Ploymer

POLYMER PROCESSING

The processing of polymeric materials -plastics, elastomers and composites- is characterized by a wide variety of distinct methods or techniques. Techniques involving the continuous manufacture of a product basically have uniform cross section, which include extrusion, extrusion covering, film blowing and calendering; techniques involving the shaping of a deformable polymer perform against a mold surface, which involve coating and rotational molding; and, finally, techniques which involve the complete filling of a mold cavity, and include casting, compression molding, transfer molding, injection molding and reaction injection molding.Fundamental to the choice of polymer processing technique is the question of whether to use a high molecular mass starting material or a system that polymerizes in the mold.

In the liquid state, most monomers and low molecular mass polymers flow in much the same way as molten metals in that the shear stress needed to make them flow is directly proportional to the shear strain rate- they are Newtonian fluids. As their molecular masses increase their viscosities increase but at some point the long thin chains begin to rearrange themselves under the applied shear stressed to line up in the direction of flow, and the proportionality between stress and strain rate starts to change- the polymer has become non-Newtonian.The consequences of the much higher pressures needed to cast high molecular mass polymers are not difficult to appreciate. In addition, the arrangements for keeping the mold closed will need to be more robust since the pressures applied to the mold tend to force the two mold halves open during filling and feeding. And the molds themselves must be made from stronger materials to withstand being repeatedly exposed to these pressures.

Reducing the viscosity of the polymer will clearly allow higher flow rates at the same applied pressures, or permit the use of substantial machinery and tooling. The polymer chains tangle around each other and form so-called ‘mechanical cross links’, effectively strengthening the material in the solid state but making it more difficult to cast in the fluid state. So the grade polymer, which is easy to cast, is going to give inferior performance in the end product and the best performance will be obtained from a material that is more difficult to cast.

Extrusion

The extrusion process basically is continuously shaping a fluid polymer through the orifice of a suitable tool (die), and subsequently solidifying it into a product (extrudate of constant cross section). In the case of thermoplastics, the feed material, in powder or pellet form, is most commonly heated to a fluid state and pumped into the die, through a screw extruder; it is then solidified by cooling after exiting from the die.Extrusion products are often subdivided into groups that include filaments of circular cross-section, profiles of irregular cross section, axissymmetric tubes and pipes, and flat products such as films or sheets.    

Film Blowing

The film blowing process basically consists of a extruding a tube of molten thermoplastic and continuously inflating it to several times initial diameter, to form a thin tubular product that can be used directly, or slit to form a flat film.Films produced by the film blowing process are widely used for agricultural, construction, and industrial applications, including covers for silage, greenhouses, chemical/solar ponds, flat cars, etc., or for a variety of packing applications, with include wrap, can lining, fabricated bags such as garbage bags, gusseted (V-tucks, W-folds) fashion bags, and T-shirt bags (sacks) for groceries.  

Sheet Thermoforming

Sheet thermoforming, or simply thermoforming, involves the heating of a flat thermoplastic sheet to a softened state (above the glass transition temperature T_g for non-crystallizing thermoplastics or near the melting temperature T_m for crystallizing ones), followed by the deformation (forming) of the softened sheet into a desired shape by pneumatic or mechanical means, and finally its solidification into this shape by cooling.Products made by sheet thermoforming include skin and blisterpacks, individual containers for jelly or cream, vials, cups, tubs, trays and lids. As many as millions of parts per day can be produced with a tool featuring several hundred cavities. Larger products are generally made from cut sheets at much shower rates; the heating stage often is the limiting factor. Transparent products, such as contoured windows, skylights and cockpit canopies, are often made by this method.

Blow Molding

The basic principle of the blow molding process is to inflate a softened thermoplastic hollow preform against the cooled surface of a closed mold, where the material solidifies into a hollow product.Packing is the major area of application of small to medium-size disposable blow molded products. Liquid foodstuffs are increasingly packaged in narrow neck plastic PET bottles. Blow molded containers are also used for cosmetics, toiletries, pharmaceutical and medical packaging and a variety of household products.

Compression Molding

The compression molding process is used for temperature activated thermosetting polymers. Compression molding basically involves the pressing of a deformable material charge between the two halves of a heated mold, and its transformation into a solid product under the effect of the elevated mold temperature. Compression molding temperatures are often in the range 140-200 ⁰C; mold pressures can vary from 35 atm to 700 atm. Material charges are often pre-heated to speed up the initial softening stage. Compression molding is characterized by the show and moderate flow of the very viscous material charge to fill the cavity, and it is not normally suitable for making complicated parts, or parts featuring fragile inserts.

Transfer Molding

Transfer molding is often associated with compression molding, because it is used with the same two classes of materials, temperature-activated thermosets, and vulcanizable rubbers. In transfer molding a softened temperature-activated material is transferred through a narrow gate into the closed cavity of a heated mold, where it cures to a solid state.

Transfer molding is normally used with materials that have fairly high pre-curing fluidity, facilitating the flow from the loading area to cavities. This also permits the molding of complex parts, parts featuring fragile inserts.

