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WELDING OF STAINLESS STELL

Stainless steels can be bonded by resistance brazing, splice and fusion weld.In this part, the information about experiment and commendation for stainless steels.

  * Materials
  * Corrosion Resistance
  * Welding Methods
  * Other Subjects About Welding of Stainless Steels


Materials

Ferritic and martensitic structures have, apart from corrosion resitance attitudes, different from each other’s resistance and shaping specifications and thus they show variety in terms of welding ability.

Breaking elongation and ductility of the ferritic steels, especially their not stabilized types’ are lower and it is essential to be more attentive in terms of further materials and choosing of welding method and heat input so as not to cause tearing during the welding The internal structure of the austenitic steels is exactly austenitic but combination is arranged as delta ferrite on a limited scale in internal structure.Founding of this phase decreases the possibilty of hot tearing.While those kind of hot tearing cause to dent effects to ferritic steels, these cracks are lower dangerous in terms of austenitic steels so long as micro scale.For dublex steels, with the help of Schaeffer Diagram, prediction of ferrite quantity of the filler metal can enable which depend on composition.

Corrosion Resistance
Because stainless steel are durable to corrosion in so few environment passive circuit , they are not required an additional surface act.But it is essential that this passive circuit’s welded seam should not be gone bad under the effect of heat area (ITAB). Apart from this, if heat treatment is not done for the satinless steels that have more than 0,03 % carbon quantity and thick than 6 mm wall, adventure of intergranular corrosion appear especially welded seam area. That is to say that because of carbide settling of grain boundary, the quantity of chrome in the material reduces and corrosion resistance decreases

Welding Methods
Apart from some boundaries, all welding methods, used for other steels, can be used for stainless steels (apart from gas fusion weld).These are some methods which are used commonly:

1.Methods of Fusion Weld
  " Electric Arc Welding
  " Inert-gas Welding
     *Wolfram Protective Gas (TIG)
     *Metal Protective Gas
     *Plasma Arc
  " Laser Beam Welding

2.Methods of Electric Resistance Brazing
  " Resistance- Pressure Welding ( point, roller seam and flash weld)
  " Stud Welding

Methods of Fusion Weld Electric Arc Welding
Electric Arc Welding methods, because of advantages in the below, are the key qualities for welding of stainless steels.

  " Simplicity
  " Low investment to tools and apparatus
  " Practicability to ateliers and to shipbuilder s yard
  " Having electrode types for different usages
  " Using ability for hard positions
  " Low heat input (it is important especially austenitic steels)

Welding attitude and seam presence are determined by cloth of electrode
Electrode With Rutile Cloth: These electrodes have a thin dropped material flowing and thin jagged, smooth seams are obtained.Welding can be done with the direct current (Electrode +) or alternating current.It is easy to repress the breeze and partially it seperates itself. Because of these features,they are preferred for the welding of stainless steels.

Electrode Basic Cloth: Welding can be done with just direct current.Its drops are much more thin, so it is suitable for hard positions.Features of losing of space are good, so it is preferred for root seams.In comparison with Rutile Electrodes, its outlook is rough and it is difficult to repress the breeze

In both electrode types, it is worked with shorter arc.Because high electric resistance of the high-alloyed electrode, it is required to work with them as a low current density.That the current cloth is moisture can cause porosity and cold tearing by making a mess of passing of breeze cloth.Basic electrodes have low sensibility in terms of pore formation.

Inert-Gas Methods
In this methods, arc is burning under the pedigree or active protective gas cloth, and so air is repressed than arc and welding bath

Wolfram Protective Gas Methods ( TIG Welding) :
The protective gas is the argon-arc welding.Trade gases (argon- hydrogen) can be used to accelerate welding speed in the welding of machine with austenitic.The wolfram electrode is fixed to pole (-) and welding is done with direct current.This method is suitable for all welding positions and especially thin sheets and stem paso.About to 3 mm thickness, 304, 321, 316 and 316 Ti quality austenitic stainless steels can be welded without welding additional material. For 316L steels, bonding is done with additional metal

