1

Influence of Ni, Mn, and Cu on the nitride formation in 20Cr alloys relevant to modern stainless steels

Mina Khoda Karami Ali Kohanzadehmaranlou

Master of Science Thesis - Engineering Materials Science

KTH Royal Institute of Technology

Industrial Engineering and Management

SE-100 44 STOCKHOLM

2

Abstract

New experimental information on the effect of Ni, Cu and Mn on the nitride precipitation

temperature and the phase relations in the Fe-Cr-N system is presented. The samples were

heat treated under flow of pure and mixed nitrogen gas at different temperatures. The

microstructure was investigated by SEM and the nitrogen composition by LECO nitrogen

analyzer. The phase equilibria between austenite and Cr2N nitride have been studied at 1173

K and 1373 K using the sealed capsule technique to measure the N activity and SEM-EDS to

measure the chemical compositions of the individual phases followed by analysis using

LECO nitrogen analyzer. Deviations from theoretical calculations which have been done by

Thermo-Calc using the TCFE7 database were observed. E. g. Thermo-Calc predicted single

phase austenite at higher Cr content in contrast to experiments which showed austenite and

Cr2N nitride phases. Furthermore, the experimental data show slight changes on the

extension of the single phase austenite field whereas the calculations show larger effects by

addition of Ni, Cu and Mn.

3

FOREWORD

The authors would like to thank to the supervisors Staffan Hertzman , Malin Selleby and Peter

Hedström for their mentoring and inspiration in this thesis work. Special thanks to Rein Vainik

for his help and arrangements to do analysis in this work at Swerea KIMAB. The authors would

also like to thank Albin Stormvinter for his continuous help and advices and also to suggestions

from Lars Höglund.

4

ABSTRACT 2

FOREWORD 3

TABLE OF CONTENTS 4

1 INTRODUCTION 6

1.1 BACKGROUND OF THE FE-CR-N SYSTEM 6

1.2 COMPUTATIONAL THERMODYNAMICS AND THERMO-CALC 7

1.3 PURPOSE 7

2 IMPLEMENTATION 8

2.1 EXPERIMENTAL TECHNIQUE 8

2.1.1 HEAT TREATMENTS 8

2.1.2 PART ONE: FINDING NITRIDE FORMATION TEMPERATURE 8

2.1.3 PART TWO: PHASE EQUILIBRIA 9

2.2 ANALYZING TECHNIQUE 9

2.2.1 NITROGEN ANALYSIS BY LECO TC-436 NITROGEN-OXYGEN ANALYZER 9

2.2.2 PHASE IDENTIFICATION USING BACKSCATTER ELECTRONS IN SEM 9

3 RESULTS 10

3.1 EXPERIMENTAL RESULTS WITH NITRIDE FORMATION TEMPERATURE 10

3.1.1 ALLOYS WITH HIGHER CR CONTENTS 12

3.1.2 ADDITION OF NI 13

3.1.3 ADDITION OF CU 14

3.1.4 ADDITION OF MN 15

3.2 EXPERIMENTAL RESULTS OF PHASE EQUILIBRIA 16

4 DISCUSSION AND CONCLUSION 17

4.1 Discussion 17

4.1.1 COMPARISON BETWEEN EXPERIMENTAL AND CALCULATED

RESULTS FOR PART ONE 17

4.1.2 COMPARISON BETWEEN EXPERIMENTAL AND CALCULATED

RESULTS FOR PART TWO 20

5

4.1.2.1 THE FCC / Ε EQUILIBRIUM IN THE FE-CR-N SYSTEM WITH ADDITION

OF NI,CU AND MN 20

4.1.2.2 THE FCC PHASE IN THE FE-CR-N SYSTEM WITH ADDITION OF NI, CU

AND MN 26

4.2 Conclusions 28

5 REFERENCE 29

APPENDIX A: SUPPLEMENTARY INFORMATION 30

6

1 INTRODUCTION

1.1 Background of the Fe-Cr-N System

Addition of nitrogen in stainless steels as type of alloying element has been increasing because

it provides steels with higher mechanical properties and high resistivity to pitting corrosion.[1]

Solubility of nitrogen in steels depends on the composition of steel, crystal structure and partial

pressure of nitrogen. [3]

for example, nickel which stabilizes the austenite phase, decrease the

solubility of nitrogen. In addition, manganese increase nitrogen solubility while it has the similar

effect, as nickel, on austenite stability. In general, there are not much information about the

addition of alloying elements on the solubility of nitrogen in the case of solid steels. Many

research on solid steels have been done on fine powder and thin plates. [4-5-6-7-8-9]

Chromium is an element that increase the solubility of nitrogen in iron and because of this fact

it has been used broadly in illustration of high nitrogen stainless steels.( In fact nitrogen enhance

the mechanical properties of stainless steels like increasing their tensile strength and hardness in

a way that their toughness will not decrease.) It stabilized austenite. Due to the fact that nitrogen

brings a proper mixture of mechanical properties, good weld ability and higher resistivity to

corrosion, then has been an increase in the carbon restoration (full and partial) by using nitrogen.

