Cutting processes - application of oxyfuel cutting

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Cutting processes - application of oxyfuel cutting

مُساهمة من طرف Admin في الجمعة مارس 07, 2008 3:26 pm

Cutting processes - application of oxyfuel cutting






Rough-cut gear wheel cut by oxyacetylene

Oxyfuel is one of the most widely used cutting processes with the following benefits:




  • Low cost equipment
  • Basic equipment suitable for cutting, gouging and other jobs such as welding and heating
  • Portable, suitable for site work
  • Manual and mechanised operations
  • Mild and low alloy steels (but not aluminium or stainless steel)
  • Wide range of thickness (typically from 1mm to 1000mm)
It is therefore not surprising that the process can be used for a diverse range of applications from manual rough severing and scrap cutting to precision contour cutting in fully automated systems. Here, the process application is described including the choice of fuel gas and nozzle design to maximise performance. Best practice to ensure adequate quality of the cut surface is also included.

Choice of fuel gas


Basically, a mixture of oxygen and a fuel gas (acetylene, propane, MAPP propylene or methane) is used to preheat the metal to its 'ignition' temperature which is well below its melting point. A jet of pure oxygen is then directed into the preheated area which burns through the spot and the resulting molten metal and slag are removed by the high velocity oxygen stream. The cutting speed is primarily determined by the oxygen jet but as the outer fuel gas/oxygen flame determines the rate of preheating, the choice of fuel gas has a significant influence on the time taken to initiate the cutting operation. This is especially important if the designed cut begins by piercing.
The choice of fuel gas is largely made on cost, performance, ease of use and whether it is a manual or mechanised operation. However, in making the choice it should be noted that in a typical application the cost is made up of approximately:




  • 50% overheads
  • 30% handling labour
  • 18% cutting labour
  • 1-2% gas
Consideration should, therefore, be given to the choice of fuel gas type and nozzle design to speed up the initiation of the cutting operation. Labour costs can be reduced by decreasing the pierce time and/or increasing the cutting speed. Typical flame temperatures and fuel gas to oxygen ratios are shown in Fig. 1. Generally, fuel gases which generate a higher flame temperature and require a lower oxygen to fuel gas ratio, will speed up the cutting operation.







Fig. 1. Flame temperature and the fuel gas to oxygen ratio

Acetylene


Acetylene produces the highest flame temperature of all the fuel gases and generates a highly focused flame. As the pierce time is approximately one third that achieved with propane, it should be used when the pierce time is a significant proportion of the total cutting time, for example, short cuts and multi-pierce cutting operations
The high temperature, highly focused flame makes the oxyacetylene process ideal for cutting thin sheets with minimum distortion and for bevel cutting. However, the high cost and low heat generation make it less suitable for general heating of large plates.

Propane


Propane is low cost and has the advantage of being available in bulk supplies. The flame temperature is lower than for acetylene (the maximum flame temperature in oxygen is 2810°C compared with 3160°C for acetylene) which makes piercing much slower. However, it can tolerate a greater nozzle to workpiece distance which reduces the risk of molten metal splashing back onto the nozzle and causing a 'backfire'.
For similar nozzle designs, cutting speeds for oxypropane and oxyacetylene are similar. Advantages claimed for propane are smooth cut edge, less slag adhesion and lower plate edge hardening because of the lower flame temperature. The heat affected zone is much wider than for oxyacetylene.

MAPP


MAPP gas, which is a mixture of various hydrocarbons, principally, methylacetylene and propadiene, produces a relatively hot flame (2927°C). However, the lower calorific value of the inner cone compared with acetylene gives a slightly slower pierce time.
The gas is seen as an alternative to acetylene with greater tolerance to torch distance variation because of the more uniformly distributed heat between the inner and the outer cones.
Only acetylene, hydrogen and MAPP have sufficiently high flame temperature for underwater cutting. But as acetylene has a limited outlet pressure, MAPP is the only gas other than hydrogen that can be used for cutting in deep water.

Propylene


Propylene is a liquid petroleum gas (LPG) product and has a similar flame temperature to MAPP (2896°C compared to 2976°C for MAPP). It gives off a high heat release in the outer cone (72,000 kJ/m3) but, like propane, it has the disadvantage of having a high stoichiometric oxygen requirement (oxygen to fuel gas ratio of approximately 3.7 to 1 by volume).

Methane


Methane has the lowest flame temperature similar to propane and the lowest total heat value of the commonly used fuel gases. Consequently, natural gas is the slowest for piercing.

Cutting torch


The cutting torch design can be either nozzle mix or injector. In the nozzle mix torch, the fuel gas and pre-heat oxygen are mixed in the nozzle. In the injector torch, the pre-heat gases mix either in the body of the torch, within the gas delivery tubes, or within the head of the torch. Injector torches have the advantage of being able to use the higher pressure of oxygen to pull the fuel into the torch. This allows the torch to be used at low fuel gas pressures or with large pressure drops such as those experienced through long hose lengths.

Nozzle


The primary functions of the nozzle are to provide:




  • a method of preheating the metal to its ignition temperature
  • a jet of oxygen to react with the material to be cut and at a flow rate sufficient to blow away the slag
Each torch should be fitted with the appropriate nozzle for the type of fuel gas. Nozzles can be of a one- or two-piece design. The nozzle type will depend on:




  • fuel gas
  • manual or machine operation
  • manufacturer's preference
Acetylene nozzles are usually one-piece but two-piece nozzles similar to those for other fuel gases are produced for machine cutting.
The diameter of the cutting oxygen hole is selected according to the material thickness. There are two types of nozzle; standard and high speed. The standard nozzle usually has a parallel sided, central bore for the oxygen jet, which is surrounded by an annulus or a ring of smaller diameter ports for the pre-heating gas mixture, Fig. 2. There are many designs and arrangements of the preheating ports that focus the flame for heating and to protect the oxygen jet from air entrainment.









Fig. 2. Standard nozzle with central bore for oxygen jet and a ring of ports for the pre-heating gas mixture



High-speed nozzles are capable of being used with higher oxygen pressures, up to 10 bar. The essential difference is that the cutting oxygen is forced through a convergent / divergent orifice which speeds up the gas flow rate to near supersonic levels. High-speed nozzles are primarily used in mechanised equipment to exploit the higher speeds for cutting long lengths.

Best practice


Cutting conditions are normally set to produce an acceptable cut surface finish for the application but at the highest cutting speed. It is, therefore, essential that consideration is given to the following settings for the material thickness and the cutting speed:
nozzle distance - too high or too low will disturb oxygen flow
preheat flame - too high a flow can cause top edge melting
cutting oxygen - too low a flow can cause poor slag removal - too high a flow can result in poor cut finish
The typical appearances of a good and poor quality cut surface for manual cutting are shown in Fig.3. The principal features are described together with their cause and remedial measures necessary to produce the ideal square edge, smooth surface cut.




Fig. 3. Best practice guide for hand cutting








Surface AppearanceFeaturesCause
Ideal Cut Profile
Square edge, smooth cut surface, underside free of slag, small drag lines
Cutting Too Fast
Coarse drag lines at angle to surface with excessive amount of slag sticking to bottom edge of plateOxygen jet trailing with insufficient oxygen reaching bottom of the cut
Too high nozzle to plate distance
Uneven cut surface with heavy melting of top edge, coarse drag lines at bottom cut surfacePreheat is not focused on plate surface, oxygen jet easily disturbed
Too High Oxygen Flow
Excessive slag adhering to cut face, local gouging, excessive top edge meltingTurbulence between the preheat flame and the cutting jet

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