Project Report by Sumit Gupta and Anmol Kumar Shrivas

2011

Institute of

Technology,

Banaras Hindu

University,

Varanasi

[FAILURE ANALYSIS OF FORGED/CAST SAMPLES]

Training Report

Supervising Authority: S. N. Ghosh, Deputy General Manager, Quality Management Lab, CFFO, BHEL, Haridwar

By:

Anmol Kumar Shrivas and

Sumit Kumar Gupta,

B-Tech (Part IV),

Department of

Metallurgical Engineering,

IT-BHU,

Varanasi

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Contents:

Page Number

1. Abstract 2 2. Overview of BHEL, Hardwar 2 3. Manufacturing Units of BHEL 3 4. Central Foundry and Forge Plant (CFFP) 3 5. Steel Melting Shop 4 6. Steel casting 11 7. Forge Shop 15 8. Heat Treatment 17 9. Mechanical testing 19 10. Mechanical Failure Analysis of Forged/Cast Samples. 22 11. Acknowledgement 45 12. References

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Abstract:

Mechanical testing is industrially the most widely used method to provide as an

acceptance test for the material specification. These tests were performed on

several forged/cast samples of steel and compared with the specification values

given by the customer. The failed samples were then analysed for possible sources

of failure. A study of the manufacturing stages was also made for a deeper

evaluation and training exposure.

Overview of BHEL, Hardwar:

Bharat Heavy Electicals Ltd, better known as BHEL, is the largest engineering and

manufacturing enterprise of its kind in India. The company is engaged in

engineering, development for generation transmission and utilization of energy and

electrical power. Placed among the top 12 manufactures of power plant equipment in

the world, BHEL provides products systems and services in the following fields:

Power Generation, Transmission, Industry, Transportation, Telecommunication, Oil

& Gas, Casting & Forging.

In each of these sectors, BHEL offers total service to its customers on turnkey basis.

It has 12 manufacturing plants and a host of service divisions spread all over the

country. The company is manned by over 70,000 employees comprising scientists,

managers, engineers, trained technicians and skilled workmen.

In order to cater to the need for cast products to be used in BHEL as well as in

various industries, there is one of the largest foundry and forging plant in the form of

Central Foundry and Forge Plant (CFFP) at its Haridwar Unit.

The inherent potential of BHEL, coupled with its sustained performance over the

years, resulted in it being chosen as one of the “NAVRATNA” public sector units,

which are to be separated by the government in their endeavour to become future

global players.

BHEL‟s products and systems are exported to over 45 countries around the world.

BHEL has upgraded its products which are related to state of the art technology from

leading engineering institutions and organization of the world viz., SIEMENS,

COMBUSTION ENGINEERING, GENERAL ELECTICAL, NUGVO PIGNONE, ASEA

BROWN BOVERI (ABB) etc. today. The company is engaged in engineering,

development for generation, transmission and utilization of energy and electrical

power. Placed among the top 12 manufactures of power plant equipment in the

world, BHEL provides products systems and services. In each of these sectors,

BHEL offers total service to its customers on turnkey basis.

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Manufacturing Units of BHEL:

The operating and manufacturing plant units are:

HEAVY ELECTICAL EQUIPMENT PLANT – BHOPAL

TRANSFORMER FACTORY – JHANSI

HEAVY ELECTICAL EQUIPMENT PLANT – HARIDWAR

CENTRAL FOUNDRY & FORGE PLANT – HARIDWAR

HEAVY POWER EQUIPMENT PLANT – HYDERABAD

INSULATION PLANT – JAGDISHPUR

ELECTRO – PORCELAIN DIVISION – BANGALURU

CONTROL EQUIPMENT DIVISION – BANGALURU

HIGH – PRESSURE BOILER PLANT – TRIUCHHIRAPPALLI

SEAMLESS STEEL TUBE PLANT – TIRUCHHIRAPPALLI

COMPONENT FABRICATION PLANT – RUDRAPUR

BOILER AUXILLIARY PLANT – RANIPAT

Central Foundry and Forge Plant (CFFP):

CFFP, basically a metallurgical unit of BHEL, was set up in 1976 at Haridwar. It is engaged in manufacture and supply of various types of steel castings and forgings for the vital sectors like power, steel defence, nuclear and space research, shipping machine engineering. It has state of the art technology with the most up to the date equipment and facilities to meet the requirement of quality castings and forgings. It has duly been accredited by India Boiler, TUV of Germany and American and India Bureau of Shipping. CFFP department of BHEL is engaged in production of shafts for turbine. It is one of the largest companies of this type and own monopoly over market in its production.

