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Hot Work Die Steel

• satyendra
• August 21, 2021
• 4 Comments
• Chromium hot work die steels, H type steels, heat treatment, Hot forming die, Hot work die steel, Molybdenum hot work die steels, Powder metallurgy, Tungsten hot work die steels,

Hot Work Die Steel

Hot work die steels find their use in hot forming processes. Hot forming processes are among the oldest and most important metal forming processes and accounts for a large percentage of metal products. Hot metal forming consists of a forming process either by plastic deformation or solidification where the metal is shaped by tools or dies. The hot deformation process occurs above the metals recrystallzation temperature. At that temperature the metal is easy to shape, since it behaves in a perfectly plastic manner. The metals become neither internally stressed nor work hardened, and an unlimited amount of hot-working can be performed without component fracture.

Hot work die steels are high quality steels made to close compositional and physical tolerances. They are used to make dies for forming or shaping a material into a part or component adapted for a definite use. The development of the hot work die steel is closely related to the evolution of steels in general, but the beginning of die or tool steel history is normally regarded as 1740, when Benjamin Huntsman, a clock maker melted pieces of blister steel in a crucible. By melting the steel instead of heating iron in charcoal (made carbon diffuse in into the iron producing blister steel), it made the steel much more homogenous and, thus, stronger.

The earliest hot work die steels were simple, plain carbon (C) steels, but beginning in 1868, when Robert Mushet intentionally added tungsten to high carbon steel, and to a greater extent early in the 20th century, many complex, highly alloyed hot work die steels were developed. But, the understanding of the inter-relationships among carbon content, alloy composition, and processing, which developed the present day hot work die steels, came only gradually in the 19th century. Modern hot work die steels, with complex alloying and heat treatments, are much more advanced. These complex alloy hot work die steels, which contain, among other elements, relatively large amounts of tungsten (W), molybdenum (Mo), vanadium (V), and chromium (Cr), make it possible to meet increasingly severe service demands and to provide greater dimensional control and freedom from cracking during heat treating.

For typical wrought hot work die steels, raw materials (including scrap) are carefully selected, not only for alloy content, but also for qualities which ensure cleanliness and homogeneity in the finished product. Hot work die steels are normally melted in relatively small-tonnage electric arc furnaces and refined in an argon oxygen decarburization vessel to achieve composition tolerances at low cost, good cleanliness, and precise control of melting conditions. Special refining and secondary remelting processes such as electro slag remelting, and vacuum arc remelting have been introduced to satisfy particularly difficult demands regarding hot work die steel quality and performance. The medium to high alloy contents of several hot work die steels need control of forging and rolling, which frequently results in a large quantity of process scrap.

Semi-finished and finished bars are given rigorous in-process and final inspection. This inspection can be so extensive that both ends of each bar can be inspected for macro-structure (etch quality), cleanliness, hardness, grain size, annealed structure, and hardening ability. Inspection can also need that the entire bar be subjected to magnetic and ultrasonic inspections for surface and internal discontinuities. It is important that finished tool steel bars have minimal decarburization within carefully controlled limits, which needs annealing to be done by special procedures under closely controlled conditions.

Specific production practices and stringent quality controls contribute to the high cost of hot work die steels, also because of the expensive alloying elements these steels contain. Insistence on quality in the production of these specialty steels is justified, however, because hot work die steel bars normally are made into complicated forming dies worth many times the cost of the steel itself. Although some standard constructional alloy steels resemble hot work die steels in composition, they are rarely used for expensive dies since; in general, those steels are not produced to the same rigorous quality standards as are the hot work die steels. The performance of a hot work die in service depends on (i) proper design, (ii) accuracy with which the die is made, (iii) selection of the proper hot work die steel, and (iv) application of the proper heat treatment. A hot work die can perform successfully in service only when all four of these requirements have been met.

Majority of hot work die steels are wrought products, but precision castings can be used to advantage in some applications. The powder metallurgy (P/M) process also is used in making these steels. This process provides more uniform carbide size and distribution in large sections and special compositions which are difficult or impossible to achieve by melting and casting and then mechanical working the cast product.

