Fig. 1. Cellular vacuum heat treat concept

Vacuum and atmosphere carburizing are the preferred process choices for the heat treatment of gears. As companies consider which technology to invest in for the future, both technologies continue to be analyzed with respect to their respective economic and quality aspects to meet increasing demand for improved economics (i.e., lower unit cost achieved by reducing secondary manufacturing operations) and for higher quality and performance (i.e., improved part metallurgy). Achieving these goals requires focusing on tighter process control, better quenching methods and automation to remove operator induced variability.

Fig. 2. Up-time reliability in a multiple cell (ICBP600tg) vacuum carburizing system

Vacuum carburizing with either oil or gas quenching capability has demonstrated its excellent cost effectiveness for gear heat treatment, and its implementation continues to grow rapidly [1-3], supplanting atmosphere carburizing. The benefits of low pressure vacuum carburizing are realized not only with new equipment, but also with retrofits of older units. Today's managers want the heat treating operation integrated into the production flow [4], and vacuum furnaces, especially when combined with gas quenching, easily suit this concept (Fig. 1).

Fig. 3. Automotive ring gears vacuum carburized at 1760°F in 114 min cycle with 15-bar N quench; Fig. 4. Helical shafts carburized at 1760°F in 114 min cycle with 8-bar N quench

Low-pressure vacuum carburizing offers the ability to increase carbon absorption at the surface of the steel by 10 to 15%. Improvements in cycle time of up to 40% are possible by combining this higher carbon absorption with higher carburizing temperatures and/or the use of modified chemistry material.

Vacuum furnace systems can be easily integrated into a gear manufacturing operation. Flexibility is achieved by designing systems to accommodate from 2 to 12 cells to carry out various processes including carburizing, oil quenching, high pressure gas quenching, preheating, normalizing, annealing, tempering, nitriding, and carbonitriding. Reported system up-time reliability is greater than 95% (Fig. 2).

In most gear applications, especially those involving power transmission and heavy-duty performance, gears require a carburizing treatment. A properly carburized gear is capable of handling approximately 30 to 50% more load than a through-hardened gear.

Vacuum carburizing has proven advantages including improved part performance, part durability and part uniformity. Evaluation by ECM customers of results of dynamometer tests to failure for vacuum carburized ring and pinion sets show they outperform their atmosphere-carburized counterparts (Table 1). The improved part performance is a major reason why the manufacturers converted from atmosphere to vacuum technology. Table 2 shows the improvement in vacuum-carburized part quality measured using Weibull analysis of automotive gears. In addition, results of tests conducted by ECM customers (Table 3) show vacuum-carburizing process consistency and part uniformity-part-to-part and load-to-load-for identical production loads of pinion gears.

The following examples of production workloads illustrate the high-quality obtained in short cycle times using vacuum carburizing.

Fig. 5. Helical gears carburized at 1760°F in 114 min cycle with 12-bar N quench; Fig. 6. Vacuum carburized ring gears used in quenching study

Ring gears. A workload consisting of 64 type AISI 4120 (SCM420H) automotive ring gears (Fig. 3) weighing 8.8 lb (4 kg) each, totaling a gross load weight of 836 lb (380 kg), was carburized at a temperature of 1800?F (980?C) in a 3.75 hour cycle. Effective case depth was 0.030 to 0.035 in. (0.8 to 0.9 mm). A 15-bar nitrogen quench produced core hardness values of 28-31 HRC.

Helical shafts. A workload consisting of 54 type AISI 5130 (27MnCr5) helical shafts (Fig. 4) weighing 10.4 lb 4 (4.75 kg) each, totaling a gross load weight of 770 lb (350 kg) was carburized at a temperature of 1760°F (960°C) in a 1.90 hour cycle. Effective case depth was 0.020 to 0.025 in. (0.5 to 0.6 mm). An 8-bar nitrogen quench produced core hardness values of 39-43 HRC.

Helical gears. A workload consisting of 70 type AISI 5130 (27MnCr5) helical gears (Fig. 5) weighing 5.3 lb (2.4 kg) each, totaling a gross load weight of 528 lb (240 kg) was carburized at a temperature of 1760°F (960°C) in a 1.90 hour cycle. Effective case depth was 0.020 to 0.025 in. (0.5 to 0.6 mm). A 12-bar nitrogen quench produced core hardness values of 39-43 HRC.

Fig. 7. Comparison of cooling rates in different media

Tables 4 and 5 show the results of quenching studies performed on vacuum carburized parts (Fig. 6) to quantify results and to aid in the selection of quench parameters. The study included metallurgical variables (ECD at pitch line and root, surface hardness, core hardness at the pitch line and root), dimensional variables (precision index, flank form, backface run out, backface taper), gear set variables (MTE, pattern, V&H) and assembly variables (fatigue life).

Fig. 8. Optimum cooling gas mixture composition

The use of gas mixtures, such as CO2-He, further enhances high pressure gas quenching technology. CO2-He blends improve the cooling rate and produce results similar to those of oil quenching (Fig. 7). In addition, an optimum gas mixture maximizes part hardenability (Fig. 8), and (variable speed) fan styles balance cooling speed and part homogeneity while adding process flexibility.

Fig. 9. CFD modeling of a high-pressure gas quench cell. Courtesy of Air Liquide.; Fig. 10. Infracarb™ process simulator

The need to integrate heat treating equipment on the shop floor and into the production flow of the manufacturing process is a major reason for the growth of vacuum carburizing. Coupled with technical advancements such as gas mixtures, modeling of high pressure gas quenching cells (Fig. 9) and more sophisticated process simulators (Fig. 10), low pressure vacuum carburizing equipment offers amazing versatility for the heat treater. IH (cont. >)