Welding of Cast Iron

Cast iron is an alloy of iron with carbon (more than 2,11-2,14%), which usually also contains silicon (3%), manganese (1%), sulfur, phosphorus, and may contain alloying elements - chromium, nickel , vanadium, aluminum, magnesium, iron, etc., without alloying or heat treatment - rather brittle material with low strength, hardness and ductility.

cast iron parts

Carbon may be present in the iron in the form of cementite Fe 3 C and graphite. By the number of cementite and the form of graphite irons are divided into the following types:

• white;
• gray;
• malleable;
• half-hearted;
• ductile iron.

White cast iron - a cast-iron, in which all the carbon present in the form of cementite. Color break - light. White cast iron has very high hardness and can not be handled cutting tool. It is used primarily as an intermediate to produce ductile iron.

In gray iron the entire carbon or most of it is present as graphite. Color break - gray. Grey cast iron due to the high casting properties is used as base material for casting. In contrast to the white iron lends itself well to metal processing.

Ductile cast iron is obtained by casting and subsequent heat treatment of white cast iron with the formation of flaky graphite. Ductile iron is mainly used in the manufacture of automobiles, agricultural machinery and tractors.

Halfway (bleached) iron - a cast-iron, in which carbon is present in the form of graphite, and in the form of cementite. Used as a friction material for use in conditions of dry friction for increased wear resistance of parts.

In ductile iron containing spheroidal graphite formed during the crystallization process. Typically used for the production of critical parts in machinery for the manufacture of high-water pipes, oil and gas pipelines.

Weldability of cast iron
Cast iron has poor weldability processing:

• rapid cooling of the weld and heat affected zone leads to the bleached areas (areas with secretions of cementite with high hardness), which is extremely difficult to follow the machining of metal;
• due to the high brittleness of cast iron when it is non-uniform heating and cooling of the high probability of occurrence of cracks in the weld and heat affected zone;
• Iron is a flowable alloy, which makes it difficult to hold the molten metal flowing and makes the formation of joint;
• because of the intense emission of gas from the liquid weld pool, and continuing on the stage of crystallization in the weld can be formed pores;
• the oxidation of silicon, and sometimes other elements on the surface of the weld pool can form refractory oxides, leading to lack of penetration.

However, the welding of metal is very common in the repair of cast iron products, to fix the marriage of cast iron castings, and in some cases, the manufacture of welded-cast structures.

Welding of cast iron
Welding of cast iron can be carried out or covered with carbon electrodes, cored wires, equipment for gas welding and other methods.

There are three main technological areas of welding cast iron:

• welding, providing the production of pig iron in the weld metal;
• welding, providing reception in the weld metal of low carbon steel;
• welding, providing reception in the weld metal non-ferrous alloys.

An important way of dealing with the formation of bleached and hardened sites, as well as the appearance of cracks is the preheating of the workpiece. Depending on the temperature of preheating the following types of weld metal:

• Hot welding - with pre-heating temperature 600-650 ° C;
• for semi welding - the temperature of preliminary heating up to 400-450 ° C;
• Cold welding - without preheating.

Hot and welding of cast iron used for semi when required to obtain iron in the weld metal with properties similar to those of base metal parts. Preliminary heating of the product to a temperature of 600-650 ° C with a hot iron welding creates the conditions for a relatively uniform heating and slower cooling of the metal after welding, which ensures graphitization of cast iron - carbon emissions in the form of graphite - and prevent its release in the form of cementite.

When welding cast iron for semi improved graphitization of the metal is achieved by the introduction of the welding zone grafitiziruyuschih substances (silicon, titanium, aluminum) and preheat the product with a lower temperature than the hot welding.

Cold welding of cast iron is performed in cases where the iron in the weld metal is not provided, and can be applied in some cases, when the weld metal is required to obtain iron - using grafitiziruyuschih substances at low or medium-sized defects in the non-through defects or defects of the small extent of cross-cutting and depth.

Arc welding to produce pig iron in the weld metal
The process of hot-iron welding is carried out in several stages:

• preparation of workpiece;
• preheating;
• welding;
• followed by slow cooling.