In service, majority of the hot work dies are subjected to extreme high loads which are applied rapidly. The dies are to withstand these loads a large number of times without breaking and without undergoing excessive wear or deformation. In several applications, hot work die steels are to provide this capability under conditions which develop high temperatures in the dies. No single die material combines maximum wear resistance, toughness, and resistance to softening at high temperatures. As a result, selection of the proper die material for a given application frequently needs a trade-off to achieve the optimum combination of properties.

Hot work die steels are alloyed tool steels for use in applications in which surface temperature is normally above 200 deg C. During the application the die comes into contact with hot material, the temperatures of which are well above 200 deg C. Besides long term thermal load, there is the additional stress due to the periodic change of temperature. Hot work die steels for such applications have to be able to withstand not only upto the universal mechanical and abrasive stresses normally occurring in die steels, but also they have to withstand thermal load as well.

The range of applications for hot work die steel is broad and the dies manufactured with these steels are used in a variety of fields. Hot work die steels are used for the non-cutting forming of work pieces made of iron and non-ferrous metals as well as alloy derivatives at high temperatures. They are applied in processes such as hot forging, pressure die casting, and extrusion as well as in tube and glass product manufacturing.

Hot work die steels are a widely used material for construction of tools designated for shaping and forming of metal, plastic and other elements in mass production. These tools include extruding dies, pressure casting dies, moulds, punches, and various other elements for plastic shaping of the materials preheated to temperatures in the range of 250 deg C to 700 deg C. Since shape stability constitutes the basic requirement any tool has to meet, the material from which it is made from is expected to withstand loads without any plastic strain while maintaining high abrasion resistance. Additionally, a tool is to feature good hardness and strength as well as appropriate ductility and impact strength which ensure crack resistance, and these qualities are to be obtained at high working temperatures (upto 700 deg C).

During processing, hot work die steel is normally exposed to high temperatures which are higher than 200 deg C. The microstructure of this steel is to be sufficiently stable and resistant to tempering since the micro-structural changes are to be prevented at all costs. Tools made from hot work die steel are not only consistently subject to high temperatures when used but also to the fluctuating thermal stresses occurring where the die surfaces come into contact with the materials to be processed. Combined with the wear caused by abrasion or impact, these thermal stresses constitute very specific requirements of the hot work die steels.

There are a large number of different influencing factors on which the performance of the hot work dies depends. These are as shown in Fig 1. The material properties as one of the factors are mainly influenced by the choice of the steel grade.

Fig 1 Influencing factors on the die life of hot work dies

The functionality of a hot work die steel is defined by its chemical composition, the technology applied during its production, and by the subsequent heat treatment. In general, hot work die steels are of the medium and high alloy steels and majority of them have relatively low carbon content. Hot work die steels are required to have some physical characteristics such as (i) resistance to deformation at the working temperature, (ii) resistance to thermal shock, (iii) resistance to wear at the working temperature. (iv) resistance to heat treatment deformation, (v) resistance to heat checking, (vi) high temperature strength, (vii) high temperature toughness, and (viii) good machinability in the annealed condition.

However, the above properties are impossible to realize in only one type of steel at the same time and in the same manner. The requirements frequently vary considerably from one die to the other and are therefore impossible to fulfill with steels from a very limited alloy range. Steel grades thus have to be selected based on the primary demands of the die to be employed. Using high quality hot work die steel is therefore very important in order to ensure a high degree of operating efficiency and productivity during manufacturing.

An optimum combination of both mechanical as well as metallurgical properties allows for (i) during the production of the die steel easy machining because of superior machinability, safe and simple heat treatment, and excellent dimensional stability during the heat treatment, and (ii) during the use of the die steel long and uniform life of the die, and maximum security against failure.