Injection Molding

The injection molding process involves the rapid pressure filling of a specific mold cavity with a fluid material, followed by the solidification of the material into a product. The process is used for thermoplastics, thermosetting resins, and rubbers.The injection molding of thermoplastics can be subdivided into a several stages. At the plasticity stage, the feed unit operates pretty much as an extruder, melting and homogenizing the material in the screw/barrel system. The screw however is allowed to retract, to make room for the molten material reservoir. At the injection state, the screw is used as a ram for the rapid transfer of the molten material from the reservoir to the cavity between the two halves of the closed mold. Since the mold is kept at a temperature below the solidification temperature of the material, it is essential to inject the molten material rapidly ensure complete filling of the cavity. A high holding packing pressure is normally exerted, to partially compensate for the thermal contraction of the material upon cooling. After the cooling stage, the mold can be opened and the solid product removed. The cavity pressure rises rapidly during the filling stage, which is followed by the holding or the packing stage. Once the gate freezes off, the cavity pressure decays with time till the part is ejected. At high pressures, a polymer melt is compressible, allowing additional material to be packed in the mold cavity after mold filling is complete. This is necessary to reduce non-uniform part shrinkage, which leads to part warpage. Excessive packing results in a highly stressed part and may cause ejection problems whereas insufficient packing causes poor surface, sink marks, welds and non-uniform shrinkage. All thermoplastics are, in principle, suitable for injection molding, but since fast flow rates are needed, grades with good fluidity are normally preferable. Significant differences in ease of molding, and the resulting structure and properties of products are found between amorphous and crystallizing thermoplastics; they concern problems such as shrinkage, warpage, sink marks, flashing, and short shots.The pressure distribution inside the mold cavity changes with distance from the inlet gate. The figure 8 shows a simple part geometry with pressure variations among the points one, two and three respectively. Further away from the gate, pressure rises slowly and it decays quicker than at the points closer to the gate. The pressure in the mold cavity should be more uniform to minimize part warpage.

A major disadvantage of injection molded products is the incorporation of fine details such as bosses, locating pins, mounting holes, ribs, flanges, etc., which normally eliminates assembly and finishing operations. The gate freezes off first because it is thinner than the cavity. Once the part temperature is well below the polymer solidification temperature, the part is ejected.

PROBLEMS ENCOUNTERED IN INJECTION MOLDING

There are many details to pay attention in injection molding which affects the physical properties, they may even cause to the failure of the molding.

Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. This leads to part weakness, surface blemishes, and a multiplicity of internal defects.

An air trap is air that is caught inside the mold cavity. It becomes trapped by converging polymer melt fronts or because it failed to escape from the mold vents, or mold inserts, which also act as vents.A short shot is a molded part that is incomplete because insufficient material was injected into the mold. It can be caused by entrapped air, insufficient machine injection pressure (resulting from high melt resistance and a restricted flow path), pre-mature solidification of the polymer melt, and machine defects.

A sink mark is a local surface depression and a void is a vacuum bubble in the core. Sink marks and voids are caused by localized shrinkage of the material at thick sections without sufficient compensation when the part is cooling.A weld line (also called a weld mark or a knit line) is formed when separate melt fronts traveling in opposite directions meet. The formation of weld lines can be caused by holes or inserts in the part, multi-gate cavity systems, or variable wall thickness where hesitation or race tracking occurs. The weld lines are undesirable when the strength and the surface quality are important.

Identify thermoplastic and thermosetting plastics by flame test.

Experiment No :1
Identify thermoplastic and thermosetting plastics by flame test.
Materials And Equipments Required:
• Tong.                                     

• Bunsen Burner.

• Plastic samples.
• Beakers,etc.
Principle:
The experiment is based on the property of plastics.The plastic when expressed to heat if the sample is being softened and can be stretched easily,it is the thermoplastic.If the sample burns with a burning smell,it is thermosetting.
Thermoplastic:
A plastic which will soften when heated and harden when cooled.A thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak Van der Waals forces or even stacking of aromatic rings. Many thermoplastic materials are addition polymers; e.g., vinyl chain-growth polymers such as polyethylene and polypropylene.
Plastic flow takes very readily due to heating of their structure,which breaks the secondary bonds between the individual chains.Most thermoplastic consists of very long chains of carbon atoms covalently bonded together.In thermoplastic the long molecular chains are bonded to each other by seconday bond. Thermoplastics are recyclable materials that are used frequently today to create objects such as foam cups, polyethylene squeeze bottles, acrylic lenses and safety helmets. Their unique properties set them apart from the other two types of plastic, thermosets and elastomers. Heat and pressure cause the molecular structure of a thermoplastic to change, making it pliable and more useful.
Thermoplastics are the only type of plastic that can be welded. When heated to about 120 to 180 degrees Celsius, thermoplastics become pasty or liquid, and they can be molded and formed into different shapes. Each type of thermoplastic has its own properties, characteristics and welding temperature. They can withstand multiple heating and re-shaping, which makes them recyclable. Once the thermoplastic object has cooled down, the new form is set.Types of thermoplastics include polyethylene (PE), polyvinyl chloride (PVC) and polystyrene (PS), which often are used for packaging. Other groups of thermoplastics are acrylics, fluoropolymers, polyesters, polyimides and nylons. All of these types can be melted down many times and re-shaped into different forms. For example, a foam cup is a thermoplastic material that can be heated and turned into a dish.
Thermosetting:
Thermosetting plastics are simply plastics when moulded into shape and allowed to cool will not change shape when heated again they will char or burn an example of a thermosetting plastic is urea formaldehyde.These plastics have their dimensional network structures with predominantly covalent bonding. Thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat-forming.
Thermoset plastics differ in that they are not re-mouldable. Strong cross links are formed during the initial moulding process that give the material a stable structure. They are more likely to be used in situations where thermal stability is required. They tend to lack tensile strength and can be brittle.Once the curing process is complete,these plastics cannot be softened or reworked.
Some examples of thermosets are:
• Polyester fibreglass systems: (SMC Sheet molding compounds and BMC Bulk molding compounds).
• Vulcanized rubber.
• Bakelite, a phenol-formaldehyde resin (used in electrical insulators and plasticware).
• Duroplast, light but strong material, similar to Bakelite (used for making car parts).
• Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard).
• Melamine resin (used on worktop surfaces).
• Epoxy resin (used as the matrix component in many fibre reinforced plastics such as glass reinforced plastic and graphite-reinforced plastic).
• Polyimides (used in printed circuit boards and in body parts of modern airplanes).
• Cyanate Esters or Polycyanurates for electronics applications with high demands on dielectric properties and high glass temperature requirements in composites.
• Mold or Mold Runners (the black plastic part in Integrated Circuits (IC) or semiconductors).
Procedure:
1. Take different samples of plastics and label them A,B,C.
2. Take sample A and clamp it in the tong.
3. Place the sample on Bunsen flame.
4. Observe the burning of sample.
5. Note the observation that whether sample are being softened or decompose.
6. Repeat procedure with other samples.
7. Record the observations.
Observations:
        Sample No.                    Nature of Plastic
       (Thermoplastic/Thermosetting)
           A        Thermoplastic
           B        Thermoplastic
           C        Thermosetting

