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STAINLESS STEEL IN MANUFACTURING

The production which is made with stainless steels has much more different features than carbon and low-alloyed steels.This difference takes root from amended features of the material in terms of yield strenght and constipation attributions and for each of stainless steel it is needful to think one by one.The ability of cold shaping of stainless steels effect from yield strenght, tensile strenght, ductility, hardening feature and section reduction. The common features in terms of shaping out :

a) The strenght, hardness and ductility of stainless steels generally much more higher.
b)They become rigid more quickly. ( that is their strenght raise as giving shape )
At end of the production, it is taken into consideration to have an oxide film that is protective against corrosion resistance.
c) Because of these reasons, as generally, during the production, it is needful to use much more strenght, to repair tool, apparatus and equipment ever so often or to change and to take pain over surface

  * Cutting
  * Curving And Edging
  * Deep Drawing- Stretching
  * (Lathe) Plastering
  * Hot Forging
  * Machining
  * Bonding Methods
  * Welding
  * Hard Solder
  * Heat Treatment
  * Preservation of Material During Production

Slashing
There are some information about the different cutting ways of the stainless steels in the below.For all cutting processing, the common directions are given with carefulness;:

"  The getting spoiled should be impeded in the iron ( iron or steel ) pieces.
" The protective oxide film are formed again as naturally in the cut surfaces as mechanically.Forming of that film can be hasten by chemical pasivasyon processing
" The surfaces that were cut by the way of heat treatment undergo a change as chemically and metallurgical.For not to meet any negativeness, such kind of surfaces should be fend off.

Cutting With Shears :
The cutting of stainless steels sheets,because of its constipation features, are required much more force than carbonaceous sheets.Because the material is ductile, the cutting hole should not be narrow.It is offered that sheets that have thick than 2 mm, thickness’ 5 % and for much more thick sheets, thickness’ 3 % should be taken as the cutting hole.

Slashing:
The slashing is to obtain narrow lanes from a large width roll.This processing has been used to get essential narrow lanes for the production of the seamed pipe.That cutting surfaces does not have burr are very important.Thus,the cutting hole between the circular cutters, length of the cutters’plunger ( penetrasyon ) and fastness of cutters are very considerable.Also,it is offered that the thickness of the material should be approximately 5 %.The penetrasyon value can be different in accordance with the type of material and thickness.The cutting fastness can be changed between 60 – 200 m/ minutes in accordance with the type of material and thickness

Cutting With Saw:
The stainless steels can be cut by hand-saw or mechanical saw. For both of them, it is offered to use the high-speed steel saws and the cutting liquid should be used.The cutting of austenitics is much more difficult.

Cutting In The Cast:
Cutting can be done without using of lubricator ; however, if it used the force and need of power can be reduced, thus lifetime of the equipment extends.The accurate of the cutting hole should be applicated correctly.The narrow holes are required zero adjustment.The big holes cause breaking of cuttting surfaces in ductile stainless steels and in order to get the most suitable hole requires experience and this value can change in accordance with used equipment, geometry of pieces and features of the material.Also, the cast of drilling and the cutting of flake should be done much more sensivity in comparison with carbon steels.The hole of cutting should not be exceed 10 % of the thickness.For 1 mm or much more thin sheets, the essential cutting hole (on every hand) should be between 0,025 mm and 0,035 mm.In order to reduce the cutting force, the angular cast or stapler can be required and high- viscosity oilers are used in order to obviate ebullition of the material of the stapler. In the processing of drilling, the lowest diameter should not be decrease from the double of shett thickness.If there were more than one holes, the lenght of holes should be at least sheet thickness.

Cutting With Abrasives:
It can be used for the cutting of the small lanes and thin-flat products.Generally, it is impelented with suitable oil emulsion.It is important to take pain over warming of cutting side.

Curving And Edging :
Because stainless steels have a sum of chrome and melting heat of oxide of chrome is very high, the way of cutting by oxygen is in sufficient for these materials.Instead of this, cutting of metal powder can be used.In this method, a metal powder that includes iron are sprayed to the oksi-asetilen gas jet and the heat,which is formed by the burning of this gas, is enough to melt.The other choice is to cut by high-heat that is obtained the help of electric canal.So many cutting methods with canals,using different electrode and gas, have been developed.

