Using plasma to pierce metal plate, rather than oxyfuel, is most desirable as it means faster piercing times, faster cut speeds, and a cleaner finished product. However, despite plasma’s many benefits, some companies find piercing thicker material, over 1 1/4 in., difficult with plasma.
Today, thanks to recent improvements in plasma torch and consumable
design, the piercing capabilities of plasma are significantly better.
When piercing with a plasma torch, the plasma arc attaches to the top surface of the plate and transfers enough energy to melt the metal near the top. This molten material must then be removed, usually accomplished with the non-current carrying cold gas and the plasma shielding gas. As this molten material is removed, the arc transfers the energy to the bottom of the pierce hole and melts deeper into the plate.
However, as piercing takes place and the hole becomes deeper, three limiting factors impact the process. The first is associated with the energy transfer to the bottom of the hole. This transfer of energy is reduced as the hole becomes deeper and the arc transfers its energy not only to the bottom of the hole but to the sides as well, enlarging the hole near the top of the plate and slowing the rate of pierce progression. As the hole becomes deeper and wider, the distance between the torch and the work-piece lengthens, increasing the arc voltage and the chances of the arc going out. Even if the power supply has enough voltage to maintain the arc, the longer pierce times mean the torch is kept over the hot molten steel for a longer period of time, which begins to melt the consumables, particularly the shield.
The second limiting factor is associated with the fluid dynamics of removing the molten material from the hole. Cold plasma gas and shield gas are supposed to blow the molten slag out of the hole and away from the pierce. However, as the hole becomes deeper, this becomes more difficult. As a result, gas flow tends to puddle at the bottom.
The third and most significant factor is the effect of the molten material coming out of the pierce hole. Much of it winds up on the end of the torch. As the torch sits directly over the metal being pierced, heat and molten metal travel back to the torch. As the temperature of the torch, particularly the shield, increases, molten material more readily adheres to it. This transfers even greater levels of heat into the shield, creating a continuously increasing condition of slag adhesion and heat buildup, eventually negatively impacting pierce capability and cut quality.
Another problem is that any molten metal that doesn’t wind up on the torch, often winds up on top of the plate. This significant puddle, which usually forms on the top surface of the plate around the edge of the pierce hole, can cover a large area of the plate and be quite thick. If the torch runs through this slag after it has begun to solidify, damage will occur to the tip of the torch (usually the shield). To avoid this problem, a sufficiently long lead-in to the cut, preventing the torch path from crossing the slag puddle, is required. A lead in equal to the material thickness being cut is usually recommended.
The first limitation of piercing thick metal plate or the energy transfer to the bottom of the hole is primarily addressed through the HPR XD architecture. The second limitation with regards to the removal of molten material from the hole, can be addressed through various piercing techniques. Stationary piercing, low transfer/stretch arc piercing, moving and double piercing techniques can all help the removal process.
The third factor is primarily associated with the molten metal that gets blown onto the torch and consumables, especially the shield. There really is no way to keep molten metal from hitting the torch. However, engineers at Hypertherm discovered they could reduce the metal’s tendency to stick to the shield by lowering the shield’s temperature. The engineers decided to employ a closed loop cooling system which directly contacted a flange on the back side of the shield.
During testing, the engineering team used water as a coolant, controlling its temperature with a chiller/heater. During testing, the engineers employed three different temperatures: 38°F, 85°F, and 135°F. The testing involved piercing 1 1/2 in. thick mild steel plate with the HyPerformance HPR400XD. The plan was to use the 400 amp oxygen system to make 300 pierces.
The results of the test were dramatic. When the shield temperature was maintained (via the coolant) at 135°F, the sum of the slag through the duration of the test was 198 grams. At 85°F, the team saw a small but significant drop measuring 175 grams of slag after the 300 pierces. The big change came when the shield temperature was dropped to 38°F. At that point only 31 grams of slag was measured.
By incorporating this technology, called PowerPierce, into plasma torch and consumable design, the piercing capabilities of plasma are significantly extended. Hypertherm estimates it is now possible to production pierce mild steel up to two in. thick, opening up the benefits of plasma to more companies.
