- 1 Steel Definition
- 2 Types of Steel
- 3 Uses
- 4 Capabilities
- 5 History and Timeline
- 6 Limitations
- 7 Properties
- 8 How Steel is Made
- 9 What is Steel Made From?
- 10 Carbon Steel Groups
- 11 Cast Steel
- 12 Steel Forgings
- 13 Stainless Steel
- 14 Alloy Steel (Tool Steels)
- 15 Steel Annealing Process
- 15.1 Full Annealing
- 15.2 Partial Annealing
- 15.3 Isothermal Annealing
- 15.4 Steel Normalizing
- 15.5 Quenching
- 15.6 Tempering
- 15.7 Carbonitriding
- 15.8 Cyaniding
- 15.9 Flame Hardening
- 16 Induction Hardening
- 17 Nitriding
- 18 Special Steel
- 19 Free Brochures
- 20 References
Steel alloys and steel are ferrous metals, meaning they contain iron, making it an iron alloy. Ferrous metals are strong and durable, which is why steel is used in construction. The color of steel is blue-gray. Steel contains carbon in any amount up to about 1.7%.
Difference Between Ferrous vs. Nonferrous Metals
Nonferrous metals do not contain iron, while ferrous metals do contain iron.
Difference Between Iron and Steel
Steel is differentiated from cast iron by its carbon content and malleability. Steel has more or less carbon (.3 to 1.7% carbon content) than many types of iron. It has more carbon than wrought iron, but less than cast iron. To improve steel’s physical properties, carbon steel is combined with alloying elements such as nickel and chromium.
Types of Steel
- Plain Carbon Steel: Iron and carbon are the main parts of plain carbon steel. Carbon is the hardening element. Carbon steel also contains small amounts of manganese, silicon, phosphorus, and sulfur (iron sulfide or manganese sulfide.) More carbon is added for items such as metal cutting tools for lathes and drawing dies.
- Alloy Steel: Tougher alloy steel contains other elements such as chromium, nickel, and molybdenum.
As steel became more economical vs. alternatives, steel was used to build high-rise buildings, bridges with longer spans and elevated railroad crossings.
- Structural steel
- Tools likeChisels and Hammers
Depending on the type of steel, it can be welded, machined and forged.
History and Timeline
- 500 B.C.: Wootz steel is produced in India using a clap pot blast furnace
- The 1300’s: low-quality steel produced by the Egyptians called blister steel
- The 1400’s: Metallurgists removed impurities from cast iron through remelting through a process called “refining” which produced a malleable steel
- The 1700’s: Replaced brass and bronze in cannons and cannon balls made possible by improvements in steel refining.
- Three books on steel making are published:
- The Pirotechnia by Vannoccino Biringuccio
- De Re Metallica by Georgius Agricola
- Autobiography by Benvenuto Cellini (a metallurgist)
- Three books on steel making are published:
- 1740: Benjamin Huntsman develops the crucible process which reliably produces steel. Aids the adoption of steel around the world. Enables the Industrial Revolution in Western Europe.
- A modified reverberatory furnace is developed by Charles Siemens, a German settler in England. The furnace evolves into becoming the “Open Hearth” method for producing steel. The method quickly becomes the preferred approach through the 20th century and the 1980’s.
- The reverberatory furnace works by using gas burner flames over the hearth or melting steel. The steel increases in temperature by the flame and any heat reflection off of the top of the furnace. Gasses flow into brick chambers which also preheats any gas and air, producing, even more, heat for the steel production process. A significant amount of heat produced also made it easier to repurpose scrap steel.
- 1854: Henry Bessemer applies for patent for “Improvements in the Manufacture of Steel and Iron.” The Bessemer process decarburizes molten steel by blowing air over it using a fireclay pipe. It frees carbon and other impurities from the iron to produce steel.
- In the Bessemer method, heat produced by the oxidation process was enough to yield molten iron. Bessemer was pleasantly surprised when the heat produced was higher than anticipated.
- One issue was how to stop the blow at a good carbon content level. The problem is named hot-shortness, which results in steel so brittle that the steel cannot be forged when hot. The problem of hot shortness was resolved by blowing the carbon out and then by the addition of some spiegeleisen (a variety of pig iron containing 15-30 % Manganese and 4-7% Carbon) where Manganese helped the steel become malleable. The steel price declined with the use of the Bessemer process. The result was the ability to use more steel in projects such as battleship plates and girders and rails for railways.
- 1883: first skyscraper made out of steel
- 1885: Bessemer steel is officially endorsed and used by the British government for naval ships and guns.
