WELDING OF CAST IRON Cast iron is an extremely versatile material, used in thousands of industrial products. It is hard, wear-resistant, and relatively inexpensive. Like steel, it is available in many different grades and compositions. While we usually think of cast iron as being brittle (having low ductility), this is not true of all cast irons, as we shall see shortly. Cast iron, like steel, is an iron-carbon alloy. In composition and structure, and in some of its properties, it is quite different from steel. While many grades of cast iron can be welded successfully, not all cast iron is weldable, and welding of any cast iron presents problems not usually encountered in the welding of steel. Composition and Grades of Cast Iron Cast iron is by no means pure iron. In fact, there is less iron in any grade of cast iron than there is in a low-carbon steel, which may be 98% iron. Almost every cast iron contains well over 2.0% carbon; some contain as much as 4.0% . In addition, cast iron usually contains 1.2 to 2.5% silicon, 0.5 to 0.8% manganese, and (as in steel) small percentages of sulphur and phosphorous. It is the high percentage of carbon that make cast iron different from steel in many of its properties. In a finished steel, all the carbon is combined with iron in the form of iron carbides, whether those carbides are in grains of pearlite, in grains of cementite, or in scattered small particles of carbide. In cast iron, most of the carbon is usually present in uncombined form, as graphite. (Graphite is one of the two crystalline forms of carbon; diamond is the other). The differences between the general types of cast iron most widely used arise chiefly from the form which the graphite assumes in the finished iron. Gray Iron. Of the general types of cast iron, gray iron is by far the most widely used. The term ”gray iron” was adopted originally to distinguish it, by color of the fractured metal, from white iron, a form of cast iron in which all the carbon is combined. We’ll have more to say about white iron later. At this point, we wish to stress the point that gray iron is a very broad term. All gray irons contain graphite in the form of flakes. This makes the gray irons readily machinable. All gray irons have almost no ductility, again because of the flake form of the graphite, which causes the metal to break before any appreciable amount of permanent elongation has occurred. However, not all gray irons are equally strong, or equally hard. As in steel, tensile strength and hardness are closely related. In grayirons, tensile strength ranges from about 14 MPa (20,000 psi) to more than 35 MPa (50,000 psi). The hardness ofthe strongest grades is double that of the weakest grades. All gray irons have high compressive strength – three tofour times their tensile strength.While all gray cast irons contain free carbon (graphite) in flake form, they also contain combined carbon (ironcarbide) in almost every case. This combined carbon is often present in pearlite grains, such as found in mostcarbon steels. It may also be found as cementite or martensite. The composition of the cast iron, the rate at whichit cooled after casting, and heat treatment after casting all have a bearing on the structure. Small amounts ofalloying elements are used in the strongest gray irons; they tend to prevent the formation of pearlite. While thehardness and strength of steel almost always increase as carbon content rises, in the case of gray cast iron thestrongest, hardest grades have less carbon than some of the lower-strength, less expensive grades.Gray iron is usually cast in sand molds, and allowed to cool normally in the mold. Heat treatment after casting isnot always necessary, but is frequently employed, either to increase or to decrease hardness. Almost all gasolineand diesel engine blocks are gray iron castings. Whenever industry desires an intricate form which can bemachined to close tolerances, and must withstand abrasive wear, gray iron gets consideration. Only when it isessential that the finished item have some ductility and good shock resistance is some other material – such asnodular cast iron or cast steel, both more expensive – likely to be substituted.White iron, mentioned above, is about the same as gray iron in composition, but has been cooled rapidly so thatgraphite does not have time to form, and all the carbon winds up in the combined form, as pearlite, cementite, ormartensite. Many white iron castings are subsequently converted to malleable iron, which we shall take up next.However, some gray iron castings are made with white iron wearing surfaces, since white iron is much harder thangray iron, although extremely brittle. This is accomplished by inserting metal or graphite chill blocks at appropriateplaces in the mold. The molten metal that solidifies against those chill blocks cools so rapidly that white ironsurfaces are created. Plowshares, railroad car wheels, and various types of dies are often made with such chilledwhite iron surfaces.