This article was supplied by Hypertherm Inc. (hypertherm.com), Hanover, NH.
When piercing with a plasma torch, the plasma arc attaches to the top surface of the plate and transfers enough energy to melt the metal near the top. This molten material must then be removed, usually accomplished with the non-current carrying cold gas and the plasma shielding gas. As this molten material is removed, the arc transfers the energy to the bottom of the pierce hole and melts deeper into the plate.
However, as piercing takes place and the hole becomes deeper, three limiting factors impact the process. The first is associated with the energy transfer to the bottom of the hole. This transfer of energy is reduced as the hole becomes deeper and the arc transfers its energy not only to the bottom of the hole but to the sides as well, enlarging the hole near the top of the plate and slowing the rate of pierce progression. As the hole becomes deeper and wider, the distance between the torch and the work-piece lengthens, increasing the arc voltage and the chances of the arc going out. Even if the power supply has enough voltage to maintain the arc, the longer pierce times mean the torch is kept over the hot molten steel for a longer period of time, which begins to melt the consumables, particularly the shield.
The second limiting factor is associated with the fluid dynamics of removing the molten material from the hole. Cold plasma gas and shield gas are supposed to blow the molten slag out of the hole and away from the pierce. However, as the hole becomes deeper, this becomes more difficult. As a result, gas flow tends to puddle at the bottom.
The third and most significant factor is the effect of the molten material coming out of the pierce hole. Much of it winds up on the end of the torch. As the torch sits directly over the metal being pierced, heat and molten metal travel back to the torch. As the temperature of the torch, particularly the shield, increases, molten material more readily adheres to it. This transfers even greater levels of heat into the shield, creating a continuously increasing condition of slag adhesion and heat buildup, eventually negatively impacting pierce capability and cut quality.
Another problem is that any molten metal that doesn’t wind up on the torch, often winds up on top of the plate. This significant puddle, which usually forms on the top surface of the plate around the edge of the pierce hole, can cover a large area of the plate and be quite thick. If the torch runs through this slag after it has begun to solidify, damage will occur to the tip of the torch (usually the shield). To avoid this problem, a sufficiently long lead-in to the cut, preventing the torch path from crossing the slag puddle, is required. A lead in equal to the material thickness being cut is usually recommended.
The first limitation of piercing thick metal plate or the energy transfer to the bottom of the hole is primarily addressed through the HPR XD architecture. The second limitation with regards to the removal of molten material from the hole, can be addressed through various piercing techniques. Stationary piercing, low transfer/stretch arc piercing, moving and double piercing techniques can all help the removal process.
The third factor is primarily associated with the molten metal that gets blown onto the torch and consumables, especially the shield. There really is no way to keep molten metal from hitting the torch. However, engineers at Hypertherm discovered they could reduce the metal’s tendency to stick to the shield by lowering the shield’s temperature. The engineers decided to employ a closed loop cooling system which directly contacted a flange on the back side of the shield.
During testing, the engineering team used water as a coolant, controlling its temperature with a chiller/heater. During testing, the engineers employed three different temperatures: 38°F, 85°F, and 135°F. The testing involved piercing 1 1/2 in. thick mild steel plate with the HyPerformance HPR400XD. The plan was to use the 400 amp oxygen system to make 300 pierces.
The results of the test were dramatic. When the shield temperature was maintained (via the coolant) at 135°F, the sum of the slag through the duration of the test was 198 grams. At 85°F, the team saw a small but significant drop measuring 175 grams of slag after the 300 pierces. The big change came when the shield temperature was dropped to 38°F. At that point only 31 grams of slag was measured.
By incorporating this technology, called PowerPierce, into plasma torch and consumable design, the piercing capabilities of plasma are significantly extended. Hypertherm estimates it is now possible to production pierce mild steel up to two in. thick, opening up the benefits of plasma to more companies.
This article was supplied by Hypertherm Inc. (hypertherm.com), Hanover, NH.
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