Tool steel designations are useful shorthand for comparing broad material families, but the letter and number do not specify every property of a supplied bar. This chart summarizes seven Carpenter Technology grades using the manufacturer’s current alloy descriptions and listed applicable specifications. It focuses on family and hardening response because actual hardness and mechanical properties depend on product form, processing, heat treatment, and condition.

Read the grade and family together

A2 and D2 are both air-hardening cold-work tool steels, but Carpenter describes different emphases. A2 is presented as a balance of hardness and toughness, including applications needing dimensional accuracy in larger sections. D2 is a high-carbon, high-chromium grade described for high wear resistance and deep hardening. Those are distinctions for choosing which supplier data to review; they do not establish that either grade will outperform the other in every die or tool.

O1 is an oil-hardening general-purpose tool and die steel. H13 is primarily a hot-work die steel, with Carpenter describing toughness and red hardness; the supplier also notes cold-work uses where extra toughness is valued at some sacrifice in wear resistance. S2 is water hardening and shock resistant. S7 is air hardening and designed for high impact and shock resistance. M2 belongs to the high-speed tool steel family and is listed against ASTM A600 rather than the A681 scope shown for the other grades here.

What the specification column means

The specification named by Carpenter is a useful cross-check for the grade family. ASTM A681 covers alloy tool steels used for tools and dies; ASTM A600 is the specification Carpenter lists for M2 high-speed tool steel. A grade name or specification reference alone does not prove that a particular bar conforms. The purchase order, mill certificate, product form, size, delivery condition, and revision of the applicable specification still matter.

The UNS numbers in the table are material identifiers, not heat-lot certificates. The family labels are descriptive selection categories, not interchangeable acceptance criteria. For a critical tool, use the exact chemistry and property requirements for the ordered product rather than selecting by a short grade description.

Use classification as a first filter

For a cold-work blanking or forming tool, the table can help narrow the first supplier conversation to the appropriate family. Wear, chipping, impact, section thickness, dimensional change during hardening, grinding, and service temperature all affect the final choice. A tool exposed to repeated impact may need a different balance from a tool where abrasive wear dominates. A mold or hot-work insert needs evaluation at its actual working temperature and thermal cycling.

Record the grade, product form, size range, delivery condition, and requested heat treatment before comparing quotes. Then review the current data sheet for hardening practice, tempering range, machinability, attainable hardness, and dimensional-change information. Ask the supplier to confirm that the proposed designation is available in the needed form and that any special processing is documented.

Limits and material substitutions

Do not convert a family label into a heat-treatment recipe or allowable stress. Do not substitute one grade for another based only on the words “air hardening,” “shock resistant,” or “high wear.” The table does not compare certified heats, actual hardness, tensile properties, fatigue life, corrosion performance, or tool life. Carpenter’s descriptions are manufacturer-specific, and other producers may use different trade names, processing routes, and product limits.

Before release, check the current Carpenter product page and data sheet, the applicable ASTM specification, and the drawing or purchase requirements. For safety-critical tooling or a change that could affect part quality, have the material and heat-treatment decision reviewed by the responsible manufacturing or materials engineer.

Sources: Carpenter Technology alloy finder pages for A2, D2, H13, O1, S2, S7, and M2. Carpenter lists ASTM A681 for the first six selected grades and ASTM A600 for M2; confirm the current standard and ordered product documentation.