Steels for Structural Applications

Transcription

1 Lecture 5 Steels for Structural Applications Dr. Javad Mola Institute of Iron and Steel Technology (IEST) Tel: mola@iest.tu-freiberg.de

2 Applications of Structural Steels

3 Hot Finished Products Example of H-beam (I-beam or double T) processing C

4 Hot Finished Products Example of Q&T heavy plate processing Slab reheating Descaling Hot rolling Cutting Quenching and Tempering Anti-corrosion paint The Steel Book: SSAB Communications, Lena Westerlund, Printing: Henningsons Tryckeri AB, Borlänge 2012.

5 Microstructure Cooling rate after hot rolling controls the final microstructure for a given composition. If necessary, martensitic microstructures may be produced by accelerated cooling. Banded microstructure of ferrite and pearlite after hot rolling followed by slow cooling

6 Microstructure Homogenized and normalized microstructures after hot rolling Steel A Steel B

7 Effect of Cooling Rate Homogenized and normalized Austenitized 1 min at 950 C before cooling at different rates

8 Mechanical Properties Grain size Grain size Base value Base value mm Proposed for CMn steels containing %C B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

9 Partitioning of Stress and Strain B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

10 Historical Developments Riveted Riveted The original 1961 bridge with welded joints collapsed shortly after construction

11 Types of Structural Steels The trend for structural steels used in the construction of bridges and buildings is to replace mild steels with HSLA steels. For many years, ASTM A 7 (now ASTM A 283, grade D) was the most common type of structural steel. In about 1960, improved steelmaking methods resulted in the introduction of ASTM A 36, with improved weldability and slightly higher yield strength. Nowadays, HSLA steels often provide a superior substitute for ASTM A 36, because they have a higher yield strength and at the same time a good weldability. Weathering HSLA steels offer a better atmospheric corrosion resistance than carbon steels. Structural steels can be classified on the basis of their chemistry and processing in the following categories: Carbon Steels High-Strength Low-Alloy Steels Heat-Treated Carbon and HSLA Steels Heat-Treated Constructional Alloy Steels R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

12 Carbon Steels A steel may be classified as a carbon steel if: 1. the maximum content specified for alloying elements does not exceed the following: 1.65% manganese, 0.60% silicon, 0.60% copper 2. the specified minimum for copper does not exceed 0.40% 3. no minimum content is specified for other elements added to obtain a desired alloying effect. Approximate EN equivalent S235JR S275JR S275JR R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

13 High-Strength Low-Alloy Steels Low-alloy steels contain alloying elements, including C, up to a total alloy content of about 8 %. High-strength low-alloy (HSLA) steels have specified minimum yield points greater than 40 ksi (275 MPa) and achieve that strength in the hot-rolled condition, rather than by heat treatment. Because these steels offer increased strength levels with a moderate increase in price, they are economical for a variety of applications. Approximate EN equivalent S355J2WP S355J2W S355JR S355JR The most commonly-used beams in construction R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

14 Heat-Treated Carbon and HSLA Steels Both carbon and HSLA steels can be heat treated (normalized/quenched and tempered) to provide yield points in the range of 50 to 75 ksi (345 to 520 MPa). This provides an intermediate strength level between as-rolled HSLA steels and heattreated constructional alloy steels. Approximate EN equivalent P275NL, S275NL P355N L485MB R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

15 Heat-Treated Low-Alloy Steels These are low-alloy steel with sufficient hardenability which are heat treated (quenched and tempered) to obtain a combination of high strength and toughness (also known as quenched and tempered low-alloy steels and constructional alloy steels). Having a yield strength of 100 ksi (690 MPa), these are the strongest steels in general structural use. Approximate EN equivalent P690QL S690QL R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

16 Heat-Treated Low-Alloy Steels ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, Ohio

17 Tensile Properties R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

18 Stress, MPa Tensile Properties St E 690 St E 690 St E 460 St 52-3 St 37-3 Room temperature tensile properties of round specimens (DIN grade designations) St E 460 St 37-3 Strain, % St 52-3 Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

