Microstructural Characteristics and Mechanical … . 1a, M. El ... a On leave from Zagazig University, Faculty of Engineering, Materials Engineering ... of the abrasive materials

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 14 (2017) pp. 4675-4686

© Research India Publications. http://www.ripublication.com

4675

Microstructural Characteristics and Mechanical Properties of Heat Treated

High-Cr White Cast Iron Alloys

Kh. Abdel-Aziz 1a, M. El-Shennawy 1b* and Adel A. Omar 1c

1 Taif University, Engineering College, Mechanical Engineering Department, Taif, Post Code 888, KSA. a On leave from Zagazig University, Faculty of Engineering, Materials Engineering Department, Zagazig, Egypt.

b On leave from Helwan University, Faculty of Engineering, Mechanical Engineering Department, Helwan, Egypt. c On leave from Benha University, Faculty of Engineering, Mechanical Engineering Department, Benha, Egypt.

Abstract

High Cr white cast irons are an important class of wear

resistant materials currently used in a variety of applications

that requires high wear resistance and reasonable toughness.

The outstanding performance of these alloys is due to the

presence of large amounts of chromium carbides which

exhibit high hardness. The size, type and morphology of these

carbides control the wear resistance and toughness. The

microstructural characteristics of these alloys, and

consequently their wear resistance, can be extensively

changed by varying the chemical composition or the

solidification rate or by specific heat treatment after casting.

Heat treatments for all high-Cr WCI alloys are essential to

change their microstructure and therefore, to improve their

wear resistance to suit the individual application requirements.

Changing in chemical composition and heat treatment carried

out to this alloy related to microstructural characteristics and

mechanical properties of high Cr white cast iron alloys are

oresented.

Keywords: High-Cr WCI alloys, microstructural

characterization, abrasive wear resistance, alloying additions,

heat treatment, mechanical properties.

INTRODUCTION

The high alloy white irons are primarily used for abrasion

resistant applications and are readily cast in the shape needed

in machinery used for crushing, grinding and general handling

of the abrasive materials. The presence of M7C3 eutectic

carbides in the microstructure provides high hardness needed

for abrasion resistant applications. The metallic matrix

supporting the carbide phase in these irons can be adjusted by

the alloy content and heat treatment to develop the proper

balance between resistance to abrasion and toughness. All

high alloy white irons contain chromium to prevent the

formation of graphite on solidification, stabilize the carbide

and to form chromium carbides which are harder than iron

ones. Most of them also contain nickel, molybdenum, copper

or combinations of these alloying elements in order to prevent

the formation of pearlite in the microstructure [1,2]. The

chromium-molybdenum white irons (class II of ASTM A532)

contain 11 to 23% Cr and up to 3% Mo and some Ni and

Cu and can be supplied; either as-cast with an austenitic or

austenitic-martensitic matrix, or heat treated with a

martensitic matrix microstructure for maximum abrasion

resistance and toughness. These irons provide the best

combination of toughness and abrasion resistance of all white

irons and are used in hard rock mining equipment, slurry

pumps, coal grinding mills, and brick molds [3].

MECHANICAL PROPERTIES AND APPLICATIONS

OF HIGH CR IRON ALLOYS

The tensile strength of pearlitic white irons normally ranges

from about 205 MPa for high-carbon grades to about 415 MPa

for low carbon grades. The tensile strength of martensitic

irons with M3C carbides ranges from about 345 to 415 MPa,

while high chromium irons, with their M7C3 type carbides,

usually have tensile strengths of 415 to 550 MPa [4]. Limited

data indicate that the yield strengths of white irons are about

90% of their tensile strengths. These data are extremely

sensitive to variations in specimen alignment during testing.

The elastic modulus of a white iron is considerably influenced

by its carbide structure [4]. An iron with M3C eutectic

carbides has a tensile modulus of 165 to 195 GPa, irrespective

of whether it is pearlitic or martensitic, while an iron with

M7C3 eutectic carbides has a modulus of 205 to 220 GPa.

