AMM-2017-4.indd

Arch. Metall. Mater. 62 (2017), 4, 2273-2280

DOI: 10.1515/amm-2017-0335

B. MRZYGŁÓD*

, A. KOWALSKI**, I. OLEJARCZYK-WOŻENSKA*,

T. GIĘTKA***, M. GŁOWACKI*

CHARACTERISTICS OF ADI DUCTILE CAST IRON WITH SINGLE ADDITION OF 1.56% Ni

The results of examinations of microstructure and an analysis of its impact on selected mechanical properties of austempered

ductile iron (ADI) were presented in the paper. The ADI was produced from the ductile iron containing 1.56% Ni only alloying

addition. The effect of the austempering time and temperature on the microstructure and mechanical properties of the examined cast

iron was considered. Constant conditions of austenitizing were assumed and six variants of the austempering treatment were adopted.

The studyof mechanical properties included a static tensile test, Charpy impact strength test and Brinellhardness measurement.

This work complements the knowledge about alloying additions effect on microstructure and mechanical properties of ADI

and focuses on the impact of a single alloying element (Ni).

Keywords: ductile iron, heat treatment, the structure of ADI, ADI properties

1. Introduction

ADI (Austempered Ductile Iron) is a modern, low-alloy

ductile iron subjected to the austempering treatment. The chemi-

cal composition of the base cast iron is similar to a conven-

tional ductile iron with 3.6% C, 2.5% Si, up to 0.3% Mn, up to

0.015% S and up to 0.06% P. The alloying elements such as Cu,

Ni, Mo, Mn are introduced to enhance the ability of cast iron to

form an ausferritic microstructure during the process of ausfer-

ritizing.

Ausferrite is the metal matrix microstructure composed

of a homogeneous lamellar ferrite in the form of “needles” and

thermodynamically stable austenite with high carbon content

(1.6-2.2% C). This structure confers to the ADI a favourable

combination of high strength, ductility, fracture toughness and

abrasive wear resistance.

Compared with steel, ADI has a number of advantages,

making it attractive to designers. The most important are [1]:

– better as-cast machinability,

– higher damping capacity,

– lower risk of scuffing,

– lower notch sensitivity,

– higher dimensional stability after heat treatment,

– lower energy consumption during production of machine

parts.

Recently, ADI has been more and more popular as a sub-

stitute for aluminium products, and this is due to the following

reasons [2]:

– three times higher strength than the strength of aluminium

for density only 2.6 times greater, which means that with

proper design of an element, a reduction in its weight be-

comes quite realistic,

– often even 10 times lower price than that of aluminium,

meaning evident cost savings,

– higher dimensional accuracy after casting and better re-

production of shapes, which results in higher metal yield

expressed as the ratio of metal cast to metal sold in the form

of castings,

– much higher fatigue strength,

– better damping capacity,

– higher resistance to abrasive wear.

The process of making castings from ADI consists of two

main stages:

– proper selection of the chemical composition and manu-

facture of castings,

– austempering conducted according to the pre-established

regime.

Although the technical literature on the chemical composi-

tion of ADI is very abundant, studies continue in search for an

optimum content of the alloying elements.

Past experience shows that the most commonly used are

the following combinations of the additives:

– Ni – Mo [3],

– Ni – Cu [4, 5],

– Mn – Cu [6],

– Ni – Mo – Cu [7].

* AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF METALS ENGINEERING AND INDUSTRIAL COMPUTER SCIENCE, AL. A. MICKIEWICZA 30, 30-059 KRAKÓW, POLAND

** FOUNDRY RESEARCH INSTITUTE, 73 ZAKOPIAŃSKA STR., 30-418 KRAKÓW, POLAND

*** UTP UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF MECHANICAL ENGINEERING, 7 PROF. S. KALISKIEGO AV., 85-796 BYDGOSZCZ, POLAND

Corresponding author: [email protected]

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Tests also cover ADI with single additions of:

– chromium [8]

– manganese [9,10]

– boron [11]

On the other hand, the literature lacks the data on the ADI

with nickel added as a single element. Studies of this subject

have been undertaken in the present work and are expected to

make a contribution and fill at least partially the existing gap.

In addition to cognitive elements, the authors of this study

also tried to keep in mind the economic aspects, including a re-

duction in the content of the alloying elements added to cast iron

and shortening the time of heat treatment.

The results of the study can also be used as a source of

knowledge in computer systems that are developed for predict-

ing the structure and properties of alloys after casting process

[12-15].

