Arc welding: Why is electric arc preferred over fire for welding?

Question

1. Is it because arc has a higher temperature and more controllable?

   Temperature of arc used for welding: 9000°F (5000°C)

   Fire: 400°F- 9000°F (200°C to 4980 °C)

It is difficult to believe a spark causes higher heat than fire. Small arcs at home never seem to produce heat similar to say a match stick. I always see very 'narrow' arcs whereas fire is more spread-out. I might be missing a fair comparison with identical area/volume.

2. How can I roughly figure out the expression for temperature rise?

From wikipedia

In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input.

Once the air/path is ionised, wouldn't its resistance drop and ability to generate heat decrease?

$$(I^2) R \downarrow$$

Source

Answer 1

Arc and glow discharges can generate virtually unlimited power density. Cosider lightning or magnetic confinement fusion reactor heating. Chemical burning has limited reaction temperatures, spreads over wide areas and isn't as quickly controlable. Just inferior across the board unless you want to heat large portions of material, idealy evenly as in cooking, annealing, refining processes.

Temperature rise depends on the thermal impedance of the arc environment which depends on many things. For short discharges you can roughly consider no thermal dispersion and assume the entire arc energy goes into heating the immediate vicinity of the arc..So there is only the specific heat capacity of the material to oppose temperature rise.

Answer 2

We actually use fire, hot gas, for welding about as often as ordinary welding with an arc.

Three broad families can be selected: chemical energy to hot gas - torch, arc to hot gas - plasma cutter, arc to metal - welding.

Why is it that ordinary welding is seen by you more often? Probably because your interest lies in joining pieces of metal, not separating. Both torch and plasma cutter blow fast stream of gas, that blows molten metal away from the joint. It is possible to weld using them, but required control of the jet of hot gas is much more precise than with an arc.

Arc allows to transfer the heat to the molten metal without blowing it away, and leaving a void.

As to your question about temperature and power, it is best estimated using black body radiation, power and area

https://en.m.wikipedia.org/wiki/Stefan–Boltzmann_constant

If temperature is above 200 C you can ignore all other types of loss of heat in normal conditions, like gas convection. And at 2000 C heat loss from the radiation dominates so much that only boiling can match it.

In welding temperature is so hot that it produces UV, temperature is about 10 000 C of the arc itself. So the radiation dominates, and everything else can be ignored if max temp is of interest. Plasma cutter and torch can reach 3000 C, but it is still enough for most metals. So temperature is not an issue, you can use any of them to melt steel.

It is the control that people want. And arc provides this control. And ease of use. No more chaotic stream of gas nearby, so the welding is easier.

About your idea of using resistance, keep in mind that there is a few volts drop just because electrons leave or enter a metal. And that is close to what arc welding has in the first place. So even a perfectly conducting plasma will still heat the metal a lot, because of the energy of the electron that is spent by the power supply and liberated in a form of heat as electron travels the boundary between metal and non-metal.

And thats why welding polarity matters. it says on which side will electron produce this heating.