Basic Metallurgy

Crystal Structures

• In solid metal, the atoms tend to align themselves into orderly lines, rows, and layers to form three dimensional crystaline structure.

• The most common crystal structure, or phases, are :

• Body centered cubic (BCC)• Face centered cubic (FCC)• Hexagonal closest pack (HCP)• Simple cubic (SC)

BCC Unit Cell• In the Body Centered Cubic (BCC) unit cell there is

one host atom (lattice point) at each corner of the cube and one host atom in the center of the cube. Each corner atom touches the central atom along the body diagonal of the cube. Thus, the corner atoms do not touch one another.

Common BCC metals are iron, carbon sheets, chromium, molybdenum and tungsten.

FCC Unit Cell• In the Face Centered Cubic (FCC) unit cell there is one

host atom at each corner and one host atom in each face.  • The packing efficiency is about 74%.  This is the maximum

packing efficiency for spheres of equal radius and is call closest packing.  Thus a face centered lattice of atoms is also called Cubic Closest Packing (CCP).

• Common FCC metals are aluminum, copper, nickel and austenitic stainless steels.

HCP Unit Cell• The HCP unit cell is a hexagonal prism. It can be

envisioned as two hexagons (six-sided shape) forming the top and bottom of the prism. An atom is located at the center and at each point of the hexagons.

• Common HCP metals are zinc, cadmium and magnesium.

Metal / Alloy Production

Metal present in earth crust as Ores. (except Gold . . .)

Ores – compound (simple or complex oxides)

Rich ores – high concentration of desired metal.

Lean ores – low concentration of desired metal.

Rich ores – Generally uses direct Pyro-metallurgical route

Lean ores – Extractive metallurgy (Mineral dressing) + Electro-metallurgy / pyro-metallurgy.

Metal / Alloy ProductionORES

Crushing

Rich Ores Lean Ores

Grinding

Beneficiation

Leaching

Electro-MetallurgyPyro-Metallurgy

AlloyingPurification Process

Alloys Pure Metal

Iron making Blast furnace

- To produce pig iron.

Raw material

- Ore, Coke, Lime + O2

“Pig iron” is final product of B/F containing high C and impurities. It is starting material for producing Cast iron & Steel.

HOW A BLAST FURNACE WORKS• The purpose of a blast furnace is to chemically reduce

and physically convert iron oxides into liquid iron called "hot metal".

• The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom.

• The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron.

• These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions.

• Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

Blast furnace Process• Iron oxides can come to the blast furnace plant in the form of raw ore, pellets

or sinter. • The raw ore is removed from the earth and sized into pieces that range from

0.5 to 1.5 inches. • This ore is either Hematite (Fe2O3) or Magnetite (Fe3O4) and the iron

content ranges from 50% to 70%. • This iron rich ore can be charged directly into a blast furnace without any

further processing. • Iron ore that contains a lower iron content must be processed or beneficiated

to increase its iron content. • Pellets are produced from this lower iron content ore. This ore is crushed and

ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron.

• Sinter is produced from fine raw ore, small coke, sand-sized limestone and numerous other steel plant waste materials that contain some iron. These fine materials are proportioned to obtain a desired product chemistry then mixed together.

• This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches.

• The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag.

Blast furnace Process..• The coke is produced from a mixture of coals. The coal is crushed

and ground into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke contains 90 to 93% carbon, some ash and sulfur but compared to raw coal is very strong. The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter.

• The final raw material in iron making process is limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux . This flux can be pure high calcium limestone, dolomite limestone containing magnesia or a blend of the two types of limestone.

Blast furnace Process…• Since the limestone is melted to become the slag which removes

sulfur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as a low melting point and a high fluidity.

• All of the raw materials are stored in an ore field and transferred to the stock house before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.

• The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows:– 1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4 Begins at 850° F– 2) Fe3O4 + CO = CO2 + 3 FeO Begins at 1100° F– 3) FeO + CO = CO2 + Fe

    or    FeO + C = CO + Fe Begins at 1300° F

Blast furnace Process….• At the same time the iron oxides are going through these purifying

reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the bottom of the furnace.

• The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows:C + O2 = CO2 + Heat

• Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows:CO2+ C = 2CO

• The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions.

