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Trans. Indian Inst. Met.Vo\.55, NoA, August 2002, pp.225-263

HNS ' 02TP 12

MICROSTRUCTUREAND CHEMICAL CHARACTERIZATIONOF HIGH TEMPERATURE NITRIDED 12%Cr STAINLESSSTEELS

Carlos Mario Garzon, Alejandro Toro, Andre Paulo TschiptschinMetallurgical andMaterials EngineeringDepartment, University of Sao Paulo,Av. Prof. Mello Moraes 2463, CEP 05508-900, Sao Paulo, Brazil.

ABSTRACTAISI 410 (11.5 %Cr - 0.13 %C) and AISI 41OS (13.0 % Cr - 0.07 % C) martensitic stainless steels were gas nitrided in Nz atmosphere at1273-1473 K, 0.02 - 0.38 MPa and 0.9 - 172.8 ks. Nitrogen gradients were determined by chemical analysis through opticalspectrometry and WDS microanalysis. The average nitrogen content of long-term nitrided thin specimens was measured by opticalspectroscopy and fusion under inert gas. Precipitate extraction was performed by dissolution of the matrix, the precipitates beinganalyzed by fusion under inert gas and X-ray diffi 'action.The result s showed that by increasing nitr iding time and pressure, as wel l as by decreas ing temperature, both the nit rogen content at thesurface of the steels and the tendency to form precipitates increased. Thermocalc@ calculations for the Fe-Cr-N-C. system allowedpredicting nit rogen contents, as well as microst ructures of the ni trided alloys. Good agreement between calculated and experimentalvalues was observed. XRD of extracted precipitates confirmed Thermocalc calculations concerning the stabili ty of nitrides.In specimens containing precipitated nitr ides, the ni trogen content dissolved in martensite increased with the distance t rom the surface,in the region where precipi tation occurs. Beyond this region the nitrogen content decreased towards the core.

1. INTRODUCTIONNitrogen has been added as an alloying element tostainless steels to improve mechanical properties andcorrosion resistance!. Nitrogen can be introduced instainless steels by a high-temperature thermochemicaltreatrnenr,3, which allows introducing nitrogen attemperatures"aboveAc! under nitrogen partial pressuresthat typically vary in the range 0.03-0.40 MPa. After3.6-86.4ks treatments, a high nitrogen case is formed4.Underthese conditions nitrogen remains mainly in solidsolution,without forming a white layer usually found inlow temperature nitriding processes. The gas used(N2+Ar)is neither explosive nor toxic, and no gas fluxis required.Dependingon the chemical composition of the steel andthe phases in equilibrium at the nitriding temperature,eitherhigh strength austenitic or hard martensitic casescan be obtained. When applied to martensitic andmartensitic-ferriticstainless steels, this process allowscombining high hardness at the surface with good

impact resistance of the core. The resultantmicrostructure after hig!). tempt:rature nitridingtreatment depends on the material and on nitridingtimes, temperatures and N2 partial pressures. Severalauthors4.5consider the existence of stable equilibriumbetween the steel surface and the gas, which implies thatthe phases present"-.at the surface as well as theirchemical composition can be read in a phase diagramincludingmetal-gas equilibrium.According to Frisk6 the metal-gas equilibrium duringnitriding of steel powders can be predicted by drawingin a phase diagram, the nitrogen activity for specificN2partial pressures. In a similar approach, Tschiptschin5proposes that metal-gas equilibrium during high-temperature nitriding may be predicted by using ametal-gas equilibrium diagram, constructed for aspecificN2partial pressure.There is no systematic study of microstructural changesat the surface, as a function of time during high-temperature nitriding. Tschiptschin5 mentioned thatunder some specific nitriding conditions, the assumption

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,~

Ni0.100.21

TRANS. INDIAN INST. MET., VOL. 55, NO. 4, AUGUST 2002

Table 1CHEMICAL COMPOSITION OF THE BASE MATERIALS (wt"A.)