MICROSTRUCTURAL CHARACTERISTICS OF HIGH

CR WHITE CAST IRONS

As-cast austenitic and pearlitic microstructures

High Cr white cast irons are ferrous alloys containing

chromium between 12 and 30%. It is well documented that the

microstructure of all hypoeutectic high Cr white iron alloys in

the as cast condition consists of a network of M7C3 eutectic



4676

carbides in a matrix of austenite dendrites or its

transformation products [5-15]. The type, amount and

morphology of these eutectic carbides control wear resistance

and toughness [16]. There are four kinds of iron carbide

precipitate in high alloyed white cast iron according to the

contents of carbon and chromium [17,18]; (Fe, C)3C, (Fe,

Cr)7C3, (Fe, Cr)23C6 and (Fe, Cr)3C2. Other carbide forming

elements e.g. molybdenum, vanadium and manganese are

soluble in both M3C and M7C3 and may also give rise to other

hard carbides e.g. M2C and M6C [3]. Fig. 1 shows the

microstructure of high Cr alloyed white cast iron in the as-cast

state. The matrix structures can be pearlite, austenite, and

martensite or some combination of these [19]. The matrix in

alloy white cast irons in the as-cast state was primarily

austenite [20,21]. Austenitic structures are favored by [3];

faster cooling rates, high Cr/C ratio and nickel and

molybdenum.

Figure 1: High chromium iron microstructures in the as-cast; (a) with austenitic matrix microstructure, (b) With austenitic-

martensitic matrix microstructure. Both at 500X [19].

As-cast and heat treated martensitic microstructures

Martensitic structures can be obtained in as-cast, especially in

heavy section castings which cool slowly in the mold. With

slow cooling rates, austenite stabilization is incomplete and

partial transformation to martensite occurs. However, in these

alloys, martensite is mixed with large amounts of retained

austenite as shown in Fig. 1(b). To obtain maximum hardness

and abrasion resistance, full martensitic matrix structures must

be produced by full heat treatment. Martensitic matrix

microstructures with secondary carbides of heat-treated high

Cr white irons are shown in Fig. 2 [19]. All as-cast structures

contained patches of secondary carbides [6]. These secondary

carbides were precipitated during cooling in the baked sand

moulds and resulted in a local decrease of carbon content of

the matrix. Therefore, small amounts of martensite were

present in the predominantly austenitic structures of the as-

cast irons [6].

Figure 2: Microstructure of heat treated martensitic matrix microstructure illustrating fine secondary M7C3 carbides for;

(a) hypoeutectoid and (b) hypereutectoid high chromium irons. Both at 500X [19].

(b) (a)

(a) (b)



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ABRASIVE WEAR BEHAVIOR OF HIGH CR WHITE

CAST IRONS

In the abrasion wear process, the hard body may be fractured,

and the softer surface will be cracked and/or deformed, and

the material will be removed from the surface resulting in a

measurable volume loss [2,6,22]. Bulk hardness

measurements obtained for the modified HCCI alloys series

being studied are shown in Fig. 3. The variations in wear

resistance of the alloys are mainly caused by the changes in

hardness resulting from the introduction of fine carbides and

microstructure refinement [23]. The hardness of the abrasive

materials has a very strong influence on relative abrasion rates

[7]. For a harder abrasive material the abrasion resistance

increased with the material toughness, and for a softer

abrasive material the abrasion resistance increased with the

material hardness [24,25]. For a softer abrasive material the

abrasion resistance increased with carbide volume fraction but

decreased significantly with a hard abrasive material [26,27].

The as-cast structure of high Cr cast irons containing 1.7%

Mo with mostly austenitic matrix achieved the hardness of 38-

45 HRC [28]. Asensio et al [29] stated that the abrasion

resistance increased by increasing the carbon content, and the

impact toughness impaired by higher carbon levels. The

retained austenite contributed to work hardening under field

applications [30,31]. Yongxin et al [32] reported that with the

increase of Cr content, fracture toughness of M2B increases

first and then decreases.

