Metallurg Harry Brearli from England in 1913, while working on a project related to the improvement of gun barrels, accidentally discovered that adding chromium to low carbon steel gives it the ability to resist acid corrosion.Adding to the steel at least 12% chromium makes it corrosion resistant and stainless, and an increase in the chromium content up to 17% makes it resistant to aggressive environments.
- properties of stainless steel
- composition of stainless steel Types of stainless steel
- Types of austenitic stainless steel
- weldability of stainless
- Features stainless steel welding
- Manual welding of stainless steel coated electrodes
- welding tungsten electrodes in argon
- mechanical processing methods of stainless
- Prevention of defects after welding
properties of stainless steel
According to the classification of stainless steels can be carried to high-alloy steels that are resistant to corrosion.Chromium, whic
atoms of chromium and their oxides have similar dimensions, so they closely adjoin each other on the metal surface and form a stable layer which has a thickness of only a few atoms.If a scratch or cut the surface of stainless steel, the oxide film is destroyed.However, with this new created oxides which reduce the surface and protect it from oxidative corrosion.
Due to its strength and anti-corrosion characteristics, stainless steels are used extensively in industry and everyday life.Products that are made of stainless steel, can be found everywhere - from the kitchen in each apartment and finishing shops giant chemical production.
equipment for the welding of stainless steel in the modern world allows you to create complex products such as a variety of structures with high strength stainless steel, stair, stainless steel tubes, sheets, grids, stripes, angles, stainless steel tanks of various applications, stainless steel hangers.
Stainless steel with glass and certain plastics is almost indispensable material for equipment for processing and transportation of food products, manufacture of surgical tools, a variety of metal structures.This is due to high hygienic, toxicological and aesthetic requirements.
Hygiene in the food industry has the highest value.There are specific requirements relating to the washability of heavy metals such equipment, which is constantly in contact with food.Brands stainless steel, which are used in the food industry are the AISI 304 and 316.
composition of stainless steel
As part of the major stainless steel alloying element chrome acts with the content of 12 - 20%.If the chromium content is more than 17%, such alloys are resistant to corrosion in oxidizing and corrosive environments.
As part of stainless steel and there are elements that are responsible for specific physical, mechanical and increase corrosion resistance of stainless steel: nickel, molybdenum, niobium, titanium and manganese.Niobium, molybdenum, and chromium increase the corrosion resistance, and nickel reduces thermal and electrical conductivity of steel.
stainless steel chemical composition is chromium, chromium-nickel and hromomargantsevonikelevoy.Chromium stainless found use as a structural material for the manufacture of hydraulic presses valves, fittings cracking units, turbine blades, cutting tools, springs and other household items.Chrome-nickel stainless
used in various industries.There have been such properties of austenitic stainless steel.Due to its structure the stainless steel surface is considered to be of high quality and does not need further treatment for use in the food industry.Chrome-nickel austenitic
stainless steel is not capable of magnet that allows it easily distinguishable from other alloys, as well as to use such property in the industry.Particularly different steel 12X18H10T, which is used for welded structures, appliances, architecture and construction of various buildings.
There are three main types of stainless steel - austenitic, ferritic and martensitic stainless steel.These types of stainless steel microstructure are determined, as well as the predominant crystal phase.
Austenitic steel as a main phase have austenite.Such alloys contain nickel, chromium, manganese and sometimes nitrogen.The most famous stainless steel austenitic steel is 304, which is sometimes called the T304, with the content of 18-20% chromium and 8-10% nickel.This makes the content of the elements of non-magnetic stainless steel and gives it a high-corrosion properties, ductility and strength, so they are used widely in various industries.
ferritic steel as the main phase have a ferrite.These steels contain chromium and iron.The main kind of like stainless steel - steel 430, which contains 17% chromium.Ferritic steels are less malleable than austenitic steels.Steels not hardened by heat treatment and are usually used in an aggressive environment.
martensitic steels have a microstructure characteristic which is observed for the first time microscopist Adolf Martens from Germany in 1890.Martensitic stainless steel is mild steel, the main type among the 410 which is steel that contains 12% chromium and about 0.12% carbon.The martensite steel is able to give a high hardness, but at the same time reduces its toughness and makes it brittle.Therefore, this type of steel is used in slightly aggressive environment, for example, in the manufacture of cutting tools and cutlery.