This replacement in steels with higher chromium content delays M23C6 precipitations. [10]

There are some information on the Fe-Cr-N system which can be find in the paper by Tisinai et

al. who studied about the Fe-Cr-C-N and Fe-Cr-Ni-N systems on alloy with 21to 33 wt percent

Cr. In addition, precipitation of nitride and solubility of nitrogen in Fe-Cr-Mn-Ni steel alloys has

been studied under high nitrogen gas pressure by Rawers, Bennet, Doan and Siple. the resulting

phases and microstructure of steel alloys containing nitrogen were examined to conclude the

limitation of interstitial solubility of nitrogen. The formation of of Cr2N in two nitrogen alloyed

austenitic stainless steels with different carbon contents is studied by means of electron

microscopy and thin foil techniques by Kirsti Mielityine-tiitto. [11]

Some information on the Fe-Cr-N system can be obtained from the series of investigations by

Tisinai et Al. who studied the Fe-Cr-C-N and Fe-Cr-Ni-N systems in the composition range 21

to 33 wt pct Cr but also with substantial amounts of silicon and manganese present, using a

metallographic technique. In addition, nitrogen solubility and nitride formation in Fe-Cr-Mn-Ni

alloys has been studied under high nitrogen gas pressure by Rawers, Bennet , Doan and Siple.

The resulting microstructure and crystal phases of the nitrogen-iron alloys were analyzed to

determine the interstitial nitrogen solubility limit.[6]

The precipitation of Cr2N in two nitrogen

alloyed austenitic stainless steels with different carbon contents is studied by means of electron

microscopy and thin foil techniques by Kirsti Mielityine-tiitto. [11]

Presser and Silcock have been

studying the cellular precipitation of nitride precipitation in Fe-18Mn-18Cr-0.6N alloy. Also,

Rayaprolu and Hendry studied the characteristics of cellular precipitation in Nitronic 50, by use

of fixed activity of nitrogen and the process nature by x-ray diffraction and electron microscopy.

[12] Finally, Hertzman studied phase equilibria in the Fe-Cr-N and Fe-Cr-Ni-Mo-C-N systems at

1273 K. [2]

7

1.2 Computational thermodynamics and Thermo-Calc

Thermodynamics describe the equilibrium state of a system. Since every system is trying to

reach this state, it is essential for phase transformation and process simulation to have knowledge

on both meta-stable and equilibrium condition. Many mathematical models have been developed

to describe the thermodynamic properties of materials, and proven to be very useful in

developing new material since they can save the time and money for experimental work. In

general, a good thermochemical databank should consist of reliable, consistent, and high quality

data for different fields of thermodynamic, which have been always considered individually such

as geochemistry, metallurgy, ceramic and steel alloys. Thermodynamic calculations using

software packages such as Thermo-Calc can dramatically facilitate the design/development of

new materials, choosing heat treatments temperatures, optimizing yields of manufacturing

processes, and etc.

1.3 Purpose

The main ambition of the project was to experimentally determine the tendency for nitride

formation in stainless steels by nitriding a set of alloys, Table 1, and clarifying the effects of the

elements Cu, Mn, and Ni. This should then be compared to the predictions using Thermo-Calc

and its thermodynamic databases, e.g. TCFE7. In addition, an attempt has been made to study

phase equilibria of Fe-Cr-N system by addition of Ni, Mn and Cu at 1173 K and 1373 K.

Fig.1: The isothermal section Fe-Cr-N at 1000 °C.

Alloys Cr% Ni% Cu% Mn% structure

1 19.8 0.0 <0.01 0.11 ferritic

2 19.9 2.9 <0.01 0.09 ferritic

3 19.7 0.2 1.45 0.08 ferritic

4 19.9 <0.1 0.1 4.42 ferritic

5 24.3 <0.1 0.02 0.44 ferritic

6 28.7 <0.1 0.01 0.21 ferritic

8 20.08 2.98 1.48 5.12 duplex

9 19.8 10.14 0.01 0.05 austenitic

10 19.89 9.96 1.48 0.02 austenitic

11 20.05 9.99 0.016 4.7 austenitic

Table 1: Compositions of alloys (mass-%)

8

2 Implementation

2.1 Experimental Technique

The compositions of the samples were selected to examine the austenite one-phase field

boundary by variation of Ni, Mn and Cu. The samples were prepared by Outokumpu in the form

of 6 mm thick plates. Then the samples cut down in to 1mm*15mm*6 mm pieces.

