Virtually all types of tool steel can be welded by the shielded metal arc, gas tungsten arc, plasma arc, or electron beam processes. Die units used for blanking, forming, forging, drawing, embossing, coining, or hot and cold trimming can be salvaged or reclaimed using one of these processes.

Tool and die welding applications can be separated into four categories:

(1)Repairing of dies

(2) Composite fabrication of dies

(3) Correction of designs

(4) Improvement of properties by hardfacing

Die Welding and Repair. Welding professionals, along with tool engineers, have developed tool and die welding and repair methods which can be economically significant. Research, development and testing by welding equipment and electrode manufacturers have resulted in varied lines of tool and die welding electrodes, with recommended procedures for their use. This combined effort has served to minimize down-time in manufacturing facilities that use tools and dies.

Tool and Die Welding Electrodes

Tool and die welding electrodes can be divided into two categories: basic tool steel welding electrodes, and alloy welding electrodes. A combination of these two types is used for some applications.

Basic Tool Steel Welding Electrodes. This group of coated electrodes includes water-hardening, air-hardening, hot-working and high-speed steel. These electrodes are in an annealed state, and the weld metal is hardened by air quenching from the high heat of the arc. The weld deposits are “hard-as-welded,” whether they are applied to hardened or annealed tool steel, mild, medium, or high-carbon steel, or to other alloy steels. The weld deposits can be annealed to facilitate machining, then heat treated and tempered. As a general rule, weld deposits will respond to the heat treatment recommended for the average tool steel in its classification.

Alloy Welding Electrodes. Included in this group are low-alloy electrodes for plastic or zinc casting molds and flame-hardened dies. Also in this group are the more highly alloyed electrodes used to weld dies for forging, drawing and forming. These electrodes produce machinable weld deposits which are not affected by heat treatment. They are available in several types, providing a range of hardness in the weld deposits. Additional hardness is obtained by work hardening.

Combination. Other alloy electrodes are sometimes used in conjunction with tool and die welding electrodes, especially for applications on cast dies for drawing or forming. Nickel-iron electrodes, nickel electrodes, and copper-nickel electrodes can be used as foundation on cast units, then other tool and die electrodes are used to finish the castings.

Current, Coatings, and Deposits. Generally, tool and die welding electrodes should be used on direct current electrode positive (DCEP). The percentage of alloying elements lost in the weld deposits during welding can be regained by selecting an electrode which incorporates the required alloys in the coating. Mineral-alloyed coatings are preferred.

The introduction of a mineral-alloyed coating on the electrodes also helps produce a desirable spray action of the arc and forms a protective slag, which is easily removed.

Tool and die welding electrodes will produce sound homogeneous weld deposits free from porosity. In many cases, laboratory tests have revealed weld deposit structures that are superior to parent steel of the same class.

Gas metal arc and flux-cored arc welding can be used to weld tools and dies, generally using small diameter (less than 1.5 mm [0.060 in.]) wires. The plasma arc, electron beam and laser beam processes, with or without filler metal, can also be used for tool and die welding.

Factors Influencing Hardness

The hardness developed in weld deposits “as- welded” and “heat-treated” will vary according to the following principal factors:

(1) Preheat treatment (i.e., the preheating temperature)

(2) Technique during the welding sequence

(3) Admixture of the base metal with the deposit

(4) Rate of cooling and mass of the workpiece

(5)Tempering temperature after welding.

Preheating. As a crack-preventive measure, it is very important to preheat the workpieces to which tool and die electrodes are to be applied. The degree of preheat is a primary factor affecting the hardness developed in weld deposits because preheating tends to delay the rate of air quenching. For a given set of welding conditions, such as current and welding speed, the cooling rate will be faster for a weld made without preheat than with preheat. Preheating also helps to reduce or prevent shrinkage stresses and deformation.

Welding Technique. Welding technique affects the hardness of the weld deposit. Direct current electrode positive (DCEP) is recommended because it minimizes arc penetration, resulting in less admixture with the base metal.

The smallest electrode adequate for the job should be selected because it requires less heat, and this influences the ultimate hardness of the deposit.

Work positioning, travel speed, welding current, and manipulation of the arc all exert an influence on weld hardness.

Ultimate hardness and characteristics of the weld deposits can be enhanced by thorough peening while at forging temperatures. Extended deposits should not be made before peening because the metal will cool; hot metal is more ductile.

Admixture and Cooling. The admixture (dilution) of the deposits with the base metal produces weld metal that is alloyed in direct proportion to the alloys contained in the electrodes and in the parent metal. When elements such as carbon and chromium are added to steels to enhance hardenability, the percentage of these elements will be directly reflected in the “as-welded” or “as-heat treated” hardness of the deposits.

