Common Defects in Tool Steel Castings and How to Prevent Them

Tool steels are used for dies, moulds, cutting tools and punches. Their high resistance to wear and ability to maintain an edge when heated is due to a complex alloy of carbon, chrome, molybdenum, tungsten and vanadium. The same characteristics also make them difficult to cast, as a narrow range of temperature must be maintained both when melting and when solidifying. Deviations in either process can result in imperfections that may only appear after machining or heat treatment. Here we outline the most common defects, their causes and ways to prevent them.

Characteristics of Tool Steel

Tool steels, unlike mild steel, have a wide freezing range and contain significant quantities of alloying elements which tend to separate on solidification. The metal also has a lower thermal conductivity, resulting in uneven shrinkage of thick and thin sections due to differential cooling. Its high affinity with oxygen and nitrogen in the atmosphere also makes it prone to gas absorption and oxidation. Tool steel casting therefore requires a high degree of precision and control; slight variations in the temperature of the molten metal or the material and design of the mould can result in a defective casting.

Gas Porosity

Gas porosity takes the form of small round cavities that appear in or just below the surface of the casting. Hydrogen, nitrogen and carbon monoxide gases are all highly soluble in liquid steel and will remain trapped when the metal solidifies if they are unable to escape during freezing. Common causes include: wet sand or chapulcas; moist ladles or charge; and a lack of deoxidation.

Investment casting suppliers UK makes sure that prevention includes: preheating and drying the ladles and moulds; ensuring that scrap metal and fluxes are dry and free from contamination; and deoxidising the melt with aluminium or other additives before pouring. Adequate venting and a smooth steady pour will also allow trapped gases to escape.

Shrinkage Cavities

As steel moves from its liquid to its solid state, it shrinks in volume. If additional liquid metal is not available from the riser, a cavity will form in the area of the casting that solidifies last. This is likely to occur in thick sections or where they join thin ones.

The best ways to prevent shrinkage cavities are good design and riser placement. Risers should be sized and located so that they remain molten for longer than the main casting, and chills added to promote directional solidification. Sections of different thickness should be joined with gradual transitions rather than abrupt changes in section.

Hot Tears and Cracks

Hot tears are surface or internal cracks that form when the metal is still semi-solid, while cold cracks occur after the casting has cooled. Both are caused by high levels of stress that exceed the strength of the metal, a common occurrence in high carbon and high alloy steels that have low ductility. Causes of high stress include: uneven cooling of thick sections; the restraint caused by core prints or metal inserts; and rapid cooling as a result of the pouring process.

Hot tears and cracks can be prevented by minimising restraint. This can be achieved by using a flexible mould, incorporating generous fillets in corners and using a slower cooling rate in thick sections. Avoiding premature shake out or cooling in a draught will also reduce the risk of cracking.

Non-Metallic Inclusions

Inclusions appear as oxides, sulfides or bits of refractory material trapped in the metal. They act as stress raisers and, in the case of a tool or die, can cause fractures or chipping under strain. Causes of inclusions include: reoxdiation of the metal during pouring; erosion of the refractory lining; carry over of slag from the ladle; and turbulent flow of the metal.

Prevention measures include: skimming the melt of any slag; placing ceramic filters in the gating system; avoiding splashing when pouring; and ensuring that the refractory lining of the furnace is in good condition. Proper deoxidation practice will also reduce inclusions.

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Segregation and Carbide Networks

Alloying elements do not always distribute themselves evenly throughout the metal as it solidifies. This can result in chromium, molybdenum or carbon becoming concentrated between dendrite arms and causing regions of varying hardness. In some steels, carbides form large networks at grain boundaries, causing the casting to be susceptible to chipping or impact damage.

The formation of segregation and carbide networks can be minimised by promoting rapid solidification with chills or thinner moulds and using the correct pouring temperature. Some steels require a homoginising treatment and proper annealing to ensure that carbides are evenly distributed before final heat treatment.

Surface Defects and Decarburization

Surface defects include misruns (areas where the metal has failed to flow along the mould), cold shuts (where two streams of metal have failed to join) and burn-on (fusion of sand to the surface of the casting). The carbon in tool steels can also burn away at the surface of the casting when heated to high temperatures, creating a decarburised layer that lacks toughness and is prone to wear.

These surface defects can be minimised by pouring within the recommended temperature range and filling the mould quickly to avoid premature freezing. A gating system that prevents the metal stream from breaking and high-quality coatings on the mould surface will also help. Protectiveatmospheres in the furnace and generous machining allowances will allow the decarburised layer to be machined away.

Process Control and Inspection

Consistency and discipline throughout the casting process is essential to minimising defects. Chemistry should be checked with a spectrometer before pouring, since a slight variation in carbon or vanadium content can affect the response of the metal to heat treatment. Pouring temperature should also be measured; the higher or lower end of the recommended range should never be exceeded. A number of inspection techniques can then be used to check that the metal is free from imperfections:

·        Visual and dimensional inspection of the surface for imperfections

·        Dye penetrant or magnetic particle inspection for surface cracks

·        Radiography or ultrasonic inspection for internal porosity and shrinkage

·        Hardness and microstructure tests after heat treatment

These tests should be performed on a batch basis to identify trends that might reveal the source of a repeat defect, such as a damp ladle or faulty filter.

Heat Treatment as a Safety Net

Even a defect-free casting will contain residual stresses as a result of uneven cooling. Annealing or normalising relieves some of this stress and prepares the piece for machining, while hardening and tempering create the desired properties. The rate at which the metal is heated or cooled must be carefully controlled, as excessive heat or a rapid change in temperature can cause cracking in a previously sound casting. Heat treatment is an essential part of tool steel casting and should be planned alongside the pouring process.

Conclusion

Tool steel casting defects rarely arise from a single cause; gas porosity, shrinkage, cracking, inclusions, segregation and surface imperfections are typically the result of minor variations in temperature, gating and mould design or gas content of the metal. Fortunately, there are ways to address each type of imperfection, from ensuring that ladles and moulds are dry and free from contamination to using the correct riser size or pouring temperature. When these basic principles are combined with good process control and inspection, scrap rates are minimised and the performance of the finished components is improved. For those who rely on dies, moulds and cutting tools, attention to these factors at the foundry stage can save time and money in the long run.