welders---what is peening?

It is still imperative to understand the effects of introducing the heat generated by welding into any given base material. Cast iron is one of the more complex substrates. The effects on cast iron of heat, hydrogen induced cracking, and transformation of grain structure, can create problems that peening or vibratory stress relief cannot overcome. My point is that expansion and contraction is not your only adversary at times.

You see, the HAZ just below the weld deposit consists primarily of eutectiferous carbides, martensite, and bainite. The result is a metallurgical notch developing stress in a straight line. Peening or vibratory stress relief are but two ways in which a solution to those effects may be approached.

Inherently, flake type cast iron substrates are largely lacking in ductility. These types make up the majority of castings encountered in the field. They exhibit an inherent inability to absorb the residual stresses created by base metal vs. weld metal grain growth and contraction. Incorrect weld metal chemistry may be a contributor that will lead to catastrophic failure of the substrate, the weld deposit, or both. This is most common in applications involving highly restrained joints or applications requiring larger depositions of filler metal material.

Lack of knowledge pertaining to proper pre/post weld preparation or excessively high heat input can be detrimental to overall success. When attempting to control overall heat input, knowledge of flux composition and chemical effects is key. High heat inputs (concerning the use of SMAW electrodes) can be limited by utilizing the advantages of electrodes containing barium, strontium, or caesium. These elements allow the impartation of high thermionic emissivity at arcing temperature levels. Operating arc amperages and heat input to the base material are thus reduced by way of the increased ionization across the arc gap.

It is not possible to delve into all the aspects in the space of this forum. Hopefully this will be enough to plug that hole or at the very least result in a slow drip.
Steve

As a weld cools it shrinks a very small amount at a faster rate than the surrounding base metal. By peening the weld your stretching the weld face slightly so it more closely matchs the base metal contraction.

Normally with a good joint design, peening is not required. So what is a good joint design? This can discussed at length so I'll only cover the basics with a couple examples.

Thin gauge metal when butt welded has to have the correct root opening. If both pieces are butted together tight, tacked then welded, the joint will heave in the center of the weld as it's cooling creating a small "V" shape in the base metal. Now you could peen this bead and get the metal to lay flat but in doing so it will create stress pockets out of the weld zone thus creating a bubble in the base metal. Why? Your expanding two different thickness, the base metal and the weld bead. The weld bead being thicker will add force on the base metal thus creating the bubble. All metal thickness requires this root opening not just lite gauge.

Take that same butt joint design with a root opening 1/2 of the base metal thickness, tack at 25 times the base metal thickness for between tack centers, then weld, and now it will lay flat after it's been welded. Why? The root opening allows for the weld puddle to draw the two base metal pieces together and just touch as the weld cools. This joint would not require any peening.

Keeping with this thought, why peen a weld on castiron? Here we have two different metals joining together. The castiron having a very open grain structure where as the weld metal has a tighter grain structure so they pull against one another just like the thin gauge metal did. Here if we lightly peen the weld bead as the castiron cools will allow for the expansion of the weld bead to closer match that contracting rate of the castiron thus resulting in a weld with less stress. By covering the compeleted welded joint and letting it cool slowly also helps relieve the stress. If not peened or heat treated then theres a greater chance for the weld to crack away from the base metal.

T_Bone

Yes that is the root opening.

Root weld= the first bead of a weldment.

Root opening= the gap between the two base materials.

Open Root weld= The first bead without the aide of a root backing plate.

Root Backing Plate= A plate that covers the open root on the opposite side from where the weldment is placed.

The usual maxmium root opening allowed is upto 1/8" on a open root weldment.
Most welding engineers will spec a root backing plate when the root opening is wider than 1/8".

T_Bone

I took a 1-1/2" Copper? drain/waste/vent pipe, about 4" long, and flattened it in the press. I put it under the starting edge of end welds and begin welding about 1/8" out from the edge. This seems to work ok. Is this the same kind of plate material that's used with open root welds over 1/8"? How does one determine if the roots should be ground at 45 degrees or just left as straight sides?
Slowpoke

What you are refferring to is called a chill bar. I have one thats 1/2"x 2" x8" CU for backing SS & AL and one 1/2"x2"8" AL that I use for mild steel. Both will work with most metals. I also had one that had V cut into the center to use as a purge block with a cover gas but someone decided they needed it more than me. It can be used for wide root openings but it was not what I was talking about as you can get oxygen into the molten puddle with some processes as it doesn't fit tight to the base metal.

A backing plate is 1/4"x 1" FB (flatbar) that attachs to the back of the base metal and becomes part of the weldment. You penetrate deep into the root and fuse the backing plate into the root land of the weld joint. On a certification test after the weld is completed, the backing plate is cut flush and polished with the back of the base metal before the cupon is tested.

Any thickness over 1/4", AWS code calls for a 30* bevel or a minmium 60* V opening .

Depending on the joint design requirement, I use bevels on less than 1/4" if I'm wanting a fairly flat surface finish and close too 100% penetration. Alot depends on the tpye of electrode that I will use, joint requirement and design strength. On other joints I will gap open 1/16", tack, then weld if strength or surface finish is not of concern.

T_Bone

what about cooling a weld with h2o quickly after the flux is chiped off? does this make the weld stonger but more brittle?

Peening of welds on cast iron is very important as the weld and cast iron contract at different rates causing built up internal stress and possible furtur cracking. Below is a link to two guys garage, look up show TGG102 in archives, an expert from Lincoln welders shows how its done.

• Cast iron welding