• The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows:CaCO3 = CaO + CO2

• This reaction requires energy and starts at about 1600°F. The CaO formed from this reaction is used to remove sulfur from the iron which is necessary before the hot metal becomes steel. This sulfur removing reaction is:FeS + CaO + C = CaS + FeO + CO

Blast furnace Process….• The CaS becomes part of the slag. The slag is also

formed from any remaining Silica (SiO2), Alumina (Al2O3), Magnesia (MgO) or Calcia (CaO) that entered with the iron ore, pellets, sinter or coke.

• The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense.

• Another product of the iron making process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and proceed through gas cleaning equipment where particulate matter is removed from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the "hot blast stoves" which are used to preheat the air entering the blast furnace to become "hot blast". Any of the gas not burned in the stoves is sent to the boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as "cold blast" that comes to the stoves.

Blast furnace Process….• In summary, the blast furnace is a counter-current realtor

where solids descend and gases ascend. In this reactor there are numerous chemical and physical reactions that produce the desired final product which is hot metal. A typical hot metal chemistry follows:

Iron (Fe) = 93.5 - 95.0%Silicon (Si) = 0.30 - 0.90%Sulfur (S) = 0.025 - 0.050%Manganese (Mn) = 0.55 - 0.75%Phosphorus (P) = 0.03 - 0.09%Titanium (Ti) = 0.02 - 0.06%Carbon (C) = 4.1 - 4.4%

Steel making

Pneumatic / Bessemer process.

Open hearth process.

Electric Furnaces. Direct arc furnace.

Induction furnace.

Ladle treatment.

Ingot casting.

Bessemer process

• The process is carried on in a large ovoid steel container lined with clay or dolomite called the Bessemer converter.

• The capacity of a converter was from 8 to 30 tons of molten iron with a usual charge being around 15 tons.

• At the top of the converter is an opening, usually tilted to the side relative to the body of the vessel, through which the iron is introduced and the finished product removed.

• The bottom is perforated with a number of channels called tuyères through which air is forced into the converter.

• The converter is pivoted on trunnions so that it can be rotated to receive the charge, turned upright during conversion, and then rotated again for pouring out the molten steel at the end.

Bessemer process• The oxidation process removes impurities such as

silicon, manganese, and carbon as oxides. • These oxides either escape as gas or form a solid slag.• The refractory lining of the converter also plays a role in

the conversion—the clay lining is used in the acid Bessemer, in which there is low phosphorus in the raw material.

• Dolomite is used when the phosphorus content is high in the basic Bessemer (limestone or magnesite linings are also sometimes used instead of dolomite)—this is also known as a Gilchrist-Thomas converter, named after its inventor, Sidney Gilchrist Thomas.

• In order to give the steel the desired properties, other substances could be added to the molten steel when conversion was complete, such as spiegeleisen (an iron-carbon-manganese alloy).

Bessemer process• When the required steel had been formed, it was poured

out into ladles and then transferred into moulds and the lighter slag is left behind.

• The conversion process called the "blow" was completed in around twenty minutes.

• During this period the progress of the oxidation of the impurities was judged by the appearance of the flame issuing from the mouth of the converter: the modern use of photoelectric methods of recording the characteristics of the flame has greatly aided the blower in controlling the final quality of the product.

• After the blow, the liquid metal was recarburized to the desired point and other alloying materials are added, depending on the desired product.

Bessemer Furnace

Bessemer Furnace

Open-Hearth Steel-Furnace

ELECTRIC ARC FURNACE

Manufacturing process

Casting.

Metal working / forming. Forging. Rolling. Extrusion. Drawing.

etc.

Welding. / Joining.

Casting

Advantage: Final component in one step.

Precision and complete shape .

Only method for non-workable alloys e.g. CI.

Disadvantage: Thinner sections are difficult

Lower workability / ductility.

Liable to segregation of alloying elements.

Cracking due to thermal shocks.

