CORROSION ENGINEERING (MCB 4423)SEMESTER SEPT, 2015

ASSIGNMENT 1CARBON STEEL USAGE IN OFFSHORE STRUCTURES & PROPERTIES

STUDENT:THIBANKUMAR A/L ARUMUGAM 15956 MECHANICAL ENGINEERING

DATE OF SUMMISSION:TUESDAY, 29th OCTOBER 2013

Carbon SteelCarbon steel is the most common type of steel used in the industry. The properties of a certain carbon steel depend on the amount of carbon present in it. There are 3 types of carbon steels which are low carbon steel, medium carbon steel, and high carbon steel, and as their name suggest all these types of plain carbon steel differ in their carbon content.

Low Carbon SteelAlso known as mild steel. Contains carbon content up to 0.25%. Heat treatment improves the ductility but has no effect in respect of its strength properties.

Medium Carbon SteelContains carbon content ranging from 0.25% to 0.70%. Heat treatment improves the machinability. This steel is made machined and forged where surface hardness is desirable.

High Carbon SteelContains carbon content ranging from 0.70% to 1.05%. When fully treated, it improves hardness with the ability to withstand high shear and wear, and will thus be subjected to little deformation.

Relationship Between Carbon Content, Toughness, and HardnessHardness is defined as the ability of a solid matter to resist permanent change. Toughness is defined as the ability of a material to absorb energy and plastically deform without fracturing or cracking. In carbon steel, the increase in carbon content increases its hardness while reduces toughness. A decrease in carbon content sees an opposite mechanical property wear hardness decreases while toughness increases.

Figure 1: Inverse properties of Toughness to Hardness mechanical properties.

Mechanical Properties of Carbon SteelMaterialDensity(x103 kgm-3)Thermal Conductivity(Jm-1K-1s-1)Thermal Expansion (10-6K-1)Youngs Modulus (GPa)Tensile Strength (MPa)Elongation (%)

0.2% Carbon Steel7.865011.721035030

0.4% Carbon Steel7.854811.321060020

0.8% Carbon Steel7.844610.82108008

The tensile strength and yield strength of carbon steel typically decreases with an increase in temperature. This is not always true as some carbon steel might see a decrease in actual tensile strength, and then increase due to strain aging.

Mechanical Properties of AISI 1018 Mid/Low Carbon SteelMetric

Hardness, Brinell126

Hardness, Knoop (Converted from Brinell hardness)145

Hardness, Rockwell B (Converted from Brinell hardness)71

Hardness, Vickers (Converted from Brinell hardness)131

Tensile Strength, Ultimate440 MPa

Tensile Strength, Yield370 MPa

Elongation at Break (In 50 mm)15.0 %

Reduction of Area40.0 %

Modulus of Elasticity (Typical for steel)205 GPa

Bulk Modulus (Typical for steel)140 GPa

Poissons Ratio (Typical For Steel)0.290

Machinability (Based on AISI 1212 steel. as 100% machinability)70 %

Shear Modulus (Typical for steel)80.0 GPa

Creep Properties of Carbon SteelCreep can be defined simply as time-dependent strain occurring under constant stress. There are basically three stages of creep identified primary, secondary, and tertiary. Primary creep is the initial instantaneous elastic strain from the applied load, followed by a region of increasing inelastic strain at a decreasing strain rate. Secondary creep occurs when the creep rate is nominally constant at a minimum rate. Tertiary creep is characterized by a drastically increased strain rate with rapid extension to fracture.

For carbon steels, these time-dependent properties dominate the allowable stress above about 750F (400C), although creep begins to occur in carbon steels at about 700F (370C). Because the creep rupture strength is heavily influenced by temperature, the allowable stress drops off rapidly above that temperature.

Using the Larson-Miller Parameter (LMP) the life expectancy of carbon steel can be estimated.

Where C is a constant assumed to be 20 for carbon and low alloy steels, and t is the time to failure in hours.

Chemical Properties of Carbon SteelCarbon. Carbon is the most important alloying element in steel and can be present up to 2% (although most welded steels have less than 0.5%). The carbon can exist either dissolved in the iron or in a combined form, such as iron carbide (Fe3C). Increased amounts of carbon increase hardness and tensile strength as well as response to heat treatment (hardenability). On the other hand, increased amounts of carbon reduce weldability.

