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POWDER METALLURGY ASPECTS OF HIGH NITROGEN STEELS

André Paulo Tschiptschin

Metallurgical and Materials Engineering Department University of São Paulo – São Paulo, Brazil

[email protected]

ABSTRACT

Controlled addition of nitrogen to stainless steels has been encouraged over the last two decades due to the possibility of improving mechanical and corrosion properties. High-pressure techniques were developed for medium and large-scale fabrication of high nitrogen steels (HNS), but in general these procedures are very expensive and require sophisticated equipments.

One of the main challenges involved in HNS production is obtaining fully dense

components with uniform nitrogen content in volume and excellent surface properties. A uniform nitrogen distribution leads to a more homogeneous microstructure, and consequently to better mechanical properties.

Powder metallurgy has emerged as an attractive solid-state route for obtaining high-nitrogen

stainless steels, since the solubility of nitrogen in Fe-Cr austenite is much higher than in the liquid phase. As a consequence, a larger amount of nitrogen can be added to the alloy without application of high pressures. In addition the small size of the powder particles and the typical range of temperatures used (1273-1473 K) allow chemical homogenization in relatively short times.

In this paper several powder metallurgy processes are reviewed and discussed. Near-net-

shape manufacturing methods involving HIP of P/M alloys allows producing fully dense parts. Nitrogen exceeding the predicted equilibrium content can also be introduced by nitrogen gas atomization where the solidification rate is much more rapid than the nitrogen desorption kinetics of the supersaturated liquid. Since the microstructure, chemical composition and surface properties of HNS strongly depend on the nature of the production routes, the relations between processing parameters and final properties are presented.

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INTRODUCTION

High nitrogen steels are being considered a new promising class of engineering materials. When nitrogen is added to austenitic steels it can simultaneously improve fatigue life, strength, work hardening rate, wear and localized corrosion resistance. High nitrogen martensitic stainless steels show improved resistance to localized corrosion (pitting, crevice and intergranular corrosion) over their carbon containing counterparts.

Nitrogen is effective in improving the mechanical and corrosion properties of steels if it remains in solid solution or precipitates as very fine and coherent nitrides. Profuse precipitation of coarse chromium nitrides can lead to Cr and N depletion in the matrix, impairing the corrosion and wear resistance of the steel.

Nitrogen in solution can be obtained by solution annealing followed by a rapid quench to

suppress precipitation. Fine and coherent precipitates can be obtained by ageing the solubilized alloy or by tempering the quenched martensitic steel.

Compared to C the solubility of N in steel is lower in the melt but higher in the austenite,

leading to somehow distinctive HNS manufacturing practices. Normal steel making practice at atmospheric pressure does not permit to introduce high

amounts of nitrogen due to its low solubility in the melt and still lower solubility in δ ferrite, as shown in Figure 1 [1], leading to nitrogen losses during solidification. Nitrogen cannot remain absorbed in steel unless during the solidification process the L → δ reaction could be suppressed.

Figure 1 - Nitrogen solubility in Fe-Cr alloys at 0.1 MPa.[1]

Increasing N2 pressure increases the maximum nitrogen content that can be dissolved in

liquid steel, as shown in Figure 2 [2].

Figure 2 – Effect of pressure and chromium content on maximum nitrogen content of liquid iron at 1873 K. [2]

High-pressure techniques were developed for medium and large-scale fabrication of high

nitrogen steels (HNS), but in general these procedures are very expensive and require sophisticated equipments [3].

High nitrogen contents can be introduced by thermochemical treatments, exposing the steel

surface to nitrogen containing atmospheres at temperatures where austenite is the stable phase. Solution nitriding [4,5] of stainless steel parts at temperatures between 1323 K and 1423 K in 0.01 to 0,3 MPa nitrogen atmosphere is being used to enrich the steel’s surface in a way similar to case hardening of carbon steels. The nitrogen content achieved by this treatment may be very high, depending on solution nitriding temperatures and nitrogen partial pressures.

