Oil and gas are the prime sources of energy around the world. In most countries oil and gas contribute to over 66 of the total energy consumed by the country. Practically all of the gas and oil are transported over land through pipelines. Unfortunately the costs of designing these pipelines are increasing day after day with the replacement cost reaching as high as £100 000 million! This is merely due to the increase in demand for energy and so does the demand for more pipelines. Nowadays pipelines up to 5000km have been built and pipes of diameters of over 42 inches are becoming more common. However the main aim of the pipeline construction engineer is to make an excellent and reliable pipeline that is free from faults. To do so he will require great knowledge of construction as well a standard to which he will adhere and work according to it. This is to make sure that the qualities of pipelines are maintained. (1) Such standard is developed by the American Petroleum Institute (API).
"The American Petroleum Institute, commonly referred to as API, is the main U.S trade association for the oil and natural gas industry, representing about 400 corporations involved in production, refinement, distribution, and many other aspects of the petroleum industry." (2) The American Petroleum Institute publishes specifications and standards which are intended for use in the petroleum industry. The API specification 5L is the specification used for steel pipelines used for the transportation of oil and gas in the petroleum industry. This specification establishes requirements for two product specification levels (PSL1 and PSL2). This specification basically tells the manufacturer about the different mechanical and chemical properties that a pipe of certain dimensions must have in order to perform its job without failure. This report will show the different mechanical properties such Yield Strength, Ultimate tensile Strength, percentage elongation, toughness and ductility followed by various tests to measure these properties. It will also show the different chemical compositions of the different grades of steel and their effect on the mechanical properties.
Line Pipe Physical Properties (3)
API 5L Grade
Yield Strength
min.
(ksi)
Tensile Strength
min.
(ksi)
Yield to Tensile Ratio
(max.)
Elongation
min.
1
A
30
48
0.93
28
B
35
60
0.93
23
X42
42
60
0.93
23
X46
46
63
0.93
22
X52
52
66
0.93
21
X56
56
71
0.93
19
X60
60
75
0.93
19
X65
65
77
0.93
18
X70
70
82
0.93
17
X80
80
90
0.93
16
1
API 5L elongation figures vary with specimen dimensions. Values shown for 0.2 sq. in. specimen.
Table
Here Table 1 shows the different physical and mechanical properties of the different grades of steel. The far left hand side column shows the different grades of steel. A and B are the standard grades while X is used for stronger grades followed by the minimum Yield strength.
The next column shows the minimum Yield Strength for each grade. Yield Strength is defined as "the stress required to produce a very slight yet specified amount of plastic strain; where a strain offset of 0.002 is commonly used." (4) Basically it is how much pressure is needed change the length by a specific amount. We can straight away see that The minimum Yield Strength is increasing down the table with grade A having a minimum Yield Strength of 30 Ksi and grade X80 having a minimum Yield Strength of 80 Ksi. [1] The middle column shows the minimum Tensile Strength. Tensile Strength is defined as "The maximum engineering stress, in tension, that may be sustained without fracture." (4) In other words its how much force it can endure before breaking. Here the values are also increasing down the table with grade X80 having the largest minimum tensile strength of 90 Ksi. The rest of the table shows the yield to tensile ratio, which should not exceed 0.93, and the percentage elongation. Percentage elongation is a measure of the ductility of the material.
These values can be determined by performing what is called a Tensile Test and drawing a Strain -Stress diagram for each type of steel. During this Test a specimen prepared to specific dimensions is placed in a Tensile testing machine and is subjected to various loads. The change in length is recorded until fracture and the Stress and Strain are calculated using the equations Stress= and Strain=. A graph of Stress (y-axis) and Strain (x-axis) is drawn. (Figure : Typical Stress-Strain diagram)
Stress-strain diagram of medium-carbon structural steel
Figure : Typical Stress-Strain diagram (9)
As shown is figure 1 we can find the Tensile Strength ( Ultimate Strength), Yield Strength ( at the Yield Point) and the percentage elongation which is the strain expressed as a percentage. Percentage elongation is a measure of ductility. Ductility is "a measure of the degree of plastic deformation that has been sustained at fracture" (4) It is very important for the material to have good ductility so it can be made into pipes without fracturing under stress. Another property which can be found from the stress Strain diagram is toughness of the material. Toughness is simply how much energy a material can absorb before breaking. Toughness is found by calculating the area under the graph. For a material to be tough it must also show strength and ductility. Toughness could be measured by a Charpy Impact Test. During this test a specimen is prepared to specific dimensions and is placed between two horizontal bars, a swinging pendulum then hits the bar at the notch causing to fracture. The height of the pendulum before and after impact is used to calculate the impact energy. (5) Fortunately these mechanical properties could be adjusted be changing the chemical composition of the material
Most Steels used in making pipes are alloys of Iron, carbon and other elements such as manganese, chromium, vanadium and tungsten. "These elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and the form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but such steel is also less ductile than iron." (6) As Iron and carbon are the main alloying elements in steel we will look at the iron-iron carbide phase diagram to determine the different phases.
iron-ironcarbidediagram.JPG
Figure : iron-iron carbide phase diagram (10)
First of all before saying how the phase diagram connects with the properties of steel we will take a quick look at the properties of a typical iron-iron carbide phase diagram.