Metal Protective Inert-Gas Arc Welding (MIG Welding) :
The most common type is MIG Inert-Gas Arc Welding (MIG Welding ) for stainless steels. Welding current is transferred to melting wire electrode by helping of contact slide.In comparison with TIG method, higher melting power can be obtained.Also, solid and pith wire electrodes can be used. Wire diameters are between 0,8 and 1,6 mm. Welding is done with direct current and wire electrode should be put (+) pole

As a protective gas for solid wire electrode, 1-3 % oxygen or mixtures that include greatest 2,5 % CO2 are used.( if much more CO2 is found, welding bath takes carbon and corrosion resistance decreases).According to the implementation, wire electrodes are welded as spraying, short or pulsed arc.As a rule, for the runnel or horizontal positions, working is performed with spray-arc.Because of low tendency of leap provides the material passing as short circuit, thin dropped.If lesser heat input is required, short arc is used.(for example, thin sheets, root paso for hard positions).The disadvantages of short arc are tendency of leap and high thickness. Heat input can be lessened by pulsed arc. Thus thin sheets can be bonded with wall thickness easily (even hard positions).

Pith wire electrodes can be welded by in each MIG supplies, having in store.In the meantime, solid wire with wire breeding apparatus can be used. Tendency of leap is reasonably low, seams are not swollen and without dent. Surface is flat and serrated.

Plasma Arc Welding (WPL)
This method is unprecedented in comparison with TIG welding ; much more higher energy concentration is obtained when arc process of concentration is improving.As plasma gas, argon is used and a small amount of hydrogen can be added to austenitics.As the out-productive gas, argon- hydrogen mixture can be used. Generally, it is used for automatic supplies.

" Microplasma welding about 1 mm thickness
" To 10 mm I-seam for sheets, for more thick layers 5 mm root frontal height and Y-seam are suitable. Residual section is overcharged with other methods.

Generally additional metal is not used, but if root space is > (0,08x thickness) , additional metal requires.

Advantages of plasma welding :
  " High welding speed
  " Narrow seam thickness and narrow ITAB ( the area under the influence of heat)
  " Low heat input
  " Low collision

Disadvantages:
  " In comparison with TIG, more expensive supplies
  " Necessary of the preperation of sensible welding bend
  " Retaining apparatus (fixture) and necessary of skilled labour

Welding With Laser Beam
Besides decorous methods, method of laser beam is new and it is convenable in terms of fusion weld.With effect of focused beam, metal melts as local and with formation of key plug, deep boiling is obtained. Obtained welding seams are very narrow, so it is possible to bond the thickness of layers, to 15 mm, with welding by using powerful lasers.Because heat input are localized and heat clears off speedy, those features are obtained:

  " Rate of depth-width is very big narrow welding seams
  " Narrow ITAB Low thermal collision Good ability of shaping

Today, 2 type lasers have been used in the industry:
  " CO2 Laser for 1…15 mm thickness
  " Oil laser for 0,2-4 mm thickness The first type laser, the beam focus to piece by mirrror reflections. In the second type because beam transmits to weldign point by helping of fiberglass, the movement of this beam is easier and for example three dimensional process can be performed with helping of robots.

Submerged Arc Welding
In here, wire burns between electrode and piece under the breeze cloth. This is formed by a kino of powder. It is possible for only positions of gain and horizontal. It is also done in the ledge position with special apparatus. Generally wire electrode is bonded to direct current’s (+). According to the wall thickness, electrode dimensions can be chosen between 1,2- 4 mm. Density of current, in comparsion with other steels, are chosen lower.

Methods of Electric Resistance Brazing
With this method, in quality and effortless combinations are provided. Because austenitic steels have low thermal and electric conductivity, using of resistance breezing is much more suitable in comparison with other steels. Since heat input is lesser, degeneracy can not be seen in terms of surface quality. But , as thermal expansion is high, risk of collision is higher.

Sheets are bonded by stowed to each other in point and roller welding.In here,because there is a scape which is not welded , it is essential not to use it space corrosion areas.During the welding, tempering colours can be removed later.

