AISI A2 Hardware Tool Steel Sheet

ASTM A2 hardware tool steel is ASTM standard high toughness and wear-resistant tool steel, which is air-hardened section steel. The mass fraction of carbon of the steel is about 1%. After heat treatment, the hardness can reach 57~62HRC, and the mass fraction of chromium is about 5%. After polishing, the surface of the steel is darker, with excellent corrosion resistance and strong ductility. The damage resistance of the die edge is better.

The steel has good hardenability, uniform carbide distribution, certain impact and good wear resistance. It has good air-quenching performance, small size deformation after air-quenching, uniform and fine carbides.

Chemical Composition(Wt.%):

A2 Steel Techinical Details:

A2 is a cold work die steel improved on the basis of SKD11 (Cr12Mo1V1). Under normal heat treatment conditions, the retained austenite is almost completely decomposed. Generally, cryogenic treatment can be omitted, and high toughness can still be maintained under strong hardness.

A2 after quenched at 1040℃ and tempered at 520～530℃, the hardness HRC can reach 60～62, and the toughness is twice that of Cr12Mo1V1. Good, the residual stress of the electro-processed modified layer is small, the retained austenite is very small, and the carbides are fine and uniformly distributed.

Due to the complex force of the mold, some mold working parts need to have some special mechanical properties. If the standard heat treatment process is often not able to meet the ideal working performance requirements, the basic characteristics such as hardness, toughness and wear resistance must be treated by heat treatment. Properly adjust to achieve the best working condition of the mold. Quenching temperature and tempering temperature are the main process parameters of heat treatment. This article focuses on the tempering characteristics of A2.

In the experiment, some changes were made to the A2 heat treatment specification, the quenching temperature was appropriately adjusted, and the tempering temperature was set to 6 levels, namely 100°C, 200°C, 300°C, 400°C, 500°C, and 600°C. For tempering at 100℃, use 101-2 type drying oven for heating, and use SX-25-12 box-type resistance furnace to heat the rest. Take two samples for each tempering temperature.

The hardness test selects the metal Rockwell hardness test, which is carried out at room temperature, and uses the HBRVU-187.5 Blowi optical hardness tester.

The impact test adopts a 10mm×10mm×55mm unnotched sample, which is carried out on the JB30B impact testing machine, and the impact energy is 0.3 KN.m or 0.15 KN.m.

Experimental results and analysis:

Hardness value:

Take three different positions for each sample to measure the hardness, and get the hardness value at each tempering temperature. After summing up the hardness value of each sample, when A2 is tempered at 100～500℃, the hardness value does not change much; The hardness is slightly higher when tempered at 400°C at medium temperature, and the peak hardness after standard heat treatment and tempering is generally around 520°C; after high temperature tempering at 600°C, the hardness drops significantly, and the average HRC hardness value is only 52.4, so tempering The temperature should not be too high.

Impact toughness:

After tempering, the oxidized decarburization layer on the surface of the sample is ground off, and the impact value of each sample under different tempering temperatures is measured. The impact value of each sample is combined. When DC53 is tempered at 200℃, the average impact value reaches 60 Above J/cm2. When tempered at 500 ℃, the impact toughness is poor, showing a certain high temperature temper brittleness. The impact toughness of tempering above 600℃ is very good, but the hardness is greatly reduced, which does not meet the requirements of use.

The experimental results show that the overall tempering stability of A2 is good, and within a certain tempering temperature range, the hardness and impact value change little; the toughness drops significantly when tempered at 400～500℃, and temper brittleness appears; When tempered at ℃, the toughness of the sample is very high, the impact value reaches 85 J/cm2, but the hardness is greatly reduced. In production, high temperature tempering can be used for cold work molds with low hardness and wear resistance requirements and high toughness requirements; cold work molds with high hardness requirements and high toughness should be used Low temperature tempering around 200℃. The hardness and impact values ​​at other tempering temperatures can be predicted by appropriate calculation methods (such as interpolation, function approximation, etc.), and then verified by experiments. The carbides in the quenched sample are distributed in intermittent thin bands. After tempering at 200 ℃, the carbides are uniformly distributed, and there are almost no massive carbides in the structure, so the toughness is better. From the morphology of the fracture, the cleavage steps of the fracture of the tempered structure at 200℃ are far less than that of the quenched sample. The fracture in the 5000-fold metallography has some small and shallow dimples, which shows that it has a certain degree of toughness. After tempering, the retained austenite transforms more fully, and the carbides are fine and uniformly distributed, which increases the toughness.

Heat treatment Related

Forged of A2 Tool steel

Slowly heated the A2 steel material to 650-680°C (1202-1256°F) before raising the temperature to 1050-1100°C (1922-2120°F) and soak A2 tool steel material until uniformly heated. Initial hammer blows should be light due to the comparatively high hot strength of this steel.

The A2 steel forged temperature should be maintained at 1000°C(1832℉) until the A2 steel metal begins to yield. Final forged should not be done below 900°C(1652℉). Slowly cool after forged is essential to avoid the possibility of cracking.

Annealing of A2 Tool steel

Slowly heated A2 steel to 850-870 ℃（1562-1598°F） and allow enough times, let the steel to be thoroughly heated, Then cool slowly in the furnace at a rate of 4°C (40°F) per hour or less.. The A2 tool steel will get MAX 250 HB （Brinell hardness）.

Hardening of A2 tool steel

A2 steels should be heated uniformly to 790-820°C (1436-1508°F) until completely heated through. Then heated to the hardening temperature of 950-980°C(1742-1796℉), soak at the temperature enough time, let A2 steel to be thoroughly heated.

Then withdraw A2 steel tools from the furnace，either air cool or oil quench.

Tempering of A2 tool steel

Tempering of A2 steels is performed at 150-400°C (302-752°F), Soak well at the selected temperature and soak for at least one hour per 25mm of total thickness. Get Rockwell C hardness of 57 to 62. Others different Heat treatment temperature will get different hardness. Below only for reference.

Conclusion:

1. After adjusting the quenching temperature appropriately, the hardness and impact toughness of A2 are higher when tempered at 200℃; the hardness is higher when tempered at 400～500℃, and the toughness is greatly reduced; when tempering at 600℃, the impact toughness is very high. The hardness dropped significantly.
2. The molds with complex shapes, such as precision punching dies, trimming dies, and cold-rolled rollers, should be tempered at low temperature, so that the working parts of the mold can obtain high hardness, high toughness, good wear resistance, and high strength, which can effectively extend the life of the mold and prevent Early failure phenomena such as excessive wear, deformation, and cracking.
3. Complex molds subject to large impact loads can adopt low-quenching and high-return processes to obtain higher impact toughness and prevent brittle fracture of the mold

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