ISIJ International, Vol. 36 (1 996), No, 7, pp. 840-845

Nitrogen Bearing

and Properties

Martensitic Stainless Steels: Microstructure

M. B. HOROVITZ.F. BENEDUCENET0.1) A. GARBOGINIand A. P. TSCHIPTSCHIN

Departamentode Engenharia MetalOrgica e de Materiais, Universidade de S~oPaulo, P. O. Box81 74-01 065-970, S~oPaulo,Brazil. 1) Institute de Pesquisas Tecnoi6gicas do Estado de S~o Paulo, P.O. Box 71 41 -OI OOO-S~0Paulo, Brazil.

(Received on September29. 1995, accepted in final form on Alpril 12. 1996)

Three experimental nitrogen bearing martensitic stainless steels (nitrogen content ranging from 1600 to

1900ppm)were produced in an air induction furnace and the Nwasadded into the melt as Fe-Cr-N masteralloy or gas nitrogen. Thesealloys with (C + N) content equal to 0.32 wt'/• werecomparedwith acommercialAISI 420 steel. The alloys were homogenized, forged, quenched in air (alloys I and ll) or in oil (alloys lll

and AISI 420) from temperatures between I 073 to 1423 K. The austenitizing temperature of 1273Kwaschosen and the specimenswere tempered in the range of 373 to 973Kfor I h. SEMof the 773Ktemperednitrogen steels did not showany visible precipitates. TheAISI 420 alloy, however, exhibited a high densityof chromiumcarbide precipitates whenheat treated in the samemanner. TEMobservation of the alloy lll

did not show clearly fine and well distributed precipitates. The nitrogen bearing steels showedbetter

corrosion resistance in the 773Ktempered condition than the as quenchedAISI 420 as a consequenceof

lower precipitate size, stoichiometry and distribution of precipitates.

KEYWORDS:nitrogen; martensite; stainless steel; corrosion resistance.

1. Introduction

Nitrogen is an element always present in conventionalmetallurgical processes and has been considered to bedeleterious for manyyears. But, recently, nitrogen hasbeen recognisedl) as an element that improves not onlythe corrosion resistance of martensitic stainless steels,

but also their yield strength, creep strength and tough-

ness. I ~ 4)

Nitrogen and carbon are interstitial elements in rela-

tion to the iron crystal lattice and maybe kept in solid

solution in the austenite and martensite phases and/orprecipitated in the matrix as nitrides or carbonitrides. A"High Nitrogen Steel" (HNS) is defined as such if the

nitrogen content is greater than 0.08 wto/o in a ferritic

matrix andgreater than 0.4 wto/o in an austenitic matrix.4)

The production of high nitrogen stainless steels is com-plex, and high pressure metallurgy is involved.

l.1. Solubility of Nitrogen in Ferrous Alloys and Pro-duction Routes of HNS's

The thermodynamic equation to calculate the solu-5).bility of nitrogen in ferrous alloys is given by

.

KT=fN • (~N)/p~/2. . . .. . . ..

.( l)

where the equilibrium constant (KT) is determined for

each temperature and the activity coefficient (fN) is afunction of the influence of other elements on the nitro-

gen activity, determined by the interaction parametersof first (e~) and second (r~) orders. o/oN is the weight

percentage of nitrogen in the alloy, andpN, is the nitro-

gen pressure.By analysing Eq. (1)-Sievert's law whenfN = l-it is

noted that to increase the content of nitrogen in solution

by 2, it is necessary to increase its partial pressure by 4,

which shows that it is necessary to work with high

pressures of nitrogen.

