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Hot welding

In hot arc welding the part must first be evenly heated to approximately 500°C. Especially if the casting to be welded has any appreciable material thickness it is important that the preheating proceeds slowly. Cast iron has a low coefficient of thermal conductivity, and demands care in preheating. Too quick heating can cause tension cracks. The temperature must be maintained throughout the welding operation.

As the means for preheating on board will normally be limited to the welding torch, it means that only smaller parts, and which can be dismantled and brought into the workshop can be hot welded on board.

When hot welding cast iron the temperature of the workpiece must be

Arc welding of cast iron

Below is a short description of the cast iron electrodes UNITOR NICKEL-333N and UNITOR NIFE-334N to help you select the correct electrode for the work. Complete data on these electrodes are found in the section

“Technical Data for Consumables”.

Remember that cold welding of cast iron can only be done by electric arc welding.

UNITOR NICKEL-333N

For use on old, oil-impregnated cast iron and on thin material dimensions.

Use this electrode to “butter” the sides of oily cast iron to seal the surface. Then finish the filling-up to join the parts together with UNITOR NIFE-334N, which has greater tensile strength. Do not deposit more than maximum two layers for NICKEL-333N.

UNITOR NIfE-334N

To be used on cast iron that takes strain, vibrations and sudden loads.

Also to be used for joining cast iron to steel, copper alloys and stainless steel. NIFE-334N is used for multi-bead welding on heavy gauge material. It has greater tensile strength than NICKEL-333N.

Electrode binding test

Grind off a small area of the material close to the welding zone. Select a 3. mm electrode of NICKEL-333N and NIFE-334N, and deposit a 4-5 cm bead with each of them without weaving.

Use 100 Amps and the correct polarity. Use a hammer and chisel to remove the beads. The bead which exhibits the least porosity on the

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monitored by means of temperature indicator crayons or by an electronic thermometer. It is important that the temperature does not exceed 600°C, as this may result in structural changes in the base material, which may drastically reduce its strength.

Opposed to cold welding, hot welding can proceed continuously.

The finished welded part must then be allowed the slowest possible cooling down to room temperature.

Burying the piece in kieselguhr, sand or cinders will help to give it a slow cooling rate. The reason why slow cooling is important is that if the rate of cooling is high, the carbon will have no time to segregate as graphite in the crystal border areas. We will instead have the carbon bound to the iron in the form of iron carbide. This structural state will be very similar to that of white cast iron; hard and brittle.

Cold welding

If the necessary equipment for preheating or for achieving the required slow cooling rate is not available, the alternative is "cold"

arc welding. The method is so called because of the low heat input to the base material when correctly executed.

On board cold arc welding is by far the most commonly used method, and large cast iron parts, or parts which are difficult or time-consuming to dismantle should be cold welded.

Amperage setting

When cold welding cast iron a low ampere setting should be used.

Thereby deep fusion between the filler material and the base material is avoided. It should be remembered that deep fusion will dig up and bring into the weld pool more graphite than need be. This graphite will give rise to iron carbide, with resulting hard zones when the weld cools off. Deep fusion will also put lots of unwanted heat into the base material with increased risk of cracking.

In order to avoid digging into the base material, and to reduce the heat input, one can also point the electrode 10° in the direction of travel. This will cause the molten pool to blow forward and act as a cushion.

Do not overdo low amperage setting.

The molten pool must be clear of slag, and easy to control. If the deposit has a high bead contour, the setting is too 2.07

Smaller parts, free to expand, may be hot welded. NB: Allow to cool off slowly!

Large, complicated parts should be cold welded.

SOLUTIONS: Cast Iron

attached to the workpiece itself if possible.

Choice of polarity

If the current source for the welding is a welding rectifier (DC-machine), the choice of correct polarity will influence very much on the result of the work. Some electrode manufacturers make cast iron electrodes that can only be used with negative polarity. Furthermore, some years ago the general low quality of cast iron electrodes made it necessary to use negative polarity to achieve binding between base material and weld deposit.

However, the quality of Unitor's cast iron electrode is very high, and the electrodes can be used with both polarities without difficulty.

The operator must be aware of the effects of the different polarities, as the heat input and the melting of the base material varies considerably according to the polarity selected.

low. If there is excessive spatter and undercutting, the amperage is set too high.

For UNITOR NICKEL-333N and UNITOR NIFE-334N amperages should be set approx. 10% lower than those used with conventional cast iron electrodes. Recommended amperages are as follows:

UNITOR NICKEL-333N

The above settings are approximate.

Setting will vary with the size of the job, type of machine, line load, etc. It is recommended practice to select a setting halfway between the figures and make a trial weld. NICKEL-333N has high ohmic resistance. If ampere setting is too high the electrode may become red hot.

In general, the welding current should be as low as possible, consistent with easy control, flat bead contour, and good wash at the edges of the deposit. The amperages listed are for flat or downhand welding positions.

Reduce the amperage range by 5-10%

for overhead welding, and about 5%

for vertical welding.