Bükme ve Kenarlama
The curving and edging of stainless steels required much more care and 50-60 % force in comparison with other steels. Also,the quantity of back-spring is much more than carbon steels.The curving axis of the tempered steels is parallel to rolling direction.But the curving axis of the ferritic steels should be vertical to the rolling direction and thickness of curving should not be low 2 mm than sheet thickness (vide tablo 6.2).Especially, radius of stapler should not be more than 8 times than the edging radium.In edging, the hole of cast-stamp should be at least 10 % from the sheet thickness and scratch of sheet should be impeded.Cast surfaces should be burnished with hand or machine and all kind of burr,metal waste,dust,etc. Should be fend off.

Type of stainless steel

R/t Rate

Tempered Steels

0,5 – 1,5

Hardened and Strenghten Steels

Quarter-Hard

1 – 2

Half-Hard

2,5 - 4

Full-Hard

4 - 6

The processing of rotary curling,using for making of cylindrical boiler and depo, is done similar to carbon steels but it should be thought that much more back-spring can be performed.The bcak-spring should be taken account in cylinder machine and in quality viscosity oiler should be used and roller surfaces should be covered with chrome-nichel or titanium nitrur.

Deep Drawing- Stretching
Because they are ductile, the austenitic stainless steels are the most favoured for cold shaping.Streching can be implemented to these materials.With quality of 201 and 301 and with 2 axises streching,they can be shaped more than 35%; because, during shaping, the partially martensitic transformation provides resistance to shrinking and shaping out.Also, type of ferritic have the ability of shaping out; however, because they are lesser ductile, their ability of shaping out is limited; thus, tempered can be required.After tempering, cleanliness should be done and then pasivasyon. The austenitic stainless steels should be burnished by chrome oxide after deep drawing.The martensitic,having lesser alloys,stainless steels can be shaped out but the general of them may not be suitable to deep drawing.The way of shaping out should be chosen in accordance with character of the stainless steel and the thickness of the material.As mentioned above, need of power for stainless steels,especially austenitic, are higher, because they are hardened much more fastness in comparison with ferritic.

Among the all austenitic steels, quality of 301 has low chrome and nichel and it has deep drawing feature with being hardened specification.With this feature, despite of the fact that it is suitable for structural equipment,formed by shaping out, it isn’t suitable for the deep drawing processing.But some kind of stainless steels,such as 304, 304L and 305 are easy to become rigid because they include much more nichel and chrome and they are using for deep drawing processing much more.The quality 302 steel makes a solution by dint of its mechanical circumstances between 301 and 304.Also, shaping of 321 and 347 alloys,which include composition elements such as titanium,tantalum and niobium and progressive carbon to 312 and 347 alloys,have lesser decrease.

The most suitable criterion to show the ability of deep drawing of the stainless steel material is the ‘ delimiter value’ of the material.

Drawing Rate Of Deep Drawing Process:
The product’s (lamella) initial diameter is described as post-drawing diameter and for the problem-free drawing, this rate which is called as delimiter value should be chosen minor (B) value.