There are several routes for the production of highnitrogen steels; the main ones are summarisedbelow:

(1) Pressurized-electro-slag-remelting (PESR) routeand its variations.5~8) This process which operatesin industrial scales in Germany, uses Si3N4 slag andnitrogen atmospheres whosepressures are up to 4MPa(40 atm).6~8) At present this process is considered the

only one which is able to produce big ingots (up to 20 tingots). The disadvantages are:

It only allows the production of ingots;

Contamination of the alloy by Si from the slag occurs,which maybe a problem, depending on the material'sapplication.8)

(2) Powdermetallurgy route-In this case nitrogenis addedwhenthe steel is in the solid state (as powder)in the austenitic field as the solubility of nitrogen in

austenite is high. Then the powder undergoes hot iso-

static pressing (HIP).6) The stainless steel powdersmayalso be mixedwith Fe-Cr powderscontaining nitrogen.9)

(3) Thermo-chemical treatments-Here the additionof Nis madeonly to the componentsurface region andthis process is normally used for low carbon stainlesssteel.6)

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ISIJ International, Vol. 36 (1996), No, 7

Table l. Interaction parameters between N and someai-

loying elements in Fe at 1873K.5)

Wtol. N

Element e~ C~

Ti

Zr

VNbCrTaMnMoWCoNiCuAsSnSbAlSi

BCN

-0,930-0,630

0.098

-0,050-0,048

- O.033- O.024-0,013

0.002

+0.010+0.01 l+0,006

+0.010

+0,008

+0,010

+0.040

+0.043

+0,048

+0.08 3+0.118+0,13

+ 19,375

+ 13, 125

+2.042

+ I.042

+ I,OOO

+O,690

+O. 500

+0,27 l+0,042

- 0.208

- 0.229

- O, 125-0.208

- O. 167

- O,208

-0,833

- 0.896

- I,OOO

- I,729

- 2.458

2,708

1.O

0.8

0.6

0.4

0.2

austenite

ferrite

liqUid

wnl•Cr25.0

18.4

13.6

8,1

0.0

(4) Melting and ingot processes in pressurised atmo-spheres. The utilization of these prQcesseshas been pro-posed and discussed as alternative for the production ofHNS.8)

It is worthy of notice that the routes which involve

high pressures require high investment in equipment,s - 8)

Apart from elevated pressures, the solubility of nitro-

gen in the steel maybe ralsed by introducing alloying

elements into the melt. Table I shows someelementswhich affect the solubility of nitrqgen in ferrous alloys

at 1873 K; the elements which posses a negative inter-

action parameter (e~) increase the solubility of nitrogen.

Taking Cr as reference, one can see that, for example,

Cr affects the solubility of nitrogen twice as muchas Mn.Thus, the activity coefficient (,fN) maybe calculated byapplying the equivalent Cr concept, using the data in

Table I and Eqs. (2) and (3) below:

[oloCr -~ iJ- CN•[oloi].........

Now,with the Cr interaction parameters of first andsecond orders, and considering the effect of nitrogenitself, the value offN maybe calculated as follows:

log,fN = - [0.048 • o/oCr,q] +36. 105 .[o/oCr,q]2

+ 13. l0~2 . [oloN]..........

(3)

This method of calculation showedgood similarity

with experimental values.5)

In order to increase the concentration of nitrogen in

the steel melt, it would be logical that elements with high

values of C~(Table 1) such as Ti andZr should be used.

However, the majority of these elements form nitrides,

which is not desirable as these nitrides are stable and,if formed in the melt, they maybe incorporated in the

slag. In the solid state the formation of nitrides is also

undesirable as they diminish the corrosion resistance andtake nitrogen out of solution.

Only a few elements increase the solubility of nitrogen

841

o.o

11oo 1800 1500 1700 1900

Te*pe'ature KFig. l. SolubilrtyofNatO IMPamaFe13 6wt~ alloy lo)

without forming nitrides during austenitization. Themost important elements in this group are Cr, Mn, andM0.5) The solubility of nitrogen in a Fe-13.6wtoloCralloy at Iatm is shownin Fig. 1.4) Becauseof the lowsolubility of nitrogen in Fe-Cr alloys in the liquid state

the addition of nitrogen to the steel melt is not ef-

ficient.2 ~ 6) Another disadvantage in this practice is thatit is necessary to have a rigorous control over tenso-active elements such as oxygenand sulphur which delay

the introduction of nitrogen into the melt.