There are three different phases of iron present. These are Ferrite (α) which has a BCC (Body Centered Cubic) crystal structure, Austenite (ɣ) which has an FCC (Face Centered Cubic) crystal structure and delta-Ferrite (δ) which also has a BCC crystal structure. All these phases are interstitional solid solutions because the carbon atom being so small compared to the iron atom it can be found in the space between the iron atoms. There is also a Liquid phase, cementite phase ( Fe3C) and many other intermediate phases. The red lines are the solubility limits of the different phases. We can see from the diagram that the Austenite has the highest solubility of all three phases as it can dissolve up to 2wt.carbon.
Most steels have carbon content between 0.2 and 2.1 as seen in the figure above. This is because below 2 carbon, iron forms an eutectoid alloy. Which has a pearlite structure. Pearlite is a layer like structure of alternating Ferrite and cementite. If the amount of carbon increases more than 2 cementite will precipitate which is very brittle and will give the material brittle properties.
In general, as the amount of carbon increases the tensile strength and the yield strength also increase to about 0.83 after which they remain constant. This is shown in Figure 3.http://info.lu.farmingdale.edu/depts/met/met205/c-vs-hrdn.jpg
Figure : Effect of carbon on tensile strength and yield strength of steels. (7)
Figure : Effect of carbon on impact strength and ductility (7) "The tensile strength is affected as the ratio of ferrite to cementite in the structure of steel changes. As the percentage of pearlite increases in the hypoeutectoid steels, the tensile strength increases. The hypereutectoid steels show only a slight increase in strength as the cementite-to-ferrite ratio increases." (7)http://info.lu.farmingdale.edu/depts/met/met205/c-vs-impact.jpg
"Figure 4 shows the effect of carbon on the ductility and toughness of steel. The elongation and the reduction in area drop sharply with increase in carbon content, going almost to zero at about 1.5 carbon. This indicates that the carbon content of 1.5 or more will cause high brittleness. The impact resistance also decreases very sharply up to about 0.83 carbon and then levels out." (7)
Some other alloying elements are added to raise or lower the critical temperature at which austenite is most stable. Elements like manganese, nickel and cobalt are used to lower the critical temperature because of their high solubility in austenite. (7). Alloying elements are also added to improve Tensile Strength, Toughness, Hardenability and ductility. Here are some common alloying elements and there properties:
Manganese (Mn) - improves hardenability, ductility and wear resistance. Mn eliminates formation of harmful iron sulfides, increasing strength at high temperatures.
Nickel (Ni) - increases strength, impact strength and toughness, impart corrosion resistance in combination with other elements.
Chromium (Cr) - improves hardenability, strength and wear resistance, sharply increases corrosion resistance at high concentrations (> 12).
Tungsten (W) - increases hardness particularly at elevated temperatures due to stable carbides, refines grain size.
Vanadium (V) - increases strength, hardness, creep resistance and impact resistance due to formation of hard vanadium carbides, limits grain size.
Molybdenum (Mo) - increases hardenability and strength particularly at high temperatures and under dynamic conditions.
Silicon (Si) - improves strength, elasticity, acid resistance and promotes large grain sizes, which cause increasing magnetic permeability.
Titanium (Ti) - improves strength and corrosion resistance, limits austenite grain size.
Cobalt (Co) - improves strength at high temperatures and magnetic permeability.
Zirconium (Zr) - increases strength and limits grain sizes.
Boron (B) - highly effective hardenability agent, improves deformability and machinability.
Copper (Cu) - improves corrosion resistance.
Aluminum (Al) - deoxidizer, limits austenite grains growth.
Table (8)
In the end we conclude that in order to make high quality pipelines that follow the API standards, The mechanical and chemical properties of the material must be known. As we have seen in this paper we must know the Tensile Strength, Yield Strength, Toughness, Hardeness and ductility of the metal used. These properties as we have seen earlier depend on the chemical composition of the Steel. We have also seen that these properties could be altered through alloying with other metallic elements. It is obvious that pipeline engineering is not easy and requires a lot of hard work and determination but with the help of modern technologies nothing is impossible.