Hardness of the materials increased with increasing the

carbide volume fraction for the austenitic and martensitic

structures [25]. The abrasive wear loss decreased to a

minimum with increasing the carbide volume fraction up to

about 30%. Above 30% volume fraction of carbides the

abrasive wear loss increased with increasing in carbide

volume fraction. Fig. 4 shows the abrasive wear volume loss

and hardness of the irons as a function of the volume of

massive carbides of predominantly austenitic and martensitic

matrices [25]. The total percentage of carbides in the structure

as a function of the carbon and chromium content can be

estimated by the following equation [33]:

Total % carbides = 12.3 × % C + 0.55 × % Cr – 15.2 (1)

Figure 3: Bulk hardness measurements obtained for the modified HCCI alloys series being studied [23].

The mean weight loss of the tested alloys increased with

increasing the volume fraction of the network eutectic

carbides [34]. The carbide morphology had a pronounced

influence on the wear and fracture behavior of high chromium

white irons [8]. Hypereutectic alloys are characterized by the

presence of hexagonal and primary carbides [8]. For the high

Cr white cast irons the hardness provided a good indication of

the volume wear rate [35]. Sare and Arnold [36] stated that

there was a poor correlation between high stress abrasion

weight loss and material hardness for matrix produced by

changes in the austenitizing temperature.



4678

Figure 4: The abrasive wear volume loss and hardness as a function of volume of massive carbides; (a) austenitic irons,

(b) martensitic irons (heat treated at 900 oC for 5 hours, air cooled) [25].

EFFECT OF CHEMICAL COMPOSITION ON

MICROSTRUCTURE AND MECHANICAL

PROPERTIES

In many previous studies [37-39], the type and form of

eutectic carbides change from M3C to M7C3 with increasing

Cr content. In unalloyed or low Cr white cast irons containing

below 5% Cr and up to 2% C, the eutectic carbides of the

M3C type in a continuous ledeburitic form (with hardness of

HV= 1000) are present. At 8-10% Cr, the eutectic carbides

become less continuous and fracture resistance is improved.

But above 10% Cr, the form of eutectic carbides changes to

the discontinuous M7C3 type (with hardness of HV=1200-

1800). Chromium addition strongly favours the formation of

carbides during iron solidification and pearlitic matrix

formation during the eutectoid transformation specially in the

absence of alloying additions [40,41]. In the range from 9.5 to

15% of chromium content, carbides type (Cr, Fe)7C3 appears,

and above 30% chromium carbides type (Cr, Fe)23C6 solidifies

[38]. Considering the balance between the wear resistance and

toughness, the amount of eutectic carbides should be

controlled with about 35% volume fraction [10], but

according to Gahr and Eldis [25], it should be controlled with

about 30% volume fraction. But Qian and Chaochaang [34]

reported that the favorable volume fraction of the eutectic

carbides in low alloy white cast irons for impact abrasion use

should be around 10%. The addition of hard inoculants as

mixture of Ti–B–W to a high Cr white cast iron resulted in a

structure containing hard carbide particulates distributed

homogeneously in microstructure [42]. The eutectic M6C, rich

in Mo with a “fish-bone” structure, was present in the irons

with 24-28 wt.% Cr and 3-9 wt.% Mo, together with typical

eutectic M7C3 [43]. At up to 10 wt.% Mo in 20 wt.% Cr (Cr/C

ratio of about 7), Mo2C was observed as a finely dispersed

eutectic in the final stage of solidification [43]. The additions

of RE, V, Ti and B into high Cr cast iron containing 3 wt.%

Mo can refine the microstructure, reduce coarse columnar

crystals and make the carbide smaller and uniform [44]. Fig. 5

shows the impact toughness and weight loss curves of high Cr

iron alloys with different contents of RE, respectively.

Figure 5(a) Impact toughness of the high chromium iron alloys with different contents of RE; (b) Weight loss curves for the high

Cr iron alloys with different contents of RE in heat-treated conditions [44].

(a

) (b)

(a)

(b)



4679

Alloying element distributions in the matrix and the carbide of

high Cr cast iron modified RE, V, Ti, and B by the energy

spectrum analysis are shown in Fig. 6 [44]. Imurai et al [45]

studied the effects of Mo on microstructure of as-cast 28 wt.%

Cr-2.6 wt.% C-(0-10) wt.% Mo irons. They pointed out that

the iron with about 10 wt.% Mo was eutectic/peritectic,

whereas the others with less Mo content were hypoeutectic.