mechanical property of polymer and ceramic

BRITTLE FRACTURE OF CERAMICS

Both crystalline and noncrystalline ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load.The brittle fracture process consists of the formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load. Crack growth in crystalline ceramics may be either transgranular (i.e., through the grains) or intergranular (i.e., along grain boundaries); for transgranular fracture, cracks propagate along specific crystallographic (or cleavage) planes, planes of high atomic density. This may be explained by very small and omnipresent flaws in the material that serve as stress raisers—points at which the magnitude of an applied tensile stress is amplified.The degree of stress amplification depends on crack length and tip radius of curvature.

Fractography of Ceramics

A fractographic study  is normally a part of such an analysis, which involves examining the path of crack propagation as well as microscopic features of the fracture surface. It is often possible to conduct an investigation of this type using simple and inexpensive equipment—for example, a magnifying glass, and/or a low-power stereo binocular optical microscope in conjunction with a light source. When higher magnifications are required the scanning electron microscope is utilized.

STRESS–STRAIN BEHAVIOR

The stress at fracture using this flexure test is known as the flexural strength, modulus of rupture, fracture strength, or the bend strength, an important mechanical parameter for brittle ceramics. A more suitable transverse bending test is most frequently employed, in which a rod specimen having either a circular or rectangular cross section is bent until fracture using a three- or four-point loading technique.

Elastic Behavior

The elastic stress–strain behavior for ceramic materials using these flexure tests is

similar to the tensile test results for metals: a linear relationship exists between stress and strain.

MECHANISMS OF PLASTIC DEFORMATION

Crystalline Ceramics

For crystalline ceramics, plastic deformation occurs, as with metals, by the motion

of dislocations. One reason for the hardness and brittleness of these materials is the difficulty of slip (or dislocation motion). For crystalline ceramic materials for which the bonding is predominantly ionic, there are very few slip systems along which dislocations

may move.This is a consequence of the electrically charged nature of the ions.

Noncrystalline Ceramics

Plastic deformation does not occur by dislocation motion for noncrystalline ceramics

because there is no regular atomic structure. Rather, these materials deform by viscous

flow, the same manner in which liquids deform; the rate of deformation is proportional

to the applied stress.

The characteristic property for viscous flow, viscosity, is a measure of a noncrystalline

material’s resistance to deformation.

MISCELLANEOUS MECHANICAL CONSIDERATIONS

Influence of Porosity

Subsequent to compaction or forming of these powder particles into the desired shape, pores or void spaces will exist between the powder particles. During the ensuing heat treatment, much of this porosity will be eliminated; however, it is often the case that this pore elimination process is incomplete and some residual porosity will remain .Any

residual porosity will have a deleterious influence on both the elastic properties and strength.. It has been observed for some ceramic materials that the magnitude of the modulus of elasticity E decreases with volume fraction porosity.

Hardness

One beneficial mechanical property of ceramics is their hardness, which is often

utilized when an abrasive or grinding action is required; in fact, the hardest known

materials are ceramics.

Creep

Often ceramic materials experience creep deformation as a result of exposure to

stresses (usually compressive) at elevated temperatures.

Mechanical Behavior of Polymers

STRESS–STRAIN BEHAVIOR

The mechanical characteristics of polymers, for the most part, are highly sensitive to the

rate of deformation (strain rate), the temperature, and the chemical nature of the environment (the presence of water, oxygen, organic solvents, etc.) Three typically different types of stress–strain behavior are found for polymeric materials, stress–strain character for a brittle polymer, inasmuch as it fractures while deforming elastically. Second one is similar to that for many metallic materials; the initial deformation is elastic, which is followed by yielding and a region of plastic deformation.Third one is totally elastic; this rubber-like elasticity (large recoverable strains produced at low stress levels) is displayed by a class of polymers termed the elastomers.

