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High speed steels
&
HSLA
High speed steels
Introduction to High Speed Steel (HSS)
• High Speed Steel is a highly alloyed tool steel capable of maintaining
hardness even at elevated temperatures. High-speed tool steels are so named
primarily because of their ability to machine materials at high cutting
speeds. High speed steel has unusually high resistance to softening at
temperatures up to 600C. It is called, red hardness.
• They
are complex iron-base alloys of carbon, chromium, vanadium,
molybdenum, or tungsten, or combinations thereof, and in some cases substantial
amounts of cobalt.
hardening response, high wear resistance, high resistance to the softening effect
of heat, and good toughness for effective use in industrial cutting operations.
• Especially suited to applications involving complicated tool shapes: drills, taps,
milling cutters, and broaches.
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• The carbon and alloy contents are balanced at levels to give high attainable
High speed steels
Classifications of HSS
• The AISI established a classification system for the high-speed tool steels. That
system consists of a T for those steels that have tungsten as one of their primary
alloying elements and an M or those steels that have molybdenum additions as one of
their primary alloying elements.
Two basic types of HSS (AISI)
1. Tungsten type, designated T- grades
2. Molybdenum type, designated M-grades
speed tool steels designated M l, M2, M41, T l, TI5, and so on. That number does not
have any special significance other than to distinguish one from another. For
example, M1 does not mean that it is more highly alloyed than M2 or has greater
hardenability or poorer wear resistance, and so on. It merely separates the types and
attempts to simplify selection for the user.
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• In addition, there is a number that follows either the M or the T. Thus, there are high-
High speed steels
HSS 18-4-1 is a most famous steel grade for cutting tool. It consist of :18-Tungsten
4-Chromium
1-Vanadium
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Few common grades of HSS
High speed steels
Typical alloying ingredients:
1. Carbon
2. Tungsten and/or Molybdenum
3. Vanadium
4. Chromium
5. Cobalt in some grades
1.
2.
3.
4.
5.
Carbon : Forms Carbides, increases wear resistance, is responsible for the basic
matrix hardness.
Tungsten and/or Molybdenum : Improved red hardness, retention of hardness and
high temperature strength of the matrix, from special Carbides of great hardness.
Vanadium : Forms special Carbides of supreme hardness, increases High
temperature wear resistance , retention of hardness and high Temperature strength
of the matrix.
Chromium : Promotes depth hardening, produces readily soluble Carbides.
Cobalt : Improves red hardness and retention of hardness of the matrix.
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Influenced of alloying elements on the steel properties :
High speed steels
Microstructure of HSS
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The microstructure consists of undissolved excess carbides in a matrix of martensite. These carbides are
probably the types M6C (where M is either molybdenum or tungsten) and vanadium carbide (VC). If it
were tempered at about 550 0C, the martensite would contain very finely divided M2C (M is again
molybdenum or tungsten) which is the carbide responsible for the secondary hardening of high speed
steels. The present microstructure is very fine grained.
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High speed steels
Properties of HSS
 High Working hardness.
 High wear resistance.
 High retention of hardness and red hardness.
 Excellent toughness.
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Applications of HSS
Single-Point Cutting Tools
End mill
Broach insert
Tap
Drill/ Twist Drill
Reamers
Milling cutters
Saw
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are
designed to provide better mechanical properties than conventional carbon
steels.
The HSLA steels in sheet or plate form have low carbon content (0.05 −
0.25% C) in order to produce adequate formability and weldability. They
have manganese content from 1.5 − 2.0% along with 0.1 % of chromium,
nickel, molybdenum,, vanadium, niobium, titanium, and zirconium are
used in various combinations to obtain a fine grain size upto 5 um.
High-strength low-alloy steels provide specific desirable combinations of
properties such as:High strength,
Toughness,
Wear resistance,
Weldability, and
Atmospheric corrosion resistance.
These steels are not considered alloy steels, even though their desired
properties are achieved by the use of small alloy additions.
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High
Strength
Low
Alloy
(HSLA)
High-strength low-alloy (HSLA) steels, or microalloyed steels,
High Strength Low Alloy (HSLA)
HSLA attains a yield strength of 290 − 550 MPa and tensile strength of 415 − 700 MPa.
Microstructure of HSLA consists of ultra fine ferrite and pearlite. This fine microstructure is mainly
responsible for such high strength and toughness.
Major factor responsible for increased strength in HSLA are:•Fine ferrite grain size:- Fine grain size of ferrite in HSLA is achieved by control of austenite grain size by
hot rolling and by solid second phase precipitates. These fine solid precipitates (with high melting point
temperature) hinder the growth of austenite by pinning the grains boundary during heating at high
temperature and refines the grain size.
used to increase the yield strength of materials. In order to apply this treatment, the equilibrium diagram
must show partial solid solubility and the solid solubility limit should be decrease with decrease in
temperature. It has two stages process, i.e. solution treatment and aging process. In solution treatment,
material is heated to a single phase region and then is quenched rapidly to room temperature to result in a
super saturated solid solution. In second stage i.e. ageing, controlled decomposition of super saturated solid
solution takes place to form finely dispersed precipitates. These precipitates restricts the movement of thee
dislocations and thus strengthen the materials.
•Solid solution strengthening:- when solute atoms are added into solvent, local stress fields are formed that
interact with the dislocations, prevent the motion of dislocations and causes an increase in the strength of the
material, this mechanism of strengthening is called as solid solution strengthening.
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•Precipitation hardening:-Precipitation hardening, also called age hardening, is a heat treatment technique
High Strength Low Alloy (HSLA)
• Applications of HSLA steels include oil and gas pipelines,
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heavy-duty highway and off-road vehicles, construction and
farm machinery, industrial equipment, storage tanks, mine and
railroad cars, barges and dredges, snowmobiles, lawn mowers,
and passenger car components. Bridges, offshore structures,
power transmission towers, light poles, and building beams and
panels are additional uses of these steels.