- 1890: Crucible steel (from crucible process) production reaches an all-time high in the United States (passes Great Britain in amount produced)
- 1918: the first steel-made automobile
- 1930: Chrysler Building in New York City is the first major use of Nirosta stainless steel in architecture
- 1953: The Linz-Donawitz process is developed using a Basic Oxygen Furnace (BOF). In this process, a lance that is water cooled is utilized for a brief period. Many innovations were pioneered by the Dofasco company in Hamilton, Canada with the help of immigrants who arrived from the United Kingdom, Germany, and Austria. Changes continued with the process becoming the dominant technology in North American steel production through the 1970’s. The Basic Oxygen Furnace can in 20 minutes produce a charge of steel. The BOF process is now the global steelmaking standard.
- 1965: Gateway Arch, the tallest steel monument, is in St. Louis and built out of stainless steel (630 feet)
Producing highly alloyed steel is difficult.
- Low-carbon steel: 45,000 psi (310,275 kPa)
- Medium-carbon steel: 80,000 psi (551,600 kPa)
- High-carbon steel: 99,000 psi (692,605 kPa)
- Alloyed steel: 150,000 psi (1,034,250 kPa)
Steel has a melting point of 2800° F (1538°C).
How Steel is Made
Multiple types of melting furnaces are used to produce steel.
- Bessemer converter
The American iron industry was revolutionized by the development of the economical Bessemer steel manufacturing process. Alloy steel turns into a molten form in furnaces that are open-hearth. Alloy steel is melted in an induction or electric-arc furnace.
Mixtures of the following raw materials are charged into the furnace:
- pig iron
- iron ore
Once the steel reaches a molten state, it is removed (tapped) out of the furnace using a ladle. It is then poured into patterned molds for design castings or ingots. If bars are created, it is further transformed or reduced by rolling.
What is Steel Made From?
Steel is made using pig iron which has the level of impurities and carbon reduced. This is done when alloys are added to molten pig iron.
The structure of a particular steel is determined by its application and the specifications developed by the following:
- American Society for Testing and Materials (ASTM)
- American Society of Mechanical Engineers (ASME)
- Society of Automotive Engineers (SAE)
- American Iron and Steel Institute (AISI)
Carbon Steel Groups
Carbon Steel Definition:
Cabron steel refers to the amount of carbon content in the steel. It can refer to a range of steel products including cast irons and pure ingot iron. Cast iron has 4.5% carbon while steel has .3% to 1.7%.
Carbon Steel is Classified into Five Groups:
- Low-Carbon Steel (carbon content up to .3%): Soft, tough and ductile, easily machined, formed, and welded, but does not respond to any type of heat-treating except case hardening. Hot or cold it can be sheared, punched, rolled, and worked. It can be welded using any method and is easily machined. It does not harden to any considerable amount; however, it can easily be case hardened.
Low Carbon Steel Tests
- Appearance: The unfinished surface of low carbon steel is similar to other steels which have a dark gray appearance. It is worked to produce a smooth finished surface and is more expensive.
- Fracture: The color is a bright crystalline gray when fracturing low-carbon steel. It is tough to nick or chip. Wrought iron, low carbon steel, and steel castings cannot be hardened.
- Spark Test: The steel gives off sparks in long yellow-orange streaks, brighter than cast iron, that shows some tendency to burst into white, forked sparklers.
- Torch Test: The steel gives off sparks when melted, and hardens almost instantly.
- Medium-Carbon Steel (carbon content ranging from .3 to .5 percent). Medium-carbon steel is strong and hard but cannot be worked or welded as easily as low-carbon steel. Medium-carbon steel is used for set screws, shafts, axles and crane hooks. After fabrication, this type of steel may be heat treated. It is also utilized for forging parts and machine work that needs strength and a hard surface. It is produced in bar form in the normalized or cold-rolled annealed condition. When welded, the welding area becomes hard if rapidly cooled. After welding, it has to be stress-relieved.
- High-Carbon Steel (carbon content ranging from .5 to .9 percent): responds well to heat treatment and with the right electrodes it can be welded., The process has to include stress-relieving procedures and preheating to prevent cracks in the welding area.This steel is used for the manufacture of:
- Hand tools and mMachine tools that are heat treated after fabrication to develop the hard structure necessary to withstand high wear and shear stress.
It is manufactured in the form of sheets, bars, and wire when in a normalized or annealed form so that it is ready for machining before heat treatment. This steel is difficult to weld because of the hardening effect of heat at the welded joint.