MACROSCOPIC DEFORMATION

Some aspects of the macroscopic deformation of semicrystalline polymers deserve our

attention. The tensile stress–strain curve for a semicrystalline material, which was initially undeformed, is shown in Figure 15.4; also included in the figure are schematic

representations of the specimen profiles at various stages of deformation. Both upper

and lower yield points are evident on the curve, which are followed by a near horizontal

region. At the upper yield point, a small neck forms within the gauge section of the specimen.Within this neck, the chains become oriented (i.e., chain axes become aligned parallel to the elongation direction, a condition that is represented schematically, which leads to localized strengthening. Consequently, there is a resistance to continued deformation at this point, and specimen elongation proceeds by the propagation of this neck region along the gauge length; the chain orientation phenomenon  accompanies this neck extension. This tensile behavior may be contrasted to that found for ductile metals ,wherein once a neck has formed, all subsequent deformation is confined to within the neck region.

VISCOELASTIC DEFORMATION

For intermediate temperatures the polymer is a rubbery solid that exhibits the combined mechanical characteristics of these two extremes; the condition is termed viscoelasticity.

Elastic deformation is instantaneous, which means that total deformation (or strain) occurs the instant the stress is applied or released (i.e., the strain is independent of time). In addition, upon release of the external stress, the deformation is totally recovered—the specimen assumes its original dimensions.A familiar example of these viscoelastic extremes is found in a silicone polymer that is sold as a novelty and known by some as “silly putty.”When rolled into a ball and dropped onto a horizontal surface, it bounces elastically—the rate of deformation during the bounce is very rapid.

On the other hand, if pulled in tension with a gradually increasing applied stress, the material elongates or flows like a highly viscous liquid. For this and other viscoelastic materials, the rate of strain determines whether the deformation is elastic or viscous.

Viscoelastic Relaxation Modulus

Stress is found to decrease with time due to molecular relaxation processes that take place within the polymer.We may define a relaxation modulus a time-dependent elastic modulus for viscoelastic polymers.

Furthermore, the magnitude of the relaxation modulus is a function of temperature;

and to more fully characterize the viscoelastic behavior of a polymer, isothermal stress relaxation measurements must be conducted over a range of temperatures.

Viscoelastic Creep

Many polymeric materials are susceptible to time-dependent deformation when the

stress level is maintained constant; such deformation is termed viscoelastic creep.

This type of deformation may be significant even at room temperature and under modest stresses that lie below the yield strength of the material. For example, automobile tires may develop flat spots on their contact surfaces when the automobile is parked for prolonged time periods.

FRACTURE OF POLYMERS

As a general rule, the mode of fracture in thermosetting polymers (heavily crosslinked networks) is brittle.For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing a ductile-to-brittle transition. Factors that favor brittle fracture are a reduction in temperature, an increase in strain rate, the presence of a sharp notch, increased specimen thickness, and any modification of the polymer structure that raises the glass transition temperature.

One phenomenon that frequently precedes fracture in some thermoplastic polymers

is crazing

DEFORMATION OF ELASTOMERS

One of the fascinating properties of the elastomeric materials is their rubber-like elasticity.That is, they have the ability to be deformed to quite large deformations, and

then elastically spring back to their original form.In an unstressed state, an elastomer will be amorphous and composed of crosslinked molecular chains that are highly twisted, kinked, and coiled. Elastic deformation,upon application of a tensile load, is simply the partial uncoiling, untwisting,and straightening, and the resultant elongation of the chains in the stress direction, Part of the driving force for elastic deformation is a thermodynamic parameter called entropy, , which is a measure of the degree of disorder within a system; entropy increases with increasing disorder. As an elastomer is stretched and the chains straighten and become more aligned, the system becomes more ordered.Several criteria must be met for a polymer to be elastomeric: (1) It must not easily crystallize; elastomeric materials are amorphous, having molecular chains that are naturally coiled and kinked in the unstressed state. (2) Chain bond rotations

must be relatively free for the coiled chains to readily respond to an applied force.

(3) For elastomers to experience relatively large elastic deformations, the onset of

plastic deformation must be delayed

Vulcanization

The crosslinking process in elastomers is called vulcanization, which is achieved by

a nonreversible chemical reaction, ordinarily carried out at an elevated temperature.

In most vulcanizing reactions, sulfur compounds are added to the heated elastomer; chains of sulfur atoms bond with adjacent polymer backbone chains and crosslink them

Unvulcanized rubber, which contains very few crosslinks, is soft and tacky and has poor resistance to abrasion. Modulus of elasticity, tensile strength, and resistance to degradation by oxidation are all enhanced by vulcanization.

Basic Oxygen Furnace (BOF)

Basic Oxygen Furnace (BOF) is a steel making furnace, in which molten pig iron and steel scrap convert into steel due to oxidizing action of oxygen blown into the melt under a basic slag.
The Basic Oxygen Process (Basic Oxygen Furnace, Basic Oxygen Steelmaking, Basic Oxygen Converter) is the most powerful and effective steel making method. About 67% of the crude steel in the world is made in the Basic Oxygen Furnaces (BOF).

