Low-pressure carburizing may gradually replace traditional atmosphere-carburizing technology and oil-quench hardening in two-chamber furnaces (sealed or integral quench). In order to achieve the same or better results, vacuum-furnace quenching-system designs must be improved to achieve the same cooling efficiency as oil using gas as a modern and more environmentally friendly medium. Gas-quenching systems outperform oil in almost every aspect. Nonetheless, current technology performance is not as strong as oil quenching given the limitations of carburized-case applications in some steel grades and/or the part dimensions.
For the purpose of measurement and comparison, many methods and coefficients help to determine the efficiency of a given system and quenching medium. These include: Grossmann’s Number (H), cooling rate at given temperature (typically at 705°C), l coefficient and heat-transfer coefficient (a) as the most objective. Quenching parameters of typical oil systems were determined with a coefficient within the range from 1,000-2,500 W/m2K according to traditional division for slow- (1,000-1,500 W/m2K), medium- (1,500-2,000 W/m2K) and fast-speed oil (2,000-2,500 W/m2K).
HPGQ vacuum furnaces may be classified as two types depending on design. Single-chamber furnaces (heat treatment and quenching occurs in one chamber without dislocation of charge) have slower cooling due to construction and material limitations. The more efficient two- and multi-chamber furnaces utilize a separated, dedicated cold quenching chamber.
At present with current technology, an average a coefficient of 600-800 W/m2K can be obtained in HPGQ separated chambers for nitrogen under 20 bar and slightly above 1,000 W/m2K for helium. Single-chamber furnaces with 15-bar nitrogen have a coefficient of 400-700 W/m2K and below 1,000 W/m2K for helium.[2,3,4,5] These parameters still differ from those available with oil.
Single-chamber furnaces have more simple construction, are less expensive and find application for batch and flexible production. Multi-chamber systems are more complicated in construction, more expensive and are typically used for mass-production applications.
Taking this into consideration, SECO/WARWICK designed a single-chamber furnace providing parameters similar to those obtained in separated quenching chambers and even comparable to oil-quenching systems. It is appropriate for mostly small and medium companies but also has applications in heavy industry and mass production, especially in these challenging times.
Testing FurnaceTests were carried out with an industrial furnace made by SECO/Warwick S.A., type 25.0VPT-4035/36IQN. This is a universal, single-chamber vacuum furnace designed for LPC under FineCarb® and PreNitLPC® technology (intensive high-temperature carburizing with control of grain growth), hardening with high-pressure gas and tempering within one cycle. The furnace allows for an advanced quenching technique with temperature control (marquenching, austempering) and gas heating (convection). The furnace has been equipped with an innovative power-management system that reduces the consumption of electrical energy. The construction consists of a cylindrical heating chamber with graphite insulation and circular, flat heating elements. The furnace has been equipped with a closed-loop, nozzle-type gas-cooling system and has been adapted for operation with nitrogen and helium (future use, hydrogen) at a pressure of 24 bar (Fig. 1). The technical specification of the furnace is as follows:
- Working space (W/H/L) - 600/600/900 mm (24/24/36 inches)
- Charge mass (gross) - 800 kg (1,760 pounds)
- Rated temperature - 1320°C (2400°F)
- Working vacuum - range 10-2 mbar (10-2 torr)
- Heating system power - 150 kW
- Cooling pressure - 24 bar, N2, He, H2
- Cooling blower motor power - 220 kW
Cooling SystemThe furnace cooling system consists of the following: a blower located at the rear that forces gas flow into the charge area through a closed loop, cylindrical-nozzle-injected system located in the sidewalls and front of the heating chamber; a back hatch for the gas outlet; and a water heat exchanger. Cooling gas circulates in the following order: blower ® nozzles ® charge ® back hatch ® heat exchanger ® blower. The nozzle cooling system is characterized with excellent evenness and penetration in a densely packed charge due to the proper location of the nozzles and high-acceleration gas flow. Gas velocity at the nozzle outlet is around 70 meters/second (230 feet/s) for nitrogen and can be increased to over 100 meters/second (330 feet/s) for helium.
Measurement of Alpha Coefficient in Ambient TemperatureTests were carried out with a 25 mm (1 inch) diameter heated a probe invented by SECO/Warwick. Probes were located in 10 positions of the working zone on two levels – at the corners and center – of the reference charge for three load-density arrangements: 0%, 50% and 100%. The reference charge of 100% density was prepared with 25-mm steel cylinders and a 150 mm (6 inch) length located vertically on a two-level tray with 768 pieces and 500 kg (1,100 pounds) gross total mass, as shown in Figure 2.
Tests were conducted using various conditions of pressure and velocity of nitrogen and helium, and the obtained results are shown in Table 1. Results confirmed the impact of pressure (density) and gas velocity on a to 0.7 power and an increase of a by 30-35% when nitrogen is replaced with helium. In addition, the application of helium resulted in a blower motor loading decrease by about six times, which enabled acceleration of the fan (by 50%) and a proportional increase of gas velocity.
Under conditions of blower maximum power (220 kW), the average a coefficient for nitrogen (24 bar and 65% velocity) was 940 W/m2K and at some locations over 1,000 W/m2K. For helium (24 bar, 150% velocity), it resulted in 1,800 W/m2K and a maximum above 2,000 W/m2K. Cooling uniformity within the working space was +/-15%. Charge packing density did not have a significant impact, but for nitrogen with loads of 50% and 100%, an increase of a coefficient occurred as compared to an empty chamber (0%). For helium, a decrease of a occurred with 50% charge packing. At 100%, a increased up to values obtained with the empty chamber.
