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Welding-stainlesssteels and their properties:SOLUTIONS with Effective, Powerful Advice
Welding-stainless, stainless steel welding, stainless steels explained, classes of stainless steels, stainless types, austenitic, ferritic, martensitic, processes for welding stainless steels, sensitization, heat affected zone (HAZ), hot cracking, welding links, welding tips, improving welding results, joining questions needing answers: these are some of the items developed in this Site for the benefit of interested readers. What is in here for me? Welding-stainless steel should not be complicate, we agree. Although some may think of stainless as a well defined material, in effect there are so many different types and sub-species, that just keeping track of them could be a little confusing. So that trying to put some order in the field might be helpful. This is what we intend to do. What benefits can be found here? Essentially a short overview of Welding-stainless steels, to help find one's way in the forest. And also tips on how one welds each type. However as types and conditions of various stainless steels can be very different, so could be the problems.
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Before dealing with Welding-stainless, one should offer a loose description of stainless steels: they represent a class of iron base materials which have a certain resistance to rusting and to corrosion in some environments, due to the presence in their composition of at least 12% of the element Chromium. The reason of this behavior is that chromium helps produce on the material's surface a tough and impervious layer of chromium oxide, which is the shield protecting the surface from rust and corrosion. Tip!: When dealing with Welding-stainless, one should be aware of the fact that the expression STAINLESS STEEL represents a huge class of different materials, it is not a technical term, therefore it cannot identify any specific metal, and as such cannot be used for any practical purpose i.e. for purchasing. Three classes... The three more general classes of stainless steels (and a mixed one called Duplex) are indicated by reference to their metallurgical structure. More specifically they are called by using an identifier which refers to the appearance of their micro-structure as seen under the microscope or by another very special technique called x-ray diffraction. This of course has influence on their Welding-stainless properties. The use of the names of the classes, called "Austenitic", "Ferritic", "Duplex" and "Martensitic", sheds at least some information on some of their characteristics and uses. It should be clarified that the microstructures identified by those names may be present at the same time in various proportions in a certain steel, and that therefore they are used to indicate the prevailing structure. What is Austenitic... When Welding-stainless is involved, Austenitic stainless steels are considered the most weldable of their category. They are known as the "300 series" which refers to a standard classification originated by AISI - American Iron and Steel Institute and by SAE - Society of Automotive Engineers. An important sub-class is also known as "18/8" (meaning that the important alloying elements of these steels are about 18% Chromium and 8% Nickel). Main characteristics of Austenitic stainless steels are as follows:
In Welding-stainless, these two last characteristics variously affect the outcome, producing larger distortion than as found in other steels. Not all austenitic stainless steels of the 300 series are equally well weldable. The addition of sulfur or selenium used to improve machinability (as in Type 303) results in severe weld hot-cracking, which makes this particular material "non-weldable". Beware of... The corrosion resistant characteristics of stainless steels may be adversely affected by the sensitization process occurring, while Welding-stainless, in a certain temperature interval from about 600 to 900 0C (from about 1100 to 1650 0F) which promotes the precipitation (gathering) of chromium carbides at grain boundaries and the parallel loss of anticorrosive chromium from the base metal. The above range of temperature occurs naturally not at the Welding-stainless location, where temperature is higher and lasts only for a short time, but in two strips of metal on both sides of the weld bead. This is the so called Heat Affected Zone (HAZ) where the harmful effects take place. In a sensitized joint the chromium, which is the main "stainless" ingredient, becomes sequestered or taken out of play and locally unavailable for the protective action. If not addressed correctly, Welding-stainless 18/8 steels may thus cause the loss of their protective property along sensitized paths. The welded material becomes prone to intergranular attack in a corrosive environment. The Big Three... Three strategies are usually employed to oppose this tendency. One is to use a very low carbon version (i.e. 304L where L stands for low-carbon) where not much carbon is available for making chromium carbides. Another is to use a different type of base metal including an amount of titanium (type 321) or columbium (also known as niobium) (type 347) which tend to form readily titanium carbides (or columbium carbides) (and by this action the carbon becomes unavailable for chromium) at sensitization temperatures leaving the chromium free to perform its anticorrosive task. Note that the filler metal for this material, if required, should be always of type 347. Why? Because titanium (in type 321) being reactive, is not readily recovered during deposition, so that it would not be available when it is needed most. Columbium however is not reactive, it will stay put through melting, and, when the material is heated to the "sensitization" temperature, will do its job of producing columbium carbides in preference to chromium carbide, and so it will save the day. The third strategy for safely Welding-stainless is to perform a solution heat treatment at elevated temperature (about 1050 oC or 1900 oF), for repairing a condition of corrosion susceptibility. This puts again in solution (called solid solution) the chromium carbides originated during Welding-stainless sensitization of regular 18/8 stainless (like types 302 or 304). This process however must contend with problems of heavy oxide formation if not done in vacuum or protective atmosphere, and of distortions. Type 309 and 310, used for elevated temperature applications, and type 316 or better type 316L used for enhanced corrosion resistance, are generally not prone to sensitization and are used with filler wires of similar composition. Now Ferritic... Other stainless steels, called Ferritic, are ferromagnetic but cannot be hardened by heat treatment. A limited amount of ferritic structure, when present in an otherwise mainly austenitic structure, is considered beneficial in that it reduces the chances of hot cracking. Welding-stainless ferritic steels can readily be performed using arc processes, either with ferritic or with austenitic filler metal, except that a post weld heat treatment may be needed for improving properties. Car exhaust components may be made out of these materials. Then Duplex... Duplex means double and, when referred to Stainless Steel, it indicates a mix in about equal proportions, of the two kinds of metallographic structures just introduced, called by metallurgists ferrite and austenite. Compositions are identified with reference to the Unified Numbering System. These are Stainless Steels with quite a substantial proportion of Chromium, the main ingredient for imparting stainless properties, and additional balanced quantities of Nickel, Molybdenum, Copper and sometimes Nitrogen. The base is obviously Iron. Carbon is kept low as are also Sulfur and Phosphorus. The main properties are improved corrosion resistance and mechanical properties (up to twice as strong), when compared with regular austenitic stainless steels. Other important differences are thermal conductivity and thermal expansion, midway between those of carbon and austenitic stainless steels. Although weldability of Duplex steels is good, attention should be devoted to sensitivity to degradation of properties, due to excessive time at heat. An important precaution is to limit as much as possible holding time at intermediate temperature, between 300 and 980 0C. Filler metals are chosen either with matching compositions or overalloyed with slight excess of nickel to promote more austenitic structure. Thorough documentation is recommended before starting a project and full qualification following procedure development, to assure acceptable properties in the complex structures that benefit most of the use of Duplex Stainless Steels.
