WELDING OF TOOL STEEL

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General

The steels used to manufacture tools usually contain high amounts of alloy elements. These elements are added to improve properties specific to their working conditions through cutting, forming, stamping, rolling, extruding or other operations.

The main alloy elements employed are carbon, manganese, chromium, molybdenum, tungsten, vanadium, silicon, cobalt, copper and nickel.

Classification

The classification method used by both the AISI (American Iron and Steel Institute) and the SAE (Society of Automotive Engineers) is the most widely used system for distinguishing the various tool steels. This system is based either on the quenching medium or the working conditions. The following table presents the AISI / SAE system.

table1t

However, most tool steels are purchased under their trade names because each producer adjusts the composition of their steel to obtain a unique, high-performance product. The tool steel 1 table shows the main alloy element composition limits for the most common tool steels.

Properties and typical applications of the different categories

The tool steel 2 table qualitatively illustrates the main properties used to select steel for tool manufacturing. The table also shows the hardness range normally used for each steel.

The thermal and electrical conductivities and coefficients of thermal expansion for tool steels fall somewhere between those of mild steel and stainless steel, depending on the alloy content.

The tool steel 3 table shows some typical applications for the various tool steel categories.

Weldability of tool steels

Surface and joint preparation

It is important to clean the weld surface thoroughly of all traces of oil, grease, rust, dirt, liquid penetrant inspection solution, or paint using the appropriate solvents or by milling. When the weld parts have polished surfaces, they must be protected with an anti-adhesive to prevent weld spatter from damaging the finished surface.

All defects or cracks must be removed. It is also preferable to make ”U” grooves and to round off the edges to minimize cracking. For sections greater than 1/2 inch (13 mm), ”U” or ”J” grooves should be machined on both sides to minimize the amount of filler metal and shrinkage stresses.

There are several methods available for removing defects and chamfering. The edges to be welded can be machined, grinded or arc gouged (512Plus). Arc gouging is the quickest, but can cause a thin hard film to form, in which case the film must be removed by mechanical means prior to welding. In addition, when grinding is used to prepare the part, it is good practice to eliminate any grinding wheel marks or residue with a file.

Finally, using a jig or fixture adapted to the part, to invert its curvature before welding and preheating, will also help to reduce the risk of distortion.

Tool steel 1 table : Tool steel composition

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Tool steel 2 table : Tool steel properties

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Tool steel 3 table : Typical applications and preheat temperature

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Preheating

When welding tool steels, part of the heat affected zone tends to transform into a fragile structure (martensite) thus creating a greater risk of cracking. This transformation occurs during cooling and is promoted by their high carbon content, somewhat greater alloy elements content, and speed with which the heat dissipates through the part’s section.

To prevent this transformation, we recommend preheating the steel part to a temperature above the point where martensitic transformation normally starts to occur, and maintaining this temperature until the welding operation is complete. This preheating will sometimes reduce the hardness of the part, but it will greatly reduce the risk of cracking.

A good way to proceed is to shield the part in an enclosure made of refractory bricks with one section open to allow access for welding. The part can be heated by inserting heating elements inside the enclosure.

The tool steel 3 table shows the temperature ranges normally used for various type of tool steels. These temperatures may vary depending on the complexity of the part and its metallurgical history.

Welding method

To minimize overheating of the part, we recommend using the smallest diameter electrode possible for the initial passes that are in contact with the tool steel. 3/32 inch (2.5 mm) electrodes or the GTAW (TIG) process work well in this respect. For the same reason, you should use the lowest possible welding current that will still produce good wetting and perfect anchoring (fusion). The entire surface and the edges of the groove should be completely covered before switching to larger diameter electrodes. To minimize dilution and prevent the part from overheating, it is better not to weave the electrode when welding, direct the arc into the weld pool or make narrow beads. It is also a good idea to peen the beads with a round-head tool while they are still hot enough, over 700°F (370°C).

The purpose of peening is to deform the bead with compression forces in order to reduce the effects of shrinkage stresses created during cooling. Be sure not to use a pointed tool such as the hammer used to remove slag, because the pointed indentations it makes will act like crack initiators.

It is important to strike the arc in the groove to avoid creating a weak spot on the part, and to make sure that every crater ir re-melted. A striking plate (starting tabs) can be used to prevent arc strikes.

To repair sharp edges, we recommend starting from one end and working toward the center, then starting from the other end and finishing by re-melting the crater from the first pass. Using copper or graphite plates to support the bead will also make the job easier. However, such plates must be preheated along with the part so that they do not sink away the heat.

For minor repairs, it is important to remember that the heat affected zone (HAZ), no matter how small, will behave differently from the rest of the part during surface finishing operations such as shot blasting or photoengraving. The only way to remedy the situation is to do a thorough annealing to homogenize the structure, then to repeat the heat treatment to re-harden the part.

