Influence of Composition and Aging Heat Treatment on the ... · microstructure-stainless steels, material science, nano and composite materials. ... Table 1 Chemical composition of

Journal of Materials Science and Engineering A 7 (9-10) (2017) 271-279 doi: 10.17265/2161-6213/2017.9-10.005

Influence of Composition and Aging Heat Treatment on

the Microstructure and Strength of Innovative-Carbon

Free 10% Cobalt-Maraging Steel Powder Composites

Waleed Elghazaly1, Omyma Elkady1, Sabine Weiss2 and Saied Elghazaly1

1. Central Metallurgical R & D Institute-, P.O. Box 87-Helwan, Cairo, Egypt

2. Head of Material Science & Technology Dept., TU Cottbus-Germany

Abstract: Good combinations between strength and toughness are always the aim of all researchers working in the field of material science. Maraging steel grades (200-300) are one of the well known steel alloys proved to have good strength and toughness and are known as 18% Ni-Co-Mo steel family. Maraging steels production, import, and export by certain countries such as USA is closely monitored by international authorities because it is particularly suited for use in gas centrifuges used for uranium enrichment and in aviation technology. In this research an effort is paid to produce innovative carbon-free maraging steel alloy composites that can compete the well known 18% Ni-8% Co standard (250-300) maraging steel alloy with higher strength and superior toughness. The experimental maraging steel composites having different Ni (18-25%) and Al (0.5-1.5%) together with or without Ti and Mo contents are produced by consolidation from the nano-elemental powders. The mechanism of strengthening in Iron- Nickel- Cobalt-Aluminum composite alloys is studied, however, the changes in microstructures after solution treatment and aging-heat treatment are emphasized using metallurgical microscopy and SEM-TEM aided with EDX analyzing unit. The effect of induced deformation on the properties of the as-sintered samples is also studied. Fracture toughness, impact toughness, hardness, and strength are measured for all alloy composites under investigation and compared with the standard nominal properties for conventional maraging series (250-300).

Key words: High-Strength steels, maraging steels, mechanical properties, microstructure, fracture toughness, precipitation hardening, solution treatment, aging heat treatment.

1. Introduction

The ever increasing demand for superior steel in

aviation and automotive industries requiring high

strength, sufficient ductility, and good weldability

initiated development of maraging and precipitation

hardenable stainless steels [1]. PH (precipitation

hardenable) steels have a unique advantage over

others where they are hardened without quenching and

the absence of distortion and decarburization [2].

These steels rely on the precipitation of intermetallics

compounds and generally contain high levels of cobalt,

Corresponding author: S. El-Ghazaly, Prof., researcher,

research fields: steel metallurgy, steelalloys, superalloys, mechanical failures, microstructures, heat treatment, microstructure-stainless steels, material science, nano and composite materials.

molybdenum and nickel [3]. Elements such as

titanium, vanadium, aluminum and niobium have been

added to enhance the precipitation process thus

increasing the strength. Those steels are generally

hardened by aging at approximately 450-500 °C [4].

Maraging steels based on iron-nickel martensite

constitute a very important family of high-strength

steels, which distinguishes itself by demonstrating an

unparalleled combination of excellent fabricability [5],

high strength and fracture toughness after heat

treatment.

Heat treatment of these steels has now been

perfected to ensure consistently high levels of strength,

ductility, and toughness in a variety of product shapes

and sizes [6]. Cobalt-free variants have been

commercialized as part of efforts to save production

D DAVID PUBLISHING

Influence of Composition and Aging Heat Treatment on the Microstructure and Strength of Innovative-Carbon Free 10% Cobalt-Maraging Steel Powder Composites

272

costs.

Further knowledge has been generated on 18%

nickel maraging steels regarding phases precipitating

during aging, thermal embrittlement, thermal cycling

and austenite reversion/retention and their effect on

mechanical properties [7-9]. The well known alloy

composition of 18% Ni-Co-Mo maraging steels was

designed for maximum strength to toughness ratios due

to their tough Fe-Ni martensite structure which is

hardened at low temperature aging.

