In this study, expanded austenite was prepared by an industrial low-temperature plasma nitriding process in 304L and 904L austenitic stainless steels. The current investigation focuses on the assessment of the thermal stability and related phase evolution of the expanded austenite layer during isothermal annealing in protective argon atmosphere at temperatures ranging from 450 to 600°C for 24 and 168 h. Characterisation of the original expanded austenite and the decomposed surface layers was performed. Denitriding, inward N-diffusion and Cr-compounds precipitation occurred at different extent, depending on annealing conditions and alloy composition. Expanded austenite in 304L exhibited a near complete eutectoid decomposition after a short annealing time, while 904L showed significantly better thermal stability. A fine dispersion of small CrN precipitates resulting from expanded austenite decomposition at relatively low annealing temperature or short duration could further positively affect the surface hardness of both materials. Precipitate growth reduces hardness at higher annealing temperatures/times.
LiX. Y., ThaiwatthanaS., DongH. and BellT.: ‘Thermal stability of carbon S phase in 316 stainless steel’, Surf. Eng., 2002,
18, (6), 448–451. doi: 10.1179/026708402225006259
2.
SunY. and BellT.: ‘Dry sliding wear resistance of low temperature plasma carburised austenitic stainless steel’, Wear, 2002,
253, (5–6), 689–693. doi: 10.1016/S0043-1648(02)00170-9
3.
MentheE., BulakA., OlfeJ., ZimmermannA. and RieK.: ‘Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding’, Surf. Coat. Technol., 2000,
133–134, 259–263. doi: 10.1016/S0257-8972(00)00930-0
4.
YangW. J., ZhangM., ZhaoY. H., ShenM. L., LeiH., XuL., XiaoJ. Q., GongJ., YuB. H. and SunC.: ‘Enhancement of mechanical property and corrosion resistance of 316L stainless steels by low temperature arc plasma nitriding’, Surf. Coat. Technol, 2016,
298, 64–72. doi: 10.1016/j.surfcoat.2016.04.045
5.
AdachiS. and UedaN.: ‘Combined plasma carburizing and nitriding of sprayed AISI 316L steel coating for improved wear resistance’, Surf. Coat. Technol., 2014,
259, 44–49. doi: 10.1016/j.surfcoat.2014.07.010
6.
BorgioliF., GalvanettoE. and BacciT.: ‘Low temperature nitriding of AISI 300 and 200 series austenitic stainless steels', Vacuum, 2016,
127, 51–60. doi: 10.1016/j.vacuum.2016.02.009
7.
AdachiS. and UedaN.: ‘Formation of expanded austenite on a cold-sprayed AISI 316L coating by low-temperature plasma nitriding’, J. Therm. Spray Technol., 2015,
24, (8), 1399–1407. doi: 10.1007/s11666-015-0278-3
8.
Díaz-GuillénJ. C., Vargas-GutiérrezG., Granda-GutiérrezE. E., GonzálezM. A., Díaz-GuillénJ. A. and Alvarez-ContrerasL.: ‘Effects of pulse length on low frequency plasma nitrided 316L steels', Surf. Eng., 2015,
31, (8), 623–627. doi: 10.1179/1743294414Y.0000000432
9.
IchiiK., FujimuraK. and TakaseT.: ‘Structure of the ion-nitrided layer of 18–8 stainless steel’, Technol. Reports Kansai Univ., 1986,
27, 135–144.
10.
ZhangZ. L. and BellT.: ‘Structure and corrosion resistance of plasma nitrided stainless steel’, Surf. Eng., 1985,
1, (2), 131–136. doi: 10.1179/sur.1985.1.2.131
11.
CollinsS. R., WilliamsP. C., MarxS. V., HeuerA., ErnstF. and KahnH.: ‘Low-temperature carburization of austenitic stainless steels', in ‘ASM handbook’, Vol. 4D, ‘Heat treating of irons and steels', (ed. DossettJ. and TottenG. E.), 451–460; 2014, Materials Park, OH
, ASM International.
12.
BosM. J.: ‘Case hardening of austenitic stainless steel pump components', World Pumps, 1998,
1998, (378), 30–34. doi: 10.1016/S0262-1762(99)80557-4
13.
LebrunJ. P.: ‘Applications of low-temperature surface hardening of stainless steels', in ‘Thermochemical surface engineering of steels', (ed. MittemeijerE. J. and SomersM. a. J.), 633–647; 2015, Cambridge
, Woodhead Publishing Limited.
14.
DongH.: ‘S-phase surface engineering of Fe-Cr, Co-Cr and Ni-Cr alloys', Int. Mater. Rev., 2010,
55, (2), 65–98. doi: 10.1179/095066009X12572530170589
15.
MaistroG., NyborgL., VezzùS. and CaoY.: ‘Microstructural characterization and layer stability of low-temperature carburized aisi 304 l and aisi 904 l austenitic stainless steel’, La Metall. Ital., 2015,
11/12, 21–30.
16.
CaoY. and NorellM.: ‘Role of nitrogen uptake during the oxidation of 304L and 904L austenitic stainless steels', Oxid. Met., 2013,
80, (5–6), 479–491. doi: 10.1007/s11085-013-9391-1
17.
