Application Of High-Pressure Gas Quenching

The introduction of high-pressure gas quenching in vacuum furnaces in 1977 has given a new horizon to the vacuum heat treatment of tools. Before then, only some hot-work steels and a few other tool steels of small cross-section could be hardened satisfactorily. Nowadays, almost all cold-work, hot-work and high-speed steels can be vacuum hardened successfully using high-pressure gas quenching up to 20 bars. Multi-chamber furnaces with high-pressure gas and oil quenching facilities are still available for the vacuum heat treatment of low-alloy steels.

Heat treatments in salt baths and traditional protective-atmosphere furnaces are being replaced increasingly by processing in vacuum furnaces with high-pressure gas quenching, for a variety of reasons:

• Vacuum heat treatment guarantees perfect surface protection, eliminating the risk of decarburisation and oxidation.
• Surface roughness and distortion can be virtually eliminated, thereby reducing finishing costs.
• Degassing of surfaces and perfect vacuum conditions produce better toughness at similar hardness levels and give longer life.
• Reliable process data acquisition and reproducibility, through automatic heat treatment and accurate control of all critical factors, ensures uniform and optimum properties of tools and components, without rogues.
• Special possibilities include combined brazing and hardening in a single operation, as well as safe annealing treatments, without decarburisation, of used tools for refinishing purposes.
• Ideal working conditions with no uncomfortable heat, no environmental pollution and no effluent problems.
• Pre-programmed automatic processing, instant readiness of the installation and flexible operation result in significant savings in labour costs.
• No idling costs.

Cooling Methods and Rates

Three basic means of cooling the workload of a vacuum furnace

• Vacuum cooling,
• Static cooling, and
• Forced cooling.

Three methods used to increase the rate of heat removal

• Increase the mass flow of the gas (i.e., gas velocity),
• Increase gas pressure and,
• Use a gas with a higher thermal conductivity.

Effect of quench pressure on cooling rate

The significant effect of the gas pressure on heat transfer during gas quenching was recognized in the mid 1970’s. For the first time, an increase of the cooling gas pressure from 1 to 2 bars was applied in practice. A further pressure increase up to 10 or even 20 bars led to the development of a single-chamber vacuum furnace with a new gas recirculation system, in which the cooling gas pressure through the charge vertically from above with high velocity. Investigations carried out with similar installations show that the cooling rate measured at the surface of components during high-pressure gas quenching, at nitrogen pressures of up to 10 bars in the temperature range of 1200 to 500ºC, is proportional to the quenching pressure, where the pressure dependence is a function of the exponent > 1. Calculations of the heat transfer coefficient H show that this is related to the velocity V and the pressure P of the cooling gas via the following equation:

H = A (V x P)m

where m and A are constants depending on furnace design, workload and properties of the gas.

Effect of blower capacity on cooling rate

A quench blower consists of a motor-driven turbine wheel sealed within a vacuum-tight enclosure. The blower’s capacity is governed by the size of the wheel, which is usually rated in cubic feet per minute at a fixed RPM. Through design calculations, the horsepower of the motor is matched to suit the size of the wheel and blower enclosure.

Pressure quench furnaces currently available come with a wide variety of blower sizes. The same size of six-bar furnace built by different manufacturers may be equipped with blower motors ranging in size from 100 to 400hp. The recent trend has been to equip furnaces with larger blowers under the assumption that this will yield improvements in cooling performance.

Effect of heat-exchanger efficiency on cooling rate

The next major component in the quench loop that is assumed to have a significant effect on cooling performance is the heat exchanger. Most vacuum furnace heat exchangers are of the water-cooled tube fin design.

Effect of quenched-gas composition on cooling rate

The optimum quench gas will have a low density to minimize the amount of power required to blow it through the quench loop. It will also have a high specific heat, which can be considered as a measure of its capacity for heat removal from the load. It must also be non-reactive with the materials that it is used to cool.

Based on availability, density and specific heat properties, hydrogen would appear to be good choice as a quenchant. However, because of the explosive risks associated with hydrogen, it is seldom used as a quench gas in commercial heat treating. Nitrogen is the most popular choice, primarily because it is readily available and inexpensive. Argon is used in some special applications but does not quench as effectively as nitrogen and is considerably more expensive. Helium also has excellent properties as a quench gas; it is used relatively frequently, especially for materials that have borderline hardenability in gas quenching.

Effect of chamber design and load distribution

The most-difficulty-to-quality factor that affects cooling performance in a vacuum furnace is the chamber itself. Both field trials and computer-modeling studies show that the flow patterns of quench gas through the load can have a marked effect on how quickly the load will cool. This effect exists regardless of quench pressure, blower size or any of the other factors already discussed.

Load distribution can affect cooling rate, even when load mass is unchanged. For example, a load distributed throughout three baskets will cool differently than the same load distributed in four baskets. Even a different load arrangement within each basket will have an effect on cooling. This conclusion was determined during pressure-quench field trials where the distribution of the load was changed while load mass and all other quench parameters were fixed. In most extreme case, the faster-cooling location in the load could be made the slowest-cooling location simply by re-arranging the load. The extent of this effect was very load-dependent and could not be generalized.