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Influences on Composition, Microstructure and Properties

 

Influence of Current Type and Polarity on the Composition, Microstructure and Mechanical Properties of C-Mn-Ni and C-Mn MMA Deposits

TWI Industrial Member Report Summary 292/1985

By D J Abson

Background

Two C-Mn and one C-Mn-Ni commercial basic-coated low hydrogen manual metal arc electrodes were used to study the effects of increasing the arc voltage on oxygen and nitrogen pick-up using d.c. positive, d.c. negative and a.c. power supplies. With increasing arc voltage, the oxgyen and nitrogen contents were found to increase most rapidly for d.c. negative welding for all three consumables; this is attributable to the greater absorption of nitrogen when the cathode is the electrode tip. Double-V butt welds were produced in the flat position in 25mm thick BS 4360 50E C-Mn-Nb steel plate, using the C-Mn-Ni and one of the C-Mn electrodes, with no back-gouging. For each electrode type, panels were produced by d.c. positive, d.c. negative and a.c. welding, employing the same current type or polarity for the whole of thejoint; the chemical composition, microstructure and mechanical properties of these panels were investigated. For the C-Mn-Ni deposits, there was a progressive reduction in manganese and silicon and an increase in nitrogen, in the order d.c. positive, a.c. and d.c. negative. The influence on strength, hardness and ductility was minimal. There was generally also a progressive decrease in Charpy toughness and in CTOD at -10°C, but little change in microstructure. For the C-Mn deposits, the progressive change in composition was less pronounced. Whilst the Charpy toughness was similar to that of the C-Mn-Ni deposits, the CTOD levels at -10°C showed considerably greater scatter; once again, the d.c. negative deposit displayed lower CTOD toughness than the d.c. positive deposit, and there was minimal change in microstructure.

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