Although austenitic stainless steels have fastness being hardened, they can reach high drawing values in the first drawing processing B=2,1. If the material is known as just austenitic and but there is not so much details, it should not be exceeded over 1,8 value as drawing value.The equipment that are used for the depp drawing process of the satinless steels should be made much more stronger than normal carbonaceous steels.Because it is expected that the operative forces should be greater than carbonaceous steels as 50…100%.If the drawing were being done more than one stage,the pieces which are among stages should be tempered between 1000-1100 degree celcius as 7 and 10 minutes.Before tempering,the purgation should be done to get rid of oil waste.The fastnees of shaping out should be low; otherwise, there may be an abrasion equipment.Also, pressure of the blunder circle should be high in order to prevent the folding.If there were any problem during the first drawing stage, there could not be met any problem in other stages.But if there is a blunder,it can be impossible to overcome in other stages.The difference of between drawing and flowing resistance of ferrtic steels are lesser than austenitic steels; that is, they are hardened less.But their ductility is much more lesser.Among sheet materials,the most consumption material in use are qualities of  409 and 430 steels.Although their usage is not much common in comparison with the qualtiy of 304 austenitic steels, especially the quality of 403 sheets are preffered mostly for every kind of stainless steels implementing.But the ferrtic steels are not so good even the irst drawing process.That is to say that the value of deep drawing should not be determined as well as austenitic steels. The value of ? can be determined over 1,55 value rarely. For large stages,the deep drawing can be progressed between 750-800 degree celcius; for the second stage, B= 1,28 and for the third stage, B=1,2 values can be used.In order to a well-shaping for the material, half-hot (soft) deep drawing can be used in some circumstances.The heat can be chosen as 100 degree celcius for thin sheets and for the thick sheets heat can be approximately 300 degree celcius.Among martensitic qualities , just 403,410 and 414 qualities can be chosen for cold shaping.They are much more stronger than austenitic,ferritic steels and even carbonaceous steels in terms of resistance of flowing and drawing and essential energy and power for shaping.Because having much carbon, the cold shaping of the other martensitic steels are low and generally they can be process as half-hot (soft).From the lower carbonaceous steels, 440A,440B and 440C qualities have the ability of shaping much more low.If the thickness of sheet piece,which is made of from these steels, is insufficient after hardened, they can be heightened to 60 HRC by giving water in the air.Their ductility,glut and becoming rigid features are as the same as austenitic and ferritic steels’ values.Also, radium of corner- edge of the sheets should be made bigger than austenitic and if more than 25% shaping is required, letup temper should be used. The shaping of the age-hardenable steels are implemented randomly in comparison with other 4 alloys.This condition is to discussed especially when thickness of martensitic steels is insufiicient.Besides type of alloys, the internal structures are also important.Among them,the most important thing is grain size.The more grain size is getting bigger , the pebbling problem can be occured and when grain size is minor, deep drawing process become hard due to improving of the yield strenght.According to the ASTM Standards,the grain size is determined 00 as the biggest grain size and 13 as the smallest grain size.In accordance with this scale, the sheets that are between 6-10 grain size are suitable for the deep drawing and the sheets that are between 9-12 are suitable for (mechanical) cutting.Also the quality of 304, 305, 316, 403 sheets are used in deep drawing and plastered surfaces commonly and their DQ (draw quality) and DDQ (deep drawing quality) are represented commercial forms.The other specification in the deep drawing is direction dependence (anizotropi), that is to say that features of material show  discrepancy to the direction of hot working and opposed it.This feature’s effects are much more evident for thin sheets in comparison with the sheets that are milled and the effects can be controlled by adjustment of the pressure of the blunder circle.The other matter is the back-spring in deep drawing for the designing of deep drawing.According to the carbaneous steels the tensile strenght and high flowing required the compensation of back-spring in cast designing with the same rate or higher pressure of blunder circle during the shaping process.For stainless steels’ deep drawing process, the demanding subject are : cast covering, greasing, material that is going to be shaped and cleanliness of the environment. Among offered deep drawing, plastering and edging cast, there are titanium carbur (TİC) that is covered by CVD, titanium nitride (TİN), titanium carbon nitride (TİCN).For casts,used skimmer and blunder, the blunder surfaces should be covered by caoutchouc or plastic and surface detriment should be prevented.

The surface of stanless steels are easy to scuff in comparson with other steels because of the friction thus oiling is very important for shaping.With this aim, lube oil is insufficient mostly so special oil that is durable to high pressure and include polymer and hydrocarbon can be used.There are many experiments for the usage of special oil.Banishing graphite-water mixture, it can be waited so as to dry.Also it can be performed by adding some graphite for other oil,used in drawng process.If the letup tempering is required, the most beneficial thing is to use graphite and other oil should be fend off before tempering.Meanwhile, the most important thing is to clean dust and waste of carbon steels and they stick to steel surface and as a consequence of this red rust can be formed in the press ateliers.If the carbon steel is operated in the same atelier or factory worktables should be fend off and also if it is possible, different carriage- transshipment machines or vehicles should be used and if this impossible, these machines should be cleaned before carring the stainless steels.During the production, carriage- transshipment activities should be done regularly and planned and cleanliness should be done nicely.Because susceptibility of stainless steels in terms of shaping fastness is better than carbon steels, it is offered that low rate should be used at press shaping.Therefore, it can be seen that the required power decrease and ductility is developed mostly.Also it is suggested that drawing space should be used as more than 20…35% of the thickness.It is suggested that the radius should be 5 and 10 times of the thickness for matrix corners. Moreover,it is suggested that the rolling radius should be used 5 times of the thickness for punch edges

(Lathe) Plastering
The austenitic steels are easy to plastered in the lathe because of its ductility; however, they can be required much more power because of their high-hardening specification.Therefore, their back-spring is higher.But it is too hard to plaster of the ferritic steels and limited.Using a good oiler is very important.Also it is required letup tempering in terms of their total shaping quantity.Before tempering, it is essential to clean oil on the piece.

