A8 Air Hardening Tool Steel

A8 tool steel is an outstanding representative of air-hardened cold working tool steels, which is an important tool material classified under the ASTM A681 standard.

The core advantage of A8 steel is that it has achieved an excellent balance between high wear resistance, good toughness and minimal heat treatment deformation, which makes it an ideal choice for manufacturing precision, complex and long-life cold working molds, especially in applications where dimensional stability is extremely high.

A8 STEEL CHEMICAL COMPOSITION

Grade	C	Si	Mn	P	S	Cr	Mo	W
A8	0.5-0.6	0.75-1.1	0.2-0.5	≤0.03	≤0.03	4.75-5.5	1.15-1.65	1.0-1.5

Core Element Characteristics

Carbon: 0.50 – 0.60% (provides the basis for hardness and wear resistance)
Chromium: 4.75 – 5.50% (mainly improves hardenability, wear resistance and certain corrosion resistance)
Molybdenum: 1.15 – 1.65% (significantly improves hardenability, wear resistance, resistance to temper softening and high temperature strength)
Tungsten: 1.0 – 1.5% (refines grains, improves wear resistance and red hardness)

A8 Steel Physical and Mechanical Properties

Density: Approx. 7.85 g/cm³
Coefficient of thermal expansion: (20-100°C) Approx. 11.0 x 10⁻⁶ /K
Thermal conductivity: (100°C) Approx. 29.0 W/(m·K)
Modulus of elasticity: Approx. 210 GPa
Typical hardness: HRC 57 – 62 after quenching + tempering (specific value depends on tempering temperature).

A8 STEEL FORGING

Forging process	Temperature range	Operation requirements
Preheating	400-650°C (slow)	Step heating to prevent thermal stress cracking (especially for large sections).Fully burn through to ensure uniform core temperature. Insulation time: 1.0-1.5 minutes/mm (section thickness).
Initial Forging Temperature	1050-1100°C	Maximum temperature should not exceed 1150°C to avoid overburning and grain coarsening.
Final Forging Temperature	≥900°C	Absolutely forbidden to be lower than 850°C! Low temperature forging is prone to cracking and forms work hardening.
Cooling After Forging	Furnace cooling/sand cooling	Cool in sand/furnace to ≤500°C, then air cool. Water cooling or rapid cooling is prohibited!

A8 STEEL HEAT TREATMENT

A8 Steel Annealing

Usually heated to about 820-845°C, then slowly furnace cooled to about 480°C at a rate of ≤15°C/hour, and then air cooled. The annealed hardness is about HB 200-230, and the structure is spherical pearlite, which is easy to process.

Quenching Process For A8 Steel

Preheating: 650-700°C (to prevent thermal stress cracking)
Austenitizing temperature: Typically 925-955°C. Exact temperature depends on desired hardness, toughness balance and workpiece size.
–Lower temperatures (925-940°C): Finer grains, higher toughness, slightly lower hardness. Recommended for complex shaped dies or where toughness is a priority.
–Higher temperatures (940-955°C): Dissolves more carbides, resulting in higher hardness and wear resistance, but slightly coarser grains and slightly lower toughness. Recommended for relatively simple dies with extremely high wear resistance requirements.
Holding time: Ensure that the workpiece is fully heated through, and avoid over-heating to prevent grain growth, decarburization and oxidation.
Cooling: Cooling in static or slightly circulating air. When the cross-section is too large or the furnace is densely loaded, strong air cooling or positive pressure gas quenching may be required, but oil quenching or water quenching should be avoided, otherwise it will cause extremely high internal stress, severe deformation or even cracking, and lose the core advantage of low deformation of A8 steel

A8 Steel Tempering Process

Tempering must be done immediately! After quenching, the workpiece should be tempered when it cools to a temperature that can be touched by hand (~50-70°C).

It is strongly recommended to temper at least twice (the second tempering temperature can be 10-20°C lower than the first). After the first tempering, the retained austenite is transformed into martensite. The second tempering can eliminate the stress generated by the new martensite and further stabilize the microstructure.

Each tempering holding time is greater than 2 hours/inch thickness (to ensure that stress is fully eliminated), and air cool to room temperature after tempering

The commonly used tempering temperature range is 175°C to 540°C. The tempering temperature directly affects the final hardness and toughness:

Tempering temperature (°C)	Typical hardness (HRC)	Performance characteristics	Applicable scenarios
175 - 205	60 - 62	Highest hardness, best wear resistance, lowest toughness	Extremely high wear resistance, impact-free precision blanking die
205 - 370	59 - 61	High hardness, good wear resistance, medium toughness	General precision blanking, forming die
370 - 480	57 - 60	Good hardness/toughness balance	Most commonly used range! High-demand bending, stretching, cold heading dies
480 - 540	55 - 58	Best toughness, good impact resistance	Dies that withstand large impact loads