To ensure good electric conductivity the return cable clamp should be

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Desired Bead Contour.

Reverse polarity (Anode) Straight polarity (Cathode)

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If the electrode is connected to the minus pole of the machine (straight polarity) we get high, concentrated heat input to the base material. This will cause excessive melting and digging into the material. In addition to contaminants such as phosphorous and sulphur, cast iron contains quantities of the gassed nitrogen, oxygen, carbon dioxide and carbon monoxide. Excessive melting will bring unwanted quantities of these impurities into the weld. The more impurities contained in the base material the lower the quality of the weld. The high heat input will also cause the formation of iron carbides, with hard zones in the weld and the heat affected area.

If we connect the electrode to the plus pole (reverse polarity) we get a wide, shallow weld zone with minimal amounts of graphite, phosphorous, sulphur and gasses. The low heat input till reduce the formation of iron carbides.

When welding with DC, reverse polarity to the electrode should be the first choice. However, in cases where the cast iron is heavily contaminated and has poor weldability, straight polarity may be tried in the first run in order to use the higher heat input and melting to achieve bonding between the base material and the weld deposit.

Length of arc

To reduce the voltage across the arc, and to help minimize heat input into the base material, the shortest practicable arc should be maintained.

By experience it may be found that the first pass should in some cases be done with a somewhat longer arc than the following runs. This would be

in cases where the composition of the base material makes good bonding more difficult, and the firs bead has to be "painted" on to it.

Correct size electrode

Always use the largest size diameter electrode that the groove can accept (but not at the expense of not getting down into the groove!). Using a large size diameter means that you reduce the heat input in relation to the amount of filler metal deposited. F.

inst. a 4,0 mm electrode deposits four times as much weld metal at only two times as much amperage compared to a ,5 mm electrode. However, if the first bead shows porosity, a small diameter electrode, low amperage setting and high welding speed should be used for this run to reduce heat input.

The welding

Remember that cast iron is very brittle, with only 1– % elongation.

Should the shrink forces exceed the tensile strength of the cast iron it will crack. Hence, when welding it is crucial to the success of the operation that the heat input to the base material is kept at a minimum to avoid cracking.

The way to achieve this is to avoid putting down long, continuous beads (as when welding mild steel), and instead to weld short, straight stringer 2.07

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SOLUTIONS: Cast Iron

beads of maximum length 0–30 mm (1") at a time. Weaving should be avoided or kept to the minimum required to "wash out" the deposit and to catch the sides of the groves.

Do not wave in excess of one-half electrode diameter to each side of the direction of the weld. When each bead of 0–30 mm has been deposited, fill the crater and withdraw the electrode a little backward on the bead before breaking the arc.

While the bead is still hot, peen it with a round-nosed peen hammer.

Since the casting is quite rigid, the filler metal must be ductile. Peening the bead immediately after depositing will stretch it to accommodate some dimensional change in the weld area, and will provide some stress relief.

Always peen from the crater back to the starting point. Use rapid moderate blows; just hard enough to leave a slight indent on the weld deposit is usually sufficient. Too heavy blows may cause cracks, while to light peening will have little or no effect in relieving stresses.

On thin dimensions the peening should not be vertical on the bead, as vibrations will reduce the effect:

After the initial bead has been deposited and peened, the next bead should not be put down until the bare hand can be laid alongside the

R = 3 mm

Recommended shape of head.

Rounded edge

Weld contraction may cause cracks.

Peening offsets contraction forces.

Wrong Correct

first bead with comfort. If you burn yourself, it is too hot to go on with the welding. Take your time, do not spoil your work by trying to rush it.

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Sometimes more than one short run can be done at a time, but only when the length to be welded is considerable, and the beads can be

spaced well away from each other to prevent heat build-up. This technique is called skip-welding and speeds up the work considerably:

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Repairing crack with free end Start welding at tied end, then backstep towards free end.

Repairing crack with tied ends Put the two first beads ( - 3 cm each) starting from the hole drilled at each end of the crack. Then weld alternatively from each end of the crack, using the backstep technique.

Repair of separated parts Starting from one edge, use the backstep technique across the part to the opposite edge.

Parts subject to high stresses To strengthen the transition zone transverse grooves may be cut in the sides of the welding groove, using chamfering electrode UNITOR CH-

38. First fill up these grooves with the electrode selected for the first layer, and then cover the entire surface of the groove with a deposit of the same electrode, without peening.

Then proceed to fill up the groove.

Remember the build-up should always be done with UNITOR NIFE-334N

Cut transverse grooves in the sides. Fill them first, then cover the entire surface before proceeding to build up and join.

SOLUTIONS: Cast Iron

cracks. Grind the edges of the casting at an angel of 45° halfway down its wall thickness. Remember to file the ground area to remove surface graphite. Seal in the contaminants, using UNITOR NICKEL-334N. Using a piece of cardboard cut a template to obtain the correct shape for the steel patch.