2. Objectives

The objectives of this work are:(1) To investigate the possibility of producing high

nitrogen martensitic stainless steel, corresponding to the

AISI 420 steel, under atmospheric pressure, by using gasnitrogen injection in the melt or by addition of FeCr-Nmaster alloy;

(2) To comparethe mechanical properties and cor-rosion resistance of these materials to those of the corn-mercial AISI 420 steel.

3. Experimental Procedure

The material was produced in a 15kWinductionfurnace, under nitrogen atmosphere, using magnesiacrucibles whosecapacities were 6kg. The injection of

gas nitrogen wasdone by an alumina lance for the runI and by a porous plug for the run II. The chemicalcompositions of the AISI 420 steel as well as that of runs

I and 11 are shown in Table 2. It was not possible to

achieve in runs I and II, chemical compositions similar

to that of the AISI 420. Run111 wasdone, therefore, in

order to obtain a stainless steel with quite the samealloy

content of the AISI 420 but with reduced carbon andhigh nitrogen as shown in Table 2. (O.ll wto/o CandO.170/0 N). Nitrogen was introduced in run 111 by the

addition of Fe-Cr-N master alloy.

It wasexpected a decrease in the solubility of nitrogen

during the solidification of the nitrogen steel, due to the

formation of 8ferrite. Howeverthis was not observed,

probably because of the high cooling rate which wasresponsible for the austenitic solidification mode. The

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ISiJ International, Vol. 36 (1996), No. 7

Table 2. Chernical composition of the studied steels. (wt"/.)

Steels oloC o/oN oloMn o/oSi o/oP o/oS o/oCr o/oNi

II

III

AISI 420

O. 130,16O, I l0,33

O. 19O. 160.170.02

0.0170.0170.3900.35

O. 130.08

0.40

O.24

0,0250,026

0.0100.0100.0040.018

15.80

14,70

13.00

12.60

0.ll

O. I l

0.23

austenitic solidification mode(L->L + y) maybe inducedby the chemical composition of the material and/or thecooling rate and it is a critical stage in the processingof these steels.

By using Eqs. (1), (2) and (3), it was possible tocalculate the nitrogen content in equilibrium at 1873Kin the melt, which were respectively 1930, 1750 andl 720ppmfor runs I, 11 and 111 for a nitrogen partial

pressure of I atm. Thesevalues are very similar to thoseexperimentally obtained.

The ingots were homogenizedfor 5.5 hat 1423Kandthen forged into bars of 25.4 mmdiameter. TheAISI 420steel wasbought from an outside supplier and wasalso

a 25.4mmdiameter bar which had been rolled.

Cubic specimenswith dimension 10 x 10 x 10mmwereused to plot the hardness of the material against theaustenitizing and temper temperatures.

The austenitizing temperatures varied from I 073 to

l 423K and the austenitizing time was I h, after whichthe specimens were air or oil quenched to room tem-perature. The tempering temperatures varied between373 and 973K, the holding time was also Ihand thenthe specimens were cooled in air to room temperature.The heat treatments were carried out in a tube furnacewhoseinternal diameter was 25.4mm,under argon at-

mosphereand thermocouples in contact with the speci-

menswere used to monitor the temperature. TheVickershardness (HVIOO)wasmeasuredand the values are the

average of 7measurements.Samplesfor scanning electron microscopy (SEM)were

ground on SiC papers (180, 240, 320, 400 and 600grades), polished with diamond paste (6, 3and I ,Im),

etched with Vilella solution for 30sec and then ultra-

sonically cleaned. Samplesfor transmission electron mi-

croscopy (TEM) were also prepared using the 100/0

perchloric acid +90o/o acetic acid solution. The speci-

menswere observed in a 200kVTEM.The polarization curves were determined by using a

potentiostat PAR-273with a scanning velocity of ImV/sin a INH2S04solution. Thesurface finish of the samples

was that resulting from grinding them on 600 grade SiC

paper immediately before immersion.Thecorrosion tests were carried out using samples in

the following conditions:

- condition A: The samples were only quenched from1273K in air (runs I and II) or in oil (AISI 420 steel

and run 111).

