Influence of the Gas Pressure of Plasma Nitriding on the Structural, Mechanical and Tribological Surface Properties of AISI 316L

Abstract

In this paper, the influence of gas nitriding (80% H 2-20% N 2) pressure (between 3 and 7 Torr) on the modification of surface properties of AISI 316L stainless steel is examined. The structural, mechanical, tribological and electrochemical properties of the layer produced in disk samples were studied. The samples' characteristics were determined by scanning electron microscopy, X-ray diffraction, electrochemical potential curves, microhardness, roughness and wear tests. When pressure increases, the microhardness, elasticity and thickness also increase. The maximum thickness of the nitrided layer was 6.4 µm, the hardness reached 2200 HV 0.0036 and the mass loss decreased until 1% relatively to the unnitrided sample. The X-ray diffractograms showed the presence of the expanded austenite in all the samples. The correlation between these diffractograms and the corrosion results corroborate to the most recent previous conclusions of our systematic study, which evince the interconnection between the improvement of the corrosion resistance and the presence of nitrides on the top of the nitrided layer. More significant presence of such nitrides was observed for the sample nitrided at 6 Torr, whose corrosion performance was better than for the other samples. This pressure seems to be the most indicated to improve the properties under study. Moreover, the results verified not only that the sample geometry is an important factor for enhancing these properties but also that the border effect, which causes the rings on the disk border, depends on the pressure, but not on temperature or time from the plasma nitriding process.

Study of the Thermochemical Surface Treatment Effect on the Phase Precipitation and Degradation Behaviour of DSS and SDSS

“…It is relevant to discuss the oscillatory region observed between ~0.0 and ~0.3 V for the carburized samples. Such behaviour is observed previously [42,43] and is typical of metals immersed in chloride solutions. However, the DSS C + TT sample does not present such behaviour.…”

Laser Surface Alloying of Austenitic 316L Steel with Boron and Some Metallic Elements: Properties

“…The various physical techniques of surface treatment of austenitic steel were compared in Table 1 , taking into account the maximal hardness and averaging depths of the produced surface layers. The maximal hardness of laser-alloyed layers with boron (595–796 HV0.1) or with boron and selected metallic elements (675–911 HV0.1) ([ 60 ] and this work) was relatively low in comparison with the surface layers, produced on the austenitic stainless steel using other physical techniques such as LTPGN processes (572–2175 HV or 720–1100 HK at different loads) [ 2 , 3 , 5 , 7 , 8 , 9 , 10 , 23 ], LTPGNC process (962 HV) [ 7 ], HTPGN processes (1060–1340 HV) [ 3 , 5 , 14 , 15 ], LTPGC process (11–11.8 GPa) [ 26 ] or PPB process (28.093 GPa) [ 31 ]. Many hybrid treatments with the use of plasma processes also resulted in higher hardness of the fabricated surface layers, e.g., shot peening (SP) followed by LTPGN or sequential LTPGC and LTPGN (1615–1662 HV or 7.5–11.5 GPa, respectively) [ 17 , 19 ], cold spraying (CS) of 316L steel followed by LTPGN, LTPGC and LTPGNC processes or their various combinations (800–1350 HV) [ 21 ], LTPGN process followed by a multi-arc ion plating (MAIP) (2280 HV) [ 23 ] as well as TiN coatings produced by PVD technique (18.7–26 GPa) [ 23 , 24 ].…”

“…The different techniques and methods of wear evaluation were reported in literature data regarding the wear behavior of the surface layers produced in austenitic steels. The samples, subjected to LTPGN process, were tested using the “block-on-ring” technique with AISI 52100 steel as a ring-shaped counter-sample [ 2 , 3 ], “ball-on-disc” method using AISI 52100 steel [ 5 , 9 ] or Al 2 O 3 [ 7 ] in the shape of ball as a counter-sample or “pin-on-disc” technique using AISI 1045 steel as a counter-sample in the shape of pin [ 8 ]. Unfortunately, in the case of very interesting technique of LTPGN using an active screen [ 10 , 11 , 12 , 13 ] the wear resistance of the layers was not studied.…”

“…Therefore, the only way to harden such a steel is via adequate surface treatment in order to produce hard and wear resistant surface layers. It is relatively easy using the physical techniques of surface treatment, especially if the surface is saturated with nitrogen, carbon or boron under glow discharge conditions [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. Such techniques are also called plasma or ion processes [ 32 ].…”

“…All the techniques, mentioned above, were applied in order to increase the hardness of the surface layers produced, and, consequently, to improve the wear resistance of the austenitic stainless steels. However, the experimental procedure of wear tests differed, including “block-on-ring” [ 2 , 3 , 31 , 52 , 60 ], “ball-on-disc” [ 5 , 15 , 21 , 22 , 23 , 38 , 53 , 62 ] as well as “pin-on-disc” [ 8 , 20 , 41 , 57 , 61 ] techniques. The tribological properties of the surface layers produced on the 316L steel were evaluated using various measured quantities, such as linear wear [ 2 ], volumetric wear [ 9 , 19 , 39 , 44 , 45 , 62 ], volumetric wear per unit of axial force and friction track (also called specific wear rate [ 7 , 20 , 21 , 22 , 35 , 40 , 54 , 61 ]), mass loss [ 8 , 41 ], relative mass loss [ 60 ], mass loss per unit of friction track (also called specific wear rate [ 53 ]), coefficient of friction (CoF) [ 7 , 8 , 9 , 16 , 37 , 41 , 54 , 57 ], percentage of the volume removed on carburized samples regarding the noncarburized material […”