Transcript No Slide Title

Mech 473 Lectures
Professor Rodney Herring
High Strength Low Alloy Steels (HSLA)
HSLA steels are low carbon steels that contain up to 10 % of alloying
additions.
The alloying elements permit HSLA steels to be quenched and
tempered to obtain high levels of strength and impact toughness.
The “hardness” of martensite and bainite is determined by the
carbon content and not by the alloying elements.
The alloying additions simply enable martensite and bainite
to form during quenching.
High Strength Low Alloy Steels (HSLA)
Even so, the hardness of low carbon martensite and lower
bainite (RC 50) is greater than the hardness of both course
pearlite (RC 20) and fine pearlite (RC 40) so the strength of
HSLA steels can be increased above the limits obtained in hotworked steels containing fine pearlite.
High Strength Low Alloy Steels (HSLA)
Note the low C
We are going to look at the properties of these steels in detail in the
following slides.
High Strength Low Alloy Steels (HSLA)
A533 grade B contains small amounts of Ni and Mo, which give it
sufficient hardenability to form a ferrite plus bainite
microstructure on quenching.
The bainite is tempered to improve the toughness, giving a better
strength, but similar ductility to the hot worked low-carbon plaincarbon steels.
The steel is used for nuclear vessels and steam generators.
High Strength Low Alloy Steels (HSLA)
Ferrite and tempered bainite form in A533 grade B quenched
from 900 oC and tempered at 620 oC.
High Strength Low Alloy Steels (HSLA)
Grades A543 class 1 and A517 grade F have very high yield and
tensile strengths for low carbon steels in addition to good
toughness.
The high strengths of these steels are achieved by alloy
additions of Ni, Cr, and Mo with further additions of V, Zr,
and B
The Ni, Cr, Mo + V in steel A543 enables a mixture of
martensite and bainite to form on quenching, while the
additional Zr and B in A517 steels enables 100% martensite
to form on quenching to give even greater strength.
Together Zr and B enhance strength by forming a precipitate at
high temperatures in the liquid phase that will nucleate
austenite to form a fine grain structure.
Zr and B are commonly used in many types of alloys for this
purpose.
High Strength Low Alloy Steels (HSLA)
The toughness of A543 class 1 and A517 grade F steels is
developed by tempering the bainite and/or martensite at
relatively high temperatures of 600-650 oC.
These steels are used in plates, shapes, forgings and for weld
constructions including bridges and nuclear pressure vessels.
Hummer vehicles and personal vehicles used in Iraq have been
found to be insufficient to block shrapnel from explosives
such as land mines so new light armor vehicles have been
built, which have HSLA steel plates for panels and doors
instead of the low-C steels used previously.
People are still dying in these vehicles even
though the vehicles remain in tact – do you
know why?
Tempered bainite and
martensite formed in A543
class 1 quenched from 850
oC and tempered at 650 oC.
Tempered martensite formed in
A517 grade F quenched from 925
oC and tempered at 650 oC.
High Strength Low Alloy Steels (HSLA)
Steels A203 grade D and A553 type 1 contain Ni to improve low
temperature notch toughness.
The presence of 3.5 %Ni in Steel A203 does not improve the
strength above that of a hot worked plain carbon steel
because Ni does not significantly improve the hardenability,
i.e., ability to form martensite.
but after tempering, the ductile-brittle transition
temperature of this steel is lowered to below -20 oC.
This steel is used for a variety of relatively low-stress, low
temperature applications.
High Strength Low Alloy Steels (HSLA)
The increased Ni content of 9% in steel A533 improves the
strength by “solid solution strengthening” up to the level of
the Ni-Cr-Mo HSLA steel and also gives A533 a higher
ductility so that its ductile to brittle transition temperature is
lowered to below –200 oC.
This relatively expensive steel is used for high-stress lowtemperature applications such as pressure vessels and for the
transport of liquified natural gas (-170 oC)
High Strength Low Alloy Steels (HSLA)
Steels A542 class 1 is quenched and tempered to give a high
strength with good ductility similar to A543 but also contains
Cr and Mo to increase its resistance to high temperature
creep and corrosion resistance.
The steel is used for high pressure chemical reactors and
refinery vessels.
The mechanism whereby the creep properties are improved is
due to “interphase precipitation hardening”, which is
discussed next for 0.15 to 0.75 Vanadium and Tungsten
HSLA steel.
High Strength Low Alloy Steels (HSLA)
HSLA steels can also be strengthened by “interphase
precipitation” in which other carbides such as VC, WC, etc
form in preference to Fe3C.
As seen in the expanded Fe- Fe3C phase diagram below, an alloy
with 0.02C heated to 1150 oC will be in the g-phase at point a.
On slow cooling, the alloy enters the g
+ a two phase region at point b.
On further cooling, below point c, it
enters the a–phase region.
Quenching the alloy from point a to d
will suppress the g  a
transformation, which will then occur
isothermally at the temperature d.
(cont’d)
Note: we’re on the far left
hand side of the Fe-C phase
diagram.
High Strength Low Alloy Steels (HSLA)
If the steel contains the microalloying elements, V, Ti, Nb, Cr, Mo
and W, the g  a transformation will still occur isothermally
at temperature d but at the same time, the stable alloy carbide
phases, VC, TiC, NbC, WC etc will also precipitate.