CaoY., MaistroG., NorellM., Pérez-GarciaS. A. and NyborgL.: ‘Multi-technique characterization of low-temperature plasma nitrided austenitic AISI 304L and AISI 904L stainless steel’, Surf. Interface Anal., 2014,
46, (10–11), 856–860. doi: 10.1002/sia.5462
18.
MartoranoM. A., TavaresC. F. and PadilhaA. F.: ‘Predicting delta ferrite content in stainless steel castings', Adv. Mater. Forum VI, 2013,
52, 733–738.
19.
OdegardB. C.: ‘Some observations on the formation of surface martensite in 304L stainless steel’, Metallography, 1974,
7, 129–135. doi: 10.1016/0026-0800(74)90002-0
20.
ChristiansenT. and SomersM. A. J.: ‘Low temperature surface hardening of stainless steel’, in ‘Thermochemical surface engineering of steels', (ed. MittemeijerE. J. and SomersM. A. J.), 557–579; 2015, Cambridge
, Woodhead Publishing Limited.
21.
ChristiansenT. and SomersM. A. J.: ‘Controlled dissolution of colossal quantities of nitrogen in stainless steel’, Metall. Mater. Trans. A., 2006,
37, (3), 675–682. doi: 10.1007/s11661-006-0039-5
22.
JegouS., ChristiansenT. L., KlausM., GenzelC. and SomersM. A. J.: ‘Determination of composition, residual stress and stacking fault depth profiles in expanded austenite with energy-dispersive diffraction’, Thin Solid Films, 2013,
530, 71–76. doi: 10.1016/j.tsf.2012.06.029
23.
ChristiansenT. L., HummelshøjT. S. and SomersM. A. J.: ‘Expanded austenite, crystallography and residual stress', Surf. Eng., 2010,
26, (4), 242–247. doi: 10.1179/026708410X12506870724316
24.
PyeD.: ‘Nitriding techniques, ferritic nitrocarburizing, and austenitic nitrocarburizing techniques and methods', in ‘Steel heat treatment metallurgy and technologies', (ed. TottenG. E.), 475–538; 2006, Boca Raton, FL
, CRC Press.
25.
McGuireM. F.: ‘Stainless steels for design engineers', 2008, Materials Park, OH
, ASM International.
26.
KösterK., KaestnerP., BräuerG., HocheH., TroßmannT. and OechsnerM.: ‘Material condition tailored to plasma nitriding process for ensuring corrosion and wear resistance of austenitic stainless steel’, Surf. Coat. Technol., 2013,
228, S615–S618. doi: 10.1016/j.surfcoat.2011.10.059
27.
SpiesH.-J., EcksteinC. and ZimdarsH.: ‘Structure and corrosion behaviour of stainless steels after plasma and gas nitriding’, Surf. Eng., 2002,
18, (6), 459–460. doi: 10.1179/026708402225006286
28.
ChristiansenT. L. and SomersM. A. J.: ‘Decomposition kinetics of expanded austenite with high nitrogen contents', Int. J. Mater. Res., 2006,
97, (1), 79–88.
ÖztürkO. and WilliamsonD. L.: ‘Thermal stability of the high-N solid-solution layer on stainless steel’, Surf. Coat. Technol., 2002,
158–159, 288–294. doi: 10.1016/S0257-8972(02)00185-8
31.
WilliamsonD. L., OzturkO., WeiR. and WilburP. J.: ‘Metastable phase formation and enhanced diffusion in f.c.c. alloys under high dose, high flux nitrogen implantation at high and low ion energies', Surf. Coat. Technol., 1994,
65, (1–3), 15–23. doi: 10.1016/S0257-8972(94)80003-0
32.
ChristiansenT. L. and SomersM. A. J.: ‘Determination of the concentration dependent diffusion coefficient of nitrogen in expanded austenite’, Int. J. Mater. Res., 2008,
99, (9), 999–1005. doi: 10.3139/146.101729
33.
OddershedeJ., ChristiansenT. L., StåhlK. and SomersM. A. J.: ‘Extended X-ray absorption fine structure investigation of nitrogen stabilized expanded austenite’, Scr. Mater., 2010,
62, (5), 290–293. doi: 10.1016/j.scriptamat.2009.11.021
34.
LiX. Y.: ‘Low temperature plasma nitriding of 316 stainless steel – nature of S phase and its thermal stability’, Surf. Eng., 2001,
17, (2), 147–152. doi: 10.1179/026708401101517746
35.
LeeI.: ‘The effect of molybdenum on the characteristics of surface layers of low temperature plasma nitrocarburized austenitic stainless steel’, Curr. Appl. Phys., 2009,
9, (3 Suppl.), S257–S261. doi: 10.1016/j.cap.2009.01.030
36.
ErnstF., CaoY., MichalG. M. and HeuerA. H.: ‘Carbide precipitation in austenitic stainless steel carburized at low temperature’, Acta Mater., 2007,
55, 1895–1906. doi: 10.1016/j.actamat.2006.09.049
