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7/18/2019 Formation of Precipitates in Multiple Microalloyed Pipeline Steels http://slidepdf.com/reader/full/formation-of-precipitates-in-multiple-microalloyed-pipeline-steels 1/9    P   u    b    l    i   s    h   e    d    b   y    M   a   n   e   y    P   u    b    l    i   s    h    i   n   g    (   c    )    I    O    M     C   o   m   m   u   n    i   c   a    t    i   o   n   s    L    t    d Formation of precipitates in multiple microalloyed pipeline steels C. O. I. Emenike J. C. Billington Introduction  An investigation has been carried out to identify the precipitates in multiple microalloyed steels. The microalloying elements and interstitials included aluminium, niobium, titanium, vanadium, carbon, and nitrogen.. It was found that the precipitates are complex in nature and they were rationalised on the basis of mutual solubility probably enhanced by non-stoichiometry. The precipitate morphologies were interpreted mainly in terms of steel compositions. Steels quenched from 1250° C contained titanium rich precipitates accompanied by the evolution of new niobium rich precipitates after hot rolling and quenching. A  parameter K [1 indicative of solute participation in the precipitation phenomenon was established and showed excellent correlation between steel and precipitate analyses. A sequence of precipitation in multiple microalloyed steels was achieved using solubility relationships as a premise. MSTj803 © 1989 The Institute of Metals. Manuscript received 30 October 1987; in final  form 20 July 1988. At the time the work was carried out the authors were in the  Department of Mechanical and Production Engineering, Aston University,  Birmingham. Dr Billington is now a Consultant Metallurgist. Experimental procedure The worldwide increase in the demand for energy has led to a continuous increase in the production of steel pipelines for the transportation of oil and gas from production site to  potential user. This has led to the demand for thinner walled, large diameter pipes which can resist hostile environments while increasing their fuel carrying capacities to meqt the economic restrictions imposed. These advances have~been made possible by the development of high strength low alloy (HSLA) steels which form the basis of the materials used together with improvements in the thermomechanical treatments applied to achieve the final  properties. Microalloying additions, such as niobium, titanium, and vanadium which, in the presence of C and nitrogen, achieve the control of austenite grains (and, subsequently, ferrite grains) and precipitation strengthening of the ferrite by controlled formation of nitrides/carbides. For improved grain refinement, controlled rolling was introduced to  permit the use of much lower finish rolling temperatures. Thus, modern pipeline steels are produced having relatively small ferrite grains, formed by the transformation of the original austenite grains. Final properties can be achieved  by the precipitation of vanadium (and niobium) carbo- nitrides below 900°C improving strength and toughness. Knowledge of the interactions between the microalloying elements and aluminium is far from complete, especially in terms of their dependence upon processing variables. The  present work was undertaken to elucidate the sequence of formation of precipitates during the rolling of a series of steels containing fixed carbon and niobium contents with varying (aluminium), titanium, and vanadium concentra- tions. The precipitates formed in as cast specimens and specimens quenched from 1250°C with and without defor- mation were examined. Five 18kg pipeline steel ingots were produced as described  previouslyl and their compositions are given in Table 1. The ingots were hot rolled in four passes, with interpass reheating, at 1250°C to give an overall reduction of 66% and a final thickness of 11 ± O'2 mm. ELECTRON MICROSCOPY A scanning transmission electron microscope (STEM) with EDX attachment (Philips Model EM 400) was used to study the morphology and composition of the precipitates. Conventional carbon extraction replicas were prepared from selected specimens which were etched in 2% nital  before being carbon coated, scored, and stripped in 5% nital. Individual precipitates were quantitatively analysed in the Philips microscope for 200 live seconds and the elemental counts corrected for atomic numbers were sub- sequently processed through a computer to give the analy- ses presented in Tables 2-4. Because the objective of this work was to study the morphology and chemistry of the  precipitates, they were classified into three sizes only: coarse, intermediate, and fine. To elucidate further the production of precipitates and their compositions during the various thermo mechanical treatments applied to the steels, a series of rolling and quenching experiments was made on steels 1 and 4. Two samples from each cast steel (initial thickness 18mm) were heated to 1250°C, one sample was quenched after soaking and the other was hot rolled in a laboratory two-high mill to give 17% reduction before quenching. The emergent temperature of the specimens from the rolls was 1150± 10°C.Specimens from the"sematerials were prepared for electron microscopy. The remaining materials were reheated at 1250°C and given a second pass of 23% Table 1 Compositions of steels used in present investigation, wt-% Cast no. C Si Mn P S Ni AI Cu N Nb Ti V 0 1* 0·09 0·24 1·22 0·014 0·002 0·03 0·030 0·27 0·007 0·042 0·007 0·009 0·0051 2 0·09 0·20 1·13 0·013 0·002 0·04 0·042 0·29 0·012 0·047 0·007 0-073 0·0040 3 0·09 0·21 1·20 0·015 0·002 0·02 0-034 0·28 0-010 0·047 0·007 0·043 0-0040 4 0·09 0·21 1-13 0-011 0-003 0-02 0·059 0·26 0·010 0·043 0'074 0·0043 5 0·09 0·23 1·23 0-013 0·003 0·03 0·060 0·26 0·011 0·048 0-037 0·0032 * Base steel. 566 Materials Science and Technology June 1989 Vol. 5 Ref. Ref.
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Page 1: Formation of Precipitates in Multiple Microalloyed Pipeline Steels

7/18/2019 Formation of Precipitates in Multiple Microalloyed Pipeline Steels

http://slidepdf.com/reader/full/formation-of-precipitates-in-multiple-microalloyed-pipeline-steels 1/9

   P

  u   b   l   i  s   h  e   d   b  y   M  a  n  e  y   P  u   b   l   i  s   h   i  n  g

   (  c   )   I   O   M    C

  o  m  m  u  n   i  c  a   t   i  o  n  s   L   t   d

Formation of precipitates inmultiplemicroalloyedpipeline steels

C. O. I. Emenike

J. C. Billington

Introduction

 An investigation has been carried out to identify the precipitates in multiple

microalloyed steels. The microalloying elements and interstitials included 

aluminium, niobium, titanium, vanadium, carbon, and nitrogen.. It was found that 

the precipitates are complex in nature and they were rationalised on the basis of 

mutual solubility probably enhanced by non-stoichiometry. The precipitate

morphologies were interpreted mainly in terms of steel compositions. Steels

quenched from 1250° C contained titanium rich precipitates accompanied by the

evolution of new niobium rich precipitates after hot rolling and quenching. A

 parameter  K [1 indicative of solute participation in the precipitation phenomenon

was established and showed excellent correlation between steel and precipitate

analyses. A sequence of precipitation in multiple microalloyed steels was achieved 

using solubility relationships as a premise. MSTj803

© 1989 The Institute of Metals. Manuscript received 30 October  1987; in final

 form 20 July 1988. At the time the work was carried out the authors were in the

 Department of Mechanical and Production Engineering, Aston University,

 Birmingham. Dr Billington is now a Consultant Metallurgist.

