Iron ore is obtained in the conventional method of open cast or underground mining and conveying the ore to the surface preparation are where it is crushed, washed, and transported to the smelter.
Here the ore is put into a blast furnace along with limestone and coke and subjected to hot air blasting at 1000C (which is can be heated using the exhaust gasses from the furnace much like a boiler economiser) is blasted into the bottom of the furnace through several tuyeres.
The coke is used as a fuel to produce the high temperatures required by the process of smelting the ore in a blast furnace. It produces CO which adds heat, as well as acting as a reducer removing the oxygen content from the ore.
Limestone is used as a flux to separate the gangue from the molten iron ore, the accumulated slag and the molten iron being tapped from two tap-holes at the bottom of the furnace. The slag goes to a disposal area and the molten iron is directed into molds known as pigs where it solidifies to pig iron and is transferred to the next stage of processing.
Converting Pig Iron to Wrought Iron and Steel.
The pig iron can be further refined to produce steel or wrought iron. Both these methods reduce the carbon content of the pig iron, which in turn reduces the brittleness property of the metals.
Wrought Iron Process
Wrought iron is produced by pummeling the pig iron using mechanically driven hammers in a wrought iron works. This process prepares it for the next stage which is the heating and forging to the desired wrought iron designs and shapes.
This used to be an art of the blacksmith, but is now normally carried out by machines.
Steel Process
This process is used to remove impurities from the pig iron produced in a blast furnace. It has developed from the early methods used such as an open hearth, or a Bessemer furnace. Today we normally use an Electric Arc Furnace (EAF) or a Basic Oxygen Furnace. (EAF)
Both methods remove a high percentage of the carbon content from the pig iron, along with the impurities of aluminum, calcium (produced during tapping), sulphur, silicon, manganese, and phosphorus.
- Electric Arc Furnace (EAF)
The vertical circular steel furnace is brick-lined and has a roof which contains graphite electrodes. The electrodes are withdrawn and the roof is lifted to facilitate the loading of the pig iron to the bottom of the furnace. This is followed by a known quantity by scrap steel, which is dependent on the final grade of steel required.
Once the roof is replaced, the electrodes are lowered and an initial voltage is applied to them, and when the scrap steel has started to melt, the main voltage is applied.
When the metal is molten, oxygen is injected into it, with the impurities being converted to oxides and forming a slag. The oxygen also reacts with the carbon forming CO which is combusted adding heat to the process. Oxygen injection also reduces the carbon, nitrogen and hydrogen content on completion of the process, samples are taken and analyzed, and if acceptable, steel with content of up to 1% of carbon is poured into ingots.
- Basic Oxygen Furnace (BOF)
If the furnace has been designed to be linked to the blast furnace process, molten iron is carried in crucibles and poured into the BOF. This is a much more efficient method of removing the bulk of carbon and impurities rather than using solid pig iron
The crucible of molten iron can undergo pretreatment by the injection of magnesium, iron oxide to remove the sulphur, phosphorus, and silicon, or these impurities can be removed during the BOF process.
The BOF furnace is filled up to 20% scrap steel, and then the molten iron is poured into the furnace from the crucible.
Oxygen is injected through a water cooled steel lance into the molten metal promoting the same effects as in the EAF steel processing system.
CARBON
When carbon is added to ferrite(pure iron), it combines to form iron carbide or cementite, which lies side by side with ferrite in laminations. the whole structure is called PEARLITE. As carbon is increased, more cementite, thus more pearlite is formed, with a reduction in the free ferrite. At 0.83%, ferrite doesnt exist, and the whole structure is only pearlite. Further increase in carbon increases free iron carbide with decrease in pearlite
When carbon is added to ferrite(pure iron), it combines to form iron carbide or cementite, which lies side by side with ferrite in laminations. the whole structure is called PEARLITE. As carbon is increased, more cementite, thus more pearlite is formed, with a reduction in the free ferrite. At 0.83%, ferrite doesnt exist, and the whole structure is only pearlite. Further increase in carbon increases free iron carbide with decrease in pearlite
Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite. This means carbon content can be both a blessing and a curse when it comes to commercial steel.
And while there are steels that have up to 2 percent carbon content, they are the exception. Most steel contains less than 0.35 percent carbon.Most commercial steels are classified into one of three groups:
- Plain carbon steels
- Low-alloy steels
- High-alloy steels
Plain Carbon Steels
These steels usually are iron with less than 1 percent carbon, plus small amounts of manganese, phosphorus, sulfur, and silicon. The weldability and other characteristics of these steels are primarily a product of carbon content, although the alloying and residual elements do have a minor influence.