2. Test materials and methods

2.1. Test material

Ductile iron of the chemical composition given in Table 1

was used in this study. Melting was performed at the Foundry

Research Institute in Cracow using a RADYNE medium fre-

quency induction furnace with crucible of 100 kg capacity and

an inert lining. The spheroidizing treatment was performed by

Sandwich method. All these operations were made in a slender

ladle at 1400°C. From the manufactured ductile iron, test coupons

were cast according to ASTM A897 standard.

TABLE 1

Chemical composition of the tested ductile iron

Elements, wt%

CSiMnMgPSNi

3.55 2.55 0.31 0.063 0.025 0.009 1.56

2.2. Determination of heat treatment parameters based

on the results of dilatometric studies

For dilatometric studies, 15 ductile iron samples with

dimensions of 3×10 mm were cut out. Dilatometric tests

(dilatometer L78 RITA) comprised heating the samples to a tem-

perature of 1100°C at a rate of 0.08°C/s and plotting of respec-

tive dilatographs to determine the values of critical temperature

, A

c1s

). Then, one cycle of the continuous cooling at a rate

of 100°C/s to ambient temperature was run to determine the M

point. The next phase of the work included dilatometric analysis

of samples held under isothermal conditions in the temperature

range of 700-300°C.

Dilatometric studies and the TTT diagram obtained as a re-

sult of these studies (Fig. 1) allowed planning the heat treatment

parameters (time and temperature of austenitizing and austemper-

ing). Constant austenitizing conditions (temperature: T

= 920°C,

time: τ

= 120 min) were selected and 12 variants of austempering

were established. Choosing the temperatures of isothermal hold-

ing, the area of the occurrence of bainitic transformation was

divided into two ranges, i.e. the upper range and lower range.

For each of the ranges, three values of the temperature and two

times of the austempering were selected, different for the lower

and upper range. Samples for the strength and toughness tests

were austempered according to conditions presented in Table 2

and illustrated by diagram shown in Fig. 2.

The applied variants of austempering treatment and their

reference numbers are shown in Table 2.

Each single variant of the austenitizing and austempering

treatment included three test pieces prepared for the impact

test and three test pieces prepared for the tensile test. Heat

treatment was carried out in the Department of Ferrous Alloys

at the Foundry Research Institute in Cracow. Austenitizing

was made in a NABERTHERM Multitherm N41/M furnace

with a tight retort and a protective atmosphere of inert gas to

prevent decarburizing of the sample surface. Samples were

heated with the furnace to 920°C and held at that temperature

Fig. 1. TTT diagram of examined ductile cast iron

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for 2 h. A protective atmosphere of compressed argon was used

in the furnace.

Austempering (ausferritizing) was carried out in a salt

bath consisting of a mixture of potassium nitrate and sodium

nitrite. The salt bath temperature and isothermal cooling times

are shown in Table 2.

2.3. Microstructure of base ductile iron and ADI after

different variants of the austempering treatment

To reveal the microstructure of the test material, metallo-

graphic sections of both base ductile iron and ADI after differ-

ent heat treatment variants were prepared. The metallographic

sections of base cast iron and of cast iron after heat treatment

were etched in the Mi1Fe reagent according to PN-61/H-04503.

Selected metallographic sections of ADI were also etched in the

B-M reagent of the following chemical composition: 100 ml

of base solution (5 parts by volume of H

O, 1 part by volume

of concentrated HCl), 2 g of NH

F • HF, 1 g of K

. This

reagent does not dye austenite and carbides, while bainite and

tempered martensite are coloured in brown, and martensite is

coloured in blue. Sometimes fine martensite needles assume

not a blue but light brown colour, and if this is the case, then

the assessment of microstructure should be based on the studies

of its morphology.

Microstructure in both as-cast state and after heat treatment

was examined with an AXIO OBSERVER Z1M metallographic

microscope.

2.4. Testing of mechanical properties

Testing of mechanical properties included:

– static tensile test carried out at ambient temperature on an

Instron 8502 type testing machine. The cut out test pieces

were prepared in accordance with PN EN 1564. Tests con-

sisted in loading at a constant speed the samples mounted

in chucks of a hydraulic testing machine. The speed of the

tensile test was 0.02 mm/s. The elongation of the sample

was measured with an extensometer mounted on a measur-

ing section of the sample. During tensile tests, the instan-

taneous values of the loading force and elongation of the

sample were recorded. The tensile test was carried out until

the sample fracture. The parameters determined included

the tensile strength R

, yield strength R

p0.2

and elongation

. The results of the strength measurements were recorded

on three samples taken for a given heat treatment variant,

calculating next the mean values.

