Customer Information - Tool Steel Info

Hardening of Tool Steels

Available in high-quality grades, these specifically-designed steels can be grouped broadly according to their intended application:

• High-speed steels (BM and BT series in BS 4659) for drilling/cutting, with an ability to retain hot hardness
• Cold-work steels (BA, BD and BO series) for stamping , blanking, pressing and forming.
• Hot-work steels (BH series) for hot forming and precision die casting · Plastic-moulding steels (BP series) for plastic-moulding and highly-polished dies, where toughness is required.
• Shock-resistant steels (SR series) or chisels, punches and tools subject to impact loading.
• Hammer die steels for cold forging, hammering and stamping.

What are the Treatments?

All tool and die steels must be treated to develop optimum properties in terms of hardness, strength, toughness and wear resistance. Almost all are hardened and tempered.

Hardening involves controlled heating to a critical temperature dictated by the type of steel (in the range 760-1300°C) followed by controlled cooling. Dependant on the type of material, appropriate cooling rates vary from very fast (water quench) to very slow (air cool).

Tempering involves reheating the hardened tool/die to a temperature between 150-657°C, depending on the steel type. A process which controls the final properties whilst relieving stresses after hardening, tempering can be complex; some steels must be subjected to multiple tempering operations.

In some cases, a sub-zero treatment can be incorporated into the hardening and tempering cycle in order to develop maximum hardness and optimize dimensional and metallurgical stability.

What are the processing options?
Most tools and dies must be protected from oxidization and decarburization during treatment. The heat transfer uses four basic types of furnace with various processing media to meet this requirement:· Salt Baths - the traditional route capable fo treating the complete range of tool steels with tight control.

• Fluidized beds - a more recent development capable of treating a wide range of tool steels other than those requiring high hardening temperatures.
  
• Sealed-quench furnaces - applications restricted by lower hardening temperatures and the choice of oil quenching or "still" gas cooling.
  
• Vacuum furnaces - the cleanest route, mainly employing gas quenching; the recent introduction of high-pressure gas quenching has widened the range of steels which can be successfully treated.

What are the limitations?

Hardenability
The measure of a steel’s ability to harden in depth, hardenability can very depending on the type of tools steel used. For example, low-hardenability BW grades will only hardened to a depth of a few millimeters, even with a sever water quench, whilst high -hardenability steels, such as BH grades, can harden through a section in excess of 1 metre with gas quenching.

Considered in conjunction with section size, steel hardenability can limit the choice of processing route. It is recommended that the requirements be discussed with the heat treater at an early stage.

Hardening Temperature
Some high-speed steels require extremely high hardening temperatures which can restrict the processing route options.

Physical Size
Contract heat treatment furnaces come in a variety of sizes, as do customers job's. Always check the availability of appropriate capacity at an early stage.

What problems can arise?

Distortion
Distortion of hardened and tempered tools and dies can arise from a variety of factors. Many of these are outside the control of the heat treater who cannot therefore accept responsibility for its prediction or it consequences.Complex shapes and sharp changes in section will generate stress, and hence distortion, during rapid cooling for hardening. If it is impossible to avoid such stress-raisers, select a high-hardenability steel so that slower cooling rates can be utilized. The possibility of distortion can also be reduced by specifying stress relieving prior to final machining.

Cracking
Cracking usually results from factor such as:

• Poor-quality or incorrect steels
  
• Defects in the steel
  
• Decarburisation - usually because of insufficient or unequal metal removal during initial machining ro "black" billet.
  
• Poor design and material selection
  
• Poor post-heat-treatment practice, such as incorrect grinding or EDM
  
• Incorrect Heat Treatment.

How Can I ensure successful Treatment?

• Do use good-quality steel from a reputable supplier
  
• Do design for heat treatment by eliminating features such as sharp corners and abrupt changes in section.
  
• Do talk to your heat treater before design and specification are decided.
  
• Do specify a steel capable of giving the required hardness in the section size involved.
  
• Do remove all "black" and decarburised layers and surface defects - ensure the initial section size is large enough to allow this.
  
• Do consider intermediate stress relieving to minimize distortion.
  
• Do allow for any post-heat-treatment grinding etc. when the tool/die is produced.
  
• Do ensure all your requirements are specified correctly.

How do I specify?
If uncertain, consult your heat treater before producing a specification. Always include:

• the material used, , other standard designation or trade name
  
• the hardness required (HRC, Hb or HV), quoting a realistic range
  
• The processing route required, if this is relevant (e.g. "vacuum treat" or "salt-bath treat")
  
• Any special requirements (e.g. "area to be kept soft", "press temper to keep flat")
  
• Any area where testing must or must not be applied · any special certification or testing requirements.
  
  

[Customer Information - Tool Steel Info]