casting is absolutely essential. Since a higher level of preheat is required for oxy-acetylene welding then for arc welding, arc welding is likely to be chosen where fusion welding is essential (as it is whenever good color match is desired). For many repair jobs, however, oxy-acetylene braze welding is the ideal method. Much less preheating is required; in many cases, preheating can be done with the torch. If the work is properly done, the braze-welded joint will have a strength equal to that of the base metal, and excellent machinability. Welding of gray iron castings which have chilled white iron surfaces is seldom attempted, since the desirable properties of white iron will always be affected by welding temperatures. Welding of white iron generally is limited to malleable iron foundries, where castings may be reclaimed by welding before conversion to malleable iron takes place. Malleable Iron. The chemical composition of malleable cast iron is much the same as that of a typical gray iron, but its properties are much different. It is tough; it can resist shock; it has ductility approaching that of mild steel. How is such a remarkable change achieved? By cooling the original casting so rapidly that white cast iron, with no free carbon, is formed; then heating the casting to about 8000C and holding it at that temperature for several days.Under those conditions, virtually all the carbon is released from the iron carbide to form fine rounded particles ofgraphite (sometimes called temper carbon) scattered among grains of ferrite. Malleable iron has good wearresistance, and is widely used for parts where the toughness of steel is required, and the economy of casting(instead of forming or machining) will result in lower cost. However, malleable iron is substantially more expensiveto make than gray iron, and is usually selected only where its toughness and ductility are essential.Malleable iron cannot be successfully fusion welded and retain its unique properties; to put it another way, you canweld malleable iron as easily as you can weld gray iron, but in the act of welding you will convert some of themalleable iron casting into a gray iron casting. Seldom will that yield a satisfactory result. However, malleable ironcastings can usually be braze welded successfully.You may wonder how to tell a malleable iron casting from a gray iron casting. There’s one almost infallible method:use a high-speed grinder to make a spark test. The difference between the spark streams produced by gray ironand malleable iron is quite pronounced. Spark testing is covered in the Appendix.
Nodular cast iron is made by inoculating the molten metal, just before casting, with a small amount of magnesium or cerium. This causes the free carbon in the finished casting to appear as rounded nodules of graphite, rather than as flakes. Each nodule is surrounded by a zone of ferrite (carbon-free iron) with the balance of the metal usually in the form of pearlite. Nodular iron has less ductility than malleable iron (which can have almost as much ductility as mild steel) but far more than ordinary gray iron, which has virtually none. It usually has high strength; in fact, the yield strength of a nodular iron is almost always greater than that of mild carbon steel. All nodular irons have one property which clearly sets them apart from most gray irons; they have a high modulus of elasticity. In simpler terms, they have excellent stiffness, a property much desired in parts like propeller shafts or forming rolls. Where most gray irons are much more elastic (less stiff) than steel, nodular cast iron is nearly as stiff as cast steel. Like malleable iron, nodular iron cannot be fusion welded and retain all of its original properties. This is especially true of nodular iron castings which have been heat-treated after casting. A fusion weld made in nodular iron may not cause loss of tensile strength, but will almost always reduce the shock resistance of the part. Braze welding can be used on nodular iron if some sacrifice of tensile strength can be tolerated. Alloy Cast Irons. Alloying ingredients – chromium, nickel, molybdenum, and, occasionally copper or aluminum – are added to cast iron for three principal purposes: to increase wear resistance, to increase resistance to scaling in high-temperature service, and to increase corrosion resistance. In some alloy cast irons, the silicon level is also increased substantially. Some of the extra-hard, abrasion-resistant alloy irons are white irons; they appear almost white when fractured because they contain virtually no free carbon. Others may have the general appearance of gray cast iron. The range of compositions is so great that no general statement about the weldability of alloy cast iron can be made. So far as the oxy-acetylene process is concerned, fusion welding is not recommended; braze welding will not permit retention of all the properties for which the alloy iron was originally specified The Importance of Preheating For Fusion Welding. If you are called on to weld cast iron, the material to be welded will almost always be gray iron. Gray iron is brittle; it has virtually no ductility. If the forces of expansion or contraction, as generated during the welding operation or in cooling after welding, are concentrated in one area of the casting, cracking of the casting, or of the cooling weld, will almost certainly occur. Even at elevated temperatures, gray cast iron has little ”give”; it will break, rather than stretch, when the force of expansion or contraction exceeds its yield strength. Therefore, whenever a casting must be fusion welded, it is usually necessary to preheat the entire casting, slowly and evenly, before welding is started, and then allow the casting to cool slowly after welding has been completed. This will permit all sections of the casting to expand and contract at a reasonably uniform rate. The temperature to which a casting must be preheated depends somewhat upon the welding process to be used. Oxy-acetylene fusion welding puts more heat into the casting than does arc welding, and therefore requires a higher level of preheat, usually to about 6000C (11000F). The preheat temperature level is also somewhat dependent upon the size and form of the casting. Rather simple castings, without major variations in section thickness, usually require less preheat than complex castings. If a suitable furnace is not available for preheating a casting, one can be improvised out of fire brick, as suggested in Chapter 13. If the casting is preheated in a furnace, and then withdrawn for welding, it is essential that as much of the casting as possible be insulated during the welding operation, to hold the preheat as well as protect the welder. Asbestos paper will be found almost indispensable during the fusion welding of cast iron. For Braze Welding. When a casting is to be braze welded, some preheating is usually required, but the level of preheat temperature can be much lower, and many jobs can be done without preheating the entire casting. In braze welding, there is no danger of weld cracking. Bronze weld metal has extremely high ductility, and is capable of absorbing any contraction stresses to which it may be subjected. Because the temperature of the casting itself, even in the metal immediately adjacent to the weld metal, need never exceed 9000C, changes in the physical properties of the casting metal will seldom occur. That is why malleable iron castings can often be braze welded.