High-carbon Steel Tests:
- Appearance: High-carbon steel’s unfinished surface similar to other steel with a dark gray color. It is usually worked to a smooth surface and tends to be more expensive than other steels.
- Fracture Test: High-carbon steel typically yields a fracture that is whiter than low-carbon steel and fine-grained. Tool steel is more brittle and harder other low-carbon materials or plate steel. High-carbon steel can be hardened by heating to a good red and then by water quenching.
- Spark Test: High-carbon steel gives off many bright orange-yellow sparks.
- Torch Test: High-carbon steel looks porous when the surface is melting and a brighter appearance than when compared to low-carbon steel. Sparks are whiter, and it sparks more freely than low-carbon (mild) steels.
- Very High-Carbon Steel (also called Tool Steel, .9 to 1.55 percent): Very high-carbon steel is similar to high-carbon and responds well to heat treatment. It can be welded with special electrodes, but the process must include preheating and stress-relieving procedures to prevent cracks in the weld areas. Both very high and high-carbon steel is used to produce knives, chisels, railroad car wheels, shear blades, large taps, wood turning tools, razors, mill tools and cutting tools. Anything where hardness is needed to maintain a sharp cutting edge. The American Society for Testing and Materials (ASTM) covers High-strength steel specifications.Welding very high-carbon steel is difficult due to the high carbon content. When used in a spark test, you will see a significant amount of white sparks along with a large volume of repeating, fine bursts.
- Low-Alloy, High-Strength, Tempered Structural Steel: This is a type of special low-carbon steel that has a low percentage of alloys. Steel beams and other structures made of high-strength steel have small cross-sectional areas, but strength which is equal to or greater than low-carbon steel.
Carbon Steel Shapes, Types and Uses
The ability to weld cast steel depends on the alloys:
1) If steel castings have over .30% carbon and 20% silicon, it could be a challenge to weld.
2) If the steel alloy contains molybdenum or nickel, and with low carbon content, the steel is easily welded.
3) If the steel has vanadium or chromium, then welding is challenging.
4) Manganese steel is almost always in the form of steel castings due to its wear resistance. It is very tough.
Cast Steel Identification Tests
- Appearance: The surface of cast steel is brighter than cast or malleable iron and sometimes contains small, bubble-like depressions.
- Fracture Test: Bright crystalline gray is the color of a cast steel fracture. Cast steel does not easily break into small pieces since it is tough. It is tougher than malleable iron. Any steel chips produced by using a chisel will curl. If the metal is manganese steel, it is so tough that it can’t be fractured using a chisel or when it is machined.
- Spark Test: When melted, cast steel sparks and hardens quickly.
Alloy steels or carbon steel is used to produce steel forgings. Alloy steel forgings are brittle and harder than low carbon steels.
Steel Forging Identification Tests
- Appearance: Steel forging surfaces are smooth. When drop forging surfaces are not finished, and you will see a “fin” that is the result of metal being squeezed through the two forging dies. The “fin” is eliminated by the trimming dies. However, enough of the sheared surface should be identifiable. Forgings are covered with black or reddish-brown scale unless they have been cleansed before being inspected.
- Fracture Test: A steel forging fracture test produces a color that is somewhere between silk gray and bright crystalline. Any chips that are produced are tough. It is harder to create a cast steel chip, and you will notice when you do that it has a fine grain. Forgings are either high-carbon steel, low-carbon steel or alloy steel. Tool steel is more brittle and harder than any low-carbon steel or plate steel. Tool steel is usually finer grained and whiter. Tool steel uses heat to harden the metal to a reddish color following by water quenching. Steel castings, wrought iron, and low-carbon steel are not often useful when hardened.
- Spark Test: The sparks given off are long, yellow-orange streamers and are typical steel Sparks from high-carbon steel (tools steel and machinery) are much brighter than those from low- carbon steel.
- Torch Test: Steel forgings spark when melted, and the sparks increase in number and brightness as the carbon content becomes greater.
Stainless Steel Definition
Stainless steel was discovered in the early 1900’s after noticing the superior corrosion resistance of chromium-iron alloys. Patents were awarded in 1912 and the first product was produced in 1908. Corrosion resistance is due to a surface that is covered with transparent passive chromium oxide film.