Note that doing a complete annealing treatment to a tool steel part will greatly reduce the risk of cracking, but at the same time, it will require re-hardening of the part, thus creating a risk of dimensional changes that are costly to repair. These two facts must be considered whenever the repair to be effected by welding demands a significant amount of weld metal that could result in dimensional changes or a significant reduction in hardness caused by the thermal input from welding.

Slag cleaning

Slag on the weld deposit can be removed with round pointed hand tools and a stainless steel brush. When doing multipass welding, it is vital to remove all traces of slag from the weld before making the next pass. To ensure that the part has virtually uniform hardness, and to improve its toughness, it is best to post-weld anneal the part.

Postheating

After welding, the part must be allowed to cool very slowly from 35 to 50°F (20 to 30°C) per hour, down to about 175°F (80°C). Next, it should be reheated to about 25°F (15°C) below the tempering temperature the part was subjected to during hardening heat treatment to eliminate as much residual stress as possible. If the latter temperature is not known, the same temperature that was used for preheating can be used. In general, the part should be maintained at that temperature for about 60 minutes per inch (25 mm) of thickness and then allowed to be air cooled or oven cooled.

Filler metal

The choice of a filler metal will depend on several factors :

  • capacity to preheat ;
  • the size of the repair ;
  • the type of repair to be made ;
  • the part working conditions.

In general, the filler metal(s) chosen should enable the part to resist the various stresses it will encounter in service.

If the parts must undergo heat treatment for hardening or surface treatment (shot blasting, photoengraving, etc.), or if the repaired area must wear in the same manner as the rest of the part, a uniform structure is necessary and the filler metal chosen must have similar characteristics to the steel part is made of (Sodel H13, Sodel O1, Sodel P20, etc.)

When the part cannot be preheated, you must use a filler metal that will minimize the risks of cracking, such as Sodel 335, for the initial passes. Once the groove and its edges are well covered, you can continue welding with the appropriate electrode.

To join two pieces of tool steel, it is best to use a filler metal with high resistance to cracking, good tensile strength and good ductility, like Sodel 335. For this type of repair, it is important to leave enough room for two or three layers of filler metal having characteristics that are at least equal to those of the base metal. In a number of situations, using a filler metal like Sodel 245, which is stronger than the base steel, will help to compensate for the difference between the base steel and the filler metal used as a cushion layer or for joining.

For some repairs, a composite tool can be created. The tool is first fabricated from low-allow steel that can be hardened by heat treatment. The parts of the tool that are subject to special stress are covered with a layer of Sodel 335 and two or three layers of a stress-resistant tool steel (Sodel 245, Sodel H13, Sodel P20, etc.)

For parts working at high temperatures, it may be advantageous to use Sodel 3500 as the under layer instead of Sodel 335. Consult Sodel Technical Service to verify the appropriate product for your application.

Finally, as is the case for most repairs, it is very important to determine the type of stress the tool will be subjected to in order to make the best possible choice.

PRACTICAL TIPS FOR WELDING TOOL STEELS

  1. Clean all traces of grease or oxide from the weld surface.
  2. Protect polished surfaces with an anti-adhesive to prevent spatters from damaging the finished surface.
  3. Preheat the part in accordance with the tool steel 3 table and maintain this temperature until the welding operation is complete.
  4. When preheating is not possible, use Sodel 335 to make the initial passes.
  5. Use the smallest diameter electrode possible and the lowest recommended weld current for the initial passes that are in contact with the tool steel.
  6. Always use perfectly dry electtrodes
  7. Slag must be completely removed after each bead during multipass welding.
  8. For sharp edge repairs, use copper or graphite plates to support the bead.
  9. To join two pieces of tool steel, use Sodel 335 because of its high resistance to cracking, tensile strength and good ductility. Finish with Sodel 243 or Sodel 245 to make the surface of the part resistant to wear.
  10. After welding, allow the part to cool very slowly, then reheat to about 25°F (15°C) less than the initial tempering temperature to eliminate as much residual stress as possible.

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Standard

PART 2 :WELDABILITY OF ALUMINUM ALLOYS

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The various series of aluminum alloys are all weldable, but some variants of the 7XXX series are more difficult to weld due to their high tendency to cracking. Generally, the operational weldability of aluminum is good; however, certain precautions are necessary when welding aluminum.

The family of aluminum alloys that are easiest to weld are the 1XXX, 3XXX and 5XXX series in which no heat treatment has been applied. The 6XXX series can be welded readily, but welding reduces their mechanical properties.

The high-strength 4XXX and 2XXX series are weldable, but special precautions must be taken. In the 7XXX family, only the 7039 and 7005 alloys are weldable. In addition, welds age naturally, and after 30 to 90 days, only 70 to 90% of their previous mechanical strength remains.

Overall weldability

The heat required to melt the base metal when welding lowers the mechanical strength within the heat-affected zone through annealing. With alloys that have been cold worked to obtain their mechanical properties, the properties in this zone cannot be regenerated through heat treatment.