• Furnace structure
• Refractory lining
• Chemical and physical processes
• Operation

Structure of a Basic Oxygen Furnace

The scheme of a Basic Oxygen Furnace (BOF) is presented in the picture.

Typical basic oxygen furnace has a vertical vessel lined with refractory lining.
Only 8-12% of the furnace volume is filled with the treated molten metal. The bath depth is about 4-6.5 ft (1.2-1.9 m). The ratio between the height and diameter of the furnace is 1.2-1.5. The typical capacity of the Basic Oxygen Furnace is 250-400 t.

The vessel consists of three parts: spherical bottom, cylindrical shell and upper cone. The vessel is attached to a supporting ring equipped with trunnions.
The supporting ring provides stable position of the vessel during oxygen blowing.
The converter is capable to rotate about its horizontal axis on trunnions driven by electric motors. This rotation (tilting) is necessary for charging raw materials, sampling the melt and pouring the steel out of the converter.

The top blown basic oxygen furnace is equipped with the water cooled oxygen for blowing oxygen into the melt through 4-6 nozzles. Oxygen flow commonly reaches 200-280 ft³/(min*t) (6-8 m³/(min*t)). The oxygen pressure is 150-220 psi (1-1.5 MPa). Service life of oxygen lance is about 400 heats.

The bottom blown basic oxygen furnace is equipped with 15-20 tuyeres for injection of oxygen (or oxygen with lime powder). The tuyeres are cooled by either hydrocarbon gas (propane, methane) or oil supplied to the outer jacket of the tube.
Refractory lining of a Basic Oxygen Furnace

The refractory lining of basic oxygen furnaces work in severe conditions of high temperature and oxidizing atmosphere. The lining wear is fastest in the zone of contact with the oxidizing slag (slag line).

Refractory bricks for lining basic oxygen furnaces are made of either resin bonded magnesite or tar bonded mixtures of magnesite (MgO) and burnt lime (CaO). The bonding material (resin, tar) is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from chemical attack of the molten metal.

The following measures allows to prolong the service life of the lining:

• Control of the content of aggressive oxidizing oxide FeO in the slags at low level.
• Addition of MgO to the slags.
• Performing “slag splashes” - projecting residual magnesia saturated slag to the lining walls by Nitrogen blown through the lance.
• Repair the damaged zones of the lining by gunning refractory materials.

Properly maintained lining may serve 20000 heats.

Chemical and physical processes in a Basic Oxygen Furnace

The basic oxygen furnace uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel. Iron and its impurities oxidize evolving heat necessary for the process.

Oxidation of the molten metal and the slag is complicated process proceeding in several stages and occurring simultaneously on the boundaries between different phases (gas-metal, gas-slag, slag-metal). Finally the reactions may be presented as follows:
(square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas)

1/2{O₂} = [O]

[Fe] + 1/2{O₂} = (FeO)

[Si] + {O₂} = (SiO₂)

[Mn] + 1/2{O₂} = (MnO)

2[P] + 5/2{O₂} = (P₂O₅)

[C] + 1/2{O₂} = {CO}

{CO} + 1/2{O₂} = {CO₂}

Most oxides are absorbed by the slag.

Gaseous products CO and CO₂ are transferred to the atmosphere and removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio: {CO₂}/({CO₂}+{CO}).

Basic Oxygen Process has limiting ability for desulfurization. The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. However the slag formed in the Basic Oxygen Furnace is oxidizing (not reducing) therefore maximum value of distribution coefficient of sulfur in the process is about 10, which may be achieved in the slags containing high concentrations of CaO.

Operation of a Basic Oxygen Furnace

• Charging steel scrap (25-30% of the total charge weight).
• Pouring molten pig iron from blast furnace.
• Charging fluxes.
• Starting oxygen blowing. Duration of the blowing is about 20 min.
• Sampling. Temperature measurements (by disposable thermocouple) and taking samples for chemical analysis are made through the upper cone in tilted position of the furnace.
• Tapping - pouring the steel to a ladle. Special devices (plugs, slag detectors) prevent penetration (carry-over) of the slag into the ladle.
• De-slagging - pouring the residual slag into the slag pot. The furnace is turned upside down in the direction opposite to the tapping hole.

The Basic Oxygen Furnace has a capacity up to 400 t and production cycle (tap-to-tap) of about 40 min.

Desulfurization of steel

• Sulfur in steel
• Desulfurization of steel by slags
• Desulfurization of steel by injection of active agents

Sulfur in steel

Sulfur (S) may dissolve in liquid iron (Fe) at any concentration. However solubility of sulfur in solid iron is limited: 0.002% in α-iron at room temperature and 0.013% in γ-iron at 1832°F (1000°C).

When a liquid steel cools down and solidifies the solubility of sulfur drops and it is liberated from the solution in form of iron sulfide (FeS) forming an eutectic with the surrounding iron. The eutectic is segregated at the iron grain boundaries. The eutectic temperature is relatively low - about 1810°F (988°C).
Fe-FeS eutectic weakens the bonding between the grains and causes sharp drop of the steel properties (brittleness) at the temperatures of hot deformation (Rolling, Forging etc.).