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Steel Melting Practice

Operation:

Scrap metal is delivered to a scrap bay, located next to the melt shop. Scrap generally comes

in two main grades: shred (white goods, cars and other objects made of similar light-gauge

steel) and heavy melt (large slabs and beams).Composition of all scrap is reported. Grading is

also done based on chemical composition.

Raw materials

Coke

Graphite powder

Turning-boring (scrap from machining)

Return scrap (false bottom + hot top)

Skull

Alloying elements

Charging

The scrap is loaded into large buckets called baskets, with "clamshell" doors for a base. Care

is taken to layer the scrap in the basket to ensure good furnace operation; heavy melt is

placed on top of a light layer of protective shred, on top of which is placed more shred. These

layers should be present in the furnace after charging. After loading, the basket may pass to a

scrap pre-heater, which uses hot furnace off-gases to heat the scrap and recover energy,

increasing plant efficiency.

The scrap basket is then taken to the melt shop, the roof is swung off the furnace, and the

furnace is charged with scrap from the basket. Charging is one of the more dangerous

operations for the EAF.

Melting

After charging, the roof is swung back over the furnace and meltdown commences. The

electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore

into the layer of shred at the top of the furnace. Lower voltages are selected for this first part

of the operation to protect the roof and walls from excessive heat and damage from the arcs.

Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are

shielded by the scrap, the voltage can be increased and the electrodes raised slightly,

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lengthening the arcs and increasing power to the melt. This enables a molten pool to form

more rapidly, reducing tap-to-tap times. Oxygen is also supersonically blown into the scrap.

Slag Making

An important part of steelmaking is the formation of slag, which floats on the surface of the

molten steel. Slag usually consists of metal oxides, and acts as a destination for oxidised

impurities, as a thermal blanket (stopping excessive heat loss) and helping to reduce erosion

of the refractory lining. For a furnace with basic refractories, which includes most carbon

steel-producing furnaces, the usual slag formers are calcium oxide (CaO, in the form of

burntlime) and magnesium oxide (MgO, in the form of dolomiteand magnesite). These slag

formers are either charged with the scrap, or blown into the furnace during meltdown.

Another major component of EAF slag is iron oxidefrom steel combusting with the injected

oxygen. Later in the heat, carbon (in the form of coke or coal) is injected into this slag layer,

reacting with the iron oxide to form metallic iron and carbon monoxide gas, which then

causes the slag to foam, allowing greater thermal efficiency, and better arc stability

andelectrical efficiency. The slag blanket also covers the arcs, preventing damage to the

furnace roof and sidewalls from radiant heat.

Oxidation

To correct the steel chemistry and superheat the melt above its freezing temperature in

preparation for tapping, More slag formers are introduced and more oxygen is blown into the

bath, burning out impurities such as silicon, sulphur, phosphorus, aluminium, manganese, and

calcium, and removing their oxides to the slag. Removal of carbon takes place after these

elements have burnt out first, as they have a greater affinity for oxygen. Metals that have a

poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through

oxidation and must be controlled through scrap chemistry alone, such as introducing the

direct reduced iron and pig iron mentioned earlier. A foaming slag is maintained throughout,

and often overflows the furnace to pour out of the slag door into the slag pit. Temperature

sampling takes place via automatic lances. Oxygen and carbon can be automatically

measured via special probes that dip into the steel,

Deoxidation

In this stage deoxidisers like Si (Ferro silicon) and Al are added for deoxidation. They

combine with Oxygen to form their oxides which finally combines with lime to form calcium

silicate/aluminate.

http://en.wikipedia.org/wiki/Refractory

http://en.wikipedia.org/wiki/Carbon_steel

http://en.wikipedia.org/wiki/Carbon_steel

http://en.wikipedia.org/wiki/Calcium_oxide

http://en.wikipedia.org/wiki/Lime_%28mineral%29

http://en.wikipedia.org/wiki/Magnesium_oxide

http://en.wikipedia.org/wiki/Dolomite

http://en.wikipedia.org/wiki/Magnesite

http://en.wikipedia.org/wiki/Iron_oxide

http://en.wikipedia.org/wiki/Coke_%28fuel%29

http://en.wikipedia.org/wiki/Coal

http://en.wikipedia.org/wiki/Carbon_monoxide

http://en.wikipedia.org/wiki/Foam

http://en.wikipedia.org/wiki/Thermal_efficiency

http://en.wikipedia.org/wiki/Electrical_efficiency

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Sulfur

http://en.wikipedia.org/wiki/Phosphorus

http://en.wikipedia.org/wiki/Aluminium

http://en.wikipedia.org/wiki/Manganese

http://en.wikipedia.org/wiki/Calcium

http://en.wikipedia.org/wiki/Carbon

http://en.wikipedia.org/wiki/Nickel

http://en.wikipedia.org/wiki/Copper

http://en.wikipedia.org/wiki/Oxidation

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Slagging off & Tapping

Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle

through tilting the furnace. For plain-carbon steel furnaces, as soon as slag is detected during