Experimental procedure

The worldwide increase in the demand for energy has led toa continuous increase in the production of steel pipelinesfor the transportation of oil and gas from production site to potential user. This has led to the demand for thinner walled, large diameter pipes which can resist hostileenvironments while increasing their fuel carrying capacitiesto meqt the economic restrictions imposed. These advanceshave~been made possible by the development of high

strength low alloy (HSLA) steels which form the basisof the materials used together with improvements in thethermomechanical treatments applied to achieve the final

 properties.Microalloying additions, such as niobium, titanium, and 

vanadium which, in the presence of C and nitrogen, achievethe control of austenite grains (and, subsequently, ferritegrains) and precipitation strengthening of the ferrite bycontrolled formation of nitrides/carbides. For improved grain refinement, controlled rolling was introduced to

 permit the use of much lower finish rolling temperatures.Thus, modern pipeline steels are produced having relativelysmall ferrite grains, formed by the transformation of theoriginal austenite grains. Final properties can be achieved 

 by the precipitation of vanadium (and niobium) carbo-nitrides below 900°C improving strength and toughness.

Knowledge of the interactions between the microalloyingelements and aluminium is far from complete, especially interms of their dependence upon processing variables. The

 present work was undertaken to elucidate the sequence of formation of precipitates during the rolling of a series of steels containing fixed carbon and niobium contents withvarying (aluminium), titanium, and vanadium concentra-tions. The precipitates formed in as cast specimens and specimens quenched from 1250°C with and without defor-mation were examined.

Five 18 kg pipeline steel ingots were produced as described  previouslyl and their compositions are given in Table 1.The ingots were hot rolled in four passes, with interpassreheating, at 1250°C to give an overall reduction of 66%and a final thickness of 11± O'2 mm.

ELECTRON MICROSCOPY

A scanning transmission electron microscope (STEM) withEDX attachment (Philips Model EM 400) was used tostudy the morphology and composition of the precipitates.Conventional carbon extraction replicas were prepared from selected specimens which were etched in 2% nital before being carbon coated, scored, and stripped in 5%nital. Individual precipitates were quantitatively analysed in the Philips microscope for 200 live seconds and theelemental counts corrected for atomic numbers were sub-sequently processed through a computer to give the analy-ses presented in Tables 2-4. Because the objective of thiswork was to study the morphology and chemistry of the precipitates, they were classified into three sizes only:coarse, intermediate, and fine.

To elucidate further the production of precipitates and their compositions during the various thermo mechanicaltreatments applied to the steels, a series of rolling and quenching experiments was made on steels 1 and 4. Twosamples from each cast steel (initial thickness 18 mm) wereheated to 1250°C, one sample was quenched after soakingand the other was hot rolled in a laboratory two-high millto give 17% reduction before quenching. The emergenttemperature of the specimens from the rolls was1150± 10°C. Specimens from the"sematerials were prepared for electron microscopy. The remaining materials werereheated at 1250°C and given a second pass of 23%

Table 1 Compositions of steels used in present investigation, wt-%

Cast no. C Si Mn P S Ni AI Cu N Nb Ti V 0

1* 0·09 0·24 1·22 0·014 0·002 0·03 0·030 0·27 0·007 0·042 0·007 0·009 0·0051

2 0·09 0·20 1·13 0·013 0·002 0·04 0·042 0·29 0·012 0·047 0·007 0-073 0·0040

3 0·09 0·21 1·20 0·015 0·002 0·02 0-034 0·28 0-010 0·047 0·007 0·043 0-0040

4 0·09 0·21 1-13 0-011 0-003 0-02 0·059 0·26 0·010 0·043 0'074 0·0043

5 0·09 0·23 1·23 0-013 0·003 0·03 0·060 0·26 0·011 0·048 0-037 0·0032

* Base steel.

566 Materials Science and Technology June 1989 Vol. 5

Ref.

Ref.

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Emenike and Billington Formation of precipitates in pipeline steels 567

Table 2 STEM-EDX precipitate analyses of as cast

steels 1, 2, and 4

Table 3 STEM-EDX precipitate analyses of steels 1-5

as rolled at 1250°C

Precipitate form

 Analyses

wt-% at.-%

Precipitate yield

quotient ~ Precipitate form

 Analyses

wt-% at.-%

S t e e l 1

Fine, spherical Nb 85 ± 4 73 ± 3

Ti 15±1 25±1

 AI 0'4±0'1 1·2±0·4

Intermediate, spherical Nb 80 ± 6 63 ± 4

Ti 15 ± 1·2 23 ± 2

 AI 5'3±0'8 14±2

2024

2143

13

1905

2143

177

S t e e l 1

Fine, spherical

Intermediate, spherical

Nb 91±6

Ti 7±1

 AI 3±1

Nb 91 ±2

Ti 9±0'2

 AI 0'6±0'1

81±512±1·4

8±2

83±2

16±0'4

2±0'2

2167

957

832167

1257

20

Results

reduction before quenching for subsequent microscopic

examination. Further experimental details are given

elsewhere.2

PRECIPITATE MORPHOLOGIES

The precipitates of the base steel 1 were predominantly

spherical (see Tables 2 and 3 a nd Fig. la), while various

shapes were found in the other steels (see Tables 2-4 and 

Fig. Ib). Similar morphologies have been cited in the

literature.3-

6

S t e e l 2

Fine, spherical Nb 84 ± 1

Ti 7±1'5

 AI 6±1·4

V 4±1'1

Intermediate, spherical Nb 35±6

Ti 3±1

 AI 60±4

V 3'2±1

S t e e l 4

Fine, spherical Nb 61 ±4

Ti 38±2

 AI 2±0'4

Intermediate, spherical Nb 59±2

Ti 40±1 AI 1·1 ±0'2

69±9

11 ±2·4

15±4

6±2

14±2'5

2±1

82±6

2·3±1

43±3

53±2

4'2±1

42±2

56±1

2·5±0·4

1787

957

131

51

745

357

1429

44

1419

514

29

1372

541

19

S t e e l 2

Fine, spherical

Coarse, angular, hexagonal

Coarse, plate, cuboid

S t e e l 3

Fine, spherical

Coarse, long plate (needle),

Coarse, plate (cuboid)

