Electrode quality steel for core wire of covered electrodes Jun16

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Electrode quality steel for core wire of covered electrodes


Electrode quality steel for core wire of covered electrodes

Covered welding electrode is a delicate tool which while in use combines physical, chemical, and metallurgical processes of the flux, core wire and parent metal to achieve a durable weld joint or surfacing. Covered welding electrodes are used in manual metal arc welding (MMAW) process.

During the 1890’s, electric arc welding was accomplished with bare metal electrodes. The welds produced were porous and brittle because the molten weld puddle absorbed large quantities of oxygen and nitrogen from the atmosphere. Since 1927, many improvements have been made and many different types of electrodes have been developed and produced through variations in the formulations of the covering and the amount of covering on the mild steel core wire. Commercial production of welding electrodes was started in India in early 1960s.

Mild steel covered electrodes, also commonly called coated electrodes, consist of only two major elements namely the core wire or rod and the flux covering. The raw material for the core wire is hot rolled wire rod which is supplied in coil form. Wire rod is cleaned, drawn down to the proper electrode diameter, straightened, and cut to the required electrode length. The most popular diameter of wire rod is 5.5 mm, although larger diameters are also used in a limited way. In covered welding electrode the product mix is of mild steel electrodes (around 60 %), mild steel high tensile and low hydrogen low alloy (around 30 %) and special type (around 10 %). Fig 1 shows EQ steel wire rod, EQ steel drawn wire and covered welding electrode.

EQ steel

Fig 1 EQ steel wire rod, EQ steel drawn wire and covered welding electrode.

 Core wire is an important component of a manual electrode since it significantly influences the electrode performance and weld metal properties. Electrode quality (EQ) steels consist of both rimming and non rimming quality steels which are used for the core wire of the mild steel and low alloy steel electrodes although rimming quality steel is preferred. The starting material for rimming steel core wire is ingot whereas for non rimming quality steel core wire it is either bloom or billet. The characteristic features of the rimmed steel are a high oxygen content (typically 200 -400 ppm) and a rim of almost pure iron on the surface.

It is preferred to produce semi killed or fully killed EQ steels because they are cheaper and easier to produce and also because they confer optimum performance characteristics on the electrode. Since it is possible to obtain any desired alloy steel chemistry of the weld metal by incorporating suitable alloying elements in the flux coating, covered electrode manufacturer uses a single grade of rimming quality steel to produce a wide range of mild steel, low alloy steel and hardfacing electrodes. EQ steels are having low percentage of carbon, silicon and aluminum. Further sulphur and phosphorus control is very important in EQ steels. The sulfur and phosphorous levels must be kept very low since they can cause undesirable brittleness in the weld metal. Integrated steel plants have established metallurgy, drawability and consistency of producing non rimming killed EQ steels by Concast route. This route of EQ steel production has led to tramp element free steel.

Composition of rimming and non rimming steels used for core wire is given in Tab. 1

Tab 1 Composition of electrode quality steel for core wire

Element

Unit

Rimming

Non rimming

C

%

0.1 max

0.1 max

Si

%

0.03 max

0.03 max

Mn

%

0.38-0.62

0.38-0.62

P

%

0.03

0.025

S

%

0.03

0.025

Cu

%

0.15 max

0.15 max

V

%

0.005 max

0.005 max

Ti

%

0.003 max

0.003 max

Al

%

0.012 max

0.012 max

Quality of wire rods of EQ steels needed for core wire of covered electrodes

Besides chemical composition there are several quality requirements from the wire rods of EQ steels which are expected by the EQ steels user. These are enumerated below.

  • The chemical composition of the wire rod must meet special weldability requirements and, at the same time, it must not affect the microstructure of the manufactured wire rod to maintain the reduction capabilities as required.
  • The EQ steel wire rods should be free of injurious impurities (tramp elements) and should contain stable amount of components
  • The wire rods are to be produced by slow cooling operation in the heat treatment section of the wire rod mill to ensure the good workability of wire rod during its drawing.
  • Wire rods should have good surface quality as well as internal soundness. In order to improve the surface quality of wire rods, the surfaces of the billets are descaled by shot blasting. Then the surface is inspected for any surface flaw and imperfection by a magnetic particle tester. The flaws are removed from the surface by using a grinding machine. In addition, the continuous ultrasonic testing is conducted to guarantee the internal quality of the billets.