tapping the furnace is rapidly tilted back towards the deslagging side, minimizing slag

carryover into the ladle. For some special steel grades, including stainless steel, the slag is

poured into the ladle as well, to be treated at the ladle furnace to recover valuable alloying

elements. During tapping some alloy additions are introduced into the metal stream, and

some more lime is added on top of the ladle to begin building a new slag layer. Often, a few

tonnes of liquid steel and slag is left in the furnace in order to form a "hot heel", which helps

preheat the next charge of scrap and accelerate its meltdown. During and after tapping, the

furnace is "turned around": the slag door is cleaned of solidified slag, repairs may take place,

and electrodes are inspected for damage or lengthened through the addition of new segments;

the taphole is filled with sand at the completion of tapping. For a 90-tonne, medium-power

furnace, the whole process will usually take about 60–70 minutes from the tapping of one

heat to the tapping of the next (the tap-to-tap time).

Testing

At the end of each stage melting onwards sample is taken using automatic lance.For chemical

composition this ―chill" sample—a small, solidified, cylindrical sample of the steel—is

analysed on an arc-emission spectrometer.

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Main facilities available at SMS BHEL

Description Capacity

EAF 70T

EAF 30T

EAF 10T

VAD1 70T

VAD2 70T

VOD 70T

Effect of Alloying Elements on the properties of Steel

Carbon

The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.

Manganese

Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)

Chromium

Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.

Nickel

Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to

http://www.chasealloys.co.uk/steel/austenitic/

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alloy steels.

Molybdenum

Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.

Titanium

The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.

Phosphorus

Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.

Sulphur

When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.

Selenium

Selenium is added to improve machinability.

Niobium (Columbium)

Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.

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Nitrogen

Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.

Silicon

Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels.

Cobalt

Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually approximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.

Tantalum

Chemically similar to niobium and has similar effects.

Copper

Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.

Sources of failure

Inappropriate compositions of constituent materials

Improper slagging practices

Oxidations of alloying elements during tapping

Improper cleaning practices

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Steel Casting

The making of a steel casting is a long and complex process. A large investment in

capitolequipment is required for the melting of steel, manufacturing of cores and molds and the cleaning and heat treating of castings. Additional major investments for support equipment and facilities are required for sand reclamation systems, dust collection devices and bulk material handling systems. A typical casting begins when an order is entered into the Southwest Steel Production Control program. This entry adds it to the production schedule which in turn creates a demand for raw materials (i.e. sand, binder, scrap steel etc.) and manufactured items such as cores.

In addition, pattern and core boxes are taken to the Pattern Shop and 'prepped' for the production run.

Typically, the core room reacts first, getting the necessary cores ready for setting as the molds are being made. Next, molds halves (upper and lower) are made and sent to the assembly area. At the assembly area, molds are flow coated and cores are set in place. The mold is then closed up for pouring.

As the assembled molds are being staged on the pour-off lines, a heat is melted in the arc furnace. Molten steel from the arc furnace is brought to the molds on the pouring lines in a refractory lined pouring ladle. Once poured, the molds are allowed to cool before next being sent to the shakeout. At the shakeout, the castings are separated from the sand mold. The sand is sent to a reclamation system so that it can be reused in the molding process.

As castings are removed from the shakeout they are sent to the cleaning room where they are 'finished' to the customer's specifications. Processing in the cleaning room includes shot blasting, cut-off, welding, heat treating and inspection.

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Sand casting

Sand casting, also known as sand molded casting, is a metal casting process

characterized by using sand as the mold material

Basic steps

Placing a pattern in sand to create a mold. Making of a gating system Removing the pattern. Filling the mold cavity with molten metal. Allow the metal to cool. Break away the sand mold and remove the casting.

Patterns

Patterns are the imitations of the actual component to be prepared, made of wood, metal, or

a plastic such as expanded polystyrene. BHEL uses wooden patterns. They are of oversize

by calculated amount, this oversizing is to provide necessary allowance for metal shrinkage,

pattern removal etc.

Making of pattern is highly specialized job done with high precision because it affects all the

succeeding operations. A ready pattern has markings for ingates, chills etc .