HAZ

Operational weldability

The alumina (Al2O2) layer covering aluminum and its alloys is about 0.0004 inch (0.01 mm) thick and tends to increase with temperature. This coating acts as a thermal and electrical insulation and is insoluble in the molten metal. It is also less dense, which explains why it remains on the surface of the weld metal.

To weld aluminum, you must therefore eliminate this layer which hampers welding, either with a mechanical device like a grinder or stainless steel brush, or through chemical methods such as strongly alkaline solutions.

When welding with gas (Sodel 480) or coated electrodes (Sodel 118), the flux contained in the rod itself, or in the slag from its melted coating, prevents the oxide layer from re-forming. In a similar manner, in the GTAW (TIG) or GMAW (MIG) weld processes, the gas shield protects the weld metal from air contamination and helps to minimize re-formation of the oxide layer. In addition, the use of alternating current for GTAW (TIG) welding breaks the oxide layer that forms at high temperatures.

Another welding problem with aluminum is its high thermal conductivity. To counter heat losses due to thermal diffusion, the piece to be welded must often be preheated to 400 to 500°F (200 to 260°C). Preheating stabilizes the welding arc by making it less erratic and promotes good penetration. When working with aluminum pieces that are to be heat treated or may age, it is important to understand the consequences of preheating on the mechanical properties of the alloy in question. Sometimes an excessively high temperature causes embrittlement or a reduction in certain mechanical properties.

Metallurgical weldability

Welding aluminum alloys can result in a lowering of the mechanical strength of the base metal if the latter is cold worked, whenever the part’s temperature rises above 660°F (350°C) during welding. It is impossible to regenerate the mechanical properties obtained through cold working by heat treating after welding. Depending on the application, it may be important to take into account this loss of strength in the HAZ.

It is important to remember that if an alloy has undergone one or more heat treatments prior to welding, welding will undo all the effects of the previous heat treatment. The larger the heat affected zone, the more this reduction of mechanical properties will lower the part’s performance in service. However, small zones have limited impacts in terms of reducing mechanical properties. This is why it is always important to limit the HAZ as much as possible.

For precipitation-hardened aluminum alloys, it is usually preferable (although sometimes difficult in practice) to follow one of the two sequences shown below, in the order given :

-homogenizing                                                -welding an alloy that has already been                -welding                        or                                 solution  annealed (T4)

-solution annealing                                         -aging

-aging

Filler metal selection

The choice of a filler metal primarily depends upon weldability, desired mechanical properties, or corrosion resistance.

To minimize hot-cracking, Sodel 118 can be used for shield metal arc welding (SMAW); Sodel 480 can be used for gas welding. The high silicon content of these three products provides excellent de-oxidizing power and lowers the melting point of the filler metal. Their low magnesium content also reduces the likelihood of hot cracking.

PRACTICAL TIPS FOR WELDING ALUMINUM

1- For heat-treated alloys :

– Select the alloy that will be least affected by the weld thermal cycle.

– Design assemblies so that weld joints are located in places less subject to stress.

– Use as short a weld cycle as possible

2- Aluminum-copper alloys (2XXX series) are highly susceptible to burn through. To minimize the risks, pass through the solidification range quickly.

3- If the base metal temperature exceeds 660°F (350°C), recrystallization of the cold worked zone occurs causing a lowering of mechanical strength.

4- When welding, annealing may occur, thereby undoing all the effects of previous heat treatments.

5- Tendency to hot cracking is greater when the weld metal contains from 0.5 to 1.5% silicon and/or 0.5 to 2.5% magnesium.

6- Aluminum-magnesium alloys (6XXX series) have the highest resistance to atmospheric corrosion. However, they are anodic with respect to several non heat-treated alloys and can corrode when welded to the latter.

7- Using filler metal containing silicon (4043) when welding an alloy of a type other than aluminum-silicon produces blackening of the weld during the post-weld anodizing treatment.

8- Careful joint preparation ensures good penetration and strong mechanical properties (see Operational weldability section)

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Standard

PART 1 : TYPES OF ALUMINUM AND THEIR CHARACTERISTICS

AL1

General

Aluminum used to be considered unweldable because of its refractory oxide layer (alumina). Later on, a flux (stripper) was used to remove the alumina, thus enabling gas welding of aluminum alloys. Through electronic stripping, it is now possible to weld aluminum using the GTAW process or even with coated electrodes. Aluminum can readily be welded these days, and it is easy to create a high-quality, good-looking bead using the SMAW, OFW, GTAW or GMAW processes.