There are several ways in which a hot forming die can be damaged. For example wear, plastic deformation, gross cracking, thermal fatigue, and mechanical fatigue. But, thermal fatigue (frequently called heat checking) is probably the most common failure mechanism in all hot forming processes and can be defined as fatigue produced by the repetition of stresses which are thermal in origin, i.e. stresses which arise when the expansion or contraction from the heating or cooling is constrained. As these stresses accumulate by each repetition they eventually cause either excessive distortion or thermal fatigue cracking. Thermal fatigue crack is normally recognized as a network of surface cracks and is generally facilitated by creep and environmental effects, such as oxidation. The thermal fatigue mechanism can roughly be divided into three stages namely (i) sub-structural and micro-structural changes, (ii) crack nucleation, and (iii) crack growth.

Common for the two processes, hot forging and die casting (Fig 2) is that they both have a die or a tool, which gives the product its final shape. These dies are normally very complex and expensive and in order to lower the production costs, they need to last for a long time. The materials used in the dies for hot forming are today completely made of a special type of steel. The selection of the steel for the dies is a very significant decision in the production of precise components by the processes of forging or die casting. Appropriate selection of steel for the dies is imperative to get acceptable die life at reasonable cost. Die wear is mostly influenced by the hardness of the die material and other material properties such as toughness and ductility. Selection of proper die materials is very important for reducing the production costs and setting narrow tolerance for the formed part.

Fig 2 Hot forging and die casting processes

For a given application, selection of the appropriate die material depends on three variables namely (i) variables related to the process itself which includes factors such as size of the die cavity, type of machine used, deformation speed, initial stock size and temperature, die temperature to be used, lubrication, production rate, and number of parts to be produced, (ii) variables related to the type of die loading, including speed of loading, i.e., impact or gradual contact time between dies and deforming metal, maximum load and pressure on the dies, maximum and minimum die temperatures, and number of loading cycles to which dies is being subjected, and (iii) mechanical properties of the die material, including hardenability, impact strength, hot strength, and resistance to thermal and mechanical fatigue.

Hot work die steels have been developed to withstand the combinations of heat, pressure, and abrasion associated with such operations. Presently, there exist several types of hot work die steels, but the desire to increase the performance of these steels still remains. The steels used for hot forming is a special type of die steel, made to withstand a combination of heat, pressure, and abrasion and has been classified by American Iron and Steel Institute (AISI) as hot work die steel AISI type ‘H’. All the hot work die steels are to be used in a quenched and tempered condition. The most essential properties for these types of steels are high levels of hot strength, ductility, toughness, thermal conductivity, creep strength, temper resistance, and also low thermal expansion. AISI type ‘H’ die steels normally have medium carbon contents (0.35 % to 0.45 %) and chromium, tungsten, molybdenum, and vanadium contents ranging from 6 % to 25 %.

Since the hot work die steels need to maintain its properties at high temperatures, they need an increased temper resistance so that an appropriate strength can be achieved after tempering at 550 deg C/650 deg C. The most convenient technique is to use a secondary hardening reaction involving the precipitation of alloy carbides. A good secondary hardening effect is achieved by strong carbide forming elements such as chromium, molybdenum, vanadium, and tungsten. These elements play an important role when the hot work die steel is subjected to high temperatures, since they precipitate as fine alloy carbides, which not only retards the softening but also increases the hardness.

The AISI type H steel is divided into three subgroups named after the dominant alloying element namely (i) chromium, (ii) tungsten, and (iii) molybdenum. H1 to H19 steels are chromium based, H20 to H39 steels are tungsten based, and H40 to H59 steels are molybdenum based. The chromium based steels have the best toughness of all these steels but their highest temperature of usage is the lowest (upto 475 deg C) among the three categories. The tungsten or molybdenum steels are used for service upto around 600 deg C but their toughness is inferior to chromium steels.

Chromium hot- work die steels – These steels are well adapted to hot work of all kinds such as dies for the extrusion of aluminium and magnesium, but also as die-casting dies, forging dies, and hot shears. This group of the hot work die steels contain chromium and in some cases the steels are with additions of tungsten, molybdenum, vanadium, and cobalt (Co). The carbon (C) in this group is held relatively low, around 0.35 % to 0.4 %, and this, together with the relatively low total alloy content, promotes toughness at the normal working hardness ranging between 41.7 HRC to 54.5 HRC.