19 Section Sensitivity of Strength For a given chemical composition, higher strength levels may be achieved in smaller sections. Martin Anderson, Charles J. Carter, Are you properly specifying materials?,

20 High-Strength Low-Alloy Steels High-strength low-alloy (HSLA) steels, also known as microalloyed steels, are used in applications where a high strength and a moderate level of formability is required. HSLA steels are basically low C structural CMn steels with less than 0.1 %C, %Mn, and microalloying additions of Nb, Ti, or V which are strong carbideand nitride-forming elements. Role of microalloying elements: - Strengthening by grain refinement (ferrite grain sizes below 10 m) - Solid solution strengthening - Precipitation strengthening Example applications: Plates for naval vessels and offshore structures, forgings and welded connectors in tension-leg platforms, pressure vessel steels used for transportation of compressed natural gas, steels for line-pipe, formable steels used for automotive & truck parts.

21 CMn Steel vs. HSLA Typical mechanical properties for standard S235 constructional steel (4 mm thick) Chemical composition of a typical constructional steel and its HSLA equivalent. Typical mechanical properties for S460MC HSLA steel (4.55 mm thick) B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

22 CMn Steel vs. HSLA B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

23 Controlled-Rolling Process for Grain Refinement Controlled-rolling process and associated changes in the microstructure T nr H. Sekine, T. Tanaka, C. Ouchi, eds., in: Thermomechanical Process. High-Strength Low-Alloy Steels, Butterworth-Heinemann, 1988

24 Delayed Static Recrystallization of Austenite The main effect of the microalloying elements is the retardation of the recrystallization of deformed which eventually leads to the development of a fine-grained microstructure. HSLA steels are hot rolled at relatively low temperatures in the homogenous -field. Below a composition-dependent temperature known as T nr, there is a strong retardation of the static recrystallization of the deformed, so that the interpass time (interval between consecutive rolling passes) is not sufficiently long to allow for the recrystallization of austenite. An empirical relationship for recrystallization stop temperature is as follows: T nr ( C)=887 + (6445%Nb) + (890%Ti) + (732%V) + (464%C) + (363%Al) - (357%Si %Nb %V)

25 Delayed Static Recrystallization of Austenite Effect of solute Nb on the softening at 900 C of a 20 ppm C steel due to static recrystallization of austenite. Effect of microalloying elements Nb, Ti, and V on the recrystallization stop temperature of a Fe-1.4Mn-0.25Si-0.07C steel. B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

26 Softened fraction Delayed Static Recrystallization of Austenite Softened fraction Interpass time, sec Interpass time, sec

27 Precipitation Strengthening by Microalloy Carbides Effect of cooling rate on the increase in yield strength due to precipitation strengthening in a 0.15%V steel. Effect of Nb on yield strength for various sizes of niobium carbide particles. ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, Ohio

28 Precipitation Strengthening by Microalloy Carbides The reduced solubility of TiC and NbC carbides in ferrite compared to austenite results in further precipitation of these carbides during the transformation. The formation of fine carbides during transformation acts as an additional contribution to the strength by dispersion hardening. These precipitates which most often form at / interphase boundaries appear as well-defined rows of precipitates in the final ferritic microstructure. Rows of vanadium carbides/nitrides 100 nm B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

29 Hot-Rolled Microstructure The final hot rolled microstructure can be adjusted by controlled cooling after hot rolling on the run-out table. A low coiling temperature inhibits the coarsening of fine precipitates. B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

30 Application of HSLA Steels Proportion of HSLA steel used for different applications relative to carbon and alloy steel (1986 statistics)