Brittleness of steel at hot metal forming operations due to the presence of low-melting iron sulfides segregated at grain boundaries is called hot shortness.
In order to prevent formation of low-melting iron sulfide manganese (Mn) is added to steel to a content not less than 0.2%.
Manganese actively reacts with iron sulfides during solidification of steel transforming FeS to MnS according to the reaction:

(FeS) + [Mn] = (MnS) + Fe

(square brackets [ ] - signify concentration in steel, round brackets ( ) signify concentration in slag)

The melting temperature of manganese sulfide is relatively high - about 2930°F (1610°C) therefore the steels containing manganese may be deformed in hot state (no hot shortness).
Unfortunately MnS inclusions are:

• Brittle (less ductile than steel);
• They may have sharp edges;
• They are located between the steel grains.

All these factors determine negative influence of sulfide inclusions on the mechanical properties. Cracks may be initiated at brittle sharp edge inclusions. Sulfide inclusions especially arranged in a chain form also make easier the cracks propagation along the grain boundaries.
The negative effect of sulfur on the steel properties becomes more significant in large ingots and castings, some zones of which are enriched by sulfur (macrosegregation of sulfur).

The properties negatively affected by sulfur:

• Ductility;
• Impact toughness;
• corrosion resistance;
• Weldability.

 

Desulfurization of steel by slags

The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. Basic slag is a slag containing mainly basic oxides: CaO, MgO, MnO, FeO.
A typical basic slag consists of 35-60% CaO + MgO, 10-25% FeO, 15-30% SiO₂, 5-20% MnO.

Transition of sulfur from steel to slag may be presented by the chemical equation:

[S] + (CaO) = (CaS) + [O]

The equilibrium constant K_S1 of the reaction is:

K_S1 = a_[O]*a_(CaS)/a_[S]*a_(CaO)

Where:
a_[O], a_[S] - activities of oxygen and sulfur in the liquid steel;
a_(CaS), a_(CaO) - activities of CaS and CaO in the slag.

The same reaction in ionic form:

[S] + (O^2-) = (S^2-) + [O]

The equilibrium constant K_S2 of the reaction is:

K_S2 = a_[O]*a_(S2-)/a_[S]*a_(O2-)

Where:
a_(S2-), a_(O2-) - activities of S^2- and O^2- in the slag.

Capability of a slag to remove sulfur from steel is characterized by the distribution coefficient of sulfur:

L_S = (S)/[S]

Where:
(S) - concentration of sulfur in slag;
[S] - concentration of sulfur in steel;

As appears from the above equations desulfurization is effective in deoxidized (low (O)) basic (high (CaO)) slags. Therefore ability of Basic Oxygen Process (BOP) to remove sulfur is low due to its highly oxidized slag.
Desulfurization may be effectively conducted in the reducing slag stage of the steel making process in Electric-arc furnace. At this stage the oxidizing slag is removed and then lime flux is added to form basic slag with high CaO content.

Deep desulfurization by slags may be achieved in ladle:

• The refining (desulfurizing) slag with high content of CaO and no FeO is prepared and placed in an empty ladle.
• The molten steel is poured into the ladle filled with the refining slag.
• Energy of the falling steel stream causes mixing the slag with the steel, during which sulfur is removed from the steel to slag phase.

Effect of desulfurization may be enhanced by additional stirring, for which electromagnetic (induction) stirrers or argon bubbling are used.
Desulfurization of steel by injection of active agents

Injection of desulfurizing agents to a molten steel is the most effective method of sulfur removal.Injection methods usually combine supply of a disperse desulfurizing agent (powder) with stirring by argon blowing.

Deep desulfurization by injection of active agents are achieved due to the following factors:

• High chemical activity of the desulfurization agents (Ca, Mg);
• High contact area between the steel and slag phases;
• Stirring providing good kinetic conditions of desulfurization;
• Presence of basic non-oxidized slag capable to absorb the products of the desulfurization reaction (CaS, MgS).

The following materials are used as desulfurizing agents:

• Slag mixtures CaO (50-90%) + CaF₂ (10-20%) + A₂lO₃ (0-30%);
• CaSi;
• CaC₂;
• CaC₂ + Mg;
• Lime (CaO) + Mg;
• Ca + Al;
• Ca;
• Mg.

The desulfurizing agents are injected into molten steel either in form of powder transported by an argon blown to the steel through a lance or in form of a cored wire containing powder of desulfurizing agent. In the latter method stirring by argon bubbling from the porous plug mounted in the ladle bottom is used.



Chemical reactions between desulfurizing agents and sulfur dissolved in steel may be presented by the following equations:

Ca + [S] = (CaS)

Mg + [S] = (MgS)

Injection of desulfurizing agents allows to achieve ultra-low concentrations of sulfur in steel (0.0002%).

Structure of killed steel ingot

 Typical ingot structure consists of five zones:

• Zone of small equiaxed grains

The thin layer of small crysrtals forms when a melt comes to a contact with a wall of a cold metallic mold. The crystals (grains) have no favorable direction (equiaxed) and their chemical composition is close to that of the liquid steel. Heat liberated as a result of Crystallization depresses the nucleation and crystal growth.