The American Iron and Steel Institute (AISI) classifies stainless steel into two series:
- Stainless Steel 200-300 series – most common type of stainless steel — known as Austenitic [aw-stuh-nit-ik]. This type of steel is very ductile and tough in the as-welded condition and is ideal for welding. Annealing is not required when there are regular atmospheric conditions.The most widely used are the generally nonmagnetic chromium-nickel steels. Nickel is added as a strong austenitic stabilizer along with chromium. It contains 16% to 26% chromium and up to 35% nickel. The nickel composition drives up the cost of this metal, a factor that limits its’ use. It is nonmagnetic and not hardenable with heat treatment. In solution is also contains nitrogen, contributing to high corrosion resistance.The word austenitic refers to the structure of the metal (face-centered cubic or fcc.)The most common type of austenitic stainless steel is the 304 or 18/8 grade, which contains 18% chromium and 8% nickel.Austenitic stainless steel is weldable and extremely formable. It can be used in extreme cold and heat conditions such as on a jet engine.
Typical uses are in dairy equipment and food processing plants.
- Stainless Steel 400 series — further subdivided according to their crystalline structure into two general groups:
- Ferritic Stainless Steel [fer-rit-ik] – referred to as bcc or body-centered cubic iron base alloys): When properly finished and heat treated, it resists corrosive attacks from corrosive media, resists oxidation and is magnetic in nature.Contains 12% to 18% Chromium and .15 to .2% carbon besides iron and usual amounts of silicon and manganese. It is nickel free. Ferritic stainless steel is non-hardenable by heat treatment and normally used in the annealed or soft condition. Is is an iron-chromium alloy that cannot be hardened by heat treatment.Ferritic stainless steel is frequently used for decorative trim and equipment subjected to high pressures and temperatures.Ferritic stainless steel is used in places which have less critical anti-corrosion applications such as auto trim and architectural uses.
- Martensitic Stainless Steel [mahr-tn-zit-ik]These steels contain 12% to 18% chromium and .1% to 1.8% carbon. Martensitic stainless steel is readily hardened by heat treatment although this decreases corrosion resistance. They are magnetic and used where high strength, corrosion resistance, and ductility are required.Uses of martensitic stainless steel are instruments under corrective conditions and high temperatures, ball bearings, cutlery, surgical instruments, turbines, wrenches, and springs.
Properties of Stainless Steel
- Excellent surface finish
- Full range of hardness and strength
- High formability and ductility
- High corrosion resistance
- Excellent thermal conductivity
- Good creep resistance
- Good machinability
- High cold & hot workability
- No. 1: – Descaled, annealed and Hot-rolled
- No. 2D Finish -Descaled, annealed and Hot-rolled
- No. 2B Finish – Bright finish and Cold-rolled
- Bright Annealed Finish – Cold-rolledA bright finish retained by final annealing takes place in a carefully regulated atmosphere furnace
- No. 3 Finish – One or both sides, Intermediate polished finish, usually 120 grit
- No. 4 Finish- Polished finish for general use, usually 180 grit, one or both sides
- No. 6 Finish- Tampico brushed, Dull satin finish, one or both sides
- No. 7 Finish- Near mirror or High luster finish (has some lines)
- No. 8 Finish- Mirror finish
- Pots, pans, flatware and utensils
- Kitchen and restaurant equipment
- Sinks and drains
- Fittings, clamps, and connectors for sanitary and processing applications
- Tanks, tubing, piping for sanitary, food service chemical and petrochemical processing
- Automotive components
- Enclosures, housings, and cabinets
- Angle: alloys often added of molybdenum and nickel to improve corrosion resistance and pitting
- Beam: used in sterile environments and provides excellent structural support
- Channel: also used in sterile and corrosive environments
- Bar: high-temperature resistance, easy to weld, stamp and fabricate
- Plate: used in extreme conditions
- Sheet: common in many applications and products
- Pipe: used when there is pressure within the pipe
- Tube: Tubing comes in square, round or rectangular shapes. Used for support and structural applications.
- Grating: Minimal carbide precipitation and corrosion resistance. In press-locked, swage locked riveted and welded types.
- Expanded & Perforated Metal: stretched and slit into diamond-shaped openings
- Grip Strut (safety grating): for load-bearing safety grating such as used on platforms
Alloy Steel (Tool Steels)
The properties of alloy steels come from the alloy that is added in addition to carbon. These elements are added when the steel is manufactured to achieve the required characteristics. Tool steels can become hard and maintain hardness when temperatures are high, particularly when the steel is being cut.
Production of alloy steels occurs in plates, sheets, structural sections, and bars. The bars are used in what is called an “as-rolled” condition. When compared to hot-rolled carbon steels, tool steels usually have better physical properties.