It is reported that the primary precipitate responsible

for the strengthening is Ni3Mo, however Cobalt

enhances its precipitation by decreasing the solubility

of Molybdenum in the matrix. Another secondary

hardening by formation of Ni3Al and Ni3Ti

intermetallic precipitates was also reported [10].

In this research some carbon-free maraging steel

alloy composites with different compositions were

produced by sintering them from their powders at

about 1,350-1,450 °C to emphasize the effect of Ni,

Al and Mo contents as well as aging condition on their

microstructures, tensile and fracture toughness.

Microstructure characteristics were investigated on the

light, scanning and transmission electronic

microscope.

2. Experimental Work

2.1 Materials Processing

Maraging steel bars composites 55 10 10 mm

were prepared by consolidation from their powder

constituents by HIP (hot isostatic pressing) at 1,200

MPa. The powders (100-200 microns) were blended

in a tumbling mixer in dry basis for 1.5 h and then

compacted under 1,200 MPa uniaxial pressures in a

special steel die to final product .The bar samples

were then sintered at 1,400 °C for 3 h under vacuum.

The final adjusted compositions of the experimental

steel composites sinter are shown in Table 1.

Some of the sintered bars were forged in the range

1,100 °C-850 °C to about 60% reduction in area to

study the differences in densification, shrinkage,

microstructure and tensile properties as well. Standard

testing specimens were cut and machined from the

forged bars using normal procedures.

2.2 Heat Treatment

Samples of sintered steel rods C-1 to C-4 and others

forged ones were solution treated (900 °C, 120 min,

water cooling). The as-sintered and as-deformed

solution treated samples are then aged at 500 °C for 5

hours and then air cooled to strengthen their matrices

by precipitation hardening mechanism through

forming series of intermetallic phases. Fig. 1

illustrates the procedure of solution treatment and

aging heat treatment of both as-sintered and forged

steel bars.

3. Results and Discussions

3.1 Sintring

Variations in dimensions of the consolidated bars of

produced experimental maraging steel bars were

subjected to changes during processing depending on

the sintering temperature at constant pressure (1,200

MPa) and powders grain size as shown in Fig. 2.

Densities of about 7.8 and 8.3 gm/cm3 were reached

for C-4 maraging steel under such conditions and at

Table 1 Chemical composition of experimental sintered maraging steel forged bars and standard 18%Ni Maraging (250) steel. Element, wt.% Material

Ni Co Mo Ti Al S Cr C

Composite-1 25 10 ---- ---- 1.50 0.003 ---- ----

Composite-2 20 10 4.50 0.55 1.50 0.004 ---- ----

Composite-3 18 10 4.50 0.62 1.50 0.003 ---- ----

Composite-4 20 10 4,50 0.50 0.50 0.005 ---- ----

Standard maraging (250) 18.5 8.4 5.00 0.72 0.12 0.002 ---- 0.03


273

Fig. 1 Solution and aging heat treatment of the experimental maraging steel alloys.

Fig. 2 Variations in density and shrinkage of C-4 steel after sintering .

sintering temperatures 1,200 °C and 1,300 °C

respectively, while shrinkage of about 22% in such

steel was measured after sintering. The compactness

and density of for example C-4 maraging steel bars,

was upgraded to reach 9.1 gm/cm3 after 85%

reduction in thickness by drop forging at 850 °C.

3.2 Microstructure Variations

Cobalt, Nickel and Molybdenum dissolved in liquid

Iron to form series of solid solutions and intermetallic

compounds as shown in the Fe-Co, Fe-Ni, Fe-Mo and

Fe-Ni-Co equilibrium phase diagrams in Fig. 3 [11].

