CE 470 Lect 1.pdf

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CE 470 STRUCTURAL DESIGN IN METALS Fall 2006

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Introduction; manufacturing of steel, alloying, and definitions

CE 470 - Lecture #1

Steel

Steel is an alloy made primarily from iron and carbon (the alloy).

Typically, the percent of carbon in steel is relatively low, less than 2% carbon.

Many other elements within common structural steels such as manganese (1%) and small

amounts of silicon, phosphorus, sulfur and oxygen.

Composition controlled by ASTM Standards.

Henry Bessemer, a British inventor is typically credited with the invention of steel in1856 (Bessemer Steel Company in Sheffield, England). Although steel was produced

before that time, it was his patented Bessemer process which is still used today. In short,

the process involves blowing air through molten pig iron to oxidize the material andseparate impurities.

Steel is different than Wrought Iron and Case Iron.

Wrought Iron - Iron that is almost pure (less than 0.15% carbon). Can be shaped and forge

welded with ease, but is soft and does not harden in the same way as Steel. The properties of

Wrought Iron are partially attributed to the Slag inclusions that result from Puddling and Forge

Welding. Widely used in Bridges, Axles and Ships plates before the development of Bessemerand Siemens Steel. The last commercial production of Wrought Iron in the U.K. ceased in 1976.

Cast Iron - Iron with a high Carbon content (above 2% to 2.5% but usually less than 6%).

Identical, in most cases, to Pig Iron, it is easily cast to almost any shape and melts at a lowertemperature to other type of iron and steel.

Cast Iron is extremely hard and brittle. Machining is difficult and it is easily shattered, revealingits crystalline structure. Chilled Cast Iron is even harder and is produced by cooling the castings

to increase the speed at which the iron solidifies. Cast Iron is still is wide use for numerous

casting, from drains covers through to engine blocks and water pipes. Can be Grey, White orMalleable.

Pig Iron - The name used for the iron directly produced from a blast furnace. Originally cast

into 'pigs' around the base of the furnace, lasted casting machines were developed to producepigs but iron is now generally transported while still molten and converted into steel on the same

site. The name is derived from the impression given of piglets feeding from the sow by the ironbeing run off the furnace into the original style of sand moulds. Pigs were traditionally sized tobe man handled, but size increase later. Pig Iron changes its name to Cast Iron when re-melted,

although no actual processing takes place. Iron casings can be created directly from the blast

furnace.

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Back to Steel

Steel is made via two basic routes - from raw materials - iron ore, limestone and coke by the

blast furnace and basic oxygen furnace (BOF) route. If a mill can produce virgin (brand new)

steel as well as roll the steel, it is referred to as an integrated mill or plant.

However, today much steel (about 34% in 2003) is obtained from recycled scrap using the

electric arc furnace (EAF) method. The second technique is much easier and faster since it onlyrequires scrap steel. Recycled steel is introduced into a furnace and re-melted along with some

other additions to produce the end product.

Iron Ore

Simply rock that happens to contain a high concentration of iron. A few common ores are listed

below.

Common Iron OresHematite - Fe2O

3- 70 percent iron

Magnetite - Fe3O

4- 72 percent iron

Limonite - Fe2O

3+ H

2O - 50 percent to 66 percent iron

Siderite - FeCO3- 48 percent iron

Note these all have oxygen attached. To get the iron, we have to get rid of the Oxygen

The more advanced way to smelt iron is in a blast furnace. A blast furnace is charged withiron ore, coke (coke is charcoal made from coal) and limestone (CaCO

3). Huge quantities of

air are blasted into the bottom of the furnace. The calcium in the limestone combines with thesilicates to form slag. At the bottom of the blast furnace, liquid iron collects along with a layer

of slag on top. Periodically, you let the liquid iron flow out and cool.

To create a ton of pig iron, you roughly start with 2 tons of ore, 1 ton of coke and half-ton oflimestone. The fire consumes 5 tons of air. The temperature reaches almost 3000 degrees F

(about 1600 degrees C) at the core of the blast furnace! The pig iron produced is tapped from

the furnace. Recall Pig iron is high in carbon and is very brittle, so the carbon must be removed.

Bessemer Furnace (Process)

The process method of producing steel from a charge consisting mostly of pig iron,

however limestone and iron ore are also added.

The process is carried on in a large container called the egg-shaped Bessemer converter,which is made of steel and has a lining of silica and clay or of dolomite.