S t e e l 4Fine, spherical

Coarse, needle

Coarse, plate (cuboid)

Nb 89±5

Ti 4±1

 AI 3'3±0'4

V 6'3±1

Nb 48±3

Ti 2±0·2

 AI 46±1

V 4'3±0'4

Nb 54±2

Ti 1'4±0'2

 AI 41 ±1

V 4±0·3

Nb 56±9

Ti 7'3±2

 AI 7·3±1·4

V 9±2

Nb 43±2

Ti 2±0·2

 AI 54±1

V 1'2±0'3

Nb 62±3

Ti 2·1 ±0'3

 AI 33±1

V 3±0·3

Nb 57±11

Ti 40±5

 AI 3·4±1·4

Nb 48±4

Ti 51 ±2

 AI 1'3±0'3

Nb 50±3

Ti 49±2

 AI 1·1 ±0'2

74±4

6'3±1

10±2

10±1

22±1

1'4±0'2

73±2

4±0'3

26±1

1'3±0'2

69±1

3·2±0·3

50±713±2

23±4

14±3

18±1

1'4±O'1

80±2

1 ±0'1

34±2

2·3±0·3

61±23±0'3

39±1

53±6

8±3'2

32±3

65±3

2'9±1

34±2

64±2

2'5±0'5

1894

543

79

150

1021

214

1095

102

1149

200

976

88

1191

1043

215

202

915

243

1588

29

1319

300

971

60

1326

541

58

1116

689

22

1163

662

19

PRECIPITATE ANALYSES

It is evident from Tables 2 and 3 that the precipitates of 

multiple micro alloyed steels are of mixed composition, thus

supporting previous work on similar steels.7-

10

S t e e l 5

Fine, spherical

Coarse, plate (cuboid)

Nb 56±6

Ti 30±2

 AI 1'1±0'2

Nb 55±5

Ti 26±2

 AI 0'9±0'4

40±5

41 ±3

2'5±0'5

40±5

37±0'4

2'3±1

1167

811

18

1146

703

15

SOLUTE DISTRIBUTION

The solute content of the precipitates tended to show size

dependence consistent with other findings. 7.8 The highest

vanadium containing particles appeared to be fine and 

spherical (Table 3) which contrasted with the behaviour of 

the highest titanium and aluminium precipitates. Niobium

Table 4 STEM-EDX analyses of quenched steels 1 and 4

did not show such preferred morphologies. In the as rolled 

steel 4, it could be argued that titanium was partitioned 

approximately 50 : 50 in both fine and coarse particles and 

a possible explanation for this was that the Ti/N ratio was

hyperstoichiometric. Consequently, the excess titanium

Treatment

S t e e l 1

Held at 1250°C for 50 mins, WQ

Soaked + 17% reduction, WQ

Steel 4Held at 1250°C for 50 min, WQ

 Analyses

Precipitate form wt-% at.-% K 1

Fine, dense, and spherical Nb 43±3 28±2 1024

Ti 56±2 71 ±2 8000

 AI 0'7±0'2 1'6±0'5 23Fine, dense, and spherical Nb 47±3 32±2 1119

Ti 52±2 67±2 7429

 AI 0'6±0'2 1'4±0'5 20Fine, less dense, and spherical Nb 81 ±4 68±3 1929

Ti 18±1 30±2 2571

 AI 0'7±0'2 2'3±0'5 23

Coarse, plate (cuboid) Nb 26±1 16±0'5 605

Ti 73±1 83±1 986

 AI 0'5±0'1 1·0±0·1 9

Materials Science and Technology June 1989 Vol. 5

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568 Emenike and Billington Formation of precipitates in pipeline steels

a

b

a spherical precipitates in steel 1; bvarious morphologies in steel 4

(massive cuboids are (NbTi) rich particles)

Precipitate morphologies observed in steels 1 and 4

(carbon extraction replicas)

would form titanium carbide (in the presence of carbon)with a solvus below that of titanium nitride, but approxi-mately the same as that of niobium carbide, at equivalentlow activities of 0·01 referred to the pure solid standard state (see Appendix). It should be pointed out that thealuminium content of the precipitates in the as rolled steels4 and 5 was insensitive to size. However, aluminium nitrideformation and precipitation is the subject of another paper 1

and therefore will not be discussed here.

The distribution of niobium between fine and coarse precipitates in the as rolled steels 2 and 3 in Table 3suggests that the niobium containing particles formed during the initial stages of precipitation, i.e. at the higher 

temperatures, had a high probability of nucleating on tita-nium nitride particles. These high temperature precipitateswould be more vulnerable to particle growth and, in fact,the presence of aluminium in these precipitates has beensuggested to accelerate coarsening.8 The fine particles weredevoid of aluminium and probably formed during the later stages of precipitation and tend to support this claim. This

explanation might be advanced also for the behaviour of niobium in steels 4 and 5 (Table 3), but, in addition, theniobium was distributed apparently equally between fineand coarse precipitates. These assertions have beensubstantiated further in the Appendix.

The affinity of vanadium for small particles can be under-stood from the calculations summarised in the Appendixwhich indicate the bulk of its precipitation as carbide in theferrite. Such late forming particles were likely to remain fine

 because of the improbability of Ostwald ripening.Generally, it can be stated that the original precipitates

and/or those out of solution at the soaking temperaturehad a greater tendency to coarsen and this was accentuated 

 by contamination (e.g. the presence of aluminium in tita-

nium nitride). This is partially supported by the analysis inthe Appendix which indicates that the titanium nitridesolvus was higher than the soaking temperature of l250°C,approaching a maximum value for steel 4 corresponding toa maximum of titanium-nitrogen solubility product.

The strong relationship between the micro alloyingelements in the steels and the precipitates is reflected in the

 parameter  Kh

the precipitate yield quotient, defined as theratio of the weight percentage of a micro alloying element inthe precipitates to that in the steel. Evidently, K 1 is ameasure of the participation of the microalloying elementsin precipitate formation. The values of K 1 are plotted as bar charts in Figs. 2-4. Of great interest are the high values of K 1 ~ 8000 for the lowest titanium containing steel (0'007%),especially in the quenched condition and shown in Figs. 4a

and  b, compared with that of 0·074%Ti (Fig. 4d ) which hasa value of K 1 ~ lOOO.This high degree of participation in precipitation exhibited by the former may provide anexplanation for the advocation of low titanium additions~O'Ol% for elevated temperature austenite grain pinning.This dimensionless parameter which can be applied to anyreaction that obeys the solubility relationship givenll,12could therefore prove to be an important design require-ment, because it provides information concerning thechemical identity of the alloy additions and particles.

