bull head hammer

8/13/2019 Bull Head Hammer

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Introductionto bull head hammer at VSP:-Out of the spares supplying by ES & F, bull head hammer is the important spares which is used

in hammer crusher of sinter plant for crushing of

Dolamite (MgCo3)

Limestone (CaCo3)

These hammers are made in forge shop and central machine shop (CMS) .

In forge shop, raw material is forge to the required shape, stresses relieved or annealed and

send to CMS for further operations like drilling and heat treatment .

The raw material for bull head hammer is being procured from M/s VSNL, BHADRAWATHI.

Bull head hammer is generally made of material 50 Cr Mo 4.

Average life of Bull-head hammer is 60 hrs.

COMPOSITION OF 50 Cr Mo 4 :

C : 0.46-0.54

Si : 0.15-0.4

Mn : 0.5-8,

P : 0.03 Max,

S : 0.03 Max,

Cr : 0.9-0.12,

Mo: 0.25-0.3, (hot rolled, spherodoised annealed)

Technical specifications OF BULL HEAD HAMMERS as per

Drawing :-


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1. Hammer is to made by stamping.

2. Hammer should be weighed and weights. Punched on bull head hammers.

3. Surface defects like laps and cracks are not allowed.

4. The compensation hole is to be drilled if the weight exceeds 17.6 kg.

5. Hammers are to be heat treated up to 380- 530 BHN.

6. sharp corners should be avoided by providing sufficient radii.

Qualities required by BHH are :-The bull head hammers should have the followings qualities :-

1. Hardness

2. Toughness

3. Wear resistance

4. Ultimate tensile strength

HARDNESS-It is a fundamental property which is closely related to strength. It is defined as

resistance to abrasion / penetration. It also includes resistance to cutting & scratching.

Hardness of bull head hammer should be 380 to 530 BHN range.

TOUGHNESS It is defined as amount of energy required for fracture. The toughness

of a material is its ability to withstand both plastic & elastic deformation. Since the particles

of dolomite and Limestone are continuously hitting by the hammer and it is more prone to

fracture at neck area, So it is a desirable quality for bull head hammer.

WEAR RESISTANCE Bull head hammer should have sufficiently wear resistance.

If it is not there then it will be worn out faster, leading to the low life of BULL HEAD


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hammer. The wearing of bull head hammer in sinter plant may be due to high

temperature generated and strong impact between hammer and dolomite. We can

cool it by circulating cool air the spark (heat) generated will be given less time to

propagate through the material and change the grain size and deformation.

UTS ULTIMATE TENSILE STREGTH) The strength of material is its capacity to

withstand destruction under the action of external load. The stronger the material the

greater the load it can withstand. It therefore determines the ability of a material to

withstand stress without failure. The maximum stress that any material will withstand

before destruction is called its ultimate strength. This is the desired quality of BHH as it

is under continuous stress while crushing of limestone and dolomites in hammer

crusher.

MANUFACTURING PROCESS OF BHH :-

Raw material is procured from VISL. Material is 50Cr MO4. Raw material size is

6000x100x 100mm.

Process involved in forge shop :-

1. Length is reduced to 3 m by gas cutting.

2. Heat treatment is done in bogie type hearth furnace in order to reduce hardness from

range of (236 to 246) to 170 BHN.

3. Shearing of billet. Size of the billet is reduced to 100x100x217mm.

4. Heating is done in fixed hearth type furnace for forging operation. Heating is done up

to 12500C for 5 hrs. At the rate of 2000C / hr. heating is done then half an hour soaking

is done.


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5. With the help of manipulator and open die hammer, manual forging is done.

Only 90 mm size is forged. Shank portion width is made to 36 mm. And total length is

made to 360 mm. shank portion length is kept as per drawing equal to 193.5 mm. for

checking the size during the process template is used. After this process the BHN of

BHH is approx. 350BHN.

6. Annealing is done after forging in order to reduce hardness from 350 to 220. So that

drilling may take place successfully. Room temp loading is done for annealing, it is

heated up to 5000C at the rate of 1500C /hr. then half an hr. soaking again heating at the

same rate up to 850 .c then one hr. soaking. Then furnace cooling is done for 38 hrs.

Due to this annealing hardness come down to 210-220 BHN.

7. Inspection is done with respect to shape and size of BHH.

8. BHH is sent to Central Machine shop after inspection. They are sent in batch as

Batch size is 400 jobs per batch.

