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Physics of Electrode Melt-off
Richard Holdren, PE/Senior Welding Engineer
ARC Specialties Applied Automation Workshop – Submerged Arc Welding
March 5, 2013
Introduction
• Submerged arc welding (SAW) is known for its inherent high productivity and quality
– Ability to produce welds at much higher deposition rates than most other processes while still providing welds with excellent soundness and mechanical properties
• While amperage is the primary factor affecting deposition rate, the size and form of the electrode is also an important factor
ARC Applied Automation - Submerged Arc - 2013
Definitions
• deposition rate (DR). The speed with which a welding filler metal is consumed and deposited to form a weld. Deposition rate will be expressed in terms of lb/hr.
• melt-off rate (MOR). The speed with which an electrode is melted to form a weld. Melt-off rate will be expressed in terms of lb/hr.
• deposition efficiency. Percentage of MOR contributing to DR. SAW typically ~99%.
ARC Applied Automation - Submerged Arc - 2013
Factors affecting melt-off rate
• Welding current type
– DCEP – most common
– DCEN – melts electrode more efficiently (roughly double), but arc more erratic and potential for incomplete fusion
– AC – average of the other two. Often employed with tandem and multi-electrode setups
• Current level (amperage)
– Greatest effect – squared function
ARC Applied Automation - Submerged Arc - 2013
Factors affecting melt-off rate
• Electrode resistance
– For a given alloy, dependent upon • Diameter
• Length (electrode extension)
• Form (solid vs. cored)
• For given current type, electrode type and flux type heat for melting electrode basically determined by: I2 x R
– At a given current, electrode resistance determines melt-off rate (and deposition rate)
ARC Applied Automation - Submerged Arc - 2013
Effect of electrode size
• Resistance of a conductor (including an electrode) inversely related to its size
– Therefore, at a given current level, a smaller electrode will yield a higher deposition rate
– Typical deposition rates for carbon steel electrode at 500A
– 5/64 in = 18.2 lb/hr
– 3/32 in = 17 lb/hr
– 1/8 in = 14.5 lb/hr
– 5/32 in = 12.5 lb/hr
ARC Applied Automation - Submerged Arc - 2013
Effect of electrode form
• With solid electrode, entire cross section conducts current
• With metal core electrodes, current is conducted through outer sheath
– Most metal core electrodes have ~20% powder fill, so cross-sectional area of conductor is reduced increased resistance increased melt-off
• At given current, metal core electrodes will increase deposition rate 15-20% ARC Applied Automation - Submerged Arc - 2013
Deposition rate comparison – solid vs metal core
Amperage Diameter, in [mm] Solid* Metal Core* % Increase
400 3/32 [2.4] 11 12.5 14
5/32 [4] 7.5 11 47
500 3/32 [2.4] 13.5 18 33
5/32 [4] 11 14 27
535 3/32 [2.4] 15 21 40
550 5/32 [4] 13 17 31
600 5/32 [4] 15 18 20
700 5/32 [4] 18 22 22
800 5/32 [4] 22 26.5 20
900 5/32 [4] 25 32 28
1000 5/32 [4] 30 40 33
ARC Applied Automation - Submerged Arc - 2013
* Deposition rate in lbs/hr
Effect of power source output
• Conventional SAW power sources provide DCEP output
– Greater melting rate when operating DCEN, however potential for incomplete fusion
• Manufacturers have variable polarity power sources that take advantage of enhanced melting/deposition rate provided by DCEN
ARC Applied Automation - Submerged Arc - 2013
Deposition rate comparison – DCEP vs Variable Polarity
ARC Applied Automation - Submerged Arc - 2013
Amperage Diameter, in [mm] DCEP* VP (50/50)* % Increase
450 3/32 [2.4] 11.8 15.2 29
1/8 [3.2] 10.1 14.2 41
5/32 [4] 9.2 12.4 35
500 3/32 [2.4] 13.1 17.6 34
1/8 [3.2] 11.5 16.3 42
5/32 [4] 11.1 14.7 32
600 1/8 [3.2] 14.8 21.3 44
5/32 [4] 14.7 18.8 28
700 1/8 [3.2] 18.8 26.1 39
5/32 [4] 18.2 23.7 30
800 5/32 [4] 22.1 28.6 29
* Deposition rate in lbs/hr
Effect of electrode extension
• An electrode of a given size and form has a resistance per unit length
• Increasing the length of electrode extending beyond the contact tip increases the resistance in a linear relationship
– Increased resistance preheats the electrode to result in increased melting/deposition rates
• However, arc becomes unstable as columnar strength of preheated electrode is reduced
ARC Applied Automation - Submerged Arc - 2013
TipMate
• TipMate Systems provides ceramic extensions that attach to the contact tip to support the electrode as it extends beyond the end of the contact tip
ARC Applied Automation - Submerged Arc - 2013
Deposition rate comparison – Conventional vs Extended
ARC Applied Automation - Submerged Arc - 2013
Amperage Diameter, in [mm] Conventional * Extended* % Increase
450 3/32 [2.4] 1 in / 11.8 2.25 in / 17.6 49
1/8 [3.2] 1 in / 10.1 2.5 in / 15.1 50
500 3/32 [2.4] 1 in / 13.1 2.25 in / 20.6 57
1/8 [3.2] 1 in / 11.5 2.5 in / 16.3 42
5/32 [4] 1.25 in / 11.1 2.75 in / 13.1 18
600 1/8 [3.2] 1 in / 14.8 2.5 in / 22.5 52
5/32 [4] 1.25 in / 14.7 2.75 in / 18.3 24
700 1/8 [3.2] 1 in / 18.8 2.5 in / 26.8 43
5/32 [4] 1.25 in / 18.2 2.75 in / 26.8 47
800 5/32 [4] 1.25 in / 22.1 3.5 in / 36.5 65
* Deposition rate in lbs/hr
Deposition rate comparison – Solid / MC / VP / EE
ARC Applied Automation - Submerged Arc - 2013
Amperage Diameter, in [mm] Solid Metal Core Variable Polarity EE / TipMate
400 3/32 [2.4] 11.0 12.5
5/32 [4] 7.5 11.0
450 3/32 [2.4] 11.8 16.0 15.2 17.6
1/8 [3.2] 10.1 14.2 15.1
5/32 [4] 9.2 13.0 12.4 12.1
500 3/32 [2.4] 13.1 18.0 17.6 20.6
1/8 [3.2] 11.5 16.3 16.3
5/32 [4] 11.1 14.0 14.7 13.1
600 1/8 [3.2] 14.8 21.3 22.5
5/32 [4] 14.7 18.0 18.8 18.3
700 1/8 [3.2] 18.8 26.1 26.8
5/32 [4] 18.2 22.0 23.7 26.8
800 5/32 [4] 22.1 26.5 28.6 36.5
Summary
• Numerous ways to improve productivity
• It all boils down to physics ---- or how efficiently the electrode can be melted
• To determine the most economic approach, one must compare productivity increase with the cost associated with that change
– Solid to metal core cost of consumable
– Conventional to variable polarity cost of power source
– Use of electrode extension cost of TipMate extensions
ARC Applied Automation - Submerged Arc - 2013