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Stainless Steel1723305147.presentationppt
- 1. Stainless Steels Present by Weldingdepartment
- 2. What are stainlesssteels ? “ stain less” ❖ Steels containing 10.5 - 30% Chromium ❖ The chromium oxide forms a thin passive layer on the surface when exposed to atmosphere. ❖ This prevents corrosive attack and gives the steel its property. ❖ Minimum 10.5% Cr needed to protect against atmospheric corrosion. ❖ Elements like Ni, Mo, Cu, Nb, Ti etc added to improve mechanical properties and corrosion resistance
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- 5. Carbon steel VSStainless steel
- 6. ❖ Corrosion resistant ❖Very durable ❖ Temperature resistant ❖ Easy formability and fabrication ❖ Low-maintenance (long lasting) ❖ Attractive appearance ❖ Environmentally friendly (recyclable) General properties of Stainless Steel
- 7. FERRITE FORMER ➢ Chromium ➢Titanium ➢ Niobium/Columbium ➢ Silicon ➢ Molybdenum
- 8. AUSTENITE FORMER ➢ Nickel ➢Nitrogen ➢Copper ➢ Carbon ➢ Manganese
- 9. Effect of AlloyingElements and their Purpose Chromium (Cr) ❖ Oxidation & Corrosion Resistance Nickel (Ni) ❖ Increases resistance to mineral acids ❖ Produces tightly adhering high temperature oxides ❖ Increases Toughness properties
- 10. Effect of AlloyingElements and their Purpose Molybdenum(Mo) ❖ Increases resistance to chlorides. ❖ Resistance to pitting corrosion Copper (Cu) ❖ Provides resistance to sulfuric acid. Manganese (Mn) ❖ Increases the solubility of nitrogen
- 11. Effect of AlloyingElements and their Purpose Sulfur (S) ❖Improves resistance to chlorides. ❖ Improves weldability (Penetration) of certain austenitic SS. ❖ Improves the machinability of certain austenitic SS. Titanium (Ti) ❖ Stabilizes carbides to prevent formation of Cr carbide. ❖ Precipitation hardener
- 12. Effect of AlloyingElements and their Purpose Niobium (Nb) & Columbium (Cb) ❖ Carbide stabilizer. ❖ Precipitation hardener Aluminum (Al) ❖ Deoxidizer – Precipitation hardener Carbon (C) ❖ Carbide former and Strengthener
- 13. Types of stainlesssteels • Martensitic • Ferritic • Austenitic • Duplex (Austenite +Ferrite) • Precipitation hardening
- 15. Welding of stainlesssteels •All standard welding processes is SMAW, GMAW, FCAW, GTAW, PAW and SAW maybe used depending on the application. •Generally corresponding grades of filler metal composition are used to match corrosion and / or heat resistance properties along with strength. •Weldability problems are different for different types of stainless steels eg martensitic, Ferritic, austenitic, duplex and precipitation hardening. •Weldability considerations are similar for wrought and cast alloys
- 17. How differentiation toFerritic & Martensitic Stainless steel ❖ Ferritic SS – 4xx ❖ Martensitic SS – 4xx Ferritic SS – Cr > 12.5% Low Carbon High Chromium Martensitic SS – Cr < 12.5% High Carbon Low Chromium
- 18. Ferritic stainless steels FerriticSS – Cr > 12.5% Low Carbon High Chromium Ferritic SS – 4xx
- 19. ❖ AISI 405,409, 430, 446 grades ❖ Ferritic structure - higher ductility and resistance to SCC & pitting corrosion. ❖ Used as thin sheet for corrosion, oxidation & heat resisting applications and decorative purposes eg. Automobile exhausts, Solar water heater, catalytic converters and automobile trim. Ferritic stainless steels
- 20. Welding of Ferriticsteels ❖ Softer and more ductile than martensitesteels but poorer formability than austenitic steels. ❖ Corrosion resistance and formabilityimproved by increasing Cr & Mo content and lowering levels of interstitialelements e.g. C & N to below 100 ppm. Low interstitialor Super Ferrites. ❖ Ferrite phase does not transform to martensite but susceptibleto 475 deg embrittlementand sigma phase formation in higher chromium grades. ❖ Problem of grain growth during welding leading to brittle structure in HAZ. ❖ Grains may be refined only by cold work and re-crystallization. ❖ Generally welded with austenitic SS electrodes or TIG process with restricted heat input.
- 21. Applications of FerriticSS Automobile exhausts Solar water heater
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- 23. Martensitic stainless steels ❖AISI 403, 410, 416, 420, & 440 A/B/C grades ❖ Martensitic structure - higher carbon grades used in tempered condition. ❖ Used for cutlery, surgical instruments, steam, gas & hydel turbine blades, ball bearings and races.
