Jim Cuhel

Welding Engineer

Miller Electric Mfg. Co.

Short Circuit Transfer

Time(mS)

Vo

lta

ge

(v

olt

s)

Cu

rre

nt

(am

ps

)

Typical STD MIG Waveform150 IPM .035" S-6

912 914 916 918 920 922 924 926 928 9300 0

6 50

12 100

18 150

24 200

30 250

36 300

42 350

VoltageCurrent

Short Arc RMD

Taking Control

Time(mS)

Vo

ltag

e (v

olt

s)

Cu

rren

t (a

mp

s)

Typical STD MIG Waveform150 IPM .035" S-6

912 914 916 918 920 922 924 926 928 9300 0

6 50

12 100

18 150

24 200

30 250

36 300

42 350VoltageCurrent

Time(mS)

Vo

ltag

e (v

olt

s)

Cu

rre

nt

(am

ps

)

Typical RMD Waveform150 IPM .035" S-6

108 109.5 111 112.5 114 115.5117 118.5120 121.5 1230 0

5 50

10 100

15 150

20 200

25 250

30 300

35 350

40 400VoltageCurrent

What Is RMD?

Current

Voltage

Cu

rren

t (a

mp

s)

Time (Ms)

300 301.5 303 304.5 306 307.5 309 310.50

50

100

150

200

250

300

WET

PINCH CLEAR

BLINK PREDICT

BLINK ARC

BALL

BACKGROUNDPRE-SHORT

dl

Wire

Puddle

Necking Region

neckneckneck r

Tdl

A

TdlR

2

)()(

neckweldneck r

TdlIP

22 )(

Power Density in the Necking Region

Molten Wire

Time

Pow

er in

Nec

king

Reg

ion

Necking Begins

Short Circuit Clears (standard MIG)

Detect Clearing Event

Reduce Current (hence, power)

Short Circuit Clears at Much Lower Power Level (RMD)

Heat Input

dttIe

kTVtItH anodein

)()

2

3()()( 2

Arc Heating TermResistive Heating

How Much Energy is Needed to Burn off the Incoming Wire?

First we need to bring the wire temperature from something near room temp up to the melting point of the wire:

)( ambmelt TTT Temperature Change:

Heat Input Required to Effect Temperature Change:

TMCQ wireT

(where C=specific heat of material and Mwire = Mass of the wire being heated)

Then, we need to supply sufficient energy to cause a phase transformation from solid to liquid (we need to melt it):

wiremeltmelt MHQ (where Hm=latent heat of fusion of material)

)()( meltwiremeltTTotal HTCMQQQ

So, the total* energy required to burn off the incoming wire is:

Putting This Knowledge to Use

Heat In = + I2(t)*βI(t)*α

ARC ON Little ‘l’

Little ‘l’

I(t)

dte

kTVtItH anodearcin

)

2

3()()(

I(t)*α

I2(t)*β1/2”

Σ

-Jset

Arc Heating

Resistive HeatingJset = (Heat in @ ½” Stickout)

Error Term

ARC OFF

Jset

Actual End of the Wire

Virtual Wire

Holds info from the last i2Rdt

Holds info from the second to last i2Rdt

Holds i2Rdt info from 128 samples ago

History Array (microprocessor world)

12

3

Holds i2Rdt info from 127 samples ago

Holds i2Rdt info from m samples ago

m

m+1

m-1

128127

l

Wire Segments (Real World)

Each array element represents .01” of wire(1.28” total)

dt=1/(100*v(in/sec))after each time interval (dt), each array element shifts down one position. The last one gets dumped and an empty one is inserted on top

Keeping Track of Wire Heating History

Determined by controller based upon current feedback. Calibrated to read out in units of inches times 100. This tells us how far back in time to go with our summing of array elements

N

j

j dtiContentHeat R1

2**_

Typically, there are multiple array elements per wire segment

100*__ inchesinlengthN

Tball

Width of ball pulse is adjustedin response to the heat input in the wire

Duration of Ball Phase is Modified Based Upon Heat Content of Wire

Time(mS)

Vo

ltag

e (v

olt

s)

Cu

rren

t (a

mp

s)

Typical RMD Waveform150 IPM .035" S-6

108 109.5 111 112.5 114 115.5 117 118.5 120 121.5 1230 0

5 50

10 100

15 150

20 200

25 250

30 300

35 350

40 400VoltageCurrent

Stick out variation video

Constant Voltage GMAW Comparison

RMD: 0.035” ER70S-6 on 8” Sch. 80

Conventional GMAW

Establishing Good Technique

• As with any welding process, success with RMD process requires establishing and maintaining good preparation and welding techniques.

• The following guidelines lead to proven success and increased productivity for welding pipe

Joint Configuration

• Standard 75 degree included angle• Land: 0” – 3/32”• Root Opening: 1/8”

Five Critical Items For Stainless Steels• The techniques for welding carbon are the

same for stainless alloys• To qualify procedures for welding 300 series

stainless steel piping – Without backing gas, fabricators should do the following:1.) Ensure a minimum 1/8” gap around the entire

circumference of the joint. This gap allows the shielding gas to flow through to protect the backside of the joint from oxidation

Five Critical Items Cont.

2.) Clean the pipe both inside and out to remove any contaminates or unwanted substances. Use a wire brush to clean at least 1 in. back from the edge of the joint

3.) Use only a stainless steel wire with a high silicon content, such as 316LSi or 308LSi. Higher silicon contents helps the puddle wet out and acts as a deoxidizer

Five Critical Items Cont.

4.) For optimum performance, use a “Tri-H” gas that’s 90 He/ 7 ½ Ar/ 2 ½ CO2

Alternatively, use 98 Ar/ 2 CO2

5.) For best results, use a tapered nozzle for the root pass because it localizes the shielding gas coverage. Tapered nozzles with built-in gas diffusers provide exceptional coverage

Thank You

Any Questions