EFFECTS OF THERMAL CUTTING ON STEELS BASIC INFORMATION

Fabrication of steel structures usually requires cutting of components by thermal cutting processes such as oxyfuel, air carbon arc, and plasma arc. Thermal cutting processes liberate a large quantity of heat in the kerf, which heats the newly generated cut surfaces to very high temperatures.

As the cutting torch moves away, the surrounding metal cools the cut surfaces rapidly and causes the formation of a heat-affected zone analogous to that of a weld. The depth of the heat-affected zone depends on the carbon and alloy content of the steel, the thickness of the piece, the preheat temperature, the cutting speed, and the postheat treatment.

In addition to the microstructural changes that occur in the heat-affected zone, the cut surface may exhibit a slightly higher carbon content than material below the surface. The detrimental properties of the thin layer can be improved significantly by using proper preheat, or postheat, or decreasing cutting speed, or any combination thereof.

The hardness of the thermally cut surface is the most important variable influencing the quality of the surface as measured by a bend test. Plate chemistry (carbon content), Charpy V-notch toughness, cutting speed, and plate temperature are also important.

Preheating the steel prior to cutting, and decreasing the cutting speed, reduce the temperature gradients induced by the cutting operation, thereby serving to (1) decrease the migration of carbon to the cut surface, (2) decrease the hardness of the cut surface, (3) reduce distortion, (4) reduce or give more favorable distribution to the thermally induced stresses, and (5) prevent the formation of quench or cooling cracks.

The need for preheating increases with increased carbon and alloy content of the steel, with increased thickness of the steel, and for cuts having geometries that act as high stress raisers. Most recommendations for minimum preheat temperatures are similar to those for welding.

The roughness of thermally cut surfaces is governed by many factors such as (1) uniformity of the preheat, (2) uniformity of the cutting velocity (speed and direction), and (3) quality of the steel. The larger the nonuniformity of these factors, the larger is the roughness of the cut surface. The roughness of a surface is important because notches and stress raisers can lead to fracture.

The acceptable roughness for thermally cut surfaces is governed by the job requirements and by the magnitude and fluctuation of the stresses for the particular component and the geometrical detail within the component. In general, the surface roughness requirements for bridge components are more stringent than for buildings.

The desired magnitude and uniformity of surface roughness can be achieved best by using automated thermal cutting equipment where cutting speed and direction are easily controlled. Manual procedures tend to produce a greater surface roughness that may be unacceptable for primary tension components. This is attributed to the difficulty in controlling both the cutting speed and the small transverse perturbations from the cutting direction.

EFFECTS OF WELDING ON STEELS BASIC INFORMATION AND TUTORIALS

Failures in service rarely, if ever, occur in properly made welds of adequate design. If a fracture occurs, it is initiated at a notchlike defect. Notches occur for various reasons.

The toe of a weld may form a natural notch. The weld may contain flaws that act as notches. A welding-arc strike in the base metal may have an embrittling effect, especially if weld metal is not deposited.

A crack started at such notches will propagate along a path determined by local stresses and notch toughness of adjacent material.

Preheating before welding minimizes the risk of brittle failure. Its primary effect initially is to reduce the temperature gradient between the weld and adjoining base metal.

Thus, there is less likelihood of cracking during cooling and there is an opportunity for entrapped hydrogen, a possible source of embrittlement, to escape. A consequent effect of preheating is improved ductility and notch toughness of base and weld metals, and lower transition temperature of weld.

Rapid cooling of a weld can have an adverse effect. One reason that arc strikes that do not deposit weld metal are dangerous is that the heated metal cools very fast. This causes severe embrittlement.

Such arc strikes should be completely removed. The material should be preheated, to prevent local hardening, and weld metal should be deposited to fill the depression.

Welding processes that deposit weld metal low in hydrogen and have suitable moisture control often can eliminate the need for preheat. Such processes include use of low-hydrogen electrodes and inert-arc and submerged-arc welding.

Pronounced segregation in base metal may cause welds to crack under certain fabricating conditions. These include use of high-heat-input electrodes and deposition of large beads at slow speeds, as in automatic welding.

Cracking due to segregation, however, is rare for the degree of segregation normally occurring in hot rolled carbon-steel plates. Welds sometimes are peened to prevent cracking or distortion, although special welding sequences and procedures may be more effective.

Specifications often prohibit peening of the first and last weld passes. Peening of the first pass may crack or punch through the weld.

Peening of the last pass makes inspection for cracks difficult. Peening considerably reduces toughness and impact properties of the weld metal. The adverse effects, however, are eliminated by the covering weld layer (last pass).

(M. E. Shank, Control of Steel Construction to Avoid Brittle Failure, Welding Research Council, New York; R. D. Stout and W. D. Doty, Weldability of Steels, Welding Research Council, New York.)

CORROSION OF IRON AND STEEL AND PREVENTION BASIC INFORMATION AND TUTORIALS

What causes corrosion on Iron and Steel?

Principles of Corrosion.

Corrosion may take place by direct chemical attack or by electrochemical (galvanic) attack; the latter is by far the most common mechanism. When two dissimilar metals that are in electrical contact are connected by an electrolyte, an electromotive potential is developed, and a current flows.

The magnitude of the current depends on the conductivity of the electrolyte, the presence of high resistance “passivating” films on the electrode surfaces, the relative areas of electrodes, and the strength of the potential difference. The metal that serves as the anode undergoes oxidation and goes into solution (corrodes).

When different metals are ranked according to their tendency to go into solution, the galvanic series, or electromotive series, is obtained. Metals at the bottom will corrode when in contact with those at the top; the greater the separation, the greater the attack is likely to be.

Table 4-14 is such a ranking, based on tests by the International Nickel Company, in which the electrolyte was seawater.