PROCESS INVOLVED IN CENTRAL MACHINE SHOP :-

(a) A hole of 50 mm dia. is made in shank portion of BHH as per drawing. If required

another hole is also drilled in order to control the weight of BHH.

(b) HARDENING: - Heat treatment is done in CO gas fired fixed hearth type furnace. it

is twin chamber type.

Heat treatment cycle for BHH is-

(1) Heating the hammers up to 6000c at a constant rate of 150.c /hr.

(2) Soaking at 600.c for 30 minute.


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It acts very much like Cr but is more powerful in action. Italso increases depth of hardness after heat treatment. Mo finds its greatest use when

combined with the alloying elementLike Cr, Ni, or both. Mo increases critical range of temp. Except for carbon it has the

greatest hardening effect and results in the retention of a great deal of toughness.

SILICON It is added to all steels as deoxidizing agent. When added to very low

carbon steels it produces a brittle and a high magnetic permeability. The principal use

of silicon is with other alloying element, such as manganese, chromium, and

vanadium to stabilize carbides.

MANGANESE-It is added to all low and the manganese content is high over one

percent, then it is classified as a manganese alloy. It lowers the critical range of

temperature.

SPHERODOISED ANNEALING The mach inability of high carbon tool steel is

at its best condition when the structure is composed of grained or globular pearlite. An

alloy steels, including those of the carbide class, as well as ball bearing steels should

have a structure of globular pearlite in the deliverable state. The process of producing

a structure of globular pearlite is known as spheredoising or spherodoised annealing.

HEAT TREATMENT PROCESS:-Heat treatment is defined as an operation of combination of different

processes involving the heating and slow or rapid cooling of a metal or its alloys in solid state for

the purpose of obtaining required structure and desirable properties.


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Heat treatment usually results in the change or transformation of the microstructure of

the metal or alloys.

All heat treatment processes have mainly three main stages :-

1. The heating of the metal to the pre-determined temperature.2. The soaking of metal at that temperature until the structure becomes uniform through out

the mass.

3. The cooling of metal at some pre-determined rate to cause the desired structure.Types of heat treatment process :-

1. Normalizing2. Annealing3. Hardening4. Tempering5. Spheroidizing6. Case hardening7. Carburizing8. Nitriding9. Cyaniding10.Induction hardening11.Flame hardening

REQUIRMENT OF HEAT TREATMENT PROCESS:-1. To relieve the stresses set up in a material in order to improve machinability and

ductility.

2. To improve mechanical properties likea. Tensile strengthb. Hardnessc. Ductility


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d. Shock resistance3. To modify the structure of material to improve electrical and magnetic properties.4. To improve the quality of metal to provide better wear resistance, heat corrosion5.

To refine grain structure.

6. To soften and toughen high carbon steel.SELECTION CRITARIA FOR HEAT TREATMENT PROCESSES:-

1. The Softening ProcessesAnnealing

Normalizing

2. The Hardening ProcessesHardening

Tempering

The Softening Processes:-

AnnealingUsed variously to soften, relieve internal stresses, improve machinability and to develop

Particular mechanical and physical properties.

In special silicon steels used for transformer laminations annealing develops the particular

Microstructure that confers the unique electrical properties.

The process of annealing involves heating the metals slowly to the required temperature, then

holding at that temp for long enough to enable the internal changes to take place and finally

cooling slowly. Annealing reduces the hardness, increases ductility and usually reduces its

strength.

NormalizingAlso used to soften and relieve internal stresses after cold work and to refine the grain size

and metallurgical structure. It may be used to break up the dendritic (as cast) structure of

castings to improve their machinability and future heat treatment response or to mitigate


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banding in rolled steel.

This requires heating to above the as temperature, holding for sufficient time to allow

temperature equalization followed by air cooling. It is therefore similar to annealing but with a

faster cooling rate

The Hardening Processes:-

Hardening -Hardening is a heat treatment process .It is made to develop high hardness to resist wear as

well as to improve strength, elasticity, ductility and toughness to the material. Hardening

process consists of heating the steel to a temperature above critical point, holding at this temp

for considerable period and finally quenching in water, oil or molten salt bath.

TEMPERINGWhen a steel specimen has been fully hardened, it is very hard and brittle and has high

residual stresses. The steel is unstable and tends to contract on aging. The internal stress

can be relieved by an additional heating process. This heating process is called

tempering. After specimen has been fully hardened by quenching from above critical

temp, it is reheated to some temp below the critical temp for a certain period of time and

then allowed to cool in still air. The temp to which it is reheated depends upon the

composition and the degree of hardness or toughness desired.

