1st semester 2015/2016 Arc welding SMAW CTU in Prague Faculty of Mechanical Engineering Ing. Petr Vondrouš, PhD., IWE
SMAW welding Method no. standardized acc. ČSN EN ISO 4063 111 (Metal arc welding with covered electrode) used names MMA SMAW Manual Metal Arc Welding EU Shield Metal Arc Welding USA
The Arc Welding Circuit power source electrode arc base material ČVUT, Fakulta strojní, Ústav strojírenské technologie, skupina svařování 3
Basic Electricity Voltage DC - Current Polarity DC- (Direct Current Electrode Negative) DC+ (Direct Current Electrode Positive) AC DC+ AC (Alternating Current) ČVUT, Fakulta strojní, Ústav strojírenské technologie, skupina svařování 4
Basic parameters Positions Base material Welded thickness Current Voltage Current Diameter Welding source : all, depends on electrode : very wide availability structural steels, low, high alloyed materials, Ni, Cu, Al alloys : 2-100 mm : 50-450 A (40 A /1 mm) : 15-40 V : DC, AC : 1,6-8 mm (standard 2 6,3 mm) : steep
Use Main advantage experinence, availability, ease of use, flexibility Pipelines Maintanance Field welds Concrete Reinforcements
Functions of covering Protective shielding Metallurgy Electrical Operational purifying WM dezoxidation, denitridation, P, S removal protection Slower cooling Forming the WM flow Arc stability, direction of arc, WM
Types of electrodes Core composition non alloy alloyed Covering composition Rutile (R) Acid (A) Bazic (B) Cellulose (C) Combined RC, RA, RB
Acid, bazic, rutile Acid - Mainly Fe, Mn, Si oxides = acidic components Ease of slag removal,strenghts upto max. 440 MPa Bazic CaF 2, CaCO 3, Fe, Si oxides = basic reaction High strenghts - weld metal cleanliness due to deoxidation, low P, S Difficult slag removal, hygroscopic slag Rutile TiO 2, Fe, Si oxides basic reaction Good operation quality
Weld metal transfer Acid electrodes Basic electrodes
Electrode rebaking
Electrode selection 1. BM 2. Weld type, load, size 3. Welding positions 4. Further weld processing 5. Price Developement of electrodes Ease of use Hygroscopy Stability Metallurgy
Impurities in WM and their influence As basic impurities and detrimental elements are considered: Gases oxygen, hydrogen, nitrogen Inclusions nitrides, oxides, phosphates, sulfites Some metals - Cu, Sn Entraped slag Source of these can be air, moisture, contaminated WM, electrodes, improper welding technigue Influence on WM properties is crucial strength, ductility, toughness
Influence of Oxygen If oxygen (Molecules, atomos, ions) is present in areas adjacent to WM: Creation of oxidic compounds with many metals different properties of oxides and metals Source of oxygen: air moisture rust Creation of FeO, Fe 2 O 3, Fe 3 O 4 cause of brittleness from 0,23 % Dezoxidation by FeSi, FeMn, FeTi Creation of SiO 2, MnO, TiO 2 SiO 2, MnO, TiO 2, FeO. SiO 2 floating on the WM surface as slag, non dissolved in WM.
Influence of Nitrogen N2 present in atmosphere Molecule can create porosity, change of gas solubility with temperature When dissociated in the arc - atom gas creates Fe 4 N (800 až 500 C) dissolved in ferite, inclusions With other metals (Mn, Si, Al, Ti) nitrides created as well Nitrides increase strength, decrease of ductility, toughness NOT GOOD In some cases nitrides as precipitation hardening can be used, yet maily for very ductile austenitic alloys
Influence of Hydrogen Hydrogen highly detrimental gas in welding Source: Air Moisture coverings, flux Grease, moisture of BM One of most observed parameters is Hydrogen presence during fabrication of filler wire wire drawing, moisture in flux data hydrogen 5 ml/100 g Atomic H disolves easily (dia 1 A), diffuses, decrease of temperature solubility decrese, creation of molecular H 2 big size dia 2,7 A Fish eye cracking Cold cracking Low hydrogen filler wire, flux, alloying with fluorides slow cooling
WM Impurities-S, P S present from steel production - coke With iron - low melting eutectic FeS at grain boundary Tt = 890 C. Removal: Acid : Bazic : FeS + MnO = MnS + FeO FeS + CaO = CaS + FeO FeO + SiO 2 = FeO. SiO 2 FeO + TiO 2 = FeO. TiO 2 MnS, CaS undissolved - floats up into the slag P present from steel production - ore With iron hot short compound Fe 3 P, Fe 2 P, FeP created at grain boundaries Removable only by basic covering 2 Fe 2 P + 5 FeO = P 2 O 5 + 9 Fe P 2 O 5 + 3 CaO = Ca 3 P 2 O 8 or Ca 4 P 2 O 9 Removed into the slag