Whether heat treatment is carded out in-house or subcontracted, procedures must be drawn up and adhered to if the heat treatment process is to be properly controlled. Subcontract heat treatment should only be given to reliable heat treaters with experience and the right equipment to ensure that optimum results are obtained.
The Variety of Heat Treatment Processes Available for Tool Steels
The variety of heat treatment equipment suitable for tool steel includes vacuum furnaces, fluidised bed furnaces, sealed quench furnaces and salt pots of various make-up. Added to this is the necessity to have a wide variety of quenching facilities aimed at producing the best type of quench to achieve the desired cooling rate for a given tool steel.
Design Aspects and Heat Treatment
Tool design should be discussed with the heat treater, particularly in respect to choice of materials, geometry of the tool and subsequent effects on distortion. Cracking can result from poor die design in relation to heat treatment requirements, sudden changes of section, stamping marks and blending radii can all contribute to major problems.
Volume changes occur when steel is heated to the austenitisation temperature prior to quenching. These changes result from thermal expansion and structural changes when steel is heated through phase change boundaries. For these reasons, particular attention must be given to preheat conditions so that tooling is allowed to stabilise at various temperatures below the austenitisation temperature to achieve uniform heating. Proper preheating and soaking on tooling with varying sections reduces the risk of cracking and distortion.
Tempering and Heat Treatment of Tool Steels
Tempering is another aspect of tooling heat treatment which requires careful planning. Many of the more complex tool steels such as the high speed and hot work varieties require double tempering to completely transform austenite to martensite. These steels reach maximum hardness after first temper and are designated as secondary hardening steel the second temper is aimed at reducing the hardness to the desired working level Many authorities believe that a third temper is necessary for secondary hardening steels to ensure that any new martensite formed as a result of austenite transformation in tempering is effectively tempered. This is a matter of individual choice and involves minimum additional cost.
Cooling After Quenching
Tooling should never be allowed to cool to room temperature after quenching as this will lead to cracking. The tool must be transferred to the tempering furnace whilst still warm - about 50-80°C. During multiple tempering operations tooling may be allowed to cool to room temperature between tempers.
Stress Relief and Heat Treatment
In instances where tooling has been subject to a large amount of machining it is possible for residual stresses to be built up in the tool. These stresses need to be removed prior to heat treatment and it is recommended that a stress relief heat treatment at 500-550°C be carried out allowing the tooling to cool to room temperature after stress relief.
Spark Eroding Processes and Stresses in Tool Steels
Due to the nature of spark eroding processes in die making, tools made by this method are likely to possess a highly stressed surface. This can be minimised by reducing the electrode current density (amperes) in the final stages of die sinking followed by a stress relief heat treatment.
The Need for Normalising Heat Treatments
Before attempting to harden any tool steel it is necessary to ensure that the material has been normalised and/or annealed prior to machining. The purpose of this type of heat treatment is to grain refine and to ensure that the micro constituents are evenly dispersed throughout the steel as excessive segregation can lead to distortion due to differential hardening rates and cracking.
Generally tool steel as received from the tool supplier is in the annealed condition and this may be confirmed by hardness testing.
The Need for Annealing Heat Treatments
Tool steels which have been hot forged or cold hobbed must be annealed before proceeding with further operations such as machining or hardening. Many grades of tool steel are air hardening or partially air hardening which results in build up of internal stress in die blocks or tools forged and air cooled prior to machining. Dies and tools which may need to be rehardened for various reasons must be annealed prior to rehardening if subsequent heat treatment defects are to be avoided.
Normalising Heat Treatment
The normalising heat treatment cycle involves heating slowly and carefully to the normalising temperature for that particular steel, holding at temperature sufficient to allow homogenisation to occur and then air cooling to room temperature.
Normalising is generally carried out where required on the AS1239 grade 'W' carbon tool steels but is not usually carried out on other grades of tool steel particularly the hot work grades unless the normalising treatment is used as a preliminary to annealing. As stated above, many of the tool steel grades will air harden when air cooled from austenitising temperatures.
Annealing Heat Treatment
The full annealing heat treatment process involves heating the steel slowly and uniformly to a temperature above the upper critical transformation point and holding until complete austenitisation and homogenisation occurs. Cooling after heating is carefully controlled at a particular rate as recommended by the steel manufacturer for the grade of tool steel involved.
Cooling at this specified cooling rate is continued down to 550°C when the steel may be removed from the furnace and air cooled to room temperature.
This may occur during all heat treatment processes and is to be avoided due to subsequent detrimental effect on the finished tool unless removed by machining. The use of protective atmospheres will minimise or eliminate decarburisation. Other techniques such as the use of a borax or glass coating will also reduce the effects of decarburisation.