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: Quenching of Hot-Work Tool Steels
January 28, 2016
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Hot-work steels range from high to extremely high in hardenability, so it is possible for them to achieve full hardness by cooling in still air. However, this is not recommended, because intense embrittlement occurs after slow cooling from hardening temperature.
Even with those types having the highest hardenability, sections of die blocks may be so large that insufficient hardening results. In such instances, pressurized nitrogen gas (in vacuum furnaces) or an oil quench is required to achieve full hardness. Hot-work steels are never water-quenched. The figure shows the Continuous Cooling Transformation (CCT) diagram for AISI 6F3.
Volume 4D, Heat Treating of Irons and Steels ⇒Tool Steels ⇒ Heat Treating of hot-Work Tool Steels ⇒ Heat Treatment of H Series and Low-Alloy Hot-Work Steels
Hot-work steels range from high to extremely high in hardenability, so it is possible for them to achieve full hardness by cooling in still air. However, this is not recommended, because intense embrittlement occurs after slow cooling from hardening temperature.
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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: 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: 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: Atmospheres for Water-Hardening Tool Steel
December 3, 2015
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Excellent results are obtained by austenitizing water-hardening tool steels in a slightly oxidizing atmosphere, as indicated by the data in the graph, obtained in tests on type W2. The graph shows the effect of furnace atmosphere on the surface carbon content of W2 tool steel. Specimens were heated at 790°C (1450°F) for 1 hour, quenched in brine, annealed in lead at 705°C (1300°F), and machined in 0.13 mm (0.005 in.) cuts for analysis.
Oxidizing atmospheres are inexpensive, and are usually produced by controlled direct-firing burners. The light scale that is produced is removed by the vigorous water or brine quench. Endothermic atmospheres also are used, but close control is necessary to match the carbon potential of the atmosphere to the carbon content at the surface.
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 -> Water-Hardening Tool Steels
Excellent results are obtained by austenitizing water-hardening tool steels in a slightly oxidizing atmosphere, as indicated by the data in the graph, obtained in tests on type W2.
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One Minute Mentor: Austenitizing Water-Hardening Tool Steels
November 30, 2015
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Austenitizing temperatures for water-hardening tool steels normally vary from 1400 to 1550°F. Higher temperatures are sometimes used for special purposes. Hardenability increases as austenitizing temperature increases. The optimum time at austenitizing temperature is from 10 to 30 minutes. Preheating is unusual except for very large tools or those with intricate cross sections.
The graph shows the maximum section thicknesses of three classes of water-hardening tool steels that develop minimum hardness of 60 HRC when oil quenched from various austenitizing temperatures.
It is particularly important to protect shallow-hardening steels against scaling and decarburization. Severe scaling can interfere with heat transfer during quenching and slow the required high rate of cooling. Decarburization will produce a soft surface on any tool steel, but in a deep-hardening steel it can be ground off until the underlying hard high-carbon area is reached. Grinding a shallow-hardening steel will frequently expose the soft core.
Volume 4D, Heat Treating of Irons and Steels -> Tool steels -> Introduction to Heat Treating of Tool Steels -> Water Hardening Tool Steels
Austenitizing temperatures for water-hardening tool steels normally vary from 1400 to 1550°F. Higher temperatures are sometimes used for special purposes.
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Buehler introduces fully automated hardness tester
November 24, 2015
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Buehler, Lake Bluff, Ill., an ITW company, created the fully automatic Wilson VH3300 Vickers/Knoop hardness test system to meet virtually any requirement in the area of automated precision micro hardness testing. The VH3300 is equipped with patent-pending collision protection technology, a high-speed sample stage, and DiaMet universal testing software. The new system features a six-position test head with up to three indenters and three zoom objectives, an automated X/Y sample stage available in two sizes (180 x 180 mm as standard) and a powerful image analysis system. A high-resolution digital camera provides up to 2000× magnification. The entire system is controlled through the DiaMet hardness automation software to ensure high accuracy acquisition and analysis of test data. A wide range of test loads from 5 g to 60 kg (depending on configuration) renders the sturdy system suitable for a wide variety of applications. A purpose-designed collision protection system helps to avoid accidental damage to the indenters and objectives due to operator errors. The VH3300 hardness testing system provides a superior, high capacity testing platform capable of performing 150 make-and-measure indents per hour using the fully automated test program. For more information, visit www.buehler.com or email [email protected].
Buehler, Lake Bluff, Ill., an ITW company, created the fully automatic Wilson VH3300 Vickers/Knoop hardness test system to meet virtually any requirement in the area of automated precision micro hardness testing.
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One Minute Mentor: Hardening Type W1 Tool Steels
November 19, 2015
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W1 steels are capable of hardening to high surface hardness and soft core which is useful in some shock applications. They are low cost tool steels with fair to good wear resistance as carbon content increases. As they are water quenched and have poor dimensional stability, their use is limited to fairly uniform sections with minimum amount of stress risers; in other applications quench cracking can occur. The graph shows an isothermal transformation diagram for a W1 steel.
Volume 4D, Heat Treating of Irons and Steels -> Heat Treating of Cold-Work Tool Steels—Low- and Un-Alloyed Water and Oil Hardening Steels -> Water-Hardening Tool Steels
W1 steels are capable of hardening to high surface hardness and soft core which is useful in some shock applications.
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