Casting: Defects

POROSITY / BLOW HOLES SHRINKAGE INCLUSIONS. COLD SHUT. CORE SHIFT. SCAB UNFUSED CHAPLETS, MIS-RUN. SEGREGATION

Porosity • Porosity is small smooth-faced cavities,

generally smaller than 1.5 mm diameter. Porosity is usually caused by the release of gas from the molten metal as it cools. Gases such as hydrogen may be dissolved in the liquid metal. As the metal cools, the dissolved gas separates out to form bubbles, which are trapped in the solidifying metal.

• The preferred NDT method for detecting porosity is radiography. Ultrasonic testing may also detect porosity.

Gas holes • The main distinction between gas holes and porosity is

the size. Gas holes are smooth-faced cavities greater than 1.5 mm diameter. Typical causes are:

• Evolution of gas from molten metal during solidification. • Gas trapped as the molten metal enters the mould. • Reactions between the metal and the mould, also known

as blowholes. • Again the best method to detect gas holes is

radiography. Ultrasonic testing can also be used.

Air locks

• Air locks are cavities formed by air trapped in the mould during pouring of the casting. Air locks tends to form just below and parallel to the top surface during casting. They have a smooth surface and can be quite large.

Shrinkage cavities • Shrinkage cavities form during solidification as a result of

the reduction in volume when metal changes from the liquid to the solid state. Shrinkage cavities occur in situations where molten metal is not available to compensate for the volume decrease during solidification. Shrinkage flaws typically occur where there is a localized variation in section thickness but may occur in parallel sections where penetration of the liquid feed metal is difficult.

• Shrinkage defects vary in form from open cavities (piping) to branched interconnected fine cavities. The defects tend to have a rough surface profile.

Hot tears • These are jagged crack type defects resulting from

stresses imposed on the cast metal when it is just below the solidification temperature and so is in a weak condition. The stresses usually arise when the casting is restrained during contraction by the mould, or by an already solid thinner section. The defect occurs mainly at or near a change of section and may or may not extend to the surface.

• The best NDT method for detecting hot tears, if they are at the surface, is magnetic particle testing for ferromagnetic materials or liquid penetrants for other metals. If the defects are sub-surface radiography or ultrasonic testing should be used.

Metallic projections• Joint flash or fins. Flat projection of irregular thickness,

often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect.

• Possible Causes– Clearance between two elements of the mold or between mold

and core; – Poorly fit mold joint.

• Remedies– Care in pattern making, molding and core making;– Control of their dimensions; – Care in core setting and mold assembly; – Sealing of joints where possible.

Cold shuts

• These are in effect a ‘lack of fusion’ defect caused by the failure of a stream of molten metal to form a continuous bond with a second stream, or solid metal such as an internal chill or splash. They are most prevalent in thin-walled castings.

• The preferred NDT method for detecting cold shuts is magnetic particle testing for ferromagnetic metals and liquid penetrant testing for other metals.

Cavities• Blowholes, pinholes. Smooth-walled cavities,

essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are most often isolated; the smallest (pinholes) appear in groups of varying dimensions. In specific cases, the casting section can be strewn with blowholes of pinholes. The interior walls of blowholes and pinholes can be shiny, more or less oxidized or, in the case of cast iron, can be covered with a thin layer of graphite. The defect can appear in all regions of the casting.

Cavities..• Possible Causes

– Blowholes and pinholes are produced because of gas entrapped in the metal during the course of solidification:

– Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.); Dissolved gases are released during solidification;

– In the case of steel and cast irons: formation of carbon monoxide by the reaction of carbon and oxygen, presents as a gas or in oxide form. Blowholes from carbon monoxide may increase in size by diffusion of hydrogen or, less often, nitrogen;

– Excessive moisture in molds or cores; – Core binders which liberate large amounts of gas; – Excessive amounts of additives containing hydrocarbons; – Blacking and washes which tend to liberate too much gas;

Cavities…• Possible Causes

– Insufficient evacuation of air and gas from the mold cavity; -insufficient mold and core permeability;

– Entrainment of air due to turbulence in the runner system. • Remedies

– Make adequate provision for evacuation of air and gas from the mold cavity;

– Increase permeability of mold and cores; – Avoid improper gating systems; – Assure adequate baking of dry sand molds; – Control moisture levels in green sand molding;– Reduce amounts of binders and additives used or change to

other types; -use blackings and washes, which provide a reducing atmosphere; -keep the spree filled and reduce pouring height;

– Increase static pressure by enlarging runner height.

