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The series of American die steels and their selection criteria were initially established in the 1940s. Analysis of Materials Used in the U.S. Molding Industry The United States classifies mold steels into three main categories based on the conditions under which they are used. The Tool Steel Committee of the American Iron and Steel Institute identifies these three categories as cold-work mold steels, hot-work mold steels, and plastic mold steels. Among them, cold-work die steels are further divided into 12 subcategories, hot-work die steels into 9 subcategories, and plastic mold steels into 2 subcategories. The selection of materials for each sub-category depends on three main factors: size and complexity of shape, the material to be processed, and the requirements regarding durability or design life. The classification of cold work die steels involves five groups: Group W, Group O, Group A, Group D, and Group S. Group W refers to water-quenched tool steels, which include 11 different types of steel; 7 of these are carbon tool steels with carbon contents ranging from 0.7% to 1.3%. Group O refers to oil-quenched die steels (commonly known as oil steels), which include 4 different steel grades with a carbon content ranging from 0.85% to 1.55%. Group A refers to air-quenched alloy cold-working die steels, comprising 9 different steel grades with carbon contents ranging from 0.5% to 2.25%. Group D refers to high-carbon, high-chromium cold work die steels, which include 7 different steel grades with carbon contents ranging from 0.9% to 2.5%. Group S refers to impact-resistant tool steels, which include 7 different steel grades with a carbon content of 0.4–0.6%. For cold working dies, there are also high-speed steel (HSS group) and super-high-speed steel (SHSS group), cobalt-based cemented carbides and steel-bonded cemented carbides (HA group), powder steel and engineering ceramics (PIM group), tungsten-carbon tool steel (F group), and special-purpose tool steel (L group). Selection of cold work die steels The main category of cold work die steels is the high-hardness type, which is used primarily in dies that require high compressive strength and wear resistance, with a hardness level of over HRC60-62. For molds that require high impact resistance and toughness, with a hardness below HRC60-62, category S steels, some category A steels, as well as the most common quenched and tempered steels, spring steels, hot-work die steels, or matrix steels are used. For large stamping dies, such as automotive body stampings, cast iron is primarily used. For simple designs or those with a limited number of lifespans, zinc-based alloys or polymer composite materials are used. The use of high-speed steel and ultra-high-speed steel in cold working dies is growing rapidly. It mainly has a high ratio of \"compressive strength/hardness\". Moreover, the hardness can be selected between HRC60 and 70. Powder die steel possesses excellent wear resistance, with a hardness of no more than HRC60-62, and is widely used. Carbon tool steel still has a certain range of applications in punches with a service life of 100,000 pieces or stamping dies for soft materials. Classification of hot work die steels In the United States, hot work die steels are divided into two main categories: hot work die steels and super-high strength alloys. Selection of hot work die steels: Since hot work dies operate in high-temperature conditions, the materials used must possess heat strength and heat wear resistance. To ensure the longevity of these dies, they need to be cooled; however, exposure to repeated cycles of heat and cold can cause cracks to form in the dies, namely heat fatigue cracks. Therefore, the materials must also have crack resistance and heat fatigue resistance. Material selection is carried out according to the main series arranged by heat resistance: low-alloy quenched and tempered die steels (6G, 6F2, 6F3) — medium-chromium hot work die steels (H11, H12, H13) — tungsten hot work die steels (H21, H22). Non-standard hot work die steels: such as age-hardening type 6H4 for hot heading dies. When H11, H12, and H13 fail to meet the requirements for heat and wear resistance, 6H1 and 6H2 can be chosen. When the mold is required to have high heat-resistant wear resistance, D2, D4—M2, M4—powder steel can be chosen. Steel-bonded cemented carbides and cobalt-based cemented carbides have very high high-temperature wear resistance, but their thermal fatigue resistance (i.e., resistance to fatigue cracks caused by temperature changes) is poor, preventing them from being used in conditions of rapid heating and cooling. Classification of plastic mold steels The United States was the first to include steels specifically designed for plastic molds among tool steels; these are denoted by the letter P, and there are five categories in total. Carburized plastic mold steels: P1, P2, P3, P4, P5, P6. These types of steel have a very low carbon content; they were used in the United States in the early days, and the extrusion molding method was employed for their production. They require good cold formability and high extrusion properties. After molding, surface carburizing and quenching are carried out to increase surface hardness, thereby extending their service life. The ultra-low carbon content in the core minimizes deformation during quenching. Quenched and tempered plastic molds: P20, P21. Currently, P20 is widely used in plastic molds and has become the standard material, with most of it being used in a pre-hardened state. Medium-carbon alloy tool steel is used for thermosetting plastic molds. The steel grades include H13, while L12 and S7, O1 and A2 are also used. The characteristics of this category are: 1. It basically belongs to the category of second-hardening steels, with good heat resistance at 500–600°C. 2. It has a high chromium content, resulting in good resistance to atmospheric corrosion. 3. It has excellent hardenability, making it suitable for large modules. Stainless steel is used in plastic molds that require high corrosion resistance; the main grades include 420, 414L, 440, and 416. Age-hardening steel achieves high performance through aging treatment. There are two types; one is P21 low-carbon Ni-A1 aged steel ； Another category is 18Ni maraging steel. The latter is a carbon-free, high-purity, high-strength, and high-toughness material used in the aerospace industry. Used in plastic molds where high requirements are placed on mechanical properties, dimensional accuracy, surface finish, and corrosion resistance. Selection of plastic mold steel: For thin-walled plastic enclosures with production volumes of less than 100,000 units, P20 or P21 in pre-hardened condition (HB250-300) is used; 414L is employed when there is high corrosion. High-lifespan ordinary plastic molds, made of P6 or P20; after carburizing and quenching, their hardness ranges from HRC54 to 58 ； When the plastic parts are not too large, O1 and S7 can be used. Use 420 when corrosion is high. Non-high-temperature thermosetting molds are made of P6 and P20 and are used after carburizing and quenching. For high corrosion resistance, use 420. H13 and S7 or carburized steel P4 for high-temperature thermosetting plastic molds. These, with a high chromium content, possess good temper resistance and resistance to high-temperature oxidation.