Plaın Carbon Steels
06 Kasım 2007PLAIN CARBON STEELS
Plain carbon steels are steels in which the alloying elements do not play a significant role in determining the properties of the metal. The two systems used are both based on carbon content. The phase diagram of the iron/carbon alloy indicates that at 0.8% carbon a eutectoid composition exists, and a pearlitic structure forms upon slow cooling. With carbon content less than 0.8%, steels are sometimes termed "hypo-eutectoid" and with carbon content greater than 0.8% we have "hypereutectoid" steels.
Unfortunately the classification based on eutectoid structure is coarse and little used for practical purposes. Instead, low, medium and high carbon steel are the preferred groupings used.
MEDIUM CARBON STEELS
These steels usually include 0.25-0.5% carbon have high strength and toughness in a quenched and tempered condition. Quenching can be broughtabout by cooling in water, forced air, oil, etc.. Therefore, caution should be taken when cooling a work piece after welding. Large sections that have been welded may also cool rapidly due to heat, dissipation. It is, therefore, recommended that heavier sections are first given some preheat to minimize this effect and to allow a slower cooling rate. These higher carbon steels are generally used where higher tensiles, yield strength, hardness and impect values are required and a detrimental element against obtaining some of these higher values is hydrogen. During welding hydrogen can be absorbed in and around the weld pool and during cooling this hydrogen can be trapped, leaving pockets of trapped gas, some of which then forces its way out to the surface, causing cracks to open up. This can still occur hours or even days later resulting in surface and underbead cracks. The remaining trapped hydrogen leaves fissures and pockets inside the metal. It is most important, therefore, to select. Special low hydrogen electrodes for welding fine grain steels. Basic coated electrodes are designed specifically for this purpose and can be relied upon to give the required results.
Standard Chemical Composition (%) Mechanical Properties
JIS ASTM AISI C Si Mn S P Cr Ni Mo V Yield Strength Kg/mm3 Tensile Strength Kg/mm3 Elongation % Brinell Hardness
SCC3 90-60 1040 0.30-0.40 0.30-0.60 0.50-0.80 0.04 max 0.04 max — — — — 28-40 56-65 15 min 146-192
SCC5 105-85 1050 0.40-0.50 0.30-0.60 0.50-0.80 0.04 max 0.04 max — — — — 30-45 63-70 10 min 163-201
Figure.1 Properties of some medium carbon steels
Structure of Medium Carbon Steel
The essential difference between ordinary steel and pure iron is the amount of carbon in the former, which reduces the ductility but increases the strength and the susceptibility to hardening when rapidly cooled from elevated temperatures.
Above about 820°C, the microstructure consists of austenite. This transforms to ferrite as the steel cools. The amount of ferrite increases as the temperature decreases, while the amount of austenite decreases. The solubility of carbon is much lower in the ferrite than in the austenite, so the carbon concentration of the austenite increases as the temperature decreases.
At approximately 730° the remaining austenite, which now has a carbon concentration of about 0.8wt%, transforms for pearlite. This is the eutectoid of ferrite and iron carbide, Fe3C. The iron carbide is also known as cementite.
At low magnifications, the pearlite is the dark phase, and the light phase is the ferrite. The amount of pearlite in this steel is moderate due to the 0.4wt% carbon content.
At higher magnifications, the lamellar eutectoid structure of the pearlite can be observed. This structure is due to the simultaneous formation of ferrite and Fe3C from the austenite. The spacing of the lamellae increases with slower cooling rates.
Figure.2 Transformation of a 0.4% C hypoeutectoid
plain-carbon (medium carbon steel) steel with slow cooling.
The structure in figure.3 consists of two constituents, the white one being the ferrite, and the dark parts representing the constituent containing the carbon, the amount of which is therefore an index of the quantity of carbon in the steel. Carbon is present as a compound of iron and carbon (6-67 %) called cementite, having the chemical formula Fe3 C. This cementite is hard (Brinell hardness 600 +), brittle and brilliantly white.
Figure.3 0,4% carbon steel. Ferrite + Pearlite
PRODUCTION OF STEEL
Removing impurities, such as carbon, from raw or pig iron, produced by a blast furnace, produces steel. The main industrial process is the basic oxygen process, in which molten pig iron and scrap steel is placed in a container lined with heat-resistant, alkaline (basic) bricks. A pipe or lance is lowered near to the surface of the molten metal and pure oxygen blown through it at high pressure. The surface of the metal is disturbed by the blast and the impurities are oxidized (burned out). The open-hearth process is an older steelmaking method in which molten iron and limestone are placed in a shallow bowl or hearth. Burning oil or gas is blown over the surface of the metal, and the impurities are oxidized.