2.1.1 Heat treatments

The samples were heat treated in a high temperature furnace in the range 1050-1210 °C. A

flowing nitrogen gas atmosphere was applied in the furnace during the heat treatment process to

diffuse into samples. The heat treatment time needed to reach the equilibrium state can be

determined by the time needed for the elements to diffuse through the thin plates of stainless

steels. This can be estimated from the Einstein equation for random walk:

X=√

where D is the diffusion coefficient of the element in the phase with a specific composition at a

certain temperature. The values for both N and Cr were calculated from the mobility database in

the DICTRA software. X is the diffusion distance the element diffuses in time t. With the above

equation, the estimated heat treatment time is 10 hours for N. However, the samples were heat

treated for 44 hours to ensure that equilibrium was reached and then quenched in water

2.1.2 Part One: Finding the Nitride Formation Temperature

The experimental measurements have been done in this project to clarify the effect of Ni, Mn

and Cu on the nitride formation temperature in high Cr stainless steel alloys. Sets of specimens

were put on a holder and covered by a stainless steel foil in order to prohibit the oxidation. The

samples were then heat treated under two flows of gas: pure nitrogen to get the nitrogen activity

of 1 and gas mixture of 0.25 atm nitrogen and argon-mixture, to get the nitrogen activity of 0.5,

for 44 and 48 hours. To estimate the heat treatment temperature range, an attempt was made to

calculate the nitride precipitation temperature by using Thermo-Calc and TCFE7. The heat

treatments continued for temperature interval of 10 °C until each specimen showed single-phase

austenite. The slightly oxidized surface layers were removed by grinding and samples were

identified metallographically using SEM and for their nitrogen content by LECO nitrogen

analyzer.

Fig.2: Experiment design and sample preparation

9

2.1.3 Part Two: Phase Equilibria

The purpose of the experiments was:

a) To setup experimentally the effect of Ni, Cu and Mn on phase equilibria in the system

Fe-Cr-N at 1173 K and 1373 K.

b) To supply information for position of FCC/FCC+Cr2N phase boundary.

SEM-EDS and LECO analysis have been used to acquire direct data on the tielines between

equilibrium phases. All samples have been pre-nitrided in an open system at at 1373 K (1100°C)

for 44 hours for aN = 1 and 48 hours for aN = 0.5. Then they were ground and sets of them were

put in each silica tube, which was evacuated. After sealing the capsules, the samples were

equilibrated for 1 month at 1373 K and 5 months at 1173 K and quenched in brine. In order to

get information about the nitrogen activity for a tie line, an additional sample with 10% Cr

composition was used which was in one phase at both 1173 and 1373 K. By using Thermo-Calc

calculations, the nitrogen activities for this alloy at both temperatures were calculated and these

activities were used for the rest of the analysis. The phases were identified for their chemical

compositions by SEM-EDS and for their total nitrogen content by LECO. An attempt was made

to determine compositions of each phase using SEM-EDS, but the low intensity of Nitrogen

makes this determination problematic. So, using some single phase alloys which have known

nitrogen content as the reference samples in SEM-EDS analysis, correction has been done for the

phase’s compositions obtained by SEM.

2.2 Analyzing Technique

2.2.1 Nitrogen analysis by LECO TC-436 Nitrogen-Oxygen analyzer

The inert gas fusion method was employed to calculate nitrogen and oxygen contents of various

materials for many years. LECO Corporation introduced its first inert gas fusion oxygen and

nitrogen determinators in the mid-twentieth century. This methodology is referred to as a thermal

evolution method in ISO terminology. This method was originally developed for rapid nitrogen

and oxygen perseverance of steel and iron (ASTM E1019). However, a variety of ferrous and

nonferrous materials are analyzed using LECO inert gas fusion instruments as well (ASTM

E1409 and E1937).

The high-purity graphite crucibles which have been used to hold the sample, while the fusion

process is done, are an integral part of the determination. They are actually carbon resistors that

make the heat necessary for the fusion of the sample as well as the carbon for the reduction of

oxygen in the sample. The crucibles are made from high purity graphite and they are available in

different sizes and shapes.

To determine the nitrogen content of heat treated alloys, all samples have been analyzed by

LECO TC-436 Nitrogen-Oxygen analyzer at SWEREA/KIMAB.

2.2.2 Phase identification using backscattered electrons in SEM

The scanning electron microscope (SEM) is a tool that can create larger image of sample

surfaces by using electron beams instead of light. This electron beam is produced by the electron

gun microscope, then it passes vertically through vacuum in the microscope. By passing through

the electromagnetic fields and lenses it will reach the surface of samples. During the time of

contacting the sample, electrons and X-rays are excluded from the sample. Finally, the detectors

collect these X-rays, primary electrons and the electrons which are produced from collision of

10

primary electrons with the surface of specimen and they will turn them to the signals which are

carried to the final image in the screen of TV. Rapid analysis of samples, images with high

resolutions and good detection limit for alloying elements are the obvious advantage of the using

scanning electron microscopy.

3 RESULTS

3.1 Experimental results with nitride formation temperature

After experiments, the surface of all the samples revealed a thin layer of oxide. This was due to

the impurity in the nitrogen gas. Then the samples were cut into pieces and were mounted in

bakelite plastic resins ground and then polished by automatic polishing machine and

subsequently analyzed by SEM for their structures. The BSE images of all samples for the two

activities are shown in Figure 4 and 5.