Hot Forging
Almost all of the stainless steels are shaped with forging.But if the alloy quantity is raised, forging becomes hard.Especially, there can be cracks formation on the surfaces of the high-alloyed materials at the begining of the process.

The heat of forging of the steel,used in the industry commonly, has been summarized in the table 6.3.The width of heat at the austenitic steels depend on the shaping ability of the material and whether allotropic alternation is formed or not.Some commercial 18-8 alloys, when processing heat is raising to 1260 degree celcus, this heat alloy rate decrease at in quality because of metallurgical change that causes crack formation on surfaces

The delta ferric phase in some austenitic steels emasculates open-cast forging.There can be a large amount of ferric in 304, 309, 317, 321 types. The exaggerated amount ferric is the cause of cracks formation in the beginning of the shaping process. But mostly it can be beneficial when austenitic change is provided by homogeneous tempering between 1150 degree celcius.The austenitic steels should be tempered and gained ductiliyty of corrosion resitance after shaping.It can be unnecessary if the shaping is completed between approximately 870 degree celcius and if the material heat is decreased tdo 425 degree celcius in order to prevent formation of carbide.Because there could not be seen formation of carbide in such stabilized types as 321, 347 and 348 and 304L, 316L, there is no need a thermal process after forging.While low-carbonate martensitic types can be forged in low heat, high-carbonate types (such 420 and 440) can be forged more narrow heat space.Because these steels are hardened in air, it is obligatory to do thermal process ( tempering for soften, improvement process,etc).

The space shaping of ferrticis steels are large; but, because of the grain size during high temperature, it is suggested to flog in low temperature in comparison with austenitic steels.After forging, tempering should be done for ferritic steels.Owing to the fact that there is tender martensitic in forgeable condition in the internal structure despite of the fact that it can not be seen an improvement for the hardness.

Cold Forging And Mass Shaping
Among the successive cold shaping methods in terms of stainless steels are head hoarding,pulling, extrusion and clench.As mentioned before, in comparison with carbon steels, because of its high strength and hardening, the needful strength and power are much more. However there is some limits which depend on ductility and hardening features almost all stainless steels can be used in cold head hoarding.When a large deformation is needed, the problem of necessity of high strength and oiling can be seen. The attitudes of ferritic and low-carbon martensitic steels similar to the carbon and down ayyoyed steels.While austenitic and ferritic types can be used as literally; after shaping,heat treatment is implemented to the martensitic types generally.

Material

Heat (X)

301, 302, 3Û2B, 303, 304, 305, 308, 321, 347

930- 1180

309,310,330

1000 - 1150

314

950- 1100

316,317

950-1150

403,410, 416

875- 1150

414,431

915-1150

405, 420, 440

960-1125

430, 430F, 442, 446

810- 1125

Tablo 6.2 : Recommended Forging Heat for variegated stainless steelsı

Machining
The machining of stainless steels’ high strenght, as is the case with shaping, are more difficult than carbon steels in terms of hardening features and ductility.Besides there are many dissimilarities among the types such as higher strenght, lower cutting fastness, also surface quality problems that appeared during cutting, accumulation of material on cutter team equipment can be snown

The cutting process of the stainless steels in order to prevent vibration it is important the rigidity of tool workbench and cutter tools.It is offered to use cracker of metal filings or bending or plates because there is rough and continuous metal filings especially austenitic and high-alloyed types.In most practices carbide cutter tools preferred because of their good corrosion resistance.

So as to prevent the hardening that may be formed on the surfaces of material, it is important to pay attention some points.The brush movement of material before cutting of tool bring about crushing and hardening in the layer which is under the surface, detonation of surface and burning of tool without concentrate to the surface.Possibility of hardening during the cutting can be prevented by decreasing cutting fastness and lower feeding more than carbon steels.