- 24. Welding of MartensiticSteels ❖ Higher carbon grades used in the quenched and tempered condition ❖ Problem of Hydrogen induced cold cracking in HAZ. ❖ Pre-heat and post-weld heat treatment required if welded with matching composition martensitic SS electrodes. ❖ Austenitic SS electrodes generally used which avoids cracking problems without pre and post heating
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- 26. Duplex stainless steels 60%austenite + 40% ferrite structure
- 27. Duplex stainless steels ❖Half the nickel content of austenitic steels ❖ Cr 18 – 28% ❖ Ni 4.5 – 9.0 % ❖ Almost twice the strength of austenitic steels ❖ Excellent pitting + SCC resistance ❖ Used for plant and piping in oil and gas production, corrosive applications to resist chloride ion media. Higher strength structurals
- 28. Weldability of Duplexsteels ❖Duplex stainless steels have fairly good weldability. ❖All standard welding processes can be used. ❖Not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding. ❖All grades ➢Solidify as ferrite, austenite formation during cooling ➢Austenite/Ferrite ratio dependent on 2 primary variables ❑Alloying effects – Cr & Ni equivalents ❑Heat input/cooling rate
- 29. Welding metallurgy ofDuplex steels •The longer the cooling time between 900 and 1100 C the higher the austenitecontent •Faster cooling rate produces higher Ferrite which leads to reduced low temperature impact strength and corrosion resistance •Slow cooling through 1050 – 550 C produces carbides, nitrides, sigma etc which affect corrosion resistance and cause embrittlement Fast cooling Correct cooling Slow cooling Too high Ferrite Between 30 -60 % ferrite Nitrides, Carbides & intermetallics
- 30. Welding of DuplexStainless Steels –Overmatch nickel in filler metal –Control heat input and cooling rates carefully –Use N2 in shielding gas –Ensure low impurities in base and filler
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- 32. Precipitation Hardening stainless steels Cr12 –18% C 0.05 – 0.15% Ni 3.0 – 27.0% + Mo, Cu, Al, V, Cb, Ti ❖ This sub-group provides a combination of austenitic and martensitic properties. ❖ Hardening is achieved by adding one or more elements such as aluminium, molybdenum, niobium, titanium, and copper. ❖ It is capable of developing high tensile strength through heat treatment ❖ Available as forgings, castings, bar and plate and used for compressor blades, pumps, gears for metering chemicals etc
- 33. Stainless Steel Castinggrades Alloy Type Wrought grade Casting grade Martensitic 12Cr 410 CA15 Ferritic 21Cr 442 CB30 Austenitic 19Cr 9Ni 304 CF8 Austenitic 19Cr 9Ni Nb 347 CF8C Austenitic 19Cr 12Ni 2.5Mo 316 CF8M Austenitic 25Cr 20Ni 310 CK20 do Heat Resistant - HK40 Duplex 25Cr 5Ni 2.5Mo 3 Cu 329Cu CD4MCu Duplex 22Cr 5Ni 3Mo 0.15N Alloy 2205 Alloy 2205
- 34. Wrought Grade C –Corrosion service H – High – temperature service Example: CA15 C - Corrosion service A - Refer Fig 15 – 0.15%Carbon (max)
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- 37. ❖ AISI 304,310, 316, 321 & 347 grades ❖ Austenitic structure gives good weldability with excellent ductility and toughness down to cryogenic temperatures. ❖ Nickel improves general corrosion resistance ❖ Widely used for chemical, petrochemical, fertilizer plant and food processing. Also used for nuclear and cryogenic plant Austenitic stainless steels
- 38. •Has 50% highercoefficient of linear expansion, than carbon steels •Has poor thermalconductivity,30% less than carbon steels •Results in much higher distortionafter welding •Steps to prevent distortion - closer tacking - greater use of jigs and fixtures - use of balancedand skip welding techniques - use of copper chill bars to help remove heat - limit heat input by use of low currents and stringer beads Physical properties of austenitic stainless steels
- 39. Welding of Austeniticsteels Generally good weldability as there is no martensitic transformation but following problems encountered: ❖ Sensitization leading to inter-granular corrosion –IGC ❖ Hot cracking ❖ Stress corrosion cracking –SCC ❖ Sigma phase formation leading to embrittlement ❖ Higher distortion during welding
- 40. Sensitization and inter-granular corrosion ❖Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the alloy to be susceptible to intergranular corrosion. ❖ austenitic stainless steel may become sensitised if they are heat treated or used at temperature in range 450- 900c. the heat affected zones of welds may also be sensitised in some circumstance
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- 42. Steels to preventIGC ❖ Standard grades 304 – 18Cr / 8Ni 316 – 18Cr / 12Ni / 2.5Mo – for pitting resistance ❖ Steels with elements having higher affinity for carbon eg. Ti , Nb/Cb – stabilised steels. Form carbides in preference to Cr. 321 grade – Ti stabilised 347 grade - Nb stabilised ❖ Steels with low carbon 304L & 316L grades( 0.03% C max)
- 43. Hot cracking inaustenitic welds ❖ Hot cracking or solidification cracking is caused due to low melting eutectics formed at the grain boundary. ❖ As the weld solidifies, in combination with shrinkage stresses, leads to cracks in fully austenitic welds ❖ Promoted by S, P, Nb, Ti, N etc. ❖ Prevented by adjusting weld metal composition to give 5 – 10 % ferrite phase in the deposit. ❖ Also prevented by reducing heat input and controlling design stress.