Fig.4: Structural images of all samples from all heat-treatments at different temperatures (°C) aN=0.5. (appendix)

11

Fig.5: Structural images of all samples from all heat-treatments at different temperatures (°C) aN=1. (appendix)

12

3.1.1 Alloys with higher Cr contents

Results of phase identifications by SEM and nitrogen composition analysis by LECO nitrogen

analyzer are listed in Table 2 and 3. At the higher temperatures alloy with 20% Cr shows fully

austenitic while two other with higher chromium have fractions of nitrides. Addition of

chromium increases the nitride formation temperature. BSE images of these three alloys at aN=1

and 1160 °C are shown in Fig.6. The black area in samples 5 and 6 are Cr2N phase. The desired

nitrides have to be along the grain boundaries but in alloy 6 there are some nitrides within the

grains with lamellar structures, which can be formed during cooling. BSE images of these three

samples at aN=0.5 , 1090°C also are given in Fig.7. to show the effect of Cr. As it is given in

Tables 2 and 3, alloys with higher chromium content dissolve more nitrogen, significantly at

lower temperatures.

Fig.6: Effect of Cr at 1160 °C. A) Alloy 20% Cr is fully austenitic. B) Alloy 25% Cr has some fraction of nitride

with austenite. C) Alloy with 30% Cr has a higher fraction of chromium nitrides, aN=1

a b

a b

c

13

Fig.7: Effect of Cr at 1090 °C. A) Alloy 20% Cr is fully austenitic. B) Alloy 25% Cr has some fraction of nitride

with austenite. C) Alloy with 30% Cr has a higher fraction of chromium nitrides, aN=0.5

3.1.2 Addition of Ni

The BSE images of sample 1, 2 and 9 at aN=1 and 1130 °C and aN=0.5 and 1070 °C are shown

in Fig. 8. and Fig. 9. Having more Ni in the stainless steel alloys will increase the presence of

chromium nitrides at higher temperatures. In contrast with chromium, Ni decreases the nitrogen

solubility in stainless steels. Alloys 9 and 2 have lower nitrogen content than alloy 1 which does

not have any nickel.

Fig.8: Effect of Ni at 1130 °C. A) Alloy 1 with 20% Cr is fully austenitic. B) Alloy 2 with 3% Ni has a small

fraction of nitride with austenite. C) Alloy 9 with 10% Ni has a higher fraction of chromium nitrides, aN=1.

a b

c

c

14

Fig.9: Effect of Ni at 1070 °C. A) Alloy 1 with 20% Cr is fully austenitic. B) Alloy 2 with 3% Ni has some

fraction of nitride with austenite. C) Alloy 9 with 10% Ni has a higher fraction of chromium nitrides, aN=0.5.

3.1.3 Addition of Cu

The chemical analysis and structural results from experiments indicates alloys 3 and 10, which

contain copper, have two-phase structure at higher temperature while alloy 1 shows one phase

austenite. The BSE images of sample 3 and sample 10 at aN=1 and aN=0.5 are given in the Fig.10

and Fig.11. By comparing Figures 10 and 11 it can be concluded that addition of Cu increases

the nitride solubility temperature in stainless steel alloys. However, this effect is not so much and

Cu showed a slight increase on nitrogen solubility.

Fig.10: Effect of Cu. A) Alloy 3 with 1.5% Cu shows some amount of nitrides at 1130 °C. B) Alloy 10 with 1.5%

Cu and 10% Ni show more nitrides in comparison with alloy 9 at 1140 °C. aN=1

a b

c

a b

15

Fig.11: Effect of Cu. A) Alloy 3 with 1.5% Cu shows some amount of nitrides at 1070 °C. B) Alloy 10 with 1.5%

Cu and 10% Ni show a higher nitrides in comparison with alloy 9 at 1070 °C. aN=0.5

3.1.4 Addition of Mn

Manganese also increases the nitride solubility temperature. Fig.12 and Fig.13 compares

images of sample 4 and 11 at aN=1 and aN=0.5 which also declare the effect of Mn addition in

stainless steel grades. Alloy 11 with 5% Mn and 10% Ni has some fractions of nitrides while

alloy 9 with 10 % Ni is fully austenitic under the same conditions. Similar trends can be

understand at aN=0.5 by referring to Figure 13. Nitrogen solubility of stainless steels is

increasing by addition of manganese. The experiments of this report show that alloys having Mn

have higher nitrogen content comparing with alloy 1 where they heat treated under the same

conditions.

Fig.12: Effect of Mn. A) Alloy 4 with 5% Mn has a higher content of nitrides at 1122 °C while alloy 1 has less. B) Alloy 11

with 5% Mn and 10% Ni has considerable fraction of chromium nitrides at 1140 °C where alloy 9 is fully austenitic. aN=1.

Fig.13: Effect of Mn. A) Alloy 4 with 5% Mn has more content of nitrides at 1080 °C while alloy 1 has less. B) Alloy 11 with

5% Mn and 10% Ni has considerable fraction of chromium nitrides at 1140 °C where alloy 9 is fully austenitic, aN=0.5.