EVOLUTION OF PRECIPITATE ANALYSES AND

SIZESIn Table 4 are given the analyses of complex precipitatesobtained in steel 1 after soaking at 1250°C and quenching.They portray fine, dense, spherical, titanium rich particles

f in e in te rm e d ia te in te rm e d ia tel a ) I b ) Id )

2 Variation of  K. for steels 1, 2, and 4 in as cast condition

2 0 0 0

1 8 0 0 S tee l S tee l1 2

1 6 0 0

1 4 0 0

1 2 0 0

~1 0 0 0

8 0 0

6 0 0 N b Ii N b Ii N b Ii N b A l

4 0 0

2 0 0A l A l

Ii

N b

Ii

fin eIe)

S tee l4

A l

N b

Ii

A l

in term ed iateI f )

Materials Science and Technology June 1989 Vol. 5

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Emenike and Billington Formation of precipitates in pipeline steels 569

2000S te e l S te e l S te e l S teel

1800 1 2 2 4

1600

1400

~ - 1 20 0N b N b N b

1000

8 00 .

60 0 T i T i N b A I N b A I N b N b N b

40 0 T i T i T i T i

20 0 A IA I T i T i A IA I A I A I

f in e in t e rm ed ia t e f in e c o a rs e c o a rs e f in e c o a rs e c o a r s e(a ) ( b ) (c ) ( d ) (e ) (f ) ( g) ( h )

3 Variation of  K1 for steels 1, 2, and 4 in as rolled condition

(70 at.-%Ti and 30 at.-%Nb). This analysis contrasts strik-

ingly with those of the as cast and the as rolled specimens of the same steel (see Tables 2 and 3) in which the niobiumand titanium values are reversed. These precipitates areshown in Fig. 5a.

After 17% deformation followed by quenching from the

rolls, a new set of fine, spherical, but less dense, precipitatesemerged and the resulting microstructure contained amixture of these precipitates and titanium rich precipitates.The new precipitates were niobium rich (about 70 at.-%Nband 30 at.-%Ti) and their chemistry was close to that of the

 precipitates in the as cast steel 1 given in Table 2. These particles a"re shown in Fig. 5b . The increased number of  precipitates can be seen from Fig.5b, thus confirming thework of Yamamoto et al.13 Therefore, it is probable that, as

well as increasing the population of precipitates, rollingaccelerated the precipitation of niobium rich particles in thetemperature range 1250-1150°C. Even the lower limit ismuch greater than the finish precipitation temperature of 

 NbC or Nb(C,N)oog7 (see Table 5).

Steel 4, also quenched after soaking at 1250°C, contained 

titanium rich precipitates (about 83 at.-%Ti and 16 at.-%Nb) which deviated markedly from those obtained in the as cast or as rolled material. The quenched particlesare shown in Fig. 6.

The effect of a second thermomechanical deformation

and quenching cycle for steel 1 on the precipitate size and distribution is shown in Figs.7a and  b. By comparingFig. 7a with Fig. 5a, it can be inferred that the intermediateheat treatment nullified the previous rolling. By comparing

Fig. 7b with Fig. 5b it can be deduced that titanium nitrideor titanium-niobium nitride grew slowly, hence, there wasno major difference in particle size and in Fig. 7b the

 presence of more dense spherical precipitates can beobserved, suggesting that the intermediate reheating did 

not completely redissolve all the prior niobium rich precipi-tates. In both cases, the distribution of the precipitatesappeared to be even with some of the particles pinningthe prior austenite grain boundaries or lath martensite

 boundaries.

a after soaking at 1250°C, WQ; b, C  after soaking at 1250°C, rolling

(17%), WQ; d  after soaking at 1250°C, wa

4 Variation of K1 for steels 1 and 4 after water quench

from 1250°C

f i n e , d e n s e(a)

8000

7 5 00

7 000

65 00

6 000

5 000

~ - 2 0 00

1800

1600

1400

1200

1000

80 06 00 N b

40 0

20 0

T i

A I

S teel

Ii

N b

A I

f in e , d en se( b J

 N b T i

A I

f in e , l e s s d e ns e(c)

S te el 4

 N b T i

A I

c o a r s e(d)

a

b

a after soaking at 1250°C, wa, insoluble dense precipitates and less

dense soluble precipitates can be observed; b after soaking at

1250°C, rolling (17%), wa, increase in number of dense precipitates

can be observed

5 Microstructure of steel 1 after different heat

treatments {carbon extraction replicas}

Materials Science and Technology June 1989 Vol. 5

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570 Emenike and Billington Formation of precipitates in pipeline steels

Table 5 Data for various types of precipitate in steels 1-5

Microalloy Microalloying

Start Finish content Weight of element, wt-%

Precipitate Weight of  of each microalloying

(ppt) 8x(C/N) TstarvOC 8x(C/N) '7finish, °C ppt, g ppt, wt-% element, g Theory Actual*

Steel 1TiN 1·0 1578 0·1 747 0·00904 12·5 Ti 0·007 12·0 8

Nb(C,N)o'87 0·137 1578 0·9 1043 0·03985 55·1 Nb 0·03205 70·7 91 AIN 0·072 1578 0·01 742 0·00205 2·8 AI 0·00135 2·3 1·8NbC 0·5 1004 0·5 720 0·01047 14·5 V {0'00927 15·1VC 0·01 931 0·01 720 0·01088 15'1 0·00881

(Fe3C 1·0 720 1·2468)Total 0-07229 0·05848

Steel 2

{ 5·4 { 5·1TiN 1·0 1633 0·1 747 0·00904 5·4 Ti 0·007 5·3 2·5

{34'5 {34'2Nb(C,N)o'87 0·122 1633 0'6 725 0·05803 35·0 Nb 0·047 35·5 64

{ 6'6 { 5·4 AIN 0-050 1633 0·3 720 0·01112 6·7 AI 0·0074 5,6 30

{ 0·1 { 0·0728 { 53·1NbC 0·1 810 0·01 720 0·00017 0'1 V 0·0709 53,6 4·9

{0·01 1098 0·01 720 0·08992 { 53·4

VC 0·1 925 0·1 720 0·08760 52·8(Fe3C 1·0 720 {1'0140 )