– measurement of the Charpy impact strength at ambient

temperature. The measurement of impact strength was

performed on 55x10x10mm samples V-notched to a depth

of 2 mm and with a fillet radius of 0.25 mm. To carry out

the tests, Charpy pendulum was used. The results of impact

test measurements were recorded on three samples taken for

a given heat treatment variant, calculating next the mean

values.

– Brinell hardness measurement. Brinell hardness measure-

ments were taken using an INNOVATEST/Nexus 703A

hardness tester. The test consisted in pressing a calibrated

D = 2.5 mm diameter ball into the polished surface of the

sample at a load of P = 187.5 kg. The operation time of full

pressure was 15 s. Hardness measurements were conducted

on samples broken in the impact test. For each variant,

15 Brinell hardness measurements were taken, calculating

next the mean.

3. Test results

3.1. As-cast microstructure

Images of as-cast ductile iron microstructure are shown

in Fig. 3. The microstructure is composed of metal matrix with

particles of spheroidal graphite distributed in this matrix. Most

of the graphite particles are surrounded by polygonal ferrite

grains. The metal matrix is formed of pearlite, inside which oc-

casionally occur ferrite grains and particles of non-metallic inclu-

sions.

Fig. 2. Variants of the austempering treatment

TABLE 2

Variants of the austempering treatment

Variant

Austempe-

ring

temperature

[°C]

Austem-

pering

Time

[min]

Variant

Austempe-

ring

temperature

[°C]

Austem-

pering

time

[min]

W11

400 15

W12

400 90

380 15

W10

380 90

360 15

360 90

330 30

330 120

300 30

300 120

260 30

260 120

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3.2. Microstructure of ADI after different variants

of the austempering treatment

The photos of microstructure in Table 3 illustrate the effect

of the time and temperature of bainitic transformation on the ADI

matrix. As regards the temperature of the transformation, it is

clear that the higher is this temperature, the longer and thicker are

the “needles” of ferrite, and the higher is the austenite content,

reaching at 380-400°C even 30%. This type of ADI has a very

high ductility (Table 4). On the other hand, austempering in the

range of 260 – 300°C significantly increases both strength and

hardness of this material (Table 4). The austempering time of

15 minutes is definitely far too short and not used in practice.

In this study it was used only for cognitive reasons. The photos

of microstructure show that with the time of treatment so short,

complete bainitic transformation is not possible.

The austempering time of 90 minutes is applicable in the

case of austempering treatment ranging from 360 to 400°C,

while the time of 120 minutes (and longer) gives satisfactory

results when the treatment is carried out in the temperature range

of 260-300°C.

Both theory and practice show that this is related to the rate

of carbon diffusion, which at higher temperatures of the bainitic

transformation is proceeding more quickly, and more slowly at

lower temperatures.

It should be emphasized that studies were conducted on

samples of f 10 mm, tested for the mechanical strength. Castings

with thicker walls usually require longer times of austempering.

Fig. 3. As-cast microstructure of the tested cast iron

TABLE 3

Microstructure of material after different variants of the austempering treatment

Variant 1000× Variant 1000×

W11

= 400°C,

= 15 min

nital

W12

= 400°C, τ

= 90 min

= 380°C,

= 15 min

nital

W10

= 380°C,

= 90 min

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= 360°C,

= 15 min

B-M

= 360°C,

= 90 min

B-M

= 330°C,

= 30 min

nital

= 330°C,

= 120 min

= 300°C,

= 30 min

nital

= 300°C,

= 120 min

= 260°C,

= 30 min

nital

= 260°C,

= 120 min

3.3. The results of mechanical tests

3.3.1. Static tensile test

The following as-cast mechanical properties of the ductile

iron were obtained: the yield strength R

p0,2

= 478 MPa, the tensile

strength R

= 701 MPa and elongation A

= 6.9%.

Table 4 gives the results obtained in the static tensile test

carried out on specimens heat-treated according to the pro-

posed variants. The values given in Table 4 are the arithmetic

mean values calculated from the measurements taken on three

samples.

TABLE 3. CONTINUED

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TABLE 4

The results of mechanical tests

Variant A

, %

MPa

p0,2

MPa

Variant A

, %

MPa

p0,2

MPa

W11 1,8 868 595 W12 7,8 950 597

W9 1,8 810 675 W10 4,9 960 681

W7 1,3 952 757 W8 6,0 1068 783

W5 2,4 1201 779 W6 4,2 1255 968

W3 1,4 1065 835 W4 3,9 1381 1017

W1 1,1 737 W2 2,0 1343 1343

Fig. 4. ADI impact elongation vs the time and temperature of austem-

pering

Fig. 5. ADI tensile strength vs the time and temperature of austempe-

ring

Fig. 6. ADI yield strength vs the time and temperature of austempe-

ring

3.3.2. Hardness

As-cast hardness of the ductile iron was 202 HB. Table 5

gives the results of hardness measurements for samples heat

treated according to Figure 2 Austempering diagram. The results

are the mean calculated from the measurements taken on three

samples.