Equipment manufacturers use alloy steel due to the durability and high strength when compared to carbon steel. Tool steel also weights less. Of note is manganese steel, which is an alloy that is always used as a cast.
Common Alloy Steels
These alloys are used in the machine or forming of metals when strength is a requirement. Examples include:
- Crane booms
- Bulldozer blades
- Railroad cars
What is high-speed steel?
High-speed Steel (H.S.S.) is the name for common tool steel. It can cut steel at high cutting speeds. These steels:
- Contain relatively significant amounts of molybdenum or tungsten, together with vanadium, cobalt or chromium.
- Are resistant to wear
- Maintain hardness at elevated temperatures around 650oC
- Excellent hardenability
- Have high allowed content
H.S.S. is typically comprised of:
- carbon (.75)
- vanadium (1%)
- chromium (4%)
- tungsten (18%)
Common Alloy Steels:
- Nickel Steels: Nickel increase the ductility, strength, and toughness of steels. It lowers the hardening temperatures so than an oil quench, rather than a water quench is used for hardening. Aircraft parts like frame support members and propellers are made with nickel steel.
- Chromium Steels: Used for the races and balls in anti-friction bearings; highly resistant to corrosion and to scale. As an alloy in carbon steel it helps to improve corrosion resistance, shock resistance and improves hardenability. It also increases strength with a minimal reduction in ductility.
- Chrome Vanadium Steel: Chrome Vanadium Steel is used for its high strength in items such as axles, gears, sockets, high-quality tools (sockets, wrenches) and crankshafts.
- Tungsten Steel: Tungsten steel is used in milling cutters, lathe tools, cutting tools and drills. It is costly to produce.
- Molybdenum: Molybdenum is used in place of tungsten to make the cheaper grades of high- speed steel and in the carbon molybdenum high-pressure tubing. Heat treatment improves hardenability. However, if the steel alloy has more than .60% molybdenum; impact, fatigue is compromised. Wear resistance does improve if the level of molybdenum content rises past .75%. Molybdenum is also combined with vanadium, tungsten or chromium.
- Molybdenum High-Speed Steel: Molybdenum High-Speed Steel contains 2% vanadium, 4% chromium, 6% molybdenum and 6% tungsten. This type of high-speed steel is cheaper than other types of steel. Uses include tapping and drilling operations.
- Manganese Steels: Manganese is alloyed in steel for improved toughness, easy hot rolling, easier forging and wear resistance. The more manganese in the steel, the harder it is to weld. Properties of Manganese depend on the quantity that is in the steel:
- Small amounts produce strong, free-machining steels.
- Larger amounts produce a somewhat brittle steel.
- Still more significant amounts produce steel that is tough and very resistant to wear after proper heat treatment.
- Columbium and Titanium (niobium): These metals are used as additional alloying agents in corrosion resistant low-carbon steels. After subjected to prolonged high temperatures, these metals resist any intergranular corrosion.
- Vanadium: Vanadium controls Grain size. This alloy improves hardenability, resists tempering and causes marked secondary hardness. It is also added to steel during manufacture to remove oxygen.
- Silicon: To get better hardenability and corrosion resistance, silicon is added to steel. Silicon is frequently used with manganese to obtain a strong, tough steel. Cutting tools use high-speed tool steels that have special alloy compositions. The carbon content ranges from 0.70% to 0.80%. To improve weldability, they are welded using the furnace induction method. Otherwise they are difficult to weld.
- Constructional Alloy Steels (high yield strength, low alloy structural steels): This type of steel is much tougher than low carbon steels. Low-carbon steels are called a constructional alloy. Steel is tempered and quenched to obtain a tensile strength of 689,500 to 965,300 kPa (100,000 to 140,000 psi) and a yield strength of 620,550 to 689,500 kPa (90,000 to 100,000 psi) depending upon shape and size. When these high strength steels are fabricated in structural members they may have smaller cross-sectional areas than conventional structural steels and still have equal strength. These steels are more abrasion resistant and corrosion resistant. In a spark tests, constructional steel alloys appear similar to low carbon steels.
Alloy Steel Identification Tests
- Appearance: Alloy steels appear the same as drop-forged steel.
- Fracture Test: Alloy steel is usually very close-grained, at times the fracture appears velvety.
