One Minute Mentor: Heat Treatment of a Premium Grade H13 Die
February 4, 2016
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The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F). It shows the effect of intermediate cooling rates on the microstructure and toughness of H13 tool steel austenitized at 1075°C (1970°F). (a) Continuous cooling transformation (CCT diagram) shows variation in microstructure with varying cooling rates. (b) Variation in toughness as a function of carbides ejected from austenite. Hardness was maintained at a constant value.
Hot-work die steels containing 5% Cr are normally used (H13 is the most common in the United States) because they have:
• A good combination of toughness and strength for safety and service
• Good hardenability in thick sections
• Good hot hardness and strength to resist cyclic stressing at operating temperatures up to 400 °C (750 °F)
• Good resistance to tempering during service that gradually lowers the surface hardness and the fatigue strength
Carbide precipitation, other than that produced during tempering, is known to be deleterious to the fatigue life of H13 tool steel. Fast quenching rates required to suppress intermediate precipitation of carbides (such as proeutectoid grain-boundary carbide, pearlite, or bainite) also produce large thermal gradients within the tool. Fast quenching rates can cause distortion or even cracking. The larger the tool, the greater the risk of cracking.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Hot-Work Tool Steels ⇒ Examples of Heat-Treating Procedure for Hot-Work Tools
The graph shows a continuous cooling test in which the austenitizing temperature was approximately 1040°C (1900°F).
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One Minute Mentor: Secondary Hardening in Tool Steels
January 22, 2016
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As shown schematically in the graph, high-temperature hardness is made possible only by a very important straightening mechanism: secondary hardening, promoted by the precipitation of fine alloy carbides. The stronger the secondary hardening (meaning more intense carbide precipitation), the higher the tempering resistance of hot-work tool steels. Such precipitation intensity depends on the amount of alloy elements in solid solution, which is related to the alloy composition and heat treating practice obtained by molybdenum, vanadium, or tungsten alloying.
The graph shows a schematic of hardness after tempering and the effect of secondary hardening in high-alloy steels. Observe that high-temperature hardness is only possible through precipitation hardening caused by alloy carbides (secondary hardening). At low temperatures, hardness is less than martensite due to the presence of retained austenite, which is eliminated after high-temperature tempering.
Volume 4D, Heat Treating of Irons and Steels ⇒ Tool Steels ⇒ Heat Treating of Hot-Work Tool Steels ⇒ Introduction
As shown schematically in the graph, high-temperature hardness is made possible only by a very important straightening mechanism: secondary hardening, promoted by the precipitation of fine alloy carbides.
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One Minute Mentor: Heat Treating of Hot-Work Tool Steels
January 15, 2016
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The ability of a given grade of hot-work tool steel to maintain hardness for a prolonged time at high temperatures is called tempering resistance. A simple comparison of tempering resistance may be performed by analyzing tempering curves such as those shown in the graph. Higher tempering resistance is related to curves dislocated to the right. High mechanical strength at high temperature is fundamental for hot-work steels.
Toughness in hot-work tools is an essential property for avoiding the cracks and fractures that can be common in normal working conditions. Unexpected and very unstable in cold-work tools, cracks may be encountered after a given operation time for hot-work dies. The most important example is fine crack networks, large in number but small in length, observed on a tool surface after the tooling operation. This kind of cracking is known as heat checking and is typically found in die-casting dies as well as in forging dies or other hot-work tools. The cause is thermal fatigue at the tool surface, caused in turn by repeated temperature fluctuations (heating and cooling) during tool operation.
Depending on the intensity, heat checking can lead to tool failure, which is typically the case in die-casting cavities. In such situations, the casting metal, normally aluminum, enters into the heat-checking cracks, making part extractions difficult or impairing part surface quality. In cases of a lack of toughness, gross cracking may also occur, leading to complete loss of the working tool.
