and exhibits superplasticity was used as the superplastic material. The SPC was conducted at temperatures ranging from 1123 K to 1223 K for various durations. Initial loads of 25 MPa, 49 MPa and 74 MPa were applied to give the superplastic deformation effect on the carburized specimens. Metallographic studies revealed a thick, uniform, smooth and dense morphology of hard carbon layer formed at the surface of fine grain DSS. The resulting case depth of carbon layers were between 15 μm to 76 μm. A remarkable increase in surface hardness was observed in the range of 600 HV to 1600 HV. The kinetics of this process in terms of carbon diffusion and its variation with processing time and temperature was achieved using Arrhenius equation. Activation energy (Q) was determined as 151.87 kJ/mol. Based on the results obtained, SPC process can significantly enhance the surface properties of DSS.