Steel properties

ALTA HT515(TM) steel is a kind of hot work tool steel for large section hot-forging dies, it have high hardenability, high thermal stability, high thermal fatigue performance and impact toughness. 

This steel strength is higher than 5CrNiMo/5XHM steel but slightly lower than H11/1.2343 steel, and the impact toughness is higher than 5CrNiMo, H11/1.2343 steel. Its high-temperature strength improved by 64% than 5CrNiMo, the thermal stability temperature increased by 150 to 170°C than 5CrNiMo which is very close to H11/1.2343 steel.

The grow tendency of austenite grain size is small during heating. And the heat treatment temperature range is wide, which is conducive for large-size molds to long-time heating and soaking.

Applications

² Suitable for manufacturing large-section hot working dies for presses and die forging hammers

Chemical composition

Chemical Composition per %

Heat treatment

Coefficient of thermal conductivity : 33.5λ W/(m·K) (room temperature 20°C)
Specific heat [capacity] : 501.6 Cp (J/Kg·K) (room temperature 20°C)

Quenching process specification :

     The quenching heating and cooling processing is the key to affect the molds deformation or even if cracking, and also influence the obtaining of ideal quenching structure and mechanical properties.

Due to the large size of hammer forging die, the generated thermal/structural stress during cooling process are also very big. Thus, an appropriate pre-cooling is necessary before quenching cooling.

Generally, it should be pre-cooled to near the AC3 temperature of steel (~ 880℃). Pre-cooling way :

1. Pre-cooled with furnace to uniform temperature, and then cooled again after comes out from furnace and done quenching.

2. Pre-cooled in the air after comes out from furnace.

   --small molds (≤250mm) pre-cooling time 3 ~ 5 minutes

   --big molds (≥300mm) pre-cooling time 5 ~ 8 minutes

Then, put it into oil (oil temperature 30 ~ 80℃) for further cooling. And, the best is to stirring and cooling to be cooled evenly.

Generally, molds should be taken out Of the oil tank when cooled to 150 ~ 200℃, and be tempered immediately.

(at this moment, it can be determined based on experience: when the mold lifts the oil surface, it emits light smoke and no longer catches fire, it can also be determined by using an infrared thermometer)

Please refer to the following table for the quenching and cooling time of the forging die in oil.

** The Structure of HT515 steel after quenching in the temperature range of 960-1010℃ is lath martensite + twinned martensite and a small amount of residual austenite is expected. The hardness after quenching is 54-61HRC.

Tempering process specification :

    HT515 steel has good tempering resitance which still can maintain high hardness 51~53HRC after tempering at 550℃, 47~48HRC after 600℃ tempering, and it drops to 42~44HRC after 650℃tempering. But 5CrNiMo/5XHM steel hardness is less than 30HRC after 650℃ tempering.

The thermal stability temperature of HT515 steel is higher by 150℃ than 5CrNiMo/5XHM steel.

When HT515 tempered in 400~500℃, it precipitates from the matrix Carbides such as M2C and VC which have high dispersion and can produce a secondary hardening effect.

Recommended tempering specification :

1. Pre-heating to 280±10℃ (soaking time 1.5h/100mm thickness)
2. Heating to 640±40℃ (soaking time 3h/100mm thickness) and then air cooled

Second round of tempering

3. Pre-heating to 280±10℃ (soaking time 1.5h/100mm thickness)
4. Heating to 640±40℃ (soaking time 3h/100mm thickness) and then air cooled

In general condition, the quenching stress of steel is very large, it will generated new stress if the heating speed is too fast during tempering, thereby casuing molds deformation or cracking oftenly.

Therefore, an isothermal preheating segmented heating tempering form should be applied. And, the preheating temperature should not be higher than 350℃, if so, the retained austenite in the center of the mold will transform to upward bainite, which will not only reduce the mold strength, but also reduces the impact toughness significantly.

Therefore, the isothermal preheating temperature is generally around 280°C. At this temperature, the retained austenite will transform into lower bainite which has high strength, toughness and impact toughness. It is helpful to improve the molds service life.

Because of HT515 steel contains 0.80~1.20 Mo%, it's not sensitive to second time of tempering, so air cooling can be applied after tempering. Although slow cooling will reduce the impact toughness slightly, but the absolute value still meets the technical requires.

In production, one-time tempering be used oftenly, however, due to insufficient tempering (especially for medium to large-sized molds), it often occurs cracking after one-time tempering (a short tempering time). Therefore, the second time of tempering needs to be carried out after the first-time tempering and molds be cooled down to room temperature, to allow the residual austenite to get a fully transform. The second tempering temperature should be about 10°C lower than the first tempering temperature.

HT515 steel can obtain a high comprehensive mechanical properties after at 960~1010℃ quenching and tempering at 600~680℃.

HT515 mechanical properties (985℃ quenching)

HT515 high temperature strength :

    The high-temperature strength of HT515 steel is similar as 5CrNiMo/5XHM steel when test

temperature below 500℃. But when the test temperature is higher than 600℃, the high-temperature strength of HT515 steel is double more than 5CrNiMo/5XHM steel. This is cause of the M3C carbide in 5CrNiMo/5XHM steel is easy to gather to grwo up at high temperature.

Comparison of high temperature strength Rm (Mpa)

** HT515 steel 985℃ quenching, 630℃tempering

** 5CrNiMo/5XHM steel 850℃ quenching, 550℃ tempering

HT515 high temperature impact toughness :

   500-550℃ is the working temperature range of the mold working surface. At this temperature,

the impact toughness of 5CrNiMo/5XHM steel is at the lowest range, while the impact toughness of HT515 steel only drops slightly, which is twice as high as 5CrNiMo/5XHM steel.

Comparison of high temperature impact toughness (Aku (J))

** HT515 steel 985℃ quenching, 630℃tempering

** 5CrNiMo/5XHM steel 850℃ quenching, 550℃ tempering

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Features

² High thermal fatigue resistance

² High thermal stability

² High hardenability

² Good impact toughness