The nature of the electrolyte may affect the order to some extent. It also should be recognized that very subtle differences in the nature of the metal may result in the formation of anode-cathode galvanic cells: slight differences in composition of the electrolyte at different locations on the metal surface, minor segregation of impurities in the metal, variations in the degree of cold deformation undergone by the metal, etc.

It is possible for anode-cathode couples to exist very close to each other on a metal surface. The electrolyte is a solution of ions; a film of condensed moisture will serve.

Corrosion Prevention.

An understanding of the mechanism of corrosion suggests possible ways of minimizing corrosion effects. Some of these include:

(1) avoidance of metal combinations that are not compatible,

(2) electrical insulation between dissimilar metals that have to be used together,

(3) use of a sacrificial anode placed in contact with a structure to be protected (this is an expensive technique but can be justified in order to protect such structures as buried pipelines and ship hulls),

(4) use of an impressed emf from an external power source to buck out the corrosion current (called cathodic protection),

(5) avoiding the presence of an electrolyte—especially those with high conductivities, and

(6) application of a protective coating to either the anode or the cathode.  

STEEL STRAND AND ROPE BASIC INFORMATION AND TUTORIALS

What are steel strand and steel ropes?

Iron and Steel Wire. Annealed wire of iron or very mild steel has a tensile strength in the range of 310 to 415 MPa (45,000 to 60,000 lb/in2); with increased carbon content, varying amounts of cold drawing, and various heat treatments, the tensile strength ranges all the way from the latter figures up to about 3450 MPa (500,000 lb/in2), but a figure of about 1725 MPa (250,000 lb/in2) represents the ordinary limit for wire for important structural purposes.

For example, see the following paragraph on bridge wire. Wires of high carbon content can be tempered for special applications such as spring wire. The yield strength of cold-drawn steel wire is 65% to 80% of its ultimate strength. For examples showing the effects of drawing and carbon content on wire, see Making, Shaping, and Treating of Steel, U.S. Steel.

Galvanized-Steel Bridge Wire. The manufacture of high-strength bridge wire like that used for the cables and hangers of suspension bridges such as the San Francisco–Oakland Bay Bridge, the Mackinac Bridge in Michigan, and the Narrows Bridge in New York is an excellent example of careful control of processing to produce a quality material.

The wire is a high-carbon product containing 0.75% to 0.85% carbon with maximum limits placed on potentially harmful impurities. Rolling temperatures are carefully specified, and the wire is subjected to a special heat treatment called patenting.

The steel is transformed in a controlled-temperature molten lead bath to ensure an optimal microstructure. This is followed by cold drawing to a minimum tensile strength of 1550 MPa (225,000 lb/in2) and a 4% elongation.

The wire is given a heavy zinc coating to protect against corrosion. Joints or splices are made with cold-pressed sleeves which develop practically the full strength of the wire. Fatigue tests of galvanized bridge wire in reversed bending indicate that the endurance limit of the coated wire is only about 345 to 415 MPa (50,000 to 60,000 lb/in2).

Wire Rope. Wire rope is made of wires twisted together in certain typical constructions and may be either flat or round. Flat ropes consist of a number of strands of alternately right and left lay, sewed together with soft iron to form a band or belt; they are sometimes of advantage in mine hoists.

Round ropes are composed of a number of wire strands twisted around a hemp core or around a wire strand or wire rope. The standard wire rope is made of six strands twisted around a hemp core, but for special purposes, four, five, seven, eight, nine, or any reasonable number of strands may be used.

The hemp is usually saturated with a lubricant, which should be free from acids or corrosive substances;

this provides little additional strength but acts as a cushion to preserve the shape of the rope and helps to lubricate the wires. The number of wires commonly used in the strands are 4, 7, 12, 19, 24, and 37, depending on the service for which the ropes are intended.

When extra flexibility is required, the strands of a rope sometimes consist of ropes, which in turn are made of strands around a hemp core. Ordinarily, the wires are twisted into strands in the opposite direction to the twist of the strands in the rope. The makeup of standard hoisting rope is 6 X 19; extrapliable hoisting rope is 8 X 19 or 6 X 37; transmission or haulage rope is 6 X 7; hawsers and mooring lines are 6 X 12 or 6 X 19 or 6 X 24 or 6 X 37, etc.; tiller or hand rope is 6 X 7; highway guard-rail strand is 3 X 7; galvanized mast-arm rope is 9 X 4 with a cotton center.

The tensile strength of the wire ranges, in different grades, from 415 to 2415 MPa (60,000 to 350,000 lb/in2), depending on the material, diameter, and treatment. The maximum tensile efficiency of wire rope is 90%; the average is about 82.5%, being higher for 6 X 7 rope and lower for 6 X 37 construction.

The apparent modulus of elasticity for steel cables in service may be assumed to be 62 to 83 X 106 kPa (9 to 12 X 106 lb/in2) of cable section. Grades of wire rope are (from historic origins) referred to as traction, mild plow, plow, improved plow, and extra improved plow steel. The most common finish for steel wire is “bright” or uncoated, but various coatings, particularly zinc (galvanized), are used.

DIFFERENT TYPES OF STAINLESS STEELS BASIC INFORMATION AND TUTORIALS

Iron-base alloys containing between 11% and 30% chromium form a tenacious and highly protective chrome oxide layer that gives these alloys excellent corrosion-resistant properties. There are a great number of alloys that are generally referred to as stainless steels, and they fall into three general classifications.

Austenitic stainless steels contain usually 8% to 12% nickel, which stabilizes the austenitic phase. These are the most popular of the stainless steels. With 18% to 20% chromium, they have the best corrosion resistance and are very tough and can undergo severe forming operations.

These alloys are susceptible to embrittlement when heated in the range of 593 to 816°C. At these temperatures, carbides precipitate at the austenite grain boundaries, causing a local depletion of the chromium content in the adjacent region, so this region loses its corrosion resistance.