leave the casting with residual stresses which might cause cracking at a later time) if some preheating is not performed. If preheating of the complete casting is feasible, it should be done, although the temperature need not be raised to more than 300-4000C. In most cases, thorough preheating of the metal adjacent to the weld zone, using the welding torch, will be sufficient. Braze Welding Practice If you have already braze welded pieces of steel plate, as suggested in the preceding chapter, the braze welding of cast iron should present no new problems. If possible, get some coupons of cast iron, about 13mm (1/2 in.) thick, which some foundries cast especially for welding practice. If not, locate some pieces of a broken casting and use them. Prepare the edges of the joint carefully. The included angle of the weld vee should be a full 900, and the edges must be thoroughly cleaned. Grinding, followed by filing, is recommended. (The file will remove any loose particles left by the grinding wheel, as well as any graphite flakes which might interfere with proper tinning of the metal.) Be sure to remove all traces of grease or paint from the metal surface immediately adjacent to the weld vee. For braze welding cast iron about 13mm (1/2 in.) thick, we suggest that you use a welding tip which consumes 15 cfh of acetylene, and a slightly oxidizing flame. (To secure that, adjust the flame carefully to neutral, then throttle the acetylene flow enough to shorten the flame inner cone slightly.) You must also have a good braze welding flux, such as OXWELD BRAZO flux or OXWELD Cast Iron Brazing Flux. (The latter is more expensive, but contains bits of bronze ”spelter” which help you to determine when the casting has reached the proper temperature for tinning.) Since tackwelding will seldom be called for in actual repair work on castings, we suggest that you merely space the two pieces of cast iron so that there is a gap of about 1.6 mm (1/16 in.) at the starting point of the weld, and a gap of 5 to 6 mm (3/16 to 1/4 in.) at the finishing point. If you are using a welding table with a cast iron top, be sure to raise the finishing end of the joint a bit above the table, lest you actually weld the specimens to the table. If the cast iron coupons or the pieces of casting are at least 13 mm (1/2 in.) thick, we suggest that you plan to make your weld in three passes.
holding the tip of the inner cone at least 13 mm (1/2 in.) away from the metal surface. Then concentrate the flame in an area about 8 cm (3 in.) in diameter at the starting end. The cast iron will be ready for tinning just as it starts to turn a very dull red color. (To spot this point in a brightly lighted room, through the dark lenses of welding goggles, isn’t always easy. It will take practice to acquire the knack of instantly recognizing that first glow.) You should have heated the end of the welding rod and dipped it in flux while you were preheating the metal. Now melt just a little of the bronze onto the surface of the vee. If it balls up and tends to roll down the surface, the metal isn’t hot enough. Withdraw the rod and continue heating for a few seconds. Dip the rod in flux again and make a fresh start. If the molten rod tends to bubble up on the cast iron surface, and run around like drops of water on a fairly hot stove top, the cast iron is too hot. You must withdraw the flame and let the iron cool down a bit before trying to deposit more filler metal. Once proper tinning action has been started, continuing the first pass is largely a matter of maintaining the tinning action and melting in the right amount of bronze. Always try to get tinning action which extends a good halfway up each side of the vee, but deposit no more bronze than necessary to achieve a concave weld contour. If you do not tin the sides of the vee enough, and then try to melt in too much metal, you’ll arrive at a convex weld metal surface and find it very hard to make the second pass without running molten bronze onto parts of the iron surface that have not been properly tinned. On the second pass, tinning action should be carried to the top of each side of the vee, and enough bronze melted to secure another concave surface, not a convex surface. Be sure that the additional filler metal added in this pass in is completely fused to the bronze deposited in the first pass. If there is not complete fusion between the first- pass bronze and the second- pass bronze, what appears to be a good weld may actually be less than full strength. In making the third pass, try to achieve a good ripple and a good shape for the top surface of the weld. Carry the weld just a bit past the top of the vee on each side, making sure that the cast iron surfaces tin before the puddle passes the top of the vee.