Select a piece of low carbon steel, one half the thickness of the casting's wall. Using the template as a guide cut the patch out of the piece of steel. It should fit the hole with a clearance of  - 3 mm. Grind the edge of the patch 45°. The patch should be "doomed" slightly by striking with a ball-peen hammer until a bulge is formed.

Place the patch in the hole and secure it in position but do not tackweld it. As this will prevent the patch from expanding to heat. Now weld the patch to the casting, using the backstep technique as indicated in the sketch. Be careful to let each bead of  - 3 cm cool off before proceeding with the next bead. Use UNITOR NIFE-334N for the build-up.

A way to reduse shrinkage stress In order to reduce the shrinkage stresses, do not put down a complete root bead first, before proceeding to build up. "Stack" the beads stepwise as shown in the sketch, advancing each bead (starting always with the root bead)  - 3 cm forward at a time.

Take care to keep the heat down (hand warm before proceeding each new bead!) and hammer each bead while still hot. A small pneumatic hammer is suitable for this kind of hammering.

Steel insert technique

It is sometimes desirable to insert a steel patch in the centre section of a large housing or motor block. A gear may have shattered and pushed out an area in the housing. A frozen block may have suffered the same damage.

Usually the pushed-out section has been broken in so many pieces that it is not practical to try to join the pieces. The least expensive method or repairs is to fit a steel patch into the hole. This is done as follows:

Remove all damage material on the casting. Avoid leaving sharp corners, round off wherever possible. Clean off dirt, paint, oil and grease from the area where the repairs is to take place. Use a Crack Detection kit to make sure there are no hidden

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Hammering

Stacking

Bevel edges to 45°.

Casting steel-patch

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Build up of missing section Occasionally, it will be necessary to rebuild parts where sections have been worn down or broken off. In explaining the salvage process a broken-off cog on a wheel is taken as an example.

Cut stress-relieving grooves in the area to be built up, using UNITOR CH- 38. If the area is large enough to accept more than one groove, there should be a minimum of 6 mm (1/4") between the groves. Do not use a grinder for cutting stress relieving grooves. The groves will not be geometrically correct for the dispersion of stresses from the casting surface.

Fill in the grooves. Then "paint" on weld metal by using a long arc and fast zigzag movements. Apply thin deposits (1,5 mm) to seal in all contaminants. Do not quench.

Built up section, use stringer beads.

When repairing defects in raised areas, such as bosses, which must be machined after welding, it is unwise merely to chip out the defects and fill the defective areas. It is better to prepare the area for repair by machining to a dept slightly below the desired final surface. Then build up the surface  mm (1/16") above the required dimension to allow for machining. All machining will then be done on the solid weld metal deposit, which is fully machinable.

Alternatively the parts may be rebuilt by braze welding, using UNITOR Castiron 68 S or UNITOR FC-Wearbro 6.

filling holes that penetrate clear through the casting

Begin to weld at one side and run straight stringer beads side by side until the hole is closed. Weld from one side only. Do not alternate sides or weld around the edges. Such action will create stress cracks. Peen each weld pass lightly to reduce shrinkage stresses. Remove slag.

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SOLUTIONS: Cast Iron

In braze welding the cast iron is heated to bonding temperature. This is the minimum temperature to which it must be heated to form a bound (surface alloy) between the filler metal and the cast iron. The phenomenon, which then occurs, is called "tinning" (also called

"wetting out" or "bonding"). Actually, it refers to the almost microscopically thin layer in which the alloys of both the cast iron and the molten filler metal intermix.

While there is several processes for braze welding, the use of an oxy-acetylene torch to provide the necessary heat will be the method employed on board.

Preparation of the workpiece Remove oil, paint and rust from the surface. Grind off casting skin to a width of 0 mm on each side of the edges to be welded. If the damage is in the form of a crack in the material, it is recommended to use the crack detector set to find the complete extent of the crack. Drill 3 mm holes at each end of the crack, at a distance of about 3 mm from the ends to prevent the crack from opening further during brazing.

Groove preparation

Prepare a 70°–90° groove. Remember that braze welding is a surface bonding, and that the lager the surface of a joint the better the bonding. If the part has been broken into two or more pieces, bevel each side of the fracture to 45°.

Use a file to remove surface graphite from the groove, and round off edges and sharp corners.

Braze welding of cast iron General

The name braze welding comes from the relatively slow-flowing nature of the filler material, which gives the joining process much in common with ordinary gas welding.

A condition for using braze welding on cast iron is that it must be possible to preheat an area on each side of the weld zone to 300–400°C. In practice this limits braze welding to smaller parts and thinner dimensions. For larger components "cold" electric arc welding is recommended.

In braze welding the cast iron is not melted, and braze welding is thus a form of mechanical bonding, as opposed to gas welding, where the parent metal is melted and forms a chemical bonding with the filler metal.

The use of braze welding on cast iron has the decided advantages of low heat and ductility, both of

The use of braze welding on cast iron has the decided advantages of low heat and ductility, both of

In document Elvira Moreno Moreno (página 164-170)