Chromium hot work die steels have good resistance to heat softening because of their medium chromium content and the addition of carbide-forming elements such as molybdenum, tungsten, and vanadium. The low carbon (C) and low total alloy contents promote toughness at the normal working hardnesses of 40 HRC to 55 HRC. Higher tungsten and molybdenum contents increase hot strength but slightly reduce toughness. Vanadium is added to increase resistance to washing (erosive wear) at high temperatures. An increase in silicon content improves oxidation resistance at temperatures upto 800 deg C). The most widely used types in chromium hot work die steels are H11, H12, H13, and, to a lesser extent, H19.

The high chromium in this group, coupled with low carbon, ensures depth of hardening, and hence these steels can be air hardened to full working hardness. The H11, H12, and H13 steels can be air hardened to full working hardness in section sizes upto 150 mm, other type H steels can be air hardened in section sizes upto 300 mm. The higher tungsten and molybdenum contents of the H10 steels increase the red hardness and hot strength, but tend to slightly reduce toughness. The air-hardening qualities and balanced alloy contents of these steels result in low distortion during hardening. The main advantage of this group is the ability to resist continued exposure to temperature upto 540 deg C, and at the same time, provide a tough and ductile die at this temperature.

In this group, the H11, H12, and H13 steels perhaps represent the highest tonnage used in all hot work die steels. The air hardening qualities and balanced alloy content are responsible for low distortion during hardening. Chromium hot work die steels are especially well adapted to hot die work of all kinds, particularly dies for the extrusion of aluminum and magnesium, as well as die casting dies, forging dies, mandrels, and hot shears. Most of these steels have alloy and carbon contents low enough so that dies made from them can be water cooled in service without cracking.

The H11 steel is used to make certain highly stressed structural parts, particularly in aerospace technology. Material for such demanding applications is produced by vacuum arc remelting of air melted electrodes, which provides extremely low residual-gas content, excellent micro cleanliness, and a high degree of structural homogeneity. The main advantage of H11 over conventional high-strength steels is its ability to resist softening during continued exposure to temperatures upto 540 deg C and at the same time provide moderate toughness and ductility at room temperature tensile strengths of 1,720 MPa to 2,070 MPa. In addition, because of its secondary hardening characteristic, H11 steel can be tempered at high temperatures, resulting in nearly complete relief of residual hardening stresses, which is necessary for maximum toughness at high strength levels. Other important advantages of H11, H12, and H13 steels for structural and hot work applications include ease of forming and working, good weldability, relatively low coefficient of thermal expansion, acceptable thermal conductivity, and above-average resistance to oxidation and corrosion.

The H10 steel is relative newcomer to this family of steels, being introduced first in the USA. This steel grade provides steel with improved toughness, and it is so important in various applications that several steel suppliers are marketing modifications within the H10 steel nominal composition, to give a range of selected properties. Tab 1 gives typical chemical composition of chromium hot work die steels.

Tungsten hot work die steels – The tungsten hot work die steels are used to make mandrels and extrusion dies for high temperature applications, such as the extrusion of brass, nickel alloys, and steel. The main alloying elements of tungsten hot work die steels (types H21 to H26) are carbon, tungsten, and chromium and in some cases vanadium. The higher alloy contents of these steels make them more resistant of high temperature softening and washing when compared with the straight chromium steels. However, high alloy content also makes the tungsten hot work die steels more prone to brittleness at normal working hardnesses (45 HRC to 55 HRC).

Although tungsten hot work die steels can be air hardened, they are normally quenched in oil or hot salt to minimize scaling. When air hardened, these steels show low distortion. Further, these steels need higher hardening temperatures than do the chromium hot work die steels, making the former more likely to scale when heated in an oxidizing atmosphere. Also while these steels have much higher toughness, in many characteristics they are similar to high speed steels. In fact, H26 steel is a low carbon version of T1 high speed steel. If tungsten hot work die steels are preheated to operating temperature before use, breakage can be minimized. These steels have been used to make mandrels and extrusion dies for high-temperature applications, such as the extrusion of brass, nickel alloys, and steel. These steels are also suitable for use in hot forging dies of rugged design.