The manipulating of the  low-alloyed martensitic and ferritic steels are very similar to carbon steels by removed the machining.Embrittlement of these types provide broken pieces shavings.Using tempered low-alloyed martensitic steels with appoximately 38 HRC water, it can be obtained high quality products and dimension tolerance.Due to the fact that high-alloyer martensitic steel have high tempered hardness (such 420 and 440), it is much more difficult to take out shavings for high chrome ferritic steels (such 446) in terms of high ductility.Also, manipulation of steels that are austenitic and solution hardening show discrepancy in accordance with types of the material.The most easy steels in terms of  the taking of shavings are automat steel types.The austenitic steels such as 304 and 316 have 550-620 MPa yield strength as tempered.In these materials, the cutting become hard because of the huge differance between strenght of yield and tensile and hardening.Because austenitic,ferritic, martensitic steels or double phases can found in the internal structure, the cutting features change.In most of the time, as in case with the martensitic steels, it is suitable to harden with the heat treatment,then machining.Having gained automat steels such as 416,430F and 303 as a result of adding sulfur and selenium to the mixture, the machining is much more free of problem and easier in comparison with the others.The sulfur which is found in the mixture as manganese sulphide formation provides broken pieces shavings and prevents the accumulation on the cutter tool

Selenium provides to get the best surface quality.The automat steels which are more expensive than others provide advantages cost reduction because of their easy machinability.These materials,generally, should be preferred when more than 10 % shavings are going to be taken.While choosing material, besides production, the features of usage scopes should be taken into consideration.The automat steels that have low corrosion resistance in comparison with other steels, apart from selenium types, are not suitable to the shaping processes such as head hoarding.

Bonding Methods
Stainless steels can be shaped by welding technique and soldering methods as generally.Among them, the most common one is arc welding because it provides void-free and linkage that have productive junction.Apart from these, resistance brazing is preferred in austenitic types and this method provides reasonably high strenght, quick and cheap linkage. In terms of the strenght corrosion, it is necessary to obey the precautions and operation details in each method

Welding
All of the stainless steels can be shaped with any welding arc but it should be taken care to some points such as corrosion resistance in the filler metal and effect for heat, residual tension,warping and seam fracture.The welding arc of stainless steels.Also the resistance brazing of the stainless steels is too common.After the carbon steel, the resistance brazing is implemented to stainless steel at most.For the austenitic steels, it is sufficient to weld the low welding current than carbon steels, because low heat conduction, high electiric resistance and being not magnetic. But there is problem of warping because their high coefficient of thermal expansion. Since their welding time is short, there s not a huge reduction in terms of corrosion resitance because of the carbide settling. But interval corrosion can be a trouble during the spot weld in determined points.However it is not so common, the resistance brazing can be done to martensitic and ferritic types.If formation martensite is in question during the colding, after welding, the second current manipulation can be provided for tempering. The operation of inert-gas welding is not implemented generally to the stainless steels.It is too hard to form a welding atmosphere that protect the filler metal from oxidation or carburetation in the inert-gas welding.

Among stainless steels, the bonding with welding is done for the austenitic types commonly.But these types have different welding attitudes than carbon and lox-alloyed steels.The most important thing is to prevent carbide settling in terms of grain limit during an average temperature.The carbide can settle in place that is close to seam and warm in 650-870 degree celcius.This condition decreases the corrosion resistance in so few circumstances,especially acidic environment used for antioxidant.But,because this area is so narrow, pieces are used as welded mostly.Also, most of stainless steels can be bonded with welding arc as similar to austenitic types that solution hardening can be implemented them.Generally, by choosing the filler dope metal similarly, the response is provided to thermal effects.After welding, a full heat treament is done, thus,however there is no ductility, it is provided to be same mechanical features of seam as the main metal.

Hard Solder
All stainless steels can be bonded with hard solder but this method has been opted mostly to bond stainless stells with other metals.Generally the hard solder is implemented in furnace since there is a stopping facility to oxidation of the stainless steel by working in protective atmosphere ( usually hydrogen or vacuum ).Austenitic steels are soldered roughly when there is a risk of carbide settling and they have power of corrosion resistance.Because there is no possibility to unfastening temper in high temperature it is necessary to use types of in moderation or low carbon for soldering.The martensitichardening can be prevented by using hard solder material melts in under 830 degree celcius or martensitic and ferritic stainless steels. Remaining of heat during the soldering of pre-hardened steels bring forth tempering and most probably reduction of hardening of the material.

