Gas vs. Liquid Quenching: A Direct Comparison in Jominy Hardenability
Hardenability is the ability of steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface when cooled under a certain condition. The “gold standard” test for all hardenability results has always been the “Jominy End-Quench Test.” The information gained from this test is necessary for metallurgists when selecting the proper combination of alloy steel and heat-treatment parameters in order to minimize their thermal stresses and distortion in the manufacture of components of various sizes.
The Jominy end-quench test involves a normalized cylinder specimen that measures 1 inch in diameter by 4 inches in length, heated to the austenitizing temperature for that given material. Once thoroughly heated, the specimen is quickly transferred to a test stand that holds the specimen vertically. One end of the sample is then sprayed with a controlled flow of water. This cooling spray of a portion of the sample simulates the effect of quenching a larger steel component in water. Once at room temperature, the specimen is ground to a depth of 0.015 inches on parallel surfaces. The hardness is then measured at 0.0625-inch increments from the quenched end of the bar.
High hardness occurs where high volume fractions of martensite develop – always nearer the quenched site. The lower hardness indicates transformation to be incomplete. Structures such as bainite or ferrite/pearlite microstructures usually develop farthest from the quench site (Fig. 1). With this valuable information, the metallurgist can make critical design decisions. Steels with higher hardenabilities are needed for higher-strength, larger components, whereas steels with low hardenabilities may be selected for smaller components.
It is well known that the medium used for quenching a material influences cooling rate due to varying thermal conductivities and specific heats. Liquid quenchants such as brine, water or oil cool much more quickly than air quenching. Today, it is also well understood that eliminating two of the three phases of liquid quenching (vapor and vapor-transport phases) and cooling only via conduction can dramatically reduce distortion. Therefore, with nearer net-shaped parts, higher-alloyed steels are preferably hardened primarily by gas quenching within vacuum furnaces. As the cooling parameters within vacuum furnaces continue to increase due to such factors as increased gas velocities, increased pressures, various pedigrees of gases and better fan designs, why then would the Jominy end-quench test not adapt to changing times?
Description of the Test
In order to differentiate the hardenability of materials when gas quenched, a new Jominy test method is necessary. That new test for this experiment will be a “Jominy Air End-Quench Test.” The first phase of this experiment will examine how various gas velocities affect hardenability on various alloy steels. The three alloy steels chosen for this test were 4130, 4140 and 4340. All specimens were normalized at 1700±10ºF for one hour. The respective austenitizing temperatures and times for the test specimens were as follows.
4130 per AMS 6370N: 1600±10˚F for one hour
4140 per AMS 6349C: 1550±10˚F for one hour
4340 per AMS 6415S: 1525±10˚F for one hour
A total of four tests were performed utilizing all three materials. Each respective material was cut from the same bar, maintaining heat-lot integrity. The baseline was established by a typical Jominy end-quench test performed with water (Fig. 2). For the next three tests, an air-quench test stand was built (Fig. 3). All dimensions for the test stand were identical to the standard Jominy stand, including matching the orifice size for the gas jet while maintaining the identical distance to the end of the test specimen.
To better understand the effect of gas velocities on hardenability values, three vacuum furnaces were selected. All furnaces had identical internal hot zones measuring 36 inches wide x 36 inches high x 48 inches deep. All gas velocities were measured by an anemometer capable of measuring wind speed up to 300 mph (Fig. 4). The internally quenched (IQ) vacuum furnaces represented three distinct vintage furnaces with varying fan designs, nozzle configurations and hot-zone designs. Furnace number one, a 2-bar vacuum furnace manufactured in the 1990s, produced a gas velocity of approximately 50 mph. Furnace number two, a 10-bar vacuum furnace manufactured circa 2000, measured gas speeds at approximately 100 mph. Furnace number three, a brand-new, state-of-the-art 20-bar vacuum furnace, measured gas velocities approaching 200 mph (Fig. 5). These same gas velocities, supplied by banks of 2,500-psi nitrogen cylinders, were then measured at the air-quench test stand. The three alloys were heated to their corresponding austenitizing temperatures and quenched at gas speeds of 50, 100 and 200 mph.
Jominy End-Quench Results
Three charts display the Jominy curves that were generated from the tests. Figure 6 reveals that the gas hardenability tests did not even begin to approach the water-quench baseline (red line). Due to the lack of carbon in the 4130 alloy steels, this less-hardenable alloy is not a good candidate for gas quenching even at 200-mph gas velocities.
Figure 7 shows that the only gas velocities that match the water-quench baseline for 4140 alloy steels are gas speeds in excess of 200 mph.
The results for the most-alloyed and therefore most-hardenable steel alloy (4340) are shown in Fig. 8. Although the 50-mph and 100-mph tests fared much better, the 200-mph gas velocity ultimately matched the water-quenched baseline.
Jominy Test with Pressure
The next step in this testing protocol was to merge the results that were performed at atmospheric pressure into the protective atmosphere of a vacuum furnace. In a vacuum chamber, the objective was to duplicate the same gas velocity that was produced at atmosphere while identifying how elevated pressures (10 bar) could affect cooling rates and, therefore, hardenability values.
A special “Jominy Vacuum Furnace” was designed and built by Solar Manufacturing. The vacuum furnace was equipped to heat the same-sized Jominy bar within an internal fixture. A gas orifice, identical to the external test stand, streamed pressurized nitrogen at the Jominy bar end at various speeds (Fig. 9).
After multiple tests, it was quickly discovered that the desired results could not be achieved. The vacuum chamber was so small (10 inches ID x 12 inches deep) that the incoming desired gas speed of 200 mph could not be maintained consistently (Fig. 10). Additionally, with the need to rapidly vent the large volumes of incoming nitrogen gas, the desired operating pressures could not be maintained. The decision was made to cease the Jominy pressure/velocity testing phase and not to build a larger chamber.
At atmospheric pressure, gas velocities of up to 200 mph must be attained to duplicate traditional Jominy end-quench results for 4140 and 4340 alloy steels, but these velocities are not sufficient to duplicate traditional Jominy end-quench results for 4130 alloy steels. Inconclusive results were attained attempting to evaluate Jominy hardenability values when increasing both gas velocity and gas pressure within a minimally sized vacuum furnace. IH
For more information: Contact Robert Hill, president, Solar Atmospheres of Western PA, 30 Industrial Road, Hermitage, PA 16148; tel: 1-866-982-0660; fax: 1-724-982-0593; e-mail: firstname.lastname@example.org; web: www.solaratm.com