Many publications suggest that LPC is 30% or so more efficient than conventional carburizing and is cheaper even when the higher capital equipment cost is taken into consideration [1-3]. It has been shown to be significantly faster even at the same carburizing temperature [4], particularly for thin cases where carbon transfer is the rate-controlling step. Vacuum technology also allows the use of higher temperatures and hence even faster carburizing.

However, speed and cost are not the only concerns in industrial processing. In a world where energy is in ever-increasing demand, the amount consumed to process a given part can be a critical factor.

Fig. 1. A sealed-quench furnace

Energy Sources

Sources of available energy depend on location. To improve comparability, this study assumes, except where stated otherwise, that the ultimate energy source is natural gas. The energy required to extract and distribute it can therefore be disregarded. The efficiency of electricity generation has been taken as 50% (high for conventional gas-fired power station but low for a CHP station, and a reasonable mean for all forms of electricity generation [5, 6]).

Fig. 2. Double-chamber vacuum carburizing furnace (photo courtesy of SECO/WARWICK)

Furnace Equipment

While it is tempting to compare two furnaces with the same loading capacity, they are not really comparable. The slightly longer cycle time of a low-pressure carburizing furnace (floor-to-floor) does not allow it to carburize as much product as a conventional furnace in a given time, even though the actual carburizing time is shorter. In this study, a typical gas-fired, sealed-quench furnace using an endothermically generated atmosphere (Fig. 1) is compared to an acetylene-based vacuum carburizing unit using a 15-bar nitrogen quench (Fig. 2), both having the same total throughput. The initial study assumes an optimal continuous 24/7 output. The effect of the sub-optimal loading typical of a unit integrated into a production line is discussed later.

Capital Cost and Energy

No comparison of processing routes, in terms of either the total energy used or the total processing cost, can ignore the equipment the process uses. This is widely understood in terms of the processing cost where depreciation is averaged across all the units produced. The energy used to produce the equipment itself, however, is ignored by most studies because it is difficult to define accurately [7]. This article assumes that the total energy cost of the equipment is equal to the energy cost of producing steel components (65 MJ/kg)[8] and that the equivalent of the entire structure is replaced by spare parts over the equipment’s lifetime of 20 years.

Firing

As the vast majority of sealed-quench furnaces are gas fired and most vacuum furnaces are electrically heated, these have been taken as the fuels. The energy requirement for both furnaces is made up of several elements:
  • The energy to heat the furnace from its loading condition – hot in the case of the sealed quench and room temperature in the case of the vacuum furnace – to the operating temperature
  • The energy to heat up the load to its operating temperature
  • The energy to maintain the temperature over the period of the treatment
All these data are best determined experimentally as, apart from the energy to heat up the load, they are highly dependent on the furnace and in particular on its insulation [4,9].

In addition to the firing itself, other ancillaries require energy including control equipment, the fan in the sealed quench and the vacuum pumps. These need to be assessed on a per cycle basis as the power required – particularly for the vacuum pumps – varies during the cycle.

Atmospheres

In comparing the energy consumption for the atmospheres, only the consumables are considered because the equipment was included previously. The elements of the energy consumption of endothermically generated gas are well known – the natural gas for firing and for reaction gas and the electricity for the compressor, etc. It has been assumed that the generator runs 24/7 and that an average of 5% natural gas is added to the endo to control carbon potential.

The energy represented by the gas mixture for the LPC (acetylene, ethylene and hydrogen) is complex. The energy requirement of acetylene is high because of the large energy investment to make its precursor, calcium carbide. As most calcium carbide is made using hydroelectric power, the high efficiency of this type of plant – typically 90% [10] – has been used in the calculations. For the ethylene, its calorific value has been used. The energy required to make hydrogen was calculated based on electrolysis of water.

Quenching

The sealed-quench furnace uses little energy for quenching. All that is needed is to recirculate the oil. The small amount of dragout has been incorporated into the maintenance energy requirement.

The vacuum furnace is more energy intensive. A high-power blower is needed to drive the nitrogen through the load. Additional energy is required to separate the nitrogen from air, liquefy it and transport it to the plant. In this latter calculation, plant efficiencies and transportation distances typical of the U.S. have been used.

Fig. 3. Furnace load after LPC processing and high-pressure gas quenching (photo courtesy of Ipsen International)

Washing

Finally, the parts may need to be tempered and, if necessary, washed. As both processing routes use the same tempering energy, however, this has been ignored in the comparison. As Fig. 3 shows, the parts from the LPC treatment are clean, so only the parts treated in the sealed-quench furnace and quenched in oil need to be washed.

Fig. 4. Comparison of the energy used per kilogram of treated product for conventional and low-pressure carburizing at maximum efficiency

Results

A comparison of the total energy required by each processing route is shown in Fig. 4. It is immediately obvious that the LPC process uses less energy overall than the conventional sealed-quench processing route. The primary reason for this is that the majority of the energy used for the LPC process is in the form of electricity. Electricity must be generated from a primary energy source, in most cases, at fairly low efficiency. The conventional route uses natural gas firing, however, which is even less efficient. This is not to say that the LPC process is not more cost efficient as that depends largely on the relative cost of the fuel gas and electricity.

Fig. 5. Comparison of the energy used per kilogram of treated product for conventional and low-pressure carburizing under typical operating conditions

One of the big advantages of a system that uses electricity and industrial gases as the major costs is that when the system is turned off, it uses no consumables. A system that uses natural gas, however, must remain in standby mode, assuming that the stoppage periods are short (such as over a weekend). When the energy audit is re-based on a more typical 14-shift pattern, the energy for the LPC process changes little but that for the conventional technology increases sharply (Fig. 5). In this more typical operating environment, the conventional carburizing process uses well over twice as much energy – per kilogram of product – than the low-pressure carburizing/high-pressure gas-quenching route. IH

For more information: Paul Stratton is the project manager for research and development for Linde AG - Linde Gas Division, Carl-von-Linde-Strasse 25, 85716 Unterschleissheim, Germany; tel: +44 1484 328736; e-mail: paul.stratton@boc.com; web: www.boc-gases.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: low-pressure carburizing, endothermic atmosphere, high-pressure gas quench