REFRACTORY STEELS AND ALLOYS

UDC 620.186.1:669.14.018.44

FORMATION OF LAVES PHASE IN A REFRACTORY AUSTENITIC STEEL

DUE TO LONG-TERM HEATING

L. V. Tarasenko1 and A. B. Shalkevich1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 21 24, March, 2011.

Steels of the Fe Cr Ni Mo Nb Al C system are studied by methods of phase physicochemical analy-

sis and electron microscopy with the aim to determine the causes of changes in mechanical properties after

long-term heating at a temperature of 600 700C. Grain-boundary formation of particles of a Laves phase is

shown to cause decrease in the impact toughness and transformation of particles of -phase under conditions

of creep. The effect of alloying elements on the chemical composition of the multicomponent Laves phase is

studied depending on the temperatures of hardening, aging, and subsequent heating. Concentration correspon-

dence between the chemical composition of the austenite and the intermetallic tcp phase formed in aging is

discovered. A computational scheme for predicting the possibility of formation of Laves phases in

multicomponent alloys is suggested.

Key words: refractory austenitic steel, phase analysis, Laves phase, concentration correspondence.

INTRODUCTION

Austenitic steels hardened in aging due to formation of

-phase are enriched with refractory elements, i.e., molybde-

num, tungsten and niobium, that provide solid-solution hard-

ening. However, their joint introduction into the metal may

exceed the solubility limit and yield an intermetallic Laves

phase not expected in the composition. The Laves phases

(1

, 2

, 3

) are tetrahedrally (topologically) close-packed

intermetallics (tcp phases) characterized by the presence of

two types of positions for atoms in their structure, i.e., the

coordinate number (CN) may be 12 or may exceed 12 and be

equal to 16, 15, or 14.

In refractory steels of the austenitic class the formed

Laves phases have structural type 1

based on binary com-

pounds MoFe2

and WFe2. The Laves phases segregate from

multicomponent austenite by an aging reaction and are also

multicomponent ones. Their common formula can be written

in the form

(A, A1, A

2... A

i)m

(B, B1, B

2... B

i)n

,

where A is the main phase-forming element (Mo, W); B is

the element of the matrix of the alloy (Fe); A1, A

2... A

iare

elements of groups IV VI of the periodic system that sub-

stitute the A-component in the phase in positions with coor-

dinate numbers 16, 15, and 14; and B, B1, B

2... B

iare ele-

ments of groups VII VIII that substitute iron in positions

with coordinate number 12.

The effect of the alloying elements on the solubility of

the phase in multicomponent steels depends on the propor-

tion of the Gibbs energies of formation of the intermetallic

phase (Gf

0)ph and of the solid solution (G

f

0). If the main

components form a thermally stable compound, the Gibbs

energy of which is low, the other elements of the alloy cannot

affect substantially the process of formation of the phase [1].

The Gibbs energy of formation of tcp phases based on

Am

Fen

in iron alloys, including Laves phases, is commensu-

rable with the excess Gibbs energy of the binary solutions

formed between the A-components and the elements of the

Cr Co Ni triad, which are an inseparable part of the com-

position of a refractory alloy. Therefore, we may expect that

the alloying elements should affect considerably the forma-

tion of tcp phases in alloys based on -Fe.

The aim of the present work consisted in studying the ac-

tion of alloying elements on formation of a multicomponent

Laves phase in steels of the Fe Cr Ni Mo Nb Al C

system due to standard heat treatment and long-term heating,

determining the effect of this phase on the mechanical pro-

perties, and creating a method for predicting the formation of

Laves phase in long-term operation of the steels.

Metal Science and Heat Treatment, Vol. 53, Nos. 3 4, July, 2011 (Russian Original Nos. 3 4, March April, 2011)

123

0026-0673/11/0304-0123 2011 Springer Science + Business Media, Inc.

1N. . Bauman Moscow State Technical University, Moscow, Rus-

sia (e-mail: tarasenko@bmstu.ru; lvt@freemail.ru).

METHODS OF STUDY

We checked the correctness of the suggested computation

technique for predicting formation of Laves phase for heats

of chromium-nickel austenitic steel Kh15N36M6T2YuB

with different contents of molybdenum (5.9 6.4%), tita-

nium (2.10 2.65%), and niobium (0.85 1.00%). We per-

formed mechanical tests and determined the ultimate rupture

strength and the impact toughness by standard methods after

the main heat treatment and subsequent heating operations.

We also tested the steels for creep resistance. The main heat

treatment consisted of hardening from 1050C and aging at

750C. Additional heating was performed at 600, 650 and

700C for from 100 to 1000 h. We used the methods of trans-

mission electron microscopy and physicochemical phase

analysis (PCPA), isolated the carbides and the intermetallic

phases, and determined their crystal structure, chemical com-

position, and content. The Laves phase was identified by tak-

ing electron diffraction patterns from foils2 and powder x-ray

diffraction patterns of anode precipitates.

