{"id":2661,"date":"2021-08-03T04:49:32","date_gmt":"2021-08-03T04:49:32","guid":{"rendered":"https:\/\/www.jstconsultancy.cz\/?p=2661"},"modified":"2026-08-01T06:58:33","modified_gmt":"2026-08-01T06:58:33","slug":"heating-of-dies-in-a-vacuum-furnace-part-ii","status":"publish","type":"post","link":"https:\/\/intelligent-kalam.80-240-27-133.plesk.page\/en\/heating-of-dies-in-a-vacuum-furnace-part-ii\/","title":{"rendered":"Heating of dies in a vacuum furnace, part II"},"content":{"rendered":"

In the first part of the heating, we achieved that the inserts are heated to a temperature corresponding approximately to the setup furnace temperature Tp = 650 \u00b0C.<\/strong> However, the surface temperature Ts<\/strong> may not reach the furnace temperature Tp<\/strong> and will lie somewhere in the interval Tp> Ts> Tc<\/strong>. The temperature in the core Tc<\/strong> then decides on the start of the hold time countdown. It does not have to be significantly long; the goal is to heat the parts in full cross-section above 600 \u00b0C<\/strong> so that the material of the insert goes into a plastic (ductile) state. Usually, after reaching the difference Tp – Tc < 50 \u00b0C<\/strong>, when the condition Ts – Tc <110 \u00b0C<\/strong> is automatically met, a holding of 15 minutes is set. Its extension makes no sense and is uneconomical, because by reducing the temperature difference Tp – Ts<\/strong> and Ts – Tc<\/strong>, the heating efficiency is reduced. Our interest is not only the proper heating of components in the furnace, but also the economics of this heating.<\/p>\n

Just to give you an idea, for possible calculation it is necessary to take into account the heat transfer by radiation between the heating elements and the heated parts, expressed by the equation in which the temperatures are in the fourth power Q = \u03c3 * \u03b5 * S * (Tg4<\/sup> – Ts4<\/sup>) [W],<\/strong> where Tg<\/strong> is the temperature of the graphite heating elements, \u03c3<\/strong> represents the Stefan-Bolzmann constant, \u03b5<\/strong> the emissivity of the surface, and S<\/strong> then the irradiated area.<\/p>\n

Furthermore, it is necessary to take into account the heat transfer from the gaseous medium to the surface of heated parts according to the equation for convection heating Q = \u03b1 * S * t * (Tp – Ts) [W],<\/strong> where \u03b1<\/strong> is the heat transfer coefficient for the given environment, and at the same time heat inside the components according to the relation Q = \u03bb * S * (Ts – Tc) \/ d [W],<\/strong> where \u03bb<\/strong> is the coefficient of thermal conductivity for a given steel, which is not constant and changes with the heating temperature, and d<\/strong> is the wall thickness.<\/p>\n

In all these equations, therefore, there is a difference in temperature, but most significantly in the transfer of heat by radiation, when the temperatures are in the fourth power. Therefore, more important than dealing with the theoretical calculation is to measure temperatures with thermocouples Ts <\/strong>and Tc<\/strong>, and to control the heating process from these thermocouples.<\/p>\n

When programming the individual heating steps on a vacuum furnace, the so-called conditional dwell is usually used. In practice, this means that meeting a condition triggers the required step. However, since most vacuum furnaces do not have such programming capabilities, it is necessary to find a heat treatment supplier who has furnaces so programmable. The usual trigger for the heating step is one of the thermocouples, in our case Tc<\/strong>. When the difference Tp – Tc < 50 <\/strong>\u00b0<\/strong>C<\/strong> is reached, the step time starts automatically.<\/p>\n

Whether Ts<\/strong> in the first step reaches a temperature Tp = 650 \u00b0C<\/strong> is therefore not decisive, it is decisive that the previous condition Tp – Tc < 50 \u00b0C<\/strong> is fulfilled. After its fulfillment, it is possible to continue heating, regardless of the temperature Ts<\/strong>, but with the knowledge that the optimal temperature difference between the heating elements and the surface Ts<\/strong> will be guaranteed. Thus, the heating will be sufficiently efficient according to the above equation for heat transfer by radiation.<\/p>\n

However, since by fulfilling the condition Tp – Tc < 50 \u00b0C<\/strong> it is confirmed that Tc > 600 \u00b0C<\/strong>, it is thus also confirmed that the heated material is already over the transit temperature of the transition from solid to plastic (ductile) state and is therefore heated to the temperature range, where it is no longer necessary to take into account dT<\/strong>, and thus also the stress from the temperature difference dT = Ts – Tc.<\/strong><\/p>\n

In the next heating phase, for a hold on Tp = 850 \u00b0 C<\/strong>, we can heat the parts at any speed and the heating ramp for this second delay can be significantly higher than for heating up to 650 \u00b0C<\/strong>. If in the first step up to 650 \u00b0C<\/strong> it is usually 6 -10 \u00b0C\/min<\/strong>, and it is additionally controlled and limited by a ramp to Tc <220 \u00b0 C \/ hour = 3.66 \u00b0 C\/min, <\/strong>then in the second heating step to 850 \u00b0C<\/strong> we can set a ramp of 10 to 20 <\/strong>\u00b0<\/strong>C\/min<\/strong> on the furnace and we no longer have to check the ramp on Tc<\/strong>.<\/p>\n

The aim of the second hold at \u00a0850 \u00b0C is therefore not to reduce the stress in the heated parts, but only to thermally homogenize the material before we start heating it to the austenitization temperature.<\/p>\n

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Fig. 1<\/u><\/strong> \u2013 Theoretical (ideal) diagram of heating of dies in a vacuum furnace<\/p>\n