31 (YS/UTS)x100 YS/UTS Ratio In conventional structural design the working stress is usually taken as a proportion of the yield stress; typical values are 60% YS in normal loading and up to 80% in severe loading. The YS/UTS ratio is largely irrelevant for such elastic cases. More recently, structures have been designed using plastic design concepts whereby the ability of the structure to yield and redistribute load without catastrophic failure is required. In such cases, the post-yield strain-hardening behavior of the steel is of increasing importance. Bainite Ferrite + Pearlite Bainite Tempered Martensite Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, Yield Strength, MPa

32 Crack opening in cracked plates, mm Plastic Design Concept At temperatures where the material is relatively ductile and the development of a critical strain is required for fracture, a high strain hardening exponent (high n, low YS/UTS) increases the energy required to produce failure. In the DBTT transition regime and lower temperatures, however, a critical stress law is valid and a low n may enhance the resistance to crack propagation. HIGH LOW above DBTT A.C. Bannister and S.J. Trail, Structural Integrity Assessment Procedures For European Industry: The Significance Of The Yield Stress/Tensile Stress Ratio To Structural Integrity, British Steel plc Report Total strain, %

33 Toughness: C Normalized steels, carbon levels above solubility limit ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, Ohio

34 Toughness: P and Si Optimum Si in view of toughness B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

35 Toughness: Mn, P, and C Fe 3 C-free Increased grain boundary cohesion Containing Fe 3 C B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

36 Toughness: Grain Size DBTT, C Lower strength Easier crack propagation (lower chance of crack deflection at grain boundaries) Fe-C-Mn-Nb steel Ferrite Grain Size, µm

37 Toughness Impact energy, J Toughness of some structural steels (DIN designations) St E 690 St E 355 St E 460 Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, St 52-3 RSt 37-2 Temperature, C

38 Toughness Positive effect on toughness Grain refinement by transformation at lower temperatures (Mn, Cr, Mo) by accelerated cooling after hot rolling by micro-alloying and controlled rolling Alloying with Ni (Fe-9Ni martensitic steels for cryogenic applications) Alloying with Mn Negative effect on toughness Non-metallic inclusions Banded microstructure Presence of pearlite at higher C contents Precipitation strengthening Prior deformation Si solid solution strengthening B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

39 Anisotropy of Toughness Impact energy, J l 1 l 2 t 2 t 1 Anisotropy of toughness in St 52-3 steel with 0.02%S. l and t indicate longitudinal and transverse specimens, respectively. Subscript 1 : high rolling reduction ratio Subscript 2 : low rolling reduction ratio Temperature, C Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

40 Weathering Steels Small amounts of Cu, Cr, Ni, Si or P can substantially improve the atmospheric corrosion resistance of structural steels by the formation of a stable protective rust layer on the steel surfaces exposed to the periodic wetting and drying cycles that characterize atmospheric corrosion. This allows them to be used in non-painted conditions as they slowly develop an adherent brownish patina. The addition of 0.35%Cu is very effective in improving the resistance to atmospheric corrosion. Improvement of atmospheric corrosion resistance in presence of the above elements is because they catalyze the formation of a compact nanoscale rust layer containing the dense and stable -FeOOH and -FeOOH variants of iron oxy-hydroxide B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

41 Atmospheric Corrosion Resistance Average Reduction in Thickness, mils (10-3 in.) Time of Exposure, years R. L. Brockenbrough, Structural Steel Designer s Handbook: Properties Of Structural Steels And Effects Of Steelmaking And Fabrication, 3rd ed., McGraw-Hill Professional, 1999.

42 Welding

43 Welding Effect of a change in the peak temperature of weld thermal cycles from 1000 C to 1400 C on the hardenability characteristics 1000 C (fine-grained austenite) 1400 C (coarsegrained austenite)

44 Welding Hardness HV10 Peak Hardness HV10 Welding speed 0.4 m/min Underbead crack St %C St E TM 0.08 %C Position Welding Speed, m/min Measurement points HAZ Heat Input, J/cm Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

45 Peak Hardness in the Heat-Affected Zone, HV3 Welding Steel Plate Thickness mm Yield Strength MPa %C %Mn %Nb %Mn/%C CE CE=%C+(%Mn/6) St E 355 Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, Mn-Nb steel Preheat Temperature, C An alternative crack-susceptibility parameter proposed by Ito (increased negative effect of C): d: plate thickness in mm, H: H content in cm 3 /100g

46 Welding Ratio of bend angle (bending angle if welded divided by bending angle in the unwelded condition) for normalized steel plates. A high value of the ratio indicates proper weldability. Thickness, in.