Manganese. Steels usually contain at least 0.3% manganese, which acts in a three-fold manner: it assists in de-oxidation of the steel, prevents the formation of iron sulfide inclusions, and promotes greater strength by increasing the hardenability of the steel. Amounts up to 1.5% are commonly found in carbon steels.

Silicon. Usually, only small amounts (0.2%, for example) are present in rolled steel when silicon is used as a deoxidizer. However, in steel castings, 0.351.0% is common. Silicon dissolves in iron and tends to strengthen it. Weld metal usually contains approximately 0.5% silicon as a deoxidizer. Some filler metals can contain up to 1.0% to provide enhanced cleaning and de-oxidation for welding on contaminated surfaces. When these filler metals are used for welding of clean surfaces, the resulting weld metal strength will be markedly increased. The resulting decrease in ductility could present cracking problems in some situations.

Sulfur. This is an undesirable impurity in steel rather than an alloying element. Special effort is made to eliminate or minimize sulphur during steelmaking. In amounts exceeding 0.05%, it tends to cause brittleness and reduce weldability. Additions of sulfur in amounts from 0.1% to 0.3% will tend to improve the machinability of steel but impair weldability. These types of steel can be referred to as free machining.

Phosphorus. Phosphorus is also considered to be an undesirable impurity in steels. It is normally found in amounts up to 0.04% in most carbon steels. In hardened steels, it tends to cause embrittlement. In low-alloy, high-strength steels, phosphorus can be added in amounts up to 0.10% to improve both strength and corrosion resistance, although it is not generally added for this reason in carbon steels.

Chromium. Chromium is a powerful alloying element in steel. It is added for two principal reasons: first, it greatly increases the hardenability of steel; second, it markedly improves the corrosion resistance of iron and steel in oxidizing types of media. Its presence in some steels could cause excessive hardness and cracking in and adjacent to the weld. Stainless steels contain chromium in amounts exceeding 12%.

Molybdenum. This element is a strong carbide former and is usually present in alloy steels in amounts less than 1.0%. It is added to increase hardenability and to elevate temperature strength.

Nickel. Nickel is added to steels to increase their hardenability. It performs well in this function because it often improves the toughness and ductility of the steel, even with the increased strength and hardness. Nickel is frequently used to improve steel toughness at low temperatures.

Vanadium. The addition of vanadium will result in an increase in the hardenability of steel. It is very effective in this role, so it is generally added in minute amounts. In amounts greater than 0.05%, there can be a tendency for the steel to become embrittled during thermal stress relief treatments.

Columbium. Columbium (also called niobium), like vanadium, is generally considered to increase the hardenability of steel. However, due to its strong affinity for carbon, it can combine with carbon in the steel to result in an overall decrease in hardenability.

Other alloying elements. Some carbon steel specifications allow additions of certain other elements, but they are not deliberately added. Other specifications might list these elements as a specified addition to the steel, but the addition would be minor in carbon steels.

Carbon Equivalent of Carbon SteelCarbon equivalent is used to understand how the different alloying elements effect the hardness of the steel. Higher concentrations of carbon and other alloying elements tend to increase the hardness of the steel. Each of these elements tend to influence the hardness of the steel to different magnitudes. The formula to determine the carbon equivalent of carbon steel is as below.

A 106 Grade B PipeA 106 Grade B Pipe conforms to American Society for Testing and Materials standards and specifications.

ApplicationsA 106 Grade B pipes are used in power plants, boilers, petrochemical plants, oil and gas refineries, and ships where the piping must transport fluids and gases that exhibit high pressures and temperatures.

Figure 1: Example of an ASTM A 106 Grade B pipe.