Patterns also have core prints that create registers within the molds into which are placed

sand 'cores.

http://en.wikipedia.org/wiki/File:Haandform-e.png

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Molding box and materials

A multi-part molding box (known as a casting flask the top and bottom halves of which are

known respectively as the cope and drag) is prepared to receive the pattern. These boxes

are made of cast iron. Different parts of a mold b ox can be attached to each other by

welding or using bolts.

Mold

Mold is a hollow replica of job made of molding material (sand) in molding box. When liquid

melt is poured in mold and is allowed to solidify it takes the form of job and is referred as

cast job.

Chills

To control the solidification structure of the metal, it is possible to place metal plates, chills, in the mold. The associated rapid local cooling will form a finer-grained structure and provides directional cooling. In ferrous castings the effect is similar to quenching metals in forge work.

Cores

To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Designs are made to avoid the use of cores, due to the additional set-up time and thus greater cost.

Molding material

Green sand is not green in color, but "green" in the sense that it is used in a wet state (akin to green wood). Unlike the name suggests, "green sand" is not a type of sand on its own, but is rather a mixture of:

silica sand (SiO2), or chromite sand (FeCr2O), or zircon sand (ZrSiO4), 75 to 85%

bentonite (clay), 5 to 11% water, 2 to 4% inert sludge 3 to 5% anthracite (0 to 1%)

For different casting material proportion may differ. The coal typically referred to in foundries as sea-coal, which is present at a ratio of less than 5%, partially combusts in the presence of the molten metal leading to off gassing of organic vapours.

For making runner and ingates small tubes of different shapes mad of refractory material are joined together.

http://en.wikipedia.org/wiki/Silica

http://en.wikipedia.org/w/index.php?title=Zircon_sand&action=edit&redlink=1

http://en.wikipedia.org/wiki/Bentonite

http://en.wikipedia.org/wiki/Clay

http://en.wikipedia.org/wiki/Anthracite

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Design requirements

The parts and pattern design must take care of each step and must allow smooth removal of pattern without disturbing sand . A slight taper, known as draft, must be used on surfaces perpendicular to the parting line & for cores, to serve the same purpose. The sprue and risers must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Molding material must allow the easy removal of gases.

Common Casting Defects

CRACKS Discontinuities caused by rupture.

COLD SHUT When two streams of metals fail to units.

MISRUN A failure to completely form the casting.

POROSITY - An area of local or general cavitations.

SHRINKAGE Gross the cavitations.

BLOW HOLES Small rounded cavities formed due to entrapping of gases.

Sources of defects during Casting, causing mechanical

failure of components

Segregation during cooling

Non directional solidification

Presence of blow holes

Improper cleaning

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Forging

Forging is done to convert ingots into useful shapes either by impact or compression. For application of load several types of hammers and presses can be used. To reduce the load requirement for certain amount of deformation ingot are heat treated before forging.

Facilities available:

Description Number Capacity Max Max temperature

Light forge press with manipulator

1 2650T

Heavy forge press with manipulator

1 7500T

Heat Treatment furnace(LDO)

7 140T 980-1000

Re Heating Furnace(LDO)

5 200T

Vertical H.T Furnaces (electrical )

2 1200-1250

Mist Quenching installations

1

„ESR‟ Equipment 1 50T

Electrical overhead travelling cranes

2

Energy sources The stock is heated to correct forging temperature in a furnace, heated by gas, oil, electrical resistance or induction heating. Gas and oil (esp. Light Diesel Oil) are cheap, easily controlled and mostly used as fuel. The formation of scale due to the heating process, especially on steels created problems in subsequent forging. A non oxidizing atmosphere is, therefore maintained for surface protection. New designs of gas fired furnace have been developed to reduce scaling. Forging furnaces are built so as to ensure a temperature up to 1350ºC in the working chambers. They are heating of the work piece. Each heating furnace consists of Firebox, working chamber, chimney, flues, recuperator or regenerator and various auxiliary arrangements. In BHEL, most of the reheating furnaces are of gas fired type. These furnaces are usually constructed of a rectangular steel frame, lined with insulating and refractory bricks. 4 burners are provided on either side of the chamber and the work piece is handled in and of the furnace by means of the Electric Overhead Travelling (EOT) cranes

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Major Forging Operations Blooming: Blooming is a process of decreasing the diameter of a bar.

Upsetting: Upsetting (or Heating) is the process of increasing the thickness of a bar at the expense of its length and is done by end pressure.

Edging: Edging is the process uses shape the ends of bars and to gather the metal.

Fullering: Fullering is the process used to reduce the cross sectional area of a portion of the stoke. The metal flow is outwards and away from the centre of the Fullering die.

