Factors Affecting the Building up of Roll
- 1. The size of electrode.
- 2. The RPM of the work roll.
- 3. The current and the electrode feed speed.
- 4. The arc voltage.
- 5. The off set of electrode from the centre and pitch of the built up welds.
Pre-heating of Rolls
Pre-heating of rolls is to be done at 250°C-400°C depending upon carbon content and size of the roll. Preheating of roll, before welding is required for the advice due to following reasons:
- 1. To remove internal str esses generated in it as a result of its sendee in the mill.
- 2. To avoid martensitic structure which in prone to cracking due to rapid quenching.
- 3. To reduce the magnitude of shrinkage cracks.
Post heating or Stress Relieving of Rolls
Stress relieving is done by heating below critical temperature and then allows it to slow cooling. It is desired for following reasons:
- 1. Thermal stress-due to temperature change during welding.
- 2. High internal stress-due to contraction of the weld metal.
- 3. Hydrogen embrittlement.
Process of Arcing
An arc is struck between the roll and electrode (normally the wire) when they are connected in an electrical circuit. The arc melts both the wire and to that parts of rolls. The str eam of flux, forming a thick layer on the welding surface, protects the metal from the harmful effects of air, maintains the heat of the arc and also prevents splashing of the molten metal. The arc melts and the flux and creates around itself a void encircled by elastic film of liquid flux and thus protects the molten metal. The film of the molten flux does not prevent escaping of the entrapped gases.
For mechanized welding, any welding apparatus having mechanical feed of the electrode is used. The most commonly used welding apparatus is which has a constant speed of electrode feed, irr espective of the voltage. This apparatus can be installed on any discarded turning lathe or on any other machine, capable of rotating the roll to be welded at the given speed.
Fig. 3.23 Welding of Rolls.
They are categorized into three main groups as follows :
- 1. Carbon steel wires.
- 2. Alloy steel wires.
- 3. Highly alloyed steel wires.
- 12.7.1 Types of Wires
- 1. CCMS (Copper coated mild steel)
- 2. Alloy steel wire.
- 12.7.2 Chemical Composition
Chemical composition of alloy steel wires is as follows:
C-0.25 to 0.35%, Mn-0.8 to 1.1%, Si-0.9 to 1.2%, Cr-0.8 to 1.1%, Ni- 0.3%, S-0.25%, P-0.03%.
Flux is one of the important elements of the welding process; which determine the quality of the weld metal and of the welded seam to a large extent.
- 12.8.1 Functions of Flux
- • Protect the molten metal from the undesirable effects of the atmospheric gases like oxygen, nitrogen etc.
- • Ensure good formation of welded head and the stability of the arc.
- • Prevent splashing of the molten metal.
- • Ensure easy separation of the slag incrustation from the weld.
- • Slow down the crystallization of the molten metal and to provide the better condition for the escape of gases from the molten metal.
12.8.2 Composition of the Flux
Composition of the flux should be such that it should ensure easy removability of slag and to form the uniform shape of the deposited layer. Generally neutral flux is used, alongwith alloyed electrode.
12.8.3 Type of Flux
The fluxes are categorized into two main types, depending upon the method of manufacture:
(a) Fused fluxes-are prepared by melting the charge in the furnace, followed by granulation by pouring in water. Mostly flux used are fused flux. The chemical composition of fused flux is given below:
Si- 41 to 43%, Ca 0-5.5%, Mn 0-34.5 to 37.5%, Mg 0-5.5 to 7.5%, Clay- 3%, Ca 2-35-5.5%, S<0.15%, P<0.12%.
(b) Agglomerated fluxes-are mechanical mixture of ferro alloys with natural minerals. These fluxes are the mechanical mixture of the powders with liquid glass.
Preparation of the Job for Weld Deposition
The roll received for welding after its service in the mill and is consequent a worn out roll. Selection for a weld deposit depends on the characteristics of the wear out; as explained below:
- (u) Rolls having a heavy network of fire cracks and other cracks should be turned down till the fire cracks and other cracks are completely eliminated. Cracks extending to a considerable depth must be removed by turning annular grooves. The sides of the annular grooves will be having a taper of 30-45°.
- (b) Roll which has been previously welded and has brought again for rewelding after providing service in the mills. These rolls should be prepared for weld deposition with given consideration to the characteristic of the wear out, as explained above. If blow holes or slag incrustations are noticed on the surface, there these should be removed completely.
- (c) The rolls must be thoroughly cleaned and free from grease, graphite, dirt and rust before they are taken for welding.
- (<7) After cleaning etc., the roll has to be heated before and after welding to relieve the str esses and strains developed during rolling.
Welding of Rolls
The process of welding starts with the proper setting up of the roll on the machine and proper clamping of it on the face plate and in centers. After the roll has been properly set on machine and checked for proper clamping on the face plate, the welding should be set in accordance with technological chart. The welding head should set for the proper welding regime.
Fig. 3.24 Welding of Rolls.
Defects of Welding
Causes, prevention and rectification of defects of the welded surface are given below:
12.11.1 Cold Cracks
They are located at right angles to the seams. Sometimes even extending upto the entire depth and even get extended to the parent metal.
Causes: In-sufficient heating prior to welding and rapid cooling after welding.
Preventions: Ensuring proper heating of the entire roll, slow cooling in the controlled cooling pit.
Rectification: Removal of the crack portion by machining and then goes for re-welding.
12.11.2. Hot Cracks
They are generally located circumferentially along the seam with propagation at random angles confined only to the welded seam.
Causes: Insufficient heating of the roll, high welding current for the first layer. Higher welding pitch. Welding wire not corresponding to the parent metal.
Prevention: Ensure the proper heating of the roll. It is not desirable to have high current and low voltage during deposition of the first layer.
Rectification: Removal of the crack portion by machining and re-welding.
12.11.3 Porosity of the Deposited Metal
Along the seam circumferentially as well as across it.
Causes: Higher arc voltage, moist flux contaminating with rust or oil. Insufficient flux coverage over the arc.
Prevention: Measures to ensure that flux not become dirty or moist, wire getting msted. Avoiding high voltage and open arc.
Rectification: Removal of the crack by machining and re-welding.
12.11.4 Slag Entrapments
At any place, has irregular shapes and size over 1 mm.
Causes: Incomplete removal of the slag incrustation. Bad quality of flux, flux not corresponding to the wire.
Prevention: Preventing over-heating of the roll. Watching for proper welding regime and position of the electrode. Ensuring proper composition and quality of flux and wire.
Rectification: Chipping/drilling and re-welding.