• Zone of columnar grains

Columnar grains start to grow when a stable and directed heat flow is formed as a result of heat transfer through the zone of small equiaxed grains. Direction of the columnar grains growth is oppsite to the direction of heat flow. Columnar grains continue to grow untill the heat flow decreases due to the following causes:

• Large width of the solidified metal;
• Heating the mold wall;
• Formation of an air gap between the ingot and the mold wall. The air gap is a result of shrinkage caused by solidification.

When the temperature of the melt, adjacent to the solidification front, increases due to the liberation of the latent heat, constitutional undercooling will end and the columnar grains growth will stop.

• Zone of large equiaxed grains

Low temperature gradient (low heat flow) and low cooling rate of the solute-enriched liquid in the cenral zone of the ingot result in formation of equiaxed grains. This process is slow due to slow heat extraction therefore the number of nuclei (seed crystals) is low and the grains size is large.
Zone of large equiaxed grains is enriched by the impurities (sulfur, phosphorous, carbon).

• Bottom cone

This cone-shaped zone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.
Bottom cone is characterized by negative segregation of the impurities.

• Shinkage cavity zone

Shrinkage cavity is located in the top part of the ingot (which is later discarded) where last portion of liquid solidifies. The mold design should provide upwards direction of solidification at its last stage. Below the shrinkage cavity the zone of shrinkage porosity is located. This zone forms when the feeding of solidifying metal by the residual liquid is insufficient. Isolated pockets of liquid metal separated from the liquid pool by “bridges” form their own shrinkage cavities (shrinkage pores).
The mold shape, which is wider in upper levels and thermal isolation of the “hot top” favor to diminish the shrinkage porosity.

Fabrication of large steel ingots

Large steel ingots are required for manufacturing electric power plant turbine shafts, generator rotor shafts, nuclear pressure vessels, chemical pressure vessels, ship parts and other heavy machinery parts.
Metalforming technology used for final shaping of large ingots is Forging.

The largest ingot (570 metric tons) was produced in 1980 by “Kawasaki Steel” (Japan).
Solidification of a large mass of steel is characterized by significant development of micro and macro-defects of the ingot structure:

• Non-metallic inclusions
• Macrosegregation
• Hydrogen in steel

The structure defects decrease the reliability of the part manufactured from the ingot. Since such parts work in equipment, failure of which is potentially catastrophic (nuclear equipment, electric power plants, chemical equipment, large scale machinery), Technology of large ingots fabrication should provide minimum degree of steel structure defects.

Non-metallic inclusions

Non-metallic inclusions in steel are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, oxysulfides, carbonitrides).

Despite small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:

• Tensile strength
• Deformability (ductility)
• Toughness
• Fatigue strength
• corrosion resistance
• Weldability
• Polishability
• Machinability

The following parameters of non-metallic inclusions influence on the properties of parts made of large steel ingots:

• Chemical composition (oxides, sulfides etc.)
  • Oxides
  • Sulfides
  • Oxysulfides
  • Carbides
  • Nitrides
  • Carbonitrides
  • Phosphides
• Inclusions size

Size of non-metallic inclusions is determined by the processes of nucleation, growth and coalescence/agglomeration. High surface energy causes the nucleation at higher supersaturation of the solutes (oxygen, sulfur, nitrogen, aluminum, silicon, titanium, vanadium, etc.) and favors coalescence and agglomeration of the inclusions.

• Shape (morphology) of non-metallic inclusions
  • Globular inclusion form in liquid state at low concentration of aluminum. Globular shape inclusions exert moderate influence on the steel properties therefore globular morphology is preferable.
  • Platelet shaped inclusions form at the grain boundaries as a result of eutectic transformation during solidification. Platelet shaped inclusion exert adverse effect on the steel properties and therefore this morphology is undesirable.
  • Dendrite shaped inclusions form at high concentration of aluminum. Their shape is characterized by sharp edges, which may cause concentration of stresses during the ingot forging and decrease of ductility, toughness and fatigue strength of the steel part fabricated from the ingot.
  • Polyhedral inclusions are result of modification of the dendrite shaped inclusions by addition of strong deoxidizers and rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. The effect of polyhedral inclusions on the steel properties is less than that of the dendrite shaped inclusions.
• Distribution (homogeneous, clusters, chains)

Homogeneous distribution of non-metallic inclusions is most desirable. Clusters of inclusions are unfavorable since they may result in local drop of mechanical properties such as toughness and fatigue strength.

• Physical and mechanical properties (hardness, ductility, melting point)

Microscopic hard inclusions (carbides, nitrides) strengthen the metal however larger hard inclusions may cause drop of the steel ductility without increase of the strength and hardness. Ductile and brittle inclusions behave different during plastic deformation (steel Forging). Ductile inclusions elongate in the direction of deformation. Brittle inclusions break to fragments and form chains.

Macrosegregation

The following factors favors macrosegregation in large steel ingots:

• Large absolute amount of solutes (sulfur, phosphorous, carbon). Steel is being enriched with the solutes rejected by the moving solidification front therefore at the final solidification stage the residual liquid contain large solute content.
• Low cooling rate of the metal in the central ingot zone. Low cooling rate results in larger spaces between the dendrite arms filled with the liquid metal enriched with the solutes. Partition of solutes between the dendrite arms and interdendritic liquid is called Microsegregation, which is the main cause of macrosegregation. Additionally solidification conditions at low cooling rate are closer to equilibrium resulting in decrease of Equilibrium Partition Coefficient and higher microsegregation.
• Large distances, which liquid metal and separate dendrite crystals and their fragments may be transferred. Transfer of liquid and solid phases in a solidifying ingot is the result of solidification shrinkage, buoyancy forces acting on the liquid metal, sedimentation of solid crystals homogeneously nucleated in liquid phase and fragments of broken or melted off dendrites.