SteelAI814108AI81410

Cr13.011.5

C0.070.13

of metal-gas stable equilibrium is not a goodapproximation.The microstructure of a region below the surface can bepredicted in a phase diagram, if the mean nitrogencontent of that region is known. Furthermore, theni~ogen distribution in the nitrided case can beestimated by solving the Fick's second law for diffusionof nitrogen from the surface towards the core.The aim of the present work was to study the effects ofnitriding temperature, Nz partial pressure and time, onthe microstructure of high temperature nitrided anddirect-quenched AISI 410 and 410S martensiticstainless steels.2. EXPERIMENTALTwo martensitic stainless steels were used: AISI 410bars, 12.7 mm in diameter and AISI 410S sheets, 9 mmthick. Table I shows the chemical composition of thesealloys. The AISI 410 and 410S steels were high-temperature nitrided in Nz atmosphere and directquenched in water. Two sets of specimens were used:thick pieces 9 - 13mm thick and thin pieces 0.3 - 0.7mm thick. With the aim of obtaining steels with a highnitrogen case, the thick specimens were nitrided under0.25 MPa Nz partial pressure at 1273 -1473K for0.9-86.4 ks. The thin pieces were long-term (172.8 ks)nitrided under 0.02 - 0.38 MPa Nz partial pressure at1273 - 1473 K, in order to obtain steels with bulkhomogeneous nitrogen contents.Metallographic preparation of the specimens consistedof grinding in emery paper up to 2400 grit, followed by1J.1IIliamond polishing.

Mn0.40.6

. Siro:-250.36

The microstructure' of the specimens were examined byoptical and scanning electron microscopy and by X-raydiffraction. The prior austenite grain size wasdetermined according to the E 112ASTM standard. Thecomposition of the phases after nitriding wasinvestigated by wavelength dispersive spectroscopy(WDS).Nitrogen gradients in the thick specimens weredetermined by chemical analysis through opticalspectrometry (OS) and WDS microanalysis. Thenitrogen content of long-term nitrided thin specimenswas measured by OS and fusion under inert gas.Precipitate extraction was performed by the chemicaldissolution of the matrix, and the precipitates wereanalyzed by fusion under inert gas and X-raydiffraction. For precipitate extraction, specimensweighing 1-2 g were dissolved in a Berzelius typesolution at 300 K for 43.2 ks. The reagent was preparedby dissolving 320 g of CuClz2HzO,280 g of KCl, and 20g of HOOC(CHOH]zCOOH in 1.85 I of distilled waterand 150 mI of HCI. A magnetic wrist-action sbaker wasused to agitate the specimens inside the reagent in aflask. The solution was suctioned through a 45 mmdiameter PTFE filter with 0.45 nun maximum pore size.The residue was washed with 0.25 N hydrochloric acidand water and then. transferred to a watch glass anddried at 393 K for 24 h.Thermocalc@ was used to predict phase stability andchemical composition of the nitrided specimens. Thegas-metal equilibrium was calculated for one specificNz isobar allowing predicting the phases present and themaximum nitrogen content at the surface of the steel.The stability and the compositions of the phases in otherregions inside the case were calculated assuming

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Table 2THERMOCALC @ CALCULATED AND WDS MEASURED NITROGEN CONTENTS AT THE END OF THE MARTENSITIC LAYER

Temperature(K)1473137313531273

Calculated (wt%)0.090.120.150.20

Measured (wt%)0.11 :t 0.050.16:t 0.050.20 :t 0.050.22 :t 0.05

thermodynamicequilibrium as well. All thermodynamiccalculationswere performed using the TCFE database.3. RESULTSANDDISCUSSION3.1Microstructure and ChemicalCharacterization of the ThickSpecimens

Thick specimens of nitrided AISI 410 steel showedmartensiteboth in the high- nitrogen case and in the lownitrogen core (Fig. 1). Different volume fractions ofretained austenite, MX and M2X type precipitates werealso observed in the case, depending on the nitridingconditions.

Fig.I : Optical micrograph of the AISI 410 steel nitrided at1473 K for 6 h at 0.25 MPa, showing the highnitrogen case (HNC) and the low nitrogen core(LNC).

Thick specimens of nitrided AISI 4IOS steel showed amartensitic case and a martensitie-ferritic core (Fig. 2).

Three zones with different microstructures wereobserved from the surface towards the center of thespecimens: a martensitic surface layer, a transition zone,and a dual-phase martensitic-ferritic inner zone. Themicroconstituents in the martensitic surface layer werethe sameobserved in the high-nitrogen case of AlSI 410steel. The transitio!l zone was consisted of a martensiticmatrix with isolated regions of untransformed ferrite, asshown in Fig. 2. No precipitates were observed underSEM. The inner zone showed a dual phase martensitic-ferriticmicrostructure containing 40-50% ferrite.

Fig.2 : Micrographs of the AISI 410S steel nitrided at 1473 Kfor 3 h at 0.25 MPa. (a) Optical micrograph showingthe high nitrogen martensitic surface layer (MSL), thetransition zone (TZ) and the low nitrogen core (LNC);(b) Scanning electron micrograph showing ferrite (cx)and marten site (cx') in the transition zone.