High-quality steel is made in an electric furnace. A large electric current flows through electrodes in the furnace, melting a charge of scrap steel and iron. The quality of the steel produced can be controlled precisely because the temperature of the furnace can be maintained exactly and there are no combustion by-products to contaminate the steel. Electric furnaces are also used to refine steel, producing the extra-pure steels used, for example, in the petrochemical industry.
The steel produced is cast into ingots, which can be worked when hot by hammering (forging) or pressing between rollers to produce sheet steel. Alternatively, the continuous-cast process, in which the molten metal is fed into an open-ended mould cooled by water, produces an unbroken slab of steel.
Blast Furnace
Smelting furnace used to extract metals from their ores, chiefly pig iron from iron ore. The temperature is raised by the injection of an air blast.
In the extraction of iron the ingredients of the furnace are iron ore, coke (carbon), and limestone. The coke is the fuel and provides the carbon monoxide for the reduction of the iron ore; the limestone acts as a flux, removing impurities.
The principle of reducing ferrous oxides by carbon has been known for thousands of years, but the present blast furnace was introduced c.1400. The fuel was originally charcoal and the resulting iron was either used for casting or converted to wrought iron or steel. Production increased with the use of coke as fuel in the 18th century.
Figure.4 Blast Furnace
Two methods of making steel are dominant in modern steel industries all over the world; Basic Oxygen Furnace and Electric Arc Furnace.
Figure.5 Pie chart to show relative amounts of steel produced by EAF and BOS methods.
The numbers are in millions of metric tones in 1997
Basic Oxygen Furnace
There are three main stages in the operation of the furnace.
1. Charging the furnace:
The BOS converter is charged first with scrap. This is used as a coolant. It helps to control the very high temperatures produced by the violently exothermic reactions in the furnace. After the scrap, three or four times as much hot metal (up to 300 tonnes) is poured into the furnace from a ladle.
2. Blowing:
After charging, the furnace is blown by blasting oxygen through a lance that is lowered into the molten metal. The furnace needs no heating because the oxygen combines very exothermically with the impurity elements, carbon, silicon, manganese and phosphorus. Carbon is oxidized to carbon monoxide and much of the carbon in the metal escapes as this gas. The other impurity elements also form oxides. These are acidic and are separated from the metal by adding basic calcium oxide (lime) to the furnace. This combines with the oxides and removes silicon, manganese and phosphorus in a slag.
3. Tapping the furnace:
After the blow has continued for about 20 minutes, the metal is sampled. The BOS process is now complete and the furnace can be tapped. Steel is run out of the tap hole into a ladle, separating it from the lighter slag, which is later emptied as waste.
Figure.6 Basic Oxygen Furnace
Electric Arc Furnace
The Electric Arc Furnace (EAF) offers an alternative method of bulk steel manufacture. It makes steel from what would otherwise be unsightly and environmentally damaging scrap metals. It also consumes much less energy than the BOS furnace. Every tonne of EAF steel uses about 7.4 GJ of energy compared with about 16.2 GJ for every tonne of BOS steel.
Furnace Design: The EAF is a kettle-shaped structure with a removable lid. The three graphite electrodes that heat the furnace pass through this lid, which can be swung back when the furnace is being charged. The hearth of the EAF, where the metal is melted, is lined with a chemically basic and refractory material.
The sequence of operations is similar to that in the BOS furnace, except that, after charging, the charge must be melted down. The furnace charge melts when an electric arc passes between the electrodes and the scrap metal. The temperature around the arc rises to 1200oC and a 100 tones charge can be melted in about 60 minutes.
The four main stages are:
1. Charging with a mixture of metal and lime.
2. Melting the metal and scrap using electric arcs from the graphite electrodes. After this some more lime is added to clear out the oxides in the next step.
3. Blowing with oxygen to oxidize elements such as carbon, silicon and manganese in the scrap metal. As in the BOS furnace, carbon monoxide escapes as a gas. The oxides of the other elements are acidic and combine with the basic lime to make a neutral slag, which is poured off the surface.
4. Tapping the metal itself by running it out through the furnace spout into a ladle. The liquid in the ladle is now ready for secondary steelmaking and casting. Further treatments of the metal from the EAF take place in much the same way as steel from the BOS plant.
Continuous Casting
The modern steel industry uses continuous casting, which is more efficient. This technique allows molten steel from the ladle to be cast directly into the basic shape that the customer wants. By adjusting the water-cooled moulds in the continuous caster, steel sections can be produced in the three basic shapes; slabs, blooms and billets.
