Surface & Coatings Technology 204 (2010) 1375–1379

Contents lists available at ScienceDirect

Surface & Coatings Technology

j ourna l homepage: www.e lsev ie r.com/ locate /sur fcoat

Modifying the structure of glow discharge nitrided layers produced onhigh-nickel chromium-less steel with the participation of an athermalmartensitic transformation

Tomasz Borowski a,⁎, Jerzy Jeleńkowski b, Marek Psoda a, Tadeusz Wierzchoń a

a Faculty of Material Science and Engineering, Warsaw University of Technology, 141 Woloska Str., 02-507 Warsaw, Polandb Institute of Precision Mechanics, 3 Duchnicka Str., 01-796 Warsaw, Poland

⁎ Corresponding author. Tel.: +48 22 234 87 02; fax:E-mail address: borowski.tomasz@wp.pl (T. Borows

0257-8972/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2009.09.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 January 2009Accepted in revised form 8 September 2009Available online 15 September 2009

Keywords:Plasma assisted glow-discharge nitridingHigh-nickel chromium-free steelTRIP effectDeformation martensiteAthermal martensiteNitrogen austenitePrecipitate hardening

High-nickel steels composed of metastable austenite are used in the nuclear power industry, aircraftindustry, arms industry, and also in the manufacture of surgical instruments. These steels can be exploited atboth low and elevated temperatures. Their important advantage lies in that, during plastic deformation,austenite transforms into martensite (TRIP effect). When subjected to a phase transformation combined withprecipitation hardening these steels can achieve very high strength indices (R0.2≈2200 MPa).The material examined was Ni27Ti2AlMoNb steel. The steel was subjected to cold treatment. This treatmentresulted in the formation of athermal martensite (α′a). The treatment conducted in liquid nitrogen gave aphase structure of the duplex type (martensite–austenite). After the transformation, the steel was subjectedto glow discharge-assisted low-temperature nitriding (≤450 °C). The structure thus produced, composed ofthe phase transformed steel substrate and a nitrided surface layer, was complex in respect of its phase aswell as chemical composition and showed unique physical and mechanical properties.By using the martensitic transformation we can modify the depth to which nitrogen diffuses into the steelsubstrate during the glow discharge nitriding. This possibility permits improving its resistance to frictionalwear and corrosion.

+48 22 234 87 05.ki).

l rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

High-nickel TRIP (Transformation Induced Plasticity) steels, whichare chromium-less and practically carbon-free, are still little known asfar as their behavior in thermo-chemical and thermo-mechanicaltreatments are concerned. They have an untypical structure, which inalloys with the nickel content above 25% is built of metastableaustenite, can chiefly be modified through martensitic transformationinduced athermally, isothermally or by cold treatment. An improve-ment of the strength properties of steels with additives such as Ti, Al,Mo and Nb can be achieved by precipitate hardening [1]. The greatestand durable increase of hardness can be obtained when the γ′-Ni3(Ti,Al,Mo,Nb), Ni(AlTi), and Laves phases, which coagulate most slowly,are formed. Aluminum which is present in the γ′ phase, in additionopposes the formation of the η phase (Ni3Ti). The TRIP effect alsooccurs in this steel, which means that the martensitic transformationinduced by plastic deformation may take place here. An importantparameter of this transformation is the temperature Md, above which

austenite does not transform into deformation martensite. Steels withthese properties have been used in nuclear power industry, aircraftindustry, arms industry and in the production of medical instruments[1].

By combining low-temperature glow discharge assisted nitriding[2–4] with a martensitic transformation we can produce layersconsiderably thicker and with a higher concentration of the diffusingelement (and hence with better strength properties). The participa-tion of the martensitic transformation in addition permits us tomodify the structure and the properties of the steel core. A athermalor deformation-induced transformation can also modify the magneticproperties of the steels [5].

Every surface treatment of the high-nickel TRIP steel results indeformation martensite being formed in the surface zone. Evenmechanical polishing yields martensite (up to 40%) in a thin (1 µmthick) surface layer. The most effective treatment is sliding burnishingwhich leads to the formation of martensite in the amount even above90% (Fig. 1).

The results of investigations presented in this paper constitute acomplement to the up-to-date knowledge concerning nitriding ofTRIP steels [6–9] and give an estimate how the athermal martensite,produced by cooling, affects the formation of surface layers during thelow-temperature glow discharge-assisted nitriding process.

Fig. 1. Content of the deformation martensite produced in the surface layer of steel bypolishing and slide burnishing (determined by grazing angle geometry X-ray diffraction)[10]. Fig. 2. Dilatometric diagramobtained for Ni27Ti2AlMoNb steel; steel heated to 180 °C at a

rate of 2 °C/min, then cooled together with the furnace and then in liquid nitrogen (curvedenoted by LN2).

1376 T. Borowski et al. / Surface & Coatings Technology 204 (2010) 1375–1379

2. Examination methods

The samples were made of austenitic chromium-less Ni27Ti2Al-MoNb steel. The chemical composition of the steel (Table 1) wasdesigned so as to obtain metastable austenite with the temperatureMs lower by several tens of centigrades than room temperature, andso that the austenite undergoes transformation when subjected toplastic deformation and is effectively precipitate-hardened while theprecipitation of the disadvantageous η phase (Ni3Ti) is hampered.