Drawing Down: It is a process of concurrent increase in the length of a bar at the expense of its cross sectional area.

Swaging: The process of drawing out when carried out in concave dies to produce bars of small diameters is known as Swaging.

Piercing and punching: It is a process of producing holes, generally cylindrical, by using a hot punch passing through

a hole of required size in the anvil

the cylindrical die

another hole of same size in the swage block

A reduction ratio of 4 is desirable in the forging operations, which is difficult to achieve in a single step so sequence of blooming and upsetting are repeated to achieve desirable reduction ratio. Reduction ratio=initial diameter of bar/final diameter of bar FORGING DEFECTS LAPS AND FOLDS - Discontinuities caused by forging overlapping surfaces. CRACKS - Discontinuities cause due to rupture. BLISTERS - Raised spots on the surface caused by expansion of gases in Sub-surface zone. ELECTOSLAG RESMELTING (ESR) UNIT Electro slag resmelting is a secondary refining process for electrode ingots of essentially the same composition as the final product. The basic requirements of the ESR furnaces are

a high amperage, low voltage electric supply

an open bottom water cool copper mould that contains the molten slag and metal. In this furnace the mould rests on the starting plate at the beginning of melting and moves steadily upward progresses.

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Heat Treatments

Major heat treatments carried out here are:-

Different types of Heat Treatment are:

Annealing

This process consists of heating the steel slightly above the critical temperature, holding it at this temperature for a long period and furnace cooling.

The main purposes of annealing are

1. To soften the steel

2. To improve machine ability

3. To relieve internal stresses

4. To refine grain size

5. To reduce structural inhomogeneity

Normalizing

In this process steel is heated above the critical temperature for long duration then air cooled. It is done particularly for the following purposes

1. To eliminate coarse grained structure

2. To remove internal stresses that may have been caused by working

3. To improve mechanical properties of steels

4. To achieve better strength than annealed product

Quenching

The primary purpose of quenching is to cool the piece rapidly enough that no transformation occurs above the marten site range, the first requisite of a good quenching medium is a sufficient cooling rate to accomplish the result. The necessary cooling rates is, in turn, determined by the size and harden ability of the piece being quenched, so that the choice of a quenching medium is primarily determined by these factors. The quenching media most commonly used are water, oil or brine. Brine quenching is the most severe, although when thoroughly agitated, as by submerged pressure spray, water approaches it in severity, Oil is considerably less severe.

Objective:

To achieve martensitic structure

To achieve hardness

To avoid the formation of pearlite ,ferrite etc

Tempering

Tempering is the process of heating hardened steel to a temperature below the lower critical

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Temperature and then cooling at desired rate.. If a fully hardened steel, that is, one in which the structure consists entirely of marten site, is heated to successively higher temperatures above room temperature, sub- microscopic particles of cementite will be expelled from the BCTlattice, and the lattice will become BCC. With continued heating, the particles of cementite grow to microscopic size by a process of coalescence and growth.

Objectives:

To restore ductility.

To achieve high toughness.

To achieve desired micro structural constituents.

To relieve the residual stresses.

Rate of heating and cooling depends on equivalent diameter that is the diameter of largest sphere that can be inscribed inside the given job.

Sources for Mechanical failure

Improper heat treatment resulting in different microstructure constituents than desired.

High rates of cooling (quenching) resulting in cracks.

Improper scale removal.

Improper forging resulting directional property differences.

Inhomogeneous cooling rate throughout the job during quenching.

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Testing Methods Used for failure analysis

Tensile testing is a test in which a sample is subjected to uniaxial tension until

failure. The results from the test are commonly used to select a material for an

application, for quality control, and to predict how a material will react under other

types of forces. Properties that are directly measured via a tensile test are ultimate

tensile strength, maximum elongation and reduction in area. From these

measurements the following properties can also be determined: Young's modulus,

Poisson's ratio, yield strength and strain-hardening characteristics.

The test was carried on universal tensile testing machine (USTM)

A typical tensile test specimen A typical tensile curve

Engineering Stress=load/cross section area of specimen

Engineering Strain=change in gauge length/actual gauge length

Yield Strength (YS): It is the stress required to produce specific amount (0.2%)

plastic strain.

Ultimate Tensile Strength (UTS): It is the maximum stress that the given material can

bear in a tensile test. It is signified by maxima in tensile curve.