Discontinuities• Hot cracking. A crack often scarcely visible because the

casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.

• Possible Causes– Damage to the casting while hot due to rough handling or

excessive temperature at shakeout. • Remedies

– Care in shakeout and in handling the casting while it is still hot; – Sufficient cooling of the casting in the mold; – For metallic molds; delay knockout, assure mold alignment, use

ejector pins.

Defective Surface • Flow marks. On the surfaces of otherwise sound

castings, the defect appears as lines which trace the flow of the streams of liquid metal.

• Possible Causes– Oxide films which lodge at the surface, partially marking the

paths of metal flow through the mold.

• Remedies– Increase mold temperature;– Lower the pouring temperature; – Modify gate size and location (for permanent molding by gravity

or low pressure); – Tilt the mold during pouring; – In die casting: vapor blast or sand blast mold surfaces which are

perpendicular, or nearly perpendicular, to the mold parting line.

Incomplete Casting • Poured short. The upper portion of the casting is missing. The

edges adjacent to the missing section are slightly rounded, all other contours conform to the pattern. The spree, risers and lateral vents are filled only to the same height above the parting line, as is the casting (contrary to what is observed in the case of defect).

• Possible Causes– Insufficient quantity of liquid metal in the ladle; – Premature interruption of pouring due to workman’s error.

• Remedies– Have sufficient metal in the ladle to fill the mold; – Check the gating system; – Instruct pouring crew and supervise pouring practice.

Incorrect Dimensions or Shape • Distorted casting. Inadequate thickness, extending

over large areas of the cope or drag surfaces at the time the mold is rammed.

• Possible Causes– Rigidity of the pattern or pattern plate is not sufficient to

withstand the ramming pressure applied to the sand. The result is an elastic deformation of the pattern and a corresponding, permanent deformation of the mold cavity. In diagnosing the condition, the compare the surfaces of the pattern with those of the mold itself.

• Remedy– Assure adequate rigidity of patterns and pattern plates,

especially when squeeze pressures are being increased.

Inclusions or Structural Anomalies • Metallic Inclusions. Metallic or intermetallic inclusions

of various sizes which are distinctly different in structure and color from the base material, and most especially different in properties. These defects most often appear after machining.

• Possible Causes– Combinations formed as intermetallics between the melt and

metallic impurities (foreign impurities); – Charge materials or alloy additions which have not completely

dissolved in the melt; – Exposed core wires or rods; – During solidification, insoluble intermetallic compounds form and

segregate, concentrating in the residual liquid.

Inclusions or Structural Anomalies

• Remedies– Assure that charge materials are clean; eliminate

foreign metals;– Use small pieces of alloying material and master

alloys in making up the charge;– Be sure that the bath is hot enough when making the

additions;– Do not make addition too near to the time of pouring;– For nonferrous alloys, protect cast iron crucibles with

a suitable wash coating.

Metal working: Defects

LAMINATION, LAPS. CRACKS. POROSITY. PIPES. FLOW THROUGH

Laps • These are found in rolled or forged products. Laps in hot

rolled bars are longitudinally oriented folds on the surface of the product due to rolling over of projections on the surface.

• In cross-section laps tend to ‘hook’ under the surface. They generally contain oxide or scale and may be partially welded at the tip.

• Because of their method of formation, laps tend to be very long although they are usually quite shallow, say less than 1 mm in depth.

• The preferred NDT to detect laps in steel is magnetic particle testing. Eddy current testing is the best method for non-ferrous metals. Penetrant testing is generally not suitable as laps usually contain scale or oxide.

Seams • Seams are longitudinal grooves or lines in the surface of hot rolled

products due to the elongation of defects such as gas holes, or inclusions on the ingot surface.

• With continuous casting of rolled steel products (where the product is fully-killed), seams are much less common. However, ingots which are semi-killed are prone to produce seam defects.

• Seams are generally quite shallow, up to about 0.75 mm deep, and tight-lipped, but like laps they can be very long. Sometimes numerous seams exist in a bar surface.

• Seams differ from laps in that, in cross-section, they tend to extend into the metal at right angles to the surface. Like laps they generally contain oxide or inclusions.