Types austenitic stainless steels
most popular austenitic group, an additional number indicating the chemical composition:
- Stainless Steel A1, as a rule, used in mobile and mechanical components.Due to the high sulfur content of such steel has a low resistance to corrosion than other types of stainless steel.
- A2 Stainless steel is the most common, non-toxic, non-magnetic, nezakalivaemoy, corrosion-resistant steel, which is easy to weld and then does not become brittle.A2 shows the magnetic properties after machining.Clamps and articles made of stainless A2 are not suitable for use in acid environments, and which contain chlorine, such as salt water and swimming pools.Suitable A2 for temperatures up to minus 200 degrees Celsius.
- Steel A3 differs similar properties as the stainless steel A2, and additionally stabilized with titanium, tantalum and niobium.This improves the quality of resistance to corrosion at high temperature.
- Stainless steel A4 is similar to stainless steel A2, but its composition has 2-3% molybdenum.This gives it a large degree of high ability to resist acid and corrosion.Rigging products and fasteners A4 used in shipbuilding.Suitable stainless steel A4 for the temperature to minus 60 degrees.Corrosion
- A5 has similar properties inherent steel A4, and further stabilized by tantalum, niobium and titanium, but with different content of alloying elements to improve its resistance to high temperatures.
weldability of stainless
Before you begin welding stainless steel with their hands, it is recommended to get acquainted with its features.Welding of stainless steel is quite difficult occupation that depends on many parameters.The most important among these acts weldability - the ability of the metal to form a weld, the weld material which has the same or similar mechanical properties to the metal substrate.
weldability of stainless steel affected by a number of characteristics that it possesses:
- very important indicator of linear expansion and a significant casting shrinkage that occurs because of this, high casting contribute to the growth shrinkage deformation of the metal during welding and after it.If between the welded parts with significant thickness is not enough clearance, it can form a huge crack.
- thermal conductivity, which is reduced as compared with low carbon steels 1.5 - 2 times, can cause heat concentration and enhance the penetration of metal in the welding zone.When welding stainless steel because of this there is a need to reduce the power of 15 - 20% of the current compared with the current for the conventional steel.
- High electrical resistance provokes a very strong heating of the electrodes of high-alloy steel.To reduce the negative effect, the electrodes are made from chrome nickel rods having a length not greater than 350 millimeters.
- important property of stainless high-chromium steel stands tendency to lose their own anti-corrosion properties of the application of the thermal regime of improper or incorrect use of the machine for welding stainless steel.This phenomenon is called intergranular corrosion.Its nature is that at temperatures over 500 degrees Celsius at the edges formed by the chromium carbide grains and iron which become subsequently foci of the stress-corrosion cracking and corrosion.With such phenomenon fighting various methods, for example by rapid cooling of the weld any procedure until pouring water loss reducing corrosion resistance.
Features stainless steel welding
When welding stainless steel is recommended to take into account the differences between some of its physical properties from those of carbon rolled.For example, it is necessary to take into account that the electric resistance giving approximately 6 times greater at 100 degrees less than the melting point, thermal conductivity reaches one-third of that of carbon rolled.Thermal expansion in length is 50% greater.
welding stainless steel at home perform in different ways.Manual arc welding with tungsten electrodes stainless steel in an inert atmosphere is usually used, when the thickness of the material is greater than 1.5 millimeters.For welding pipes and thin sheets using arc welding in an inert gas.
pulse arc welding in an inert gas is used for sheets which have a thickness of 0.8 millimeters.Short arc welding with consumable electrodes in an inert atmosphere is registered for sheets whose thickness 0.8-3.0 mm, and welding of the spray of metal consumable electrodes in an inert gas - for plates that have a thickness of more than 3.0 millimeters.
