# Vacuum Heat Treatment (2): How to choose the vacuum heating temperature and heating time?

In the previous article: Vacuum Heat Treatment Knowledge (1), we introduced in detail what is vacuum heat treatment, the features of vacuum heat treatment, and how to choose vacuum level. Today, we continue to share the knowledge of vacuum heat treatment (2): How to choose the vacuum heating temperature and heating time?

## How to choose the vacuum heating temperature？

In general information, the heating temperature we see is within a range. When setting the process parameters, we have to take into account the performance requirements of the workpiece heat treatment. Technical requirements and the service conditions to be worked in order to find out the best heating temperature. Without affecting the mechanical properties and with minimum deformation, it is advisable to choose the conventional lower temperature. After all, most of the parts heated under vacuum conditions are finished parts, which do not require post-processing and have high requirements for deformation. Such as molds, smelting tools, cutting tools, etc.

For vacuum heating, normally it is divided into several stages.

• When the final holding temperature of the workpiece is 1000 ~ 1100℃, 800℃ can be used as a preheat.
• When the final quenching temperature of the workpiece is above 1200℃, a preheating of 850℃ can be used for simple shapes.
• When the final quenching temperature is above 1200℃, and the workpiece with large size, large quantity, complex shape, and high deformation requirement should be preheated twice: the first time 500 ~ 650℃, the second time 800 ~ 850℃ or multiple preheating, so as to ensure the balanced heating of the workpiece under vacuum.

## How to choose the vacuum heating time？

The vacuum heat treatment holding time mainly depends on the effective size and shape of the workpiece and the size of the furnace charge. So far there is a lack of a unified standard. Generally, there are three equations to calculate the vacuum heat treatment holding time.

T1 = 30 + (2.0 ~ 1.5) x D (1)
T2 = 30 + (1.5 ~ 1.0) x D (2)
T3 = 15 + (0.5 ~ 0.8) x D (3)

T1 is the first preheating time, T2 is the second preheating time, T3 is the final austenitizing time (min). D is the effective thickness of the heated workpiece (mm).

According to the relevant coefficients: cylindrical workpiece according to the diameter calculation, tube-shaped workpiece when the height/wall thickness ≤ 1.5mm, calculated by the height.

When the height/wall thickness ≥ 1.5mm, calculated by 1.5mm wall thickness.

When the outer diameter/inner diameter >7, calculated by solid cylinder, the hollow inner cylinder is calculated by multiplying the outer diameter by 0.8.

The data in the formula are preheating coefficients, and the lower limit (large value) is chosen when the workpiece shape is complex, large, and dense in formula (1)(2); the upper limit (small value) is chosen when the shape is simple and loosely placed. Style (3) in the high alloy steel choose the lower limit (large value); high-speed steel choose the upper limit (small value).

The preheating constants 30, 30, 15 are preset lag times (min) based on the heating characteristics of the endothermic vacuum furnace. This equation sometimes has a shortcoming in the actual calculation. That is, the final austenitizing time heating coefficient of the upper and lower limits of the selection of improper deviation, the deviation will cause a lack of hardness or reduce efficiency. This requires the furnace operator to observe diligently. That is, to observe the furnace heating chamber (workpiece, material basket, heating chamber lining, and heating elements, etc.) whether the color is uniform, timely adjustment of the heating time to obtain a more accurate burning time, to ensure the quality of the heat treatment.

To be continued…

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