Use of “extra low carbon” grades and grades containing stabilizing additions of strong carbide-forming elements such as niobium minimizes this problem. These alloys are also susceptible to stress corrosion in the presence of chloride environments.

Ferritic stainless steels are basically straight Fe-Cr alloys. Chromium in excess of 14% stabilizes the low-temperature ferrite phase all the way to the melting point. Since these alloys do not undergo a phase change, they cannot be hardened by heat treatment. They are the least expensive of the stainless alloys.

Martensitic stainless steels contain around 12% Cr. They are austenitic at elevated temperatures but ferritic at low; hence they can be hardened by heat treatment.

To obtain a significant response to heat treatment, they have higher carbon contents than the other stainless alloys. Martensitic alloys are used for tools, machine parts, cutting instruments, and other applications requiring high strength. The austenitic alloys are nonmagnetic, but the ferritic and martensitic grades are ferromagnetic.

STRUCTURAL STEEL FABRICATION BASIC INFORMATION

When considering fabrication, as well as erection of the fabricated product, the designer must taken into account contractual matters, work by others on the construction team, schedule implications of the design, and quality assurance matters.

Fortunately, there are well established aids for these considerations. Contractual questions such as what constitutes structural steel, procedures for preparing and approving the shop detail drawings, and standard fabrication procedures and tolerances are all addressed in the AISC’s Code of Standard Practice.

Insights on economical connection details and the impact of material selection on mill material deliveries are generally available from the fabricator’s engineering staff. These engineers are also able to comment on unique erection questions.

Quality assurance questions fall into two categories, fabrication operations and field operations. Today, sound quality control procedures are in place in most fabrication shops through an AISC program which prequalifies fabricators.

There are three levels of qualification: I, II and III, with Level III being the most demanding. Fabricators with either a Level I or Level II certification are suitable for almost all building work. Most engineers incorporate the AISC’s Code of Standard Practice in their project specification.

Shop Detail Drawings

Detail drawings are prepared by the fabricator to delineate to his work force the fabrication requirements. Because each shop has certain differences in equipment and/or procedures, the fabricator develops details which, when matched with his processes, are the most economical.

To accomplish this end, the design drawings need to be complete, showing all structural steel requirements, and should include design information on the forces acting at connections. Designers should avoid specifying deck openings and beam penetrations through notes on the drawings. This is a frequent cause of extra costs on fabrication contracts.

Fabrication Processes

Mill material is cut to length by sawing, shearing, or flame cutting. Columns may also be milled to their final length. Holes for fasteners are drilled or punched. Punched and reamed holes are seldom used in building construction. Cuts for weld preparation, web openings, and dimensional clearances are flame cut.

AISC guidelines for each of these processes are associated with the AISC’s fabricator prequalification program. Welding for building construction is performed in accordance with the provisions of the AWS Structural Welding Code, D1.1. Most requirements can be satisfied using pre-qualified welding procedures.

HEAT TREATMENT OF STEEL BASICS AND CIVIL ENGINEERING TUTORIALS

DIFFERENT HEAT TREATMENT OF STEEL BASIC INFORMATION
What Are The Different Heat Treatment Of Steel?


Heat Treatment of Steel
Properties of steel can be altered by applying a variety of heat treatments. For example, steel can be hardened or softened by using heat treatment; the response of steel to heat treatment depends upon its alloy composition.

Common heat treatments employed for steel include annealing, normalizing, hardening, and tempering. The basic process is to heat the steel to a specific temperature, hold the temperature for a specified period of\ time, then cool the material at a specified rate.

Annealing
The objectives of annealing are to refine the grain, soften the steel, remove internal stresses, remove gases, increase ductility and toughness, and change electrical and magnetic properties. Four types of annealing can be performed, depending on the desired results of the heat treatment:

Full annealing requires heating the steel to about 50°C above the austenitic temperature line and holding the temperature until all the steel transforms into either austenite or austenite–cementite, depending on the carbon content.

The steel is then cooled at a rate of about 20°C per hour in a furnace to a temperature of about 680°C, followed by natural convection cooling to room temperature. Due to the slow cooling rate, the grain structure is a coarse pearlite with ferrite or cementite, depending on the carbon content.

The slow cooling rate ensures uniform properties of the treated steel. The steel is soft and ductile. Process annealing is used to treat work-hardened parts made with low carbon steel (i.e., less than 0.25 percent carbon). The material is heated to about 700°C and held long enough to allow recrystallization of the ferrite phase.

By keeping the temperature below 727°C, there is not a phase shift between ferrite and austenite, as occurs during full annealing. Hence, the only change that occurs is refinement of the size, shape, and distribution of the grain structure.

Stress relief annealing is used to reduce residual stresses in cast, welded, and cold-worked parts and cold formed parts. The material is heated to 600 to 650°C, held at temperature for about one hour, and then slowly cooled in still air.

Spheroidization is an annealing process used to improve the ability of high carbon (i.e., more than 0.6 percent carbon) steel to be machined or cold worked. It also improves abrasion resistance. The cementite is formed into globules (spheroids) dispersed throughout the ferrite matrix.

3.3.2 Normalizing
Normalizing is similar to annealing, with a slight difference in the temperature and
the rate of cooling. Steel is normalized by heating to about 60°C (110°F) above the
austenite line and then cooling under natural convection. The material is then
air cooled. Normalizing produces a uniform, fine-grained microstructure. However,
since the rate of cooling is faster than that used for full annealing, shapes with varying thicknesses results in the normalized parts having less uniformity than could
be achieved with annealing. Since structural plate has a uniform thickness, normalizing
is an effective process and results in high fracture toughness of the material.

Hardening
Steel is hardened by heating it to a temperature above the transformation range and holding it until austenite is formed. The steel is then quenched (cooled rapidly) by plunging it into, or spraying it with, water, brine, or oil. The rapid cooling “locks” the iron into a BCC structure, martensite, rather than allowing the transformation to the ferrite FCC structure.