As the alloy carbide phase is
nucleated at the boundary between
the austenite and the ferrite phases,
this is called
“interphase precipitation”.
Note: we’re on the far left
hand side of the Fe-C phase
diagram.
Interphase Precipitation in 0.15C–0.75V HSLA Steel
g-phase
a-phase
precipitates
Shown is a
micrograph of
the steel after
quenching to 725
oC and holding
for 5 min.
Interphase Precipitation in 0.15C–0.75V HSLA Steel
The heat treatment for this steel would be to:
• Austenitize at 1150 oC and then quench from 1150 oC to
700
– 850 oC and hold at this temperature.
During the holding treatment, the following reactions occur
simultaneously:
• Isothermal transformation of g  a and interphase
precipitation of VC.
The g  a transformation occurs by the movement of ledges 5 –
50 mm thick that sweep along the boundary between the two
phases.
Interphase Precipitation in 0.15C–0.75V HSLA Steel
The precipitation forms on the a/g boundary in the time interval
between the passing of successive ledges, and then grows into the a
–phase where diffusion of the alloying element is more rapid than in
What enables higher diffusion in a?
austenite.
As the precipitates grow while the ledges move away, the size of the
particles increases as the front continues to move, as shown.
g-phase
a-phase
precipitates
Other Methods of Strengthening HSLA Steels
Solid Solution Strengthening
Solid solution strengthening is achieved by the addition of
elements such as Mn, Ni and Co, which partition to the ferrite
phase rather than the carbide.
AISI 1020 is a plain carbon steel containing 0.3-0.6 %Mn and
0.18-0.23 %C.
The equivalent HSLA steels are:
AISI 1320 containing 1.6-1.9 %Mn and 0.18-0.23 %C
AISI 2317 containing 0.4-0.6 %Mn, 0.18-0.23 %C and 3 %Ni
To increase weldability and formability for auto body
manufacturing, the carbon content of these HSLA steels is held
below 0.2 %C.
Other Methods of Strengthening HSLA Steels
Strain “Ageing”
The dislocation density of the substructure of HSLA steels can also
be increased by strain ageing.
In this process, the steel is given a light cold roll, or a final cold
pressing, as the finishing manufacturing process.
It is then aged at room temperature for two to three weeks before
assembly into a finished product. – Because this occurs,
Dr.
Hubert King studied this for his PhD project.
During ageing, interstitial carbon atoms in the ferrite phase
diffuse to the dislocations developed during the final cold
working process and increase the yield strength by Cottrell
locking.
What is Cottrell locking? Recall how is it seen on a stress-strain
curve?
Other Methods of Strengthening HSLA Steels
Dual Phase Steels
These HSLA steels have a typical composition of 0.12 %C, 1.7
%Mn, 0.58 %Si, 0.04 %V (Vanadium is used for
microalloying).
Their microstructure is composed of islands of martensite
embedded in a matrix of ferrite, which is produced by giving
the steel a “subcritical anneal” at ~800 oC (in the two phase g-a
region) and then it is quenched to room temperature.
Other Methods of Strengthening HSLA Steels
Dual Phase Steels (cont’d)
The ferrite is unaffected by the treatment but the austenite grains
transform to martensite during the quench as shown by the
light regions below and the steels are usually tempered at low
temperatures to increase ductility.
Dual phase steels have a yield strength
of 415-900 MPa with excellent work
hardening properties, which make
them very suitable for the
manufacture of pressed auto bodies.
Non-Metallic Inclusions in Structural Steels
Non-metallic inclusions such as oxides, nitrides, sulphides and
silicates are often embedded in structural steels.
In forming processes such as rolling and drawing, these become
strung out along the working direction causing a severe
reduction in ductility and fatigue properties, particularly in the
transverse direction.
Treatments to de-oxidize (e.g., Al – killed steels) or de-sulphurized
steels are based on the greater affinity of the alkaline earth
metals (e.g., Mg, Ca) for oxygen and sulphur.
Non-Metallic Inclusions in Structural Steels
Mg and Ca in the form of hydroxides, combine with the oxygen and
sulphur to form stable oxides or sulphides.
Carbonates or carbides are added to liquid steel after deoxidization to form stable oxides.
As these oxides and sulphides are less dense than the liquid metal,
they rise to the surface and become incorporated into the slag.
The slag is crushed, mixed with tar and used to make the surface of
our roads.
The slag is also ground to a fine powder to make cement, eg.,
Portland cement.
So the steel industry is indirectly responsible for our roads and
concrete buildings.
The Ca and Mg treatment reduces the remnant oxygen to less than
0.002% and the sulphur to less than 0.005% (i.e., by 1/10th).
Non-Metallic Inclusions in Structural Steels
In addition, the Ca and Mg oxides and sulphides tend to be
equiaxed compared to MnS, which forms as small rods. So, any
non-metallic inclusions remaining after the Ca or Mg treatment
do not increase the anisotropy of the mechanical properties of
the steel.
Manganese is an indispensable addition in steels because it reacts
with the sulfur remaining in the steel to form MnS. Without
Mn, the sulfur reacts with FeS, which is a liquid at the normal
hot-rolling temperature, which will induce the steel to split
during hot-rolling. MnS remains solid and deforms with the
steel during hot rolling.
Manganese was the first alloying addition (after C), which enabled
steel to be ductile sufficiently for many applications including
ship building.
The End
(Any questions or comments?)