Do we know how much energy we need for heat treatment?

What about the energies in the furnace? Do we even know how much energy we use per 1 hour of furnace operation or per 1 kg of processed amount? Do we know how furnaces behave, how they differ from each other, and how to choose the source of processing so that we have optimal or minimum energy costs? Or does it matter?

To clarify the terminology, this is a very simplified view based on knowing the cycle time and the total cycle energy consumption in kWh. Whether it’s natural gas or electricity. The cycle time is calculated from the process START  to the STOP. By the simple share of the total energy consumption per cycle from START to STOP and the cycle time, we get the average hourly furnace consumption, regardless of the fact that in some phases of the cycle the consumption is higher and in others smaller. Therefore, we do not deal with consumption during furnace evacuation, furnace heating, consumption within holding time on temperature or consumption during hardening given by fan power input, oil heating or oil bath mixer. It is therefore an indicative indication of how much energy on average-maintained furnaces we have to supply during each hour of the cycle so that the cycle runs technologically.

Because the size of the batch in kg, without baskets, is also known, then we are also able to calculate the energy consumption per 1 kg of parts processed in the furnace.

I did such a small analysis of processes and their consumption. I included hundreds of furnace cycles in my analysis, whose data I had available. The aim was to compare different types of furnaces and different types of processes.

And what the result is?

Fig. 1

Explanations of abbreviations

• SQ – Multipurpose furnaces
• V – Vacuum furnaces
• C + H <1.5 mm – gas carburizing with CHD <1.5 mm, typical SQ furnaces, size 7 or 8
• C + H <1.0 mm – gas carburizing with CHD <1.0 mm, typical SQ furnaces, size 7 or 8
• AH – Hardening under protective atmosphere in SQ furnaces, typical SQ furnaces, size 7 or 8
• CN – Carbonitriding in gas with CHD <0.5 mmm, typical SQ furnaces, size 7 or 8
• AT – Tempering without or with protective atmosphere, typical chamber furnaces size 7 or 8
• VH – Vacuum hardening in various vacuum furnaces with a typical size of 600x900x600 mm
• VT – Vacuum tempering in various vacuum furnaces with direct heating and with a typical size of 600x900x600 mm

Logically, the highest energy costs are for processes that require long times. It is a carburizing  or carbonitriding. It can be seen from Figure 1 that 2.54 kWh / kg falls on carburizing with CHD <1.5 mm. This is followed by carburizing to CHD <1.0 mm and carbonitriding with depths up to 0.5 mm. For direct hardening in multi-purpose furnaces, we need about 1 kWh per 1 kg of parts.

There are also some interesting results, which are logical, but in practice not everyone will notice them. For example, vacuum hardening consumes 2x more energy per kg of charge than multi-purpose hardening furnaces. Why? For multi-purpose furnaces, we load into a furnace heated to at least 750 C, for a vacuum furnace we always start from zero. From this point of view, if SQ furnaces are still on temperature, this type of hardening is significantly less energy-intensive per 1 kg of charge.

On the contrary, it is necessary to think about the concept of vacuum furnaces. In order to get to the level of energy consumption of SQ furnaces, we must think of at least a two-chamber solution.

Fig. 2 – Example of single-chamber and double-chamber vacuum furnace Ipsen

In the case of vacuum, on the other hand, the energy consumption of tempering furnaces is better. The difference is almost 24% compared to multi-purpose furnaces. The reason is certainly in the concept of the furnace and in the insulation materials used.

Surprisingly, the energy consumption of AT (Atmosphere tempering) furnaces is almost the same as for AH (Atmosphere hardening) tempering furnaces. Why? Not an easy answer without details investigation.

But it is true that I could only include furnaces that are available in this analysis, and they are usually 10 years or more old. However, the technology is moving fast forward, and therefore the insulation of the furnaces will certainly improve. Still, it’s a result to think about.

Fig 3 – Comparison of vacuum hardening furnaces with TAV H8-S

An example of the impact of the new furnace concept is shown in the following figure. Here I compared 7 types of different vacuum hardening furnaces with the newly installed H8-S furnace from TAV Vacuum Furnaces. It is a surprise to me that this furnace has 32.5% lower consumption per unit weight of charge than other furnaces.

The last column of the graph then shows the energy consumption when we are doing the tempering process in the hardening furnace. Consumption is 32% lower than during hardening (Fig. No. 3), but at the same time 38% higher (Fig. No. 4) than if we tempered in a vacuum tempering furnace. However, we must also take into account that the tempering time in the hardening furnace will be 20 to 30% longer than if we performed the same process in a dedicated furnace, ie in a vacuum tempering furnace with direct heating.

Fig. 4 – Comparison of tempering furnaces with TAV H6-T

What does this mean?

• Each furnace, for hardening, carburizing, or for tempering and annealing, shows completely individual values. These values ​​may not be optimal at all.
• In terms of consumption, the furnaces will differ according to the furnace concept, and according to the quality and condition of the insulation. This is one of the reasons why new devices with better insulation properties will have better conditions for reducing consumption than older devices.
• As furnaces are depreciated over 20 years today, the question is whether a shorter depreciation period would not be the answer. In 20 years, this is a step change in innovation, and in my humble estimate, the furnace, after 20 years of operation, can no longer meet the requirements of today.
• To find out, we need to measure the energy consumed. Whether in natural gas or electricity
• For the above calculation, we need a clear link between energy consumption and charge weight. This cannot be done without our ERP system being able to register cycles, with reporting the total batch weight as the sum of all production order weights in the batch, and being able to assign the consumption result from natural gas and electricity measurements to the cycle ID in the ERP system.
• According to Janusz Kowalewski of Ipsen USA, the trend for this year will be vacuum brazing of aluminium heat exchangers for e-cars, large vacuum furnaces for the aerospace or energy industries, and small vacuum furnaces for processing 3D output or for medical purposes. I no longer register LPC or movement from atmospheric furnaces towards vacuum, except for nitriding processes.
• From the above results, however, it is probably necessary to add multi-chamber vacuum furnaces, as this is one of the few ways to reduce the energy costs per kg of goods to the level of multi-purpose furnaces
• It is also necessary to think about whether for such high-energy equipment as heat treatment furnaces there should be classification of energy consumption, as we know it today, when we buy a refrigerator, washing machine, etc. Given the acquisition values, it would there was certainly interesting information for customers and investors, including the fact that the time is approaching when we will have to audit our carbon footprint in the heat treatment shops.

February 15, 2022

Jiří Stanislav