Technical Information - Fabrication (Cutting)

FABRICATION - CUTTING


Stainless steels are highly alloyed materials, the various types possess different mechanical and physical properties. Further, these properties are, in most instances, vastly different from low carbon (mild), medium carbon and low alloys steels, with a corresponding effect on the cutting methods and procedures.
It must be emphasised that the information and recommendations given here serve as a guide to aid in the cutting of Stainless Steels. Many of the common problems may thus be avoided. Procedures and results which have been successful in actual practice should be adhered to. Experience, type and condition of the equipment utilised may indicate slight change of modification to the information given in this article.


SAWING
· High quality blades of High Speed Steel should be used. Sharp teeth are essential.
· An emulsion of soluble oil is used as a cutting fluid. More diluted emulsions are needed for cutting Austenitic (300 series) steels to improve the cooling rate.
· All grades of Stainless Steels, both wrought and cast, can be sawn.
· The sawing of Austenitic grades (300 Series) is made more difficult due to their tendency to work harden. In cutting these grades the cut must be initiated without any riding of the saw on the work, a positive feed pressure must be maintained, and no pressure, drag or slip should occur on the return stroke.


HAND HACKSAWING
Generally used for random cutting of light gauge material, small diameter bar, tube and pipe. A blade with a wavy set is preferable. For thin gauge sheet and thin wall tubes a fine 32 teeth per 25mm blade is necessary. As the thickness of the material being cut increases, the coarseness of the blade should be increased to 24 teeth per 25mm.

POWER HACKSAWING
Cutting fluid should be flooded on the cut to maximise the cooling, particularly in cutting the Austenitic grades.
More than one tooth should be in contact with the work at all times. This necessitates small pitched blades for cutting thinner gauges and small diameters. As the material thickness or diameter increases the tooth spacing should increase to give better clearance and to minimise chip packing:
Up to 6mm thick/diameter 10 teeth per 25mm
6 - 20mm thick/diameter 10-8 teeth per 25mm
20 - 50mm thick/diameter 6 teeth per 25mm
over 50mm thick/diameter 4 teeth per 25mm


SHEARING
Stainless Steels have greater strengths than low carbon (mild) steels.
Further the tendency of the Austenitic grades to work harden has a significant effect on the shearing of these steels.
More power is therefore required, and it is necessary to derate the shears (guillotines) against their nominal capacity, which is usually given in terms of the thickness of low carbon (mild) steel which they are capable of shearing.
Indicative relative derated capacities are as follows:
Low carbon (mild) steel 10mm thick material
Corrosion Resisting Steel (3CR12) 7mm thick material
Ferritic Stainless Steel (eg 430) 7/8mm thick material
Austenitic material (eg 304) 5/6mm thick material
Corrosion resisting (3CR12) and Ferritic Stainless Steels tend to fracture after being cut through approximately half their thickness. In this respect they are similar to carbon and low alloy steels.
Austenitic Stainless Steels are typified by a high ductility, and hence a greater resistance to fracture. A greater degree of penetration therefore takes place before fracture occurs. The clearance setting of the blades is therefore important. For shearing thin gauge sheet a clearance of 0.025 to 0.050mm is suggested.
Closer clearance tends to induce blade wear, whereas larger clearances allow the material being sheared to drag over to an excessive degree, resulting in excessive wear of the blades and a poor cut.
As the material thickness increases the clearance should be increased accordingly and adjusted to best suit the specific piece of equipment being used, consistent with minimum roll over, burr height and distortion (camber, twist and bow).
The nominal suggested clearances for such thicker material are:
3CR12 Corrosion resisting steel 2.5% of material thickness
Ferritic/Austenitic Stainless Steels 3 - 5% of material thickness
To counteract the greater shearing force required, the hold down pressure on the clamps may have to be increased, particularly when shearing the Austenitic grades.
The higher power requirements can to some extent be countered by altering the rake/shear angle. A rake of 1 in 40 is a shear angle of approximately 1½ °. This is the suggested least rake which should be used. Small rake/shear angles necessitate higher power/force, but cause less distortion, whereas larger rakes/shear angles (eg 1 in 16 or 3½ °) reduce the power/force required, but need higher hold down pressure on the clamps and tend to increase distortion.
Blades MUST BE SHARP. Blunt blades increase the roll over, burr height and distortion (camber, twist and bow).
The moving blade should be provided with as large as possible back clearance/rake angle, without causing chipping of this blade.


ABRASIVE CUTTING
Abrasive discs, rotating at high speeds, can be used for both cut-off operations on relatively small section sizes, and for straight line cutting of sheet and thin plate material. * (The cutting of large radius curves may also be undertaken).
It is therefore a useful method for cutting thinner thicknesses to length (or to a mitre), and for making cuts of limited length on the shop floor during fabrication.
The use of Aluminium Oxide (Alumina) discs is recommended.
Cut-off operations are normally done wet, using a soluble oil emulsion. Rubber-based discs are used.
Random straight line cutting of sheet and this plate is normally done dry. Vitrified or resinoid-bonded discs are used. Care must be exercised not to induce excessive over-heating of the cut edge.
Dedicated discs (i.e. uncontaminated by cutting of other material) must be used.
Random cutting done by hand must employ safety measures, as the discs can jam and break in the cut groove.
* Note: Straight line cutting of thick plate (from 20 - 100mm thick) can be accomplished by abrasive cutting. This necessitates the use of high cost, specialised equipment.


THERMAL CUTTING
In conventional Oxy-cutting the metal is first heated by the flame, then an excess of oxygen is supplied. This causes exothermic (heat generation) reactions which generate the heat necessary to melt the oxides formed, which are then removed from the cut by the velocity of the gas jet.
Stainless Steels having a high level of Chromium (Cr) cannot be cut by simple oxy-cutting methods due to the refractory nature (very high melting point) of the Chrome Oxide which is formed.
Modified or other methods therefore have to be employed.


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