• As for laps the preferred NDT technique for seams in steel is magnetic particle testing and for non-ferrous metals eddy current testing is the preferred method.

Pipe/lamination • These two defects are grouped together since they have the same

origin. Piping is a cavity formed during solidification of an ingot due to the fact that when molten metal solidifies there is a reduction in volume called shrinkage. Piping may be open at the ingot top when it is called a primary pipe. It may also be within the ingot when it is called a secondary pipe.

• Ingot pipe can persist in material right through a rolling sequence from the ingot stage to fine wire or thin sheet to produce a pipe or lamination defect. In some cases secondary pipe can weld up and so disappear during rolling operations.

• The difference between pipe and lamination is that pipe occurs in sections such as rounds, hexagons and squares and lamination occurs in flat products such as plate or sheet.

• Pipe and lamination defects are a by-product of ingot steel production. Modern steelmaking practice uses continuous casting technology where these defects are much less common.

• Both pipe and laminations defects are centrally located and, in the case of lamination the defect is planar and parallel to the flat faces.

• The preferred NDT method for pipe and lamination is ultrasonic testing. In smaller sections pipe may also be detected by radiography.

Inclusions • These are non-metallic material such as:• Products of steelmaking reactions, for example,

sulphides, silicates, slag. • Refractory material dispersed through the metal. • Inclusions are always present to some degree in steel

but are of concern in gross form or at excessive levels.• Inclusions tend to be orientated in the direction of metal

working • The preferred NDT method for detecting gross inclusions

is ultrasonic testing. For smaller sections radiography may be used.

Hydrogen flakes • Hydrogen flakes are also called snowflakes because when viewed

on a fracture surface they tend to glisten like snowflakes.• Hydrogen flakes form in the central portion of steel bars as a series

of very fine, circular shaped cracks of longitudinal orientation. In severe cases the individual flakes may measure up to 50 mm in diameter.

• Flakes are usually confined to higher carbon and alloy steel grades in larger sections such as blooms, slabs and billets. They are rare in the lower carbon steels such as mild steel. They are caused by situations where hydrogen dissolved in steel during the steelmaking process has not had sufficient opportunity to diffuse out of the metal to the atmosphere.

• They can be avoided by minimising moisture entering steelmaking furnaces and also slower cooling of susceptible grades after hot rolling.

• The best NDT technique to detect hydrogen flakes is ultrasonic testing. Magnetic particle testing can be used on cross-sections of the product.

Forging bursts • These are surface or internal ruptures due to the inability

of metal to withstand internal tensile stresses generated in forging. They are promoted by such factors as processing at too low a temperature, excessive working in forging or forging steels with higher sulphur contents (hot shortness).

• Bursts are often large and seldom heal during subsequent working. They may take the form of an open cavity or a tight faced crack and may be longitudinally or transversely orientated. The best method of detection is ultrasonics, or radiography in smaller sections.

Welding: Defects

BURN THROUGH. LACK OF ROOT FUSION / LACK OF PENETRATION. ROOT CONCAVITY (SUCK BACK). ICIECLES / GLOBULES / LUMPS. EXCESSIVE PENETRATION. POROSITY / GAS HOLES. INCLUSIONS. UNDERCUT / ROOT UNDERCUT. LACK OF SIDE WALL FUSION INTERBEAD FUSION CRACKS

Porosity• This occurs as a series of fine cavities, generally

spherical, but sometimes tubular in form (worm holes). Porosity can occur in various patterns, for example, linear porosity, scattered porosity and start porosity. The defect is caused by such factors as:– Excessive gas content generated by chemical reactions in the

weld. – Gases or other hydrocarbon contamination.– Damp flux.

• The preferred NDT techniques are radiography, ultrasonic testing and, if the porosity is at the surface, liquid penetrants.

Trapped slag • A number of welding processes deliberately form a flux

or slag covering over the molten weld pool as it solidifies. This isolates the weld metal from the atmosphere and helps purify the weld metal. Some of this slag can be trapped in the weld metal due to insufficient slag removal between runs or insufficient back gouging of the root. Depending on the circumstances of formation the slag is generally in an isolated or linear pattern. Slag can be classed as a ‘volume’ defect.