The relatively low diffusion coefficients of nitrogen in Fe-Cr austenite, shown in Table 1

[6],[7] lead to very high thermochemical treatment times and/or to heterogeneous distribution of nitrogen in steel, introducing a shortcoming in these solid state processes.

Table 1 - Diffusion coefficients and activation energies for Fe-Cr alloys. [6*],[7‡]

2.0 X 10-13 3.5 X 10-13 1473

297 5.0 X 10-12280 1.8 X 10-12 1373

1.8 X 10-11 1.3 X 10-11 1273

Q(kJ/mol) (‡) D(m2/s)(‡)Q (kJ/mol) D(m2/s)(*) T(ºC)

POWDER METALLURGY OF HNS

Powder metallurgy is a near net shape technique that enables fabricating high quality,

complex parts to close tolerances in an economical manner. Among the various metalworking technologies, powder metallurgy is the most diverse manufacturing approach. The powder metallurgy process is neither energy nor labor intensive, it conserves material, it is ecologically clean, and it produces components of high quality with homogeneous and reproducible properties [8]. The process effectively uses automated operations with low energy consumption, high material

utilization and low capital costs. Through P/M processing it is possible to process specialty alloys, typically composites containing mixed phases or compound structures. The challenging demands for new and improved processes and materials of high integrity for advanced engineering applications, including those in the aerospace field, stimulated the research and development of P/M processing.

Difficulties to fabricate the unique high nitrogen steels by conventional metallurgy have

contributed a large part to the growth of P/M processing of HNS. The distinguishing characteristic of the P/M process applied to HNS reside in the possibility of introducing high nitrogen levels in solid state steel, at temperatures where the solubility of nitrogen is very high, Figure 1. Besides being a consolidation process, sintering becomes also a synthesis one, where the final chemical composition is reached during the process.

When nitriding is performed during the sintering stage of steel powders one can obtain,

high-nitrogen steels with relative high densities. There is no need of using capital intensive, high-pressure metallurgy equipment needed to manufacture high-nitrogen steels directly from the melt. The solid-state sintering/nitriding process is cheaper, flexible and allows obtaining a wide range of nitrogen contents in the alloy. Moreover, the short diffusion distances involved in the process guarantee minimization of segregation, shorter processing times and fine-grained microstructures of homogeneous composition.

When Hot Isostatic Pressing (HIP) is performed after the sintering-nitriding treatment, P/M

processing becomes a manufacturing method that allows producing near-net shape, fully dense forms. Machining is avoided or minimized, which becomes an advantage for these highly alloyed, high hardness HNS with poor machinability.

The process has also some disadvantages. The main disadvantage is that the nitrogen content

of the product strongly depends on the processing temperatures and nitrogen partial pressures. Strict control of processing parameters is needed to guarantee reproducible composition, mechanical and corrosion properties of the P/M processed alloys. Usually the best conditions to obtain high nitrogen levels in High Nitrogen Steels are not the same that conduct to fully dense materials. Selecting processing parameters is not an easy task and depends on good knowledge of equilibrium and phase diagrams of the systems being considered.

The relative easiness of contamination of the steel’s powder surface, during manufacturing,

with impurities, especially oxygen, which leads to formation of oxides that prevent nitriding and sintering, constitute another drawback of the P/M process. It is often necessary to use high-purity gases, during milling, handling and compaction operations of the powder, and reducing gases during sintering and nitriding treatments.

The as-sintered HNS materials do not present extremely high-density levels and the

completion of the process is performed in a hot isostatic press (HIP), which limits part size and may turn the process rather expensive, as it does not allow economy of scale.

N ALLOYING BY POWDER NITRIDING IN NITROGEN ATMOSPHERES

As mentioned above the nitrogen content of P/M processed materials strongly depends on

the processing temperatures and nitrogen partial pressures and good knowledge of equilibrium and of the phase diagrams of the systems being considered is required.