- Spark Test: Characteristic sparks are produced in Alloy Steel regarding shape and color. Common alloys used in steel and their effects on the spark stream are as follows:
- Chromium: In spark tests steels containing 1% to 2% chromium has no outstanding features. Large amounts of Chromium shortens the spark stream length to one-half that of the same steel without chromium without noticeably affecting the stream’s brightness. Other elements reduce the stream to the same extent and making it dull. An 18% chromium, 8% nickel stainless steel produces a spark that is similar but half as long as wrought iron. Steel containing no nickel and 14% chromium provides a shorter low-carbon spark. An 18 percent chromium, 2 percent carbon steel (chromium die steel) has a spark that is like the spark produced by carbon tool steel but with a length that is one-third as long.
- Nickel: Just before the fork, a nickel spark has a short, sharply defined dash of brilliant light. Nickel, in the amount found in S. A. E. steels are only recognized when the carbon content is so low that the bursts are not too noticeable.
- High chromium-nickel alloy (stainless) steels: In a spark test, the sparks given off are white near the end of the streak and straw colored near the grinding wheel. The streak volume is medium with a moderate number of forked bursts.
- Manganese: Carbon steel and manganese steel alloy have a similar spark. The force of the bursts and the volume of the spark stream increase with and an increase in manganese. If in the steel there is more than the usual amount of manganese, the spark will be like the spark from high-carbon steel with low manganese content.
- Molybdenum: Steel containing this element produces a characteristic spark with a detached arrowhead similar to that of wrought iron. It can be seen even in relatively strong carbon bursts. Nickel, chromium or both are found in Molybdenum alloy steel.
- Other elements with Molybdenum: When tungsten in high-speed steel is sued to replace some of the other elements and molybdenum, the spark stream turns to the color orange. Although other items give off a red spark, there is enough difference in their color to tell them from a tungsten spark.
- Tungsten: When testing tungsten, the spark stream closest to the wheel becomes a dull red. The spark stream shortens, reduces in size or the carbon burst is eliminated. Steel that contains 10% tungsten results in curved, short orange spears points at the tail end of the carrier lines. When the tungsten content is reduced further, at the end of the spear point you will see small white bursts. The carrier lines appear to be orange to dull red, depending what other elements are in the steel, particularly when it has high tungsten content.
- Vanadium: Alloy steels containing vanadium produce sparks with a detached arrowhead at the end of the carrier link similar to those arising from molybdenum steels. The spark test is not positive for vanadium steels.
- High-speed tool steels: Near the wheel, a spark test will produce several elongated forked sparks. The sparks at the end of the stream will be straw-colored.
Steel Annealing Process
During this process, the heating phase results in fine-grained austenite. After cooling a fine-grained structure is obtained. The result is an improvement in toughness, ductility and mechanical properties. It is the process where hypereutectoid steel is heated 30–50°C above the critical temperature. At that temperature, it is held for some time which heats the metal thoroughly. Phase transformation occurs throughout the metal. This is followed by slow cooling in a furnace. The heating rate is usually 100°C/hr and the holding time is one hr/ton of metal. The cooling rate is kept from 10°C–100°C for alloy steels and can be 200°C/hr for carbon steels.
Partial Annealing is a process where steel is heated slightly above a lower critical temperature. This annealing is applied for hypereutectoid steels only. It is also applied to hypereutectoid steels where hardness is to be reduced while improving machinability. In this operation, pearlite is transformed to austenite and ferrite is partially deformed into austenite. The heating and holding period is followed by slow cooling.
In Isothermal annealing, steel is heated in the same way as it is treated in full annealing. It is rapidly cooled from 500°C to 100°C below a critical temperature. This is followed by keeping steel at this temperature for an extended period which results in complete decomposition of iron. Then this is cooled in air. Isothermal annealing results in more homogenous structure throughout the section and improved machinability.
Normalizing steel is the process of heating steel to the temperature 50°C or more above the critical temperature 723°C. A complete transformation occurs when the steel is held at this temperature for a considerable period. This is followed by air cooling of steel. In normalizing, a complete phase recrystallization takes place, and a fine-grained structure is obtained.
The rate of cooling is faster than furnace cooling. During air cooling, austenite transforms into a finer and more abundant pearlite structure in comparison to annealing. Properties obtained by normalizing depend on the size and composition of the steel. As the smaller pieces cool more rapidly because of more exposure area, fine pearlite is formed, and thus they are harder than larger pieces. The object of normalizing is to refine the structure of steel and remove strains which may have been caused by cold working.
The crystal structure becomes distorted when steel is cold worked. The metal may become unrealistic and brittle.
To efficiently transform the austenite to martensite, rapid cooling is needed, so the temperature of transformation is from about 750° to 300°C. This involves very rapid cooling and invites trouble of cracking and distortion. The factors which tend to cause the metal to warp and crack are:
- When a metal cooled it undergoes a contraction which is normally not uniform but occurs at the outside surfaces and in thin product sections.