Figure.7 Graphic of slab, bloom and billet
SOME EXAMPLES OF MEDIUM CARBON STEEL
AISI 1030
Composition
Element Weight %
C
0.28-0.34
Mn
0.60-0.90
P
0.04 (max)
S
0.05 (max)
Mechanical Properties
Properties Conditions
T (°C) Treatment
Density (×1000 kg/m3) 7.7-8.03
25
Poisson’s Ratio 0.27-0.30 25
Elastic Modulus (GPa) 190-210
25
Tensile Strength (Mpa) 463.7
25
annealed at 845°C more
Yield Strength (Mpa) 341.3
Elongation (%) 31.2
Reduction in Area (%) 57.9
Hardness (HB) 126 25
annealed at 845°C more
Impact Strength (J)
(Izod) 69.4
25
annealed at 845°C more
Thermal Properties
Properties Conditions
T (°C) Treatment
Thermal Expansion (10-6/ºC) 11.7
15-75
Typical Application
Used for engineering, forging and heat treated automotive applications. Can be flame or induction hardened. Widely used for machine components having good machined performance and having moderate strength in the as rolled condition. Class 4.6 bolts and screws.
AISI 1035
Composition
Element Weight %
C
0.32-0.38
Mn
0.60-0.90
P
0.04 (max)
S
0.05 (max)
Mechanical Properties
Properties Conditions
T (°C) Treatment
Density (×1000 kg/m3) 7.7-8.03
25
Poisson’s Ratio 0.27-0.30 25
Elastic Modulus (GPa) 190-210
25
Tensile Strength (Mpa) 485
25
cold drawn (round bar (50-75 mm)) more
Yield Strength (Mpa) 415
Elongation (%) 10
Reduction in Area (%) 30
Hardness (HB) 143 25
cold drawn (round bar (50-75 mm)) more
Thermal Properties
Properties Conditions
T (°C) Treatment
Thermal Expansion (10-6/ºC) 14.8
20-700 more
annealed
Typical Application
Intermediate carbon higher in strength and hardness than low carbon steel. Used for studs, bolts etc.
AISI 1040
Composition
Element Weight %
C
0.37-0.44
Mn
0.60-0.90
P
0.04 (max)
S
0.05 (max)
Mechanical Properties
Properties Conditions
T (°C) Treatment
Density (×1000 kg/m3) 7.845
25
Poisson’s Ratio 0.27-0.30 25
Elastic Modulus (GPa) 190-210
25
Tensile Strength (Mpa) 518.8
25
annealed at 790°C more
Yield Strength (Mpa) 353.4
Elongation (%) 30.2
Reduction in Area (%) 57.2
Hardness (HB) 149 25
annealed at 790°C more
Impact Strength (J)
(Izod) 44.3
25
annealed at 790°C more
Thermal Properties
Properties Conditions
T (°C) Treatment
Thermal Expansion (10-6/ºC) 13.6
20-1000 more
annealed
Electric Properties
Properties Conditions
T (°C) Treatment
Electric Resistivity (10-9-m) 171
20 more
Typical Application
Used for general engineering and bright drawn applications as well as for bed frame applications in angle shapes. Can be flame or induction hardened. Widely used for machined components having good machining performance and having moderate to high
strength in the as rolled condition. Commonly used for bright drawn shafting. Class 5.8 bolts
and screws oil quench 860°C and tempered 600°C.
AISI 1045
Composition
Element Weight %
C
0.43-0.50
Mn
0.60-0.90
P
0.04 (max)
S
0.05 (max)
Mechanical Properties
Properties Conditions
T (°C) Treatment
Density (×1000 kg/m3) 7.7-8.03
25
Poisson’s Ratio 0.27-0.30 25
Elastic Modulus (GPa) 190-210
25
Tensile Strength (Mpa) 585
25
cold drawn, annealed (round bar (16-22 mm)) more
Yield Strength (Mpa) 505
Elongation (%) 12
Reduction in Area (%) 45
Hardness (HB) 170 25
cold drawn, annealed (round bar (19-32 mm)) more
Thermal Properties
Properties Conditions
T (°C) Treatment
Thermal Expansion (10-6/ºC) 15.1
0-700 more
annealed
Typical Application
Widely used for machined components having good machining performance and having high strength in the as rolled condition. Commonly used for axles and shafts.