Nickel forms with Iron series of equilibrium phases

like austenite, martensite, ferritic-austenitic and even

Ni3Fe phases depending on Nickel content, however

Molybdenum prefers to form ferrite (110). The

presence of 10% Cobalt (002) in Iron also enriched

formation of ferrite, however addition of 20% Ni and

Time (Hrs)

0 2 4 6 8 10 12 14 16

Tem

pera

ture

o C

0

200

400

600

800

10002 hrs

5 hrs

SolutionTreatment

Aging

SoftMartensite(28-32 HRc)

Hardening (52 HRc)

Sintering temperature oC

1050 1150 1250 1350

Den

sity

, gm

/cm

3 , x10

30

40

50

60

70

80

90

10

15

20

25

30

Shr

ink

age

%

1100 1200 1300 1400

Density

Shrinkage


274

Fig. 3 Equilibrium phase diagrams of Fe-Ni , Fe-Co, Fe-Mo and Fe-Ni-Co systems.

5% Mo prefers to form fully austenitic (111)

equilibrium phase even at room temperature.

The microstructures of as-sintered samples

depended to a great extent on the composition of the

experimental steel as projected in Figs. 4A-4C. A

completely fine, interlocked homogeneous

microstructure was obtained at compositions C-2 and

C-4 while the worst microstructure was obtained for

alloy compositions C-1, hence a dendritic and

polygonal microstructures identified the as-sintered

samples. The main differences in microstructures

revealed the absence of segregation at grain

boundaries and the increasing homogeneity in case of

C-4 steel composition. At higher Aluminum content

(1.5%) as in compositions C-1, C-2, C-3 it was

observed the presence of hard clusters of Al2O3

dominated the steel matrix field, while at levels of 0.5%

Al together with 0.5% Ti the matrix was clean from

Alumina particulates. After forging operation, the

structure of as-sintered samples was completely lost,

however, it was observed that re-crystallization of the

slightly banded structure occurred on forging from

high temperatures (1,000-900 °C) as shown in Fig. 4A.

At low forging temperature (800 °C) the aged

microstructures of steel C-4 and C-3 were found to be

the optimal ones hence massive precipitation of

Ni3Mo along with Ni3Ti and Ni3Al took place as in

Fig. 5. Deformation of the sintered steel samples to

60% reduction in thickness altered the microstructure

of all steels to denser, homogenous and grain refined

martensitic-austenite phases which enriched the

formation of intermetallic massive precipitation

during aging process as shown in Fig. 5.

It was observed also that solution treatment of

such steels from 1,100 °C caused coarsening of the

prior austenite and lath martensite phases, however

forging at 800 °C refined the lath martensite

structures.

Fig. 4 Micro

Fig. 5 Appe

The opti

temperature

martensite

crystallograp

differences

density of

interactions

content of n

martensite

mainly she

depending o

Aging of

about 500

precipitating

Ni3Mo

Influence of of Inn

ostructures of

C-4

earance of prec

mal structur

(900 °C) s

(Fe-Ni m

phic structure

as the pres

dislocations

with interm

nickel. Mean

formed by

eared BCT

on the carbon

such carbon

°C introduc

g intermetalli

Ni3A

Compositionnovative-Carb

(A) forged (A), for

cipitations and

res were ob

solution treat

martensite) h

e as body cen

sence of tre

s, fine twin

metallics dep

nwhile, the cr

quenching F

(body cent

content.

-free martens

ed more ha

ic compounds

20,000

Al,Ni3Ti

n and Aging Hbon Free 10%

rged-aged (B)

d lath martensi

btained for

ted. The for

has the s

ntered cubic w

emendously h

ns and com

pending on

rystallograph

Fe-C alloys

ered tetrago

sitic structure

ardening thro

s inside the B

LathRetain

Heat Treatme% Cobalt-Mara

(B)and as-sintere

C-4

ite in forged-ag

low

rmed

same

with

high

mplex

the

hy of

are

onal)

es at

ough

BCC

mat

part

(Ni

of T

foun

Cob

incr

inte

the

T

reve

defo

disl

reac

h martensite (1ned Austenite

ent on the Micaging Steel P

d aged (C), exp

ged C-4 and M

trix. TEM of

ticulates that

-Fe)3Mo, Ni3

Ti, Al (N) as

nd in any

balt contribu

reasing the

ermetallics wi

ordering in th

The fine har

ealed the tr

formation on

locations an

ctions.