Classification

There are a wide variety of aluminum alloys. They are usually classified according to the Aluminum Association Alloy Number for wrought alloys. The four-digit number indicates :

XXXX             alloy series (see below) based on the major alloy element ;

XXXX             if other than ”0”, denotes a variant of the basic alloy;

XXXX             in the 1XXX series, indicates the purity of the aluminum; for series 2XXX                               through 8XXX, indicates a specific alloy within the series

Example                                   Alloy series                                            Major element

1100                                             1XXX                                                       Al-99% or more

2219                                             2XXX                                                       Copper (Cu)

3003                                             3XXX                                                       Manganese (Mn)

4043                                             4XXX                                                       Silicon (Si)

5356                                             5XXX                                                       Magnesium (Mg)

6061                                             6XXX                                                       Mg + Si

7005                                             7XXX                                                       Zinc (Zn)

8006                                             8XXX                                                       Other elements

The 1XXX, 3XXX, 4XXX, 5XXX series and some alloys of the 8XXX series, cannot be heat-treated; i.e., their mechanical properties cannot be significantly altered through heat treatment. However, the 2XXX, 6XXX, 7XXX series, and other alloys of the 8XXX, series are heat treatable. Their mechanical properties can be altered through heat treatment because they contain certain alloy elements that dissolve in the alloy at high temperatures and can then be naturally or artificially aged to improve their mechanical properties.

 

Series 1XXX alloys are often used for their good thermal and electrical conductivity. They are thus used in the manufacture of electrical overhead conductors (1350), heat exchangers and cookware (1050).

Series 2XXX includes the aluminum-copper alloys primarily found in aeronautics or public and military transportation because of their strong mechanical properties and high machinability (2017).

The aluminum-manganese alloys of the 3XXX series are valued for their corrosion resistance, weldability and formability. They can be found in the manufacture of the roof sheeting, storage tanks and cooking vessels (3003).

For engine parts, or engine blocks themselves, aluminum-silicon alloys of the 4XXX series are used since they are very easy to cast and stand up well under forging (4356).

Because of their excellent weldability, corrosion resistance and good mechanical properties, aluminum-magnesium alloys of the 5XXX series are used in the manufacture of dump truck bodies, bottle caps and scuba diving air tanks (5356).

The 6XXX series includes aluminum-magnesium-silicon alloys used for camping trailers, streetlight fixtures, and truck bodies, and certain marine applications (6061).

The aluminum-zinc alloys of the 7XXX series are used for special applications; 7020 is used in the first and second stages of the Ariane rockets, for example. These are the best all-around alloys. They are corrosion resistant, easy to cast and machine, and have good mechanical properties because of their zinc content. They are even used to make armor plate for some types of military aircraft (7075).

Metallurgical state designation

The metallurgical state designator is separated from the alloy designation by a dash. This designation (”Aluminum Association Temper Designation System) is used for wrought or cast aluminum and its alloys. It reflects the various treatment sequences used to obtain the desired metallurgical characteristics.

It is comprised of one letter indicating the basic state, and one or more numbers for cold worked or heat treated states. The following four basic states can be found :

-F, As fabricated. The physical state of these materials has not been controlled during the manufacturing process. There are no specific mechanical property requirements for such materials.

-O, Annealed or re-crystallized. Materials in which ductility and dimensional stability are more important than hardness and mechanical strength. These are the softest and weakest of the hot-worked alloys.

-H, Cold worked (hard drawn). Alloys in which mechanical properties have been improved by cold working, with or without heat treatment. The symbol ”H” is always followed by two or more numbers. The first number represents a well defined sequence of operations.

           -H1X. Cold worked. Applied to products whose mechanical properties have been                 obtained solely through cold working. The second number defines the level of cold             working : 1 is the lowest, 8 is the highest, and 9 is used for ultra hard materials.

-H2X. Cold worked and partially annealed. Products hardened to a high level                        through cold working followed by partial annealing to restore the desired                            properties. The residual cold working level is indicated by the second number, the              same as for H1X

-H3X. Cold worked and stabilized. Applies to alloys containing magnesium whose               mechanical properties have been stabilized through low-temperature heat                           treatment. The residual cold working level is indicated in the same manner as the             preceding types.

-T, Heat treated. Product that underwent various heat treatments, with or without hardening through cold working, to obtain a stable state other than ”F”, ”O” or ”H”. The symbol ”T” is followed by a number between 2 and 10 inclusively that indicated the thermo-mechanical treatment sequence to which the alloy has been subjected.

                -T2        Annealed (cast product only)

-T3       Solution annealing followed by cold working

-T4       Solution annealing followed by natural aging to a state considered stable

-T5         Artificial aging following hot forming

-T6         Solution annealing followed by artificial aging

-T7         Solution annealing followed by overaging or stabilization

-T8         Solution annealing, cold working and artificial aging

-T9          Solution annealing, tempering and cold working

-T10        Artificial aging as in T5 followed by cold working

Physical properties of aluminum alloys

Heat capacity

The heat capacity of aluminum (the quantity of heat required to increase by 1°C one gram of aluminum) is twice as high as iron. That is why it takes more heat or energy to raise the temperature of aluminum than it does when welding steel.