- When steel cools through the critical range, expansion occurs. Now if we would arrange to cool the whole volume of metal suddenly at the same instant, we should not experience much of a problem with a change in volume, etc. but unfortunately, this is not possible. When we suddenly plunge the metal into the water from the furnace at an annealing temperature, the outer portion of the metal comes in contact with water and is immediately cooled and undergoes its critical range expansion, leading a to a hard and rigid skin of metal. The inner portion of the metal, however, has not yet felt the quenching effect and is still red hot.When the quenching effect is transferred to the outer portion of the steel through the critical range the outer layer does not crack. The size, shape and quenching rate of the article affects the elimination of distortion, cracks, and hardening. A unique technique of immersing into the quenching media (may be water, oil, or brine solution ) is adopted, as described below:
- Long articles are immersed with their axis normal to the bath surface.
- Thin and flat articles are immersed with their edges first into the bath.
- The curved article’s curved portion is kept upward during the immersion.
- Heavy articles are kept stationary with the quenching media stirred around them.
Very rough surface articles do not respond to uniform hardening, therefore this factor should be taken into account before performing the quenching operation.
Martensitic structures formed by direct quenching of high carbon steel are hard and strong but also brittle. They contain internal stresses which are severe and unequally distributed to cause cracks or even fracture of hardened steel. The tempering is carried out to obtain one or more of the following objectives:
- To reduce internal stresses produced during heat treatment operations.
- To stabilize the structure of the metal.
- To make steel tough to resist fatigue and shock.
- To reduce hardness and improve ductility
Thus, tempering consists of heating quenched hardened steel in martensitic condition to a temperature below the lower critical temperature. It needs to be held it at that temperature for a sufficient time and then by cooling it slowly down to room temperature.
Tempering is classified into the following three types:
- Low-Temperature Tempering: The work is heated between 150 and 250°C for a specific time. The objective of this procedure is to relieve internal stresses and to increase the ductility with much reduction in hardness. Low-temperature tempering
is applied in the heat treatment of carbon and low alloy steel cutting tools as well measuring instrument and components that have been carburized and surface hardened.
- Medium Temperature Tempering: The work is heated between 350 and 450°C for a specific time before being allowed to cool off in the air or quenched in certain media. The martensite is converted into secondary troostite. The results provide a reduction
in hardness and strength of metal and improvement in ductility. The process is utilized in production of laminated springs and coils to ensure toughness.
- High-temperature Tempering: It is done at the temperature of 500 to 650°C which completely eliminates internal stresses and provides toughness. Hardness is practically due to prolonged heating during carburizing process grains of core become relatively coarse, and refinement of the core is essential. Refining of components is achieved by heating them to 850°C then cooling in air or quenching it in oil.
In this manner carburizing provides a hard case with a soft core. If there is brittleness of the core, it is removed by tempering normally between 180°C–270°C.
What is Steel Carbonitriding?
Steel Carbonitriding is the technique of producing a hard case by using gases to add nitrogen and ammonia on the surface of the steel . Carbonitriding uses ammonia, carbon monoxide and hydrocarbons are used for carbonitriding. The temperature for Carbonitriding is 780°C to 875°C with 840°C for 6 to 9 hours. A furnace is used with the supply of carrier gas (carbon monoxide, hydrocarbon, ammonia) under positive pressure to check and prevent air infiltration. Thus, making the process control easier.
At the furnace temperatures, the added ammonia breaks up to provide nitrogen on the surface of the steel. Nitrogen in the surface layer of steel components increases hardenability and permits hardening by oil quench (instead of water quench). Thus, the chance of cracking and distortion is eliminated. The portion of steel components which is not to be carbonitrided can be protected by a layer of copper.
What is cyaniding?
Cyaniding is the process of using a liquid cyanide bath to create a hard wear resistant case with a tough core to low carbon steels. In this process, the piece of low carbon steel is immersed in a molten soft bath containing cyanide (generally it contains 20% to 50% sodium cyanide up to 40% sodium carbonate and varying quantities of sodium and barium chloride) at 840°C to 940°C and then quenching the steel in water or oil. Before quenching the steel is kept in the bath for 15 to 20 minutes. The soaking time varies with depth of case to be hardened and size of the component. Under average conditions as discussed above, a case depth of 0.125 mm would be obtained, i.e., in 15 minutes and at 840°C. This technique is chiefly utilized for cases not exceeding 0.8 mm in thickness.