%Elongation: It signifies ductility of given material

%EL= (change in gauge length till fracture/actual gauge length)*100

%Reduction in Area: It is more accurate measure of ductility

%RA= (change in cross sectional area at fracture point/actual cross sectional

area)*100

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Charpy impact test, also known as the Charpy v-notch test, is a

standardized high strain-rate test which determines the amount of energy absorbed

by a material during fracture. This absorbed energy is a measure of a given

material's toughness and acts as a tool to study temperature-dependent brittle-

ductile transition.

Sample for this test is square cross section(10mm*10mm) containing 45 deg V notch

of 2mm depth and 0.25mm root radius Impact velocity is approximately 5m/s and

strain rate is of the order of 1000/s

A typical Charpy Tester A typical curve showing DBTT and FATT

FATT(Fracture appearance transformation temperature):It is the

temperature at which charpy sample shows 50%shear fracture and 50%

cleavage fracture.

DBTT (ductile to brittle transformation temperature): It is the temperature

at which material shows no ductility on fracture surface.

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Brinell Hardness Test characterizes the indentation hardness of

materials through the scale of penetration of an indenter, loaded on a

material test-piece.

The typical test uses a 10 mm (0.39 in) diameter steel ball as an indenter with a 3,000 kgf (29 kN; 6,600 lbf) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

Where

p= applied load

D=Ball diameter

d=indentation diameter

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Failure Analysis

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Sample: 01

Component: Breach Nut

Heat number: 23103

Specification:-- -----

Chemical composition of material: (ppm)

Chemistry C S P Si Mn Ni Cr Mo V Al

SP VAL

Min. 170 Max. Max. 150 350 500 900 650 250 Max.

Max. 250 20 20 350 850 800 1200 800 350 15

AC VAL

230 13 8 220 620 680 1100 700 180 10

Heat treatment before inspection:

PRECAUTIONARY TREATMENT FOR EQUIVALENT DIAMETER 820mm

AFTER FORGING AIR COOL TO 450°C SKIN

CHECK WITH THERMOCHALK

CHARGE IN FCE AT 450°C

HOLD AT 450°C AS PER PAGE 3/3 OF 10-50-116

HEAT @ °C/Hr 700°C

HOLD AT 700°C 20 + H2 Hrs


HOLD AT 980°C 20 Hrs

AIR COOL TO 350°C SKIN


HOLD AT 350°C AS PER PAGE 2/2 OF 10-50-118

Some confidential data is missing

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QUALITY TREATMENT FOR EQUIVALENT DIAMETER820mm

IN BLACK IF THERE ARE NO SURFACE CRACKS



WATER QUENCH ≥ 180mts

LEAVE IN AIR TILL RERAISE TEMP 200/250°C


CHARGE IN FCE AT 200/250°C

HOLD AT 200/250°C ≥ 6 Hrs


GROUP THERMOCOUPLES WITHIN ±10°C



SLOW COOL UNDER ASBESTOS CLOTH


MECHANICAL TESTING RESULTS:

Required Actual After RHT

UTS(N/Sqmm) - 828 741

YS(N/Sqmm) 550-700 742 656

%EL 15min 15 16

%RA 40min 57.3 42

BHN - 252 229

FATT 24min 90,88,78 66,40,34

Micro structural observation:

Inclusions

Permissible amount of oxide inclusions

microstructure consists of non tempered Bainite & Ferrite with little segregation

Possible causes of failure:

presence of acicular Bainite due to incomplete tempering

segregation

25 | P a g e

CORRECTIVE MEASURES:

REPEAT HEAT TREATMENT




HOLD AT 700°C 8Hrs

SLOW COOL

Comments:

Re-Tempering of the component was effective in producing

tempered Bainite

Thus brought UTS & YS in the desired range.

Application of component:

Breach nut is used to join different parts in a turbine

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Microstructure of Breach nut sample etched with 2% nital

27 | P a g e

Sample2

Component: Diaphragm

Heat no: 70538

Specification: HW19387


Chemistry C S P Si Mn Cr Mo

SP VAL Min. 80 Max. Max. Max. 400 2000 900

Max. 150 15 25 500 800 2500 1100

AC VAL 120 9 20 310 660 2150 940


HEAT TREATMENT FOR EQUIVALENT DIAMETER 910mm

AFTER FORGING AIR COOL TO 450°C SKIN CHECK WITH THERMOCHALK CHARGE IN FCE AT 450°C HOLD AT 450°C AS PER PAGE 3/3 OF 10-50-116 HEAT @ °C/Hr 650°C HOLD AT 650°C 23 + H2 Hrs HEAT @ °C/Hr 980°C HOLD AT 980°C 23 Hrs AIR COOL TO 200/250°C CHECK WITH THERMOCHALK HOLD AT 200/250°C 4Hrs HEAT @ °C/Hr 660°C GROUP THERMOCOUPLES WITHIN ±10°C HEAT @ °C/Hr 700°C HOLD AT 700°C 25 Hrs AIR COOL

Some confidential data is missing.