Volume 4D, Heat Treating of Irons and Steels -> Tool Steels -> Heat Treating of Hot-Work Tool Steels-> Introduction
The ability of a given grade of hot-work tool steel to maintain hardness for a prolonged time at high temperatures is called tempering resistance.
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One Minute Mentor: Relation Between Tempering Temperature and Hardness
January 8, 2016
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The graphs show the relation between tempering temperature and hardness for D2 and D3 tool steels. Steels were austenitized in an air furnace; a recirculating pit-type furnace was used for tempering. The curves represent steel from three suppliers. An average of five hardness measurements were made on each specimen from each supplier. Specimens were 25 mm (1 in.) in diameter and 38 mm (1.5 in.) long.
It can be seen that certain steels exhibit higher hardness after being tempered at about 540°C (1000 °F) than after being tempered at temperatures 55 to 110°C (100 to 200°F) lower. This reversal in the usual relationship is known as secondary hardening and is caused by transformation of retained austenite and the precipitation of fine alloying carbides during tempering at the higher temperatures, near 540 °C (1000 °F).
Volume 4D, Heat Treating of Irons and Steels -> Tool Steels -> Heat Treating of Cold-Work Tool Steels -> Medium-Alloy Air-Hardening, High-Carbon High-Chromium and High-Vanadium-Powder Metallurgy Steels -> Recommended Heat Treating Practices
The graphs show the relation between tempering temperature and hardness for D2 and D3 tool steels.
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One Minute Mentor: Tool Steel Tempering Practices
January 4, 2016
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Multiple tempering is effective in decreasing the amount of austenite retained in A and D steels and is a common practice in heat-treating them. The general precautions and tempering practices outlined for O steels are also followed for the A and D steels.
However, because most of the steels in groups A and D (except A4, A5, and A6) soften less rapidly than the group O steels with an increase in tempering temperature, as shown in the graph, higher tempering temperatures can be used for the A and D steels. A minimum tempering temperature of 205°C (400°F) is a common requirement for A2, A7, and D steels. Tempering temperatures as high as 550°C (1025°F) are frequently used, and even higher temperatures are used for special requirements.
Volume 4D, Heat Treating of Irons and Steels -> Tool Steels -> Heat Treating of Cold-Work Tool Steels -> Medium-Alloy Air-Hardening, High-Carbon High-Chromium and High-Vanadium-Powder Metallurgy Steels -> Recommended Heat Treating Practices
Multiple tempering is effective in decreasing the amount of austenite retained in A and D steels and is a common practice in heat-treating them.
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One Minute Mentor: Austenitizing Powder Metallurgy Steels
December 17, 2015
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Steels of groups A and D as well as 8%Cr or High-V-PM steels are usually austenitized in vacuum furnaces or in various types of furnaces using gaseous atmospheres.
Whereas for conventionally produced cold-work tool steels the adjustment of the working hardness is usually done by a combination of different austenitizing and tempering temperatures, PM steels are typically tempered in a small tempering range, from about 550 to 560°C (1020 to 1040°F) with a variation in austenitizing temperature to adjust the hardness level , as shown in the graph.
This different procedure is explained by the toughness results, which increase when austenizing temperature is reduced. Furthermore, because of high content of fine dispersed stable carbides, grain growth at high temperatures does not take place in these steels. Typical austenitizing temperatures therefore range from 1000 to 1180°C (1830 to 2160°F).
Volume 4D, Heat Treating of Irons and Steels -> Tool Steels -> Heat Treating of Cold-Work Tool Steels—Medium-Alloy Air-Hardening, High-Carbon High-Chromium and High-Vanadium-Powder Metallurgy Steels -> Recommended Heat Treating Practices
Steels of groups A and D as well as 8%Cr or High-V-PM steels are usually austenitized in vacuum furnaces or in various types of furnaces using gaseous atmospheres.