Martensite has a very hard and brittle structure. Since the cooling occurs more rapidly at the surface of the material being hardened, the surface of the material is harder and more brittle than the interior of the element, creating nonhomogeneous characteristics.

Due to the rapid cooling, hardening puts the steel in a state of strain. This strain sometimes causes steel pieces with sharp angles or grooves to crack immediately after hardening. Thus, hardening must be followed by tempering.

Tempering
The predominance of martensite in quench-hardened steel results in an undesirable brittleness. Tempering is performed to improve ductility and toughness. Martensite is a somewhat unstable structure.

Heating causes carbon atoms to diffuse from martensite to produce a carbide precipitate and formation of ferrite and cementite. After quenching, the steel is cooled to about 40°C then reheated by immersion in either oil or nitrate salts. The steel is maintained at the elevated temperature for about two hours and then cooled in still air.

Example of Heat Treatment
In the quest to produce high-strength low-alloy steels economically, the industry has developed specifications for several new steel products, such as A913. This steel is available with yield stresses ranging from 50,000 to 75,000 psi.

The superior properties of A913 steel are obtained by a quench self-tempering process. Following the last hot rolling pass for shaping, for which the temperature is typically 850°C (1600°F), an intense water-cooling spray is applied to the surface of the beam to quench (rapidly cool) the skin.

Cooling is interrupted before the core on the material is affected. The outer layers are then tempered as the internal heat of the beam flows to the surface. After the short cooling phase, the self-tempering temperature is 600°C (1100°F).

BRIDGE STEELS BASIC AND TUTORIALS

BRIDGE STEELS BASIC INFORMATION
What Are The Type Of Steels Used In Constructing Bridges?


Steels for application in bridges are covered by A709, which includes steel in several of the categories mentioned above. Under this specification, grades 36, 50, 70, and 100 are steels with yield strengths of 36, 50, 70, and 100 ksi, respectively.

The grade designation is followed by the letter W, indicating whether ordinary or high atmospheric corrosion resistance is required. An additional letter, T or F, indicates that Charpy V-notch impact tests must be conducted on the steel.

The T designation indicates that the material is to be used in a non-fracture-critical application as defined by AASHTO; the F indicates use in a fracture-critical application.

A trailing numeral, 1, 2, or 3, indicates the testing zone, which relates to the lowest ambient temperature expected at the bridge site. (see Table Below)

As indicated by the first footnote in the table, the service temperature for each zone is considerably less than the Charpy V-notch impact-test temperature.

This accounts for the fact that the dynamic loading rate in the impact test is more severe than that to which the structure is subjected.

The toughness requirements depend on fracture criticality, grade, thickness, and method of connection. A709-HPS70W, designated as a High Performance Steel (HPS), is also now available for highway bridge construction.

This is a weathering plate steel, designated HPS because it possesses superior weldability and toughness as compared to conventional steels of similar strength.

For example, for welded construction with plates over 21⁄2 in thick, A709-70W must have a minimum average Charpy V-notch toughness of 35 ft lb at 10 F in Zone III, the most severe climate.

Toughness values reported for some heats of A709-HPS70W have been much higher, in the range of 120 to 240 ft lb at 10 F. Such extra toughness provides a very high resistance to brittle fracture.

(R. L. Brockenbrough, Sec. 9 in Standard Handbook for Civil Engineers, 4th ed., F. S.
Merritt, ed., McGraw-Hill, Inc., New York.)

STEEL STRUCTURES ERECTION EQUIPMENT CIVIL ENGINEERING TUTORIALS

STEEL STRUCTURES ERECTION EQUIPMENT TUTORIALS
What Are The Steel Structure Erection Equipment?

If there is a universal piece of erection equipment, it is the crane. Mounted on wheels or tractor threads, it is extremely mobile, both on the job and in moving from job to job.

Practically all buildings are erected with this efficient raising device. The exception, of course, is the skyscraper whose height exceeds the reach of the crane.  Operating on ground level, cranes have been used to erect buildings of about 20 stories, the maximum height being dependent on the length of the boom and width of building.


The guy derrick is a widely used raising device for erection of tall buildings. Its principal asset is the ease by which it may be ‘‘jumped’’ from tier to tier as erection proceeds upward. The boom and mast reverse position; each in turn serves to lift up the other.

It requires about 2 h to make a two-story jump. Stiff-leg derricks and gin poles are two other rigs sometimes used, usually in the role of auxiliaries to cranes or guy derricks. Gin poles are the most elementary— simply a guyed boom.

The base must be secure because of the danger of kicking out. The device is useful for the raising of incidental materials, for dismantling and lowering of larger rigs, and for erection of steel on light construction where the services of a crane are unwarranted.

Stiff-leg derricks are most efficient where they may be set up to remain for long periods of time. They have been used to erect multistory buildings but are not in popular favor because of the long time required to jump from tier to tier.

Among the principal uses for stiff legs are (1) unloading steel from railroad cars for transfer to trucks, (2) storage and sorting, and (3) when placed on a flat roof, raising steel to roof level, where it may be sorted and placed within each of a guy derrick.

Less time for ‘‘jumping’’ the raising equipment is needed for cranes mounted on steel box-type towers, about three stories high, that are seated on interior elevator wells or similar shafts for erecting steel.

These tower cranes are simply jacked upward hydraulically or raised by cables, with the previously erected steel-work serving as supports. In another method, a stiff-leg derrick is mounted on a trussed platform, spanning two or more columns, and so powered that it can creep up the erected exterior columns.

In addition to the advantage of faster jumps, these methods permit steel erection to proceed as soon as the higher working level is reached.

IRON CARBON EQUILIBRIUM DIAGRAM BASICS AND TUTORIALS

IRON CARBON EQUILIBRIUM DIAGRAM BASIC INFORMATION
What Is Iron-Carbon Equilibrium Diagram?


