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GAS_METAL_ARC_WELDING_OF_STAINLESS_STEEL.pdf

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GAS_METAL_ARC_WELDING_OF_STAINLESS_STEEL
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GAS METAL ARC WELDING OF STAINLESS STEELT A B L E O F C O N T E N T S 1 Introduction 1 2 Base Metals 2 A. Alloying Elements B. Stainless Steels1. Ferritic Alloys2. Austenitic Alloys3. Martensitic Alloys4. Precipitation Hardening Alloys5. Duplex Stainless Steels 3 Electrical Characteristics 7 A. Constant Voltage Power Supply Basics B. Constant Voltage Power Supply Settings C. Electrical Stick-Out D. Constant Voltage Power Supply Characteristics1. Slope2. Inductance3. Heat Input 4 Shielding Gases 13 A. Shielding Gas Functions B. Flow Rates C. Gas Losses 5 Electrodes 19 A. Filler Metal Alloying Additions B. Wire Designations and Chemistries C. Solidification of the Weld Puddle Section Topic Page Section Topic Page 6 Metal Transfer 23 A. Short-Arc B. Globular Transfer C. Spray Transfer D. Pulsed Spray Transfer 7 Welding Stainless Steels 29 8 Technique and 33 Equipment Set-Up A. Torch Angle B. Feed Roll Tension C. Burnback D. Arc and Puddle Position E. Vertical Down Welding F. Gaps G. Crater Filling H. Arc Starting 9 Weld Discontinuities 38 and Problems A. Lack of Fusion B. Porosity C. Burn-Through D. Undercut E. Spatter F. Cracking G. Sensitization of Alloying Elements 10 Precautions and Safe Practices 43 11 Conclusion 47S E C T I O N 1 Introduction 1 s As you learn more about GMAW, it will become apparent that this is a sophisti- cated process. Welders who have used “stick” welding (Shielded Metal Arc Welding or SMAW) are sometimes of the opinion that the GMAW process is sim- pler; but to deposit a high quality bead requires as much knowledge as, or more than, with the SMAW process. The reason for this is the number of variables that affect the arc and the degree of control the operator has over those variables. The purpose of this manual is to make you a better welder by increasing your knowl- edge of how the GMAW process works. A more knowledgeable welder can be more productive by working smarter, not harder. Figure 1 shows why your company is interested in educating you in welding stainless steel. Your labor and overhead account for about 80% of the cost of depositing weld metal. Any knowledge you gain from this course not only helps you, but also helps to make your company more competitive in a very tough marketplace. If you should have any questions in the future that this manual or your supervisor cannot answer, please feel free to have him contact your Praxair regional engineering staff for further assistance. This training program was written to give you a better understanding of the MIG welding process. MIG is an acronym for Metal Inert Gas, which is not technically correct for stainless steels because shield- ing gases for these materials contain an active gas such as oxygen or carbon dioxide. The correct term according to the American Welding Society (AWS) is Gas Metal Arc Welding (GMAW). We will use the correct terminology as defined by the AWS and also explain the slang used so that you will be familiar with all the terms applicable to this process. Figure 1 – Breakdown of Stainless Steel Welding Cost 1 Equipment 4% Overhead and Labor 80% Wire and Gas 14% Power 2%Stainless steels were accidentally discov- ered in the 1800s when cannon barrels were being cast. They noticed that the castings from a certain ore didn’t rust. They had found a load of ore that was high in chromium. Steel is classified as a stainless steel when its chromium content exceeds 10.5%. At this level a thin, invi- sible oxide layer forms on the surface of the base metal. This adherent oxide layer effectively stops the oxidation (rusting) process. This protective film is self-healing in the presence of oxygen (20.9 %of the make-up of air is oxygen). Carbon steels, of course, form a loose, flaky oxide that we call rust. As this rust flakes off, more of the base metal is exposed to the atmosphere, which allows the corrosion process to continue. Stainless steels are defined as steel alloys where the chromium content ranges from 10.5% to 30%. There are five distinct types of stainless steel. Each is iron-based with alloying additions designed to modify specific characteristics. The major grades are as follows: 1. Ferritic 2. Austenitic 3. Martensitic 4. Precipitation Hardening 5. Duplex Ferritic Stainless Steel – Ferritic stainless steel contains from 10.5 to 30% chromium, is low in carbon, with some alloys containing major amounts of molybdenum, columbium and titanium. These alloys are typically used where corrosion is not severe, such as in automo- bile exhaust systems, cookware, architec- tural applications and automotive trim. Ferritic stainless steels are magnetic at room temperatures due to their body- centered cubic crystal microstructure. Austenitic Stainless Steel – Austenitic stainless steels contain from 16% to 26% chromium, up to 35% nickel, and have very low carbon