without too many unsightly protrusions, you should find a way to test it. The methods suggested for testing welds in steel sheet and plate are not really suitable for braze welds. For one thing, you can’t cut out a coupon with your cutting torch or attachment. More important, any test which involves hammering the specimen is likely to cause the cast iron to fracture, even if the weld is less than perfect. For a very rough test (if welding was done on flat pieces) you can place the specimen in a heavy vise, with the centerline of the weld parallel to the vise jaws and level with the top edge of the jaws. Then strike the specimen, above the weld line, with a heavy hammer. If you can break the specimen, and the break occurs in the cast iron, not along the weld zone, you know that the weld was at least passable. A far better method of testing, if you have the means for doing it, is illustrated in Fig. 15-3. If a steadily increasing force is applied, the specimen will ultimately break, and if you have made a really good weld, it will break through the cast iron, with no evidence of bronze on either side of the fracture. If one surface of the fracture shows bits of bronze, indicating that the weld broke through the bronze-cast iron bond at that point, it indicates that you did not attain complete tinning of the cast iron during the welding operation. On your first weld, lack of proper fusion at the bottom of the vee will usually be the result of overheating the cast iron, rather than underheating it. Tips on Braze Welding. A cast iron surface that has been exposed to fire may prove hard to tin, even after thorough mechanical cleaning. Spreading a strong oxidizing agent, such as powdered potassium chlorate, on the well- heated surface, just ahead of the weld puddle, will often help. The chlorate will foam; once foaming starts, tinning will proceed normally. A cast iron surface which has been exposed to oil and grease for a long period will actually absorb some of that material, and normal cleaning methods will not remove it. The answer to that problem is to heat the surface bright red before any attempt is made to weld it. That will usually vaporize and burn the grease out. Before repairing any casting, take time to study the job in advance, and decide how to clamp it or support it so that welding can be done most easily. If there are a series of cracks, try to plan the welding sequence so that welding one crack isn’t likely to create expansion forces which will enlarge another crack. No matter how you preheated a casting for braze welding, always do everything possible to permit slow, even cooling. Frequently it will help to play the torch flame gently over the surface of the metal for a considerable area surrounding the weld, to bring the piece as a whole to a more even heat level. Whenever possible, cover the part with asbestos paper, or, if it is small, bury it in dry slaked lime. Always protect the part from drafts. Fusion Welding The general rule for the oxy-acetylene fusion welding of gray iron castings is that the entire casting be preheated in a furnace to dull red heat (about 8000C), that the actual welding be done under conditions which will allow the retention of most of the preheat, and that the casting be allowed to cool slowly after the welding. Whenever possible, the casting should be reheated to a uniform temperature of about 750-8000C after welding, and cooling to room temperature should require at least one full day. When these conditions can be met, the results should be good. In some cases, depending on the size of the casting, and the thickness of its various sections, fusion welding can be done successfully with only local preheating to a dull red color. However, it would be unwise to attempt fusion welding without full preheat unless you have had considerable experience in the fusion welding of cast iron, and feel thoroughly competent to assess the effects of expansion and contraction on the whole casting. Few oxy- acetylene welders get the chance to acquire that kind of experience. You do not, however, require previous experience, or a preheating furnace, to acquire the basic skills involved in making a fusion weld. The welding action is quite different from that with which you are familiar (if you have previously welded only steel, or braze welded cast iron) so we suggest that you make a few practice welds in small pieces of cast iron. Materials Required for Practice Welds. You will need two or more pieces of 13mm (1/2-in.) cast iron, about 3 by 6 in. in area, with edges bevelled to an angle of 45 degrees (the same as suggested for braze welding practice). Torch tip size should be the same as that you used for braze welding 13-mm (1/2-in.) cast iron. Filler metal should be cast iron rod especially formulated for welding (either gray iron or nodular iron). A flux designed specifically for fusion welding of cast iron is required, such as OXWELD Ferro Flux. (Do not attempt to use a brazing flux. It will not serve this purpose.) Practice. The bevelled edges of the pieces should be filed thoroughly. If the bottom of the bevelled edge is sharp, it should