Table 2 shows the Thermocalc@ calculation and WDSmeasured values of nitrogen content at the end of themartensitic layer (beginning of the transition zone) forthe AIS 4IOS specimens, nitrided at 0.25 MPa for 21.6ks. A good agreement between the calculated and themeasured valueswere observed.

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TRANS. INDIAN INST. MET., VOL. 55, NO. 4, AUGUST 2002

Table 3 shows the microconstituents at the surface of XRD andWDS techniques. Loweringthe temperaturethe thick specimens nitrided at 0.25 MPa, identified byTable 3

MICROCONSTITUENTS AT THE SURFACE OF THICK SPECIMENS NITRIDED AT 0.25 MPa

Table 4PRECIPITATES IN THE CASE OF SPECIMENS NITRIDED AT 0.25 MPa

Identified PrecipitatesT(K)1473137313531273

AISI 410NoneMXMXMX

AISI410SNoneM2XMX + M2XMX+M2x

enhanced nitride precipitation; the volume fractionincreased with increasing nitriding time, as shown inFigs.3 and 4 for the AISI 410 stet:!(nitrided at 1273K,0.25MPa).In the precipitate-free specimens (nitrided at 1473 K)grain growthoccurred reaching a size of 2-4 ASTM. Onthe other hand, in specimens with intense precipitation(nitrided at 1273 and 1353 K) grain growth wasinhibited,and grain sizes of 5-6 ASTM were obtained.Fig. 5 shows the effect of nitriding time and temperatureon prior austenite grain size for the AISI 410 steelnitridedat 0.25 MPa.Table 4 shows the precipitates formed inside the entirecase of specimens nitrided at 0.25 MPa for 21.6 and

4tl

II\I

Fig.3 : Scanning electron micrographs at the surface of theAISI 410 steel nitrided at 1273 K, 0.25 Mpa for (a)10.8 ks and (b) 43.2 ks.

free cases were obtained in both the steels. For the othernitriding temperatures, MX and M2Xprecipitates wereidentified in AISI 410S steel, while only MX typeprecipitates were found inAISI 410 steel.

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AISI 410 steel AISI 410 S steelT(K) Time (ks) Micro constituents Time (ks) Micro constituents1473 0.9/1.8/3.6 a' 0.9 a'1473 7.2/10.8/86.4 a' + 'Yretained 1.8/3.6/10.8/86.4 a' + 'Yretained1373 3.6 a' 3.6/6.3 a'1373 6.3/10.8/21.6/86.4 a'+MX 10.8/21.6/86.4 a' + M2X1353 3.6/10.8/21.6/86.4 a'+MX 3.6 a' +M2X

6.3/10.8/21.6/43.2 a'+MX1273 1.8/3.6/63 a' + M2X 3.6 a' + M2X. 1273 10.8/21.6/86.4 a'+MX 6.3/10.8/21.6/86.4 a'+MX

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0.15c0U0.10I!....Co);:..~ 0.05;:,'0> 00 20 40 60 80 100

Nltrldlng time, (ks)Fig.4 : Effect of nitriding time on the amount of precipitates

at the surface of the AISI 410 steel nitrided at 1273 K,0.25 Mpa.

25 2.5

~2

lAlSI 410 1473K

! / cfS / "1.0 '& I / 1~ Ki 05 .." "Z

12.01:11.5!8 1.0! 05

AISI410S /

/'///

0 0 . ...............10 .20 30 40MItrkIInu time, (~.)

10 20 30M_lno time. (D)

,so

Fig. 6 : Effect of nitriding time and temperature on nitrogencontent at the surface of the specimens nitrided at 0.25 MPa.

at 0.25 MPa, 1273 - 1473 K. The nitrogen contentincreasedwith nitriding time up to a saturation value,whichdecreasedwith increasing nitriding temperature.The nitrogen content decreases towards the core, asshownin Fig. 7 for three representative specimens. Inspecimens containing precipitated nitrides, bothnitrogenand chromium contents in martensite increasedwiththe distance from the surface in the region whereintense precipitation occurs. Beyond this region themartensitenitrogen content decreased towards the core

Figure 6 shows the relation between nitriding time andnitrogen content at the surface of the specimens nitrided200

1473Ki.&. 150!c 100'i0)ui! 50cI 0 0

1353K, Q o 1273K

10 20 40 500Nitriding time, (ks)

Fig. 5 : Effect ofnitriding temperature and time on austenite grainsize in the case of the AISI 410 steel nitrided at 0.25 MPa.- 1.2'#1 1.01! 0.8~8 0.6c 004fit2 0.2:t:::Z

-0- (0) AISI410S -1473 K4>-(bIAISI410 -1473K-Q- (1;1 AIS! 410S - 12731