- condition B: Sameas condition A, but the samples

were also tempered at 773K for I h.

C 1996 ISIJ 842

700

600

oe 500

>= 400

300

200

=L- I -e- II

-~III HI-420

lOOO IIOO 1200 1300 1400 1500TemperatureK

Fig. 2. Hardness vs. austenitizing temperature for runs I,

II and 111 and AISI 420 steel.

4. Results and Discussion

Figure 2showsthe hardness vs. austenitizing temper-ature curves. It is noticeable that the AISI 420 steel is

harder than the high nitrogen martensitic stainless steels

at the higher austenitizing temperatures. This maybedue t06:

- The atomic concentration of carbon and nitrogenin the high nitrogen steels, and consequently the numberof occupied interstitial atomic positions, is lower thanthe atomic concentration of carbon in the AISI 420steel.

- The nitrogen atomic radius is slightly smaller thanthat of carbon which would cause less lattice deforma-tion in the martensitic reaction.

- TheM*temperatures for the steels containing nitro-

gen are higher than the M~for the AISI 420 steel, there-

fore the self-tempering tends to be morepronounced in

the steels containing nitrogen.

The microstructures of the samples from which thehardness was measured(to draw the hardness vs, aus-tenitizing temperature curve) showed ferrite with highdensity of precipitates for austenitizing temperatureslower than 1173K. As the austenitizing temperature in-

creased, formation of martensite and increasing dissolu-

tion of precipitates were observed. Figure 3 shows the

micrograph (SEM)of the AISI 420 steel quenchedfrom1273K where only martensite and primary carbides

(M23C6type )(marked as p) can be seen. Figure 4showsthe structure of RunI steel containing nitrogen. It is also

possible to see primary precipitates (p), but in this case,they are, probably, nitrides or carbonitrides (MX, M2Xtype). Higher austenitizing temperatures favoured the

formation of increasing quantities of retained austenite,

which decreased the hardness. The steels containingnitrogen showedlath martensite, with a dislocated sub-

structure, typical for low carbon martensites, as shownin Fig. 5(TEM).

After tempering the high nitrogen steels showedhigher

ISIJ International, Vol. 36 (1996), No, 7

Fig. 3.

SEMAISI 420steel quenchedfrom 1273 K. Mar-tensite wlth prlmary carbides (p). Etchant: Villela.

2KX.

