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