Mo addition promoted the formation of M23C6 and M6C

[43,45]. At 1 wt.% Mo content, multiple eutectic carbides

including M7C3, M23C6 and M6C were observed. Mo addition

increased Vickers macro-hardness of the irons from 495 to

674 HV30. Mo dissolved in austenite and increased the

hardness by solid solution strengthening [46].

Wear performance of high Cr-Mo irons depends on the type,

morphology, amount and distribution of hard eutectic and

other carbides in the microstructure, and also on the nature of

the supporting matrix [5,26,47]. Multicomponent Cr-Mo

white cast irons with reduced C and Cr levels and additions of

other strong carbide-forming elements such as W, V and Nb

have been developed as wear resistant materials for special

applications [18,48]. Their microstructures contain relatively

discontinuous networks of hard alloy eutectic carbides giving

improved wear resistance, fracture toughness and thermal

fatigue resistance [49]. Fig. 7 outlines the chemical

compositions and the Cr/C ratios of Mo-containing, high Cr

cast irons investigated in previous studies, plotted on the

liquidus surface developed by Jackson [50] and subsequently

revised by Thorpe and Chico [51]. A high Cr cast iron with

Cr/C ratio of 7 has been studied by Ikeda et al [10] and by

adding 1 wt.% Mo, M2C was found. On the other hand,

addition of 1 wt.% Mo to as-cast Cr iron containing 20 wt.%

Cr (Cr/C ratio of 8) [52] and to Cr iron containing 16 wt.% Cr

(Cr/C ratio of 6) [53] did not significantly change the structure

of the alloys in their as-cast state. The addition of

molybdenum to high chromium cast iron led to the formation

of Mo2C that crystallized in a finely dispersed form as eutectic

in the final stage of solidification [10]. By increasing the

vanadium content in an alloy, the structure became finer and

the volume fraction of eutectic M7C6 and V6C5 carbides

increased, and the morphology of eutectic colonies changed

[54].

Figure 6: Energy spectra of the matrix and the carbide of high chromium cast iron modified with 0.8% RE, 0.4% V, 0.3% Ti, and

0.1% B: (a) matrix; (b) carbide [44].



4680

Niobium addition to high Cr white iron increased its wear

resistance because of niobium formed very hard carbides

(NbC) of hardness around HV 2400 and niobium dissolved in

the matrix and increased the matrix microhardness from HV

693 at 0% Nb to HV 899 at 3.47% Nb [55,56]. The addition

of small amount of boron modified the morphology of eutectic

carbides [57,58]. Therefore, boron addition resulted in an

increase in the abrasion wear resistance.

EFFECT OF HEAT TREATMENT ON

MICROSTRUCTURE AND MECHANICAL

PROPERTIES

The role of the matrix surrounding the eutectic carbides is to

provide a sufficient mechanical support to prevent their

cracking, deformation and spalling [21]. The matrix may be

extensively altered through the destabilization and subcritical

heat treatments commonly. The main objectives of the

destabilization heat treatment is the precipitation of secondary

carbides and transformation of the primary austenitic matrix

to martensite. The destabilization heat treatments are usually

performed in the range of 800-1100oC, for 1-6 hrs. [59-61].

The matrix in the microstructure of high-Cr WCI after

destabilization treatment is generally composed of martensite

and retained austenite. Destabilization heat treatment of high

Cr cast irons normally consists of heating to a high

temperature or austenitizing, air quenching and tempering [7].

The successful heat treatment produces austenite

destabilization by precipitation of fine secondary M7C3

carbides within the austenite matrix. Class II irons containing

12 to 23% Cr are austenitized in the temperature range 950 to

1010 °C [62]. It is necessary to ensure that the soaking time is

sufficient to ensure a complete secondary carbide

precipitation. Typical continuous cooling transformation

(CCT) diagram illustrating the hardenability in an alloy for

class II are presented in Fig.8 [62]. Air quenching of the

castings from the austenitizing temperature to below the

pearlite temperature range is highly recommended.