Tempering reduces the elastic limit and ultimate strength lightly but they are still higher than

they were before drawing. Hardening with subsequent tempering serves the following

purposes for bullhead hammer. It develops high hardness to resist wear and to enable it to

cut another metals. It also improves strength, elasticity, and ductility.

Anneal Normalise Harden Temper


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Low Carbon 0.5% yes yes yes yes

Low Alloy yes yes yes yes

Medium Alloy yes yes yes yes

High Alloy yes may be yes yes

Tool Steels yes no yes yes

Stainless Steel (Austenitic eg 304, 306) yes no no no

Stainless Steels (Ferritic eg 405, 430 442) yes no no no

Stainless steels

(Martensitic eg 410, 440) yes no yes yes

Analysis of heat treatment process required for bull head hammer :-(A) The heat treatment should be done at a proper temp. The best temp suitable for heat

treatment is 7230C. The current system heat treatment is done at 8160C.

Since martensite is formed from the austenite by diffusion less, two step shear transformation it

has the same composition as austenite. Martensite is appreciably harder than the equilibrium

combination of ferrite +carbide.

During quenching martensite is formed. The formation will be more if the time

given for quenching is less by means of relatively cool water than we are using now. The outer

surface will be harder.

We should also take care about the brittleness if we cool fast it will be more brittle. So tempering

should be done to reduce the brittleness. So our point out interest is to maximize the amount of

martensite in the quenched hammer.

(b) Heat treatment as per proposed cycle is difficult to achieve due to poor functioning valves,

actuators.


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(c) Heat transfer rate is difficult to achieve in furnaces since there is no facility of considering

time factor during heat treatment. to control the heat treatment , pyrometer may be used for

checking temp of different bullhead hammer at different areas of furnace. According to this

measured temp, different burners of heat treatment furnace may be kept on or off so that

uniform temp of each bull head hammer may be achieved as per proposed heat treatment

cycle.

(d) Tempering furnace also has no facility to control the heat transfer rate. Air cooling is not

being done.

(e) Temperature control of hammers inside furnace is difficult to achieve uniformly.

Procurement of a heat treatment furnace, which can be controlled automatically, will reduce the

problems related to heat treatment of bull head hammers.

(f) Magnetic testing is not done.

Quench cracks also come during quenching of bull head hammer... Magnetic testing may be

done in order to avoid any quench cracks.

(g) Toughness testing is also not being done.

Toughness is important property in consideration of bull head hammer failure. If it is less it is

more prone to failure. Right now there is no provision for checking of toughness of bull head

hammer. Izod impact test / charpy test may be done on sample basis.

(f) Improper quenching due to formation of water bubble.

(g) Proper care during heat treatment can be done; pyrometer may be used for checking temp

of different bullhead hammer at different position. According to this measured temp different

burners of heat treatment furnace may be kept on or off so that uniform temp of each bull head

hammer may be achieved as per proposed heat treatment cycle.


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(h)While doing quenching, it is proposed that quenching should be in up to max 100 mm in head

area, but due to boiled water bubble formation length may goes up to high. So proper checking

of stand dimension, proper height should also be maintained carefully.

(i) Due to not proper recirculation of water, in quench area steam layer forms, which resists

further cooling and contact of water. So it is desired that proper recirculation of water may take

place to perform proper quenching.

Conclusion :-

1. Range of hardness as per drawing is 380 to 500 BHN. This large range may lead to variation

of properties of each and every bull head hammer. This range should be minimized in order to

get uniform properties of each and every bull head hammer.

2. Forging is done for the handle portion only. If the head can be forged it will give better

mechanical properties for the hammer. Chances of better hammer life will be more.

3. Manufacturing of hammers by stamp forging instead of open die forging may also result in

improvement of life time due to the formation of uniform grain structure throughout its length.

4. Productivity may also be increased by crushing the raw material in two stages.

5. It is very difficult to change the shape of hammer without changing the composition.

6. The hammer is made of 50crmo4 while the shield is made of 40cr4. The hammer which is

rotating at 1000rpm is wearing quickly than the shield which is stationary the idea is to make the

hammer with the same material as that of shield.

7. There would be increase in downtime of crushers due to frequent hammer charging.

8. LOW LIFE and High WEAROUT of the hammers is identified as the root cause for all

problems initiating alternatives for making a better BHH.

bull head hammer

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