5 Sept 2013
4 Sept 2013
Dual Phase Steels
Dual Phase Steels
Microstructure
•
This class is
characterized by a tensile strength value of approximately 550 MPa (80 ksi) and
by a microstructure consisting of about 20% hard martensite particles dispersed
in a soft ductile ferrite matrix.
Properties
•
These steels
have a low yield strength and continuous yielding behavior; therefore they form
just like low-strength steel, but they can also provide high strength in the
finished component because of their rapid work-hardening rate.
•
Typical
as-shipped yield strength is 310 to 345 MPa (45 to 50 ksi) and tensile strength
550 MPa (80 ksi) .
•
A higher total
elongation than other HSLA steels of similar strength.
Production
•
Dual-phase
steels can be produced from low-carbon steels in three ways:
Ø Intercritical austenitization of carbon-manganese
steels followed by rapid cooling
Ø Hot rolling with ferrite formers such as silicon and
transformation-delaying elements such as chromium, manganese, and/or molybdenum
Ø Continuous annealing of cold-rolled carbon-manganese
steel followed by quenching and tempering
Composition
•
In general,
these steels have a carbon content of less than 0.1%, which ensures that they
can be spot welded.
•
Manganese in
amounts of 1 to 1.5% is added to ensure sufficient hardenability so that
martensite is formed upon rapid cooling.
•
Chromium and
molybdenum have also been added in amounts that are usually under 0.6%.
•
Silicon is added
to provide solid solution hardening.
•
Small amounts of
microalloying additions, such as vanadium, niobium, and titanium, may be added
to provide precipitation hardening and/or grain size control.
•
Nitrogen may be
added to intensify the precipitation-hardening effects of vanadium.
Microalloyed Ferrite-Pearlite Steels
Microalloyed Ferrite-Pearlite Steels
Composition
•
These steels use
additions of very small amounts of alloying elements such as niobium and
vanadium (<0.10% each) to increase strength (and thereby increase
load-carrying ability) of hot-rolled steel without increasing carbon and/or
manganese contents.
•
Minor alloying
elements refine the grain microstructure and/or facilitate precipitation
hardening.
•
Carbon content
thus could be reduced to improve both weldability and toughness because the
strengthening effects of niobium and vanadium compensated for the reduction in
strength due to the reduction in carbon content.
Properties
•
Along with
microalloying elements, the mechanical properties of microalloyed HSLA steels
depend upon austenite conditioning.
•
Austenite
conditioning depends on the complex effects of alloy design and rolling
techniques.
•
The high yield
strength of 485 MPa (70 ksi) is achieved by the combined effects of fine grain
size developed during controlled hot rolling and precipitation strengthening
that is due to the presence of vanadium, niobium, and titanium.
Classification
The various types of microalloyed ferrite-pearlite
steels include:
Ø Vanadium-microalloyed steels
Ø Niobium-microalloyed steels
Ø Niobium-molybdenum steels
Ø Vanadium-niobium microalloyed steels
Ø Vanadium-nitrogen microalloyed steels
Ø Titanium-microalloyed steels
Ø Niobium-titanium microalloyed steels
Vanadium-titanium microalloyed steels
HSLA Steels (high strength low alloy steel)
HSLA Steels
General Introduction
•
High-strength
low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better
mechanical properties and/or greater resistance to atmospheric corrosion than
conventional carbon steels.
•
These are a
group of low-carbon steels that utilize small amounts of alloying elements to
attain yield strengths greater than 275 MPa (40 ksi) in the as-rolled or
normalized condition.
•
These are
developed by controlled chemical composition and mechanical treatment to obtain
desired better mechanical properties.
•
They are not
considered to be alloy steels in the normal sense because they are designed to
meet specific mechanical properties rather than a chemical composition
Composition
•
Primarily
low-carbon (C≤ 0.20%) steels with about 1-2% Mn and small quantities (<
0.50%) of other elements.
•
Small quantities
of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium,
and zirconium are used in various combinations.
Properties
•
These steels
have better mechanical properties and sometimes better corrosion resistance
than as-rolled carbon steels.
•
Yield Strength=
290-480 MPa, Tensile Strength= 415-620 Mpa.