Figure 7: Chemical composition and the Cr/C ratio of Mo containing high chromium cast irons investigated in previous studies

[10,47] and plotted on the Fe–C–Cr liquidus surface [50,51].



4681

Figure 8: Continuous-cooling transformation diagram for high Cr white cast irons (Class II) [62].

Effect of destabilization heat treatment

High chromium white cast irons must be heat treated to

develop the full hardness and maximum wear resistance. The

destabilization (hardening) heat treatment is used with high Cr

and Cr-Mo white cast irons to transform the as-cast austenitic

or pearlitic matrix structures to martensite [36,37,60-63].

Better wear resistance was obtained for the alloyed heat

treated iron compared with that of the as-cast and unalloyed

irons [63]. The responsible for such better wear behavior is

the strengthening of the matrix by the MC carbides plus the

precipitation of secondary M7C3 carbides and the partial

transformation of matrix from austenite to martensite after

heat treatment. Fig. 9 shows specific wear rate against applied

load for the high chromium white irons.

Figure 9: Specific wear rate against applied load for the

investigated iron alloys [63].

As the austenitizing temperatures increased, the amount of

carbon dissolved in the austenite increased [54]. Decreasing

the austenitizing temperature from the optimum gave a lower

hardness due to the lower carbon martensite formed on

quenching. This temperature would generally lie in the range

950-1050 oC and would depend on the particular composition

of the alloy being treated. Hakan Gasan and Fatih Erturk [64]

reported that the morphologies of secondary carbides, the

crystallographic properties of the phases, and the proper

combination of the amount of martensite, retained austenite,

and carbides were the principle parameters that affect the

hardness and wear behavior of the alloy. The effect of the

destabilization temperatures on the hardness is shown in Fig.

10 [64]. A destabilization heat treatment caused the

transformation of austenite to martensite and the precipitation

of M7C3-type secondary carbides, while eutectic carbides

remained unchanged [64]. Fig. 11 shows SEM micrographs

of the alloys subjected to destabilization treatments at various

temperatures 900 and 1000oC. With increases in the treatment

temperature, the precipitated carbides became coarser and less

abundant and the amount of retained austenite increased. The

martensitic matrix reduces wear of matrix and it in turn

reduces the carbide fracture. The secondary carbides also

influence the wear behavior; they strengthen the martensitic

matrix which in turn increases the mechanical support to the

carbide phase. Dynamic fracture toughness of white cast irons

in both as cast and heat treated conditions is determined

mainly by the properties of the matrix [65]. Therefore, the

austenite is more effective in this respect than martensite. The

volume fraction of the eutectic carbide phase (M3C or M7C3)

have an important influence on the wear resistance of white

iron alloys under low stress abrasion conditions [65]. Huang

and Wu [66] reported that the heat treatment of 28Cr 1Mo

white iron changed the structures and properties. The austenite

matrix destabilized when the iron was treated at a high

temperature because there were two secondary phases; χ and

M7C3II, precipitated from the matrix, and the austenite

transformed into martensite during air cooling. The higher the

temperature of heat treatment the higher was the volume of



4682

precipitated secondary phases and transformed martensite

[66,67]. With increasing austenitizing temperature, the

secondary carbides in high Cr iron alloys became coarser, and

the amount of retained austenite in the denderitic areas

increased [53,67,68]. The optimal heat treatment process is 2

h quenching treatment at 1000℃, followed by a subsequent 2

h tempering at 400 ℃ [66].

Figure 10: Effect of destabilization temperature on the

hardness of a forced air 20Cr 1.7Mo 2C iron [64].

The precipitation of secondary carbides during austenitising

left the matrix depleted in carbon and chromium, thus raising

the Ms temperature above room temperature and subsequent

cooling resulted in the transformation of matrix to martensite

[69]. The holding time at the destabilization temperature

appeared to affect the retained austenite content of the alloy

15Cr 3Mo [20]. The short holding times with the appropriate

destabilization temperature might only be required to achieve

a hardened structure with the maximum hardness.

