Modelo de Stufflebeam-Modelo C.I.P.P. (Contexto-Insumos-Procesos-Productos) 110

Oxy-fuel cutting cuts or removes metal by the chemical reaction of oxygen with the metal at elevated temperatures. Temperature is provided by a gas flame which preheats and brings the material up to the burning temperature (approximately 850^oC). Once this is achieved, a stream of oxygen is released which rapidly oxidises most of the metal and performs the actual cutting operation. Metal oxides, together with molten metal, are expelled from the cut by the kinetic energy of the oxygen stream. Moving the torch across the workpiece produces a continuous cutting action.

Oxy-fuel cutting.

A material must simultaneously fulfil two conditions to be cut by the oxy-fuel cutting process:

 Burning temperature must be below the parent material melting point.

 Melting temperature of the oxides formed during the cutting process must be below the parent material melting point.

These conditions are fulfilled by carbon steels and some low alloy steels.

However, the oxides of many of the alloying elements in steels, such as aluminium and chromium have melting points higher than those of iron oxides. These high melting point oxides (which are refractory in nature!) may shield the material in the kerf so that fresh iron is not continuously exposed to the cutting oxygen stream, leading to a decrease of the cutting

Oxygen

Fuel gas and oxygen

Heating flame

Slag jet

effectively limited to low alloy steels containing <0.25%C, <5%Cr, <5%Mo,

<5%Mn and <9%Ni.

Advantages

 Steels can generally be cut faster than by most machining methods.

 Section shapes and thicknesses difficult to produce by mechanical means can be cut economically.

 Basic equipment costs are low compared with machine tools.

 Manual equipment is very portable so can be used on site.

 Cutting direction can be changed rapidly on a small radius.

 Large plates can be cut rapidly in place by moving the torch rather than the plate.

 Economical method of plate edge preparation.

Disadvantages

 Dimensional tolerances significantly poorer than machine tool capabilities.

 Process essentially limited to cutting carbon and low alloy steels.

 Preheat flame and expelled red hot slag present fire and burn hazards to plant and personnel.

 Fuel combustion and oxidation of the metal require proper fume control and adequate ventilation.

 Hardenable steels may require pre and/or post-heat adjacent to the cut edges to control their metallurgical structures and mechanical properties.

 Special process modifications are needed for cutting high alloy steels and cast irons (ie iron powder or flux addition).

 Being a thermal process, expansion and shrinkage of the components during and after cutting must be taken into account.

15.1.1 Requirements for gases

Oxygen for cutting operations should be 99.5% or higher purity: Lower will result in a decrease in cutting speed and an increase in consumption of cutting oxygen thus reducing the efficiency of the operation. With purity below 95% cutting becomes a melt-and-wash action that is usually unacceptable.

The preheating flame has the following functions in the cutting operation:

Factors to be considered when selecting a fuel gas include:

 Preheating time.

 Effect on cutting speed and productivity.

 Cost and availability.

 Volume of oxygen required per volume of fuel gas to obtain a neutral flame.

 Safety in transporting and handling.

Some of the more common fuel gases used are acetylene, natural gas (methane), propane, propylene and methylacetylene propadiene (MAPP) gas.

Fuel gas characteristics and applications

Fuel gas Main characteristics Applications

Acetylene

Highly focused, high temperature flame

Rapid preheating and piercing Low oxygen requirement

Cutting of thin plates Bevel cuts

Short, multi-pierce cuts

Propane

Low temperature flame, high heat content

Slow preheating and piercing High oxygen requirement

Cutting of thicker sections (100-300mm), long cuts

MAPP Medium temperature flame Cutting underwater Propylene Medium temperature flame Cutting of thicker sections Methane Low temperature flame Cutting of thicker sections

15.1.2 Oxy-fuel gas cutting quality

Generally, oxy-fuel cuts are characterised by:

 Large kerf (>2mm).

 Low roughness values (Ra <50µm).

 Poor edge squareness (>0.7mm).

 Wide HAZ (>1mm).

The face of a satisfactory cut has a sharp top edge, drag lines, which are fine and even, little oxide and a sharp bottom edge. Underside is free of slag.

A satisfactory cut is shown in the centre. If the cut is too slow (left) the top edge is melted, there are deep grooves in the lower portion of the face, scaling is heavy and the bottom edge may be rough, with adherent dross. If the cut is too fast (right) the appearance is similar, with an irregular cut edge. Plate thickness 12mm.

A satisfactory cut is shown in the centre. If the preheating flame is too low (left) the most noticeable effect on the cut edge is deep gouges in the lower part of the cut face. If the preheating flame is too high (right) the top edge is melted, the cut irregular and there is an excess of adherent dross. Plate thickness 12mm.

A satisfactory cut is shown in the centre. If the blowpipe nozzle is too high above the work (left) excessive melting of the top edge occurs with much oxide. If the torch travel speed is irregular (right) uneven spacing of the drag lines can be observed together with an irregular bottom surface and adherent oxide. Plate thickness 12mm.