Mechanical Properties for A 106 Grade B PipeValue (MPa)

Tensile Strength, min 415

Yield Strength, min240

Longitudinal (%)Transverse (%)

Elongation in 50mm, min Basic minimum elongation transverse strip tests, and for all small sizes tested in full section When standard round 50mm gage length test specimen is used For longitudinal strip tests For transverse strip tests, a deduction for each 0.8mm decrease in wall thickness below 7.9mm from the basic minimum elongation of the following percentage shall be made30

22

a

16.5

12

1.00

Footnote:a. where=minimum elongation in 50.8mm in percentage, rounded to the nearest0.5%=cross-sectional area of the tension test specimen in mm2=specified tensile strength in MPa

Chemical Requirements by Composition Percentage for A 106 Grade B PipeComposition (%)

Carbon, maxa0.30

Manganese0.29-1.06

Phosphorus, max0.035

Sulfur, max0.035

Silicon, min0.10

Chrome, maxb0.40

Copper, maxb0.15

Molybdenum, maxb0.15

Nickel, maxb0.40

Vanadium, maxb0.08

Footnotes:a. For each reduction of 0.01% below the specified carbon maximum, an increase of 0.06% manganese above the specified maximum will be permitted up to a maximum of 1.35%.b. The five elements combined shall not exceed 1%.

It is to be noted that there are several grades for this pipe, which are Grade A and Grade C with their own mechanical and chemical properties as tabulated in the table below.

Mechanical Properties for A 106 Grade A & A 106 Grade C PipeGrade A: Value (MPa)Grade C: Value (MPa)

Tensile Strength, min 330485

Yield Strength, min240275

Longitudinal (%)Transverse (%)Longitudinal (%)Transverse (%)

Elongation in 50mm, min Basic minimum elongation transverse strip tests, and for all small sizes tested in full section When standard round 50mm gage length test specimen is used For longitudinal strip tests For transverse strip tests, a deduction for each 0.8mm decrease in wall thickness below 7.9mm from the basic minimum elongation of the following percentage shall be made35

28

a

25

20

1.0030

20

a

16.5

12

1.00

Footnote:b. where=minimum elongation in 50.8mm in percentage, rounded to the nearest0.5%=cross-sectional area of the tension test specimen in mm2=specified tensile strength in MPa

Chemical Requirements by Composition Percentage for A 106 Grade A & A 106 Grade C PipeGrade A: Composition (%)Grade C: Composition (%)

Carbon, maxa0.250.35

Manganese0.27-0.930.29-1.06

Phosphorus, max0.0350.035

Sulfur, max0.0350.035

Silicon, min0.100.10

Chrome, maxb0.400.40

Copper, maxb0.150.15

Molybdenum, maxb0.150.15

Nickel, maxb0.400.40

Vanadium, maxb0.080.08

Footnotes:c. For each reduction of 0.01% below the specified carbon maximum, an increase of 0.06% manganese above the specified maximum will be permitted up to a maximum of 1.35%.d. The five elements combined shall not exceed 1%.

API 5L X65The API stands for American Petroleum Institute, while API 5L addresses seamless and welded steel line pipe for pipeline transmissions in the petroleum and natural gas industries. The two digits after the X indicates the minimum yield strength in thousands psi (000 psi). In comparison to ASTM A 106 graded pipes, the API 5L graded pipes have stricter requirements where rolled grades are not acceptable, and reworks are not allowed.

In addition to that, API 5L graded pipes is further segregated into two specific groups which are PSL 1, and PSL 2. PSL 1 and PSL 2 are standards that are placed to help with the even more stricter requirements of each grade of pipes. In general terms, PSL 1 is a loose standard quality for line pipes while PSL 2 contains additional testing and requirements such as chemical and physical requirements, different upper limits for the mechanical properties, requires specific Charpy Impact Test conditions.

ApplicationsUsed for conveying gas, water and oil in the natural gas and oil industries. It is preferred in long pipelines due to inexpensiveness, and resistance to crack propagations. In addition to that, it is also used especially for sour pipelines.