Thermal expansion coefficient

This parameter is also about double that of steel. However, steel deforms to about the same extent as aluminum because aluminum’s melting point, 1220°F (660°C), is about half that of steel, 2800°F (1540°C).

Thermal conductivity

Aluminum conducts heat three to five times as much as steel. This is why heat dissipates very quickly in the areas adjacent to the weld. More heat must therefore be provided because of the heat capacity, but it must also be provided quickly to minimize heat losses on each side of the weld.

Solidification range

Some alloys have a high solidification range; as aluminum-copper alloys with a 140°F (60°C) solidification range. Within this range, the alloy is still liquid and has no mechanical resistance. This range must be passed through as quickly as possible to prevent burn through within the zone.

Next part (November 11) : Weldability of aluminum alloys

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Standard

PART 3 : CAST IRON WELDING METHOD

To minimize heating of the piece, the smallest possible diameter electrode should be used for the first passes that are in contact with the cast iron; 3/32 inch (2.5 mm) electrodes are best for this purpose. Similarly, you should select the lowest current setting possible that will provide good wetting and perfect adhesion. Adhesion is often enhanced by using alternating current. Avoid weaving the electrode when welding; direct the arc into the weld metal to minimize dilution; and make beads a minimum of 3/4 to 2 inches (20 to 50 mm) in length. The beads should be peened with a round-heated tool while they are still hot enough : over 1000°F (540°C).

The purpose of peening is to deform the bead through compression forces to reduce the effect of the shrinkage stresses created during cooldown. Be sure not to use a pointed tool such as a slag-removing hammer, because any holes it may leave behind could become sources of cracking.

If the deposited metal contains porosities, this means that the base metal is contaminated or impregnated with sand. You must then remove the bead with a cold chisel then seal this part of the piece with a steel-core electrode designed for cast iron (Sodel 352) before continuing welding.

The arc should be struck within the groove to prevent creation of a brittle spot on the piece. To minimize the effect of local overheating, successive beads should be spaced out over the entire area of the groove, taking care always to lay down the beads in the same direction, ensuring that the beads are not aligned so that they do not create a rupture plane, and making sure to fill in each crater.

It is sometimes preferable, especially with thick pieces, to butter the faces of the groove with a nickel type electrode (Sodel NI99, Sodel 355) before finishing the fill with a ferronickel type electrode (Sodel NI60, Sodel 35, Sodel CU89). This way, the metal deposited in the buttering will absorb part of the stresses by deforming readily, thus reducing the risks of cracking.

The use of nickel type electrodes (Sodel NI99, Sodel 355) is not recommended when more than three layers are required because deposits with very high nickel content are subject to hot-cracking. It is better to complete the fill using a ferronickel type electrode (Sodel NI60, Sodel 35, Sodel CU89) or to alternate nickel and ferronickel type electrodes.

Welding must always be done beginning in the most restrained areas and working toward those which are less restrained. The following diagram, below, illustrates this principle.

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When replacing part of a piece with a steel plate, the best technique is to make a hole in the center of the plate, cut it into four segments, then join the four parts to

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 the piece before welding them together, as per the sequence shown in the diagram. It is also important to minimize heating using the method described at the begin

ning of this section. The holes prevent crack propagation; they are filled in when the rest of the weld complete.

 

When the thickness of the piece requires filling in several overlapping passes, this must be done from the outset by progessing over the full thickness of the groove to allow the opening as much play as possible during welding (see diagram below).

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If the hot welding method is used, bead length can vary from 3 to 5 inches (75 to 125 mm). This welding method is otherwise the same, except that cooldown should be as slow as possible and never greater than 30-55°F (15-30°C) per hour. To achieve this cooling rate, you can use insulating blankets.

With hot welding, an oxyacetylene rod that deposits grey cast iron (Sodel 65FC) can be used. This way, the deposited metal will have a metallurgical structure and color similar to that of the piece.

You must, however, select high preheat temperatures between 1000 and 1200°F (540 and 650°C), use wider groove angles (up to 120°), and round off the edges and base of the joint well.

The interpass temperature should not fall below 600°F (315°C) and must be careful not to overheat the piece beyond 1250°F (675°C).

The flame used must be neutral or slightly reducing.

First, form a 1 inch (25 mm) weld metal pool at the root of the groove, keeping the inner flame of the torch 1/8 – 1/4 inch (3-6 mm) from the surface. Then gradually move the flame from one face to the other until they melt into the weld metal pool. Then direct the flame toward the rod to add filler metal to the weld pool. Each pass should not exceed 3/8 inch (10 mm) in thickness.

Slag cleaning

Slag on the deposit can be removed with hand tools and a stainless steel brush. When making multipass welds, all traces of slag must be removed from the weld before going over the bead.