content. Some of these steels are also alloyed with a minor amount of molybdenum, columbium and titanium. Austenitic stainless steels are used where corrosion can be severe. They are easily weldable. All alloys of this type are non-magnetic due to their face cen- tered cubic structure at room temperature. Austenitic grades include the 200 and 300 series of stainless steels. The 304 and 316 grades are very commonly used in welded fabrications. Over 80% of today’s stainless steel welding applications are done with these types of grades. Martensitic Stainless Steel – Martensitic alloys contain from 12% to 17% chromium, up to 4% nickel and .1% to 1.0% carbon. Some alloys will also have minor additions of molybdenum, vanadium, columbium, aluminum and copper. These alloys are used where high mechanical strength, hardness and corro- sion resistance are required. They are not easily weldable. S E C T I O N 2 Base Metals 2 s 2Precipitation Hardening Stainless Steel – Precipitation hardening alloys contain between 11% and 18% chromium, 3% and 27% nickel and low carbon content. Some of the alloys will also have minor additions of molybdenum, vanadium, columbium, aluminum and copper and boron. Duplex Stainless Steel – Duplex stainless alloys have 18% to 28% chromium, 2.5% to 7.5% nickel and low carbon contents. Some of the alloys will also have additions of nitrogen, molybde- num and copper. Duplex alloys have a ferritic and austenitic make-up. They offer the high strength properties of the ferritic stainlesses combined with some of the corrosion properties of the austenitic stainlesses. How are Stainless Steels Formulated? In order to understand how stainless steels resist corrosion, let’s look at the basic metallurgy of these materials. All metals are crystals, meaning that the atoms are arranged in an ordered matrix. An easy way to visualize a metal is to think of layers of balls with each ball in the layer touching its four neighbors (see figure 2). The balls represent the atoms of iron, chromium, nickel, molybdenum and other metallic alloying elements. Carbon and nitrogen are much smaller than the metal atoms, and fit into the open spaces between them. These atoms are called interstitials. The layers above and below the first layer are arranged identi- cally, except that they are shifted on a 45 degree angle to fall into the areas where the first layer of balls intersect. Now each ball in the second plane is touching four balls in its layer and four balls in the layers directly above and below it. The orderly manner in which metals are arranged in crystals is one of the reasons that they are so strong. When a metal yields, or deforms plastically, the planes of atoms slip in relation to adjacent planes. The only single crystal materials used today are for turbine blades. They are extremely strong due to the orderliness of the matrix. They are also extremely expensive to make. The stainless steel alloys that are used in fabrication are actually made up of grains or groups of crystals. During welding, grains begin to grow into the molten puddle from the solid base metal at the edge of the weld. When two grains contact each other, they stop growing. The inter- sections of these grains are called grain boundaries. In the types of alloys that we use, a lot of the “slip” or shifting of atoms occurs at these grain boundaries. Because of grain boundaries, the actual strength of these steels is typically 25% to 50% of the theoretical strength of a single crystal of iron. The following section lists some of the elements that are commonly added to stainless steels to produce alloys that give us the desired properties. Figure 2 – Iron Face Centered Cubic Unit Cell 3 Corner Atom Face Atom Unit cell (4 atoms)Carbon Carbon strongly promotes the formation of austenite that influences material properties. Carbon can strengthen the matrix through the formation of particles such as iron carbide (Fe 3 C). The larger size of the carbide compound pins the layers within the metal matrix and makes it much more difficult for the material to yield. Higher levels of carbon in austenitic stainless steel can lead to the formation of chromium carbide (Cr 23 C 6 ) and a subsequent deterioration in corrosion resistance. Austenitic grades are typically < 0.20% C, ferritics are < 0.12% C and the martensitic grades are either < 0.15% C or from 0.6 to 1.2% C. Chromium Additions of chromium increase corrosion resistance, strength, wear resistance, heat resistance, and the hardness when added to stainless steels. Chromium promotes the formation of ferrite and can form carbide particles that affect the strength of the material. Chromium forms a very tight, adherent oxide layer that resists further oxidation. At elevated temperatures, this layer grows thicker and changes color. This can be seen on motorcycle exhaust pipes. First a straw color is produced (about 700 F), then a blue (about 1,000 F). If chromium levels are decreased in localized areas below about 10% due to chromium carbide precipitation at the grain boundaries, corrosion resistance is