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The most common method of cutting steel plate
is thermal cutting, especially oxy-acetylene and plasma. These methods provide an accuracy of +1/8" on length and width, which is suitable for most industrial wear plate applications. For applications requiring tighter
tolerances on cutting and reduced heat affect on the edges, alternate methods are available including friction sawing, water jet cutting and laser cutting. The various cutting methods are described below.
One consideration in selection of cutting method is the effect on the hardness of the edge. The cutting operation creates a heat affected zone (HAZ), similar to that created in welding. Generally, the depth of this zone is related to the amount of heat input into the plate during cutting. On the surface of the cut edge, EB-450 hardens, and effect similar to flame hardening. Behind this hardened zone is a tempered zone where the hardness decreases below the base metal hardness. The depth of the hardened zone varies from about 1/8" to about 3/16". The full HAZ, including the softened region may be up to 1/2" in thick plates.
The EB-450 steel in the HAZ softens because
steel is "tempered" when its temperature rises above about 400°F. This is evident in the graph of hardness versus tempering temperature for furnace tempering heat treatments. This graph show that for short heating times typical of thermal
cutting, that the plate temperature needs to reach about 600°F to reduce the hardness of the steel below advertised hardness.
During the cutting operation the temperature of
the plate edge varies from the melting point at the cutting line to ambient (room) temperature some distance away. The exact temperature profile depends on the travel speed of the cutting torch. AWT conducted a study during a typical oxy-acetylene cutting operation of a 3/8" thick EB-450 plate using tempil
sticks crossing the cutting path ahead of the torch. The melting of the various tempil sticks indicated a temperature profile as shown in the following table.
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Temperature Profile of Oxyacetylene Cutting Operation on a 3/8" thick EB-450 plate |
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Temperature of Plate
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Distance from the cutting line
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Over 475°F
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0.5 inch
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Over 400°F
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1 inch
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Over 350°F
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1-1/4 inch
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Over 300°F
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1-1/4 inch
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Over 250°F
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1-1/2 inch
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Over 200°F
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2 inches
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The study above shows that the steel exceeds 400°F only
within 0.5 inches of the edge, which corresponds roughly to the maximum extend of the HAZ as measured by metallographic examination or hardness measurements. The hardness profile of two 3/8" thick EB-450 plates are shown in the graphs below. Consistent with the tempil stick study, the heat affected zone is restricted to within about 0.5
inches of the cutting edge.
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Thermal Cutting - Oxy-fuel
Common methods include oxygen-fuel and plasma cutting. the common fuel used with oxygen is acetylene because it yields the hottest flame. Other fuels used successfully include natural gas and chemolene Plasma cutting achieves an even higher temperature, which will melt steels without reliance on
combustion of the carbon dissolved in the steel. Paradoxically, the hotter the cutting flame, the less heat is transferred into the plate. The cutting proceeds faster with a hotter flame. Thus the plate is exposed to the flame's heat for a
shorter time.
Prior to cutting, preheating is recommended. A pre-heat of
250°F for plates up to 1 inch thick and of 300°F for plates over 1" is recommended. In warm weather, EB-450 can often be successfully cut without preheat.
Astralloy has had good experience with the following settings:
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Plate Thickness
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Tip Size
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Pressure (psi)
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Travel Speed (ipm)
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Approximate Tip & Plate Distance
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1/8" - 3/16"
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#0
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65
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25-30
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3/16"
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1/4" - 3/8"
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#1
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75
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22
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1-4"
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1/2" - 1"
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#2
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75
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18-20
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5/16"
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1-1/8" - 1-1/4"
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#3
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80
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16
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5/16"
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Thermal Cutting - Plasma
The hottest cutting method is plasma cutting. This method uses
an electric arc, like welding, to heat a gas, such as nitrogen, to a plasma state. The level of heat generated is sufficient to melt the steel plate. Plasma cutting can be specified for applications, such as ballistic test plates, where
full hardness is required to the edge. For industrial applications where the edge will be welded and a HAZ will be obtained anyway, oxyacetylene is entirely
satisfactory.
Water Jet Cutting
Steel plate can be cut by directing a small stream of water,
containing fine abrasive particles, at high velocity. The water cools the plate as the abrasive particles cut the steel; therefore eliminating heat affected zone. Water jet cutting can hold a dimensional tolerance of 0.001";therefore, it can be used in lieu of conventional edge milling and hole drilling. Water jet cutting is not limited by plate thickness; however, cutting speed is reduced as plate thickness increases. for example, cutting speed may be 20 inches per minute for a thin plate, but may only proceed at 1/4" per minute on a 12" plate.
Laser Cutting
Laser cutting is another thermal process, but cuts at such a
high temperature that the HAZ is quite small. Laser cutting can hold tolerances comparable or better than water jet cutting.
Product Information
Product Specifications
Welding Guidelines
Machining
Forming
Heat Treating
Printable Specs Sheet
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