be filed to give you square edges, at least 2 mm (3/32 in.) deep, at the root of the weld. The pieces should be positioned so that there is a gap of about 1.6 mm (1/16 in.) at the weld starting point and about 5 mm (3/16 in.) at the finishing point. For fusion welding of cast iron, a neutral flame should be used, not the slightly oxidizing flame suggested for braze welding. First preheat the entire weld zone thoroughly with the torch flame. Try to reach dull red heat along the entire length of the welding vee. Then heat the bottom of the vee, at the starting end, until the actual melting has started. Angle the torch flame as you did in steel welding; keep the inner cone at least 3 mm (1/8 in.) from the metal, however. When a small puddle has formed at the base of the vee, move the flame from side to side to melt down the sides of the vee gradually. Only after you have a fair-sized puddle should the rod, which has been preheated in the flame and dipped in the flux until it is well-coated, be introduced into the puddle. From this point on, your aim must be to keep the rod in the puddle, and to allow the heat from the puddle, not the flame itself, to do the actual melting of filler metal. Try to avoid withdrawing the rod from the puddle except when more flux is needed on the rod. Never hold the rod above the puddle and allow it to melt into the puddle drop by drop. Direct the flame against the puddle, and against the sides of the vee. You must make the weld in one pass, not two or three. Therefore, you must not allow the puddle to advance too rapidly along the root of the vee. Keep the rod in the puddle, fill the vee completely for a length of perhaps one inch, then redirect the flame to melt the lower edges of the vee and allow the puddle to advance. You will find the puddle more fluid than the puddle you handled in steel welding, since cast iron does not have the fairly wide ”mushy” range which make steel welding quite easy. Therefore, extra care to avoid letting the puddle run ahead and roll onto metal which has not yet reached fusion temperature is required. While making the weld, you may see gas bubbles or white specks in the puddle. During your first weld, we suggest that you ignore them. Thereafter, you must take pains to work them out as you go along, by adding flux to the rod, and by playing the flame around the specks until they float to the very top of the puddle. Once they float to the top, skim them off with the tip of the welding rod, and tap the end of the rod gently on the welding table to dislodge them. Removal of such visible particles (usually dirt, or impurities in the base metal) is essential if a full-strength weld is to be secured. Once the weld has been completed, reheat the entire weld with the torch until it glows faintly. Then place the welded specimen between sheets of asbestos paper to allow it to cool as slowly as possible. After the weld has cooled completely, wire brush the surface of the weld on both sides, and examine it carefully. Note particularly the appearance of the underside. If thorough fusion between the bottom edges of the vee has not been obtained, the defect can be clearly seen. The bottom of a good weld will show little round beads of weld metal protruding through. Test your weld by clamping the specimen in a large vise, with the centerline of the weld flush with the tip of the jaws. Strike the upper part of the specimen with a heavy hammer until the part breaks. If you have made a good weld, the break will probably occur in the base metal, not in the weld. If it breaks through the weld, examine the fracture carefully for inclusions, gaps, or blowholes. If the break occurs in the base metal, remove the specimen from the vise, nick it with a hacksaw, on both sides of the weld zone, then return it to the vise and break it across the weld. Examine the fractured weld metal carefully to see whether it appears sound, with no slag or oxide inclusions or blowholes. Practical Hints If you are preheating a casting with the torch, or in an improvised furnace, watch carefully to make sure that you do not overheat any part of the casting. It should never get more than dull red. If it gets too hot, it may warp from its own weight, and become completely unrepairable. Try to keep the thinner sections farthest from the heat source if an improvised preheating furnace is being used. Just as the foundryman must rely on experience, and the use of correct foundry practices, to feel quite sure that a finished casting with no visible defects is sound, so a welder must follow correct procedures, with emphasis on proper preheating and cooling, if he is to feel confident that a good-looking fusion weld in cast iron will stand up in service. Of course, he can leak- test a weld in a water jacket. A weld which must be leaktight, but cannot be tested under pressure in the repair shop, can be checked rather well by applying kerosene to one side of the weld. Kerosene will work its way rapidly through even a slightly porous weld. But such tests cannot be conclusive as to the overall soundness of the weld, and the final condition of the repaired casting.