Postheating

Post-weld annealing can be done to :

  • improve the ductility of the heat-affected zone ;
  • enhance the machinability of the deposit and the heat-affected zone ;
  • relieve residual stresses

The temperature, time at temperature, heating and cooling rates vary according to the type of cast iron and its alloy content. The manufacturer is the best person to contract for the choice of heat cycle. Generally, for unalloyed grey cast iron, annealing at 900°F (500°C) followed by cooling in free air reduces residual stresses by 30%, while annealing at 1100°F (600°C) reduces them by 50%. To eliminate them almost completely, you must :

  • raise the temperature to 1650°F (900°C) ;
  • hold the piece at that temperature for 60 minutes per inch (25 mm) thickness;
  • then air cool

For ductile cast iron, the cycle that gives the piece maximum ductility consists in :

  • heating to between 1650 – 1750°F (900-950°C) for one hour plus one hour for each inch (25 mm) thickness ;
  • oven-cooling down to 1275°F (890°C)
  • holding at this temperature for five hours plus one hour for each inch (25 mm) thickness ;
  • oven-cooling down to 650°F (345°C) at the rate of 100°F (55°C) per hour ;
  • allowing it to air cool

 

PRACTICAL TIPS FOR WELDING CAST IRON

1- To clean grease or other contaminants from cast iron, heat the pieces uniformely to between 700 – 1000°F (370 – 540°C) until all volatizing stops (about one hour). Please refer to Surface and joint preparation.

2- If there are cracks on the piece before welding, stop them from propagating by making holes in their line of extension at about 3/8 inch from their apparent ends.

3- To minimize cracking risks during welding, use ”U” shaped preparations and round off the edges.

4- To detect the presence of a hardened zone, use a drill to check whether the bit can penetrate the piece.

5- When a hard layer (hardened zone) exists in the joint, remove it before welding

6- Remove any apparent traces of grinding using a chisel or file before welding.

7- To improve the mechanical strength of the joint, insert studs into the groove surface.

8- Cast iron can be welded without preheating – see the Preheating section

9- Using alternating current for welding cast iron produces very good adhesion.

10- To limit heat input, make beads about 1 inch (25 mm) long and lay them down one after another in a random and discontinuous process.

11- Buttering the pieces using a nickel electrode before welding helps lower the risks of cracking by reducing the stresses within the cast iron.

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Standard

PART 2 : CAST IRON WELDABILITY

Surface and joint preparation

It is important to clean the surface to be welded properly to remove any trace of oil, grease, rust, dirt, liquid-penetrant inspection solution, or paint using the appropriate solvents. Because of its porous nature, cast iron tends to become impregnated with such contaminants and should therefore be heated to between 700 and 1000°F (370 and 540°C) for about one hour or until volatizing no longer occurs. Be careful not to cause localized overheating, or to heat or cool the piece too quickly, to prevent cracking (see preheating section).

All defects or cracks must be removed. To prevent cracks from propagating during preparation or welding, it is best to block them by drilling holes ahead of their line of propagation about 3/4 inch (10 mm) from the apparent end of the cracks. The diameter of these holes should be 1/4 inch (6 mm) for lesser thicknesses and larger when the piece’s cross-section is greater. If available equipment does not allow drilling, a bead 1 to 11/2 inches (20 to 40 mm) long can be welded across the crack at each end. If a bead cracksndue to welding stresses, the end of the crack must be located again and the operation must be repeated.

If the part has already been welded, we must check for the presence of a hard layer near the welded bead. A quick and efficient method to detect a hardened zone is to use a drill to check whether the bit can penetrate the piece. All hardened zones must be removed before welding.

Since cast irons are brittle, it is better to make grooves in the shape of a ”U” and to round off their edges. Also, when nickel-based filler products are used, groove angles should be about 30% more open than for steel,  and the root face of the joint should be thinner to ensure thorough penetration. For thicknesses greater than 1/2 inch (13 mm), using ”U” or ”J” grooves on both sides will minimize the amount of filler metal and shrinkage stresses.

For grey cast irons with a rupture load less than 40 000 psi (275 MPa), you can groove only two-thirds of the thickness for pieces over 1/2 inch (13 mm) when ferronickel type welding electrodes (Sodel Ni60, Sodel 35, Sodel CU89) are used.

There are several available methods for removing defects, cracks, mill scale, or oxide films, and for chamfering. The edges to be welded can be machined, ground, chiseled or arc gouged using a Sodel 512Plus electrode. Arc gouging is the fastest method but can result in the formation of a thin hardened film. When such a hardened film forms, it must be removed by mechanical means prior to welding. When grinding is used as the preparation method, grinding traces or residue should be removed with a file or cold chisel.

If the piece shows extensive cracking, a part of it can be removed and replaced with a mild steel plate. To minimize the risks of cracking, it is important to round off the corners of the plate well. Given the high tensile strength of mild steel, the thickness of the plate can be up to 33% less than that of the piece for grey cast iron with a tensile strength less than or equal to 40 000 psi (275 MPa).

