Hot ductility of tool steels

Four tool steels (A2, D2, M2 and W1) representing a wide variety of compositions (and car hide content) were hot deformed in torsion between 900 and 1200 degreesC at strain rates of 0.1, 1 and 4 s(-1). Carbides contribute to an increase in strength but to a lower ductility. The plain carbon tool steel (W1) with no carbides was very ductile at all test conditions, but the alloy steels displayed poor ductility due predominantly to void formation around the carbides at lower and medium temperatures and incipient melting at higher temperatures, The effects of carbides on ductility depend on their morphology and distribution. Alloy tool steels experience eutectic solidification of the networks of primary carbides that are broken up by hot working thereby improving ductility. Hot rolled stocks experience directional ductility; such anisotropy is reduced by fur ther working. The alloy steels display parabolic ductility curves for each strain rate: the ductility increases with decreasing strain rate and a rise in temperature up to a maximum of about 1000 degreesC for A2 and D2 and about 1050 degreesC for M2. The M2 steel, despite having the highest volume fraction of carbides and strength, displays the greatest ductility followed relatively closely by A2. D2, though close to M2 in carbide volume and strength, has much lower ductility.