Materials 8

MATERIALE I
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FOUNDRY CAST IRONS
Foundry cast irons: Fe – C alloys with more than 2% C destined to cast parts and containing
graphite in the structure;
The graphite amount determined by the Si (graphitizing) / Mn (anti- graphitizing) ratio
Si (+ high carbon content) – ferrite + graphite structures are favoured
Mn – favours the formation of cementite (pearlite included) opposite to graphite
Classification according to the shape of graphite:
1. Grey cast irons – lamellar graphite; [+ vermicular, coral]
2. Malleable cast irons – temper graphite;
3. Ductile cast irons - spheroidal graphite;
GREY CAST IRONS
Composition: 2.8 – 3.5% C (generally); 0.5 – 3.5% Si; 0.1 – 1% P; 0.02 – 0.15% S
Classification of graphite: according to size, shape, distribution
Lamellar graphite in grey cast irons
a – fine lamellar graphite with uniform distribution;
b – coarse lamellar graphite with uniform distribution;
c – rosette-like lamellar graphite;
d – non-oriented inter-dendritic graphite;
e - oriented inter-dendritic graphite;
a, b, d – differ by the germination capacity (different cooling rate, particles for heterogeneous
nucleation)
c – for hyper-eutectic cast irons
d, e – hyper-eutectic cast irons; only heterogeneous nucleation;
Size of graphite – essential for strength (and toughness): fine graphite → tougher cast iron
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MATERIALE I
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Metallic matrix (for standardized marks): F + P, P, [+ steadite]
Steadite = ternary eutectic Feα – C - P; favours castability but brittleness also
Increase of the pearlite amount – increase in strength
Structure of grey cast irons: a – pearlitic (500x); b – pearlite and ferrite (500x); ferritic (500x);
pearlite, ferrite and steadite (300x)
Strength of grey cast irons: 100 – 400 MPa
Ductility: very low (brittle) A = 0.2 – 0.5%
Hardness: 100 – 300 HB
Cast irons with inter-dendritic graphite (high strength) – inoculated
Inoculation = altering of the characteristics of the as-cast structure by inserting of small amounts of
inoculants (less than 1%)
Inoculants for grey cast irons: SiO2, Al2O3, CaO – heterogeneous nuclei
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MATERIALE I
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Standardization of grey cast irons
SR EN 1561:1999 (Lamellar graphite cast irons)
Ex. EN-GJL-100, 150, ..., 350
EN-GJL- Rm [MPa]
There exist also other symbols - numerical or expressing hardness
MALLEABLE CAST IRONS
Contain graphite in irregular agglomerates – TEMPER GRAPHITE (GM)
Graphite is obtained through heat treatments – MALLEABILIZATION ANNEALING – applied to
white cast irons
Classification – according to the aspect of the fracture
1. Black heart cast irons – F + GM (b.); P + GM (a.)
2. White heart cast irons – F + P + GM (c.)
a.
b.
c.
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MATERIALE I
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White heart cast irons - malleabilization performed in oxidizing environment
~ 3.2%C;
Malleabilization annealing: first malleabilization
stage – decomposition of the free cementite
(secondary / ledeburitic cementite);
Rm = 270 – 570 MPa
A = 3 – 16%
200 – 250 HB
Black heart cast irons - malleabilization in neutral environment
~ 2.8%C
The most used – cost of heat treatment, favourable compromise between Rm / A
Malleabilization annealing:
Stage 1 - decomposition of the free cementite
Stage 2 – decomposition of the pearlitic cementite
Rm = 300 – 550 MPa
A = 1 – 10%
150 – 320 HB
Pearlitic malleable cast irons – variant of black heart malleable cast irons
~ 2.7%C, malleabilization – first stage of malleabilization, neutral environment
Strongest malleable cast irons: Rm = 450 – 650 MPa
Low ductility: A = 2 – 4%
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MATERIALE I
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Standardization of malleable cast irons
I – decarburised cast irons (white heart)
II – non- decarburised cast irons (black heart + pearlitic)
I. EN-GJMW- Rm [MPa] – A [%]
Ex. EN-GJMW-350-4, EN-GJMW-550-4
II. EN-GJMB- Rm [MPa] – A [%]
Ex. EN-GJMB-300-6; EN-GJMB-500-5
SR EN 1562:1999
SPHEROIDAL GRAPHITE (DUCTILE) CAST IRONS
Cast irons containing ~3.5% C, with graphite nodules obtained through inoculation
Inoculants: Mg, Ce, ... – generate films on the graphite surface
Most favourable shape of graphite (minimum stress concentration effect)
Rm = 370 – 700 MPa; A = 2 – 18%; 140 – 300 HB
F + GN
F + P + GN
P + GN
Standardization of ductile cast irons
SR EN 1563:1999: EN-GJS- Rm [MPa] – A [%]
Ex. EN-GJS-350-22, EN-GJS-350-22-LT (resilience determined in cold conditions)
THEORY OF HEAT TREATMENTS
Heat treatments: technological processes that comprise heating, maintaining at various
temperatures and cooling under defined conditions, targeting the improvement of certain properties
by altering the structure.
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Classification:
1. According the position of the HT in the manufacturing process
Preliminary
Intermediate
Final
2. According to the mechanisms during the treatments
2.1 simple HT
Annealing
Quenching
Tempering
2.2 thermochemical treatments
2.3 thermophysical treatments
Thermo-mechanical
Thermo-magnetic
DIFFUSION
(Generally): Changing of the position of atoms / ions in solids, liquids or gases;
Only for large groups of atoms (ions)
Heterodiffusion – determined by a concentration gradient
Self-diffusion
Mechanisms:
Reciprocal replacing
Through interstitials
Cyclic
Through vacancies
Most of the transformations in materials (solidification, solid state transformations,
recrystallization,...):
with diffusion
Transformation with diffusion: requires a longer process time
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Diffusion coefficient: number of atoms that diffuse through a unit surface that is perpendicular to
the atoms flux during a second, for a unit concentration gradient
D  D0  e

Types of diffusion:
Qa
RT
Surface
Inter-granular
Volume
Qs < Q i < Qv
Ds > Di > Dv
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