ISO/TR 945-3:2016

TECHNICAL ISO/TR
REPORT 945-3
First edition
Microstructure of cast irons —
Part 3:
Matrix structures
Microstructure des fontes —
Partie 3: Structures de matrice
PROOF/ÉPREUVE
Reference number
©
ISO 2016
© ISO 2016, Published in Switzerland
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ii © ISO 2016 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Designations and descriptions of cast iron microstructures . 1
2.1 Ferrite . 1
2.2 Pearlite . 1
2.3 Austenite . 2
2.4 Acicular ferrite . 2
2.5 Ausferrite . 2
2.6 Bainite . 2
2.7 Cementite . 2
2.8 Ledeburite . 2
2.9 Martensite . 2
3 Sampling and preparation of samples . 3
3.1 Samples taken from castings and cast samples . 3
3.2 Sample preparation . 3
4 Matrix structures . 4
Annex A (informative) Spheroidal graphite cast irons: Evaluation of pearlite content .33
Annex B (informative) List of European and some national cast iron material designations
corresponding to the ISO designations .35
Bibliography .43
Foreword
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bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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The committee responsible for this document is ISO/TC 25, Cast irons and pig irons.
ISO 945 consists of the following parts, under the general title Microstructure of cast irons:
— Part 1: Graphite classification by visual analysis
— Part 2: Graphite classification by image analysis [Technical Report]
The following parts are under preparation:
— Part 3: Matrix structures [Technical Report]
— Part 4: Determination of nodularity in spheroidal graphite cast irons
iv PROOF/ÉPREUVE © ISO 2016 – All rights reserved

Introduction
The designation of cast iron matrix structures as given in this part of ISO 945 is in conformity with the
[1][2][3] [4]
designations published by several national foundry organisations or other publishers .
This Technical Report aims to
— give the designations, precise descriptions and reference micrographs of the matrix structures of
cast irons, and
— facilitate the discussion and to avoid misunderstanding between manufacturer and purchaser
regarding the identification of matrix structures.
TECHNICAL REPORT ISO/TR 945-3:2016(E)
Microstructure of cast irons —
Part 3:
Matrix structures
1 Scope
This Technical Report gives the designations, descriptions and reference micrographs of the matrix
structures of cast irons.
It applies to the following types of cast irons:
— grey cast irons (Table 4.1);
— spheroidal graphite cast irons (Table 4.2);
— austenitic cast irons (Table 4.3);
— malleable cast irons (Table 4.4);
— compacted (vermicular) graphite cast irons (Table 4.5);
— ausferritic spheroidal graphite cast irons (Table 4.6);
— abrasion-resistant cast irons (Table 4.7).
Each matrix structure is defined with explanations and micrographs.
Unless otherwise stated in Clause 4, the micrographs shown correspond to samples etched with a
solution of 2 % nitric acid in ethanol (Nital).
2 Designations and descriptions of cast iron microstructures
2.1 Ferrite
Ferrite also known as α-ferrite (α-Fe) or alpha iron is a materials science term for iron, or for a solid
solution with iron as the main constituent, with a body-centred cubic crystal structure. It is this
crystalline structure which gives to steels and cast irons their magnetic properties, and is the classic
example of a ferromagnetic material.
Since pearlite has ferrite as a component, any iron-carbon alloy will contain some amount of ferrite if
it is allowed to reach equilibrium at room temperature. The exact amount of ferrite will depend on the
cooling processes the iron-carbon alloy undergoes when it cools from liquid state.
2.2 Pearlite
Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of alpha-
ferrite (according thermal dynamical condition 88 % by mass) and cementite (12 % by mass). The
lamellar appearance is misleading since the individual lamellae within a colony are connected in three
dimensions; a single colony is therefore an interpenetrating bicrystal of ferrite and cementite. Pearlite
is a common microstructure occurring in many grades of cast irons.
If cast iron is cooled very slowly or as a result of heat treatment, the cementite can occur in globules
instead of in layers. This structure is designated as globular pearlite.
2.3 Austenite
Austenite, also known as gamma phase iron (γ-Fe), is a non-magnetic allotrope of iron or a solid
solution of iron, stabilized by an alloying element, e.g. nickel. Austenite is the face-centred cubic crystal
structure of iron.
2.4 Acicular ferrite
Acicular ferrite is a microstructure of ferrite that is characterized by needle shaped crystallites or
grains when viewed in two dimensions. The grains, actually three dimensional in shape, have a thin
lenticular shape. This microstructure is advantageous over other microstructures because of its chaotic
ordering, which increases toughness.
2.5 Ausferrite
Ausferrite is a special type of multi-phase microstructure that occurs when cast irons with a silicon
content of about 2 % or higher are austempered.
Austempering consists of rapidly cooling the fully austenitic iron to avoid the formation of pearlite to
a temperature above that of martensite formation and holding for the time necessary to precipitate
the ausferrite matrix. This microstructure consists primarily of acicular ferrite in carbon enriched
austenite.
2.6 Bainite
Bainite is a multi-phase microstructure, consisting of acicular ferrite and cementite that forms in cast
irons during rapid cooling. It is one of the decomposition products that can form when austenite is cooled
rapidly below the eutectoid temperature, but above the martensitic starting (M ) temperature. Bainite
s
can also form from the decomposition of ausferrite upon extended heating above the temperature at
which it was formed.
2.7 Cementite
Cementite, also known as iron carbide, is a compound of iron and carbon, with the formula Fe C.
By mass, it is 6,7 % carbon and 93,3 % iron. Cementite has an orthorhombic crystal structure.
In the iron-carbon system cementite is a common constituent because ferrite contains maximum 0,02 %
by mass of carbon. Therefore, in cast irons that are slowly cooled, a part of these elements is in the form
of cementite. In the case of white cast irons, cementite precipitates directly from the melt. In grey cast
irons or spheroidal graphite cast irons, cementite forms either from austenite during cooling or from
martensite during tempering, or from the decomposition of ausferrite. An intimate mixture of cementite
with ferrite, the other product of austenite, forms a lamellar structure called pearlite (see 2.2).
2.8 Ledeburite
Ledeburite is an eutectic mixture of austenite and cementite and is formed when the melt at least partly
solidifies according the metastable Fe-C-Si system.
2.9 Martensite
Martensite is formed from austenite by rapid cooling (quenching) which traps carbon atoms that do
not have time to diffuse out of the crystal structure. The martensite lattice is body-centred tetragonal
composed of ferrite and carbon. This martensitic reaction begins during cooling when the austenite
reaches the martensite start temperature (M ) and the parent austenite becomes mechanically
s
unstable. At a constant temperature below M , a fraction of the parent austenite transforms rapidly,
s
after which no further transformation occurs. When the temperature is decreased, more of the
austenite transforms to martensite. Finally, when the martensite finish temperature (M ) is reached,
f
2 PROOF/ÉPREUVE © ISO 2016 – All rights reserved

the transformation ends. Martensite can also be formed by application of stress in ausferritic spheroidal
graphite cast irons (SITRAM effect: stress induced transformation from austenite to martensite). Thus,
martensite can be thermally induced or stress induced.
3 Sampling and preparation of samples
3.1 Samples taken from castings and cast samples
The location from which samples are taken should be agreed between the manufacturer and purchaser
and should take into account the requirements specified in the appropriate material standard. If an
examination report is required, the location from where the final sample is taken shall be recorded.
The sample should be of sufficient size to provide a true representation of the matrix structure in the
agreed location from which it is taken.
3.2 Sample preparation
Attention should be paid to the careful cutting, grinding, p
...