- 44. Sigma Phase embrittlement ❖Sigma phase is a non-magnetic composed mainly of iron and chromium which forms in Ferritic and austenitic stainless steels during exposure at 475ºC-980ºC. ❖ Many stainless steels and other iron-chromium alloys are susceptible grain boundary phenomenon known as sigma-phaseembrittlement. ❖ Alloy elements such as chromium, molybdenum, titanium and silicon promote the formation of sigma phase
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- 46. Sigma Phase embrittlement CAUSES ❖Loss of ductility and toughness ❖ Corrosion resistance resulting in cracking ❖ Failure of components ❖ Especially those subjected to impact loads or excessive stress PREVENTION ❖ Nitrogen and carbon reduce as it tendency to form. ❖ Annealing treatment
- 47. THE BASICS OFCOLD WORKING AND ANNEALING ❖ Cold working makes alloys stronger and harder while reducing the material’s ductility. ❖ annealing reduces strength and hardness while restoring ductility. ❖ The sequence of change in microstructure and mechanical properties comprises three steps: Cold work, heating the material to its recrystallization temperature, and soak time at the elevated temperature
- 48. AUSTENITIC STAINLESSSTEEL SOLUTION ANNEALING ❖For the more conventional stainless steels, such as AISI 201, 202, 301, 302, 303, 304, 304L, 305, and 308, recommended solution-annealing temperatures are around 1010 to 1120 ºC(1850 to 2050 F). ❖ For higher Cr-containing steels such as the AISI 309 and 310 or steels containing molybdenum such as AISI 316, 316L, 317, and 317L, the recommended solution-annealing temperatures are around 1040ºC - 1120ºC (1900 to2050ºF). ❖ For the stabilized stainless steels (titanium-stabilized AISI 321 type, which are more prone to secondary recrystallization or abnormal grain growth, the solution annealing temperature range should be at a lower level, between 955 and 1065ºC (1750 and1950ºF), ❖ For the niobium-stabilized AISI 347 and 348 (nuclear grade) type , between 980and 1065ºC (1800 and 1950ºF),
- 49. FACTORS THAT AFFECTGRAINSIZE A heavily cold-worked structure has elongated grains and a large amount of residual stress . A moderate amount of annealing causes the elongated grains to recover and new grains to form (centre). Extended annealing is associated with grain growth
- 50. Pre Cleaning ❖ Whenworking with this material,one of the first things to ensure is that it is clean, clean, clean. ❖ When working with stainless it is essential that the material and surrounding environment be clean ❖ Ensuring you have a clean (carbon-free)atmosphere is very important, as is cleaning of the stainless steel to remove impurities that may cause oxidation (rust) laterand prohibit the rebuildingof the passive layer, which creates the protective layer to minimize oxidation.” ❖ Removing the oils, Dust ❖ Contaminants on the stainless can lead to oxidization, but they also present a problem during the welding process, potentially causing defects. ❖ So it is important to clean the surface before starting to weld.
- 51. Pre Cleaning ❖ Shopenvironments are not always the cleanest, and cross- contamination can be problematic when working with both stainless steel and carbon steels ❖ a shop will have a number of fans running or an air conditioner to cool down workers, which can push contaminants across the floor or cause condensation to drip or build up on raw material ❖ This is particularly challenging when carbon steel particles are blown onto stainless ❖ Keeping these materials separate and in a clean environment will make all the difference when it comes to effective welding.
- 52. Post Cleaning ❖ stainlesssteel, have an oxide layer at the surface. Heat tinting causes this naturally occurring oxide layer to become thicker which causes discoloration. The temper colors are a direct result of light interference effects as light bounces off the metal surface ❖ welding industry, various terms are used to describe this phenomenon, such as discoloration, oxide scale, and rainbow effect. All these terms are talking about the same thing, a change in the color of the top layer of stainless steel ❖ Light reflected from the oxide film’s normal surface and that which is reflected from the oxide metal interface created by welding cause a variety of colors depending on the oxide layer thickness ❖ It usually occurs around the weld bead and within the surrounding zone that has been affected by the heat.
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- 54. Types of weldCleaning ❖ Mechanical Weld Cleaning ❖ Chemical Weld Cleaning ❖ Electrochemical Weld Cleaning
- 55. Mechanical Weld Cleaning ❖Mechanical weld cleaning is a common and low cost method used for cleaning stainless steel ❖ involves grinding machines and abrasives to clean the top layer of metal surfaces where rust and other slag particles can form
- 56. Chemical Weld Cleaning ❖chemical pickling paste for cleaning after a welding job. The paste is applied to the affected areas using a brush or spray and left on the surface for some time to interact with the metal ❖ chemical pickling paste contains a variety of toxic acids, including hydrofluoric, nitric, and sulfur acids. These chemicals are quite dangerous for the human body and they can cause serious, long-term damage to the skin and internal organs if they are consumed or breathed in.
- 57. Electrochemical Weld Cleaning ❖Electrochemical weld cleaning, also known as electropolishing, is considered the most effective method of cleaning stainless steel ❖ It is faster, safer and preferred by welders, compared to the other two methods. It doesn’t pose any major health risks for the welder.
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