The iron-carbon equilibrium diagram in Figure below shows that, under equilibrium conditions (slow cooling) if not more than 2.0% carbon is present, a solid solution of carbon in gamma iron exists at elevated temperatures.

This is called austenite. If the carbon content is less than 0.8%, cooling below the A3 temperature line causes transformation of some of the austenite to ferrite, which is substantially pure alpha iron (containing less than 0.01% carbon in solution).

Still further cooling to below the A1 line causes the remaining austenite to transform to pearlite—the eutectoid mixture of fine plates, or lamellas, of ferrite and cementite (iron carbide) whose iridescent appearance under the microscope gives it its name.

If the carbon content is 0.8%, no transformation on cooling the austenite occurs until the A1 temperature is reached.

At that point, all the austenite transforms to pearlite, with its typical ‘‘thumbprint’’ microstructure.

At carbon contents between 0.80 and 2.0%, cooling below the Acm temperature line causes iron carbide, or cementite, to form in the temperature range between Acm and A1,3. Below A1,3, the remaining austenite transforms to pearlite.

STEEL MAKING METHODS BASICS AND TUTORIALS

STEEL MAKING METHODS BASIC INFORMATION
What Are The Basic Steel Making Techniques And Methods?


Structural steel is usually produced today by one of two production processes. In the traditional process, iron or ‘‘hot metal’’ is produced in a blast furnace and then further processed in a basic oxygen furnace to make the steel for the desired products.

Alternatively, steel can be made in an electric arc furnace that is charged mainly with steel scrap instead of hot metal. In either case, the steel must be produced so that undesirable elements are reduced to levels allowed by pertinent specifications to minimize adverse effects on properties.

In a blast furnace, iron ore, coke, and flux (limestone and dolomite) are charged into the top of a large refractory-lined furnace. Heated air is blown in at the bottom and passed up through the bed of raw materials.

A supplemental fuel such as gas, oil, or powdered coal is also usually charged. The iron is reduced to metallic iron and melted; then it is drawn off periodically through tap holes into transfer ladles.

At this point, the molten iron includes several other elements (manganese, sulfur, phosphorus, and silicon) in amounts greater than permitted for steel, and thus further processing is required.

In a basic oxygen furnace, the charge consists of hot metal from the blast furnace and steel scrap. Oxygen, introduced by a jet blown into the molten metal, reacts with the impurities present to facilitate the removal or reduction in level of unwanted elements, which are trapped in the slag or in the gases produced.

Also, various fluxes are added to reduce the sulfur and phosphorus contents to desired levels. In this batch process, large heats of steel may be produced in less than an hour.

An electric-arc furnace does not require a hot metal charge but relies mainly on steel scrap. The metal is heated by an electric arc between large carbon electrodes that project through the furnace roof into the charge.

Oxygen is injected to speed the process. This is a versatile batch process that can be adapted to producing small heats where various steel grades are required, but it also can be used to produce large heats.

Ladle treatment is an integral part of most steelmaking processes. The ladle receives the product of the steel making furnace so that it can be moved and poured into either ingot molds or a continuous casting machine.

While in the ladle, the chemical composition of the steel is checked, and alloying elements are added as required. Also, deoxidizers are added to remove dissolved oxygen. Processing can be done at this stage to reduce further sulfur content, remove undesirable nonmetallics, and change the shape of remaining inclusions.

Thus significant improvements can be made in the toughness, transverse properties, and through-thickness ductility of the finished product. Vacuum degassing, argon bubbling, induction stirring, and the injection of rare earth metals are some of the many procedures that may be employed.

Killed steels usually are deoxidized by additions to both furnace and ladle. Generally, silicon compounds are added to the furnace to lower the oxygen content of the liquid metal and stop oxidation of carbon (block the heat).

This also permits addition of alloying elements that are susceptible to oxidation. Silicon or other deoxidizers, such as aluminum, vanadium, and titanium, may be added to the ladle to complete deoxidation.

Aluminum, vanadium, and titanium have the additional beneficial effect of inhibiting grain growth when the steel is normalized. (In the hot-rolled conditions, such steels have about the same ferrite grain size as semikilled steels.)

Killed steels deoxidized with aluminum and silicon (made to finegrain practice) often are used for structural applications because of better notch toughness and lower transition temperatures than semikilled steels of the same composition.

(W. T. Lankford, Jr., ed., The Making, Shaping and Treating of Steel, Association of Iron and Steel Engineers, Pittsburgh, Pa.)

CORROSION RESISTANCE METHODS FOR STRUCTURAL STEEL BASIC AND TUTORIALS

CORROSION RESISTANCE METHODS FOR STRUCTURAL STEEL BASIC INFORMATION
What Are The Corrosion Resistance Methods For Structural Steel?

Since steel contains three of the four elements needed for corrosion, protective coatings can be used to isolate the steel from moisture, the fourth element. There are three mechanisms by which coatings provide corrosion protection (Hare, 1987):

1. Barrier coatings work solely by isolating the steel from the moisture. These coatings have low water and oxygen permeability.

2. Inhabitive primer coatings contain passivating pigments. They are low-solubility pigments that migrate to the steel surface when moisture passes through the film to passivate the steel surface.

3. Sacrificial primers (cathodic protection) contain pigments such as elemental zinc. Since zinc is higher than iron in the galvanic series, when corrosion conditions exist the zinc gives up electrons to the steel, becomes the anode, and corrodes to protect the steel.

There should be close contact between the steel and the sacrificial primer in order to have an effective corrosion protection.

Cathodic protection can take forms other than coating. For example, steel structures such as water heaters, underground tanks and pipes, and marine equipment can be electrically connected to another metal that is more reactive in the particular environment, such as magnesium or zinc.