Since the as-delivered material had a heterogeneous structurechiefly composed of martensite with high segregation of nickel, it wassubjected to homogenizing annealing at a temperature of 1250 °C in anargon atmosphere for 10 h and then cooled inwater. This process gavea structure with a uniform chemical composition, which is built ofsupersaturated metastable austenite, unstable also in mechanicalterms.

The critical temperatures of the Ni21Ti2AlMoNb steel weredetermined using the dilatometric method (Fig. 2). Steel with thebinary (duplex) structure composed of athermal martensite andaustenitewas heated to 950 °C, cooled in the furnace andfinally placedin liquid nitrogen for further cooling.The sampleswere prepared in thetwo variants:

1) Steel with an austenitic structure ground with an abrasive papergrade 800 (Fig. 3a)

2) Steel cooled in liquid nitrogen, and ground using a grade 800abrasive paper (Fig. 3b)

Cooling in LN2 gave a martensite–austenite structure with the 51%martensite content. The corrosion resistance of this ‘duplex’ steel wasconsiderably better than that of steel with an austenitic structure [11].

All the samples (both variants) were subjected to glow dischargeassisted nitriding at a temperature of 450 °C in a nitrogen–hydrogen(1:1) atmosphere (Fig. 3) for 6 h. The process temperaturewas selectedso as not to exceed the temperature As of the inverse transformation(Fig. 2). In order to study the mechanism of the formation of theindividual phase components in the nitrided layers produced at highertemperatures, we also examined, by X-ray diffraction, the phasecomposition of steels nitrided at 550 °C, i.e. above the temperature ofthe inverse transformation.

Table 1Chemical composition of Ni27Ti2AlMoNb steel (wt %).

C P S Mn Si Ni Ti Mo Al Nb Fe

0.02 0.009 0.006 0.28 0.50 26.74 2.40 0.78 1.29 0.19 Balance

The microstructure of the nitrided layers and the steel substratewas revealed using a Mi16%Fe (16.7vol.%HNO3, 33.3vol.%HF, 50vol.%H2O) etching solution, and then observed in an Olympus IX 70 opticalmicroscope and a Hitachi S-3500N scanning electron microscope. Thechemical composition was analyzed by wave-dispersion X-rayspectroscopy (WDS) using a Cameca SU-30 X-ray micro-analyzer.The constituent phases were identified in a Philips PW 1830 X-raydiffractometer using CuKα radiation whose penetration depth intothe nitrided layers did not exceed 4 µm.

3. Results

Fig. 3 shows the microstructures of the layers produced on theNi27Ti2AlMoNb steels, one with an austenitic structure (Fig. 3c) andthe other with a duplex-type martensite–austenite structure (Fig. 3d)by glow discharge assisted nitriding at a temperature of 450 °C.

The thickness of the nitrided layer produced on the austenitic steelis about 22 µm (Fig. 4a) and that of the layer on the duplex steelranges from 37 to 43 µm (Fig. 4b). It can be seen that the diffusiondepth of nitrogen depends significantly on the structure of the steel.

We can see in Fig. 5 that, in the austenitic steel, the nitrogencontent in the surface zone of the nitrided layer is about 25 at.% and itdecreases continuously along the layer depth down to about 22 µm.No rapid changes of the concentration of any element present in thelayer are observed which may suggest that no zones composed ofvarious compounds have formed. In the layer nitrided on the duplexsteel the fluctuations of the nitrogen concentration are greater whichcan be attributed to the difference in the nitrogen diffusion ratesbetween austenite which has an fcc crystalline lattice and martensitewith its bcc lattice (Fig. 6).

The concentration of nitrogen in the layer is the principal factorthat affects its hardness, but in the duplex steel the hardness of thenitrided layer is also determined by the martensite–austenitestructure: with this steel the hardness of the nitrided layer exceeds1300HV0.2 which is higher by 150HV0.2 than the hardness of thenitrided layer on the austenitic steel (Fig. 7). This is so since, duringthe nitriding, the hardness of the austenitic steel core practicallyremains unchanged whereas with the duplex steel it increases byalmost 200HV0.2. This drastic difference can be attributed to thepresence of numerous defects in the duplex steel. It contains muchmore grain boundaries, greater dislocation density and higherretained stress state, which is due to the martensitic deformationand the phase compression that occurs during this transformation.These numerous defects favor the nucleation of new phases,

Fig. 3. Flow chart of the nitriding process applied to Ni27Ti2AlMoNb steel: a) austenitic steel, b) martensitic–austenitic steel, c) nitrided austenitic steel, d) nitrided martensitic–austenitic steel.

1377T. Borowski et al. / Surface & Coatings Technology 204 (2010) 1375–1379

especially at the grain boundaries, in dislocations and in other defectsand discontinuities of the crystalline lattice [12,13].