1·0204Total { 0·16828 { 0·1342

0·16596 0·1323

Steel 3

{ 7·2 { 6·9TiN 1·0 1614 0'1 747 0·00904 7·1 Ti 0·007 6·7 3·8

{30'1 {45'5Nb(C,N)o'87 0·113 1614 0·6 1159 0-03764 29'6 Nb 0·04643 44·7 54

{ 9·3 { 7·5 AIN 0·107 1614 0·3 720 0·01159 9·1 AI 0-00763 7·3 31

{12.8 { 0·04104 {40·2NbC 0·1 1288 0·01 720 0·01601 12·6 V 0·0428 41·2 4·4

{ 0·1 890 0·1 720 0·05070 {40.6VC 0·01 1052 0'01 720 0·05287 41·6

(Fe3C 1·0 720 1'0905)

Total {0·12498 {0'10210-12715 0·10386

Steel 4TiN 1·0 1889 0·5 1132 0·0137 9·3 Ti 0·074 61·2 47

Nb(C,N)o'87 0·0426 1889 0·4 1235 0·0378 25'7 Nb 0·043 35,6 64

 AIN 0·0686 1889 0·1 720 0·0059 4·0 AI 0·0059 3·2 1·9

TiC 0-1 1593 0·01 727 0·0754 51·3

NbC 0·01 1615 0·01 720 0·0141 9'6

(Fe3C 1-0 720 1·0482)

Total 0·1469

Steel 5TiN 1·0 1812 0·4 1089 0·0195 17·5 Ti 0·037 41·5 28

Nb(C,N)o'87 0·0625 1812 0·5 1124 0·0416 37'4 Nb 0·048 53·8 55 AIN 0·0930 1812 0·03 724 0·0064 5·8 AI 0·0042 4·7 1·0

TiC 0·1 1212 0·01 732 0·0274 24·6

NbC 0·1 1210 0·003 718 0·0163 14·7

(Fe3C 1·0 720 1'1834)

Total 0·1112

* Mean of 'as rolled' precipitates.

Discussion

MORPHOLOGIES

Most of the morphologies observed in Figs. 5-7 may be

explained in terms of the compositions of the steels asevident from Tables 2 and 3 and Figs. la and  b. Thisopinion is supported by the observation that steel 1displayed predominantly spherical precipitates virtuallydevoid of aluminium. Other shapes appeared in the struc-tures when titanium and/or aluminium-nitrogen solubility

 products were relatively higher in the alloys (steels 2-5). Inthe present work, the thermomechanical treatments weresimilar and therefore could not throw any further light onthis proposition.

Materials Science and Technology June 1989 Vol. 5

 ANALYSES

The isomorphology between most of the precipitates leadsto intersolubilities and could facilitate the formation of carbonitrides. Similarities in the atomic sizes of carbon and nitrogen and in the magnitudes of their diffusivities can be

conducive to the interchangeability of carbon and nitrogenwithin the non-stoichiometric lattices of these complex

 precipitates (except for aluminium nitride). In addition tointerstitial diffusion which may take place, the existence of non-stoichiometry creates vacancies with associated enhanced diffusion, possibly increased by mutual strainingof precipitate lattices. Such changes in lattice parameter with precipitate composition have been documented earlier.14 A precipitation model based on such mixed com-

 positions has been attempted,7  but cannot be adapted to

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Emenike and Billington Formation of precipitates in pipeline steels 571

6 Microstructure of steel 4: after soaking at 1250°C,

wa, greater number of insoluble precipitates can be

observed compared with Fig. 5a (carbon extraction

replica)

the present work because of the presence of multiple micro-

alloying elements giving rise to a multiplicity of phases.

Alternatively, the precipitation of carbonitrides is a

nucleation and growth process. Solubility relationships

dictate that titanium nitride forms first and, since it is

isomorphous with other nitrides and carbides, it would 

seem realistic to assume that titanium nitride can act as

nucleation sites for subsequent precipitation. This leads to

 possible epitaxial growth or precipitate induced precipi-

tation. In fact, such a coring phenomenon has been

noted. 8,9, 15 The insignificant difference between the lattice

 parameters1!, 14 of these precipitates may enhance thiscoring. This suggestion is reinforced by the thermodynamic

analyses based on the sequence of precipitation (see

Appendix) which obviously indicate an overlapping tend-

ency of the C-curves of some of the nitrides and carbides

with the associated phenomenon of coprecipitation.

Following the model of precipitation pioneered by

Houghton et al.7  and modified in the appendix, it may be

assumed that some pure phases, e.g. titanium nitride,

niobium carbonitride, aluminium nitride, may coexist

within a complex phase. Indeed, such a conclusion has been

made,8 but the same work confirmed by chemical analysis

that most of the precipitates were of a complex nature, e.g.

titanium niobium carbonitride. Subsequent investigation by

Houghton et al.7  and calculations in the Appendix below

are in agreement with the latter finding.

PARTICLE SIZE DISTRIBUTION

Qualitatively, there is evidence of various precipitate sizes

in Table 3. Processing variations have been cited 5,8 as the

factors affecting these sizes. These claims were supported by

the discovery that, for the same titanium addition, relatively

coarse nitride particles predominated in the structure for 

sand cast ingots compared with smaller precipitates in

continuous cast billets (i.e. 62 nm compared with 8 nm).5

Also, the alloy chemistry of the steel, as dictated by the

solubility relationships, controls the - degree of super-

saturation 16 achieved. Since all the steels in Table 3 werechill cast and given the same thermomechanical treatment,

the diversity of particle sizes observed should be steel

composition dependent, but it might be difficult to interpret

them in terms of supersaturation, because of the many

 phases present in the steels.

PRESENCE OF IRON IN PRECIPITATES

From a critical examination of Table 3, it is indicated that

some of t he analyses do not add up to 100% and the

a

b

a soaking at 1250°C, rolling (17%, first pass), WO+ reheating to

1250°C, WO; b soaking at 1250°C, rolling (17%, first pass), WO+

reheating to 1250°C, rolling (23%, second pass), WO

7 Microstructure of steel 1 after different heat

treatments (carbon extraction replicas)

implicit K1 values are less than unity, since the bulk of the

alloys is comprised of iron. The deficit is accounted for by

the presence of iron in some of the precipitates which may

question the validity of some of the work on microalloy

solubility studies. Iron possibly resulted from nearby

cementite particles,l7 but' this conflicts with the finding

that the iron content increased as the particle size

decreased.18,19 A recent communication20 suggested that

the presence of iron was probably a result of contamination

of the particles by iron salts, produced during the specimen

 preparation process, which adsorbed iron onto the surface

of the small precipitates. This explanation would fit the

increase' in iron content with decreasing particle size.