Such reactive metal (sacrificial anode) experiences oxidation and gives up electrons to the steel, protecting the steel from corrosion. Figure 3.32 illustrates an underground steel tank that is electrically connected to a magnesium sacrificial anode (Fontana and Green, 1978).

Above is a diagram on Cathodic protection of an underground pipeline using a magnesium sacrificial anode.

STEEL CORROSION BASICS AND TUTORIALS

STEEL CORROSION BASIC INFORMATION
What Makes The Steel Corrode? What Is Steel Corrosion?


Corrosion is defined as the destruction of a material by electrochemical reaction to the environment. For simplicity, corrosion of steel can be defined as the destruction that can be detected by rust formation.

Corrosion of steel structures can cause serious problems and embarrassing and/or dangerous failures. For example, corrosion of steel bridges, if left unchecked, may result in lowering weight limits, costly steel replacement, or collapse of the structure.

Other examples include corrosion of steel pipes, trusses, frames, and other structures. It is estimated that the cost of corrosion of the infrastructure in the United States alone is $22.6 billion each year (corrosion costs web site, 2009).

The infrastructure includes (1) highway bridges, (2) gas and liquid transmission pipelines, (3) waterways and ports, (4) hazardous materials storage, (5) airports, and (6) railroads.

Corrosion is an electrochemical process; that is, it is a chemical reaction in which there is transfer of electrons from one chemical species to another. In the case of steel, the transfer is between iron and oxygen, a process called oxidation reduction.

Corrosion requires the following four elements (without any of them corrosion will not occur):
1. an anode—the electrode where corrosion occurs
2. a cathode—the other electrode needed to form a corrosion cell
3. a conductor—a metallic pathway for electrons to flow
4. an electrolyte—a liquid that can support the flow of electrons


Steel, being a heterogeneous material, contains anodes and cathodes. Steel is also an electrical conductor. Therefore, steel contains three of the four elements needed for corrosion, while moisture is usually the fourth element (electrolyte).

The actual electrochemical reactions that occur when steel corrodes are very complex. However, the basic reactions for atmospherically exposed steel in a chemically neutral environment are dissolution of the metal at the anode and reduction of oxygen at the cathode.

Contaminants deposited on the steel surface affect the corrosion reactions and the rate of corrosion. Salt, from deicing or a marine environment, is a common contaminant that accelerates corrosion of steel bridges and reinforcing steel in concrete.

The environment plays an important role in determining corrosion rates. Since an electrolyte is needed in the corrosion reaction, the amount of time the steel stays wet will affect the rate of corrosion.

Also, contaminants in the air, such as oxides or sulfur, accelerate corrosion. Thus, areas with acid rain, coal-burning power plants, and other chemical plants may accelerate corrosion.

TORSION TEST ON STRUCTURAL STEEL BASICS AND TUTORIALS

TORSION TEST ON STRUCTURAL STEEL BASIC INFORMATION
What Is Torsion Test Of Steel?


The torsion test (ASTM E143) is used to determine the shear modulus of structural materials. The shear modulus is used in the design of members subjected to torsion, such as rotating shafts and helical compression springs.

In this test a cylindrical, or tubular, specimen is loaded either incrementally or continually by applying an external torque to cause a uniform twist within the gauge length. The amount of applied torque and the corresponding angle of twist are measured throughout the test.

Below shows the shear stress–strain curve.

The shear modulus is the ratio of maximum shear stress to the corresponding shear strain below the proportional limit of the material, which is the slope of the straight line between R (a pretorque stress) and P (the proportional limit). For a circular cross section, the maximum shear stress shear strain and the shear modulus (G) are determined by the equations:

where

T = torque
r = radius

J = polarmoment of inertia of the specimen about its center, for a solid circular cross section.

0 = angle of twist in radians
L = gauge length



The test method is limited to materials and stresses at which creep is negligible compared with the strain produced immediately upon loading. The test specimen should be sound, without imperfections near the surface.

Also, the specimen should be straight and of uniform diameter for a length equal to the gauge length plus two to four diameters. The gauge length should be at least four diameters.

During the test, torque is read from a dial gauge or a readout device attached to the testing machine, while the angle of twist may be measured using a torsiometer fastened to the specimen at the two ends of the gauge length.

A curve-fitting procedure can be used to estimate the straight-line portion of the shear stress–strain relation.

STRUCTURAL STEEL TENSION TEST BASICS AND TUTORIALS

STRUCTURAL STEEL TENSION TEST BASIC INFORMATION
What Is Structural Steel Tension Test?


The tension test (ASTM E8) on steel is performed to determine the yield strength, yield point, ultimate (tensile) strength, elongation, and reduction of area. Typically, the test is performed at temperatures between 10°C and 35°C (50°F to 95°F).

The test specimen can be either full sized or machined into a shape, as prescribed in the product specifications for the material being tested. It is desirable to use a small cross-sectional area at the center portion of the specimen to ensure fracture within the gauge length.

Several cross-sectional shapes are permitted, such as round and rectangular, as shown in Figure 3.15. Plate, sheet, round rod, wire, and tube specimens may be used. A 12.5 (1/2 in.) diameter round specimen is used in many cases. The gauge length over which the elongation is measured typically is four times the diameter for most round-rod specimens.

Various types of gripping devices may be used to hold the specimen, depending on its shape. In all cases, the axis of the test specimen should be placed at the center of the testing machine head to ensure axial tensile stresses within the gauge length without bending.

An extensometer with a dial gauge or an LVDT is used to measure the deformation of the entire gauge length. The test is performed by applying an axial load to the specimen at a specified rate.

Mild steel has a unique stress–strain relation. As the stress is increased beyond the proportion limit, the steel will yield, at which time the strain will increase without an increase in stress (actually the stress will slightly decrease). As tension increases past the yield point, strain increases following a nonlinear relation up to the point of failure.

