Chapter 7 Alloy Steels 3 - WordPress.com · Maraging Steels • Fe-Ni martensite alloys (which...
Transcript of Chapter 7 Alloy Steels 3 - WordPress.com · Maraging Steels • Fe-Ni martensite alloys (which...
Alloy SteelsAlloy Steels
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SME 1613 : Materials Science 2007
High strength low alloy (HSLA).
� a type of steel alloy that provides many benefits over regular
steel alloys
� contain a very small percentage of carbon (less than one-tenth
of a percent) and only small amounts of very specific alloying
elements
� referred to as 'microalloyed', as they are indeed alloyed in
extremely small amounts by comparison to other main
commercial alloy steels
� group of low carbon steels that utilize small amounts of alloying
elements to attain yield strengths in excess of 275 MPa in the
as-rolled or normalized conditions
SME 1613 : Materials Science 2007
TYPICAL CHEMICAL COMPOSITION OF HSLA
SME 1613 : Materials Science 2007
MECHANICAL PROPERTIES: HSLA
• Much stronger and tougher than ordinary plain carbon
steels.
• alloying elements : alter the microstructure of plain carbon
steels, which is usually a ferrite-pearlite aggregate, to
produce a very fine dispersion of alloy carbides in an
almost pure ferrite.
• Increases the material's strength by precipitation
strengthening and by refining the grain size, which in the
case of ferrite increases yield strength by 50% for every
halving of the mean grain diameter.
SME 1613 : Materials Science 2007
Microstructure of HSLA
A517 Grade
7,quenched from
925°C,tempered at
650C = tempered
martensite.
A533 grade b, quenched
from 900C and tempered
at 620C . This is a heavy
section plate, the
microstructure consists of
ferrite and tempered
bainite.
SME 1613 : Materials Science 2007
Application of HSLA
• Applications: include oil and gas pipelines,
automotive beams, offshore structures and
shipbuilding.
• Also used in cars, trucks, cranes, bridges and other
structures that are designed to handle a lot of stress,
often at very low temperatures.
SME 1613 : Materials Science 2007
SME 1613 : Materials Science 2007
Maraging Steels
• Fe-Ni martensite alloys (which belong to the family of
iron-base alloys)additionally alloyed with cobalt,
molybdenum, titanium and some other elements.
• difference between carbon steels and maraging steels is
that the martensite that forms in carbon steels,when
quenched, transforms to a softer more ductile material
when tempered.
• maraging steels are not included in the AISI numbered
listings since they are not machine steels.
SME 1613 : Materials Science 2007
Type of Maraging Steels
Type of Maraging Steels
~very high resistance to stress corrosion
compared to carbon steels.
~these steels develop notch strenght higher
than carbon steels.
Mechanical properties
18% Ni, 8% Co, 5% Mo,
0.9% Ti
ease of welding.
~high quality to fracture toughness and
~the strenght is maintained with increase in
2. 23% Nickel
3. 12% Nickel
Composition
1. 18% Nickel
less amount of Al and Ti
12% Ni, 12% Cr, Mo,
23% Ni, 2.0% Al, 2.5% Ti
section thickness and also up to 350°C
~high hardness results from the intermetallic
compounds Ni3Mo and Ni3Ti.
SME 1613 : Materials Science 2007
• The maraging steels are strengthened by a process of martensitic transformation, followed by age or precipitation hardening. Precipitation hardenable stainless steels are also in this group.
(1) Annealing Fe-Ni alloys to 820 °C, cooling from the austenitic condition,
the alloy transforms to a fine lath type martensite.
(2)(3)(4)The iron-nickel martensite starts out ductile and become hard and tough with aging. Cooling in air for 1 hours and tempered at 480 °C for 3 hours
• The tempering results in strong precipitation hardening owing to the precipitation of intermetallics from the martensite, which is supersaturated with the alloying elements.
� By analogy with the precipitation hardening in aluminum, copper and other non-ferrous alloys, this process has been termed ageing, and since the initial structure is martensite, the steels have been called maraging.
Heat treatment: Maraging Steel
SME 1613 : Materials Science 2007
Microstructure of Maraging Steels
Fe-Ni Martensite Tempered Fe-Ni Martensite
• The maraging steels have high fracture toughness due to a
combination of the grain size of the martensite and the
dislocation density,leading to a fine
precipitation.(eg,Ni3TiAl,orthorhombic Ni,Mo)
• The steels can be nitrided. The corrosion resistance is only
slightly improved but the 12% Cr variety has been developed for
corrosion resistance.
SME 1613 : Materials Science 2007
Mechanical Properties: Maraging Steel
� Ultra-high strength at room temperature.(yield strength=1100MPa)
� Simple heat treatment, which results in minimum distortion.
� Superior fracture toughness compared to quenched and tempered
steel of similar strength level.
� Low carbon content, which precludes decarburisation problems.
� Section size is an important factor in the hardening process.
� Easily fabricated.
� Good weldability.
SME 1613 : Materials Science 2007
Application: Maraging Steel
• Applications : shafts, and substitute for long, thin, carburized or
nitrided parts, and components subject to impact fatigue, such as
print hammers or clutches.
• Also commonly used: air frame and engine components,injection
moulds and dies due to its high resistance of
decarburisation,distortion and cracking.
• Maraging steels work well in electro-mechanical components where
ultra-high strength is required, along with good dimensional stability
during heat treatment.
SME 1613 : Materials Science 2007