High Strength Low Alloy Steels (HSLA)

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 hot-worked steels containing fine pearlite.


We are going to look at the properties of these steels in detail in the following slides.

Note the low C


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 plain-carbon steels.

The steel is used for nuclear vessels and steam generators.


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 BThe 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.


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 low-temperature 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 -phase at point a.

On slow cooling, the alloy enters the + two phase region at point b.On further cooling, below point c, it enters the –phase region.Quenching the alloy from point a to d will suppress the 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 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

-phase

-phaseprecipitates

Shown is a micrograph of the steel after quenching to 725 oC and holding for 5 min.


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 and interphase precipitation of VC.The transformation occurs by the movement of ledges 5 – 50 m

thick that sweep along the boundary between the two phases.


The precipitation forms on the boundary in the time interval between the passing of successive ledges, and then grows into the –phase where diffusion of the alloying element is more rapid than in austenite.As the precipitates grow while the ledges move away, the size of the particles increases as the front continues to move, as shown.

-phase

-phaseprecipitates

What enables higher diffusion in ?

Other Methods of Strengthening HSLA SteelsSolid Solution StrengtheningSolid 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 %CAISI 2317 containing 0.4-0.6 %Mn, 0.18-0.23 %C and 3 %NiTo 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 SteelsStrain “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 SteelsDual Phase SteelsThese 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 region) and then it is quenched to room temperature.

Other Methods of Strengthening HSLA SteelsDual 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 SteelsNon-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 SteelsMg 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 de-oxidization 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 SteelsIn 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?)

Documents

High Strength Low Alloy Steels (HSLA)