'~

~~~~;~

Fig. 4.

SEMRunI quenchedfrom 1273 K. Martensitewith primary nitrides 'and/or carbonitrides (p).

Etchant: Villela. 2KX.

Fig. 5. TEMRun111 quenchedrrom 1273 K and temperedat 773 K. Lath m2lrtensite. 15 KX

ee'1>~~

700 -

600 -

500 -

400 -

300

200 -

lOO - I

I1' AN1'

--e-ll -ll--I

- -A* -420 •-e-lll

hardness than the AISI 420, for tempering temperaturesbetween 573 and 823 K-Fig. 6. Moreover, secondaryhardening at temperatures around 773 K, can be seen.

Fig. 6.

250 350 450 550 650 750 850 950

Temperature (1()

Hardness vs. tempering temperature for runs I, II

and 111 and AISI 420 steel.

650

This should be probably due to coherent precipitationinside martensite during tempering.

The microstructure (SEM)of the AISI 420 steel tem-pered at 773K is shown in Fig. 7. Onecan see undis-solved primary carbides (p) and fine secondary carbides

(marked as s) resulting from martensite decomposition.

843 @1996 ISIJ

ISIJ International, Vol. 36 (1996), No. 7

Fig. 7.

SEMAISI420 steel quenchedfrom 1273 Kandtempered at 773 K. Tempered martensite wlth

primary (p) and secondary (s) carbides. Etchant:

Villela. 8KX.

Fig. 8.

SEM= RunI quenchedrrom 1273Kand temperedat 773 K. Temperedmartensite with primary (p)

nitrides and/or carbonitrides - no visible secondaryprecipitates. Etchant: Villela. 8KX.

RLln 111, on the other hand, showsno visible secondaryprecipltation with the 8KXmagnification. Fig. 8-0nlylarge primary precipitates (p) can be seen. Thesecondaryhardenlng observed in Fig. 5, however, suggests the

existence of very small coherent precipitates. Figure 9showsthe microstructure (TEM)of the run 111 specimenafter quenching and tempering at 773 K. Even at a45 KXmagnification it Is not possible to see c]early

secondary precipitates, obscured by the high density ofentangled dlslocations, inside lath martensite.

Anodic polarlzation curves for AISI 420 and Run111

steels showthat the passive current value for the nitrogensteel is lower, in both heat treated conditions, than thosefor the AISI 420 steel, indicating that the nltrogen steel

posses higher corrosion resistance-Fig. lO. 12) Precipita-

tion of incoherent M23C6type carbide in the quenchedand tempered AISI 420 steei is responsibie for the sub-stantial decrease in the corrosion resistance, as it leaves

the matrix depleted from chromium, while small co-herent and well distributed precipitates with lower Crcontent, present in the nitrogen steel, contributes to abetter corrosion resistance, 13)

Fig. 9. TEM- Run111 quenchedfrom 1273 Kand temperedat 773 K. Lath martenslte with dislocated substructure.

45 KX.

Thedifferences of mechanical and chemical properties

betweenAISI 420 and other HNSalloys (1 and II) could

be attributed to the influence of other alloying elements

C 1996 ISIJ 844

ISIJ International. Vol. 36 (1996). No. 7

1.5

l .O

~ 0.5~~

0,0

-o 5

IRQ 420Q420QT

MQT

lO 1000 1OOOO IOOOOOlOO

i(,lA/cm')

Fig. lO. Polarization curves for run 111 and AISI 420 steel.

besides Nitrogen. Alloy 111, however, has the samechem-ical composition as the AISI 420 steel unless the carboncontent that was partially replaced by nitrogen. Withlower carbon and higher nitrogen, the precipitation ofsecondary carbides was hindered, during tempering at

773K, while precipitation of very fine MX/M2Xcoherentparticles, with lower Cr content wasenhanced. Increas-ing the N/C ratio shifts the precipitation mechanismasdescribed above.

Fromrecent workl4) on corrosion resistance of mar-tensitic HNSit waspossible to establish that Cincreasesthe passive current density while nitrogen does not have

a harmful effect over it. Replacing carbon by nitrogen,therefore, improves corrosion resistance without loss ofhardness.

5. Conclusrons

(1) It was possible to obtain nitrogen martensiticstainless steels containing O. 16 to O, 19 wto/, N, by gaseous

injection of nitrogen in the melt and by addition ofFe-CrNmaster alloy (conventional process).

(2) The nitrogen steels studied in this work showedsecondary hardening peaks, with hardness value greaterthan those of the AISI 420 steel, whentemperedbetween583 and 823 K.

(3) The corrosion testing in H2S04showed that

Run111-high nitrogen steel posses better corrosion re-sistance in the as-quenched, andquenchedand temperedcondition than the AISI 420 steel.

~)

2)

3)

4)

5)

6)

7)

8)

9)

lO)

ll)

l2)

l3)

l4)

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845 C 1996 ISIJ