a b

a b

a b

16

Nitrogen Activity = 1

Alloy Phases and Nitrogen Content

T=1100 T=1113 T=1130 T=1140 T=1150 T=1160 T=1180 T=1200

1 ɤ+ε 1.029 ɤ+ε 0.957 ɤ 0.905 ɤ 0.869 ɤ ɤ 0.804 ɤ 0.749 ɤ 0.699

2 ɤ+ε 1.07 ɤ+ε 0.963 ɤ+ε 0.869 ɤ 0.837 ɤ 0.795 ɤ 0.767 ɤ 0.719 ɤ 0.669

3 ɤ+ε 1.083 ɤ+ε 0.981 ɤ+ε 0.907 ɤ 0.865 ɤ 0.841 ɤ 0.805 ɤ 0.755 ɤ 0.704

4 ɤ+ε 1.299 ɤ+ε ɤ 1.066 ɤ 1.017 ɤ 0.952 ɤ 0.952 ɤ 0.884 ɤ 0.837

5 ɤ+ε 1.872 ɤ+ε 1.752 ɤ+ε ɤ+ε 1.472 ɤ+ε 1.305 ɤ+ε 1.252 ɤ 1.151 ɤ 1.074

6 ɤ+ε 2.529 ɤ+ε 2.412 ɤ+ε ɤ+ε 2.221 ɤ+ε 2.092 ɤ+ε 1.96 ɤ+ε 1.715 ɤ+ε 1.504

8 ɤ+ε 1.403 ɤ+ε 1.348 ɤ+ε 1.087 ɤ 1.017 ɤ 0.983 ɤ 0.962 ɤ 0.883 ɤ 0.84

9 ɤ+ε 1.066 ɤ+ε 1.001 ɤ+ε 0.792 ɤ 0.716 ɤ 0.688 ɤ 0.666 ɤ 0.61 ɤ 0.571

10 ɤ+ε 1.168 ɤ+ε 1.072 ɤ+ε ɤ+ε 0.775 ɤ+ε 0.719 ɤ 0.685 ɤ 0.642 ɤ 0.59

11 ɤ+ε 1.319 ɤ+ε 1.217 ɤ+ε ɤ+ε 0.9 ɤ 0.85 ɤ 0.821 ɤ 0.758 ɤ 0.72

Table 2: Experimental results of SEM and Nitrogen analysis (LECO) for experiments in open system at nitrogen

activity one

Nitrogen Activity = 0.5

Alloy Phases and Nitrogen Content

T=1050 T=1060 T=1080 T=1090

1 ɤ+ε 0.7550 ɤ 0.7342 ɤ 0.6840 ɤ 0.6620

2 ɤ+ε 0.7396 ɤ 0.6995 ɤ 0.6414 ɤ 0.6330

3 ɤ+ε 0.7822 ɤ 0.7306 ɤ 0.6767 ɤ 0.6707

4 ɤ+ε 0.9880 ɤ+ε ɤ 0.8117 ɤ 0.8028

5 ɤ+ε ɤ+ε ɤ+ε ɤ+ε 1.078

6 ɤ+ε ɤ+ε ɤ+ε ɤ+ε 1.706

8 ɤ+ε 1.108 ɤ+ε ɤ ɤ 0.815

9 ɤ+ε 0.7147 ɤ+ε ɤ+ε ɤ 0.540

10 ɤ+ε 0.7925 ɤ+ε ɤ+ε ɤ 0.567

11 ɤ+ε 0.930 ɤ+ε ɤ+ε ɤ 0.675

Table 3: Experimental results of SEM and Nitrogen analysis for experiments in open system at nitrogen activity

0,5

3.2 Experimental Results of Phase Equilibria

All experimental results about the phase equilibria at 1173 K and 1373 K are given in Table 4.

In addition, information about the chromium distribution between the phases are listed in this

17

Table. The first measurements which were done by SEM-EDS for calculating nitrogen content in

austenite phase were not very reliable. To get more better data and values two methods were

applied :

1) Some single-phase specimens with known nitrogen content estimated from LECO have

been analyzed by SEM-EDS for their nitrogen content and a correlation graph has been

plotted. By using that graph and EDS nitrogen analysis for austenite in two-phase

specimens, the new nitrogen content of equilibrated samples have been calculated. The

resulting values are given in Table 4.

2) WDS chemical analysis has been applied to get exact composition of each phase in two

phase specimens. Table 5 is representing the results.

Temp.

°C

Alloy

No.

Nitrogen

Activity

Nitrogen

Content

Wt Pct

phases

Composition According to SEM Measurement

Austenite (ɤ) Cr2N (Ɛ)