TABLE 5

The results of hardness measurements

Variant HB Variant HB

W11 292 W12 270

W9 369 W10 285

W7 401 W8 325

W5 375 W6 371

W3 437 W4 392

W1 523 W2 452

Fig. 7. ADI hardness vs the time and temperature of austempering

3.3.3. Impact strength

As-cast impact strength of the ductile iron was KCV =

4.2 J/cm

. The results of impact tests obtained on the heat treated

samples are shown in Table 6. For each variant, the mean was

calculated from the measurements taken on three samples.

TABLE 6

The results of impact tests

Variant KCV, J/cm

W11 4,8 W12 11,9

W9 3,8 W10 11,5

W7 3,4 W8 10,7

W5 6,3 W6 9,9

W3 5,5 W4 8,1

W1 2,4 W2 6,8

Fig. 8. ADI impact strength vs Austempering time and temperature

2279

4. Discussion

Base cast iron

The analysis of obtained results indicate that:

– the chemical composition of the base cast iron is typical

for pearlitic-ferritic ductile iron.

– pearlitic microstructure of the metal matrix and regular

precipitates of the spheroidal graphite were obtained,

– high values of the strength and ductility were obtained,

i.e. the yield strength R

p0,2

= 478 MPa, the tensile strength

= 701 MPa, elongation A

= 6.9%, Brinell hardness of

202 HB, the impact strength KCV = 4.2 J/cm

TTT diagram

To find correct parameters of the austempering treatment it

is necessary to plot a TTT diagram. A diagram like this, plotted

for the cast iron of the composition as demonstrated in Table 1,

is shown in Figure 1. From the diagram it follows that the hard-

enability to ausferrite is in this case inferior to the hardenability

obtained in the cast irons with two or three alloying elements.

Austempering

The adopted range of isothermal transformation was from

260 to 400°C at the time of austempering amounting to 15, 30,

90 and 120 minutes.

– ADI microstructure is shown in Table 4. There are clear

differences in the size of ferrite “needles” and in the austen-

ite content. The higher is the temperature of austempering,

the thicker are the “needles” and the higher is the content

of austenite.

– ADI strength achieves its peak values after austempering

at 260, 300 and 330°C, i.e. 1343, 1381 and 1255 MPa, re-

spectively, at the elongation A

equal to 2.0, 3.9 and 4.2%,

respectively. This corresponds to the ADI grade 1200-2

according to PN EN 1564,

– hardness – the results of Brinell hardness measurements

are given in Table 6. The highest values of hardness were

obtained in the cast iron austempered at 360°C (523 and

452 HB). This clearly indicates the presence of martensite

in the structure. The presence of this constituent in the

structure of ADI is disadvantageous, because it decreases

the ductility and may cause embrittlement, unless the target

is obtaining maximum abrasive wear resistance,

– impact strength, like the yield strength and elongation,

is a measure of the ADI ductility, and thus of its fracture

toughness. The results of these measurements are given in

Table 7. The highest values of this property were obtained

after austempering at 380°C (11.5 KCV) and 400°C (11.9

KCV). Yet, even after austempering at 300°C, the impact

strength of 8.1 KCV was obtained, which was a very sat-

isfactory result.

All of the above described properties are graphically de-

picted in the diagrams in (Figs. 6-10). Successive graphs illustrate

the effect of the time and temperature of austempering on the

tested mechanical properties.

5. Summary

Based on examinations results and their analysis several

conclusions were formulated:

– the conducted tests and studies have shown that ADI with

the addition of single alloying element, i.e. 1.5% Ni, can

produce only the grades 800-8 and 1200-2 (according to

PN EN 1624),

– optimum ausferritizing parameters for the grade 800-8 are

380°C at the time of 90 minutes,

– optimum ausferritizing parameters for the grade 1200-2 are

300°C at the time of 120 minutes,

– the austempering times of 15 and 30 minutes are definitely

too short,

– at the austempering temperature of 260°C, an ausferritic-

martensitic structure is obtained characterized by a hardness

of over 400 HB and high resistance to abrasive wear,

– ADI containing 1.5% Ni requires more detailed studies,

including fatigue strength and fracture toughness.

Acknowledgements

Financial assistance of the NCN, project No. 2013/11/N/ST8/00326

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