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Subject on This Issue:
* Steels & Properties
* Heat Treatment


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DIMENSIONAL STABILITY OF TOOLS & DIES/MOULDS

Dimensional stability of dies/moulds and tools in service is rising in importance with the technology advancement requiring high level of precision. Attainment of adequate dimensional stability in high precision dies and tools is often a difficult problem because such factors as material variation, design, machining, heat treatment and stamping process impose practical limitations on controlling dimensional stability since dimensional change occurs in all tools and dies made of high alloy tool steels. The harmful dimensional change could result in oversize of a finished workpiece or even catastrophic failure.

Why does the dimensional change occur

Dimensional change of tool steels is affected mainly by the level of retained austenite but not all. It is also influenced by the internal stress, transformation of retained austenite and martensite decomposition in parts. Tool steels, especially having high carbon and high chromium contents, can have high amount of retained austenite after heat treatment.

Retained austenite

Retained austenite is a specific crystalline form of iron and steel, which is soft and tough. It forms due to a combination of variables including high carbon and alloy content, long austenitizing time or high temperature, discontinuing the quench process too soon, or other factors in the process may cause the martensite start temperature (Ms) to become depressed to below room temperature.

As steel is heated up and down through the different phase ranges, it’s crystalline structure shifts back and forth from one type of crystal to another. In steel that is heated to austenite and cooled down very quickly, the crystals don’t have the time they need to shift from austenite into ferrite. Rapid cooling results in a new crystalline form that is called martensite. In high-carbon steels, martensite has a greater volume than both austenite and ferrite. Martensite is created in a special reaction that occurs during rapid cooling and begins at a relatively low temperature specific to the chemistry of the steel. For AISI D2 tool steel, Ms is around 120 degree C and for typical case-carburized bearing steels this temperature will be 260 degree C. Fig.1 shows the Time-Temperature-Transformation (TTT) diagram for a D2 equivalent steel, indicating Ms. As the steel cools below Ms, martensite begins to form. As the temperature decreases, the amount of martensite in the steel will increase until a temperature is reached where practically all of the available austenite is transformed into martensite. This is known as the martensitestop temperature. Typically, the martensite stop temperature is impracticably low (usually well below 0 degrees C). Since the martensite stop temperature is not achieved during quenching, some of the austenite in the parts remains untransformed. This austenite, which is left over from when the steel was hot, is now mingled with the newly formed martensitic structure. Since this austenite is "retained" from the heat treating process, it is commonly referred to as retained austenite.

Depending upon the steel chemistry and the specific heat treatment, the retained austenite level can vary from over 50% of the structure to nearly zero. Retained austenite is metastable. This means that when given the opportunity, it has a characteristic tendency to change from austenite into martensite. This metastability is an important characteristic of retained austenite. Its transformation can result in a corresponding expansion in volume.

The net result on volume change of a workpiece during usage is a combined effect of the considerable expansion due to transformation of retained austenite and the slight shrinkage arisen from decomposition of the newly formed martensite. These two changes take place in addition to any thermal expansion or contraction that is taking place. They can sometimes lead to unexpected growth in service, causing loss of accuracy.

1a
Fig.1 Time-Temperature-Transformation diagram (D2 equivalent steel)

Corrective measures

Fig.2 shows the chart of retained austenite against the tempering temperature for AISI D2 equivalent, indicating a 10% of austenite will be retained when tempering at 475°C and around 8% of retained austenite will be expected under the tempering temperature of 500°C, resulting in volumetric changes of 0.4% and 0.32% respectively. It is obvious that the 8% level of retained austenite is still too high for the control of dimensional stability of our parts.

With cryogenic (subzero) treatment, the amount of retained austenite can be remarkably reduced after just heat treating depending on the material and control of specifications. Subzero treatment is useful to improve the dimensional stability and wear properties by eliminating retained austenite and untempered martensite, but it increases the likelihood of cracking. Alloys with greater than 0.4% carbon require subzero treatment to finish martensite transformation. For D2 steel, martensite finish temperature Mf (a temperature at which the transformation of martensite is essentially complete) lies between -80ºC and -110ºC. At the temperature down to -100ºC, retained austenite in the as-quenched microstructure transforms to martensite, resulting in an increase in hardness and a reduction in toughness. In meanwhile, high stresses were arisen as a result of temperature change and phase change. It is very important to ensure that the die plate design will tolerate immediate cold treating rather than immediate tempering for this type of high carbon/high alloy steels. Design features such as sharp corners and abrupt changes in section should be minimized.

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Fig.2 Retained austenite vs tempering temperature

It is advisable to employ multiple tempering in conjunction with subzero treatment for the complicated parts, but subzero treatment requires the grade of steel and the product design to tolerate immediate cold treating after quenching without cracking.

Experimental results

An investigation into dimensional control of the parts made of various grades of D2 equivalent steels was carried out to identify the materials response and optimize heat treating processes and parameters. The results showed that for the good dimensional stability, the retained austenite shall be below 4%. The amount of retained austenite can be remarkably reduced to less than 4% against 10% by using a combination of two tempers and one subzero processes. Two tempers at a temperature between 450 and 525°C for a certain material may result in a hardness level of around 58 HRc in case of gas quenching. The harness of 58 – 60 HRc can be achieved by oil quenching but the risk of distortion and cracking will be higher.

 
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