And Martensitic... Martensitic stainless steels are magnetic and fully hardenable by heat treatment. Welding-stainless of this type is not recommended, although feasible with special techniques. Welding may produce cracks, especially if carbon content is not sufficiently low. Preheat and postheat may be necessary. One more class... To complete the Welding-stainless overview, one should mention a fourth class of materials, not listed above, called precipitation hardenable (PH) stainless steels, which are quite readily weldable. However precise instructions should be followed concerning heat treatments in order to develop the required properties. The following AWS Documents provide guidance in activities relative to Welding Stainless Steels:
ANSI/AWS D10.4-86(R2000) ANSI/AWS D18.1-99 ANSI/AWS D18.2-1999 The Processes... FRICTION Welding-stainless steels presents almost no problems, except for the free cutting types that should not be welded at all. It is used for Welding-stainless steel not only to itself but also to quite different materials like copper or aluminum and other combinations. One should always be aware of the type and material condition before welding and of the effects of heat near the joint and that some elements (sulfur, selenium) or very high hardness originating while welding could compromise the final soundness of welded joints. RESISTANCE process for Welding-stainless is currently used, with due adaptations deriving from differences in high electrical resistance and low thermal conductivity, high coefficient of thermal expansion, higher melting temperature and higher strength at elevated temperature. Electrode force is more elevated, while time and current are less than for low carbon steels. Resistance Welding-stainless Austenitic steel of the 300 series is readily performed. Ferritic steels are also welded, but martensitic, hardenable, stainless steels are problematic as the welds result brittle, if not softened adequately by a post weld tempering treatment. Stainless steels must not only be cleaned from external surface dirt, oil, grease or paint before welding but also from the naturally forming chromium oxide layer which should be removed with a stainless steel wire brush. GAS WELDING can be used for Welding-stainless steel but the use of a proper flux is required. This makes the process much less attractive than Gas Tungsten Arc Welding (see in the following) unless there is no other choice, because of the imperative need to eliminate all traces of residual flux on the part after welding: this introduces an additional operation which might increase the cost. ARC WELDING is commonly used for Welding-stainless steel with due attention being paid to the class and to the condition of the material and to the influence of the process on such consequences as sensitization or deformations. Filler metals... On Stainless Steels Filler Metals, the following AWS Specifications can be found
ANSI/AWS A5.9-93 ANSI/AWS A5.4-92 ANSI/AWS A5.8/A5.8M:2004
ANSI/AWS A5.22-95
AWS A5.31-92(R2003) An Article on the Selection of Stainless Steel Filler Metal was included in our Practical Welding Letter No. 02 of October 2003. To see the article click PWL#002 An Article on Filler Metals for Duplex Stainless Steels was included in the January 2005, Issue # 017 of Practical Welding Letter. To read the article click PWL#017. An Article on tips for Welding thin Stainless Steel Sheets was published (3) in the Issue 32 of Practical Welding Letter for April 2006. To read it click on PWL#032. An Article on Corrosion in Weldments was published (7) in the Issue 32 of Practical Welding Letter for April 2006. To read it click on PWL#032. An Article on Welding Effects of S Content on Stainless Steels was published (11) in issue 45 of Practical Welding Letter for May 2007. For reading the article click on PWL#045. An Article on Welding Defects in Stainless Steels was published (11) in Issue 48 of Practical Welding Letter for August 2007. Click on PWL#048 to read it. An Article on Ferrite in Austenitic Stainless Steels was published (7) in Issue No. 53 of Practical Welding Letter for January 2008. Click on PWL#053 to see it. A Contribution titled Beware of 317LMN Castings! was published (11) in Issue No. 53 of Practical Welding Letter for January 2008. Click on PWL#053 to see it. An Article on Pitting Resistance Equivalent Number was published (7) in Issue 54 of Practical Welding Letter for February 2008. Click on PWL#054 to read it. To receive regularly by e-mail at no cost the periodic publication above, please Subscribe. All types of arc processes can be employed for Welding-stainless steels with due attention to joint shape, dimensions and preparation. In particular Shielded Metal Arc Welding (SMAW) is widely used because of its flexibility. It should be noted that electrodes come in two types concerning the cover composition, which may influence the choice of the current employed:
ELECTRON BEAM Welding of stainless steels is readily performed with good results even in very deep welds. As usual the remarkably high depth to width ratio permits to join configurations not possible with other means. The heat input being low and the heat affected zone of limited extent, there is often no remarkable damage to the mechanical properties so that further heat treatment is not required. Also Laser Beam Welding is performed for Welding-stainless steels, with the usual precautions needed to insulate the weld from air and to limit the damage to properties obtained by heat treatment. NEW RESOURCES
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Welding-stainless. What are the different types? How are they welded? Which precautions must be taken? How can one identify a given stainless? Find it all here...
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