Wt Pct Cr

Wt Pct N Wt Pct

Ni Wt Pct Cu

Wt Pct Mn

Wt Pct Cr

Wt Pct N

1100

1 1 1.092 ɤ + Ɛ 16.43 0.7227 77.44 10.61

2 1 0.9908 ɤ + Ɛ 16.76 0.6967 2.98 79.7 10.66

3 1 1.0482 ɤ + Ɛ 17.15 0.6193 1.41 78.13 10.62

4 1 1.3188 ɤ + Ɛ 15.86 0.8129 4.36 75.82 10.04

5 1 1.795 ɤ + Ɛ 16.62 0.7429 - -

6 1 2.421 ɤ + Ɛ 17.29 0.8168 79.85 10.2

8 1 1.254 ɤ + Ɛ 16.02 0.5857 2.96 1.5 4.98 74.74 10.39

9 1 1.011 ɤ + Ɛ 15.7 0.4796 10.01 78.66 10.13

10 1 1.1167 ɤ + Ɛ 15.07 0.3105 10.11 1.59 79.32 10.51

11 1 1.2089 ɤ + Ɛ 15.42 0.5478 10.45 4.59 79.7 10.62

R

900

1 0.187 1.11 ɤ + Ɛ 13.81 0.2597 74.58 9.99

2 0.187 1.121 ɤ + Ɛ 13.58 0.2236 3.01 82.14 10.3

3 0.187 1.365 ɤ + Ɛ 14.03 0.1989 1.26 80.94 10.78

4 0.187 1.299 ɤ + Ɛ 12.43 0.254 4.46 84.74 10.56

5 0.187 2.017 ɤ + Ɛ 13.5 0.2424 79.26 10.16

6 0.187 2.519 ɤ + Ɛ 14.09 0.2761 81.81 10

8 0.187 1.384 ɤ + Ɛ 12.06 0.1465 3.1 1.51 5.08 72.62 9.66

9 0.187 0.977 ɤ + Ɛ 13.96 0.189 10.51 81.88 9.89

10 0.187 1.15 ɤ + Ɛ 12.74 0.0866 10.65 1.74 85.39 10.58

11 0.187 1.324 ɤ + Ɛ 11.75 0.1345 10.53 84.46 10.52

R 0.187 0.105 ɤ 9.53 0.094

Table 4 : Experimental results from SEM-EDS and LECO analysis for phase equilibria in capsules at 1173 K and

1373 K.

18

4 DISCUSSION AND CONCLUSION

4.1 Discussion

4.1.1 Comparison between experimental and calculated results for part one

The nitrogen content of specimens at heat treatment temperatures have been calculated by

using ThermoCalc with TCFE7 and a graph (Fig.14) which is comparing these calculated values

with actual ones from LECO analysis has been plotted. There are deviations between the analysis

and the calculated results, especially in alloys which contains Cu and also those ones with higher

Cr contents. However, generally the agreement for the rest of the samples which contain Mn and

Ni especially at higher temperatures are satisfactory. Both calculated and experimental results

show the same trend for the effect of Ni, Mn and Cu addition on nitrogen solubility

temperatures.

Fig. 14: Comparison between ThermoCalc calculations and experimental results for nitrogen content.

In order to do a comparison between the calculated and experimental results for Cr2N nitride

precipitation temperatures a graph (Fig.15) has been plotted. The deviation is largest for

specimens with higher chromium content and also for specimens which contain Cu (for both

activities). But for alloys which have Ni, experimental results are in accordance with Thermo-

Calc calculations. Nevertheless, both Thermo-Calc and experiments show the same trends in

predicting the effect of addition of Ni, Cu and Mn on the nitride formation temperatures in

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8

N %

in

all

oy

s fr

om

TC

N % in alloys from LECO

1100

1081

1113

1122

1130

1140

1150

1160

1170

1180

19

stainless steels. For instance, both results declare that by adding Mn to a 20 % Cr stainless steel,

nitride precipitation temperature will increase.

Fig. 15: Comparison between ThermoCalc calculations and experimental results for nitride formation temperature.

The isoactivity surfaces for all specimens are calculated by Thermo-Calc and for comparison

plotted together with equivalent experimental data in Figures 16, 17 and 18 at 1363 K, 1433 K

and 1473 K for both nitrogen activities. The agreement is satisfactory for alloys 1,2,4,9 and 11 at

aN=0.5 and for alloys 2, 9 and 11 at aN=1 which means that for alloys having Ni both results

show similar data. However, there are deviations from theoretical calculations for other alloys

containing cupper. While experiments which have been performed in this project showed that

these alloys had more dissolved nitrogen but ThermoCalc predicted that the isoactivity line is

moving towards lower nitrogen content by addition of Cu.

Fig. 16: Isoactivity lines for nitrogen in Fe-Cr-N, Fe-Cr-N-Ni, Fe-Cr-N-Cu, Fe-Cr-N-Mn, Fe-Cr-N-Ni-Cu and Fe-

Cr-N-Ni-Mn systems at different compositions and corresponding experimental data (austenite) obtained at aN=0.5

in an open system, 1363 K.

1050

1070

1090

1110

1130

1150

1170

1190

1210

1050 1070 1090 1110 1130 1150 1170 1190 1210

Exp

erim

enta

l T

emp

era

ture

Calculated Temperature by Thermo-Calc

1:1alloy 1 , a=0.5alloy 1 , a=1alloy 2 , a=0.5alloy 2 , a=1alloy 3 , a=0.5alloy 3 , a=1alloy 4 , a=0.5alloy 4 , a=1alloy 5 , a=0.5alloy 5 , a=1alloy 6 , a=0,5alloy 6 , a=1alloy 8 , a=0.5alloy 8 , a=1alloy 9 , a=0.5alloy 9 , a=1alloy 10 , a=0.5alloy 10 , a=1alloy 11 , a=0.5

20

Fig. 17: Isoactivity lines for nitrogen in Fe-Cr-N, Fe-Cr-N-Ni, Fe-Cr-N-Cu, Fe-Cr-N-Mn, Fe-Cr-N-Ni-Cu and Fe-

Cr-N-Ni-Mn systems at different compositions and corresponding experimental data (austenite) obtained at aN=1 in

an open system, 1433 K.

