mme 345 lecture 39 - bangladesh university of …teacher.buet.ac.bd/bazlurrashid/mme345/lec...
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MME 345, Lecture 39
Cast Iron Foundry Practices6. Process control in nodular iron making
Ref:
[1] Heine, Loper and Rosenthal. Principles of Metal Casting, Tata McGraw-Hill, 19670
[2] J R Brown (ed.). Foseco Ferrous Foundrymen’s Handbook, Butterworth-Heninemann, 2000
Topics to discuss today …
1. Metallurgical process control
2. Foundry process control
3. Heat treatment
4. Engineering properties
Metallurgical Process Control
Production of ductile iron is highly sensitive to process variation, requiring a
greater degree of control.
Effectiveness of magnesium treatment and inoculation is important.
The final CE value of iron is hypereutectic although the base iron composition
before Mg treatment and inoculation is often similar to grey iron and hypoeutectic.
The processes to be control are similar to those used for grey iron. However,
special attention must be given to the residual magnesium content after treatment.
spectrographic analysis of a chilled, graphite-free sample should be used
Test coupons (e.g., keel blocks or Y-blocks for making tensile test bars)
are routinely used to determine mechanical properties and to inspect graphite
shape and degree of spheroidisation.
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• Graphite shape and size and number
quality ductile iron is produced if the graphite is developed as spheroids with
a high degree of nodularity
a number of other types of graphite, including flake graphite, may be produced
if the process is not carried out properly
a high nodule count with appropriate size is also important
A number of metallurgical factors must be controlled in ductile iron production
to avoid structural imperfections.
These include:
• Carbide formation
occurrence of eutectic carbide must be prevented by maintaining high CE value
and good nodule count
• Dross formation
Mg functions first as desulphuriser and deoxidiser of base iron;
besides, Mg itself is highly oxidising
these creates dross, which adheres to the cope-surface of the casting
the situation worsen for high Mg addition, high pouring temperature, and turbulence
in the gating system and mould cavity4/26
Graphite Shape
ASTM A 247 - 67 Standard of Test Method for Evaluating the Microstructure of Graphite in Iron Castings
Up to 10% Type III, with the
remaining Type I or II has been
reported to have no noticeable
effect on properties.
Types IV and V are undesirable
and have significantly lower
mechanical properties.
I : (graphite nodules) II
III : vermicular graphite IV V : exploded graphite
Type I is the normal and usually desirable graphite form
in ductile iron, although the presence of Type II graphite
forms has little or no adverse effect on properties.
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Graphite Size
125 – 50 mm
212 – 25 mm
36 – 12 mm
43 – 6 mm
51.5 – 3 mm
6less than 1.5 mm
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nk = number of graphite particles corresponding to form number of graphite shape k.
Degree of Nodularity(As per JIS G5578)
A minimum of 75% degree of nodularity is required in order to be considered
the casting as ductile iron.
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Factors controlling graphite shape in ductile iron
• pouring temperature
• casting section size
• amount of effective Mg added
• post-inoculation
• base analysis of iron
Poorer graphite shapes are resulted for
• low pouring temperature
• heavy section size
• insufficient Mg addition
• lack of inoculation
• low CE in base analysis of iron
CE value higher than 4.6 often results graphite flotation and the development
of exploded graphite, Type V.
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combined effects of metallurgical factors on graphite nodules and properties
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Foundry Process Control
Most castings are made using green and dry sand moulds.
Moulding media are similar to those used for grey irons;
special consideration is given for moisture content, since Mg oxidises easily.
When a sufficient Mg is added, iron oxidises readily when temperature falls
below 1400 C and creates dross. So a pouring temperature of 1425 C or higher
is preferred to avoid dross formation.
Mg addition increases surface tension of iron, reducing wettability of the mould
by liquid iron, thereby reducing sand burn-in and metal penetration problems.
10/26
Feeding is complex because of the method of solidification.
Wide freezing range, shrinkage differences between hypo- and hyper-eutectic
alloys and tendency of mould-wall movement are some of the points to
consider while designing feeding system.
Ineffective Mg treatment and inoculation may often result eutectic carbide or
vermicular graphite (Type III), which alters the solidification behaviour and
increases feeding requirements. Proper treatment is therefore necessary to
control solidification shrinkage.
Design of gating system should include measures for preventing turbulence
and entry of slag and dirt into the mould cavity.
While designing gating system, the following formula can be used to determine
the average pouring time
pouring time, s = 0.65 pouring weight, lb
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Heat Treatment of Ductile Iron
Have excellent response to heat treatment; behaviour is similar to those of steels.
Matrix structure can be all ferrite, ferrite and pearlite, all pearlite, martensite,
tempered martensite, banite, and can even contain carbide or austenite.