Plain carbon steels are further subdivided into four groups:
Low. Often called mild steels - <0.30% carbon.They machine and weld nicely and are more ductile than higher-carbon steels. used in structural steels
Low. Often called mild steels - <0.30% carbon.They machine and weld nicely and are more ductile than higher-carbon steels. used in structural steels
Medium carbon steel 0.30 to 0.45% carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining. used in gearing.
High. 0.45-0.75% carbon, these steels can be challenging to weld. Preheating, postheating (to control cooling rate), and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.
Very High. 0.75-1.5% carbon content, very high-carbon steels are used for hard steel products such as metal cutting tools and truck springs. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.
Low-alloy Steels
- When these steels are designed for welded applications, their carbon content is usually below 0.25 percent and often below 0.15 percent.
- Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.
- These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them. Preheating might also prove necessary.
- Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.
- These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them. Preheating might also prove necessary.
High-alloy Steels
STAINLESS STEEL and CAST IRON
For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12% chromium and many have high nickel contents. The three basic types of stainless are:
For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel. Stainless steels are at least 12% chromium and many have high nickel contents. The three basic types of stainless are:
- Austenitic
- Ferritic
- Martensitic
Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone (HAZ).
Ferritic stainless steels have 12 to 27 percent chromium with small amounts of austenite-forming alloys.
Austenitic stainless steels offer excellent weldability, but austenite isn't stable at room temperature. Consequently, specific alloys must be added to stabilize austenite. The most important austenite stabilizer is nickel, and others include carbon, manganese, and nitrogen.
Special properties, including corrosion resistance, oxidation resistance, and strength at high temperatures, can be incorporated into austenitic stainless steels by adding certain alloys like chromium, nickel, molybdenum, nitrogen, titanium, and columbium. And while carbon can add strength at high temperatures, it can also reduce corrosion resistance by forming a compound with chromium. It's important to note that austenitic alloys can't be hardened by heat treatment. That means they don't harden in the welding HAZ.
One more high-carbon metal, cast iron. The carbon content of cast iron is 2.1 percent or more. The carbon can be in the form of cementite or graphite or both.
There are four basic types of cast iron:
One more high-carbon metal, cast iron. The carbon content of cast iron is 2.1 percent or more. The carbon can be in the form of cementite or graphite or both.
There are four basic types of cast iron:
Gray cast iron ,
which is relatively soft. It's easily machined and welded, and you'll
find it used for engine cylinder blocks, pipe, and machine tool
structures.
White cast iron, which is hard, brittle, and not weldable. It has a compressive strength of more than 200,000 pounds per square inch (PSI), and when it's annealed, it becomes malleable cast iron
Malleable cast iron, which is annealed white cast iron. It can be welded, machined, is ductile, and offers good strength and shock resistance.
Ductile cast iron, which is sometimes called nodular or spheroidal graphite cast iron. It gets this name because its carbon is in the shape of small spheres, not flakes. This makes it both ductile and malleable. It's also weldable.While most varieties of cast iron are brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.
Effects of alloys in steel
NICKEL
Low carbon steels+upto 8% Ni increases strength and toughness
Low carbon steels+(10-20%) Ni makes it brittle but high tensile strength
Med carbon steels+(20-30)% Ni used in turbine blades n I.C. engine valves
CHROMIUM
Increases hardness and resistance to wear
SHIPs STEEL
NAMC SiPS (NAMCe vasthe SiPS)
Nitrogen-Al-Mn-C-Si-P-S
nitrogen- <0.005% and makes ship brittle
Aluminium <0.5% and usu exists as impurity
Manganese purifying agent. it combines with sulphur in iron. It increases STD (strength, toughness, ductility). >0.7%
Carbon incr SEH (tensile strength elasticity hardness and hardenability) but decreases ductitlity n toughness <0.18%
Silicon oxidises with gases and thus is anti-porous. makes steel tougher. >0.3%
Phosphorous COLD SHORTNESS - highly brittle at normal and cold temp. inc S but decr TD <0.05%
Sulphur RED SHORTNESS <0.05%
As per Lloyds, grades of steel are A,B,C,D,E. have same Strength but changes in TD.
GRADE E
UTS - 400-500 N/sq.mm
Toughness 27Joules @ -40degrees
elongation 22%
N<0.005%,Al 0.015%, Mn <0.7%, C <0.18%,Si 0.35%, P &S at 0.035%
ANNEALING - improves structural homogenity and restores ductility.
Heated to 30-50degrees above higher critical temp for hypo eutectoid steel and 30-50 below lower critical temp for hyper eutectoid steel. Then held for 3-4minutes/mm of highest thickness. Cooled in a furnace (slow cooling)at a rate of 150-200degrees/hr.