•
Give good
mechanical properties with reduced weight.
•
Weldability of
many HSLA steels is comparable to or better than that of mild steel.
•
Not strengthened
by heat treatment.
Processing
•
These steels are
usually used in hot finished condition
•
Processing may
also involve special hot-mill processing that further improves the mechanical
properties.
•
These processing
methods include:
Ø The controlled rolling of precipitation-strengthened HSLA steels
Ø The accelerated cooling of, preferably, controlled-rolled HSLA steels
Ø The quenching or accelerated air or water cooling of low-carbon steels (≤0.08% C)
Ø The normalizing of
vanadium-containing HSLA steels
Ø The intercritical annealing of HSLA steels
•
HSLA steels are
also furnished as cold-rolled sheet and forgings.
Classification
HSLA steels can be divided into seven categories:
Ø Weathering steels, which contain small amounts of alloying elements
such as copper and phosphorus for improved atmospheric corrosion resistance and
solid-solution strengthening
Ø Microalloyed ferrite-pearlite steels, which contain very small (generally, less than
0.10%) additions of strong carbide or carbonitride-forming elements such as
niobium, vanadium, and/or titanium for precipitation strengthening, grain
refinement, and possibly transformation temperature control
Ø As-rolled pearlitic steels, which may include carbon-manganese steels but
which may also have small additions of other alloying elements to enhance
strength, toughness, formability, and weldability
Ø Acicular ferrite (low-carbon bainite) steels, which are low-carbon (<0.08% C) steels with an
excellent combination of high yield strengths, weldability, formability, and
good toughness
Ø Dual-phase steels, which have a microstructure of martensite
dispersed in a ferritic matrix and provide a good combination of ductility and
high tensile strength
Ø Inclusion shape controlled steels, which provide improved ductility and
through-thickness toughness by the small additions of calcium, zirconium, or
titanium, or perhaps rare-earth elements so that the shape of the sulfide
inclusions are changed from elongated stringers to small, dispersed, almost
spherical globules
Ø Hydrogen-induced cracking resistant steels with
low carbon, low sulfur, inclusion shape control, and limited manganese
segregation, plus copper contents greater than 0.26%
Applications
Primary applications for HSLA steels include
•
Oil and gas line
pipe
•
Ships
•
offshore
structures
•
Automotive crank
shaft
•
Crane and
vehicles
•
off-highway
equipment, and
•
pressure
vessels.
Selection of Stainless Steels
Selection of
Stainless Steels
•
The selection of
stainless steels may be based on corrosion resistance, fabrication
characteristics, availability, mechanical properties in specific temperature
ranges and product cost.
•
However,
corrosion resistance and mechanical properties are usually the most important
factors in selection a grade for a given application.
Factors in
Selection
•
A checklist of
characteristics to be considered in selecting the proper type of stainless
steel for a specific application includes:
•
Corrosion
resistance
•
Resistance to
oxidation and sulfidation
•
Strength and
ductility at ambient and service temperatures
•
Suitability for
intended fabrication techniques
•
Suitability for
intended cleaning procedures
•
Stability of
properties in service
•
Toughness
•
Resistance to
abrasion and erosion
•
Surface finish
and/or reflectivity
•
Magnetic
properties
•
Thermal
conductivity
•
Electrical
resistivity
•
Shrpness
(retention of cutting edge)
•
Rigidity
Alloy Steels and Its Types
Alloy Steels
General Effects
of Alloying Additions on Steels
•
Improves tensile
strength without appreciably lowering ductility.
•
Improves
toughness.
•
Improves
hardenability which permits hardening of larger sections than possible with
plain carbon steels or allows quenching with less drastic rates reducing the
hazard of distortion and quench cracking.
•
Retain strength
at elevated temperatures.
•
Obtain better
corrosion resistance.
•
Improves wear
resistance.
•
Imparts a fine
grain structure to the steel.
•
Improves special
properties such as abrasion resistance and fatigue behavior.
Stainless Steels
•
Stainless steels
are iron-base alloys containing at least 10.5% Cr.
•
Few stainless
steels contain more than 30% Cr or less than 50% Fe.