28 | P a g e

Mechanical Testing results:

Required Actual After RHT

UTS(N/Sqmm) 450-600 728

YS(N/Sqmm) 265min 613

%EL 21min 22.8

%RA - 72.9

BHN 130-180 219-224

FATT 34min 140,120,118


Presence of Oxide inclusions in permissible amount

Microstructure consists of Bainite& Ferrite


Presence of untempered Bainite

Insufficient temperature of soaking during tempering

CORRECTIVE MEASURES:





HOLD AT 710°C 10Hrs

AIR COOL


29 | P a g e

Comments:

Raising the soaking temperature during tempering served the purpose of

bringing down UTS & YS in desired range.


The steel diaphragm acts as partition wall in the turbine with a series of

tunnels cut into it.

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Microstructure of Diaphragm sample etched with 2% nital

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Sample: 03

Component: HP Rotor

Heat no: 5969



Chemis

try C S P Si Mn Ni Cr Mo V Al Cu Sn As Sb

SP VAL

Min. 270

Max.

Max.

Max.

300

500

1100

1000

250

Max.

Max.

Max.

Max.

Max.

Max. 310 7 7 100

800

750

1400

1200

350 10 120 10 20 1.5

AC VAL

285

6 7 40 410

650

1280

1050

270

9 70 7 6 7


PRECAUTIONARY TREATMENT FOR EQUI DIA 815 mm

AFTER FORGING CHARGE IN FCE DIRECTLY HELD AT 650°C HOLD AT 650°C 54 Hrs HEAT @ °C/Hr 970°C HOLD AT 970°C 22 Hrs UNLOAD AIR COOL TO 350°C HOLD AT 350°C AS PER 2/2 OF 10-50-118 HEAT @ °C/Hr 700°C HOLD AT 700°C 22+H2 Hrs SLOW COOL


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QUALITY TREATMENT FOR EQUI DIA 755 mm HEAT @ °C/Hr 950°C HOLD AT 950°C 11 Hrs MIST QUENCH = 180 mts LEAVE IN AIR TILL RERAISE TEMP 200/250°C CHARGE IN FCE AT 200/250°C HOLD AT 200/250°C ≥ 5 Hrs




HOLD AT 700°C 17 Hrs CONTROL COOL MAX TO 400°C

AIR COOL IN POST TEMPERING COOLING PIT Some confidential data is missing.


Required Actual

UTS(N/Sqmm) 686

YS(N/Sqmm) Min 530 601

%EL 24.3

%RA 69.8

BHN 211

Impact min55 110,146,110

FATT(C) 80max 30


Presence of Oxide inclusions in very small amount (permissible)

Tempered Bainite and very small fraction of Ferrite

33 | P a g e


HP Rotor is the most critical component of a steam turbine. The whole

assembly of the turbine is done around the rotor. The simplest turbines

have one moving part, a rotor assembly, which is a shaft or drum with

blades attached. Moving fluid acts on the blades, or the blades react to

the flow, so that they move and impart rotational energy to the rotor.

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Microstructure of Rotor sample etched with 2% nital

35 | P a g e

Sample: 04

Component: cylinder

Heat no: 23039

Specification: ASTM A148-08 Gr 80-50



HEATING UP TO 200C

HEATING @100C/HR UP TO 570C

SOAKING AT 570C FOR 4HRS

FURNACE COOLING TO 400C

AIR COOLING TO RT


Required Actual

UTS(N/Sqmm) 550 542

YS(N/Sqmm) 345 330

%EL - -

%RA - -

BHN - -

FATT - -

Element C S P Si Mn Ni Cr Mo V Cu Sn Al

Actual Amount

210 15 14 490 1250 180 150 90 - 80 10 27

36 | P a g e


Polished surface shows occurrence of MnS inclusions.

Microstructure consists of non-uniformly distributed ferrite and

pearlite.


Inclusions and non-uniformity in phase distribution

Corrective measures:

HEATING UP TO 200C


SOAKING AT 570C FOR 5HRS

FURNACE COOLING TO 400C

AIR COOLING TO RT

Comments:

As repeat heat treatment was still in progress its effect could not

be observed


Cylinders are used to store liquids and gases at high pressure.

Cylinders are also joined together to form pipe to carry liquid or

gas.