Cleanliness of surfaces that are going to be bonded is very important. The size of core and its shape, current density depend on time and power of electrode. For manipulation, short span of time is preferred. For the austenitic steels that have high electric resistance,in comparison with other steels, low currents are chosen. Core of height will be approximately 50 % of total thickness of sheets and current adjustment is done without passing 80 %. More higher causes leap and space ( lunker) in terms of core. When it is compared with the unalloyed steels, electrode closing pressure should be 2-3 times more.This pressure, after removing current, should be perpetuated so long as core become rigid. ( 0,5 second for thin sheets, approximately 1 second for 3 mm thickness)

For the point welding of stainless steels, copper alloys that have 400 degree celcius as electrode at least 70 HB hardness (such as CuCrZr and CuCrBe alloys). Edges of electrode, due to easy, are chosen as circular usually.

Roller seam welding (continually or on and off movements) can be done for flat surfaces or circular surfaces electrodes. Power of electrode is implemented continually, current is given by on and off.

Pieces contact surfaces are contacted so many times in the flash weld and seperate. In the meantime, if surfaces uneven at first, they become burned and flat. When edges of pieces reach to combination heat, they are appeased to each other speedy.This pressure provides to bond the edges and a sum of material spear from welding scape to out. The strenght implemented to the retaining jaws should be high (approximately 1,5-2 times than accumulation power)

For stainless steels, in comparison with unalloyed steels, lower electric current but high accumulation power can be chosen.Thus, retaining strenghts should be chosen higher..

Stud Welding
In this method, stripe pieces are bonded on large surfaces by pressure welding. The linkage can be performed when welding section can be shaped the ability of plastic shape and liquid.Among stud welding practices, arc stud welding is the method which is used mostly.

The most important advantages of this method:
  " It is enough to draw on to the main piece jus one side.,
  " For combination, it is not required to drill that can create problems of wangle.
  " A powerful linkage is formed which combines a whole section of the stripe.
  " It can be used for dimension of 0,8- 25mm stripes. (versatile usage)
  " Because its welding time is short, burning (component of lossing) and collision can be seen inconsiderable. Resistance can be developed by forming a flange in edge of the stripe.

Changing of phase and becoming rigid do not occur during the stud welding of austenitic stainless steels and heatproof steels. Speedy cooling is also an advantage in this method, carbide settling is not seen in the internal structure.Moreover, austenitic stainless steels can be shaped out easily.The filler metal of austenitic stainless steels about 10 % delta ferrite. So they are not sensitive against hot tearing.Whereas there is a danger in terms of hot tearing. Much more higher alloyed full austenitic stainless steels, in terms of appropriateness of the method should be checked.When combination of 2 materials, expected internal structure can be guessed by helping of diagrams in terms of filler metal

When welding of stud with the unalloyed steels with the stainless steels (combination of white-black) ferritic and austenitic materials mix in filler metal and breakable martensitic structures can be appeared. If each rate of materials to mixed with the filler metal is determined, internal structure can be determined. Filler metal composes from stripe ( stud) material approximately 60 %. Adjusted welding conditions, it is impossible to get rid off formation of martensite.This condition can be prevented just by alloying of stripe edge.Apart from that, a carbon passing can be formed in the section and also crackly layer can be formed which is in rich thin carbon.Thus black-and-white welding can be allowed when special conditions happen..

Other subjects about welding of stainless steels

Preparation of welding bend depend on welding method, sheet thickness and welding position. Bend edges can be arranged by mechanical ( cutting by shears, milling, polishing, water jet ) or thermal ( plasma, laser). To fend off oxide waste, polishing can be required before welding for cut as thermal. The tools which were used for other steels or include iron, can not be used. Preparation of welding bend and its cleanliness are very important. For mechanical cleanliness, brushing and for chemical cleanliness, a suitable solution can be used.

Welding Parameters

  " Stainless steels have
  " Large expansion factor
  " Low heat transmission coefficient.
  " High electric resistance..

Electrode ignition should be under the welding seam, the retaining edges should be polished and if there were, it should be cleaned from crater faults.For welding that can be formed from one side, kvk paso should be protected with protective gas.Electrode type, dimension and other welding parameters should be determined in accordance with types of steel and thickness of wall. Heat of interspace paso should be 150 degree celcius at furthest.