Fig. 18: Isoactivity lines for nitrogen in Fe-Cr-N, Fe-Cr-N-Ni, Fe-Cr-N-Cu, Fe-Cr-N-Mn, Fe-Cr-N-Ni-Cu and Fe-

Cr-N-Ni-Mn systems at different compositions and corresponding experimental data (austenite) obtained at aN=1 in

an open system, 1473 K.

21

4.1.2 Comparison between calculated and experimental results for part two:

4.1.2.1 The FCC / ε equilibrium in the Fe-Cr-N system with addition of Ni, Cu and Mn

The ε nitride solubility line in the austenite in the Fe-Cr-N system with addition of Ni, Cu and

Mn was experimentally determined and calculated. It is supposed that addition of all alloying

elements will affect the phase diagram, specifically the extension of the austenite one-phase

region. The FCC one-phase field was calculated for different content of Ni, Cu and Mn. The

results presented in Figures 19 and 20 show that the austenite/austenite+nitride phase boundary

moves to lower nitrogen content with all alloying addition. Experimental results about this phase

boundary, reported in previous part, are plotted in the same figure. The comparison have been

done between them and the corresponding calculated phase boundaries.

All the experimental results for this phase boundary locate to the right of comparable curve at

900 ° C, however at 1100 ° C it is not clear what is the influence of Ni, Cu and Mn addition by

analyzing the experimental data. Probably the equilibrium time used was much too short for the

settlement of equilibrium at 1100°C.The fcc/fcc+ε phase boundary is sensitive to the mentioned

elements addition and the austenite one phase region extends to lower Cr levels.

By comparing the experimental results with information from literature about the Fe-Cr-Ni-N

system[1,2]

, there is an unsatisfactory results from presented experiments. Part of this deviation

can be related to the analyzing technique by SEM-EDS where it is not possible to measure

correctly the nitrogen content. The way that has been used in this project was to use some

reference single phase samples with known nitrogen content in EDS analyzing and set a

correlation factor to calculate the nitrogen content in each phase in binary phase’s samples.

Fig. 19: Predicted extension of the fcc one-phase field at different content of Ni. Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=0.187, 900°C.

22

Fig. 20: Predicted extension of the fcc one-phase field at different content of Ni. Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=0.187, 900°C.

Fig. 21: Predicted extension of the fcc one-phase field at different content of Cu. Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=0.187, 900°C.

23

Fig. 22: Predicted extension of the fcc one-phase field at different content of Mn. Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=0.187, 900°C.

Fig. 23: Predicted extension of the fcc one-phase field at different content of Mn and Cu in 10% Ni. Experimental

data on the fcc / fcc+ε phase boundary are plotted for comparison at aN=0.187, 900°C.

24

Fig. 24: Predicted extension of the fcc one-phase field at different content of Ni . Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=1, 1100°C.

Fig. 25: Predicted extension of the fcc one-phase field at different content of Ni . Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=1, 1100°C.

25

Fig. 26: Predicted extension of the fcc one-phase field at different content of Cu. Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=1, 1100°C.

Fig. 27: Predicted extension of the fcc one-phase field at different content of Mn. Experimental data on the fcc /

fcc+ε phase boundary are plotted for comparison at aN=1, 1100°C.

26

Fig. 28: Predicted extension of the fcc one-phase field at different content of Mn and Cu in 10 % Ni. Experimental

data on the fcc / fcc+ε phase boundary are plotted for comparison at aN=1, 1100°C.

4.1.2.2 The fcc phase in the Fe-Cr-N system with addition of Ni, Cu and Mn

The results can be presented by plotting isoactivity lines for the Fe-Cr-N system at different Ni,

Cu and Mn level which define the combination of some of isoactivity surfaces in Fe-Cr-N plane

with Fe-Cr-3 pct Ni-N, Fe-Cr-1.5 pct Cu-N, Fe-Cr-5 pct Mn-N, Fe-Cr-3 pct Ni-1.5 pct Cu-5 pct

Mn-N and Fe-Cr-3 pct Ni-1.5 pct Cu-5 pct Mn-N planes (Figures 21 and 22). The isoactivity

lines are determined by use of the nitrogen activities got from the previous optimization and then

plotted in a same diagram with comparable experimental points to do a comparison. The

agreement between the experiments and calculated results are not proper. Part of this deviation

may be because of the fact that there was the deviation from equilibrium in the capsules since

there were the samples where high amount of nitrogen have to be carried and so the driving force

for this transportation was low.

27

Fig. 21: Isoactivity lines for nitrogen in the Fe-Cr-N system with variation of Ni, Mn and Cu and corresponding

experimental data obtained at aN=0.187, 900°C.

Fig. 22: Isoactivity lines for nitrogen in the Fe-Cr-N system with variation of Ni, Mn and Cu and corresponding

experimental data obtained at aN=1, 1100°C.