In preparing the workpiece is performed a thorough cleaning of the defective seats from dirt and cutting edge to provide access to the manipulation electrode (wire) and when exposed to the arc. To prevent leakage of molten metal weld pool, and sometimes to give some form of metal, forming the welding is performed. The forms are made of graphite plates that are held together forming a mass of a mixture of quartz sand, with liquid glass and other materials or molding in flasks molding materials used in foundries (see figure below).

Shaping places for hot welding of cast iron

At the end of molding requires dry form, with a gradual increase in temperature from 60 to 120 ° C. Further details of the shape of the heating is performed at 120-150 ° C in a furnace, forge, a special pit or other heating device. After welding, for slow cooling of the product is covered by a layer of thermal insulation (asbestos sheets and backfill of dry sand, slag, charcoal, etc.) or cooled with the heating device. Large parts can cool down from 3 to 5 days.

For manual arc welding hot iron melting electrodes are applied with rods of iron grades A or B, as well as carbon electrodes can be used. Hot welding is done continuously at high currents up to the end of the welding defect. If significant amounts of welding performed alternating welder. Cover the cast bar has a diameter of 5-20 mm alloying (silicon carbide, graphite, calcium silicon, ferrosilicon, etc.) and stabilizing materials. Electrode holder shall be provided with a shield to protect the welder's hands from the heat radiation. Welding of carbon electrodes (diameter 8-20 mm) is performed on a direct current straight polarity.

TABLE. The composition of iron rods for welding and hot for semi

Element	Grade A  hot welding	Grade B  for hot and  for semi welding
Carbon	3.0–3.5
Silicon	3.0–3.4	3.5–4.0
Manganese	0.5–0.8
Phosphorus	0.2–0.4	0.3–0.5
Sulfur	to 0.08
Chrome	to 0.05
Nickel	to 0.3

TABLE. Modes of arc welding of cast iron carbon electrode

Metal thickness (mm)	The diameter of the electrode (mm)	Current (A)
6–10	8–10	280–350
10–20	10–12	300–400
20–30	12–16	350–500
30 or more	16–18	350–600

Hot welding of cast iron allows you to weld cast iron, almost no different from the basic metal products (density, mechanical properties, machinability, etc.). However, it has some drawbacks:

• the complexity of the process associated with the need to mold the welding, the complexity of providing uniform heating of the product;
• duration and high cost of the process.

At the same time, in some cases, the welded seams of iron less stringent requirements, such as providing only a certain density or equal strength of joints. This can be achieved with the use of assistive technology and metal tools when welding with little or no heating without preheating - if for semi or cold welding of cast iron.

To prevent bleaching of iron in the weld metal can be introduced by a large number of grafitizatorov and alloying elements. For example, welding electrodes brand EMCH have iron rod with a high silicon content (up to 5.2%) and two-layer coating, which is the first layer of alloying, and the second is designed to provide protection of the gas and slag.

TABLE. The composition of electrode coatings brands EMCH

1st layer	2nd layer
Graphite - 41%  silikomagny - 40% iron slag - 14% aluminum (powder) - 5%	Marble - 50% fluorspar - 50%

In cold welding electrodes brand EMCH products of cast iron with a wall thickness of 12 mm is possible to obtain joints and weld zone without bleached and hardened areas.

When welding electrodes brand EMCH massive products of iron to produce defect-free welds require pre-heating to T = 400 ° C depending on the hardness and thickness of cast iron products.

The electrodes of nickel pig iron can obtain welds with good machinability, but it increases the likelihood of hot cracking. Welding is carried out in several layers with a reciprocating movement of the electrode.

TABLE. The composition of the electrode rods of nickel pig iron (%)

Cast iron	Carbon	Nickel	Silicon	Copper	Manganese	Iron
Ni-resist	2,0	29	1,3	7,6	0,4	59,7
Nikrosilal	2,0–2,3	19–22	5,2–6,4	–	0,5	68,8–73,3

The electrodes provide brand EMCHS doping through the coating. Their core consists of a low carbon wire, and coated with three layers: the alloying, gas and slag-forming, gas protection. When the thickness of the workpiece 8 to 10 mm for defect-free welds by means of these electrodes can be used cold welding, and for thick - hot welding.

Semi-automatic hot and cold for semi welding cast iron is performed, usually with flux cored wire PP-Anch-1, PP-Anch-2, PP-Anch-3, etc. The wires contain a set of modifying elements introduced into the charge as a ligature on the basis silicon.