NORMALISING - eliminates coarse grain str during forging/casting, and improves machinability of LOW CARBON STEEL
30-4- degrees above upper critical temp
held for 15min
cooled by stanstill air
HARDENING - develop higher hardness
heated to 30-50 degrees
above higher critical temp for hypo eutectoid steel and 30-50 below lower critical temp for hyper eutectoid steel.
held for short duration n quenched in oil or water or molten salt bath
TEMPERING - to reduce some of the hardness produced during hardening and gives toughness
heated to temp below critical temp and varies for diff metals
held till uniform temp is attained
may be quenched or cooled
NITRIDING- provides a hard surface and resistance to corrosion, erosion and fatigue.
heated to 500-650 degrees in presence of ammonia. Nitrogen combines with material alloys to form its respected nitrides.
held for 50hrs to get depth of 0.2-0.4mm
TESTS HITFB
TENSILE STRENGTH - tensile strength n ductility
UTS max load/c.s.a
% of elongation and % of contraction area
IMPACT TEST - shock absorbing capacity
used in determining differences in materials due to heat treatment, welding, and these are not indicated by tensile strength
BEND TEST
BRINELL HARDNESS TEST - resistance to wear
indenting material with 10mm hardened steel ball for 15sec. BHN=load area/surface area of indentation
White cast iron, which is hard, brittle, and not weldable. It has a compressive strength of more than 200,000 pounds per square inch (PSI), and when it's annealed, it becomes malleable cast iron
Malleable cast iron, which is annealed white cast iron. It can be welded, machined, is ductile, and offers good strength and shock resistance.
Ductile cast iron, which is sometimes called nodular or spheroidal graphite cast iron. It gets this name because its carbon is in the shape of small spheres, not flakes. This makes it both ductile and malleable. It's also weldable.While most varieties of cast iron are brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.
Effects of alloys in steel
NICKEL
Low carbon steels+upto 8% Ni increases strength and toughness
Low carbon steels+(10-20%) Ni makes it brittle but high tensile strength
Med carbon steels+(20-30)% Ni used in turbine blades n I.C. engine valves
CHROMIUM
Increases hardness and resistance to wear
SHIPs STEEL
NAMC SiPS (NAMCe vasthe SiPS)
Nitrogen-Al-Mn-C-Si-P-S
nitrogen- <0.005% and makes ship brittle
Aluminium <0.5% and usu exists as impurity
Manganese purifying agent. it combines with sulphur in iron. It increases STD (strength, toughness, ductility). >0.7%
Carbon incr SEH (tensile strength elasticity hardness and hardenability) but decreases ductitlity n toughness <0.18%
Silicon oxidises with gases and thus is anti-porous. makes steel tougher. >0.3%
Phosphorous COLD SHORTNESS - highly brittle at normal and cold temp. inc S but decr TD <0.05%
Sulphur RED SHORTNESS <0.05%
As per Lloyds, grades of steel are A,B,C,D,E. have same Strength but changes in TD.
GRADE E
UTS - 400-500 N/sq.mm
Toughness 27Joules @ -40degrees
elongation 22%
N<0.005%,Al 0.015%, Mn <0.7%, C <0.18%,Si 0.35%, P &S at 0.035%
ANNEALING - improves structural homogenity and restores ductility.
Heated to 30-50degrees above higher critical temp for hypo eutectoid steel and 30-50 below lower critical temp for hyper eutectoid steel. Then held for 3-4minutes/mm of highest thickness. Cooled in a furnace (slow cooling)at a rate of 150-200degrees/hr.
NORMALISING - eliminates coarse grain str during forging/casting, and improves machinability of LOW CARBON STEEL
30-4- degrees above upper critical temp
held for 15min
cooled by stanstill air
HARDENING - develop higher hardness
heated to 30-50 degrees
above higher critical temp for hypo eutectoid steel and 30-50 below lower critical temp for hyper eutectoid steel.
held for short duration n quenched in oil or water or molten salt bath
TEMPERING - to reduce some of the hardness produced during hardening and gives toughness
heated to temp below critical temp and varies for diff metals
held till uniform temp is attained
may be quenched or cooled
NITRIDING- provides a hard surface and resistance to corrosion, erosion and fatigue.
heated to 500-650 degrees in presence of ammonia. Nitrogen combines with material alloys to form its respected nitrides.
held for 50hrs to get depth of 0.2-0.4mm
TESTS HITFB
TENSILE STRENGTH - tensile strength n ductility
UTS max load/c.s.a
% of elongation and % of contraction area
IMPACT TEST - shock absorbing capacity
used in determining differences in materials due to heat treatment, welding, and these are not indicated by tensile strength
BEND TEST
BRINELL HARDNESS TEST - resistance to wear
indenting material with 10mm hardened steel ball for 15sec. BHN=load area/surface area of indentation
FATIGUE TEST - limiting fatigue stress
results of tests at differents stress levels are plotted in a SN curve. It gives endurance limit/fatigue limit, which is the the highest stress that a material can withstand for an infinite number of cycles without breaking —called also endurance limit
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