47 Cold Cracking after Welding Cold cracking is the term given to hydrogen-induced cracking in welds and weld heat-affected zones. Hydrogen from moisture in the air, from fluxes or electrode coatings can readily diffuse into heat-affected zone surrounding weld metal. The microstructure most sensitive to cold cracking is martensite. Trapping sites for hydrogen: alloying element atoms, dislocations, carbide and inclusion interfaces, etc. A critical combination of hydrogen concentration and triaxial stress state is required for crack initiation. An incubation period is necessary for crack initiation. This period is related to the time for hydrogen diffusion to the triaxial stress field at the root of a notch or crack. The low strength associated with hydrogen brittle fracture of steel is attributed to the weakening of the cohesive or bond strength between iron atoms by hydrogen. It is important to estimate the tendency of base metal to form martensite, essentially hardenability, in heat-affected zones. Numerous empirical carbon equivalent formulae and cracking susceptibility parameters are available.

48 Surface hardness, HV10 Thermal Cutting Embrittlement caused by thermal cutting is mainly due to: Carbon pick-up in the vicinity of the cut edge Rapid cooling and heating (residual stress and hard microstructural phases) St 52-3 (plate thickness=50 mm) St 37-2 (plate thickness=45 mm) Distance from cut surface, mm Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

49 Preheating before Welding and Cutting Preheating the steel prior to cutting and welding as well as decreasing the cutting/welding speed reduce the induced temperature gradients thereby serving to: (1) decrease the migration of carbon to hotter regions, (2) decrease the hardness, (3) reduce distortion, (4) reduce or give more favorable distribution to the thermally-induced stresses, and (5) reduce the formation of quench or cooling cracks. The need for preheating increases with: - Increased carbon and alloy content of the steel - Increased thickness of the steel, and - For parts having geometries that act as stress raisers.

50 Stress Relief Tensile or yield strength, MPa Transition temperature, C Effect of heat treatment for 1 hour at 580 C on the strength and toughness of St E 355 steel cold formed to different levels: the negligible change in strength and toughness after heat treatment shows that the microstructural changes associated with stress relieving heat treatment are small enough to ensure that the strength and toughness are not compromised. UTS YS DBTT (27J) Cold formed Cold formed +1 hr 580 C Cold forming strain, % Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

51 Stress Relief Residual stress, MPa St E 460 T: temperature in Kelvin t: time in hours St E 315 St 37-3 T(20+log t) 10-3 (Hollomon s parameter) Example: t: 1 hr T=580 C=853 K Hollomon s parameter=17060 Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

52 Steel vs Reinforced Concrete Steel: Constructability: can be erected as soon as the material is delivered on site. Strength: High strength, stiffness, toughness, and ductile properties. The high specific strength makes it the preferred choice in high-rise buildings. Fire resistance: The strength and stiffness are significantly reduced when heated to temperatures encountered in a fire scenario. The International Building Code requires that the steel is enveloped in sufficient fire-resistant materials which adds to the costs. Corrosion resistance : Steel, when in contact with water, can corrode, creating a potentially dangerous structure. The steel can be painted, providing water resistance. Reinforced concrete: Constructability: Consisting of portland cement, water, construction aggregate, and rebars, concrete is cheaper compared to structural steel. Concrete must be poured and left to set and cure (waiting time of 1-2 weeks). Off-site pre-cast concrete members may be used right after delivery to the construction site. Strength: Compared to structural steel, a larger volume is required for a structural concrete member to support the same load. This is a significant disadvantage in high-rise buildings. Reinforced concrete is often the preferred choice in low-rise buildings. Lack of tensile strength is compensated by reinforcing steel bars. Fire resistance: Excellent fire resistance properties Corrosion resistance: Excellent corrosion resistance properties (the steel reinforcement must not be exposed). Cracks in the concrete, for instance in regions under tension, may expose the steel in which case coating with epoxy resins is recommended (epoxy coated rebars). This however reduces the bond strength between rebars and concrete which in turn necessitates the use of larger and stronger reinforced concrete members.

