A2 vs. O1 Tool Steel: Which One Suits Your Factory's Needs?

A2 vs. O1 Tool Steel: Making the Right Choice | Aobo Steel

Choosing between A2 and O1 tool steel for your operations is a common consideration, and the best answer depends on your specific application requirements.

Generally, if minimizing distortion during heat treatment and achieving better toughness are top priorities, A2 is often the preferred choice, especially for more complex tooling. However, if upfront cost and ease of machining are primary drivers for less dimensionally critical parts, O1 is a capable and economical oil-hardening steel.

Let’s examine the technical details of each grade.

Understanding O1 Tool Steel

AISI O1 tool steel is a general-purpose, oil-hardening grade, often considered a low-cost material.

Key Characteristics of O1

O1 is known for its good edge-holding ability and the high hardness levels it can achieve. Its high carbon content contributes significantly to its hardness and wear resistance. Additionally, a high silicon content in O1 can increase machinability and die life. Tungsten, as an alloying element in O1, helps in attaining high abrasion resistance and very sharp cutting edges.

Common Applications for O1

O1 is widely used for applications such as surface finishing tools and woodworking knives. It’s also a common choice for blanking, coining, drawing, and forming dies and punches, as well as shear blades, gauges, and chuck jaws after oil quenching and tempering. It is the most widely used grade among the O-type tool steels. For coining low-carbon steel, O1 serves as an alternative to W1 and is often applied to smaller tools, especially for punching, coining, and powder pressing.

Heat Treatment Considerations for O1

O1 is oil-hardening and can be hardened in reasonable sizes, up to about 50 mm (2 inches). It hardens to 62-63 HRC quite readily by heating to its critical temperature (typically around 790°C – 816°C) and quenching in oil. Achieved hardness values are often in the range of 56-58 HRC, or it can reach around 60 HRC after low-temperature tempering, and even 58-62 HRC depending on the specific application. However, O1 can suffer from decarburization during heat treatment and is subject to cracking from the thermal shock of oil quenching. While low hardening temperatures and oil quenching offer relative freedom from cracking in intricate sections, double tempering may sometimes be preferred.

Mechanical Properties of O1

Machinability: Excellent, rated 90 (vs. 1% carbon steel at 100).
Dimensional Stability (Oil Quenched): Approx. 0.0015 in/in expansion.
Toughness: Rated 3 on a scale. Modulus of toughness: 68 MPa.
Strength: Higher yield and UTS vs. D2 (due to W & Mn). Yield: 758-829 MPa, UTS: 846 MPa.
Ductility: Noticeable necking before fracture (19.7% area reduction).

Understanding A2 Tool Steel

AISI A2 tool steel is a general-purpose, air-hardening tool steel. The “A” in its designation refers to it being an “air-hardening” steel.

Key Characteristics of A2

A2 is known for its minimal movement (distortion) in hardening and higher toughness than oil-hardening die steels, often with equal or greater wear resistance. It is characterized by high abrasion resistance and is considered safer for hardening with low distortion characteristics.

Common Applications for A2

A2 is suitable for a wide range of tooling applications. It’s used for inserts, bending dies, blanking dies, and extrusion dies and punches for aluminum. It’s also utilized for piercing-die bushings and stripping dies. A2 is a common alternative choice for machined dies in production quantities up to around 100,000 pieces. For coining applications, especially with stainless steel, heat-resisting alloys, or alloy steels, A2’s wear resistance makes it a good choice. It is also listed as a die material for coining aluminum and copper alloys for quantities up to 100,000. A2 is recommended for tools less demanding in wear but highly demanding in toughness.

Heat Treatment Considerations for A2

A2 has high hardenability, meaning it can be air-hardened effectively. A2 can be air-cooled from an austenitizing temperature like 995°C and tempered at 218°C to achieve a hardness of 59 to 61 HRC. Tempering A2 at about 510°C (950°F) can develop a definite but small secondary hardening peak, most intense when the steel was austenitized at higher temperatures. Typical hardness ranges for A2 in applications include 50-60 HRC or 53-56 HRC for molds, and 58-64 HRC or 56-60 HRC for bending/blanking dies. High-temperature tempering is recommended for A2 to decrease the amount of retained austenite.

Mechanical Properties of A2

Machinability: Lower than O1, rated 60 (vs. 1% carbon steel at 100).
Dimensional Stability (Air Quenched): Improved, approx. 0.0010 in/in expansion.
Toughness: Rated 4 or 5 on a scale (higher than O1).
Wear Resistance: Rated 6 on a scale. More abrasion resistant than S-series.
Carbide Formation: Lower Cr & C vs. D2 leads to small carbides, increasing toughness but decreasing wear resistance relative to D2.

Making the Right Choice: O1 vs. A2

In summary, O1 is a lower-cost, oil-hardening steel with good machinability and wear resistance, suitable for smaller tools and applications where dimensional stability is less critical and oil quenching is manageable. A2 is an air-hardening steel offering better dimensional stability and toughness, with high abrasion resistance, making it a good choice for applications requiring less distortion and higher production quantities, especially when coining tougher materials.

The choice depends on your factory’s specific requirements for wear resistance, toughness, dimensional control, and cost constraints. If you need minimal distortion during heat treatment and higher toughness for preventing cracking, A2 is likely preferable. If machinability and initial cost are paramount and your part geometry allows for safe oil quenching, O1 could be suitable.