• Preferred NDT technique for detecting trapped slag is radiography or ultrasonic testing.

Lack of fusion • This refers to incomplete fusion between the weld metal

and the parent metal or weld metal with previously deposited weld metal. Three distinct types of fusion defect occur depending on the location of the defect within the weld zone:– Lack of side wall fusion.– Lack of inter-run fusion, that is, between weld runs. – Lack of root fusion.

• Causes include such factors as:– Poor welding technique.– Incorrect electrode size.– Inadequate weld preparation.

• Lack of fusion defects are generally planar and crack-like in nature. The best NDT method is ultrasonic testing. Radiography may be used for lack of side wall and root fusion.

Lack of penetration • This is where the weld metal has failed to

penetrate into the root of a joint as opposed to lack of root fusion where weld metal has penetrated into the root area but has failed to fuse to one side.

• The causes of lack of penetration are the same as for lack of fusion defects.

• The preferred NDT technique for detecting lack of penetration is radiography or ultrasonic testing.

Hot cracking• This is also called solidification cracking because it

occurs when the weld metal has just solidified and so is in a weakened condition. Most weld metal cracks are of this type, for example, centre-line cracking, and crater cracks.

• Hot cracks result from the combined action of stress and lack of ductility of the weld metal at high temperatures. Contributing factors are:– restraint – weld chemistry (for example, high sulphur content) – weld shape, (for example, concave fillet welds).

• Preferred NDT techniques for detecting hot cracking is magnetic particle testing or liquid penetrant testing.

Heat affected zone (HAZ) cracks • These are also called underbead cracks or toe

cracks.• The heat affect zone, HAZ, of a weld is that part

of the parent metal adjacent to the weld fusion line where the metal has been heated to a sufficiently high temperature by the weld to alter its grain structure.

• Underbead cracks occur in the weld HAZ and lie parallel to the fusion line while toe cracks commence at the weld toe and angle across the HAZ as shown below. HAZ cracks form at temperatures around room temperature and may form shortly after welding or take hours or even days to form.

Heat affected zone (HAZ) cracks..• The cracks occur under the combined action of:

– Hydrogen in the HAZ – hydrogen can originate, for example, from using damp electrodes.

– Weld restraint – that is, stress. – A hard HAZ – this relates to parent metal chemistry and cooling

rate after welding. • The tendency to cracking is influenced by:

– The type of steel used (it is favoured by higher carbon and alloy steels).

– Material thickness. – Type of joints. – Type of welding process.

• The best NDT technique for underbead cracks is ultrasonic testing while magnetic particle testing is best for toe cracks.

Lamellar tearing• These are welding-related ‘cracks’ with a stepped

appearance that occurs in the parent metal, either inside or outside the HAZ.

• Lamellar tears typically occur in steel under conditions of:– T-butt or corner welds. – Thick plate material (greater than 25 mm thick). – Fusion boundary of weld parallel to plate surface, that is, parallel

to rolling direction of plate. – A relatively high level of non-metallic inclusions in the steel. – High weld restraint.

• The occurrence of lamellar tearing is associated with reduced through-thickness strength due to the presence of non-metallic inclusions in the steel.

• The best NDT method for detection of lamellar tearing is ultrasonic testing.

Check Your Progress Q.1 Which of the following types of defects are best detected using

magnetic particle testing?A. Hydrogen flakes B. Inclusions C. Laps D. Laminations

Answer: C - Magnetic particle testing is the preferred method for detecting laps and seams.

Q.2 Which of the following types of defects are best detected using magnetic particle testing?A. Porosity B. Shrinkage cavities C. Cold shuts D. Inclusions

Answer: C - Magnetic particle testing is the preferred NDT method for cold shuts in ferromagnetic material.

Check Your Progress..Q.3 Which of the following types of defects are best detected using

magnetic particle testing? A. Trapped slag B. Lack of fusion C. Lack of penetration D. Hot cracking

Answer: D - Magnetic particle testing is the preferred NDT method for hot cracking defects.

Q.4 Which of the following welding defects are best detected using magnetic particle testing? A. Quench cracksB. Fatigue cracksC. Grinding cracks D. All of the above.

Answer: D - Magnetic particle testing is the preferred NDT method for quench, fatigue and grinding cracks.