The capacity is from 8 to 30 tons of molten iron; the usual charge is 15 or 18 tons.

The wide end, or bottom, has a number of perforations through which the air is forced

upward into the converter during operation and is set on pivots (trunnions) so that it canbe tilted at an angle to receive the charge, turned upright during the "blow," and inclined

for pouring the molten steel after the operation is complete.

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As the air passes upward through the molten pig iron, impurities such as silicon,manganese, and carbon unite with the oxygen in the air to form oxides; the carbon

monoxide burns off with a blue flame and the other impurities form slag.

Open Hearth Furnace

The pig iron, limestone and iron ore go into an open hearth furnace.

Heated to about 1600 F (871 C).

The limestone and ore forms a slag that floats on the surface. Impurities, includingcarbon, are oxidized and float out of the iron into the slag.

When the carbon content is right, you have carbon steel.

Basic Oxygen Furnace

The process method of producing steel from a charge consisting mostly of pig iron.

The charge is placed in a furnace similar to the one used in the Bessemer process of

steelmaking except that pure oxygen instead of air is blown into the charge to oxidize theimpurities present.

One desirable feature of this process is that it takes less than an hour, and is thus much

faster than the open-hearth process, another important method of steelmaking.

A second advantage is that a major byproduct is carbon monoxide, which can be used as

a fuel or in producing various chemicals, such as acetic acid. The basic oxygen processalso produces less air pollution than methods using air.

Electric Arc Furnace (EAF)

Scrap is melted in an electric arc furnace (100%).

The raw material fed into the furnace may be selected but untreated scrap (old machine

parts, for example), or may be delivered as sorted, crushed and calibrated scrap with aminimum iron content of 92 percent.

The produces the molten steel, which then undergoes the same refining and grading

processes as pig iron.

The raw materials must be carefully selected for each different steel grade. Selection

depends on the type of "impurities" that any metal or ore in the scrap might contain.

About 40% of all steel in the US is from EAF

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Phosphorus increases strength and hardness and decreases ductility and notch impact toughness of steel.

The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels.

Phosphorous levels are normally controlled to low levels. Higher phosphorus is specified in low-carbonfree-machining steels to improve machinability.

Sulfur decreases ductility and notch impact toughness especially in the transverse direction. Weldability

decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions.

Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where

sulfur is added to improve machinability.

Silicon is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese

in increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to

surface quality.

Copper in significant amounts is detrimental to hot-working steels. Copper negatively affects forge

welding, but does not seriously affect arc or oxyacetylene welding. Copper can be detrimental to surface

quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts exceeding0.20%. Weathering steels are sold having greater than 0.20% Copper.

Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to

increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is

frequently used with a toughening element such as nickel to produce superior mechanical properties. At

higher temperatures, chromium contributes increased strength. Chromium is a strong carbide former.

Complex chromium-iron carbides go into solution in austenite slowly; therefore, sufficient heating time

must be allowed for prior to quenching.

Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite,

strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of

steels.

Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during

the tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated

temperatures.

Titanium is used to retard grain growth and thus improve toughness. Titanium is also used to achieve

improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than

elongated thus improving toughness and ductility in transverse bending.

Vanadium increases the yield strength and the tensile strength of carbon steel. The addition of small

amounts of Niobium can significantly increase the strength of steels. Vanadium is one of the primary

contributors to precipitation strengthening in micro-alloyed steels. When thermo-mechanical processingis properly controlled the ferrite grain size is refined and there is a corresponding increase in toughness.

The impact transition temperature also increases when vanadium is added.

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Some Useful ASTM Specifications

A36/A 36M Specification for Carbon Structural Steel

A514/A 514M Specification for High-Yield-Strength, Quenched and Tempered Alloy Steel

Plate, Suitable for Welding

A588/A 588M Specification for High-Strength Low-Alloy Structural Steel with 50 ksi [345

MPa] Minimum Yield Point to 4 in. [100 mm] Thick

A572/A 572M Specification for High-Strength Low-Alloy Columbium-Vanadium StructuralSteel

A709/A709M Specification for bridge steels

A370 Test Methods and Definitions for Mechanical Testing of Steel Products

A307 Specification for Carbon Steel Bolts and Studs, 60000 psi Tensile Strength

A502 Specification for Rivets, Steel, Structural

A668/A668M Specification for Steel Forgings, Carbon and Alloy, for General Industrial Use

Documents

CE 470 Lect 1.pdf