110), (111)

50,000

crostructure owder Comp

perimental C-3

M (250) maragin

f foils showed

had a diffrac

3Ti, Ni3(Ti-A

s projected i

intermetallic

uted to solid

activity of

ith Nickel du

he matrix as w

rd structures

emendous in

the as-sinte

nd sites for

Retained austenite

and Strengthposites

(C) 3 and C-4 steel

M250

ng steels.

d a host of s

ction pattern

Al), Ni3Al an

n Fig. 5. Co

combinatio

d solution ha

f Molybdenu

uring aging an

well.

of steels C

nfluence of

ered samples

r massive

Ni3

h 275

ls, 100.

mall discrete

for (Ni3Mo),

nd dispersion

obalt was not

ns, however

ardening and

um to form

nd increasing

C-3 and C-4

the induced

by creating

intermetallic

20,000

3Mo

5

e

,

n

t

r

d

m

g

4

d

g

c


276

Two main types of phase transformation in

maraging steels were observed, precipitation and

austenite reversion. Before either of these

transformations happens in these steels (during

heating), there is a third transformation, which is the

martensitic transformation (during cooling).

Precipitation and austenite reversion occur in this

martensite matrix, the former generally desirable as

long as it is not too complete and the latter usually

undesirable. In simple terms, precipitation leads to

hardening and austenite reversion leads to softening.

Although martensitic transformation is a prerequisite

of the functioning of maraging steels, it is easily

achievable and its details do not strongly determine

the final steel properties, at least to a far lesser extent

than precipitation and austenite reversion. Therefore,

the austenite transformation ends at around 720 °C [8].

Complete solution is ensured by heating continuously

to 900 °C and holding at this temperature for 30 min.

During cooling to room temperature, there is drastic

expansion at approximately 135 °C, due to the sudden

start of rapid transformation from austenite to

martensite.

4. Mechanical Properties

4.1 Hardness of Experimental Maraging Steels

Bulk hardness of all the specimens were measured

using IUHTM (identic universal hardness testing

machine), where all the forged hardened samples of

different compositions showed hardness in the range

40-48 HRC as shown in Fig. 6. The maximum

hardness value of about 50 HRC was measured for

solution treated and aged C-4 maraging steel, while

the worst hardness was 40 HRC for C-1 maraging

steel. The hardness of as-sintered aged samples was

found to be in the range 42-43 HRC due to moderate

intermetallic precipitation rate caused by the absence

of activation and nucleation sites created by

deformation. Hardness of composition C-4 is more

than that for standard M (250) produced

conventionally with about 10 HRC due to the more

dens, fine lath martensite and massive precipitation of

intermetallic compounds.

Maraging steel compositions

C-1 C-2 C-3 C-4

Ult

imat

e te

nsile

str

engt

h, M

pa

500

1000

1500

2000

2500

3000

800

65

55

45

40

50

Har

dnes

s, H

Rc

M250

M250

M250

32 HRcAs Sintered

60

U.Tensile Strength

HRc

Fig. 6 Variations of ultimate tensile strength and hardness with composition of experimental maraging steels compared with wrought M (250).


277

4.2 Tensile Strength of Experimental Maraging Steels

Tensile test coupons of the experimental maraging

steels in their solution treated and aged conditions

were applied to standard tensile testing using

mini-sample sizes. The results of testing steels C-1 to

C-4 were compared with those for the standard known

M (250) maraging steel as shown in Fig. 6. Tensile

values in the range 800-2,750 MPa were detected for

steels C-1 to C-4 comparing with about 1,800-2,100

MPa for M (250). The tensile strength of as-sintered

aged test samples showed only moderate values

between 890-1,000 MPa due to the depletion of

precipitates inside the matrix and in some cases due to

the bad effect of Al2O3 non-metallic inclusions at high

aluminum levels. The variations in tensile strength for

the maraging samples depend on the cleanness of the

matrix and on the ordering inside the matrix itself.

The counteraction of using aluminum as intermetallic

former with nickel and its amount forming Al2O3

inclusions must be adjusted.