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One Minute Mentor: Tempering F Tool Steels
December 11, 2015
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Because of their tungsten content, F-steels form hard tungsten carbides. In combination with the usual heat treatment procedures, they exhibit the highest hardness of all low-alloy cold-work tool steels, both in the as-quenched condition and at low tempering temperatures.
Because tools made of the F-steels (cold drawing dies, for example) are used mainly for applications requiring wear resistance, they are usually placed in service at or near their maximum hardness. Therefore, tempering temperatures higher than 400°F are seldom used. The effects of tempering temperature on hardness for the F1, F2, and F3 steels are shown in the graph, which shows tempering characteristics of carbon-tungsten special-purpose tool steels tempered for two hours after being brine quenched.
Volume 4D, Heat Treating of Irons and Steels -> Tool Steels -> Heat Treating of Cold-Work Tool Steels—Low- and Un-Alloyed Water and Oil Hardening Steels -> Carbon-Tungsten Special-Purpose Tool Steel
Because of their tungsten content, F-steels form hard tungsten carbides.
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One Minute Mentor: Austenitizing Tool Steel
October 16, 2015
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Austenitizing Tool Steel
Austenitizing is the heat treatment in which the final alloy element partitioning between the austenitic matrix (which will transform to martensite) and the retained carbides occurs. This partitioning fixes the chemistry, volume fraction, and dispersion of the retained carbides. The retained alloy carbides not only contribute to wear resistance but also control austenitic grain size. The finer the carbides and the larger the volume fraction of carbides, the more effectively austenitic grain growth is controlled. Thus, if the tool steel is austenitized at too high a temperature, undesirable grain growth may occur as the alloy carbides increasingly coarsen or dissolve into the austenite.
The graph shows that tempering by a combination of retained austenite transformation and secondary hardening, will eventually raise the hardness of as-quenched structures with large amounts of retained austenite. Not shown is the deleterious increase in austenite grain size that develops as more and more carbides dissolve at the higher austenitizing temperatures.
Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Carbon and Low-Alloy Steels -> Introduction to Heat Treating of Tool Steels -> Heat Treatment Applied to Tool Steels
Austenitizing is the heat treatment in which the final alloy element partitioning between the austenitic matrix (which will transform to martensite) and the retained carbides occurs.
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One Minute Mentor: Sinter Hardening
September 24, 2015
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Sinter Hardening
Sinter hardening refers to a process in which the cooling rate in the cooling zone of the sintering furnace is fast enough to transform a significant portion of the powder metal steel matrix into martensite.
The sinter hardening route has become increasingly widespread due to its cost effectiveness. Advantages include:
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treating of Carbon and Low-Alloy Steels ⇒ Heat Treatment of Powder Metallurgy Steels ⇒ Sinter Hardening
Sinter hardening refers to a process in which the cooling rate in the cooling zone of the sintering furnace is fast enough to transform a significant portion of the powder metal steel matrix into martensite.
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One Minute Mentor: Tempering Powder Metallurgy Steels
September 18, 2015
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Tempering Powder Metallurgy Steels
Usually, powder metallurgy steel parts having a density greater than 6.7 g/cm3 (0.242 lb/in.3) should be tempered after hardening. Recommended tempering temperatures for PM parts range from 150 to 200°C (300 to 390°F) with 175°C (350°F) being common. The effect of tempering temperature on impact properties is shown in the graph for FL-4205 at various densities.
Tempering above 200°C results in improved toughness and fatigue properties of the hardened part, while making a compromise in tensile strength and impact resistance. However, when tempering at temperatures higher than 200°C, care should be exercised as entrapped oil can ignite.
Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Carbon and Low-Alloy Steels -> Heat Treatment of Powder Metallurgy Steels -> Quenching and tempering
Usually, powder metallurgy steel parts having a density greater than 6.7 g/cm3 (0.242 lb/in.3) should be tempered after hardening. Recommended tempering temperatures for PM parts range from 150 to 200°C (300 to 390°F) with 175°C (350°F) being common.
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