decreased. Columbium + Titanium Columbium (also called niobium) is added in small percentages and stabilizes austen- itic grades by forming columbium carbides (CbC). Titanium, also a stabilizer, prefer- entially combines with carbon, before chromium, to maintain corrosion resis- tance in the material. If these stabilizers are not present and carbon levels are high, chromium carbides form in the heat affect- ed zone (HAZ) of the weld, decreasing the corrosion resistance of the material just outside the weld deposit. Copper Copper can be used as a strengthening agent in alloys that respond to precipita- tion hardening. Upon cooling or the application of some other heat treatment cycle, copper forms a precipitate in the matrix, making deformation more difficult and increasing the strength of the material. Manganese Manganese is added in small amounts as a deoxidizer, desulfurizer and strengthener. Manganese additions to austenitic stainless grades of steel also reduce the crack sensi- tivity of the weld metal. Manganese reacts with some of the available free oxygen to form manganese oxide (MnO). It will also combine with any free sulfur to form man- ganese sulfide (MnS). Sulfur is detrimental because it solidifies at low temperatures and can locate at grain boundaries where it dramatically reduces the strength of the weld metal. After combining with oxygen and sulfur, manganese, a weak carbide for- mer, will form manganese carbide (Mn 3 C) which helps to strengthen the matrix. Molybdenum Molybdenum, another strong carbide forming element, is used in alloy steels from .5% to 1.5%. Molybdenum improves yield strength and resistance to high temperature deformation (creep). Molyb- denum in stainless steels can reduce pitting (highly localized corrosion) in corrosive environments. Molybdenum is a strong ferrite former in a stainless steel weld deposit. s A. Alloying Elements 4Nickel While nickel does not form any carbide in a steel matrix, hardenability, ductility and toughness are all improved by the micro- structural changes which occur as nickel is added. Nickel is added to austenitic stain- less steels (300 series) in concentrations of 7 to 35%. Nickel additions in the 9% range encourage a fully austenitic microstructure. This non-magnetic crystalline structure maintains its strength and ductility to very cold temperatures (-300 F). Silicon Silicon is added mainly as a deoxidizer. It combines with oxygen to form SiO 2 (“glass”) which floats on the surface of the weld puddle along with manganese oxide (brown, brittle slag islands). Silicon improves the fluidity of the puddle and makes it wet the base metal more effec- tively. Silicon also promotes the formation of ferrite in a stainless steel weld deposit. Nitrogen Nitrogen is a very strong austenite former. It is added in controlled amounts in addi- tion to strong nitride forming elements to produce grain refinement and microstruc- tural modifications in duplex stainless steel. Nitrogen is 30 times more effective than nickel in stabilizing the austenite phase in duplex stainless steel. Phosphorus Phosphorus is generally considered an impurity in stainless steels, and is usually listed as a maximum allowable percentage. Phosphorus tends to segregate and push the carbon into the surrounding matrix, causing the material to become brittle. Sulfur Sulfur is also considered an impurity. Like phosphorus, it usually is specified with a maximum allowable concentration. Because of its low melting temperature, sulfur in the puddle moves to the grain boundaries of the solidifying weld metal. This segregation at the grain boundaries reduces the strength of the material. Manganese is added to prevent this as it combines with the sulfur (Mn + S = MnS) before it can react with iron. Certain steels called free machining steels, contain up to .3% sulfur; these alloys are difficult to weld and have poor strength when com- pared to other stainless alloys. Now that you have more of an under- standing of the alloys that are added to stainless steels, let’s look at some of the available alloys. 5s B. Stainless Steel Alloys Table 1 – Stainless Steel Alloys SAE/AISI Cr Ni Mn Mo C(mx) Si(Mx) Other Tensile Ferritic 405 11.5 - 14.5 – 1.0 – .08 1.0 .1 - .3 Al 60 ksi 409 10.5 - 11.7 – 1.0 – .08 1.0 .48 - .75 Ti 55 ksi 430 16.0 - 18.0 – 1.25 – .12 1.0 .6 Mo 60 ksi Austenitic 304 18 - 20 8 - 12 2 – .08 1 85 ksi 304L 18 - 20 8 - 12 2 – .03 1 80 ksi 316 16 - 18 10 - 14 2 2 - 3 .08 1 85 ksi 316L 16 - 18 10 - 14 2 2 - 3 .03 1 78 ksi 321 17 - 19 9 - 12 2 2 .08 1 .4Ti min 87 ksi Martensitic 410 11.5 - 13.5 – 1.0 – .15 1.0 100 ksi 420 12.0 - 14.0 – 1.0 – .15 1.0 250 ksi 440C 16.0 - 18.0 – 1.0 .75 .95 - 1.2 1.0 280 ksi Precipitation Hardening 15-5 PH 14.0 - 15.5 3.5 - 5.5 1.0 – .07 1.0 Cu & Nb 190 ksi 1 7-7 PH 16.0 – 18.0 6.5 - 7.5 1.0 – .09 1.0 .7 - 1.5Al 210ksi Duplex 329 23.0 - 28.0 2.5 - 5 1.0 1 - 2 .20 .75 90 ksi 6 % Addition The stainless steel designation system is based on a three-digit system for most alloys that specifies the chemistry
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