4.3 Fracture Toughness Measurements

The maraging steel alloys under investigation have

superior ductility despite their high volume fraction of

intermetallic hard precipitations like Ni3Mo, Ni3Ti and

Ni3TiAl. Therefore from fracture mechanics point of

view the most straight forward parameter to

characterize fracture toughness is the critical stress

intensity factor (K) or dynamic fracture parameter

(Kid). A fracture toughness Kic (Pa.

measurements was made at room temperature using

the well known fracture standard test, meanwhile

Charpy toughness (CVN) test was also used .The

obtained results are projected in Fig. 7. Best values of

fracture toughness 72, 76, and 80 MPa.√ were

measured for steels C-2, C-3 and C-4 respectively,

while the fracture toughness values for standard M

(250-300) are 67-71 MPa. √ . In other toughness

measures, values of about 35-42 J were obtained

for C-2, C-3 and C-4 experimental steels while

only 28-32 J were reported for M (250) maraging steel

Maraging steel compositions

0 C-1 C-2 C-3 C-4

Fra

ctur

e to

ughn

ess

(Kic

), M

pa.m

-1/2

45

50

55

60

65

70

75

80

85

M250

45

35

25

20

30

40

55

50

60

CV

N, J

M (250, 300)

M (250, 300)

As Sintered

Fracture Toughness

CVN, J

Fig. 7 Fracture toughness values for experimental steels compared with M (250).

278

Fig. 8 Fract

category. T

from lack

measured in

superior tou

steels, and t

for the as-sin

of deforme

contained ab

some appare

8B.

5. Conclus

(1) Fine

obtained at

deformation

worst mic

composition

to the prese

the absence

(2) Tensil

maximum h

measured fo

C-4 after sol

(3) Fractu

was measur

toughness ap

appearance

facets.

Influence of of Inn

(A

ture appearanc

The as-sinter

of toughnes

n average for

ughness was

the worst on

ntered sampl

d aged stee

bout 85-90%

ent precipitat

sions

e and inter

steel compos

n and then sol

rostructure

ns C-1, C-2 in

ence of Al2O

of Mo and Ti

le values of

hardness valu

or deformed

lution treatme

ure toughnes

red for comp

pproaching 5

of fracture sh

Compositionnovative-Carb

A)

ce for experim

ed forged s

ss and only

these steels.

measured f

ne was measu

es. In spite o

el C-4, the

% ductile face

te particles a

rlocked mic

sitions C-3 an

lution treated

was obtain

n as sintered

O3 nonmetalli

i in C-1 as we

about 2,500-

ue of about 4

experimenta

ent and aging

ss of about

positions C-

5 Joules, in t

howed high c

n and Aging Hbon Free 10%

mental steel C-4

samples suff

y 20-27 J w

In all cases,

for C-3 and

ured for C-1

of the hard ma

fracture sur

ets, Fig. 8A w

as shown in

crostructure

nd C-4 after 6

d, aged, while

ned for a

or deformed

ic inclusions

ell.

-2,750 MPa,

47-50 HRC w

al steels C-3

g.

75-80 MP

3 and C-4 w

the same time

content of du

DuctPrecipi

Heat Treatme% Cobalt-Mara

4.

fered

were

, the

C-4

and

atrix

rface

with

Fig.

was

60%

e the

alloy

due

and

and

were

and

√

with

e the

uctile

(4

as-s

due

prec

mar

Re

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

tile Fracture Fitates Ni3Mo, N

ent on the Micaging Steel P

4) In all c

sintered, aged

e to the absen

cipitation o

rtensite matri

ferences

Van Swam,

“Properties

Powder Meta

33-45.

Floreen, S. 1

Rev. 13 (126)

Miller, G. P

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Milenko RimMachinery aInternationalCzech RepubCorn, D. L

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370), 13. 54.

“Metals Han

Steels, and H

edition, ASM

Nedjad, S. Hand Shirazi,MicrostructurFe-10Ni-5Mn249-53. doi:1Oruč, M., RiS. 2010. Characteristic

Facets Ni3TiAl

crostructure owder Comp

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of intermata

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y

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.

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f h ,

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Influence of Composition and Aging Heat Treatment on the ... · microstructure-stainless steels, material science, nano and composite materials. ... Table 1 Chemical composition of