TABLE. The mechanical properties of the metal iron, welded with flux cored wire

Mark the wire and the temperature of heating	σ B (MPa) tensile	σ B (MPa), flexural	Hardness, HB
Welding wire PP-Anch-1,  welding without preheating	180–220	400–450	250–300
Welding wire PP-Anch-2  welding with heating at T = 350 ° C	170–250	350–450	170–190
Welding wire PP-Anch-3  Welding heated at T = 600 ° C	280–320	460–520	180–210

Gas welding
Gas welding cast iron is considered a reliable method of obtaining the metal seams, little different from the basic metal products. Compared with arc welding with heating and cooling in gas welding - a long and uniform, so that provided the best conditions for the graphitization of carbon and decreases the likelihood of bleached areas in the weld and heat affected zone.

Desirable to perform gas welding with preheating (general or local). Bevel the edges is one-sided (V-shaped), with an opening angle of 90 °. The edges are thoroughly cleaned of dirt, rust and oil with a brush or a sandblaster and heated gas flame.

Filler rods are usually cast iron rods following brands:

• " A "(for hot gas welding of cast iron, composition see table above);
• " B "(for gas welding of cast iron with a local heating, see the composition in the table above);
• " LF-1 "(for low-temperature gas welding of thin-walled castings made ​​of cast iron);
• " low-2 "(for low-temperature gas welding of thick-walled castings made ​​of cast iron);
• " Warhead "and" KhCh "(to wear welding cast iron.)

The wire diameter is selected from the range of δ / 2 and (δ / 2 + 1 mm), where δ - thickness of the base metal products.

For gas welding of metal is necessary to use flux, which performs the following functions:

• protects the weld pool from oxidation;
• translates refractory oxides of iron, manganese and silicon in low-melting slag;
• improves splavlyaemost creating micropitting oxidation and partial dissolution of the graphite inclusions of iron;
• increases the fluidity of the molten pool of metal and slag.

Some brands of fluxes used in gas welding cast iron:

• Flux FSCH-1, used mainly for welding of large defects of - calcined borax (23%), sodium nitrate (50%), sodium carbonate (27%);
• Flux FSCH-2, used for low-temperature welding and welding of small parts made of cast iron, is different from the flux FSCH-1 additive in the composition of lithium carbonate;
• gas flux of MB-1 consists of a volatile liquid organoboron.

During the welding rod should be more immersed in the flux and flux pour in the molten pool. Acetylene feed rate of 1 mm thickness of the metal should be between 100 to 120 dm 3 / h Mowing is carried out only at the edges of wall thickness over 4 mm.

It is essential that the welding flame is normal or carburizing, since oxidative flame leads to a strong local burnout of silicon with the formation of grains in the weld seam of white cast iron. The metal should be well warmed up. Welding is performed in the down position quickly, and for large parts preferably two burners simultaneously. To prevent the formation of pores in the weld metal must constantly stir welding bath filler rod end, making it easier to exit the dissolved gases.

During the welding filler rod is dipped into a bath only when it warms up to temperature end of the light-red heat, as the immersion of unheated rods can cause bleached areas. Rods are removed from the weld pool as little as possible and only to cover its flux.

Allowed periodic removal of the flame kernel to the weld pool surface, but the replacement part of the flame must always cover the surface of the bath. If excessive delay in one area of ​​the flame is burning out of carbon and silicon, which can lead to bleaching of iron.

Welding of cast iron parts of complex shape (with holes, bridges, unequal in different parts of the section) to avoid the appearance of defects caused by the uneven heating, should be performed only with the general pre-heated.

Upon completion of welding of the product cover layer of asbestos for a slow cooling.

Electroslag welding
In electroslag welding of cast iron as electrodes use a cast iron plate, and as a flux - fluoride and non-oxidative desulfurizing flux. Electroslag welding allows to obtain satisfactory properties of the joints of cast iron, hardened and bleached without the parts, pores, cracks and other defects.

See also:
welding equipment
welding wire
manual arc welding technology
semi-automatic welding
gas welding
electroslag welding

Welding of metals:
welding of cast iron
welding of aluminum
welding of titanium