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Module 8


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Module 8

  1. 1. MODULE –8 142/184-- WELDING METALLURGY Metallurgy deals with internal structure of metal. Mechanical properties of metal such as strength, Hardness, Ductility, Toughness, Fatigue strength and abrasion resistance are all affected by the metallurgical transformation as a result of welding. Mechanical properties are affected by various metallurgical factors i.e. Alloy additions, Thermal treatments, Mechanical treatments etc. Certain fabrication requirements such as Preheat, Post heat, Inter pass temp control, Heat input control, peening, thermal stress relief, and other heat treatment can result in some kind of metallurgical changes which in turn will affect the metals mechanical properties. Changes which may occur in a metal as it is heated from room temperature to a higher temperature and those occupying when a metal is cooled from higher temp to lower temperature. Other changes are the effect on the metal properties verses the rate at which these temperature changes occur. When there is a balance between attractive and repulsive forces , it is said that internal energy of the metal is at a stable level. 143/184 144/184 SOLIDIFICATION OF METALS. A metal exhibiting a small grain size will have improved room temp tensile strength, because the grain boundaries trend to inhibit the deformation of individual grains when the material is stressed. At elevated temperature atoms in boundaries can move easily and slide past one another, thus reducing the material’s strength at these high temperature. Due to these reasons fine grained materials are preferred for room and low temperature service while coarse grained materials are desirable for high temperature service. Fine grained materials generally exhibit better ductility, notch toughness and fatigue properties. ALLOYING This is an arrangement or a system by which we can alter the metallic properties by the addition of metals or non metals. Smaller atoms such as C, N, and H tend to occupy sites between the atoms that form the lattice structure of the base metal. Small amount of C can occupy the interstitial sites between iron atoms in steel. Alloying elements with atoms close to the size of those of base metals tend to occupy substitutional sites, I, e, they replace one of the base metal atom in lattice structure. This is called substitutional alloying like Cu in Ni and Ni in Cu. MICROSTRUCTURE Arrangements of grains and grain boundaries and phase present in a metallic alloy is called microstructure. Steels are having 0.008% to 2 % carbon. Based on C% steels are HYPOEUTECTOID, EUTECTOID AND HYPER EUTECTOID. The eutectoid at 0.8% is dividing line. Hypo eutectoids are alloys with less than 0.8% carbon which exists at room temperature as combination of pearlite and ferrite where as hyper eutectoid contains more than 0.8% C and exists as combination of Pearlite and Cementite. The room temperature equilibrium microstructure for an eutectoid steel is pure pearlite. The pearlite is a layered mixture of Cementite and ferrite. Transformation of Ferrite, Pearlite, Cementite and combination of these to Austenite a FCC structure. Upon heating at 1333 degree F the transformation begin to occur. Upon very slow cooling, these same changes will occur in reverse direction. When steel has been heated into the austenitic range and then allowed to cool slowly through the
  2. 2. transformation range, the resulting microstructure will contain Pearlite. Steels that are preheated to produce pearlite are generally soft and ductile. When cooling from austenite range occurs more rapidly there are significant change in this transformation for steel alloy, it occurs at a lower temperature and resulting microstructure is drastically changed and hardness and tensile strength of the steel increases significantly with corresponding decrease in ductility. At faster cooling rates the principal microstructure produced include fine pearlite, Bainite, and Martensite, with a slight increase in cooling rate we get finer pearlitic structure with lamellar more closely spaced. Now the steel is slightly harder and less ductile. At a further faster cooling rate pearlite no longer forms and Bainite is formed which has higher strength and hardness and lower ductility. If cooled very rapidly or quenching there is no diffusion, if cooling is very fast and carbon present is very high, Martensite shall be formed. The Martensitic formation is diffussionless process. Martensite has low ductility and toughness. To improve ductility and toughness without significantly decreasing hardness and tensile strength of the Martensite, TEMPERING is done . Here the steel is heated below lower transformation temperature ie 1333 degree F. 148/184-- METALLURGICAL CONSIDERATION FOR WELDING HAZ is that region of the base metal adjacent to the weld metal which has been raised to temperature from just below the transformation temperature to just below the melting point of steel, cooling rate in this heat affected zone are among the most rapid because of a phenomena known as CONTRACT QUENCHING. As the heat input increases the cooling rate decreases the use of smaller dia. electrode, lower welding currents HEAT INPUT= WELDING CURRENT x WELDING VOLTAGE x 60/ WELDING TRAVEL SPEED (Inch/ minute) In general, the use of preheat will tend to reduce the cooling rate in the weld and HAZ, resulting in improved ductility. When no preheat is used, the heat affected zone is relatively narrow and exhibits its highest hardness. In some cases Martensite may be formed. However if preheat is done then due to slower cooling rate Ferrite, Pearlite and Bainite is formed in place of Martensite. Higher the Carbon content more harden able the steel. CARBON EQUIVALENT == %C + %Mn/6 + % Ni/15 + % Cu/13 + % Mo/ 4 Above formula is used when C < 0.5%, Mn < 1.5 %, Ni < 3.5 %, Cr < 1%, Cu< 1 %, and Mo < 0.5 %. TABLE FOR SUGGESTED PREHEAT :-- Carbon Equiv. Suggested Preheat Temp. Up to 0.45 Optional 0.45 to 0.60 200 to 400 F Above 0.60 400 to 700 F Welds on thicker base metal cool more rapidly. The larger heat capacity or heat sink associated with the thicker section produces faster cooling of weld beads. When welding heavy sections preheat and inter pass requirement are normally increased to help in slowing down the resulting cooling rate. 148/184—HEAT TREATMENT
