High Strength Low Alloy Steels (HSLA)

2. 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.

3. 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.

4. 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.

5. 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 plain-carbon steels. The steel is used for nuclear vessels and steam generators.

6. High Strength Low Alloy Steels (HSLA) Ferrite and tempered bainite form in A533 grade B quenched from 900 oC and tempered at 620 oC.

7. 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, whilethe 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.

8. 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?

9. 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.

10. High Strength Low Alloy Steels (HSLA) Steels A203 grade D and A553 type 1contain 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 temperatureapplications.

11. 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-temperatureapplications such as pressure vessels and for the transport of liquified natural gas (-170 oC)

12. 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 containsCr 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.

13. 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. High Strength Low Alloy Steels (HSLA) 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 dwill 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.

14. 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. High Strength Low Alloy Steels (HSLA) 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.

15. g-phase a-phase precipitates Interphase Precipitation in 0.15C–0.75V HSLA Steel Shown is a micrograph of the steel after quenching to 725 oC and holding for 5 min.

16. 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.

17. g-phase a-phase precipitates 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 austenite. As the precipitates grow while the ledges move away, the size of the particles increases as the front continues to move, as shown. What enables higher diffusion in a?

18. 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.

19. 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 threeweeks 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?

20. 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 martensiteembedded 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.

21. 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.

22. 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.

23. 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 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).

24. 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.

25. The End(Any questions or comments?)