The present invention relates to a process for making high strength
unalloyed anstempered ductile iron. The ductile iron according to the invention
has strength exceeding 1400 mpa.
Austempered ductile iron has emerged as a new engineering material in
the family of cast irons. This material is being extensively used in developed
countries because of its high strength, wear resistance and unique
metallurgical features. The current trend toward reduced weight and increased
power output in current machine designs has placed great demands on
superior component materials. Conventional ductile iron can not meet these
demands and designers have been forced to specify expensive alloy steel
forgings in place of castings.
Austempered ductile iron is an advanced material of ductile iron with
combination of high strength, toughness and wear resistance not available in
conventional ductile iron. ADI is produced by heat treating the ductile iron.
Selection of heat treatment and other process parameters such as chemistry,
holding time etc. are the key factors for successful production of ADI. The
potential application of ADI includes connecting rod, crank shaft, heavy duty
chain cprocket, screen plate, wear guides, general machinery which is currently
dominated by wrought steel and forged steel.
The starting material for producing ADI is ductile iron. Ductile iron belongs to the
family of cast iron in which the graphites are distributed in the form of
spheroids. Ductile iron has the advantages of grey iron such as low melting
point, good fluidity and castability, excellent machinability with the engineering
advantages of steel such as high strength, toughness, ductility etc. The
spheroidal graphite matrix is responsible for the unique mechanical and
metallurgical properties of ductile iron. The Ultimate Tensile Strength (UTS) of
conventional ductile iron ranges from 37-80 Kg/mm2, with 2-17% elongation.
The ADI presently produced contains alloying additions such as nickel, copper,
molybodenum etc. and therefore the ADI containing these alloying additions has
high cost.
The applicants have found that the cost of the ADI could be significantly
reduced by providing an unalloyed composition based on Carbon-manganese-
silicon system under regulated heat treatment cycle. The applicants have
particularly found that the use selected austenization temperature and
austempering temperature provided significant benefits in the form of improved
mechanical and metallugical properties of the austempered ductile iron.
Thus the present invention relates to a process for the manufacture of high
strength unalloyed austempered ductile iron comprising the steps of
i) providing a material comprising -
Carbon (c) - 3.5 - 3.8% by wt.
Mn - 0.20-0.25% bywt.
Si - 3.00-3.20% bywt.
S - 0.01 -0.015% bywt.
P - 0.02-0.04% bywt.
Mg - 0.02 - 0.06% by wt.
Iron - Balance
ii) austenizing the material at a temperature of between 880°C and 930°C
for a time of between 1 to 2 hours ;
iii) austempering the treated material at a temperature of between 280°C
and 400°C for a time of between 2 hr. to 21/2 hours ; and
iv) cooling the mass.
The temperature of the step of austenizing and austempering may be varied
depending upon the toughness,, ductility and tensile strength requirements.
When the requirement of tensile strength of the product is high, then the
steps of austenizing and austempering are carried out at temperature ranges of
between 880°C to 900°C and 280°C to 330°C respectively.
For moderate tensile strength but high toughness and ductility requirements it
is found that the appropriate austening and austempering temperatures are in
the range of 915°C to 930°C and 370°C to 400°C respectively.
The cooling of the treated materials may be effected in the furnace by
resorting to air cooling technique.
EXAMPLE
A mixture of 3100 Kg. low sulpher, low phosphorous, low manganese steel scrap
was melted in a induction furnace. The melt was recarbonized by petroleum
coske. A bath sample of the mass was taken and there the chemistry of the mass
was adjusted by addition of ferro-silicone. 2750 Kg. of the mass was then taken
in a preheated ladle and subjected to treatment with 53 Kgs. Fe-Si-Mg alloy. The
treated mass was then subjected to the step of post inoculation by 11 Kg of Ferro
silicon. The molten mass was then quickly poured in moulds for forming Y-test
bars. The mass was allowed to cool for 24 hours. Samples were taken and
checked under microscope and when it was found that there had been 95%
nodulization the mass is subjected to heat treatment in high temperature furnace
at a temperature of 890°C for VA hrs. and anstempered at a temperature280 -
290°C for 2 hr to 21/2 hrs. in salt bath furnace and then take out and cool it to
room temperature.
The selection of heat treating temperatures, (Austenization temperature,
Austempering temperature), time and composition that influence the degree of
success of austempering on the mechanical and metallurgical property of
unalloyed ductile iron was studied in detail. The tensile strength, impact
toughness, hardness wear resistance, high temperature tensile strength and
fractography studies of various samples were evaluated. The as cast
microstructure of ductile iron, was nodular graphite in a ferritic matrix. The
microstructure of austempered ductile iron consists of bainitic ferrite, retained
austenite, nodular graphite, it has been observed that austenizing at 880°C for
one hour and Austempering at 280°C for two hour followed by furnace cooling for
unalloyed composition is suitable for high strength application. In this case, the
tensile strength has remarkably improved three fold from 48 Kg/mm2 to 144
Kg/mm2 with 7% elongation. High temperature tensile test conducted at 300°C
and 500°C also shows encouraging trend. Austenization at 930°C and
austempering at 400°C for 2 hr. followed by Air cooling can also be successfully
employed for industrial application where high toughness (8.82 Joules), higher
ductility (13%), moderate tensile strength (99 Kg/mm2) is required.
We claim:
1. A process for the manufacture of high strength unalloyed austempered ductile
iron comprising the steps of
i) providing a mateiral comprising -
Carbon -3.5-3.8% by wt.
Mn - 0.20-0.25% bywt.
Si - 3.00-3.20% bywt.
S - 0.01 -0.015% bywt.
P - 0.02-0.04% bywt.
Mg - 0.02-0.06% bywt.
Iron - Balance
ii) austenizing the material at a temperature of between 880°C and 930°C
for a time of between 1 to 2 hours ;
iii) austempering the treated material at a temperature of between 280°C
and 400°C for a time of between 2 hr. to 21/2 hours ; and
iv) cooling the mass.
2. A process as claimed in claim 1, wherein the austenizing step is carried out at
a temperature of between 880°C - 900°C when the requirement of tensile
strength of the product is high such as herein described.
3. A process as claimed in claim 2, wherein the austempering step is carried out
at a temperature of between 280°C - 330°C, when the requirement of tensile
strength of the product is high such as herein described.
4. A process as claimed in claim 1, wherein the austenizing step is carried out at
a temperature of between 915° - 930°C when the requirement of tensile
strength of the product moderate with high toughness and ductility.
5. A process as claimed in claim 4, wherein the austenizing step is carried out at
a temperature of between 370°C and 400°C when the requirement of tensile
strength of the product moderate with high toughness and ductility.
6. A process as claimed in any one of the preceding claims wherein the cooling

treatment is effected in the furnace by resorting to air cooling techniques.

A process for the manufacture of high strength unalloyed austempered ductile iron comprising the steps of providing a mateiral comprising carbon - 3.5 - 3.8% by wt. Mn - 0.20 - 0.25% by wt. Si 3.00 - 3.20% by wt. S 0.01 - 0.015% by wt. P 0.02 - 0.04% by wt. Mg 0.02 - 0.06% by wt. Iron Balance, austenizing the material at a temperature of between 880°C and 930°C for a time of between 1 to 2 hours ; austempering the treated material at a temperature of between 280°C and 400°C for a time of between 2 hr. to 2½ hours ; and cooling the mass.