Cooling Efficiency and UniformityThe test was carried out for the charge of 50% packing density. Specimens made from austentic stainless steel of 25 mm diameter (1 inch) and 150 mm (6 inch) length were located at 10 standard points of the working zone. Thermocouples were placed in the center of the specimens to monitor the temperature during the actual cooling process. Tests were carried out for helium maximum cooling rate (24 bar, 150% gas velocity). Measurement results and different interpretations were determined for cooling curves (Fig. 3.), a coefficient (Fig. 4.) and cooling rate (Fig. 5.).
Cooling curves provided data for the calculation of l coefficient that determined the cooling rate from 800°C (1472°F) to 500°C (932°F) in hundredths of seconds. The average l was 0.22 (22 seconds) with a spread from 0.18 to 0.26 for the whole working zone. However, a coefficient increased from the beginning of cooling and stabilized after 40 seconds from the start of the process due to quenching-gas temperature changes. Its average value was around 1,600 W/m2K after 80 seconds, which corresponded to results obtained at ambient temperature. On the other hand, the maximum cooling rate was achieved at 700°C (1292°F) after around 25 seconds at 12-18°C/s (22-32°F/s). Generally, considering the cooling uniformity of the whole working space determined with l, a and cooling rate, it was in the range of +/-20%.
Steel Hardening TestThe next test related to technological properties of the furnace cooling system was as-quenched hardness. For this purpose, the 50% packing-density charge was prepared, and specimens of diameter 10, 15, 25, 30, 40 and 50 mm were evenly located. Samples were made from steel grades 16MnCr5, 20MnCr5 and 18CrNi8, and the chemical compositions are shown in Table 2. Thermocouples were installed inside the core of specimens made from 16MnCr5. Hardening followed at the austenitizing temperature of 860°C (1580°F) using a 24-bar helium quench with 150% gas velocity.
The diagram in Fig. 6 presents results of temperature measurement inside the specimens of different diameters. Specimens of diameters of a given range obtained l coefficient from 0.12 to 0.39 with 0.21 for 25 mm (1.0 inch) diameter, which corresponded very well to results obtained during cooling efficiency and uniformity tests. Analysis of data indicated a maximum cooling rate of the core of 28°C/s (50°F/s) for the specimen of 10 mm (0.4 inch) diameter and approximately 9°C/s (16°F/s) for the specimen of 50 mm (2.0 inch) diameter.
As the absolute hardness value obtained after quenching depends also on steel chemical composition within a given grade (Table 2), the comparison shall provide for more objective assessment with regard to efficiency of the cooling system. For this purpose, the same charge (as in case of helium) was subjected to hardening with a two-chamber vacuum furnace equipped with a tank filled with Vacu Quench B244 oil. After quenching with helium and oil, hardness measurements were taken on the surface and core of specimens and compared (Table 3).
For 16MnCr5 steel, the 50-mm-diameter specimen was 24 HRC after helium quenching, and after oil quenching it was 21 HRC. For the 15-mm sample, hardness values were quite similar for both quenches – 29 HRC. The 10-mm sample was 38 HRC after oil quenching and 30 HRC with a helium quench. Results for 20MnCr5 and 18CrNi8 steel were similar but less dramatic due to their better hardenability (less quench-rate sensitivity).
Our findings can be explained on the basis of the critical cooling rate that appears after 20 seconds of quenching for the small-diameter specimens (10 mm). At that time, the fastest oil-quenching phase (boiling phase) took a primary role, while helium quenching had not yet reached nominal parameters, mainly due to the high temperature of the cooling gas (220°C, 428°F). For larger-diameter specimens (over 15 mm) at the critical cooling rate of 30 seconds, the third, slower cooling phase of oil – convective cooling – was effective, while helium quenching exhibited its maximum cooling rate. This was confirmed by measuring a coefficient, which for helium was twice the 600 W/m2K value found for the oil convective-cooling phase of the quench.
Summary1. Tests were conducted with the single-chamber vacuum furnace equipped with a gas-quenching system for nitrogen and helium at 24 bar.
2. Tests and processes were carried out under industrial conditions with a charge of 300-500 kg (660-1,100 pounds) and working space of 600/600/900 mm (24/24/36 inch).
3. Helium quenching efficiency was found to be similar to medium oil with a coefficient equal to 1,600 W/m2K.
4. The whole working zone indicated very good cooling uniformity (+/-20%) thanks to intensive gas penetration into the charge with the nozzle cooling system.
5. The hardening test confirmed very good gas cooling parameters. Hardness values exceeded values obtained from oil for samples above 15-mm (0.6-inch) diameter.
6. For small parts below 10 mm (0.4 inch), oil quenching provided higher hardness.
ConclusionThe single-chamber vacuum furnace made by SECO/Warwick S.A. equipped with a high-pressure gas-quenching system (HPGQ) proved capable of hardening steel grades and sizes that previously were thought to be best suited for either oil quenching or separated gas-quenching chambers in dual- or multi-chamber furnaces.
The single-chamber furnace is a device of simple construction and operation that requires a smaller investment in capital cost and at the same time produces work reliability, taking advantage of the flexibility in the technology and all of the advantages inherent with gas quenching. IH
For more information: Contact Dr. Maciej Korecki, director of research and development, SECO/Warwick S.A. Sobieskiego 8 str., 66-200 Swiebodzin, POLAND; tel: +48 068 38 20 506; e-mail: firstname.lastname@example.org; web: www.secowarwick.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: high pressure gas quenching, oil quench, low-pressure carburizing, convective cooling, critical cooling, quench efficiency