 Development of macrosegregation zones in a steel ingot and their locations are associated with the ingot grain structure.

• Bottom negative segregation

Bottom negative segregation is a result of low solute concentration in the crystals formed in the early stage of solidification and comprising bottom cone. The bottom cone is a mixture of small equiaxed garins grown as a result of the contact with a bottom of a cold metallic mold and crystals and crystals fragments, which sedimintate from other ingot zones.

• V-segregation

The central zone of ingot is enriched with solute rejected by the solidification front progressing from the mold wall to its center. The central zone consists of large equiaxed grains, which settle down to the V-shaped solidification front. The residual liquid surrounding the large equiaxed grains is solute-rich and it forms V-segregates when solidifies.

• A-segregation

A-segregates (freckles) form in the Zone of columnar grains at the regions with structure characterized by the transition from the columnar grains to large equiaxed grains. A-segrgates present channels enriched by sulfur, carbon, phosphorus and other impurities.

• Hot top segregation

Hot top segregation zone is located in the top central ingot region below the shrinkage cavity. Hot top segregation is formed at the final solidification stage from the residual liquid enriched by the solutes as a result of microsegregation (rejection by solidifying dendrites) followed by penetration of the liquid through the dendrite skeleton.

Factors allowing to diminish macrosegregation in large steel ingots:

• Lowering contents of impurities. Steel for large ingots is treated in the melting furnace and in Ladle refining stands in order to remove undesirable impurities such as sulfur (desulfurization]]), phosphorous (dephosphorization), hydrogen (degassing). Steel for large steel ingots commonly contains not more than 0.005% of sulfur, 0.005 of phosphorous and up to 2 ppm of hydrogen.
• Alloying of steel by alkaline (Ca, Mg) or rare earth (Ce,La) elements in amount of about 2*[S].
• Low concentration of silicon in steel (about 0.1%).
• Using steel grades with lower carbon content.
• Modification of the ingot dimensions. If low level of A-segregation is required the ratio of the ingot height to its diameter should be as low as possible (about 1.0-1.2).
• Decrease of the pouring temperature. Lower pouring temperature results in increase of the cooling rate, which is favorable for depressing macrosegregation.

Hydrogen in steel

Sources of hydrogen in liquid steel:

• Damp scrap;
• Fluxes and alloying additives;
• Furnace and ladle refractories;
• Atmospheric humidity;
• Fuel (if used).

Hydrogen is easily dissolved in liquid steel in dissociated (atomic) state. Solubility of hydrogen in steel drops sharply during solidification resulting in formation of gaseous hydrogen form H₂. In solid steel hydrogen is dissolved in form of interstitial solution.

Carbon, nickel, chromium (up to 10%), vanadium, titanium, zirconium, columbium, tantalum increase the solubility of hydrogen in solid steel.
Silicon, aluminum, tungsten, chromium (10% and higher) decrease the solubility of hydrogen in solid steel.
Solubility of hydrogen in austenite is much higher than in ferrite.

Both gaseous and dissolved forms of hydrogen exert adverse effect on mechanical properties of steels:

• Hydrogen flakes

Solubility of hydrogen decrease during solidification and cooling down of steel ingot. Hydrogen atoms possessing high mobility are collected at internal voids such as non-metallic inclusions (sulfides, oxides) and their clusters, shrinkage pores, cracks caused by internal stresses.
Hydrogen atoms collected at internal voids combine and form gaseous hydrogen H₂, which may cause formation of cracks (flakes) when the gas pressure exceeds the steel strength.
Hydrogen flakes is particularly dangerous for parts fabricated from large ingots. Vacuum ladle degassing methods allow to decrease the content of hydrogen to 2 ppm, which does not cause flaking formation.

• Hydrogen embrittlement

Hydrogen in dissolved form also decreases steel properties such as ductility, Fracture Toughness and fatigue strength.

Technology of large ingots fabrication

The following tasks are accomplished by the technology of large ingots fabrication:

• Dephosphorization (to about 0.005%) in a steel making furnace (melting furnace).
• Desulfurization of steel (to about 0.005%) in the melting furnace and then by a ladle refining desulfurization method (eg. Ladle desulfurization by injection of active agents, Ladle Furnace (LF)).
• Deoxidation of steel by metallic deoxidizers to the required concentrations of aluminum, silicon and other deoxidizing elements.
• Removal of non-metallic inclusions from the liquid steel in both melting furnace and Ladle refining stand.
• Vacuum treatment of steel in ladle by one of the degassing methods: Recirculation Degassing (RH), Ladle Degassing (VD).
• *Achievement of the required temperature of steel before pouring to the mold by using Ladle Furnace (LF).
• Final vacuum treatment of the steel in the Ladle-to-mold degassing operation.
• Providing non-intermittent supply of liquid steel to the pouring-to-mold operation.