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Vacuum heat treatment technology (4)

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-12-09 08:41:00
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Application of mould vacuum heat treatment technology

1. Technological parameters of vacuum heat treatment

(1). Vacuum degree

Vacuum degree directly affects roughness of mold surface, thus affecting surface quality performance. In order to prevent volatilization of alloy elements on mold surface, a reasonable vacuum degree should be selected. 

(2). Preheating temperature

When heating temperature of vacuum heat treatment is 1000 ~ 1100℃, one preheating is carried out at 800 ~ 850℃. When heating temperature exceeds 1200℃, mould with simple shape can be preheated once at 850℃, and mould with large or complex shape should be preheated twice at 500 ~ 600℃ and 800 ~ 850℃.

(3). Heat preservation

(1) Heating temperature. Heating temperature of vacuum quenching is generally lower limit of salt bath furnace and air furnace. Heating temperature of vacuum tempering, vacuum annealing, vacuum solution treatment and vacuum aging is generally same as that of conventional heat treatment.

(2) Insulation time. Normally vacuum heating time is six times that of salt bath furnace, air furnace 2 times, empirical formula is τ = KB + T

2. Vacuum quenching of mold

(1). Preheating

Low alloy steel (40Cr, 60Si2Mn, etc.), medium alloy steel (CrWMn, 9SiCr, 5CrNiMo, etc.) can be heated at two stages (such as 650℃ preheating →850℃ quenching heating).

High alloy steel (H13, Cr12MoV, etc.) can be selected three levels of heating (such as 650℃ preheating →850℃ preheating →1030℃ quenching heating).

(2). Selection of heating and holding time

The holding time should not only ensure that a certain amount of carbide is fully dissolved and alloy content in austenite is improved, but also provide a guarantee for obvious hardness rise when secondary hardening peak temperature is tempered.

(3). Cooling method

Mold steel vacuum quenching can be used oil quenching, gas quenching, water quenching, nitrate quenching and so on. Alloy die steel can be quenched with vacuum oil to obtain bright surface and reasonable performance. Compared with air quenching, oil is easy to obtain high toughness and strength because of its fast cooling speed. Less distortion can be obtained by air quenching.

(4). Vacuum tempering

Advantages of adopting vacuum furnace (such as WZH series single chamber positive pressure tempering furnace) : accurate and even temperature control; Ensure no oxidation during heating and insulation; It can be cooled slowly along with furnace, and also can be inflated to cool quickly. Cooling process can be filled with high purity N2 or a mixture of high purity N2 and other reducing gases (such as H2) to ensure no oxidation and no coloring during cooling.

The tempering heating speed is 0.8min/mm, and heart should be kept at least 2h after heating. Tempering temperature depends on hardness requirements. First and second tempering is mandatory, and third tempering depends on technical requirements and final hardness, which can also be omitted.

Keywords in the article: vacuum heat treatment technology
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