Peter Ulintz Peter Ulintz
Technical Director

The History of D2 Tool Steel…According to Research

February 8, 2024

In last month’s Tooling by Design column, “The History of D2 Tool Steel…According to ChatGPT,” I used readily available artificial-intelligence (AI) software (ChatGPT 4.0) to ask, “When was D2 tool steel invented and by whom?” I felt strange engaging in conversation with a chatbot, but it was a fun and mostly comical experience, albeit the AI responses proved unreliable and inaccurate. 

This month, I’ll explore the history of D2 tool steel using traditional means … research.

Early Tool Steels

Sterling-Steel-Company-Ulintz-Tooling-By-Design-MetalForming-MagazineTool steel refers to any steel used to make tools for cutting, forming, or otherwise shaping a material into a part or component for a specific use. The earliest tool steels were simple, plain carbon steels. Beginning in 1868, and increasingly into the 20th century, material innovators developed many complex and highly alloyed steels to meet severe service demands and provide freedom from cracking during heat treatment.

In 1868, British metallurgist Robert Forester Mushet offered the first commercially produced tool steel. Mushet steel, also known as Robert Mushet’s Special Steel, was tabbed as the first “self-hardening”—and later, “air-hardening”—steel because it could be fully hardened in still air. Prior to Mushet’s discovery, hardening steel required quenching in water or oil.

Alloy Steels

ASTM A681 Standard Specification for Tool Steels Alloy classifies three categories of cold-work tool steel: air-hardening steels (group A); high-carbon, high-chromium steels (group D); and oil-hardening steels (group O).

Development of D2, part of group D, likely would coincide with the invention of other chromium-alloyed steels such as stainless steel and high-speed steel. Chromium-alloyed steel production did not become practical until the development of ferrochromium by Pierre Berthier in 1821 and, more practically, in 1895 with the development of low-carbon ferrochromium. In 1861, Mushet reported the first commercially produced steel with a chromium addition.

Gregory Comstock of Sterling Steel Co., Pittsburgh, PA, (Fig. 1) received U.S. Patent 1,695,916 for alloy steel on December 18, 1928. According to the patent’s introductory paragraph, “the present invention relates to an alloy steel and more especially to an alloy steel particularly adapted for making dies and punches, which combines the desirable characteristics of (1) the capability for air or oil hardening, particularly air hardening, to a high degree of hardness; (2) the capability of being annealed soft enough for easy machining, and (3) a high resistance to abrasion.”

D2-Composition-Ulintz-Tooling-By-Design-MetalForming-MagazineThe patent makes several claims related to composition, including “an alloy steel containing carbon about 1.4 to 1.6%, chromium about 11.5 to 12.5%, vanadium about 0.75 to 1.25%, molybdenum about 0.75 to 1.25%, and having the capability for air hardening even in fairly large sections to over 600 Brinell and the capability for being annealed to a softness below 210 Brinell.”

American Iron and Steel Institute (AISI) designations for cold-work tool steels did not exist at the time of Comstock’s patent submission, thus no reference to D2 in the patent document. However, the chemical-composition range listed in the patent matches that of commercially available D2 steel (Fig. 2). A single value listed for elements denotes the maximum allowable percentage.

Finally, the patent claims air hardenability exceeding 600 BHN—the typical upper hardness range for D2—and annealing to 210 BHN or less, if desired. The delivery condition for AISI D2: approximately 210 BHN. 

Based on these facts, I am willing to accept—at least within the United States—that this patent answers the question: “When was D2 tool steel invented and by whom?”

ChatGPT, are you paying attention? MF

Industry-Related Terms: Annealed, Forming, Hardenability, Quenching, Stainless Steel
View Glossary of Metalforming Terms

Technologies: Tooling


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