Characteristics of Cold-Working Tool Steel
The most important characteristic of cold-working tool steel is wear resistance. The basic way to improve wear resistance is to increase hardness. Cold-working tool steel is generally quenched and tempered, and used at a hardness of about 60 HRC. Increasing the carbon content raises the hardness; adding carbide-forming elements such as Cr, Mo, V and W creates hard carbides (M7C3, M6C, MC) that further improve wear resistance. However, a large carbide fraction lowers toughness and fatigue resistance, so the grade must be selected for the application.
Carbon Tool Steel (JIS G4401)
Carbon tool steel contains 0.55–1.50% carbon with small amounts of silicon and manganese; JIS lists 11 grades. It is quenched at 750–840 °C and tempered at 150–200 °C, reaching 54–63 HRC. Because of the few alloying elements, hardenability is poor and water cooling is required - even then only the surface hardens. The most common grade is SK105 (1.05C-0.2Si-0.25Mn), used for cutting tools, steel rods, jigs and dies.
Alloy Tool Steel (JIS G4404)
JIS classifies alloy tool steel into cutting tool steel, impact-resistant tool steel, cold-working die steel and hot-working die steel. The cold-working die steels include 10 grades plus modified grades.
Low-Alloy Tool Steel
Cold-working die steels with a relatively low alloy content include SKS93 (1.05C-0.95Mn-0.4Cr) and SKS3 (0.95C-1.05Mn-0.75Cr). The manganese (about 1%) improves hardenability; SKS3 additionally contains 0.75% tungsten for hardenability and toughness. Low-alloy tool steels are used for cutting edges, gauges and dies.
High-Carbon High-Chromium Cold-Working Die Steel
The most famous grade is SKD11 (1.5C-12Cr-1Mo series), whose characteristic is good wear resistance: the high chromium forms coarse chromium carbides (M7C3) in the matrix. Its hardenability is good - full core hardness can be obtained even with air cooling. The quenching temperature of SKD11 is 1000–1040 °C, which is commonly used in vacuum heat-treatment furnaces. SKD1 (2C-12Cr series) has even higher carbon and chromium, giving more coarse carbides and higher wear resistance, but lower toughness and fatigue resistance; its quenching temperature (950 °C) differs from the common range and it must be oil quenched, so it is rarely used.
The 8% Cr Cold-Working Tool Steel
Although not included in JIS, 8% chromium steel is widely used. Based on SKD11 with adjusted composition (approximately 1C-8Cr-2Mo), it was developed in the 1970s to solve cracking problems in EDM processing of quenched and tempered die steel. High-temperature tempering effectively eliminates residual stress, but SKD11 cannot maintain the required hardness after high-temperature tempering. By reducing carbon and chromium to improve toughness and adding more molybdenum, 8%Cr steel retains about 62 HRC after high-temperature tempering. Its wear resistance is slightly lower than SKD11, but its excellent overall performance makes it a standard grade alongside SKD11.
Frequently Asked Questions
Q: What is the difference between SKD11 and SKD1?
A: SKD1 (2C-12Cr) has more carbon and chromium than SKD11 (1.5C-12Cr-1Mo), giving higher wear resistance but lower toughness and requiring oil quenching; SKD11 air-hardens and is the more widely used grade.
Q: Why was 8% Cr steel developed?
A: To eliminate cracking in EDM-processed die steel: reducing carbon and chromium improves toughness, and more molybdenum allows high-temperature tempering (which removes residual stress) while retaining about 62 HRC.
Q: Why must carbon tool steel be water quenched?
A: Because its hardenability is poor - water cooling provides the rapid cooling needed to harden the surface; oil quenching would not reach the required hardness.
Q: What is the standard quenching temperature of SKD11?
A: 1000–1040 °C, the range commonly used in vacuum heat-treatment furnaces; hardening is followed by tempering (typically 150–550 °C depending on the required hardness).