After Welding Processes
In order to get the best corrosion resistance, it is required to remove welding seams, sections effected from heat, breeze waste,leaping, temper colours and other oxidation products. If surface has little or nothing tracked and flat, corrosion resistance can be good passable.

Brushing
Stainless steels can bu used with brush but it is necessary that brushes should not be used with other materials.If oxide layer and wall waste are removed completely and if the smooth surface can be obtained in metalic cleanliness, it is settled for polishing.

Milling and Polished
It is important that used tools have not included iron and only used for stainless steels. Refinement of sanding depends on the usage area but numbers of 180-240 emeries are suitable generally.Much more smooth surfaces can be obtained by metalic or electrolitic polished.For special conditions, ( for example, danger of tensiled corrosion in chlorided environment) etching can be required after sanding

Sanding
The stainless steel, as sprayed piece, can be used with glass pieces, quartz sand and synthetic or metallic materials which do not include iron.Clean and smooth surface can etched or passivated to get better results

Etching
Thanks to etching, rough dirts and oil waste can be removed. This process is done by using immersion, spraying or etching tool or jet. Later, a mindful clearance is done with water and it is important to remove the waste by neutralization.The waste causes corrosion.Thus passivation can be done with 20 % nitric acid at the end. Then, mindful clearance with water is required

Corrosion Resistance of The Welded Section
The most important problem for joinder with welding is to be able to protect the corrosion resistance in welded seam ard its around. At these sections, materials can be sensitive in terms of intargranule corrosion. In order to prevent this, combination and welding conditions should be controlled well. Sometimes, processes after welding can be required.
Being Negative Influenced of Corrosion Resistance can be prevented by those methods

  a.Resolving Temper After Welding: Inner tensions are removed, internal structure of filler metal become well. But after this tempering, required etching is hard and expensive. In order to speedy cooling, when quencing is necessary, danger of collision can be done. Also, it is impossible to implement it for large structures. (tanks, pressured jars).
  b.Limitation of carbon quantity: If carbon quantity is low, carbide does not settled. The minumum carbon types have been developed for special conditions.( like 304L and 316L types, at most 0,03% carbon). Later, it is essential to use them for tension removal for welded joinders.
  c. Carbon stabilization:When carbide in the combination is used as titanium carbide ( at 321) or niobium carbide ( at 347 and 348 ), chrome carbide is not formed and corrosion resitance are not influenced negatively because carbon rate does not reduce in internal structure. Usage of these types are also preferred when long-time manipulations are in question at critical temperature

However , if the material, which welded combination has been done, is taken again to sensitive heat section and later if it is found determined corrosive environment, important local corrosion are seen ( like knife mark).The cause of this is the solution of stabilized carbon at so narrow section during welding.This problem can be solved by reforming of the stabilized carbon by implementing tempering after welding.

Welding Fracture
Austenitic steels are very glut and ductile, thus a problem, such cold welding fracture, can not be seen. But these materials have tendency of hot fracture while cooling to 980 degree celcius from hardening heat. In order to prevent this, it is essential to reduce the tensile stress on the seam.Apart from that, by controlling residuary elements like phosphorus, danger can be decreased.But the most effective precaution is to provide at least 3 % and 4 % ferrite in the filler metal.This quantity can be predicted by helping of diagrams ; however, the real ferrite quantity is found by magnetic analysis

This solution does not always suitable as ferrite is magnetic and it decreases the corrosion resistance in determined environment.If the material is put at high heat as long-time, sigma phase settles and embrittlement can be seen.With the tempering after welding, ferrite quantity can be decreased to 2 % and 4 % , but if it is impossible, full austenitic welding should be done

Ferritic steels are lesser ductile and much more sensitive to welding fractures than austenitic steels.The substantial amount of martensite forms in some ferritic steels, like 430, during the cooling,and after welding, and possibility of formation of cold fracture is developing. For ferritic types, between 150-230 degree celcius, pre-heating is suggested to decrease danger of forming of fracture.

Martensitic types are much more sensitive to fracture forming than ferritic types.Generally, a pre-heating is required at 200-300 degree celcius.The steels which carbon quantity is higher than 0,02, it is obligatory to do tempering after welding to them.


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