Further work on this problem would be useful.

DOMINANT EFFECT OF NIOBIUM

This effect in most of the precipitates (even in those steels

containing approximately twice its concentration of tita-

nium and vanadium) might be explained in terms of the

high stoichiometric value of niobium to carbon, the asso-

ciated electrons of the niobium atom, or the chemical

affinity of niobium for carbon and nitrogen. This domin-

ance is reflected in the values of  K1 shown in Figs. 2-4.The main exceptions being steel 1 in the quenched and as

cast conditions (see Figs. 2a,b and  4a,b).

MEASURED AND THEORETICALCOMPOSITIONS

The differences between the measured and theoretical

analyses are given in Table 5 and may be due to any or  

all of the following:

(i) insensitivity of the theoretical values to precipitate

size, thus mean values of the measured analyses were

used for comparison

(ii) the theoretical compositions are calculated assuming

equilibrium conditions exist

Materials Science and Technology June 1989 Vol. 5

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572 Emenike and Billington Formation of precipitates in pipeline steels

(iii) some precipitates may not have been detected 

(iv) premature termination of the VC precipitation

reaction as a result of the onset of Fe3C formation.

The consistently maximum difference exhibited by the

vanadium containing steels (i.e. steels 2 and 3), the magni-

tude of which increased with increasing vanadium content

of the steels, may be explained by (iii) and (iv) above. It can

 be inferred from this discrepancy between the values that

the participation in higher temperature precipitation still

left excess vanadium for low temperature precipitation.

EVOLUTION OF PRECIPITATE COMPOSITIONS AND SIZES

The analyses of the precipitates of steel 1 quenched from

1250°C reflected a stoichiometry of Tio .7 Nbo .3(NjC). These

complex high temperature precipitates were likely to be rich

in nitrogen and low in carbon, consistent with the accepted 

view. Further, the solvus of the complex precipitate was

 probably higher than 1250°C, because these precipitates

were insoluble at 1250°C.

The precipitates which were produced during or  

immediately after rolling, but before quenching, i.e. thosehaving a stoichiometry Tio.3 Nbo.7(NjC), must have h ad a

solvus below 1250°C. They were likely to be low in nitrogen

and rich in carbon, which could only have been confirmed 

 by the use of electron energy loss spectroscopy (EELS),

which was beyond the scope of the present investigation.

The driving force for these niobium rich precipitates was

likely to be the difference in free energy over the tempera-

ture gradient within the roll gap, because of the recrystalli-

sation of the austenite at this temperature reported 

 previously21.22 and confirmed in the present work. A

similar evolution of complex particles should be exhibited 

 by steel 4 given the same thermomechanical treatment, i.e.

the transition from titanium rich to niobium dominant

 particles.

The evolution of the observed spectrum of particle sizes

was due to the propensity for precipitate coarsening by the

well known Ostwald mechanism which would have a

decreasing effect in the order:

(i) insoluble precipitates at the soaking temperature

(ii) precipitates formed at high temperatures and during

rolling, e.g. titanium nitride or Ti,Nb(NjC)

(iii) low temperature precipitates, e.g. vanadium carbide

or nitride.

The presence of niobium in the titanium rich precipitates at

1250°C suggests that a transport phenomenon is involved 

in the formation of complex precipitates, thus supporting

model 2 of Houghton et al.7 

That model is based on themechanism of mixing of phases in the thermodynamic

treatment, but becomes very complex for multiple micro-

alloyed steels, because of their multicomponent nature.

From the evidence of insoluble titanium rich precipitates

at 1250°C for steels of very low (0·007%) titanium content

(see Fig. 3), it can be inferred that these particles,

Ti,Nb(NjC), were available for impeding grain boundary

motion at the soaking temperature (i.e. Zener's concept).

The usefulness of such high temperature austenite pinning,

especially during soaking or welding, has given great

impetus to the practice of making low titanium additions to

steel.

Summary

1. The precipitates in multiplemicroalloyed steels are

complex.

2. A parameter  K., precipitate yield quotient, which is

indicative of solute participation in precipitation pheno-

menon has been established.

Materials Science and Technology June 1989 Vol. 5

3. Specimens quenched from 1250°C were found to

contain titanium rich particles.

4. Hot rolling increased the population of precipitates

and effected a transition in their chemistry.

5. Microalloying elements showed a tendency to form

 precipitates differing in size. Titanium and aluminium

appeared in the coarse precipitates, but vanadium was

detected chiefly in the fine particles. Niobium showed no

 particular preference, but had a dominant chemical effect

on the other micro alloying elements.

6. Steel composition dictated the morphology of the

 precipitates for the same casting and thermomechanical

conditions.

 Acknowledgments

The authors acknowledge financial support by the Nigerian

Government to one of the authors (COlE) and the provi-

sion of research facilities by Aston University, Birmingham.They would like also to thank Professor M. H. Loretto,

University of Birmingham. Useful suggestions by Dr F. B.

Pickering, Sheffield City Polytechnic, were appreciated.

References

1. c. o. 1. EMENIKE and  J. C. BILLINGTON: Mater. Sci. Technol.,

1989, 5, (5), 450-456.

2. c. o. 1. EMENIKE: PhD thesis, University of Aston, Birmingham,1987.

3. I. WEISS, G. L. FITZSIMONS, K. MIELITYINEN-TITTO, and  A. J.

DeARDO: in 'Thermomechanical processing of microalloyed austenite' (Proc. Conf), (ed. A. J. DeArdo et al.), 33; 1981,

Pittsburgh, PA, ASTM.4. M. J. WHITE and w. S. OWEN: Me tall. Trans., 1980, l1A, 597.

5. w. ROBERTS: 'HSLA steels, technology and applications' (Proc.Conf), (ed. M. Korchynsky), 33; 1983, Philadelphia, PA,ASTM.

6. T. SIWECKI, A. SANDBERG, W. ROBERTS, and  R. LAGNEBORG: in'Thermomechanical processing of microalloyed austenite'(Proc. Conf), (ed. A. J. DeArdo et al.), 163; 1981, Pittsburgh,PA, ASTM.

7. D. C. HOUGHTON, G. C. WEATHERLY, and  J. D. EMBURY: in'Thermomechanical processing of microalloyed austenite'(Proc. Conf), (ed. A. J. DeArdo et al.), 267; 1981, Pittsburgh,PA, ASTM.