Figure 1 gives an idea of the nitrogen solubility in iron-chromium alloys as function of the

chromium content and temperature. More exact information on phase equilibria requires extensive

thermodynamic calculations to predict, N solubility in solid phases in equilibrium with nitrogen containing atmospheres and stability and composition of precipitated nitrides.

When stainless steel is exposed to a nitrogen atmosphere at high temperature, nitrogen may

be incorporated in steel through dissolution in the austenitic phase up to its solubility limit, according to equation (1):

½ N2(gas) ⇔ [N]γ (1) and through precipitation of chromium nitrides, according to equations (2) and (3):

[Cr] γ+[N]γ ⇔ CrN (2) 2[Cr] γ+[N]γ ⇔ Cr2N (3)

Nitrogen remains in solid solution depending on temperature of thermo-chemical treatment

and nitrogen pressure. Nitrogen loss or nitrogen pickup may occur according to Sieverts’ law at a given set of temperature and nitrogen partial pressure parameters.

The equilibrium of the reaction ½ N2 (gas) ⇔ [N] γ may be shifted by reactions (2) and (3) relative to CrN and Cr2N precipitation. Thermodynamic calculation of the equilibrium constants of these reactions is arduous and time consuming. Computational thermodynamics turns the task easier: Thermocalc® calculations lead to a good description of the equilibrium between nitrogen gas and steel, provided that convenient databases are selected [6, 9].

L L + gas L δ δ + gas δ δ+γ γ δ+γ 1.5 atm N2 γ + gas γ

γ + Cr2N γ + Cr2N Figure – 3 – Isopleths of the Fe-13 wt% Cr phase diagram at 0.15 MPa. (a) without considering the gas phase. (b) taking in account the N2 gas phase as an equilibrium one.

Usually the equilibrium between nitrogen gas and steel is described by overlaying N2 isobar

lines on phase diagrams calculated without considering the gas phase as an equilibrium one, as shown in Figure 3-a, for Fe-13 %Cr-N alloys. It has the advantage of representing phase diagrams for different pressures all in one. Another way of calculating equilibria takes in account the equilibrium between the gas phase and the steel as shown in Figure 3-b, calculated for a Fe-13 %Cr-N alloy at 0.15 MPa. It gives a realistic representation of the phase fields beyond the solid state.

The use of Thermocalc® allows planning experiments and selecting appropriate thermochemical treatment conditions. As soon as the nitrogen content and the microstructure of an alloy are specified, a diagram relating, nitriding temperature, nitriding pressure, nitrogen content and microstructure allows selecting the thermochemical conditions to obtain specified nitrogen contents and microstructures in the sintered/nitrided alloy. Figure 4 shows such a diagram, for a Fe – 16,2 wt% alloy (AISI 434L) that depicts the equilibrium between the powder particle surface and nitrogen gas, at different pressures and temperatures [10].

Figure 4 – Phase fields and nitrogen iso-concentration lines for a Fe-16.2 wt% Cr alloy [10]. One of the main challenges involved in HNS production by Powder Metallurgy is obtaining

fully dense components with uniform nitrogen content in volume and outstanding surface properties. A uniform nitrogen distribution leads to more homogeneous microstructures, and consequently, to better mechanical properties. When stainless steel powders are thermochemically treated, the small diffusion distances characteristic of the powder particles involved in the process allow chemical homogenization of nitrogen in relatively short times.

Fick’s second law can be used to estimate the time necessary to homogenize simultaneous

sintered-nitrided steel powder particles, as shown in Table 2, for powder particles 70 µm average size, treated at 1273 K to 1473 K. Ten minutes are sufficient to homogenize particles sintered/nitrided at 1473 K while nearly four hours are necessary when nitrided at 1273K.

Table 2 – Diffusion coefficients and times necessary to homogenize 70 µm steel particles.