28

4.2 Conclusion

Influence of Ni, Mn and Cu on nitride precipitation temperatures and phase equilibria in Fe-Cr-

N system at 1173 K and 1373 K have been studied. All experimental results and BSE images of

specimens has been given. The results of the experiments made in this work in most cases have

some deviations from calculated results by Thermocalc. Parts of deviations were related to the

nitride precipitation temperature where in the alloys having Cu Thermo-Calc at higher

temperatures predicts the two phases FCC+ε while experiments show one phase austenite.

Furthermore, in alloys with higher Cr content Thermocalc at higher temperatures is showing

single phase austenite while experimental results represented two phases. However, for the rest

of samples, especially for specimens which have percent of Ni, theoretical calculations were in

accordance to experiments data. In addition as a general view for predicting the trend of addition

of Ni, Mn and Cu there were a good agreement between both experiments and ThermoCalc

calculations.In the case of nitrogen containing specimens, data from experiments are in fair

agreement with theoretical calculations particularly at 1180 and 1200 °C where the alloys are,

with one exception, single phase austenite.

An attempt was made to study about effect of mentioned elements on change of phase diagram

of all specimens. Experimental results for phase equilibria showed deviations from ThermoCalc

calculations. Experiments represented that addition of Ni, Mn and Cu has a slightly effect on

moving of FCC/FCC+ε phase boundary while calculated results showing a stronger effect on this

boundary where it moves forward to lower nitrogen content by addition of Ni, Mn and Cu.

29

5 REFERENCES

[1] S. Hertzman, A study of equilibria in the Fe-Cr-Ni-Mo-C-N system at 1273K, Metallurgical

Transaction Vol. 18A, (1987).

[2] S. Hertzman and M.Jarl, A Thermodynamic analysis of the Fe-Cr-N system, Metallurgical

Transaction Vol.18A, (1987)

[3] D. Peckner, I. Bernstein (Eds.), Handbook of Stainless Steel McGraw-Hill, New York (1977)

Chap. 3

[4] H. Wriedt, N. Gokcen, R. Nafziger, Bull. Alloy Phase Diagr. 8 (1987), pp. 355

[5] H. Boyer, T. Gall (Eds.), Metals Handbook, Desk Edition Am. Soc. Metals, Metals Park,

Ohio (1985), p. 12

[6] J. Rawers, J. Bennett, R. Doan, J. Siple1. Nitrogen solubility and nitride formation in Fe-Cr-

Ni alloys, Acta Metallurgica et Materialia, Vol. 40, issue 6, (1992), pp 1195–1199.

[7] H. Feichtinger, A. Satir-Kolorz and Z. Xiao-Hong, Proc. Int. Conf. High Nitrogen Steel-88,

Lille, France (edited by J. Foct, A. Hendry), Institute of Metals, London (1988), p. 75.

[8] H.K. Feichtinger and X. Zheng, Powder. Metall., 22 (1990), p. 7.

[9] R. Leutenecker, G. Wagner, T. Louis, U. Gosner, L. Guzman and A. Molinari Mater, Sci.

Engng A, 115 (1991), p. 229.

[10] P. perrot, Ternary Alloy Systems, Landolt-Börnstein - Group IV Physical Chemistry

Volume 11D3, (2008), pp 184-206

[11] K. Mielityinen-Tiitto, Precipitation of Cr2N in Some Nitrogen-alloyed Austenitic Stainless

Steels, Volume 141 of Acta polytechnica Scandinavica, (1979)

[12] D.B. Rayaprolu and A. Hendry, Cellular precipitation in a nitrogen alloyed stainless steel,

Materials Science and Technology vol.5, (1989).

[13] D.Henry, Louisiana State University, J. Goodge, University of Minnesota-Duluth,

Wavelength-Dispersive X-Ray Spectroscopy (WDS), Energy-Dispersive X-Ray spectroscopy

(EDS), http://serc.carleton.edu/research_education/geochemsheets/browse.html#xray.

30

APPENDIX: SupplemEntary INFORMATION

BSE images of samples at aN=1 , 1100°C

1 2

3 4

6 5

8 9

10 11

31

BSE images of samples at aN=1 , 1113°C

1 2

3 4

6 5

8 9

10 11

32

BSE images of samples at aN=1 , 1122°C

1 2

3 4

5 6

9

10 11

33

BSE images of samples at aN=1 , 1130°C

2 1

3 4

9 8

34

BSE images of samples at aN=1 , 1140°C

2 1

3 4

5 6

8 9

10 11

35

BSE images of samples at aN=1 , 1150°C

10 11

2 1

3 4

5 6

8 9

36

BSE images of samples at aN=1 , 1160°C

2 1

3 4

5 6

8 9

10 11

37

BSE images of samples at aN=1 , 1170°C

BSE images of samples at aN=1 , 1210°C

5

6

38

BSE images of samples at aN=0.5 , 1050°C

BSE images of samples at aN=0.5 , 1060°C

1

3

2

2 1

3 4

39

BSE images of samples at aN=0.5 , 1070°C

2 1

3 4

5

8 9

10 11

40

BSE images of samples at aN=0.5 , 1080°C

4 8

9 10

11

41

BSE images of samples at aN=0.5 , 1090°C

5 6

8 9

10 11

42

BSE images of samples at aN=0.5 , 1100°C

BSE images of samples at aN=0.5 , 1150°C

5

6