For thick pieces, brittleness in the heat-affected zone can be minimized by gouging grooves in the joint face (see cast iron diagram below). These grooves are filled with the selected filler metal, then the faces are buttered and welded. The welding shall be done as described on the next article : Cast iron welding method. This method prevents the formation of continuous weakness planes in the heat-affected zone, thereby minimizing the propagation of cracks that may occur in service, or during shrinkage of the deposited metal during colldown.

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Another method of reinforcing the joint is to insert studs into the chamfer surface (see cast iron diagram below). When the studs are made of steel, there should be enough of them so that their surface area equals about 30% of the piece’s cross section in the area of the break.

The studs are screwed or pressed into pre-drilled holes to a depth equal to the diameter of the stud or more.

Furthermore, the studs should not all be inserted to the same depth and they should not be aligned across from one another. The studs should protrude from 1/8 to 3/16 inch (3 to 5 mm) above the chamfer face so that they will bond

well with the deposited metal. The studs are then

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welded in place and the surfaces of the chamfer are

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tered; the surfaces are then welded together. This technique bonds the deposited metal to the part of the piece that has not been thermally affected.

Preheating

When grey, malleable, ductile or compacted graphite cast iron is welded, part of the heat-affected zone tends to change into white cast iron or a brittle structure. This transformation occurs during cooldown and is promoted by their high carbon content, their somewhat greater alloy element content, and the speed with which heat dissipates through the piece’s cross section.

To prevent the heat from dissipating into the piece too quickly, it can be preheated to between 600 and 1200°F (315 and 650°C). It must be protected from any drafts, however, to prevent cracking problems. In addition, it is not always possible to preheat the piece because of its size or the equipment involved.

For all these reasons, it is usually better to use the cold weld method. With this method, the piece does not need to be preheated provided it is at room temperature, 68°F (20°C). Also, the temperature differential between the weld and base metal should be limited to around 90°F (50°C) so that the welder can keep his bare hand in prolonged contact with the area around the weld. In other words, for a piece at 68°F (20°C), the temperature in the vicinity of the weld should not exceed 158°F (70°C).

Even when the cold weld the cold weld method is used, it may be desirable to preheat certain parts of the piece. Doing so will cause the joint to open when these parts are heated, thereby putting the joint under compression during cooldown, thus reducing the amount of shrinkage stress. The following cast iron diagram illustrates this effect :

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Next part : Cast iron welding method (3) (October 15th)

 

 

 

 

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PART 1 : TYPES OF CAST IRON

General

Cast irons are essentially iron and carbon alloys that also contain some other elements such as silicon, manganese, sulfure and phosphorus. They are sometimes alloyed with chromium, nickel, copper, molybdenum, vanadium or other elements to enhance their resistance to wear, corrosion or high temperatures.

Unlike most metals, cast irons are not classified according to their chemical composition but instead by their microstructure. Cast iron’s microstructure appears in the form of graphite or carbide particles surrounded by a matrix of steel which can be ferritic, perlitic austenitic or martensitic, depending on the alloy elements, rate of cooling, and heat treatment received. We can therefore distinguish several types of cast irons :

White cast iron

Thus named for the white appearance of its breakage surface, its carbon remains bound to the iron to form carbides during solidification. The presence of these carbides make it very hard, but also very brittle. Its hardness makes it an ideal material for applications requiring high wear resistance such as crusher balls, drawing dies and extrusion nozzles.

Grey cast iron

So named for the grey appearance of its breakage surface. Carbon precipitates in the form of graphite flakes (see cast iron diagram, below). These graphite flakes act as weaknesses that promote crack propagation inside the material, thereby reducing its tensile strength.

However, these same flakes give it excellent vibration absorption capacity and good thermal conductivity. This cast iron is very common; it is easy to cast and machine since the graphite flakes act as a lubricant. It is also used to manufacture transmission parts, fire hydrants, housing, engine blocks, counterweights or machinery bases.

Malleable cast iron

This cast iron is obtained by annealing white cast iron through a fixed temperature and time at temperature cycle. Annealing breaks down the carbides into nodules of serrated graphite, also called temper carbon (see cast iron diagram). These nodules are less compact, however, than those found in ductile cast iron. There are two types of malleable cast iron. The most common is called ”blackheart” and is obtained by the process described above. The other type is found primarily in Europe and is called ”whiteheart”; it is obtained through decarburization of white cast iron.

Ductile cast iron

When a limited amount of special elements like magnesium or cerium are added, the carbon precipitates in the form of compact graphite nodules without having to undergo heat treatment (see cast iron diagram, below). This cast iron normally contains much less sulfur than those previously mentionned. Its mechanical properties are generally slightly better than those of malleable iron. It is used for the same applications as malleable cast iron, and also for large gears, forming dies, rollers and roll housings.