14000 3.5 x 10-13 1273

2722 1.8 x 10-12 1373

377 1.3 x 10-11 1473

time (s)D*(m2/s)T(K)

POWDER METALLURGY PROCESSES APPLIED TO HNS

Tschiptschin [11] studied the production of high-nitrogen martensitic stainless steel by simultaneous sintering and nitriding AISI 434 L ferritic stainless steel powder at temperatures ranging from 1273 to 1473 K, times of 3, 6 and 9 hours and pressures ranging from 0.1 to 0.36 MPa.

Ferritic low nitrogen iron alloys and stainless steel powder (AISI 434L ) have high compressibility and can be cold pressed leading to green compacts with 78%-80% of the theoretical density ~ 6,2 g/cm3. The effect of the compaction pressure on the relative density is shown in Figure 5.

30

40

50

60

70

80

90

0 200 400 600 800

Compaction pressure (MPa)

Rel

ativ

e de

nsity

(%)

Figure 5 – Effect of compaction pressure on the relative density of AISI 434L green compacts.

After die-compaction the cylinders are put inside a small container, in a tubular furnace

containing high purity nitrogen. During the sintering/nitriding step the alloy picks up nitrogen through a skeleton of open pores, assuring plenty of nitrogen atoms in contact with stainless steel particles, allowing nitrogen dissociation at the surface and nitrogen pick-up. The amount of nitrogen absorbed may be calculated, considering the equilibrium between the gas phase and the steel surface, which is reached in very short times.

Figures 6 shows Thermocalc® calculated isopleths of the Fe-16.2 wt% Cr – N diagrams

representing equilibrium between the steel surface and 0.15 and 0.25 MPa N2 atmospheres. They represent the condition met at the surface of the powder particles, in contact with nitrogen, inside green compacts. Nitrogen contents of sixteen AISI 434L (Fe-16.2 wt%Cr) green compact specimens, nitrided and sintered at different temperatures and N2 pressures are plotted as small squares onto the isopleths. The 1373 K and 1473 K results lay close to the solubility limit of nitrogen in austenite in equilibrium with the gas phase at 0.15 MPa. One can see that the agreement between predicted and measured values is rather good.

The lowest squares in Figure 6 represent nitrogen contents of Fe- 16.2% Cr – N specimens

thermochemically treated at 1273 K during 6 hours. These specimens did not reach equilibrium due to lower diffusion coefficients and insufficient time of exposure to the nitriding atmosphere.

δ + gas δ + gas

γ + gas γ + gas

γ γ γ + Cr2N+gas

γ + Cr2N+gas γ + CrN+gas

γ+CrN γ+Cr2N γ+CrN γ+Cr2N

γ+Cr2N+CrN γ+Cr2N+CrN

(a) (b) Figure 6 - Isopleths of the Fe-16.2 wt% Cr-N phase diagram for (a) 0.15 MPa and (b) 0.25 MPa N2 pressure. The nitrogen contents of the samples, measured after the sintering/nitriding treatment are shown as small squares.

Figure 7 shows the influence of nitriding temperature and nitrogen pressure on the nitrogen pickup of AISI 434L powder during high temperature sintering/nitriding treatment. Increasing the nitriding temperature decreases the amount of nitrogen picked up during the treatment, according to Thermocalc® predictions. Increasing the nitrogen pressure increases the nitrogen content of the sintered material.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

1223 1273 1323 1373 1423 1473 1523

Temperature (K)

% N

0,15 MPA0,20 MPa0,25 MPa0,36 MPa

Figure 7 - Influence of temperature and nitrogen pressure on the nitrogen pickup of AISI 434L powder during high temperature sintering/nitriding for 6 hours.

Figures 8 and 9 show the effect of the sintering/nitriding time and temperature on the relative densities of specimens sintered and nitrided at 0.15 MPa N2 pressure. The increase in sintering temperature has a more pronounced effect over the relative density than the increase in time for a given temperature.

79808182838485868788

1223 1323 1423 1523

Temperature (K)

Rel

ativ

den

sity

(%)

Figure 8 – Effect of sintering/nitriding treatment temperature on the relative density of green compacts treated for 6 hours.