53 Steels for Concrete Reinforcement (Rebar)

54 Steels for Concrete Reinforcement (Rebar) CE usually below 0.4 because of weldability reasons Formability (bending and in the case of mesh bars drawing) Strengthening by Microstructure control Solid solution strengthening Micro-alloying elements such as V (or Nb) and controlled rolling Work hardening by cold drawing, torsion, etc. Surface profile: Ribbed (superior bonding to concrete) Smooth Common types: Reinforcing bar Reinforcing mesh

55 Rebar Profiles Non-twisted rebar profiles Cold-twisted rebar profiles Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

56 Average Change in Yield Strength, MPa HSLA Rebars Micro-alloying with Nb and V Solubility product of various carbides and nitrides in austenite Niobium Vanadium Mass-%Niobium or Vanadium Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, ASM Handbook, Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, Materials Park, Ohio.

57 Yield Strength, MPa Tensile Strength, MPa Rebar Strengthening by Cold Deformation Strengthening of rebars by cold deformation (torsion) Nominal YS values Nominal UTS values Twisting Index Twisting Index Lay length of an intially longitudinal rib Twisting Index= Diameter of bar Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

58 Rebar Strengthening by Cold Deformation Yield Rp 0.2 or tensile R m strength, MPa Total Elongation A 10, % Change in the mechanical properties of a 0.15%C hot formed wire through cold deformation Cold deformation, % Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

59 Example Compositions and Properties Type of Rebar DIN Designation EN Designation YS, MPa UTS, MPa Total Elongation, % Rebar BSt 420 S B Rebar BSt 500 S B Mesh Rebar BSt 500 M B Typical compositions: 0.22 C, 0.60 Si max., 1.30 Mn, Nb/V microalloying 0.22 C, 0.60 Si max., 0.80 Mn, Nb/V microalloying 0.15 C, 0.60 Si max., 0.50 Mn (cold forming mesh grade)

60 Temperature, C Inducing Microstructure Gradient Through-thickness variation of microstructure Large R (near surface), martensite Ferrite+Pearlite Bainite Temperature equalization Martensite Cooling time, sec R intermediate, fine +P Small R (near core), coarse +P CCT diagram of a Fe-0.13C-1.21Mn steel rebar with a diameter of 20 mm. Cooling curves through the thickness are given. R indicates distance from the center of the bar. Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

61 Pre-Stressing Steels Why pre-stressing? To prevent the development of tensile stresses in concrete. Pre-stressing is a method for overcoming concrete's natural weakness in tension. Types of pre-stressing steel bars: Quenched and tempered low-alloy steels Cold worked (drawn) unalloyed steels Due to the high C contents and the high stresses to be carried by the pre-stressed steel, they may not be welded. This allows the use of higher C concentrations. Example types and compositions: Quenched and tempered (Q&T) Wire: 0.5C, 1.6Si, 0.6Mn, 0.4Cr Cold-drawn wire: 0.8C, 0.2Si, 0.7Mn Round bar: 0.7C, 0.7Si, 1.5Mn, 0.3V

62 Pre-Stressing Steel Profiles and Types Due to the higher stresses in the steel-concrete interface, the surface pattern of prestressing steels plays a more important role than it does in normal rebar steels. Ribbed and profiled bars and wires Stranded wire Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