8. B. LOBERG, A. NORDEN, J. STRID, and  K. E. EASTERLING: Metall.Trans., 1984, 15A, 33.

9. T. SIWECKI, A. SANDBERG, and  W. ROBERTS: 'HSLA steels,technology and applications' (Proc. Conf), (ed. M.Korchynsky), 19; 1983, Philadelphia, PA, ASTM.

10. Z. CHEN, M. H. LORETTO, and  R. C. COCHRANE: Mater. Sci.

Techno!., 1987, 3, 836.

11. K. NARITA: Trans. Iron Steel Inst. Jpn, 1975, 15, 147.

12. K. J. IRVINE, F. B. PICKERING, and  T. GLADMAN: J. Iron Steel

 Inst., 1967, 205, 161.

13. s. YAMAMOTO, C. OUCHI, and  T. OSUKA: in 'Thermomechanical processing of microalloyed austenite' (Proc. Conf), (ed. A. J.DeArdo et al.), 613; 1981, Pittsburgh, PA, ASTM.

14. H. J. GOLDSCHMIDT: 'Interstitial alloys'; 1967, London Butter-worth.

15. R. LAGNEBORG: Scand. J. Metall., 1985, 14, 289.16. N. K. BALLIGER  and  R. w. K. HONEYCOMBE: Met. Sci., 1980, 14,

121.

17. A. P. COLDREN, v. BLISS, and  T. G. OAKWOOD: in 'Thermo-mechanical processing of micro alloyed austenite' (Proc.Conf), (ed. A. J. DeArdo et al.), 591; 1981, Pittsburgh, PA,ASTM.

18. T. N. BAKER  and  R. L. REUBEN: 'Advances in the physicalmetallurgy and applications of steels', 213; 1982, London, TheMetals Society.

19. Y. C. HIRSCH and  B. A. PARKER: 'Advances in physical metal-

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lurgy and applications of steels', 26; 1982, London, The Metals

Society.20. F. B. PICKERING: personal communication, Sheffield City

Polytechnic, 1987.21. R. K. AMIN and  F. B. PICKERING: in 'Thermomechanical

 processing of microalloyed austenite' (Proc. Con f.), (ed. A. J.DeArdo et  at.), 1; 1981, Pittsburgh, PA, ASTM.

22. R. K. AMIN and  F. B. PICKERING: in 'Thermomechanical processing of microalloyed austenite' (Proc. Conf.), (ed. A. J.DeArdo et  at.), 377; 1981, Pittsburgh, PA, ASTM.

 Appendix

Modelling of sequence of precipitation inmultiple microalloyed steels

TiN precipitation starts at 2162 K (l889°C). When almostall the nitrogen has been removed, i.e. N ~ 0 (say10-6 wt-%), the titanium will have been reduced to 0'0397%(stoichiometric ratio (SR) of Ti : N = 3'43) and the end of 

TiN precipitation is given by:

log (0'0397)(10-6) = -15 200/1finish+ 3·9

where 1finish= 1345 K (l072°C). In the event of a solid solution with another compound being formed, the activityof TiN may be reduced relative to the pure solid compound. For QTiN= 0·1 the end of precipitation would  be 1203°C with Ti = 0'0397% and N = 10-6%, for bothfinish temperatures and the quantity of TiN produced 

would be 0'0443%.

Niobium carbonitride formation

where QXY is the Raoultian activity of the compound referred to the pure solid XV. In the absence of any

 practical data on the activities in the mixtures presumed to form, assumed activities are given in the followingcalculations.

where X and Yare the dissolved microalloying elementsand  A and  B are constants, T  is the absolute temperature,and it is assumed that the pure solid compound XY isformed. When the chemical potential is reduced by theformation of solid solutions, equation (2) becomes

Limited attempts to achieve this have been made,?,23 but

none incorporated multiple microalloying. Consequently,the present analysis is an extension and a modification tothe approach of Houghton et al.7 An example of the presentmethod is given below.

Alloy 4 comprised the following: 0'09C, 0'059AI,0'043Nb, 0'074Ti, 0'00430, 0'010N (see Table 1). It wasassumed that the order of precipitate formation in this steel

would be: TiN, Nb(C,N)o.87' AIN, NbC, TiC, Fe3C. It isfurther assumed that mixtures of these compounds maycoprecipitate forming mutual solid solutions with a result-ant decrease in chemical potentials of the individualcompounds referred to the pure solid state. The chemical

 potential fli is defined as

fli = flo + R T  In Qi (1)

where flo is the chemical potential of the pure solid state, Qiis the activity of the compound referred to the pure solid substance, R is the gas constant, and  T  is the absoluteternpera ture.

Most of the data in the literature on the formation of  precipitates in steels use Henrian activities for the alloyingelements in steel and the standard state chosen is the oneweight percentage (when ax = 1 at 1 wt-%). Thus, the datais in the form

log (%X%Y)= -A/T+B

log (%X%Y)/QXY = - A/T +B

(2)

(3)

It is assumed that any niobium carbonitride to be formed would be cubic l5-Nb(C,N)o'87 (Ref. 24) varying in com- position from nitrogen rich to high carbon with decreasingnitrogen in the steel. The C-curve can be represented by theequation

log (wt-%Nb) + 0·87log (wt-%C

+12/14wt-%N)=-6770/T+2'26 ..... (5)

assuming QNb(C,N)o'87= 1 for pure solid carbonitride. When Nb = 0'043% and N = 0'010% with C = 0'09%, ~tart for  Nb(C,N)o.87 is 1169°C. However, if the Nb(C,N)o.87 should  be dissolved in TiN to form a complex mixture or solid solution, its chemical potential would be reduced and itsformation temperature would increase. For example, for thecarbonitride to be coprecipitated with TiN at 1889°C,its activity would have to be 0·043 relative to the

 pure solid Nb(C,N)o'87' assuming 0'043%Nb, 0'010%N, and 0·09%C. Thus, it is possible that the titanium nitride and niobium carbonitride could coprecipitate at temperatures~ 1889°C. It can be shown that the end of TiN precipi-tation can occur at 1072 or 1203°C, depending upon itsactivity (1'0 or 0'1, respectively, referred to the pure solid compound) with the nitrogen content being 10-6% and  titanium at 0'03972%. However, during coprecipitationwith Nb(C,N)o'87 some nitrogen would react to produce thecarbonitride. From the data presented in Tables 2-4, it isnot possible to calculate how much nitrogen is used to

 produce nitride or carbonitride and therefore it is necessaryto assume a partition of nitrogen between the compoundsto obtain their individual completion temperatures. Twovalues for the partition of nitrogen going to form TiN areassumed in the following calculations: 70 and 50%. Further,the final mixture of compounds or solid solutions will causea decrease in the chemical potentials of the individualcompounds which cannot be deduced from data availablein the literature. Therefore, to illustrate the effect of different activities of the compounds (referred to their puresolid states), it is assumed that for both the 70 and 50%nitrogen used by titanium the activities of TiN in the

 product should be 0·7 and 0·5 and, for Nb(C,N)o.87' the

TiN Formation

Using Narita's derived solid data 11 for pure solid TiN (withTin K)

log(wt-%Ti)(wt-%N)=-15200/T+3'9 .... (4)

i.e.

