Hot Work Steels

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Characteristics of hot work tool steels

Generally the term “hot work steel” includes tool steels which adopt a constant temperature above 200°C (392°F) during application.

Superimposed on the latter are peak temperatures brought about by the operational cycle.  Consequently, the use of hot work steels involves, in addition to the usual stresses which tool steels are subjected to, thermal stresses due to the contact between tools and hot materials during forming.

Hot work steels must exhibit good heat checking resistance in order to delay - for as long as possible - the formation of chill cracks appearing on the surface in reticulate shape as a consequence of temperature fluctuation in the surface region.

To avoid hot cracks, i.e. tension cracks developing primarily in tools with far cavities at sectional transitions and edges and extending - contrary to chill cracks - far into the tool, hot work steels also have to feature good high temperature toughness.

For tools subjected to high impact, pressure, or tensile stresses at elevated temperatures, special attention must be paid to high strength at the various working temperatures.  If the structural state is changed by the influence of heat, the strength at ambient temperature and consequently the strength at working temperature are reduced.  This is why good high temperature strength and superior retention of hardness are prerequisites for stability of shape.  The relatively high thermal conductivity of hot work tool steels will also enhance retention of hardness, hence, the high temperature strength.

Excellent high temperature wear resistance is necessary for ensuring satisfactory tool life.

Further demands that must be met by hot work steels are low tendency to adhere to parts being processed, high resistance to erosion, high temperature corrosion and oxidation, dimensional stability during heat treatment, good machinability, and in some cases also good cold hobbing properties.





Quality of Hot Work Tool Steels




BÖHLER VMR® & ISOBLOC® - Low Microsegregation Level




Grades  Description  Download 
Available in ISOBLOC; primarily for the processing of light metal alloys; for highly stressed hot work tools such as die-casting tools, die inserts; hot shear blades; hot extrusion tools, mandrels, dies and containers for metal tube and rod extrusion; tools for manufacturing of screws, bolts and nuts, rivets; hot forging dies, core pins;  plastic moulds.  Exhibits good hot wear resistance, hot strength and very good high temperature toughness, with good resistant against heat checking.
Available in ISOBLOC; primarily for the processing of light metal alloys; for highly stressed hot work tools such as die-casting tools, die inserts; hot shear blades; hot extrusion tools, mandrels, dies and containers for metal tube and rod extrusion; tools for manufacturing of screws, bolts and nuts, rivets; hot forging dies, core pins;  plastic moulds.  Exhibits very good hot wear resistance, high hot strength and good high temperature toughness, with very good resistant against heat checking.
ESR quality with very high toughness; it is specially developed for use in die casting dies ensuring high toughness even in large tools for big challenges.
ESR quality with very high toughness; for the processing of heavy and light metal alloys; highly stressed hot work tools such as dies and punches in warm and hot forging; toolings for high speed presses; core pins and inserts in die-casting dies; extrusion toolings; tough-critical cold work applications; plastic procession toolings.
Vacuum remelted grade with extremely fine microstructure and high cleanliness, exhibits excellent heat checking resistance; processing of light metal alloys; for highly stressed hot work tools such as die-casting tools, die inserts; hot shear blades; hot extrusion tools, mandrels, dies and containers for metal tube and rod extrusion; tools for manufacturing of screws, bolts and nuts, rivets; hot forging dies, core pins; plastic moulds.


BÖHLER HOT WORK TOOL STEELS for die-casting tools

In pressure die-casting, liquid metal is injected into a die of the exact shape under pressure. Melts processed in this manner encompass materials with lower melting points such as tin, lead and zinc alloys, those with mid-range melting points such as aluminium or magnesium, and their alloys, up to copper alloys with higher melting points.

In the cold chamber die-casting process the molten metal is drawn in portions from a dosing furnace and filled into a casting chamber by means of a ladling device.  Afterwards the metal that was poured in is pressed into the die with a hydraulically driven plunger.  Due to the fact that the casting chamber does not come into contact with the liquid metal during the entire casting process, thus is cold in contrast to the melt, the term cold chamber process is used.

In the hot chamber die-casting process the casting chamber is in constant contact with the melt. This chamber is at casting temperature.

Since aluminium alloys and copper alloys react with the steel of the casting chamber, a longer contact of this melt with the tool components leads to erosion and corrosion.  For this reason these metals are processed using the cold chamber procedure.  In order to be able to guarantee even casting of the dies without premature solidification on thin gage parts as well, the melt is moulded under pressure of 200 – 300 bars.  Due to these high pressure conditions the casting processes take place correspondingly quickly.

Nowadays approximately 80% of the aluminium castings are produced using the cold chamber die-casting procedure.

 

Schematic of a Die-casting Tool





Material selection for die-casting tools




Die-Casting – Treatments and Maintenance

Preheating
In order to reduce the jolting thermal stresses from the hot metal which is being processed, and thereby reduces heat checking, the dies must be preheated according to the melting temperature of the hot metal:
• for alloys with low melting point : 150 – 200 °C
• for light-metal alloys : 250 – 300 °C
• for alloys with high melting point : 300 – 350 °C
Preheating of the dies has to be carried out slowly and thoroughly.
Cooling
It is necessary to conduct the heat away through appropriate cooling channels to avoid temperature build-up in the dies.
• Cooling medium should match shot sequence to ensure constant die temperature.
• During down-time etc., cooling cycle must be stopped; the tool should be either kept warm, or allowed to cool down slowly.
• The die cavities must be cleaned and material residue must be removed.
Rapid cooling should not occur to avoid stress cracks.
Stress relieving
Stresses built-up as a result of cyclical thermal loading need to be relieved from time to time via appropriate stress relieving treatment.
• The stress relieving temperature should be 30 – 50 °C below the highest tempering temperature, followed by furnace cooling.
• For a new die, the 1st stress relief is after 1,000 – 5,000 shots, after which stress relieve every 1/5 of the total expected die life.
Experience has shown that an intermediary stress relief improves tool life.


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Hot work tool steel applications