Rate of Cooling. The rate of cooling after welding, which is governed by the preheating temperature and the size of the workpiece, affects the ultimate hardness. The larger the workpiece, the slower the air quench.

Tempering. In welding tool steel, changes take place in the steel that require tempering. Hardening a tool steel with heat treatment requires tempering afterward. To gain the same results, weld deposits should also be tempered. Tempering yields toughness with very little reduction of hardness. It refines the grain structure and relieves stresses and strains set up in the welding process. Tempering or drawing must suit requirements. Size governs the length of time of the draw, which should never be less than one hour. Deposits of the alloy type should not be tempered, but the units on which they are applied should be stress relieved. Partial repairs should be tempered according to the draw range temperatures of the base metal; full repairs should be tempered according to the recommended draw-range temperatures for the electrode.

Fundamentals of Welding Tool Steel

Tool steels are carbon steels to which alloys have been added in varying quantities. Such elements as carbon, manganese, silicon, chromium, nickel, tungsten, vanadium, molybdenum and cobalt are added to steel to bring about such characteristics as greater wear resistance and hardness, greater toughness or strength, stabilized size and shape during changes caused by heat and cold, and “red hardness,” a condition in which the steel will remain hard while red hot.

Because of the diversified composition of tool steels, heat treating is a complex subject. However, knowledge of the fundamentals of tool steels will be of help in setting up specifications for heat treating. In practical tool and die welding, it is not necessary that the electrode match the analysis of the tool steel being welded, but in most cases, the welding electrode should match as closely as possible the heat treatment recommended for that tool steel classification. Such terms as annealing, normalizing, hardening, and tempering should be thoroughly understood.

The four general classifications of tool steels are (1) water-hardening, (2) oil-hardening, (3) airahardening and (4) hot working. It is necessary to study the analysis of the composition of tool steels iln order to become familiar with their properties and characteristics. Although hundreds of different tool steels are available, four general classes of electrodes (including high-speed steel electrodes) will generally suffice to weld them. It would be impractical to have a welding electrode to match each and every analysis, or exact specifications for heat treatment of this great variety of tool steels. In welding, however, it is not a question of matching the analysis of the steel, but of matching as closely as possible the heat treatment in its classification.

Recommended Welding Sequence. Tool and die welding is not complicated if instructions and recommendations are followed explicitly. The following basic principles should help to meet almost any tool and die welding specifications.

(1) Identify the type of tool steel to be welded. This will determine the heat treatment involved and will govern the handling of the unit in the welding sequence.

(2) Select the correct electrode. In making partial repairs of cutting edges or working surfaces, select the electrode that will match, as closely as possible, the heat treatment of the metal to be welded.

To make full repairs to cutting edges or working surfaces, choose the electrode with characteristics best suited for the type of work to which the unit will be subjected. Take into consideration any factors involving heat, abrasion, shock, and thickness of metal to be cut or formed.

For forging die repair, or facing cast or carbon-steel dies for drawing or forming, select alloy electrodes recommended by the manufacturer for these purposes.

The size of the electrode to be used for a repair will depend on the width and depth of the damaged area. In general, a 2.4 mm (3/32 in.) diameter electrode will repair a damaged area 2.4 mm (3/32 in.) wide and 2.4 mm (3/32 in.) deep. The same relation applies to other electrode diameters. Always select the smallest electrode, especially for sharp cutting edges, because less heat is required for welding. There is also less chance of creating shear marks, and less grinding will be necessary after welding.

(3) Prepare the surface to be welded. In making partial repairs of cutting edges or working surfaces, rough-grind damaged areas to allow for a uniform depth of at least 3 mm (1/8 in.) of finished deposits.

In making repairs to entire cutting edges of tool or dies, rough-grind edges to be welded to an approximate 45″ angle to allow deposits of 6 mm (114 in.) of finished metal.

On die units that require repairs over large areas, prepare surfaces so that finished deposits will be at least 3 mm (1/8 in.) deep.

For repairs to drawing and forming dies of cast structure, the edges or areas to be faced should be prepared uniformly so that finished deposits are at least 3 mm (1/8 in.) deep. To prepare for extremely long deposits, for forming edges or over large areas on castiron base metal, studding may be required. The studs should be staggered, spaced 40 mm (1-1/2 in.) apart.

When preparing damaged forging die blocks for welding, areas to be repaired should be chipped, ground or machined as uniformly as possible to a finished depth of about 5 mm (3/16 in.) for the inlay deposit, or, where necessary, to below the heat-checked depth.

(4) Preheat. Identification of the type of steel to be welded will determine the draw range temperatures involved. It is very important not to exceed maximum preheat temperature or exceed the maximum temperature of the draw range for the type of steel to be welded. Hardness will be lost if the unit is preheated to a temperature above the draw range, because the original structure of the steel will be disturbed. Maintain temperature under the minimum of the draw range in preheating, and never above the maximum for the interpass temperature while welding. This will retain the original hardness of the steel.