818283848586878889

0 1 2 3 4 5 6 7 8 9 10

Time (h)

Rel

ativ

e de

nsity

(%)

Figure 9 – Effect of sintering/nitriding treatment time on the relative density of green compacts treated at 1473 K.

While the pore structure of the compacted powder remains open and interconnected,

nitrogen can penetrate without difficulty and equilibrium between gas and the steel’s surface is attained. After 1 or 2 hours treatment the steel’s nitrogen content reaches an equilibrium level, although the material itself is not sufficiently compacted.

With increasing treatment time, sintering proceeds, the pore structure becomes closed, and

connectivity is lost, hindering nitrogen penetration. From this point on sintering becomes the main physical process occurring inside the material. Figure 10 shows schematically the kinetics of sintering and nitriding during the simultaneous treatment, performed at 1473K.

SinteringNitriding

open poresA poresAclosed

1.0

0.8

0.6

0.4

0.2

00 1 2 3 4 5 6 time (h)

1.00

0.95

0.90

0.85

0.80

0.75

%N%Neq

aaddth

Figure 10 – Kinetics of sintering and nitriding during a simultaneous sintering/nitriding treatment performed at 1473K.

Loose powder is nitrided more easily than green compacts due to a greater exposure of the particle’s surface do the nitriding media: nitrogen pick-up is higher and nitriding times are much lower. On the other hand, nitriding of loose powder promotes solid solution hardening impairing compressibility and hindering the processing of green compacts.

N ALLOYING STEEL POWDER USING NH3 GAS IN A FLUIDIZED BED

During nitriding of loose powder, sintering of the particles can occur, leading to agglomeration. Two different strategies were proposed to overcome this difficulty: Feichtinger [12] built a rotating furnace where the powder particles can be constantly stirred to prevent necking; Virta [13] proposed a process where nitrogen alloying steel powder is performed in a fluidized bed at low temperatures. The most common nitrogen source used to nitride steel powders is nitrogen gas. Ammonia has been seldom used, even though it has much higher nitriding potential than nitrogen. Nitrogen cannot be used, in this case, as its reactivity with steel is very low at low temperatures. Anhydrous Ammonia is used and it partially dissociates in the fluidized bed. The process can be carried out at atmospheric pressures and the nitrogen potential can be controlled by addition of different gases. Fluidization is necessary to guarantee heat transfer and avoid sintering the powder charge. Figure 11 [13] shows schematically the process. The small diffusion distances once more compensate the nitrogen low diffusivity at the low temperatures the treatment is performed. The resultant microstructure is heterogeneous: compound layers are formed at the surface and can be solubilized by nitrogen diffusion towards the core.

Figure 11 - Fluidized bed system for powder nitriding – VIRTA [1999] [13].

N ALLOYING BY MIXING IRON POWDERS WITH HIGH NITROGEN MASTER ALLOY.

Nitrogen alloying can also be done by mixing soft iron powder with hard chromium nitride

powders without impairing the compressibility of the blend. The master alloy may be obtained after high temperature nitriding of grinded electrolytic chromium. The fine chromium nitride particles act as nitrogen carriers after being embedded in the iron powder. The mixture is cold pressed for compaction and subsequently sintered under nitrogen gas, according to the scheme shown in Figure 12. Homogenization of the alloy takes place during sintering. Chromium nitride particles dissolve and nitrogen gets into solid solution increasing hardness, mechanical and corrosion properties of the obtained alloy.

. Figure 12 – Flowchart showing the stages of the process for obtaining high nitrogen steels by mixing iron powder with pre-grinded, pre-nitrided electrolytic chromium. NITROGEN ATOMIZED POWDERS

Nitrogen pickup is thermodynamically limited to the equilibrium nitrogen gas partial pressure. Nitrogen exceeding the predicted equilibrium content can only be introduced via liquid phase if the L → δ solidification reaction is suppressed. This can be done by forcing a rapid

solidification process where nitrogen desorption is minimized. During quenching of a stream of supersaturated liquid metal by an atomizing gas, the metal droplets solidify so rapidly that they can exhibit non-equilibrium behavior due to high solubility extensions of alloying elements or to suppression of reactions that lead to gas evolution.