Note that the decrease in stress after the peak does not mean a decrease in strength. In fact, the actual stress continues to increase until failure. The reason for the apparent decrease is that a neck is formed in the steel specimen, causing an appreciable decrease in the cross-sectional area.

The traditional, or engineering, way of calculating the stress and strain uses the original cross-sectional area and gauge length. If the stress and stains are calculated based on the instantaneous cross-sectional area and gauge length, a true stress–strain curve is obtained, which is different than the engineering stress–strain curve.

The true stress is larger than the engineering stress, because of the reduced cross-sectional area at the neck. Also, the true strain is larger than the engineering strain, since the increase in length at the vicinity of the neck is much larger than the increase in length outside of the neck.

The specimen experiences the largest deformation (contraction of the cross-sectional area and increase in length) at the regions closest to the neck, due to the nonuniform distribution of the deformation. The large increase in length at the neck increases the true strain to a large extent because the definition of true strain utilizes a ratio of the change in length in an infinitesimal gauge length.

By decreasing the gauge length toward an infinitesimal size and increasing the length due to localization in the neck, the numerator of an expression is increased while the denominator stays small, resulting in a significant increase in the ratio of the two numbers.

Note that when calculating the true strain, a small gauge length should be used at the neck, since the properties of the material (such as the cross section) at the neck represent the true material properties. For various practical applications, however, the engineering stresses and strains are used, rather than the true stresses and strains.

Different carbon-content steels have different stress–strain relations. Increasing the carbon content in the steel increases the yield stress and reduces the ductility. Below shows the tension stress–strain diagram for hot-rolled steel bars containing carbons from 0.19% to 0.90%.

Increasing the carbon content from 0.19% to 0.90% increases the yield stress from 280 MPa to 620 MPa (40 ksi to 90 ksi). Also, this increase in carbon content decreases the fracture strain from about 0.27 m/m to 0.09 m/m. Note that the increase in carbon content does not change the modulus of elasticity.


Steel is generally assumed to be a homogeneous and isotropic material. However, in the production of structural members, the final shape may be obtained by cold rolling.

This essentially causes the steel to undergo plastic deformations, with the degree of deformation varying throughout the member. Plastic deformation causes an increase in yield strength and a reduction in ductility.

This figure demonstrates that the measured properties vary, depending on the orientation of the sample relative to the axis of rolling (Hassett, 2003). Thus, it is necessary to specify how the sample is collected when evaluating the mechanical properties of steel.

COLD FORMED STEEL SHAPES BASICS AND TUTORIALS

COLD FORMED STEEL SHAPES BASIC INFORMATION
What Are The Different Cold-Formed Steel Shapes?


A wide variety of shapes can be produced by cold-forming and manufacturers have developed a wide range of products to meet specific applications.

 Figure 3.11 shows the common shapes of typical cold-formed steel framing members.

Figure 3.12 shows common shapes for profiled sheets and trays used for roofing and wall cladding and for load bearing deck panels.

For common applications, such as structural studs, industry organizations, such as the Steel Framing Alliance (SFA) and the Steel Stud Manufacturers Association (SSMA) have developed standard shapes and nomenclature to promote uniformity of product availability across the industry.

Figure 3.11 shows the generic shapes covered by the Universal Designator System. The designator consists of four sequential codes.

The first code is a three or four-digit number indicating the member web depth in 1/100 inches. The second is a single letter indicating the type of member, as follows:


framing member with stiffening lips
L = Angle or L-header
F = Furring channels
U = Cold-rolled channel
T = Track section
S = Stud or joist


The third is a three-digit numeral indication flange width in 1/100 inches followed by a dash. The fourth is a two or three-digit numeral indicating the base steel thickness in 1/1000 inch (mils).

As an example, the designator system for a 6'', C-shape with 1-5/8'' (1.62'') flanges and made with 0.054'' thick steel is 600S162-54.

COLD FORMED STEEL SPECIAL DESIGN CONSIDERATIONS BASIC AND TUTORIALS

COLD FORMED STEEL SPECIAL DESIGN CONSIDERATIONS BASIC INFORMATION
What Are The Special Design Considerations for Cold-Formed Steel?


Structural design of cold-formed members is in many respects more challenging than the design of hot rolled, relatively thick, structural members. A primary difference is cold-formed members are more susceptible to buckling due to their limited thickness.

The fact that the yield strength of the steel is increased in the cold-forming process creates a dilemma for the designer. Ignoring the increased strength is conservative, but results in larger members, hence more costly, than is needed if the increased yield strength is considered.

Corrosion creates a greater percent loss of cross section than is the case for thick members. All cold-formed steel members are coated to protect steel from corrosion during the storage and transportation phases of construction as well as for the life of the product.

Because of its effectiveness, hot-dipped zinc galvanizing is most commonly used. Structural and non-structural framing members are required to have a minimum metallic coating that complies with ASTM A1003/A1003M, as follows:

■ structural members – G60 and
■ non-structural members G40 or equivalent minimum.

To prevent galvanic corrosion special care is needed to isolate the cold-formed members from dissimilar metals, such as copper.

The design, manufacture and use of cold-formed steel framing is governed by standards that are developed and maintained by the American Iron and Steel Institute along with organizations such as ASTM, and referenced in the building codes.

Additional information is available at www.steelframing.org.

SHANLEY'S THEORY OF INELASTIC BUCKLING OF STEEL MEMBERS BASIC AND TUTORIALS

SHANLEY'S THEORY OF INELASTIC BUCKLING OF STEEL MEMBERS BASIC INFORMATION
What Is Shanley's Theory of Inelastic Buckling Of Steel Members?



Although the tangent modulus theory appears to be invalid for inelastic materials, careful experiments have shown that it leads to more accurate predictions than the apparently rigorous reduced modulus theory.

This paradox was resolved by Shanley [1], who reasoned that the tangent modulus theory is valid when buckling is accompanied by a simultaneous increase in the applied load (see Figure 3.8) of sufficient magnitude to prevent strain reversal in the member.