AISI 1050
Composition
Element Weight %
C
0.48-0.55
Mn
0.60-0.90
P
0.04 (max)
S
0.05 (max)
Mechanical Properties
Properties Conditions
T (°C) Treatment
Density (×1000 kg/m3) 7.7-8.03
25
Poisson’s Ratio 0.27-0.30 25
Elastic Modulus (GPa) 190-210
25
Tensile Strength (Mpa) 636.0
25
annealed at 790°C more
Yield Strength (Mpa) 365.4
Elongation (%) 23.7
Reduction in Area (%) 39.9
Hardness (HB) 187 25
annealed at 790°C more
Impact Strength (J)
(Izod) 16.9
25
annealed at 790°C more
Typical Application
Strain hardened, stress relieved material typically 100 KSI yield strength. Great for making hammers, dies and other applications where hardness and extreme toughness is needed.
USAGE AREAS IN THE INDUSTRY
Some examples made of medium carbon steel:
Bolts, Screws, Studs, Track Links, Pins, Bushings, Roller Shells, Sprockets, Hammers, Automotive and Machinery Applications, Fasteners, Couplings, Simple Gears, Flanges, Gas Piston Rods, Constant Velocity Joints, Shafts, Reduction Gears, Knuckles, Gears, Connecting Rods, Nuts (Car & Motorcycle Parts, Railroad Noise Wall, Bridge Concrete Pump Truck, Machinery Equipment, Machinery Axle Of Motor Speed Reducer Robot)
Fasteners: Over 90 percent of all fasteners are manufactured using carbon steel. The reason is quite simple. Steel has excellent workability, offers the broadest range of attainable comparison with other commonly used fastener materials, it’s inexpensive.
• Grade 5 Fasteners: These fasteners are quenched and tempered for the additional strength necessary for most automotive uses and other applications where strength is a moderate concern. The grade marking on the head of a Grade 5 fastener is three equally spaced lines coming out from the center of the head. Manufacturers` identifications are added for traceability.
• Grade 8 Fasteners: These fasteners are manufactured for the most demanding applications. These fasteners are then quenched and tempered to superior strength and hardness qualities. The grade marking on a Grade 8 fastener is six equally spaced lines coming out from the center of the head. Once again, the manufacturer’s mark is necessary for traceability.
Grade 5
(Three radial lines) Use for small engine repair and for automotive needs
Grade 8
(Six radial lines) Automotive needs and other applications where high tensile strength is required
Bolts:
BOLT TYPE Made of SIZE RANGE GENERAL INFORMATION
Cap Screws Steel (Grade 5) 1/4" x 1/2" to 1-1/2" x 8" Grade 5, medium-carbon heat-treated in a carbon-controlled atmosphere. Minimum tensile strength of 120,000 psi in diameters through 1", and 105,000 psi for diameters over 1" through 1-1/2". Grade 5 will also meet other specifications such as ASTM A449.
Cap Screws Steel (Grade 
1/4" x 1/2" to 1" x 7" Grade 8, alloy, heat-treated in a carbon-controlled atmosphere. Minimum tensile strength of 150,000 psi. Grade 8 will also meet ASTM A354 Grade BD.
Plow - Standard Medium carbon steel, quenched & tempered 5/16" x to 1" x Albany Steel stocks one standard type of plow bolt: the No. 3 with round countersunk head and square neck.
Plow - Dome Head Steel (Grade 5/16" x to 1" x Dome head plow bolts put extra metal at the wearing surface where it’s needed. They give added life to the assembly by resisting shock and abrasion.
Structural A325 TYPE 1 Medium carbon steel 1/2" x 1-1/4" to 1-1/4" x 8" Our fastener plants produce and stock a complete line of high-strength structural fasteners. They include bolts, nuts and washers to specification A325 Type 1 and Type 3 (Weath-R), and also to A490 specification requirements.
Figure.8 Bolt types
Weldability of Medium Carbon Steel
Welding is a materials joining process in which two or more parts are coalesced at their contacting surfaces by the suitable application of heat and or pressure
Weldability might be considered by some a subjective term but basically it refers to a material’s capacity to be welded under the conditions of design and fabrication and then to perform as expected during its service life. One of the most significant factors determining steel’s weldability is its ability to resist cracking in the weld area, and its weldability is sometimes evaluated on the basis of cracking sensitivity.
Medium carbon steel has a carbon range of 0.30 to 0.50%. Its weldability is considered good.
Although carbon content is not the only factor affecting weldability, it is generally thought that as carbon content increases, weldability decreases. When carbon levels reach the 0.30 to 0.35% range, special precautions, such as preheating, controlling heat input and postweld heat-treating, are normally required. The use of low-hydrogen electrodes is often recommended for successful welding.