High chromium, high manganese stainless steels dissolve a lot more nitrogen in the liquid phase than low alloy steels. High nitrogen levels can be introduced in these alloys in the molten state. Nitrogen atomization of a vertical melt stream of these steels disintegrates it producing very small droplets and simultaneously quenching them, avoiding δ phase solidification. The solidification rate produced by the nitrogen blow is much more rapid than the nitrogen desorption kinetics of the supersaturated liquid. Under these conditions austenite nucleates and grows directly from the melt resulting in a wide range of supersaturated alloys. Rhodes [14] reports different successful runs obtained by Rapid Solidification Process with a wide range of nitrogen contents, as shown in Figure 13 [14].

Figure 13 – Actual and predicted equilibrium nitrogen contents of P/M austenitic stainless steels [14].

MECHANICAL ALLOYING

Attrition milling can introduce extremely high levels of nitrogen in iron. When iron powders are processed in attritor ball mills in nitrogen environment, nitrogen can reach 0.4 to 2.0 wt% after 150 h. During milling a cold worked microstructure, containing dislocation forests and nano-sized cells is produced. Cold-worked particles fragment and are subsequently cold-welded refining even more the structure as shown schematically in Figure 14.

Figure 14 – Schematics of an attrition milling process [15].

Nitrogen gas molecules attached to the particle’s surface dissociate and nitrogen is introduced in the grains during cold welding. The amount of fresh surface exposed to the processing environment is proportional to the processing time. The grain surface becomes enriched in nitrogen but does not diffuse towards the grain core.

Iron powders and chromium nitrides can also be processed in Attritor Ball Mills. The iron nitride particles are broken and welded to the outside of the iron powder particles. The particles are intensively fragmented reaching a size that renders instability. The particle dissolves and nitrogen goes into solution occupying the bct lattice sites on the grain boundaries. When compound layers are formed at the surface they can be solubilized by nitrogen diffusion towards the core. The amorphization depends on an interaction parameter, which represents the difference in bonding energy of Fe-N atomic pairs and M-N pairs M being is a substitutional element). Elements with higher affinity for N strongly promote amorphization and refining [15]. INJECTION MOULDING HNS can be mass-produced by Metal Injection Moulding obtaining near-net shape parts. Uggowitzer [16] patented the process - CATAMOLD® - to obtain a high nitrogen austenitic stainless steels - PANACEIA® - for medical applications. The process consists in mixing metal powder with an organic binder and injecting the blend into the mould. Catalytic debinding process is conducted at 110ºC in an acid containing N atmosphere. After debinding the powder particles are weakly stuck together and a net of open pore channels remains in the alloy. During the sintering/nitriding treatment sintering occurs slowly while nitriding of the powder skeleton happens quickly. The process is shown schematically in Figure 15.

+

HNS

Sintering/Nitriding

SinteringNitriding

Granules

Blending

Binder

Metal powder Injection Moulding

Debinding

Figure 15 – Metal Injection Moulding applied to HNS [16]. HOT COMPACTION

Fully dense compacts may be processed by applying elevated temperature and external pressure. High temperature deformation mechanisms of the powder particles are activated allowing accommodation between particles and as a consequence pore fraction reduction. The goal is obtaining a material with 99-100% of the theoretical density. Pre-nitrided hardened powders, with low compressibility, need to be simultaneously hot pressed and sintered. Green compacts sintered/nitrided at low temperatures, do not reach relative densities compatible with applications where corrosion resistance is essential. In both circumstances it is necessary to consolidate the material using full density techniques: hot extrusion and hot isostatic pressing (HIP).