Silver,nickel, gold and copper alloys can be used as the packing material.But copper alloys that include phosphorus can not be used.In some austenitic, the copper-based alloys are never used.During hard soldering, some breaks can be formed under the material tension.Thence, pre-tempered can be implemented, stres relief annealing can be provided before the melting of solder by warming the lowest part rather slowly or a kind of solder material, not bring about this detriment,can be chosen.

Heat Treatment
Stainless steels can be subjected to tempered, giving water, stres relief annealing in accordance with their types and goal.With these operations, corrosion resistance and ductility specifications can be desirable point or mechanical and metalurgical changes can be removed as a result of shaping activities.Heat treatment of stainless steels can be done for preventing the surface detriments in controlled atmosphere mostly.

Tempering
Tempering can be implemented to almost all stainless steels.The tempering process in austenitic steels bring about crystallization once more together with softening and chrome carbides inclose austenic hard liquor.Because of the last role, it is also called as resolving tempering.Most of the time, this process is performed over 1040 degree celcius temperature, also it is done in 1010 degree celcius when a fine-grained internal structure is wanted. It is important to do this process in a very short time in order to counteract oxidation and grain growth that gives rise to the pebbling.
The tempering of austenitic steels is called as quenching.The cause of this is to prevent the chrome carbide settling after tempering and to cool the water quickly after tempering in order to prevent susceptibility of the material in terms of corrosion resistance.If the quick cooling is not done, the settling of chrome carbide decrease the corrosion resistance in matrix.Tendency of settling of chrome carbide depends on the type of material. In austenitic steels, in opposition to other types, hardening is not in question as a result of austenite.

Hardening
Martensitic steels are meliorated by austenitizing, quenching and tempering. Austenitizing heat is between 980-1010 degree celcius.If the austenitizing heat is approximately 980 degree celcius, the hardening is going to be higher after quenching.The quenching act is done as cooling mostly in the air but if the pieces are thick, cooling is done in the oil.Having adjsted the heat of tempering, hardness, toughness and corrosion resistance are tried to optimised as desirable. Because of inner tensions after quenching, tempering should be implemented so as to prevent crack formation without delay.

After tempering approximately 510 degree celcius, embrittlement is prevented by falling down under 400 degree celcius heat.

Tension Removal
The act of tension removal can be done as different in accordance with the dimension and type of material, in order to remove the waste tension, after welding or shaping, in terms of corrosion and dimension sensibility.When the pieces can not be tempered after welding, welded seams can be warmed under the normal tempering temperature.

The act of tension removal under 400 degree celcius remove the inner tension just a specific rate.Act of tension removal under 425-925 degree celcius can remove the substantially all of  waste tension that may caused to tension corrosion and collision.For example, 85 % waste tensions can be obviated with an hour tempering at 870 degree celcius.But during this process, the settled intergranular carbide cause sensibility of intergranular corrosion.Thus, the pieces ,subjected to  long-term tension removal tempering such 312, 347 and 348 as stabilized or low-carbon piece like 304L, can be made.For austentic steels,shaped as plastic, between 345- 370 degree celcius two hours “low” heat tension removal tempering should be implemented, for 425 degree celcius heat should be applied when intergranular corrosion resistance is not critical.

The tension removal in terms of ferritic and martensitic steels, concurrently, fine down some the resistance of corrosion and temper to place which s under the effect of heat and welding.

Preservation of Material During
If due diligence is not given to the production acts, different mechanical detriments (scratch, smash) can be seen on surfaces of the material and because these parts remove the protective surface oxide layer, material sensitized against corrosion resistance.Therefore, some measures should be taken for production of stainless steels unlike carbon steels

The salient measures have been determined in the below:

  " Sheets and layers can not be sinked to the ground absolutely.Material should be stocked as vertical and while taking from the depo they should not be scrolled as a clinker built.Getting dirty of iron waste should be cleaned
  " Heavy layers should be stocked on wood blocks and a direct contact of fork lift’s carrying bars to the material should be prevented.
  " For the jaw carriages,pricking of gears to the material should be prevented.
  " If it is necessary to use,hawsers should be made from synthetic material.
  " Cleaning of linen gloves should be used while carrying polished surface sheets and formation of fingerprint should be prevented.



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