Plasma welding of stainless steel can be used for a wide range of thickness and is used nowadays widely.Arc welding of stainless steel submerged arc designed for materials with a thickness greater than 10 millimeters.However, the most popular methods is the technology of welding stainless steel coated electrodes, tungsten electrodes in argon and argon semi-automatic welding stainless steel wire.
Edge preparation of stainless steel components virtually identical to the preparation of low-carbon steel products, except for one caveat - in welded joints must have a gap to provide free shrinkage joints.
surfaces prior to welding the edges taken cleaned to a shine with a steel brush and washed with a solvent - for example, aviation gasoline or acetone to remove the fat, which causes the appearance of pores in the weld and reducing arc stability.
Manual welding of stainless steel coated electrodes
Welding of stainless steel coated electrodes can provide without any problems acceptable quality welds.Therefore, if you do not show to weld specific requirements, to find another way to weld stainless steel no reason.
to coated metal electrodes for manual arc welding of stainless steel electrodes include special composition RLA-8 NIAT-1, CL-11.Choosing the recommended electrodes, providing the basic performance of the welded joint - high mechanical properties, corrosion resistance and a large heat resistance.
Welding decided to produce a direct current of reverse polarity.Aim for a smaller penetration weld welding technique involves the use of stainless steel electrodes that have a small diameter, with a minimum of thermal energy.When welding stainless steel current should be about 15-20% lower than for ordinary steel.
Using large current because of the low thermal conductivity and high electrical resistance of the electrodes can cause overheating of the coating and even otvalivanie separate pieces.Welding electrodes for this reason, have a high melting rate as compared with a conventional steel.Getting to the welding of stainless steel for the first time, you need to be prepared for this.
To maintain corrosion characteristics seam it is necessary to provide the accelerated cooling using this copper pads or blowing air.If the steel is ranked among the austenitic chromium-nickel steel, you can use water for cooling.
welding tungsten electrodes in argon
stainless steel welding by this method is used in situations where the base metal is very thin or offered for increased weld quality requirements.Stainless steel pipes are used for moving pressurized liquids or gases, it is best to weld tungsten electrode in inert atmosphere.
welding is carried out in an argon atmosphere AC and DC straight polarity.It is desirable to use as the filler material wire, which has a higher doping level than the main metal.Perform work electrodes without oscillating movements, otherwise you may compromise the cooking zone that triggers the oxidation of the weld metal and increases the cost of welding stainless steel.
as backing protects blowing argon from the air, but the stainless steel to protect the reverse side is not as critical as titanium.Exclude hit of tungsten in the weld pool.It is therefore advisable to use a non-contact or carry out arson arc ignition on graphite or carbon plate, transferring it to the base metal.
After procedures to lower consumption of the tungsten electrode shielding gas is not immediately turn off.This should be done after a certain time - 10-15 seconds.This will help eliminate the intensive oxidation of heated electrodes and extend its service life.
mechanical processing methods of stainless
Remember that the use of such workers may only accessories that are intended for the treatment of stainless steel, and which you saw in the video of the welding of stainless steel: Special grinding belts and wheels, brushed stainless steel, stainless steel shot.
Etching is the most effective method of further processing of welded joints.If properly perform etching, you can remove a zone of low chromium content and harmful oxide layer.The etching is performed by immersion in acid, coating paste or topical application depending on the conditions.
Etching often used mixed acid: nitric acid and hydrofluoric acid in such proportions - from 8 to 20% nitric acid and 0.5 - 5% of hydrofluoric acid, the water acts as the remaining components.In people with this purpose, use a strong infusion of tea.
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