log (0'074)(0'010)= -15 200/~tart + 3·9

Table 6 Final precipitation temperatures for TiN and

 Nb(C,N)o'87 formation

Finish temperature, °C

N u s ed b y T i , w t -% 8riN aNb(C,NlO'87 TiN Nb(C,N)o'87

70 0·7 0·3 1103 130770 0·5 0·5 1121 123050 0·7 0·3 1110 118150 0·5 0·5 1129 1115

Materials Science and Technology June 1989 Vol. 5

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574 Emenike and Billington Formation of precipitates in pipeline steels

corresponding activities should be 0·3 and 0·5. The end of  precipitation as represented by equilibrium finishingtemperatures is given in Table 6. It seems that changes innitrogen usage by titanium have little effect upon the final

 precipitation temperature of titanium nitride, but signifi-cantly change the temperatures for completion of carbo-nitride precipitation. Nevertheless, it does appear that

coprecipitation is a valid concept for these two compounds.

Niobium carbide and titanium carbideformation

At the end of carbonitride formation the carbon content of the steel would be 0·09- 0·00343 = 0·0866%, the titanium0,0603%, and the niobium 0·0125%.

The formation of pure titanium carbide to be coprecipi-tated is given byll

log (%Ti)(%C)=-10475/T  + 5·33 (7 )

 Aluminium nitride formation

Another compound which could be formed from thismicro alloyed steel is aluminium nitride (AIN) the C-curveof which can be represented by12

The initial weight per cent of aluminium is given by the

analysed %AI (from Table 1)minus the %AI associated withoxygen in the form of alumina, e.g. for steel 4

0·059- 54/48 x 0·0043=0·0542%AI

which, with 0·010%N, gives ~tart(AIN)=1303°C.For coprecipitation with TiN at 1889°C, the activity of 

AIN referred to the pure solid compound would have to be0·0686 and it is assumed that this is probable. However,unlike c5-Nb(C,N)o.87'aluminium nitride has a close packed hexagonal crystal structure and therefore is not isomor- phous with titanium nitride. Consequently, coprecipitationmay not be easy to achieve and this may lead to lower aluminium in the final precipitate than the equilibrium

conditions predict. Nevertheless, some nitrogen would beexpected to react with aluminium to form AIN. If this wereassumed to be 20% and the final a A1N were 0·05, the final

 precipitation temperature would be given by

log (0.0503)(10-6)/0.05 =-6770/Tfinish+ 1·03

Tfinish= 690°C, leaving 0·0503%AI and producing0·0059%AIN.

Summarising the coprecipitation' of TiN, Nb(C,N)o'87'and AIN, it follows the initial precipitation of pure TiN at1889°C, and finishes (it is assumed) when nitrogen decreasesto 10-6%. Assuming the final activities of TiN, Nb(C,N)o'87'and AIN are 0·5, 0·4, and 0·05, respectively, and the nitrogen

is divided 40% as TiN, 40% as Nb(C,N)o.87' and 20% as

AIN, the completion temperatures would be 1132, 1235,and 690°C respectively (a theoretical estimate for AIN, butit is probable that it will have stopped precipitating athigher temperatures as a result of the lower nitrogen and kinetic problems at lower temperatures). The weight per-centages of the precipitates formed would be 0·0137%TiN,

0·0378%Nb(C,N)o'87' and 0·0059%AIN leaving 0·0603%Ti,0·0125%Nb, and 0·0503%AI in the steel.

log (wt-%AI)(wt-%N) = -6770/T  + 1·03 (6 )

~tart(TiC)=1103°C. For titanium carbide to be coprecipi-tated with its nitride at 1889°C, its activity would have to

 be only 0·00171 referred to pure solid TiC and therefore it

is assumed that it is unlikely to coprecipitate at such a hightemperature, although it has a cubic crystal structure.However, when its activity is decreased to only 0·01, it is

 probable that it can form a significant part of the final precipitate at 1593°C. When the titanium content reaches10-6% and the carbon 0·07155%, the end of TiC precipi-tation will be almost complete at 567°C at an activity of 1

or at 727°C for an activity of 0·01 (the total weight would  be 0·0754 g per 100 g of steel).

Using the same data,! 1 pure NbC solubility product datais given by

10g(%Nb)(%C) = -7900/T+3·42 . . . . . . (8)

and when Nb =0·0125% and C =0·0866%, ~tart=964°Cat a NbC activity of 1 or 1615°C at an activity of 0·01referred to pure solid NbC.

Therefore, it would seem that the carbides of titaniumand niobium could coprecipitate with the nitrides and carbonitride at temperatures < 1500°C to produce a very

mixed precipitate of titanium, niobium, aluminium,nitrogen, and carbon which would continue until the }'~ (J .

transformation in the steel at 720°C. The weight of niobiumcarbide formed would be 0·014% leaving 0·0700% carbonwhich would form 1·05% of cementite in the steel. Summar-ies of the equilibrium weights of precipitates and their temperatures of formation at the assumed activities aregiven in Table 5 for steels 1-5. These calculations haveconfirmed that TiN forms in the liquid iron, but the veryhigh temperature of formation of precipitates in steel 4(1889°C) brings into question the validity of Narita's solid data 11when titanium-nitrogen solubility product exceeds acertain value. Further research is required in this area.

References

23. s. R. KEOWN and  w. G. WILSON: in 'Thermomechanical processing of micro alloyed austenite' (Proc. Conf.), (ed. A. J.DeArdo et al.), 319; 1981, Pittsburgh, PA, ASTM.

24. R. C. SHARMA, V. K. LAKSHMANAM, and  J. s. KIRKALDY: Metall.

Trans., 1985, 15A, 545.

Materials Science and Technology June 1989 Vol. 5