Simultaneously with the heating of the material of the parts, some other properties of the steel also change. Above all, its thermal conductivity changes. It increases for high-alloy steels, but decreases for low-alloy steels. However, once we reach temperatures above 700 \u00b0C<\/strong> (Fig. 2), the thermal conductivity for all types of steels is essentially the same and stabilizes at 25 – 30 W \/ (m.K).<\/strong> This is also an advantage for further heating, since all types of steels can be processed in the same way from the point of view of heating at this stage.<\/p>\n

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Fig. 2<\/u><\/strong> \u2013 Dependence of thermal conductivity of steel on temperature (<\/em>Totton’s\u00a0<\/em>Steel Heat Treatment Handbook<\/em>)<\/em><\/p>\n

The hold at 850 \u00b0C<\/strong> is again controlled from the thermocouple Tc<\/strong>. However, as soon as we reach the temperature Ts – Tc < 14 <\/strong>\u00b0<\/strong>C<\/strong> on the part, we should start counting down the dwell time. However, since usually the conditional delay evaluates the difference Tp – Ts<\/strong> or Tp – Tc<\/strong>, and not the difference Ts – Tc<\/strong> as Nadca 207 says, we will deal with the fact that again the condition of the transition to the next step will be reaching the temperature on Tc<\/strong>.<\/p>\n

The surface temperature Ts<\/strong> will again be in the interval between Tp> Ts> Tc<\/strong>, but the temperature difference range will be narrowed to only 14 <\/strong>\u00b0<\/strong>C<\/strong>. Even in this phase of heating we do not have to wait until Ts<\/strong> reaches the furnace temperature Tp<\/strong>, but it is likely that the temperature difference Tp – Ts<\/strong> will be less than 10 \u00b0C<\/strong>. The holding timer at temperature can again be minimized, eg for 15 minutes.<\/p>\n

The last heating step is heating to the austenitization temperature. This is the most important temperature of the entire heating process. We must ensure that the heated parts reach this temperature as far as possible over the entire cross-section, the holding time must be long enough to dissolve the carbides in the steel as best as possible, and at the same time not long enough for the primary grain growth. Nadca 207 therefore defines this heating phase as 30 minutes<\/strong> from the moment when Ts – Tc < 14 \u00b0C<\/strong>, and at the same time states that it must not be longer than 90 minutes<\/strong> from the time when Ts<\/strong> reaches the temperature Tp<\/strong>.<\/p>\n

If we work in the mode of Nadca 207, then the austenitization temperature is not defined by the temperature range From-To, but only by a single temperature according to Nadca 207. It is necessary to be very careful. The material sheets of steel manufacturers always refer to the temperature range for austenitization, but this range cannot be applied to die-castimg inserts processed according to Nadca 207. The specification of austenitization temperatures, as well as the determination of heating delays are given in the Nadca 207 table on page 34.<\/p>\n

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Fig . 3<\/u><\/strong> \u2013 Part of the table with temperature specifications from Nadca 207, page 34<\/p>\n

As an example, I mention steel W300 ISOBLOC from Bohler (Nadca 207, type H11 \/ 2343, steel group D), where in the material sheet the range of hardening temperatures from 1000 to 1040 \u00b0C is allowed, but Nadca 207 allows only one temperature, 1000 \u00b0C.<\/p>\n

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The ramp for heating to the austenitization temperature can be chosen again significantly higher, as in the previous step, there is no danger of stress from the temperature difference Ts<\/strong> and Tc<\/strong>.<\/p>\n

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Fig. 4<\/u><\/strong> \u2013 Ideal heating process<\/p>\n

Fig. 5<\/u><\/strong> \u2013 Heating process, when the load thermocouples never reach the required temperature<\/p>\n

The ideal heating process is shown in Figure 4<\/strong>, where Tp<\/strong> is the programmed furnace temperature, Ts<\/strong> is the surface thermocouple temperature and Tc<\/strong> is the core temperature. All 3 temperatures meet at the required temperature, e.g. 1000 \u00b0C. The austenitization time 30 minutes<\/strong> from reaching Ts – Tc < 14 \u00b0C<\/strong> starts to count down as expected, as well as the limit time of 90 minutes<\/strong> from when Ts<\/strong> reaches temperature Tp<\/strong>.<\/p>\n

But Figure 5<\/strong> shows another, not unusual, example. The furnace does not heat up and neither Ts<\/strong> nor Tc<\/strong> ever reach the desired temperature Tp<\/strong>. Because Nadca 207 does not give us the possibility the austenitization temperature tolerance, failure to reach the value Tp = 1000 \u00b0C<\/strong> can be considered as faulty austenitization. The condition that the austenitization time will be a maximum of 90 minutes<\/strong> from reaching the temperature Ts = Tp<\/strong> will never be met, only the condition Ts – Tc < 14 <\/strong>\u00b0<\/strong>C<\/strong> and the time 30 minutes<\/strong> will be met and the process will require manual intervention by the operator.<\/p>\n

Because the vacuum furnace is usually validated according to AMS 2750 F<\/strong>, it can be assumed that the desired and actual temperature in the furnace, controlled by a control thermocouple, will actually be 1000 \u00b0C<\/strong>. However, since the furnace is in Class 2<\/strong>, with a temperature tolerance of +\/- 6 \u00b0C<\/strong>, a situation may occur where the part with Ts<\/strong> will be loaded in the furnace heat chamber, where the temperature is within the tolerance band for TUS,<\/strong> but with the limit temperature near deviation -6 \u00b0C<\/strong>. The processed part itself will therefore not be at a temperature of 1000 \u00b0C<\/strong>, but only at a temperature of 994 \u00b0C.<\/strong> However, this is only provided that the batch thermocouples show the correct temperature. However, other cases may occur.<\/p>\n