In comparison to the steels in the hot work chromium group, the high tungsten content makes these steels not suitable for water cooling in service. In the hot work die steels of this group, toughness and thermal shock resistance are normally achieved by reducing the carbon content. However, in doing so, it is necessary also to adjust the tungsten and vanadium contents, since both these elements reduce hardenability by holding too much carbon in complex carbides and thus allowing insufficient carbon in the austenite matrix. The adjusted composition, hence, represents the best combination of hardness and red hardness, with a considerable degree of toughness and resistance to thermal shock. Tab 2 gives typical chemical composition of tungsten hot work die steels.

Molybdenum hot work die steels – The molybdenum hot work die steels are almost similar to tungsten hot work die steels with almost similar characteristics and uses, but have the main advantage in their lower initial cost. There are only three active molybdenum hot work die steels namely (i) H41 steel, (ii) H42 steel, and (iii) H43 steel.

The main alloying elements in molybdenum hot work die steels are molybdenum, chromium, carbon, and vanadium, together with varying amount of tungsten. Like the high speed steels, the molybdenum grades of hot work steels have almost identical characteristics and uses to the corresponding tungsten hot work die steels. Although the compositions of the molybdenum hot work die steels resemble those of various molybdenum high speed die steels, they have a low carbon content and greater toughness.

The major advantage of these steels compared to the tungsten hot work die steels is their lower costs and more resistance to heat checking, but in common with all high molybdenum steels, need more care in heat treatment particularly with regard to decarburization and control of austenitizing temperature. Tab 3 gives typical chemical composition of molybdenum hot work die steels.

Heat treatment

With few exceptions, all hot work die steels are to be heat treated to develop specific combinations of wear resistance, resistance to deformation or breaking under high loads, and resistance to softening at high temperatures. A few simple shapes can be obtained directly from hot work die steel producers in correctly heat treated condition. However, the majority of the hot work die steels are first formed or machined to produce the required shape and then heat treated as needed.

Proper heat treatment is as critical to the success of the hot work die steels as the material selection itself. Frequently the highest quality steel made into the most precise dies does not perform because of improper heat treatment. The objective of the heat treatment or hardening process is to transform fully annealed high speed steels consisting mainly of ferrite (iron) and alloy carbides into a hardened and tempered martensitic structure having carbides.

Fig 3 shows typical processing and heat treatment sequences for hot work die steels as a function of time, temperature, and phase transformation. Improper finishing after heat treatment (mainly grinding) can damage the hot work die steels through the development of surface residual stresses and cracks. Some hot work die steels are heat treated (hardened) in a blank or semi-finished state and subsequently ground, turned, or electrical discharge machined to create the final die. Although these manufacturing techniques have progressed in recent years, metallurgical damage and surface stresses are still a major concern.

Fig 3 temperature versus time graph showing sequence of operations needed for producing hot work die steels

Processing information regarding normalizing and annealing of hot work die steels is presented in Tab 4. It can be seen from the table that hot work die steels do not normalize and hence normalizing treatments are not being given to these steels. However stress relieving treatment is being given if needed to relieve the stresses of machining. Stress relieving is done by heating slowly to 565 deg C to 675 deg C, allowing the temperature to equalize, and then cool in still air.

Hot work die steels are normally supplied in the annealed condition. This condition allows the steel to be easily machined and heat treated. However, if the hot work die steels are subjected to hot or cold forming, the steels are to be fully annealed again. This procedure is important with the steels of higher alloy content, otherwise, irregular grain growth occurs and a mixed grain size (sometimes called fish scale or duplex grains) results.