Mechanical Requirements for API 5L X65 PSL 1GradeYield Strength, Minimum (MPa)Ultimate Tensile Strength, Minimum (MPa)Elongation in 50.8mm, Minimum (%)

X65448531a

Mechanical Requirements for API 5L X65 PSL 2GradeYield Strength, Minimum (MPa)Yield Strength, Maximum (MPa)Ultimate Tensile Strength, Minimum (MPa)Ultimate Tensile Strength, Maximum (MPa)Elongation in 50.8mm, Minimum (%)

X65448600531758a

Footnote:c. where=minimum elongation in 50.8mm in percentage=applicable tensile test specimen area=specified minimum ultimate tensile strength in MPa

During Bend Test, API 5L X65 should not have any cracks in any portion of the pipe. In addition to that, no opening should occur at weld areas.

Chemical Composition by Percentage of Weight for API 5L X65 PSL 1For seamless pipes, the chemical requirements are:GradeCarbon,MaximumManganese,MaximumPhosphorus, MaximumSulfur, MaximumTitanium, MaximumOther

X650.281.400.0300.0300.04a,b

For welded pipes, the chemical requirements are:GradeCarbon,MaximumManganese,MaximumPhosphorus, MaximumSulfur, MaximumTitanium, MaximumOther

X650.221.450.0250.0150.06a,b

Chemical Composition by Percentage of Weight for API 5L X65 PSL 2For seamless pipes, the chemical requirements are:GradeCarbon,MaximumManganese,MaximumPhosphorus, MaximumSulfur, MaximumTitanium, MaximumOther

X650.241.400.0250.0150.06a,b

For welded pipes, the chemical requirements are:GradeCarbon,MaximumManganese,MaximumPhosphorus, MaximumSulfur, MaximumTitanium, MaximumOther

X650.221.450.0250.150.06a,b

Footnotes:a. Columbium/Niobium, Vanadium, or any other combinations of contents may be used with prior consent of the purchaser.b. The sum of Columbium/Niobium, Vanadium, or any other combinations of contents should not exceed 0.15%

API 2H 50Also known as normalized carbon-manganese structural steel plate. Considered one of the work horse grade in offshore applications, API 2H 50 is an intermediate strength, normalized, structural steel plate used in the welded construction of offshore structures. It has a very low sulfur content and has excellent welding characteristics because of the low carbon content, which is at a maximum of 0.18%.

ApplicationsPrimarily used in critical portions in offshore structures, thus must exhibit excellent impact toughness and resistance to plastic fatigue loading and lamellar tearing. Example of use of API 2H 50 is in tubular joints, stiffened plate constructions, and other intersections where portions of the plates will be subjected to tension in the through-thickness (z-axis) direction.

Figure 1: Example of a tubular structure that would be used in constructions of offshore structures.

Mechanical PropertiesThickness, t (mm)0.2% Yield Strength (MPa)Ultimate Tensile Strength (MPa)Elongation in 50 mm, min. %Elongation in 200 mm, min. %

Less than 63.5345483 to 6202116

Greater than 63.5324483 to 6202116

Chemical Composition by Heat AnalysisCMnPSSiAlCbTiNV

%0.181.15- 1.600.0300.0100.05- 0.400.02-0.060.01-0.040.0200.012

API 2W 50Also known as TMCP steel plate. API 2W 50 is an intermediate strength, structural steel plate, produced by means of thermo-mechanical control processing (TMCP), that is commonly used in the welded construction of offshore structures. TMCP yields a fine-grained steel with high strength combined with high toughness and excellent formability. API 2W 50s low carbon content and carbon equivalence allows for better weldability.

Applications (Similar to API 2H 50)Primarily used in critical portions in offshore structures, thus must exhibit excellent impact toughness and resistance to plastic fatigue loading and lamellar tearing. Example of use of API 2W 50 is in tubular joints, stiffened plate constructions, and other intersections where portions of the plates will be subjected to tension in the through-thickness (z-axis) direction.

Figure 2: Example of a stiffened panel.

Mechanical PropertiesThickness, t (mm)0.2% Yield Strength (MPa)Ultimate Tensile Strength (MPa)Elongation in 50 mm, min. %Elongation in 200 mm, min. %

Less than 25.4345 - 5174482318

Greater than 25.4345 - 4834482318

Chemical Composition by Heat AnalysisCMnPSSiAlNiCrMoCu

%0.161.15- 1.600.0300.0100.05- 0.400.02-0.060.750.250.080.35

TiCbNB

%0.007-0.020.030.0120.0005