63 Stress, MPa Strength of Pre-Stressing Steels Example EN designations: Y1770C: cold drawn wire for pre-stressing with a tensile strength of 1770 MPa Y1770S7: 7-strand steel for pre-stressing with a tensile strength of 1770 MPa Y1230H: Hot rolled or hot rolled and processed bar with a tensile strength of 1230 MPa Strain, % Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

64 Stress drop from the initial σ i value, % Stress Relaxation Stress relaxation of a 7mm-diameter wire made from St 1420/1570 steel grade (dashed regions are extrapolated values) 70% 60% 20 C Initial stress (σ i ) as percentage of tensile strength 80% 20 C 40 C Stress relaxation test Test temperature 20 C Relaxation time, hours Years Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

65 Strengthening by Q&T Yield Strength/Proof Stress/Tensile Strength, MPa Reduction of Area, % Total Elongation, % Control of mechanical properties by heat treatment Effect of tempering on mechanical properties of quenched St 1420/1570 prestressing steel Tempering Temperature, C Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.

66 Strengthening by Cold Drawing Tensile Strength, MPa Strengthening of a 0.8%C prestressing steel wire by drawing Degree of Deformation Wire Diameter, mm Werkstoffkunde STAHL - Band 2: Anwendung Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, Reduction of Area, %

67 Pipeline Steels

68 Major Requirements Weldability High strength for pressure containment High toughness even at subzero temperatures (particularly important to pipelines in arctic regions) Resistance to hydrogen-related failure Oil and gas pipelines are a classic application of HSLA steel, and one of the first applications involved the use of acicular ferrite steel for pipelines in the Arctic regions. The development of high-strength linepipe grades has permitted the use of large-diameter pipe operating at high pressures in excess of 11 MPa (1600 psi). Grades with minimum yield strengths up to 483 MPa (70 ksi) in thicknesses up to 25 mm (1 in.) are readily available. Tensile strength is a key requirement in linepipe steels. Other requirements include weldability, fracture toughness, and resistance to sour gas attack. In addition to higher strengths, HSLA steels can provide excellent toughness, good field weldability, resistance to ductile crack propagation, and, in some cases, resistance to sour gas and oil.

69 Development Timeline of Pipeline Steels YS in ksi Development of pipeline steels

70 Mechanical Properties of Pipeline Steels B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

71 Mechanical Properties of Pipeline Steels Strengthening Microstructural factors influencing strength and toughness of pipeline steels Toughening H.G. Hillenbrand, M. Gräf, C. Kalwa, Development and production of high strength pipeline steels, Niobium 2001, Orlando, FL, USA.

72 Pipeline Steel Grades Composition in mass-% of pipeline (linepipe) steels according to the European standard. X100 and X120 grades with a low carbon bainitic microstructure (superior strength and toughness compared to polygonal ferrite and pearlite) may be produced by a combination of thermomechanical control processing (TMCP) and accelerated cooling. B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

73 Plastic Design Concept Traditionally, the limit for allowable in-service stress is set at approximately 70-80% of the yield stress, pressure containment being the main focus. In this approach, yielding and plastic strain are avoided. In actual environmental conditions, however, linepipes are often subjected to strain from seismic activities, icebergs, permafrost melting, or ice formation in initially non-frozen soil. Installation of pipes may also cause small plastic strains. The pipeline must therefore be designed to tolerate in-service plastic deformation of a few percent. More recently, the strain-based design is becoming more popular. Strain-based design (plastic design concept) requires that a low ratio of YS/UTS is maintained. The yield strength of the steel must be low enough to ensure that the weld material (which has a rapidly solidified microstructure and is prone to the presence of defects which may act as local stress concentration sites) is not highly stressed. Many specifications list a maximum YS/UTS ratio of A ratio of 0.85 can be easily achieved in CMn grades but maintaining a low YS/UTS ratio is difficult in microalloyed steels with high strength levels.

74 Pipeline Steel Design Considerations B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.

75