  3. 3. Annealing, Normalizing, Quenching, Tempering, Preheat, Post heat, and Thermal stress relief are the H T methods generally used. Annealing It is a softening treatment used to increase the metal’s ductility at the expense of its strength. To perform Annealing the metal temp is raised just into the austenitic range, held for an hour per inch of thickness or for a minimum of one hour and then cooled very slowly by keeping inside the furnace. Normalizing It softens the metal but not as much as annealing. It is considered as homogenizing heat treatment by making the metal structure very uniform throughout its cross section. The metal temperature is raised to Austenitic range, held for some time and then cooled in still air. Due to the air cooling the rate is faster than annealing hence hardness and strength is higher than annealing and possibly lower ductility compared to annealing. Normalized carbon steels are easily weld able. Quenching In this process mechanical properties like hardness and strength are increased and ductility is decreased. The metal is heated to austenitic range and held for some time and cooled rapidly by immersing it into quenching media like water, brine solution, Oil etc. Quenching is performed to produce Martensitic structure to have high hardness and strength and low ductility. To improve ductility it is essential to perform tempering. To temper the metal it is again heated (Reheated ) to a temp below lower transformation temperature, held for a short time to allow highly stressed Martensitic structure to relax some what and then cooled. Preheat and slow cooling is performed to allow for the formation of micro structural constituents other than Martensite. Post weld heat treatments are used to reduce the residual stresses and to temper hard brittle phases formed during cooling or quenching. Post weld heat temperatures are higher than those used for preheat. 149/184—THERMAL STRESS RELIEF—This is done at temperature below the lower transformation temp of 1333 F. The temperature is raised slowly and uniformly, stress relief occurs because the strength of metal is reduced as its temp is increased. The component is cooled at a moderate rate after stress relief. Diffusion Any atom in a metal when wanders away step by step from its home position, these changes of atoms position in the solid state are called diffusion. When hydrogen gets diffused in steel during welding, it often recombines into hydrogen molecule. These molecules get trapped in the metal at discontinuities like grain boundaries or inclusions. These molecules causes stresses in the internal structure of metals and if metal has low ductility then it can cause cracking. Hydrogen cracking is called under bead or delayed cracking. SOLID SOLUBILITY When steel is packed in a bed of carbon particles and then heated to a temperature of about 1600- 1700 F, some of the carbon will get defused/ dissolved in steel. This added carbon in the steel’s surface makes the surface much harder and is useful for resisting wear and abrasion. This technique is commonly called PACKED CARBURISING. The surface of steel can also be made hard by exposing steel to Ammonia environment at similar temperature to carburizing. Ammonia breaks down to its individual components N2 and H2 . The N2 atoms which enter into steel surface. This process is called Nitriding. STAINLESS STEEL Stainless steel is having at least Chromium 12%. Ferritic, Martensitic, Austenitic, Precipitation Hardening, and Duplex grades are 5 main classes of
  4. 4. Stainless steel.. Precipitation Hardening stainless steel refers to method of hardening them by an aging heat treatment , a precipitation hardening mechanism as opposed to quenching and tempering mechanism known as transformation hardening. The duplex grades are approximately half ferrite and half austenite at room temperature with improved resistance for chloride stress corrosion cracking. Austenitic steels are 200 or 300 series grade. A 416 steel is Martensitic grade. A 430 steel is Ferritic grade. Common PH stainless steel is 17-4 PH grade. Popular Duplex grade is AL6 XN. Austenitic steels are very much weldable. They can be subject to hot short cracking, this occurs when metal is hot. This problem can be solved by controlling the base metal and filler metal to promote the formation of a delta ferrite phase which helps eliminate the hot shot cracking problem. Cracking in Austenite can be avoided by selecting filler metals with a delta ferrite percent of 4 to 10 % This is Ferrite no. and is measured with a magnetic gage. Delta ferrite is HCC and magnetic where as Austenite is FCC and non magnetic. Ferrite grades are also weldable with proper filler metals. Martensitic grades are most difficult to weld and often need special preheating and PWHT. The PH and Duplex S S are also weldable. Carbide precipitation is a very common problem OF Austenitic Stainless steel welding. When base metal reaches a temp range of 800 to 1600 F, Cr and C present in SS reacts to form Chromium carbide, the most severe temp for Chromium precipitation is 1250 F. In certain corrosion environments the edge of the grains corrode at a high rate and is called inter granular corrosion attack. Carbide precipitation or Sensitization of Austenitic SS can be prevented by 1. Involving reheat, treating the complete structure by heating to 1950- 2000 F. This solution annealing breaks up the chromium carbide allowing carbon to be re dissolved in to structure. Some times this heating can cause severe distortion of welded structure. After the heating, the structure must be rapidly quenched in water to avoid re formation of Chromium carbide. 2. By adding stabilizer to base and filler metal. Titanium and Niobium are stabilizers. This addition is generally 8 to 10 times the Carbon content. These stabilizers combine with Carbon and reduce the amount of carbon available for Chromium Carbide formation. 3. 3. By reduction in Carbon content. Aluminum and its Alloys. Al has tendency to form Al Oxide very rapidly which prevents corrosion. Due to this oxide formation when ever Al has to be brazed or soldered, fluxes are used to break down the oxide film. During welding alternating current is used which results in breaking down of the oxide by current reversal of AC welding and reformation of oxide film is avoided by shielding with helium or Argon gas. The AC welding method is some times referred as a surface cleansing technique. END