Figure 11: SEM micrographs of the alloys subjected to destabilization treatments at various temperatures of: (a) 900oC, (b)

1000oC; (d) and (e) micrographs obtained after deep etching [64].



4683

Effect of subcritical heat treatment

Subcritical heat treatment (tempering) is sometimes

performed, particularly in large heat treated martensitic

castings, to reduce retained austenite contents and increase

resistance to spalling. Typical tempering temperatures range

from 480 to 550 °C and times range from 8 to 12 h. Excess

time or temperature results in softening and a drastic reduction

in abrasion resistance [62]. Insufficient tempering results in

incomplete elimination of austenite. Fernandez and Belzunce

[70] reported that compression strengths for low and high

carbon white cast irons after heat treatment at 500o C was

excellent and a significant ductility was also observed. During

tempering carbides were precipitated in retained austenite

regions which could then transform to martensite on cooling

[3]. If a prolonged tempering time was used, then the austenite

would transform to ferrite when sufficient carbide

precipitated. The structure would then consist of a tempered

martensite and softer areas of ferrite-M23C6 precipitates. Fig.

12 shows the effect of tempering on the hardness of a 20Cr

1.7Mo2C iron [3]. There was a secondary hardening peak at

450-550 oC, perhaps through the precipitation of fine alloy

carbides [3,20]. The treatments at 600oC on the destabilized

material produced a gradual drop in the hardness due to the

decomposition of martensite to a ferrite/carbide product. The

effect of subcritical heat treat parameters on hardness and

retained austenite in Mo-containing high chromium cast iron

was studied by Inthidech et al [71]. They concluded that the

hardness decreased gradually but the Vγ increased greatly as

Mo content increased in both 16 and 26 wt.% Cr cast irons in

the as cast state. In the case of subcritical heat treatment, the

hardness increased first and then decreased with an increase in

holding time. This phenomenon is due to a hardening caused

by the precipitation of secondary carbides and by martensite

transformation from the destabilized austenite during cooling.

Figure 12: Effect of tempering on the hardness of a 20Cr 1.7Mo 2C iron; (a) the effect of temperature for holding time of 4

hours, (b) the effect of holding time at 520oC [3].

CONCLUSIONS

The main concluding remarks those could be drawn from this

revision are:

Alloying additions with appropriate amounts of

carbide forming elements to high Cr white cast

irons revealed marked improvements in

mechanical properties and wear performance

through microstructure refinement and in situ

formation of fine new carbides.

The variations in wear resistance of the alloys

with respect to the added carbide forming

elements are mainly caused by the changes in

hardness resulting from the introduction of fine

carbides and microstructure refinement.

The addition of small amounts of molybdenum

(i.e. 0.5% Mo) is sufficient for suppressing

pearlite formation when the ratio of chromium to

carbon (i.e. Cr/C) is relatively high.

The carbide morphology had a pronounced

influence on the wear and fracture behavior of

high chromium white irons.

Molybdenum additions up to 10 wt.% Mo

promoted the formation of eutectic/peritectic

M23C6 and M6C carbides.

Destabilization heat treatment for high-Cr WCIs

is employed to obtain the martensitic structure

for improving the toughness and abrasion

resistance of these irons.

Subcritical (tempering) heat treatments

following the hardening are commonly utilized

to relieve the internal stresses and to control the

final hardness before service.

(a) (b)



4684

Better wear resistance is obtained for the alloyed

heat treated white cast iron compared with those

as cast and unalloyed irons. An improvement of

the wear behavior was due to the matrix change

from austenite to martensite with the presence of

secondary carbides by the heat treatment.

The hardness of white cast iron increased with

increasing the carbide volume fraction for the

austenitic and martensitic structures.

Certain level of austenite should be present in

the structure to produce the optimal abrasion

resistance. This required level of austenite was

cited as at least 20-30 %. Above this level the

abrasion resistance was independent of austenite

content.

ACKNOWLEDGEMENT

The authors would like to thank Taif University for its

financial support in the project No. 1 /734/ 2527.

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Microstructural Characteristics and Mechanical … . 1a, M. El ... a On leave from Zagazig University, Faculty of Engineering, Materials Engineering ... of the abrasive materials