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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%.


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


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)


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


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


* 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


Deposition rate comparison – DCEP vs Variable Polarity


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


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


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


Deposition rate comparison – Conventional vs Extended


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 comparison – Solid / MC / VP / EE


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


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