For electrochemical isolation of the Laves phase (to-

gether with the primary and secondary carbonitrides) we

used an alcoholic electrolyte with Cl ions; for isolating the

-phase we used an aqueous electrolyte with SO4

ions. We

used PCPA to determine the following concentration parame-

ters: the content of element in the intermetallic phase [Me]i

in wt.%, the content of phase Q ph in wt.% as [Me]i, and the

formula of the intermetallic phase.

RESULTS AND DISCUSSION

After hardening, the steels contain carbides, nitrides, and

carbonitrides of type MX with different chemical composi-

tions and crystal lattice parameters, namely, TiC0.432

,

NbC0.440

, NbN0.442

, and (Ti, Nb)(C, N)0.428

.

Heating for hardening with a hold of 2 h yields discrete

particles of a Laves phase with a size of up to 40 nm over

grain boundaries; the content of the particles is 0.5 wt.%.

During 16-h aging at 650C a hardening intermetallic

-phase forms in the steels in an amount of 8 10%. Parti-

cles with a size of 4 6 nm are distributed uniformly of the

body of grains and are coherently bonded to the matrix. In

the -phase based on the Ni3Al compound a part of the at-

oms in the sublattice of aluminum and titanium is substituted

by alloying elements. As a result, the phase has a chemical

formula (Ni, Cr, Fe)3(Al, Ti, Nb).

In the long-term heating operations that imitate opera-

tional heating the -phase is thermally stable; its particles

are characterized by low coalescence, and the coherence with

the matrix is virtually intact. The ultimate rupture strength

after long-term heating at 600 and 650C does not change.

After heating at 700C the size of the particles of the -phase

increases to 10 15 nm, the coherence is partially lost, and

rdecreases by 3 5%.

The operating capacity of the metal at elevated tempera-

tures is not controlled by the changes in the main hardening

phase but rather depends on the additional process of aging

of the austenite and formation of a Laves phase 1

, i.e.,

*

1+

**, (1)

where *

is a solid solution depleted of alloying elements as a

result of primary aging and formation of -phase, **

is a

solid solution depleted of alloying elements as a result of ad-

ditional aging and formation of 1

; and 1

is a Laves phase

based on the MoFe2

compound.

The Laves phase forms over boundaries of austenite

grains in the form of a continuous chain of particles up to

0.15 m in size; when the duration of the heating is in-

creased, the particles arranged with high density take near-

boundary regions (Fig. 1).

124 L. V. Tarasenko and A. B. Shalkevich

2The experiment was made by N. V. Sharapanova.

b

c

Fig. 1. Microstructure of near-boundary regions of steel

Kh15N36M6T2YuB after heating by the following modes (initial

state: hardening from 1050C and aging at 650C for 16 h)

( 25,000): a ) 600C, 500 h; b ) 650C, 500 h; c ) 650C, 1000 h.

The segregation of Laves phase particles over grain

boundaries causes considerable decrease in the impact

toughness of the steel after long-term heating. The impact

toughness is halved after heating to 600 and 700C with a

hold of 500 h and decreases by almost an order of magnitude

after heating to 700C with a hold of 1000 h (Fig. 2).

The content of the Laves phase after heating to 700C

with a hold of 500 h amounts to 1.7 2.0%. The size of the

particles increases after longer heating.

The segregation of Laves particles over grain boundaries

has a side effect that manifests itself in the functional part of

the specimens tested for creep. An electron microscope study

has shown the presence of chains of cuboid particles in

near-boundary regions close to particles of the Laves phase.

According to the data of a microdiffraction analysis these

particles are an -phase. The change of the round shape of

these particles for the cuboid one is possible if the misfit

parameter changes sign, i.e., the mismatching of the crystal

lattice parameters of the solid solution and of the -phase

(a

a

)a

changes from a negative value to a positive one.

The growth in the lattice parameter of the solid solution may

be connected with the fact that the cuboid -phase has segre-

gated already during creep in the near-boundary zone, the

solid solution of which (** ) is depleted of niobium, tita-

nium, and molybdenum that have passed to a chemical com-

pound, i.e., a Laves phase.

In the studied steels the Laves phase with hexagonal

crystal structure (1

) forms on the basis of the MoFe2

com-

pound. The crystal lattice parameters of the 1-phase are

a = 0.477 0.480 nm and c = 0.777 0.780 nm. In a multi-

component Laves phase molybdenum is often substituted by

titanium and niobium and iron is substituted by nickel and

chromium. According to the data of PCPA the Laves phase

in the studied steels may be described by the formula

(Mo0.54

, Ti0.35

, Nb0.11

)(Fe0.55

, Cr0.23

, Ni0.12

)2.

Titanium and niobium are elements that cause growth in

the content of the Laves phase.

The effect of titanium and niobium on formation of

multicomponent Laves phase is predetermined by the ther-

modynamic properties of binary Laves phases. It is known

that the intermetallic compounds of transition metals posses

a low, often negative, entropy, which makes it possible to use

the enthalpy for thermodynamic computations instead the

Gibbs energy. The enthalpies of formation of Laves phases

[1] are presented below

Phase H f0

, kJmole

Fe2Mo. . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7

Fe2Ti. . . . . . . . . . . . . . . . . . . . . . . . . . . 25.7

Fe2Nb . . . . . . . . . . . . . . . . . . . . . . 20.5 23.7

Cr2Ti. . . . . . . . . . . . . . . . . . . . . . . . . . . 44.0

For the TiFe2

and NbFe2

Laves phases the values of

Hf

0are 2.5 3 times lower than for the MoFe

2phase.