Compacted graphite cast iron

This cast iron too is obtained by adding minimal amounts of special elements such as magnesium, calcium, titanium, or aluminum. The carbon precipitates in the form of compacted flakes (see cast iron diagram, below). Its mechanical properties fall somewhere between those of grey cast iron and ductile cast iron. It is used to make brake discs, cylinder heads, sprocket gears, manifolds, housings or pulleys.

Alloy cast iron

Alloy elements can be added to white, grey, malleable, ductile and compacted graphite cast iron to enhance their resistance to wear or corrosion, their high temperature properties, or their mechanical properties. The most common additions are chromium (up to 35%), nickel (up to 45%), molybdenum (up to 5%), copper (up to 10%) and silicon (up to 18%). They are most often added in combination since the effect of one element will reinforce or enhance the effect of another.

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Typical chemical compositions for the cast irons listed above are given in the following table :

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Physical properties of cast iron

Thermal conductivity

Cast irons with graphite in flake form conduct heat better than those containing nodular graphite, and much better than white cast iron. Generally, any given steel will not conduct heat as well as grey cast iron with the same matrix, because the thermal conductivity of graphite is very high. It is five times greater than a ferritic matrix, eight times greater than a perlitic matrix and 50 times that of iron carbide. In addition, the presence of alloy elements in a matrix reduces its conductivity.

Electrical conductivity

The presence of alloy elements in a steel or cast iron matrix reduces the material’s electrical conductivity. Likewise, the presence of graphite reduces cast iron conductivity, and even more so when the graphite is in flake form, since there are more obstacles to the flow of current in this case.

Thermal expansion coefficient

Cast iron has a coefficient of thermal expansion similar to that of carbon steel, except for highly alloyed cast irons which can have lower or higher coefficients, depending on their alloy elements.

Next part : cast iron Weldability (2) (October 18th)

 

 

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Operation snow removal : Strategies in order to increase the durability of wear pads

bladesScraper blades of snow removal equipment are originally considered as wear (spart) parts due to their degradation they face during their operational life. However, in order to prevent direct wear degradation of these parts while improving the equipment efficiency, manufacturers have worked on the modification and the design by adapting and fixing wear pads on the rear of the scraper blades in such a way to raise them slightly above the ground level. This way of proceeding protects the blade when these pads are wearing.

Aggregates of solid particles, asphalts, sand are ice that detach from the pavement and shoulder edges slide and scratch the underside surface of the wear pads during snow removal operations. Thus, the predominant wear mechanism in such a situation consists mainly of abrasion. The wear surface is characterized by various scratches of variable depth and parallel to the moving direction. The wear depends on pad material properties and the different operating parameters such as pressure, nature of ground, sliding distance or contact time, environment, etc.

In the light of greater amplitude of sliding movement, the wear debris are mostly evacuated from the contact zone (pad/ground), some other ones are embedded on the surface prior to detaching with the progression of wear. The damaged surfaces exhibit grooves and traces of plowing parallel to the direction of movement. Wear rate is becomes roughly steady over time. Wear volume increases with the applied load (pressure) and the travelled distance.

Wear pads: materials and fabrication:

Wear pads are generally made of either fully hardened steels or from hardfaced steel base metal. In fact, the hardfacing technique consists of his last option is considered as the preferred choice to confer a higher abrasion resistant to these critical parts, leading to increase their technical lifetime and also the equipment efficiency. According to the wear conditions described above, the materials that must be selected for hardfacing consists of ferrous alloys similar to high chromium white cast irons with the addition of strong carbides-forming elements (Nb, V, W). The typical microstructure of this type of hypereutectic alloys with a high fraction of primary carbides imbedded in matrix of austenite and secondary carbides. The high carbides concentration leads the abrasive particles to slide on the wear surface instead of indenting and abrading the matrix.

As per the previous experiences, it turns out that hardfacing deposited following a dot pattern have better spalling resistance in comparison with continuous hardfacing layers deposited on the same base metal. In addition, the selected hardfacing material must be compatible with the base metal. Welding must be carried out in a way to ensure a minimum penetration and to prevent the lack of fusion on the edges of prepared holes.

Preparation and welding

  1. Piercing of holes of 1/4” deep and 1/2 – 3/4” diameter as per a pattern of holes on the underside surface of the wear pad;
  2. Chamfering of the hole edges
  3. Filling and hardfacing with Sodel 2045 Plus, Sodel 2024 Plus or other products according to the base material and the recommended strategy

For hardfacing, proceed as follow :

  1. Preheat to 400°F;
  2. Strike the arc on the bottom of the hole and start to fill the hole with a circular movement from the edge towards the centre;
  3. Deposit an excess thickness of 2-3 layers after filling the hole;
  4. Let cool down to the room temperature;
  5. Grind out the outer periphery of the hardfacing to render it in the form of a rivet head.

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Khemici.badri@sodel.com

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