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Microstructure of Cylinder sample etched with 2% nital

38 | P a g e

Sample: 05

Component: Bend

Heat no: 23161




HEAT TREATMENT

HEATING UPTO 200C

HEATING @60C/HR UPTO 920C

HEATING @30C/HR UPTO 940

HOLDING FOR 6 HRS

WATER QUENCHING TO RT


Required Actual

UTS(N/Sqmm) 590-780 856

YS(N/Sqmm) 440 744

%EL - 17

%RA - 55.1

BHN - 266

Impact 27 20-20

FATT - --

Chemistry C S P Si Mn Ni Cr Mo V Cu Sn Al

actual 170 10 12 300 750 210 1380 920 240 60 8 30

39 | P a g e


presence of Oxide and Sulphide inclusions

microstructure consists of untempered Bainite and Ferrite


Presence of untempered Bainite

Failure in impact testing suggests lack of ductility



HEATING UPTO250C


HOLDING FOR 8 HRS

COOLING@60C/HR UP TO RT

Comments:

Tempering of component is sufficient to ductility and reduce UTS

&YS

As repeat heat treatment was still in progress its effect could not

be observed


Bend is mainly to join pipes carrying liquid or gas at high

pressures

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Microstructure of Bend sample etched with 2% nital

41 | P a g e

Sample no: 06

Component: HP Rotor

Heat no: 222637



Chemistry

C S P Si

Mn-- Ni Cr

Mo--- V Al Cu Sn As Sb

SP VAL

Min. 270

Max.

Max.

Max. 300

500

1100

1000

250

Max.

Max.

Max.

Max.

Max.

Max. 310 7 7

100 800

750

1400

1200

350 10

120 10 20 1.5

AC VAL

290

6 7 40 340 580

1250

1050

300

10 80 8 7 5


PRECAUTIONARY TREATMENT FOR EQUI DIA 1030 mm

AFTER FORGING CHARGE IN FCE DIRECTLY HELD AT 650°C HOLD AT 650°C 58 Hrs HEAT @ °C/Hr 970°C HOLD AT 970°C 26 Hrs UNLOAD AIR COOL TO 350°C HOLD AT 350°C AS PER 2/2 OF 10-50-118 HEAT @ °C/Hr 700°C HOLD AT 700°C 26+H2 Hrs SLOW COOL


42 | P a g e


Required Actual After RQT

UTS(N/Sqmm) 850 max 677 731

YS(N/Sqmm) 550-700 531 558

%EL 15min 21.4 20.6

%RA 40min 69.9 67

BHN - 207,211 224,224

FATT 24min 110,124,138 200,230,280


Negligible amount of Oxide inclusions

Tempered Bainite and Ferrite


Very slow cooling Resulting in softer phase

QUALITY TREATMENT FOR EQUI DIA 985 mm



MIST QUENCH = 240 mts








CONTROL COOL MAX TO 400°C

AIR COOL IN POST TEMPERING COOLING PIT

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REPEAT QUALITY TREATMENT FOR EQUI DIA 985 mm



MIST QUENCH = 240 mts








CONTROL COOL MAX TO 400°C

AIR COOL IN POST TEMPERING COOLING PIT Some confidential data is missing.

Comments:

Repeating the Hardening and Tempering cycle brought YS in

permissible range


HP Rotor is the most critical component of a steam turbine. The whole

assembly of the turbine is done around the rotor. The simplest turbines

have one moving part, a rotor assembly, which is a shaft or drum with

blades attached. Moving fluid acts on the blades, or the blades react to

the flow, so that they move and impart rotational energy to the rotor.

44 | P a g e

Microstructure of Rotor sample etched with 2% nital

45 | P a g e

Acknowledgement:

We are very thankful to our mentor Mr S. N. Ghosh (DGM Quality

Management Lab, CFFO, BHEL, Haridwar), for his guidance and

immense support throughout the project. We are also thankful to Mr

Anurag Kushwaha (AGM, Forge Shop, CFFP, BHEL, Haridwar) and Mr

Alok Singh (AGM, Forge Shop, CFFP, BHEL, Haridwar) for providing us

this opportunity. And our sincere thanks to Mr. Pradeep Kalhan for his

guidance and valued help. We are also thankful to Mr R.D.Sharma (

Incharge Metallography lab) and all other persons who helped us in

carrying out our project at BHEL Haridwar.

References:

DieterJ.E.: “Mechanical Metallurgy” (3rd ed.) page 175-177, 326-

327, 472-473.

Tupkari R.H., Tupkary V.H. “An Introduction to Modern Iron

Making”.

Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry

H. (2000), “Machinery's Handbook” (26th ed.).

Documents

Project Report by Sumit Gupta and Anmol Kumar Shrivas