•
Stainless steel
does not corrode, rust or stain with water as ordinary steel does, but despite
the name it is not fully stain-proof.
•
They achieve
their stainless characteristics through the formation of an invisible and
adherent chromium-rich oxide surface film.
•
Other elements
added to improve particular characteristics include nickel, molybdenum, copper,
titanium, aluminum, silicon, niobium, nitrogen, sulfur, and selenium.
•
Carbon is
normally present in amounts ranging from less than 0.03% to over 1.0% in
certain martensitic grades.
Classification
Stainless
steels are commonly divided into five groups:
1.
Austenitic
Stainless steels
2.
Ferritic
Stainless steels
3.
Martensitic
Stainless steels
4.
Duplex
(ferritic-austenitic) Stainless steels and
5.
Precipitation-hardening
Stainless steels
1.
Austenitic Stainless Steels
•
These stainless
steels make up over 70% of total stainless steel production.
•
Essentially
chromium containing alloys having an FCC structure. This structure is attained
through the liberal use of austenitizing elements such as nickel, manganese,
and nitrogen.
Composition:
•
Chromium content
generally varies from 16 to 26%; nicket, up to about 35%; and manganese, up to
15% (or sufficient nickel and/or manganese to retain an austenitic structure at
all temperatures from the cryogenic region to the melting point of the alloy)
•
The 2xx series
steels contain nitrogen, 4 to 15.5% Mn, and up to 7% Ni.
•
The 3xx types
contain larger amounts of nickel and up to 2% Mn.
•
Molybdenum, copper, silicon,
aluminum, titanium, and niobium may be added to confer certain characteristics
such as halide pitting resistance or oxidation resistance.
•
Sulfur or
selenium may be added to certain grades to improve machinability.
Properties:
•
These steels are
essentially nonmagnetic in the annealed condition and can be hardened only by
cold working.
•
They usually
possess excellent cryogenic properties and good high-temperature strength.
•
Their
weldability is considered excellent.
•
They have good
corrosion resistance in most environments.
•
More expensive
than martensitic and low-to-medium chromium ferritic grades, due to the higher
alloy content of these alloys.
Applications:
•
structural
supports and containments, architectural uses, kitchen equipment, medical
products, bio implants, fasteners, etc.
2.
Ferritic Stainless Steels
•
Essentially
chromium containing alloys with bcc crystal structures.
Composition:
•
Chromium content
is usually in the range of 10.5 to 30%.
•
Some grades may
contain molybdenum, silicon, aluminum, titanium, and niobium to confer
particular characteristics.
•
Sulfur or
selenium may be added, as in the case of the austenitic grades, to improve
machinability.
Properties:
•
The ferritic
alloys are ferromagnetic.
•
They can have
good ductility and formability, but high temperature strengths are relatively
poor compared to the austenitic grades.
•
Toughness may be
somewhat limited at low temperatures and in heavy sections.
•
Ferritic steels
are not hardenable by heat treatment, due to their low carbon content.
•
These alloys
posses good resistance to stress corrosion cracking, pitting corrosion, and
crevice corrosion.
•
These alloys
possess superior corrosion resistance relative to the austenitic and
martensitic grades.
•
Ferritic
stainless steels are generally limited to service temperatures below 750°F
(400°C)
Applications
•
Used in a
variety of applications where corrosion resistance, rather than mechanical
properties (strength, toughness and ductility) is the primary service
requirement
•
automotive
exhaust systems, chemical processing, pulp and paper industries, furnaces,
refineries, trim and decorative applications, cooking utensils, etc.
3.
Martensitic Stainless Steels
•
Essentially
alloy of chromium and carbon that possess a distorted body-centered cubic (bcc)
crystal structure (martensitic) in the hardened condition.
Composition:
•
Chromium content
is generally in the range of 10.5 to 18%, and carbon content may exceed 1.2%.
•
Excess carbides
may be present to increase wear resistance or to maintain cutting edges, as in
the case of knife blades.
•
Elements such as
niobium, silicon, tungsten, and vanadium may be added to modify the tempering
response after hardening.
•
Small amounts of
nickel may be added to improve corrosion resistance in some media and to
improve toughness.