Full annealing involves heating the steel slowly and uniformly to a temperature above the transformation range, holding it at the temperature for from 1 hour to 4 hour and cooling slowly at a controlled rate followed by air cooling. Atmosphere furnaces, salt baths, vacuum furnaces, or lead pots may be used for annealing. Requirements of the heating equipment include reasonably accurate temperature control and a means of preventing decarburization.

Cracking from thermal shock can be minimized by loading the furnace at a relatively low temperature (room temperature) to permit the furnace load to heat up slowly with the furnace. Following the soak at annealing temperature, the die is to be cooled in the furnace at 22 deg C per hour to 540 deg C or lower. Below around 540 deg C, the cooling rate for most hot work die steels is no longer critical, and the die steel can then be cooled in air.

Isothermal annealing is an alternative method of cooling which consists of rapidly cooling the die steel in the furnace from the annealing temperature to a temperature just below the transformation range and holding the load and furnace at this temperature for one or more hours. Following this period of soaking at just below the transformation range, the load can be safely air cooled. This process, known as isothermal annealing, is best suited for applications in which full advantage can be taken of the rapid cooling to the transformation temperature, and from this temperature to room temperature.

The heat treatment process can be divided into four process steps namely (i) preheating, (ii) hardening (austenitizing), (iii) quenching, and (iv) tempering. Processing information regarding these four process steps during the heat treatment for hot work die steels is presented in Tab 5.

Preheating – Preheating plays no part in the hardening reaction from a metallurgical viewpoint. However, preheating performs three important functions. The first of these is to reduce thermal shock, which always results when a cold die is placed into a warm or hot furnace. Minimizing of the thermal shock reduces the danger of excessive distortion or cracking. It also relieves some of the stresses developed during forming, although conventional stress relieving is more effective.

The second major advantage of preheating is to improve the equipment productivity by decreasing the amount of time needed in the high heat furnace. The third function of preheating is that if the high heat furnace environment is not neutral to the surface of the die or part, then the preheating reduces the quantity of carburization or decarburization which normally results if no preheating is used. Preheating duration is of little importance as long as the part is heated throughout its cross section.

Hardening (austenitizing) – It is the second step of the heat treatment process. Hardening is a time / temperature dependent reaction. Hot work die steels depend upon the dissolving of different complex alloy carbides during hardening to develop their properties. These alloy carbides do not dissolve to any appreciable extent unless the steel is heated to a temperature within 28 deg C to 56 deg C of their melting point. This temperature is dependent upon the particular hot work die steel being treated. The normal recommended holding time for hot work die steels depends upon the hot work die steel type, die configuration, and cross-sectional size.

Decreasing the hardening temperature (under-hardening) normally improves the impact toughness while lowering the hot hardness. Increasing the hardening temperature increases heat-treated room temperature hardness and also increases the hot hardness.

Quenching – The quenching or cooling of the work piece from the hardening temperature is designed to transform the austenite which forms at the high temperature to a hard martensitic structure. The rate of cooling, which is to be controlled, is dictated by the analysis of the particular steel. Sometimes hot work die steels are quenched in two steps. Initially in a molten salt bath kept at around 540 deg C to 595 deg C or an oil quench, followed by the air cooling to near ambient temperature.

The least drastic form of quenching is cooling in air, although only in the smaller and / or thinner cross sections. Hot work die steels air quench rapidly enough to transform the majority of the structure into the desirable martensitic structure.

Tempering – Following hardening and quenching, the steel is in a highly stressed state and hence is very susceptible to cracking. Tempering increases the toughness of the steel and also provides secondary hardness. Tempering involves reheating the steel to an intermediate temperature range (always below the critical transformation temperature), soaking, and air cooling.

Tempering serves to stress relieve and to transform retained austenite from the quenching step to fresh martensite. Some precipitation of complex carbide also occurs, further enhancing secondary hardness. It is this process of transforming retained austenite and tempering of newly formed martensite which dictates a multiple tempering procedure.

Tempering practice can vary with size and application, but is normally performed in the range of maximum secondary hardness or higher. Double tempering is sometime needed.