In the austenitic steel, the glowdischarge assisted process results innitrogen austenite with a lattice parameter of 3.84 Å being formed in

Fig. 4. SEM image of the nitrided layers produced on steel: a) austenitic, b) martensitic–austenitic (duplex) at a temperature of 450 °C; polished non-etched samples, phasecontrast due to a BSE detector.

the surface layer, whereas in the duplex steel we have there nitrogenaustenite with two lattice parameters: 3.84 Å and 3.8 Å (Fig. 8). Thenitrogen austenite with the smaller lattice parameter probably formsdue to the inverse transformation that takes place in the duplex steelduring the nitriding. During this treatment, martensite is saturatedwith nitrogen which results in its stabilization. As steel is beingsaturated with nitrogen, the temperatures Ms and As decrease and,when the temperature As falls below the nitriding temperature i.e.450 °C, the inverse transformation takes place giving nitrogen-saturated austenite. The austenite with the greater lattice constant,on the other hand, forms from the nitrided primary austenite.

The analysis of the phase composition also revealed a smallamount of retained austenite left in the surface zone of the steel(chiefly in its duplex version) after the nitriding. This may beexplained by supposing that the austenite formed due to the inversetransformation has not been wholly nitrided.

In the nitrided layers produced at a temperature above thebeginning of the austenitic transformation but below its completion,the composition of the diffusion layers appeared to be complex(Fig. 9). The nitrogen austenite formed in the duplex steel hasevidently two different lattice parameters and contains a smallamount of retained austenite.

4. Conclusions

1. The presence of non-thermal martensite in high-nickel steelincreases the diffusion depth of nitrogen during glow dischargeassisted nitriding.

2. Duplex steel undergoes precipitation hardening much moreeffectively than austenitic steel, which is due to the difference inthe number of defects and grain boundaries between the structuresof the two steels. The nitrided layers formed of martensitic–

Fig. 5. Distribution of the concentrations of nitrogen, iron, nickel and titanium in the nitrided layer and in the steel core of the austenitic structure (WDS).

Fig. 6. Distribution of the nitrogen, iron, nickel and titanium concentrations in the nitrided layer and in the martensite–austenite (duplex) steel core (WDS).

Fig. 7. Hardness of the nitrided layers and of the steel core (austenitic and martensitic–austenitic) untreated and after nitriding.

Fig. 8. X-ray diffractograms of the nitrided layers produced at a temperature of 450 °Con: a) austenitic steel, b) martensitic–austenitic steel.

1378 T. Borowski et al. / Surface & Coatings Technology 204 (2010) 1375–1379

austenitic steel have a considerably higher hardness than thoseproduced on austenitic steel.

3. The nitrogen austenite formed in duplex steel during glowdischarge assisted nitriding occurs in two crystallographic struc-tures differing by their lattice parameters. It may be supposed thatthe nitrogen austenite with the greater lattice constant forms fromthe primary austenite, whereas nitrogen austenite with the smallerlattice parameter forms from the product of the inverse transfor-mation of athermal martensite into austenite.

4. The nitrogen austenite composed of two structures that differ bytheir lattice constants forms during the glow discharge assistedprocess irrespective of whether conducted at a temperature belowor above the temperature at which the inverse transformationoccurs in the treated material.

Fig. 9. X-ray diffractograms of the layers nitrided at a temperature of 550 °C on: a)austenitic steel, b) martensitic–austenitic steel.

1379T. Borowski et al. / Surface & Coatings Technology 204 (2010) 1375–1379

5. Besides the nitrogen austenite, the transformations occurring inthe surface layer during glow discharge nitriding also yield smallamounts of retained austenite, especially when the duplex steel istreated.

References

[1] J. Jeleńkowski, Mater. Sci. Technol. 10 (1994) 1073.[2] J. Trojanowski, A. Nakonieczny, T. Wierzchoń, Proceedings SAE International Off-

Highway Congress, Las Vegas, USA, vol. 1, 2002.[3] Y. Sun, T. Bell, Z. Kolosvary, J. Flis, Heat Treat. Met. 1 (1999) 9.

[4] M.P. Fewell, J.M. Priest, et al., Surf. Coat. Technol. 131 (2000) 284.[5] J. Jeleńkowski, J. Mater. Process. Technol. 64 (1997) 207.[6] Yu Zhiwei, Xu Xiaolei, Wang Ling, et al., Surf. Coat. Technol. 153 (2002) 125.[7] C.X. Li, T. Bell, Corros. Sci. 48 (2006) 2036.[8] S.K. Kim, J.S. Yoo, J.M. Priest, M.P. Fewell, Surf. Coat. Technol. 163–164 (2003) 380.[9] X. Yun-tao, L. Dao-xin, H. Dong, Appl. Surf. Sci. 254 (2008) 5953.[10] S.J. Skrzypek, J. Jeleńkowski, M. Marciniak, Inżynieria Materiałowa 1 (2007) 18.[11] J.D. Fritz, J.F. Grubb, R.E. Polinski, Adv. Mater. Process. (2001) 36.[12] C.M. Wayman, Mater. Charact. 9 (1997) 235.[13] C.M. Wayman, Martensitic transformations: electron microscopy and diffraction

studies, Diffraction and Imaging Techniques in Material Science, North-HollandPublishing Company, 1978, pp. 251–314.