HOT EXTRUSION Loose powder is put inside a mild steel can and consolidated by hot extrusion. The can is degassed and sealed by welding. The capsules are extruded to straight bars after being heated at high temperature. The mild steel skin is removed by machining. The low compressibility of the high nitrogen powder is bypassed in this process and densification and shaping are achieved in one operation. HIP – HOT ISOSTATIC PRESSING

Hot Isostatic pressing may be used as a final step to obtain full density P/M processed HNS. Encapsulated HIP, using a suited canning system - Figure 16 - is the only processing route, which allows pressure-aided densification over the complete range from green compact density to theoretical density. A can is filled with metal powder, evacuated, sealed and hot isostatically pressed to a relative density of 99-100%. After a suitable heat-treating cycle the can is removed. Post – HIP can be applied to preforms, which are sintered in a separate cycle to the density level (90-93%) whereby the pores are closed.

Nitriding cannot be performed during encapsulated or post-HIP pressing due to closing of

the pore structure.

Figure 16 – Schematics of the encapsulated Hot Isostatic Pressing process.

EFFECT OF PROCESSING ROUTE ON SURFACE PROPERTIES Each of the above-mentioned processes leads to specific microstructures and properties. If

sintering/nitriding is performed at low temperatures, to obtain high nitrogen contents, densification is hindered and the product shows high porosity. Encapsulated HIP must be carried out to guarantee low porosity and high density. When HIP is not available and the sintering/nitriding treatment is done at high temperatures, to assure low porosity, low nitrogen products are obtained.

One of the main applications of high-nitrogen martensitic stainless is in the field of the

mining and petrochemical industries, in components that work in contact with slurry environments. When properly processed these materials become highly resistant to erosion-corrosion in slurry environments containing chlorides. Toro et al. [17] studied the wear resistance of high-nitrogen martensitic stainless steels, processed through different P/M routes as shown schematically in the flowchart depicted in Figure 17.

PR4 PR3 PR2 PR1

1623K sintering

sintering/nitriding1273-1473K

hot uniaxial pressing 1273-1473K

hot isostatic pressing HIP – argon – 1423K

hot isostatic pressing encapsulated HIP – 1423K

pressing

stainless steel powder water atomized

HNS powder

mechanical mixture mechanical alloying

Cr and N carriers addition NCP * addition

iron powder

solid-state nitriding 1273-1473K

Figure 17 – Flowchart showing the different P/M routes used to obtain high-nitrogen martensitic stainless steels.

* NCP – Nitrided Chromium Powder

The specimens obtained through the four mentioned routes were oil quenched from 1423-

1473 K and tempered at 473 K during 1 hour. The ranges of nitrogen contents of the specimens are shown in Table 3. The relative densities obtained after each P/M route is shown in Table 4. Table 3. Range of nitrogen contents of the specimens.

Production Route wt.% N Range

1: Die compaction + sintering/nitriding + hot isostatic pressing 0.47-1.51

2: Nitriding of uncompressed powder + hot isostatic pressing 1.02-2.90

3: Nitriding of the uncompressed powder + hot pressing 0.52-2.03

4: Mechanical alloying + sintering + hot isostatic pressing 0.12-0.73

Table 4. Average relative density of the high nitrogen stainless steels

Process Relative Density (%)

Production Route 1

Sintering/Nitriding 1273 K / 6-12 h 81.0-82.0



Hot Isostatic Pressing 1423 K 98.0-99.5

Production Route 2


Production Route 3

Hot Pressing 1273 K 83.5-84.0



Production Route 4

Sintering 1623 K / 7-8 h 94.7-95.5


The specimens submitted to erosion corrosion tests in slurry environment, made of

substitute ocean water and quartz particles, allowed studying the synergism between corrosion and erosion. Non-corrosive erosion tests were carried out replacing substitute ocean water by tap water, while solids-free impingement tests were performed in substitute ocean water without quartz particles.