•
Sulfur or selenium
is added to some grades to improve machinability.
Properties:
•
These steels are
generally termed "air hardening" because when withdrawn from a
furnace as austenite, cooling in still air is sufficiently rapid to produce the
allotropic transformation into martensite.
•
They are
ferromagnetic, hardenable by heat treatments, and are generally resistant to
corrosion only to reletavely mild environments.
•
Extremely strong
and tough, highly machinable.
•
The low chromium
and low alloying element content of the martensitic stainless steels also makes
them less costly than the other types.
•
Least weldable
of the stainless steels
Applications
•
surgical
instruments, cutlery, gears, shafts, fasteners, steam, gas and jet turbine
blades, piping and valves, etc.
4.
Duplex Stainless Steels
•
Have a mixed
structure of BCC ferrite and FCC austenite. The Exact amount of each phase is a
function of composition and heat treatment.
•
Most alloys are
designed to contain about equal amounts of each phase in the annealed
condition.
Composition
•
23-30% Cr,
4.5-7% Ni, 2-4% Mo.
•
The principal
alloying elements are chromium and nickel, but nitrogen, molybdenum, copper,
silicon, and tungsten may be added to control structural balance and to impart
certain corrosion-resistance characteristics.
Properties
•
The corrosion
resistance of duplex stainless steels is like that of austenitic stainless
steels with similar alloying contents.
•
Duplex stainless
steels possess higher tensile and yield strengths and improved resistance to
stress-corrosion cracking than their austenitic counterparts.
•
The toughness of
duplex stainless steels is between that of austenitic and ferritic stainless
steels.
•
Duplex stainless
steels are weldable, but welding consumables and heat input must be controlled
to maintain the ferrite-austenite balance.
Applications
•
Duplex stainless
grades are used in applications that take advantage of their superior corrosion
resistance, strength, or both which include
Ø oil and gas pipelines
Ø on/offshore oil production
Ø petrochemical equipment
Ø heat exchangers and condensers
5.
Precipitation-Hardening Stainless Steels
Composition
•
14-18% Cr, 6-8%
Ni, 2-3% Mo, 0.75-1.50% Al
•
Chromium-Nickel
Alloys containing precipitation hardening elements such as copper, Aluminum, or
titanium
•
Precipitation-hardening
stainless steels may be either austenitic or martensitic in annealed condition
Properties
•
Unique
combination of fabricability, strength, ease of heat treatment, and corrosion
resistance not found in any other class of material.
•
Those that are
austenitic in the annealed condition are frequently transformable to martensite
through conditioning heat treatments, sometimes with subzero treatment.
•
In most cases,
these stainless steels attain high strength by precipitation hardening of the
martensitic structure.
•
Can develop very
high tensile strengths.
•
Steel can be
hardened by a single, fairly low temperature "ageing" heat treatment
which causes no distortion of the component.
•
More difficult
to fabricate than other stainless steels
Applications
•
Developed
primarily as aerospace materials, many of these steels are gaining commercial
acceptance as truly cost-effective materials in many applications which
include:
Ø
Ø Valves, gears, splines and shafts, heat exchangers
and condensers, pressure vessels, aircraft frames, surgical instruments,
turbine blades, etc.
Selection of
Stainless Steels
•
The selection of
stainless steels may be based on corrosion resistance, fabrication
characteristics, availability, mechanical properties in specific temperature
ranges and product cost.
•
However,
corrosion resistance and mechanical properties are usually the most important
factors in selection a grade for a given application.
Factors in
Selection
•
A checklist of
characteristics to be considered in selecting the proper type of stainless
steel for a specific application includes:
•
Corrosion
resistance
•
Resistance to
oxidation and sulfidation
•
Strength and
ductility at ambient and service temperatures
•
Suitability for
intended fabrication techniques
•
Suitability for
intended cleaning procedures
•
Stability of
properties in service
•
Toughness
•
Resistance to
abrasion and erosion
•
Surface finish
and/or reflectivity
•
Magnetic
properties
•
Thermal
conductivity
•
Electrical
resistivity
•
Shrpness
(retention of cutting edge)
•
Rigidity