Operating Cost Comparison Sheds Light On Carburizing Methods
Low-pressure carburizing (LPC), often referred to as vacuum carburizing, in conjunction with high-pressure gas quenching (HPGQ) has matured into a relevant, applicable technology to meet the demands of the automotive industry for high-quality heat treated parts. This is due to its ability to provide tighter tolerances on the carburizing process with notable reductions in distortion of the carburized and hardened workpieces.
While new applications of LPC continue to increase in Europe, North America still is in the initial stages of embracing this technology due to several reasons including the recent slow down in the manufacturing sector and the higher capital investment costs associated with these systems.
As with other new technologies, the role of LPC needs to be established with respect to its relevancy in certain heat-treating applications compared with conventional technologies. Applications and circumstances typically suited for LPC include:
- Applications requiring precision and uniformity of the carburizing profile
- Applications where deformation or distortion are of concern
- Applications involving workpieces illustrating "difficult" geometry with respect to applying a carburized case, including the presence of blind or through holes
- Applications where existing heat-treating equipment is not able to achieve performance specifications, or where post-production requirements make it cost prohibitive
- Applications where steel grades can achieve sufficient hardenability using high-pressure gas quenching
- Applications where new product launches mandate the need for new equipment procurement
Even though LPC has reached the advanced stages of maturity within the European heat-treating industry, many North American heat treating companies are still not convinced that LPC plus HPGQ is a cost-effective technology compared with its conventional counterparts.
Aichelin conducted a study to compare the operational cost requirements for three types of its furnaces used to carburize transmission gears. The furnaces in the study include a continuous low-pressure carburizer with HPGQ, a conventional pusher system with oil quench and an integral quench-chamber furnace line (IQ batch) with oil quench. The footprint of each furnace system is shown in Fig. 1.
The heat-treated part in the study was a 3 in. diameter by 1.6 in. high (76 by 40.6 mm) SAE 5115 (16MnCr5) automotive gear with a chemical composition range of 0.13-0.18 C, 0.70-0.90 Mn, 0.035 P, 0.04 S, 0.15-0.35 Si and 0.7-0.9 Cr. Weight of the part was 1.76 lb (0.8 kg), and expected throughput was approximately 1,000 lb/h net (455 kg/h) to meet a heat-treating specification of 0.026 in. (0.6 mm) CHD at 53 HRC.
The data presented are derived from actual and empirical industry resources. All values provided are based on typically applied engineering practices used for the particular designs of the furnace systems. Although an organization's unit costing for each individual burden (i.e., expense) may vary, the relative cost comparison reconciles these differences in providing a fair and impartial operational cost value for each applicable technology and equipment scheme.
To calculate the expected operational cost requirements for each individual furnace type, it is necessary to quantify all relevant expenses inherent to maintain a high-quality production process. This includes utilities, manpower, maintenance, depreciation, floor-space allocation and interest-bearing debt.
Process description and costs
A summary of the process parameters and requirements are shown in Table 1. Although the hourly production rates for each system are not the same, the yearly production outputs are balanced. This matching of yearly production requires an adjustment of each respective system's hours of production per year to equalize total volume output.
It is important to note that for this particular example, the low-pressure carburizing furnace heat-up stage uses a conventional atmosphere (i.e., nitrogen) operation. Therefore, it is assumed that the heat transfer wall-loss coefficient is similar for the conventional pusher furnace and the LPC vacuum furnace for the first stage of heat-up. It also is assumed that the charge load for the IQ batch furnace is preheated to 400 C (750 F) via a preheat furnace before loading into the IQ furnace.
All zones forward of the LPC furnace heat-up zone are water cooled. This results in higher heat loss through the wall and explains the higher heat requirements for this furnace design. Heating requirements for the three furnaces in Fig. 2.
The pusher and IQ batch systems accounting takes into account all electrical equipment including motors, blowers, actuators, etc., throughout the entire furnace system. The primary electrical energy user for the LPC furnace system is a high-pressure gas quench blower. The total energy-consumption values incorporate all energy used; that is, gas and electrical. This also includes all ancillary supporting systems for each furnace system including tempering furnaces, preheat furnaces, washing machines, charge cars, etc. (Fig. 3).
Water-consumption values are based on evaporative losses (a delta of 8 C, or 15 F, across cooling tower). Oil consumption is only for the pusher and IQ batch systems. Table 2 lists electrical energy, gas, water and oil consumption data, as well as heating requirements.
Conventional furnace systems require much more site preparation, ranging from building foundations and installation of ventilation and fire-suppression systems. The additional cost requirements need to be included in "up-front" capital investment calculations. Table 3 shows installation cost comparisons for the different furnace systems.
Unit costing and capital investment for the different systems are shown in Table 4 and a cost summary is shown in Fig. 4 and Table 5. At first glance, this analysis appears to indicate that an IQ batch furnace line offers slightly reduced operational costs compared with low-pressure carburizing vacuum furnace or pusher furnace systems of similar capacity. However, the analysis does not include potential costs that may be required for post-production operations including reheating, die quenching, grinding and straightening. The additional cost drivers associated with quality issues, such as scrap generation and limited utilization efficiency, also need to be considered. These additional costs often are difficult to quantify for generalized presentation purposes, because they are developed on a case-by-case basis. Given the prospects for eliminating or reducing these cost issues, LPC plus HPGQ in many applications offers much lower operational cost compared with alternative conventional processes.
For example, in this study, if the conventional IQ batch furnace system requires post-production costs in excess of $0.006/lb (or $0.01/gear), LPC plus HPGQ most likely would offer a lower operational-cost alternative. In application, the viability of LPC plus HPGQ as a favorable technology improves significantly when heat treaters take a closer look at their costs for labor, maintenance and supporting infrastructure to maintain a high-quality operation.
European heat treaters have confirmed that, in many cases, LPC plus HPGQ offers a prudent investment decision from a quality, performance and operational-cost perspective. In this regard, many industry experts predict that within the next six to eight years, LPC plus HPGQ will continue to grow and acquire commonplace status in the North American commercial and captive heat-treating industry.