Figure 4 shows the contribution of erosion, corrosion and synergism between them to the

total specific mass loss in corrosion-erosion tests. The erosion contribution (∆ME) was obtained from the results of tests performed in non-corrosive slurry, while the corrosion contribution (∆MC) came from solids-free impingement tests. The synergism (S) was calculated as the difference between the mass loss under corrosion erosion (∆MC-E) and the sum of the mass losses under solids-free impingement and non-corrosive erosion, as indicated in Eq. 1.

S = ∆MC-E - ∆MC - ∆ME Eq. [1]

From Figure 18 it can be seen that the synergistic effects between corrosion and erosion in conventional martensitic stainless steels are more accentuated than in the high nitrogen ones. Synergism can be associated to the susceptibility to intergranular corrosion and spalling of second-phase particles [17]. Besides this, the measured specific mass losses were significantly lower for all the nitrogen-alloyed specimens.

Figure 18 - Corrosion, Erosion and Synergism contributions to the total specific mass loss measured in corrosion-erosion tests. Slurry composed by substitute ocean water and quartz particles. All the specimens quenched and tempered at 473 K for 1 hour.

Porosity increments from 1% to 3% lead to expressive surface damage after 96 hours tests in substitute ocean water containing quartz particles. This behavior can be associated with the increase of synergistic effects between corrosion and erosion when large pores are present in the microstructure. BLIOGRAPHY

1) H.K. Feichtinger and G. Stein, . – HNS98 – Materials Science Forum, 318-320 (1999) p.261.

2) A.Satir-Kolorz, H.K. Feichtinger and M.O. Speidel, Giesserei Forshung, 41 (1989) p. 149. 3) M.B. Horovitz; F. Beneduce Neto; A.A. Garboggini and A.P. Tschiptschin – ISIJ Intern. 36

(1996) p. 840. 4) H. Berns - German patent letter DE40337006. 5) H.W. Zoch and H. Berns – US Patent – 5503797. 6) A.P. Tschiptschin – THERMEC 2000 - Special Issue of the Journal of Materials Processing

Technology, CD-ROM-B4, 117 3 (2001). 7) D.Heger and T.V.Duong –Defect and Diffusion Forum, 143-147 (1997), p. 443. 8) F.Thümler and R. Oberacker – Introduction to Powder Metallurgy – University Press,

Cambridge (1993) p. 3. 9) H. Frisk – Metallurgical Transactions A 21 A (1990) p. 2477. 10) C.M. Garzón and A.P. Tschiptschin – to be published. 11) A.P. Tschiptschin – Habilitation Thesis for becoming Associate Professor – University of

São Paulo – Metallurgical and Materials Engineering Department (2000) p. 90. 12) H. Feichtinger – in G.Stein and H. Witulski – High Nitrogen Steels, HNS 90 (1990) Stahl

und Eisen, Düsseldorf, p. 298. 13) J. Virta and S.P. Hannula – in H.Hänninen, S. Hertzman and J. Romu - High Nitrogen

Steels, HNS 98 (1998) – Trans Tech Publ., Switzerland p 655.

14) G.O Rhodes and W.B.Eisen - in H.Hänninen, S. Hertzman and J. Romu - High Nitrogen Steels, HNS 98 (1998) – Trans Tech Publ., Switzerland p 635.

15) R.M. German – Powder Metallurgy Science – 2nd ed. (1994) p. 462. 16) P.J.Uggowitzer, W.F.Bähre, H. Wohlfromm and M.O. Speidel - in H.Hänninen, S.

Hertzman and J. Romu - High Nitrogen Steels, HNS 98 (1998) – Trans Tech Publ., Switzerland p 663.

17) A. Toro, D.K.Tanaka,A. Sinatora, A.P. Tschiptschin – Wear, 251 (2001) p. 431. ACKNOWLEDGEMENTS The author wish too thanks FAPESP – 98/15758-4 and PADCT/CNPQ - 62.0133/98-8 for financial support.

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