(5) Welding. Generally, direct current electrode positive (DCEP) is used to apply tool-steel and alloy electrodes. However, they may also be applied with ac. Keep the temperature of the parent metal as uniform as possible during welding to assure uniform hardness of deposits.

In welding cutting edges, position the work, if possible, so that the deposit will flow or roll over the cutting edges.

Always try to work slightly upward, as gravity causes the deposit to roll back and build up evenly. Gravity also causes slag to roll out of the crater and keep it clean. There is no need to weave the electrode in an intricate pattern.

In depositing beads, a slow travel speed is used to secure an even deposit and to assure more uniform fusion of the electrode with the base metal. Keep the area clean by frequent brushing.

Thoroughly peen all deposits to offset shrinkage and stress. Ball peen hammers are generally used, but small pneumatic hammers are efficient for large areas.

It is important not to deposit excess metal in one pass. On final passes, retain beads as close as possible to finished size. This will eliminate excessive grinding.

When welding cutting edges, the arc should not be broken by rapidly pulling away the electrode. Lowering the electrode gradually as you stop welding will prevent deep craters and the searing of sharp edges adjacent to the weld area.

When repairing parts of cutting edges, the weld bead should first progress in one direction to within a short distance of the other end; then it should progress in the opposite direction and overlap the first bead. This will prevent craters and sear marks at the extreme end of the deposited metal.

When welding deeply damaged cutting edges (or drawing and forming surfaces), start at the bottom and gradually fill up the damaged areas. Use a slightly higher amperage on the first and second beads than on finishing beads. Peening while the weld metal is in the forging state also eliminates sear marks at the edges of the deposits.

If two or three dissimilar types of tool-steel electrodes are to be welded on one die unit, care must be exercised in applying the electrodes in sequence to their draw ranges; the first electrode applied must have the highest draw range, then the electrodes are applied in decreasing order to the electrode with the lowest draw range. This will prevent the annealing of previously applied deposits.

To make repairs to entire cutting or forming edges of draw rings, extrusion dies or similar circular parts, the skip-weld method should be used to ensure even distribution of heat.

 

Warping or distortion is offset by preheating to expand the units, and by peening to stretch welded deposits and to offset stresses. These are mechanical problems. Shims and clamps can be used to advantage. Peening will relieve the stresses set up in the welding operation by stretching the deposited metal. Do not weld more than 75 mm (3 in.) before peening.

(6) Post-heat or Temper Deposits or Sections. After welding, the unit is allowed to cool to approximately room temperature and is then tempered by reheating to the recommended temperature. This is important, as post-heating serves as a tempering medium for the deposited metal. Postheating refines the grain structure and relieves stresses set up by welding.

In tempering deposits made to effect a partial repair, the general rule is to temper according to the draw range temperature of the base metal. If a unit has been repaired over the entire edge or working area, temper the deposits according to the draw range temperatures recommended for the electrode used. All welded units should be tempered or drawn to meet requirements of the base metal and the electrode.

The welder should seek advice from the manufac- turer of the unit or the electrode manufacturer on heat treating specifications as to the length of time welded units should be drawn or tempered.

Preheating equipment can also be used for post- heating, tempering or drawing. A temperature-controlled furnace should be used if available.

Composite Fabrication

Sometimes die units can be fabricated as composites. Water-hardening, oil-hardening, air-hardening or hot-work tool steel electrodes can be applied to a base of a mild, medium or high-carbon steel (or SAE graded steel). The weld deposits are confined to cutting edges or working areas. The result is a fabricated composite die constructed mostly of inexpensive steel.

The same basic principles can be followed on drawing and forming dies that are used on cast structures: deposit the tool steel alloy along sharp contours, belt lines and radii. This prolongs the life of the forming surfaces, helping to withstand abrasion, scoring or fouling.

Flame- hardening dies can be fabricated by using low-alloy electrodes.

Existing tool steel units can be converted into composite units to meet unusual conditions by welding a better grade of tool steel along the cutting edges or working areas.

Because deposits made with tool-steel electrodes are hard as welded, it is not necessary to post-heat treat fabricated composite units except for tempering as recommended. To facilitate machining, however, the deposit can be annealed and subsequently heat treated with the recommended heat treatment.

The recommended welding sequence for composite fabrication is similar to that used for welding tool steel. On units with composite construction, tempering should always favor the deposited metal. The base metal acts only as a retaining medium for the cutting or working edge of the desired tool steel. For the rec ommended tempering temperatures of deposits in composite fabrication, refer to manufacturers information on each electrode.

 

 

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