Consequently, the substitution of molybdenum by titanium

and niobium atoms should cause decrease in the value of

Hf

0of the multicomponent (Mo, Ti, Nb)(Fe, Cr, Ni)

2phase

as compared to a binary phase or, in other words, should

raise the thermodynamic stability of the phase. Nickel is a

strong stabilizer of Laves phase in multicomponent systems

[1]. Chromium promotes formation of the phase only in the

presence of titanium, because the enthalpy of formation of

the Cr2Ti phase (but with another crystal lattice) has the low-

est value of Hf

0of all the binary Laves phases formed by

transition metals (see above).

Chemical analysis of the multicomponent Laves phase

has shown some concentration agreement between the pro-

portion of the concentrations of the A-components in the mo-

lybdenum sublattice with CN = 16 and the proportion of the

concentration of these elements in the initial solid solution

(austenite), i.e.,

(Mo)(Ti)1

= 1.12 1.18(Mo)(Ti), (2)

(Mo)(Nb)1

= 1.04 1.05(Mo)(Nb) . (3)

This proportion is also constant when the Laves phase is

segregated from the austenite in heating for hardening by re-

action 1

and when it is segregated in long-term heating

operations after the main aging, i.e., by reaction (1) from

austenite already depleted of the alloying elements that have

formed the -phase. After both reactions the chemical com-

pound inherits from the solid solution the proportion of the

A-components (see Table 1).

This inherited concentration may be a result of the fact

that the sublattice of the Laves phase with CN = 16 preserves

a chiefly metallic type of chemical bonding the same as in

the initial solid solution [2]. Such dependence has been de-

tected in a study of Laves phases in refractory steels of

martensitic class [3].

The limiting amount of Laves phase segregated over grain

boundaries in the steel with grain size No. 3 4, which causes

inadmissibly low decrease in the impact toughness is 1.0%.

Formation of Laves Phase in a Refractory Austenitic Steel Due to Long-Term Heating 125

1

2

3

100 500 1000

, h

1.0

0.8

0.6

0.4

0.2

KCU, J m 2

Fig. 2. Impact toughness of steel Kh15N36M6T2YuB as a function

of the temperature and duration of heating (initial state: hardening

from 1050C and aging at 650C for 16 h): 1 ) 600C, 500 h;

2 ) 650C, 500 h; 3 ) 700C, 1000 h.

In order to prevent grain boundary segregation of Laves

phase in operational heating of the existing grades of

austenitic refractory steels we can suggested the following

three ways:

(1) restriction of the service life of parts from austenitic

steels jointly containing molybdenum, tungsten and niobium;

(2) control of the chemical composition of every heat

with computation of the possible content of Laves phase ac-

cording to the suggested prediction scheme;

(3) choice of heats with the lowest limiting content of

molybdenum and niobium.

CONCLUSIONS

1. Long-term heating of refractory steel belonging to the

Fe Cr Ni Mo Nb Al C system at 600 700C

causes grain-boundary segregation of particles of a Laves

phase in the form of a continuous skeleton, which lowers

considerably the impact toughness of the metal.

2. The Laves phase in refractory steels is a multicom-

ponent compound with formula (Mo, Ti, Nb)(Fe, Cr, Ni)2.

3. Segregation of the Laves phase is caused by the joint

presence of molybdenum, titanium, and niobium the concen-

tration of which exceeds the solubility of the multicom-

ponent Laves phase in the austenite at the operating tempera-

tures.

4. We have determined concentration matching between

the multicomponent austenite and the Laves phase segre-

gated from it by the aging reaction with respect to the pro-

portion of the A-components, MoTi and MoNb.

REFERENCES

1. B. M. Mogutnov, I. A. Tomilin, and L. A. Shvartsman, The Ther-

modynamics of Iron Alloys [in Russian], Metallurgiya, Moscow

(1984), 238 p.

2. V. K. Grigorovich, Metallic Bonding and the Structure of Metals

[in Russian], Nauka, Moscow (1988), 296 p.

3. L. V. Tarasenko, Effect of alloying elements on the process of

aging with formation of Laves phase, -phase, and R-phase in

multicomponent alloys based on bcc-Fe. Part 2. Phase composi-

tion and thermodynamic parameters of solid solutions, Izv.

Ross. Akad. Nauk, Ser. Met., No. 2, 51 55 (1996).

126 L. V. Tarasenko and A. B. Shalkevich

TABLE 1. Proportion of A-Components in the Laves Phase and in

the Austenite from which the Phase is Segregated

HeatHeat

treatment

MoTi,at.%at.% in phases

MoNb,at.%at.% in phases

1

1

1 Hardening 1.46 1.30 7.8 7.5

Aging 2.30 1.95

3 Hardening 1.20 1.35 6.1 5.8

Aging 2.10 1.80

AbstractKey wordsINTRODUCTIONMETHODS OF STUDYRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES

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