Common heat treatments:
1. Stress relieving
2. Annealing
3. Normalising and tempering
4. Quenching and tempering, austempering, martempering
5. Flame/induction surface hardening
12/26
Austempered Ductile Iron
a grade of ductile iron in which heat treatment is utilized to produce a metastable
face-centered cubic matrix (i.e., austenite), which is stable at room temperature.
considerably higher strengths than the as-cast grades are obtained, yet retaining
excellent ductility.
microstructure of austempered
ductile iron (ADI)
the primary cause of this dramatic improvement in
mechanical properties stems from the presence of
the face centered cubic matrix together with a fine
scale dispersion of ferrite, commonly known as
ausferrite.
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Austempering is an isothermal heat treatment that, when applied to ferrous
materials, produces a structure that is stronger and tougher than comparable
structures produced with conventional heat treatments.
Conventional heat treaters heat the parts to about 900 C in a controlled atmosphere
and then quench them in a bath of oil or water that is near room temperature. This
produces a crystalline structure known as Martensite, a hard, brittle phase. The
parts are then tempered in another furnace at 175 to 600°C to decrease the
residual stress and “brittleness.”
Austempering starts the same way. The parts are heated to about the same
temperature in a controlled atmosphere (so they don't scale) but then are quenched
in a bath of molten salt at 230° to 400°C. This quench temperature is above the
martensite starting temperature. Therefore, a different structure (not martensite)
results. In Austempered Ductile Iron, the structure is Ausferrite (austenite and
ferrite), and in steel, it is Bainite.
14/26
Ms
Mf
transformation starts
transformation ends
TRANSFORMED
AUSTENITE
MARTENSITEBAINITE (for steel)
AUSFERRITE (for ductile iron)
Will not be included in Exam !!!
Time – Temperature – Transformation (TTT) curve
Tem
per
atu
re
Time
AUSTENITE
15/26
portion of the metastable Fe-C-2.4Si phase diagram
1. Casting is heated into the (austenite + graphite) field and held at Tg until the matrix
is fully austenitized. At this point the structure consists of austenite of 0.8 wt. % C
and graphite nodules.
16/26
portion of the metastable Fe-C-2.4Si phase diagram
2. Cool rapidly to TA, the austempering temperature and hold, allowing the reaction:
g (0.8%C) a (0%C) + g (2.0%C)
[ausferrite]
This reaction usually requires
about 1 - 2 hours to complete.
In a piece of steel heat treated
the same way, the austenite
would transform to bainite, a
two phase mixture of ferrite
and iron carbide (Fe3C).
However, in cast irons, the
presence of silicon prevents
the formation of carbides and
the austenite phase retained
as a metastable phase.
If given enough time, the
austenite will eventually
transform to bainite.
17/26
portion of the metastable Fe-C-2.4Si phase diagram
3. However, before that happens the casting will be cooled to room temperature,
a move which will retain the structure created at TA. The austempering reaction is
said to have “stabilized” the austenite.
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ausferrite
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Austempering Means Uniform Structure
During the process of quenching to Martensite, the Martensite reaction begins
immediately. It is a diffusion-less process. The result is that the outside of the part
may already be transformed while the inside is still red hot. It is this “non-uniform
phase transformation” that results in distortion and tiny micro cracks that lower
the strength of the part.
By contrast, the Austempering reaction that produces Ausferrite (or Bainite in steel)
is a diffusion-controlled process and takes place over many minutes or hours. This
results in uniform growth and a stronger (less disturbed) microstructure.
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The mechanical properties of ductile cast iron are significantly different than that
of gray iron because of the difference in shape between the graphites in these
cast irons.
In ductile irons the matrix is the continuous entity so that there are no easy crack
paths to propagate fracture.
As a result, ductile cast irons have significant ductility and toughness, properties
which place this unique material in competition with other ferrous materials such
as cast steels, forged steels, and even wrought steels.
As a result of the continuity of the matrix, the tensile properties of ductile cast iron
depend almost completely upon the microstructure of the matrix, a microstructure
which can be controlled by heat treatment.
Engineering Properties
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UTS v. % Elongation Data
summary of common as-cast and austempered grades of ductile iron22/26
composition – structure – properties of different grades of ductile iron
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Effect of Section
Section size effect on properties of casting is not as pronounced as in grey irons.
Thin sections are prone to carbide formation, and heavy sections may contain
undesirable nodule shapes.
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Other important properties to consider:
1. Machinability – superior to that of grey iron or steel
2. Corrosion resistance – equivalent to that of grey iron and better than steel
3. Wear resistance – equivalent to that of the best grades of grey iron and better than steel
4. Thermal shock resistance – good in ferritic ductile iron
25/26
Next ClassMME 345, Lecture 40
Steel Foundry Practices