When this happens, all the bending stresses and strains are related by the tangent modulus of elasticity Et , the initial modulus E does not feature, and so the buckling load is equal to the tangent modulus value Ncr,t .

As the lateral deflection of the member increases as shown in Figure 3.8, the tangent modulus Et decreases (see Figure 3.6b) because of the increased axial and bending strains, and the post-buckling curve approaches a maximum load Nmax which defines the ultimate resistance of the member.

Also shown in Figure 3.8 is a post-buckling curve which commences at the reduced modulus load Ncr,r (at which buckling can take place without any increase in the load). The tangent modulus load Ncr,t is the lowest load at which buckling can begin, and the reduced modulus load Ncr,r is the highest load for which the member can remain straight.

It is theoretically possible for buckling to begin at any load between Ncr,t and Ncr,r . It can be seen that not only is the tangent modulus load more easily calculated, but it also provides a conservative estimate of the member resistance, and is in closer agreement with experimental results than the reduced modulus load.

For these reasons, the tangent modulus theory of inelastic buckling has gained wide acceptance.

HANDBOOK OF STRUCTURAL STEEL CONNECTION DESIGN AND DETAILS FREE EBOOK DOWNLOAD LINK

HANDBOOK OF STRUCTURAL STEEL CONNECTION DESIGN AND DETAILS FREE EBOOK
Free E-Book Download Link: Handbook of Structural Steel Connection Design and Details

Handbook of Structural Steel Connection Design and Details Editorial Reviews


This book not not only gives you the best and latest methods in connection design, it supplies fabricated examples on the CD-ROM that you can use for instant application and configuration of your own designs.

Featuring a broad range of design methods and details, the Handbook demonstrates the newest techniques and materials in welded joint design and production...seismically resistant connnections...partially restrained connections...steel decks...inspection and quality control...and more.

You get the newest connection designs based on load and resistance factor AISC design methods; special methods for seismic connection design; new material on fracture and fatigue design; improved methods of connection force analysis for various structures; 400 illustrations that show you how to do the job right; and much more.

Book Description
Publication Date: April 15, 1999 | ISBN-10: 0070614970 | ISBN-13: 978-0070614970 | Edition: 1

About the Author
Akbar R. Tamboli is a senior project engineer with CUH2A in Princeton, New Jersey. He was previously vice president and project manager with Irwin G. Cantor, P.E., Consulting Engineers in New York City. A Fellow of the American Society of Civil Engineers, Mr. Tamboli is the editor of Steel Design Handbook: LRFD Method, published by McGraw-Hill.

DOWNLOAD LINK HERE!!!

STEEL STRUCTURES BRITTLE FRACTURES UNDER IMPACT LOAD BASICS AND TUTORIALSER

BRITTLE FRACTURES OF STEEL STRUCTURES UNDER IMPACT LOAD BASIC INFORMATION
What Are Brittle Fractures Of Steel Structures?


Structural steel does not always exhibit a ductile behaviour, and under some circumstances a sudden and catastrophic fracture may occur, even though the nominal tensile stresses are low. Brittle fracture is initiated by the existence or formation of a small crack in a region of high local stress.

Once initiated, the crack may propagate in a ductile (or stable) fashion for which the external forces must supply the energy required to tear the steel. More serious are cracks which propagate at high speed in a brittle (or unstable) fashion, for which some of the internal elastic strain energy stored in steel is released and used to fracture the steel.

Such a crack is self-propagating while there is sufficient internal strain energy, and will continue until arrested by ductile elements in its path which have sufficient deformation capacity to absorb the internal energy released.

The resistance of a structure to brittle fracture depends on the magnitude of local stress concentrations, on the ductility of the steel, and on the three-dimensional geometrical constraints. High local stresses facilitate crack initiation, and so stress concentrations due to poor geometry and loading arrangements (including impact loading) are dangerous.

Also of great importance are flaws and defects in the material, which not only increase the local stresses, but also provide potential sites for crack initiation.

The ductility of a structural steel depends on its composition, heat treatment, and thickness, and varies with temperature and strain rate. Figure 1.11 shows the increase with temperature of the capacity of the steel to absorb energy during impact.

At low temperatures the energy absorption is low and initiation and propagation of brittle fractures are comparatively easy, while at high temperatures the energy absorption is high because of ductile yielding, and the propagation of cracks can be arrested.

Between these two extremes is a transitional range in which crack initiation becomes increasingly difficult. The likelihood of brittle fracture is also increased by high strain rates due to dynamic loading, since the consequent increase in the yield stress reduces the possibility of energy absorption by ductile yielding.

The chemical composition of steel has a marked influence on its ductility: brittleness is increased by the presence of excessive amounts of most non-metallic elements, while ductility is increased by the presence of some metallic elements.


Steel with large grain size tends to be more brittle, and this is significantly influenced by heat treatment of the steel, and by its thickness (the grain size tends to be larger in thicker sections). EC3-1-10 [18] provides values of the maximum thickness t1 for different steel grades and minimum service temperatures, as well as advice on using a more advanced fracture mechanics [34] based approach and guidance on safeguarding against lamellar tearing.

Three-dimensional geometrical constraints, such as those occurring in thicker or more massive elements, also encourage brittleness, because of the higher local stresses, and because of the greater release of energy during cracking and the consequent increase in the ease of propagation of the crack.

The risk of brittle fracture can be reduced by selecting steel types which have ductilities appropriate to the service temperatures, and by designing joints with a view to minimising stress concentrations and geometrical constraints.

Fabrication techniques should be such that they will avoid introducing potentially dangerous flaws or defects. Critical details in important structures may be subjected to inspection procedures aimed at detecting significant flaws.

Of course the designer must give proper consideration to the extra cost of special steels, fabrication techniques, and inspection and correction procedures.