The hardness generated is due to the presence of compounds of nitrogen as well as carbon in the surface layer. The chemistry of the cyaniding process is as follows:
The generated C&N are absorbed by the surface. Inherent hardness is imparted by Nitrogen, while the absorbed carbon content in steel will respond to the quenching treatment.
- The bright finish of a machined part if required can be maintained.
- It is easy to avoid any distortion.
- Hardness from the core to the case is gradual, and we can eliminate a flaking core.
What is flame hardening?
Flame hardening is the process of surface hardening in which a hard wear resistant layer on a tough core steel component is produced by the application of heat with the flame of an oxyacetylene torch. The surface is then cooled with water. The flame is directed on the desired part without heating the remaining portion of the work.
The steel required for flame hardening generally contains 0.4 to 0.6 per cent of carbon. The component or part is heated in the austenitic range. Chances of cracking and distortion are reduced by reducing stresses by localizing the flame.
Advantages of Flame Hardening Steel Alloys
- The time taken for heating is comparatively less than when the requisite metal is heated in the furnace.
- The method is advantageous as selective surfaces can be hardened even on very large machines/components that are too inconvenient or too large to place in the furnace.
- Flame hardening is convenient when hardness is required only for a limited depth, the remainder retaining the original toughness and ductility.
Limitations of Flame Hardening
The only limitation is when precise overheating due to poor temperature control occurs it can result in cracking and distortion of the treated components.
Uses of Flame Hardening
- Open end wrenches
- Ways of lathes
- Value ends
- Steel dies
- Gear Teeth
What is induction hardening?
Induction hardening is the process where the surface hardening is achieved by placing the part in an inductor (consisting of copper) which is primary of a transformer. The components are placed in such a way that it does not touch the inductor coil. In this process a high-frequency current of about 2000 cycles/second is passed. The heating effect is by virtue of induced eddy current and hysteresis loss of the surface material.
The hardening temperature is from 750°C to 760°C for 0.5% carbon steel and 790°C to 810°C for alloy steels. The heated areas are then quenched immediately by a pressurized water spray. A depth of case of roughly 3 mm is achieved in about 5 seconds. The actual time depends upon the frequency used, power input and depth of hardening required.
Advantages of Induction Hardening
Heating time is extremely small so distortion if any is considerably reduced.
Permits automation of heat treatment process and no surface oxidation takes place.
Induction hardening provides high hardness, higher wear resistance, higher impact strength and higher fatigue limit when compared with ordinary hardened steels.
- High cost for the equipment
- Application limited to medium carbon and alloy steels
- Break drums
- Crankshaft surfaces
- Camshaft surfaces
What is Nitriding?
Nitriding is the process of surface hardening. It is used to obtain hard steel surface components. The technique is normally employed for those steels which are alloyed with metals like aluminum, molybdenum, manganese and chromium. The nitriding operation is the last operation being performed after oil hardening (at 840°C to 900°C,) tempering, rough machining, stabilizing (for removing internal stresses) and the final machining of the components.
The machined and finished steel components are placed in an airtight container of nickel chromium steel provided with inlet and outlet tubes through with NH3 is circulated (at 450°C to 540°C.) The NH3 in the furnace gets dissociated to liberate nascent nitrogen which reacts with the surface of components and form nitrides which are very hard.
The nitriding process is used in the production of machine components which require high wear resistance at elevated temperatures such as:
- cylinder lines
- airplane valves
- automobile valves
- drawing dies
- pump shafts
- roller bearing parts
- ball bearings
Very high surface hardness with excellent wear resistance.
Minimal cracks and distortion due to quenching elimination
Economical for base production, machining, and finishing
Nitrided components retain hardness up to 510°C.
The operation time is long for small depth of case hardened components and may lead to oxidation.
Applicable to steels which can form good nitrides.
Plate Steel: Welded structures such as gun carriages use plate steel.
When working with plate steel, several that contain no nickel, or commercial grades of low-alloy structural steel of not over 0.25% carbon are better for welding applications than those with a maximum carbon content of 0.30%. An example of this kind of plate is a low-carbon alloyed steel called Armor plate. This type of plate is usually utilized in an “as rolled” state.
Using a covered electrode for Electric Arc Welding may need metal preheating, followed by a post-heating proper stress-relieving heat treatment, to create a structure where the welded joint has properties equal to those of the plate metal.
Austenitic Stainless Steel
by ASM International
Stainless Steels for Design Engineers
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