Surface White-Layer Formation
The news is full of reports on the role of Russia in American politics and society. After years of speculation, The Doctor has found the answer to a seldom seen and often confusing, yet fascinating, problem in of all places Soviet science and good old-fashioned modern-day research. Let’s learn more.
Perhaps a half-dozen times over the course of The Doctor’s career have vacuum furnace users reported the presence of a white layer on the surface and near-surface of steel and stainless steel component parts after vacuum hardening and/or carburizing followed by vacuum oil quenching (Fig 1). This white layer is most often mischaracterized as surface decarburization.
The phenomenon is far from common and, as it turns out, is not new. In point of fact, it was investigated decades ago and even documented, but the explanation as with many things in Soviet science was not readily accessible to the greater metallurgical community. In these past investigations, it was found that these white layers have much higher carbon content, more carbide-forming elements and higher levels of retained austenite than the bulk microstructure of the steel.
In testing conducted both then and now, white layers of 0.0002-0.0007 inches (5-18 mm) thick were produced and analyzed. Chemical analysis showed a high carbon content in these layers directly related to carbon saturation of the surface zone of the steel. Originally believed to be related to the process of oil quenching, it is now found to be due to oil vapors present in the vacuum environment.
It was previously established that these white layers were always found after oil quenching, but when quenching from 1400-1650°F (750-900°C) these layers are very thin and cannot be easily identified without very careful sample preparation and surface-analysis techniques. They are more readily identified after quenching from above 1800°F (1000°C). Over the years, the phenomenon has also been seen in vacuum furnaces on loads that are gas cooled above the oil.
A very thin surface layer forms when the temperature of the metal surface is high enough. Metallographic examination revealed a zone of cementite-austenite on the surface of the carburized layer. The thickness of this layer is dependent on temperature, cooling rate and/or time in contact with the cooling medium above a minimum reaction temperature. The layer can be uniform, even on surfaces of complex configuration, but is most often sporadic in nature along the surface – within a given part and from part to part throughout the load.
The white-layer phenomenon occurs in both vacuum furnaces with and without vacuum-sealed inner doors (Fig. 2). Tests were conducted to more accurately determine the root cause of this uncontrolled carburization (and the resultant transfer of carbon or carbon-based compounds into the vacuum environment) and subsequently their reaction with hot parts to form the observed white layer. A furnace with a vacuum-sealed door was used in the testing.
A thorough investigation of the variables that might contribute to the formation of this white layer on carburized 5120 steel was undertaken. It included:
- Checking the tightness (leak rate) of the furnace
- Checking the operation and functionality of the carburizing system
- Looking for the presence of quench-oil vapor in the hot zone
- Checking work transport sequence and oil-quench chamber
It is well known that a large leak can cause ingress of air (oxygen) into the vacuum furnace chamber. If the temperature is high enough when this occurs, a reaction will take place between graphite and air that results in the formation of carbon monoxide, a carburizing gas. The leak rate of the test furnace was checked and found to be within acceptable limits (<20 microns/hour).
Leakage in the carburizing gas mixture injection system would be another source of carburizing gas in the vacuum furnace. As part of this test program, mass-flow controller functionality and control/shutoff valves were verified to be operating properly, which eliminated them as a potential cause of the uncontrolled carburizing problem.
Subsequent actions then focused on the possible impact of the quench oil and more specifically the oil vapor. The entire load transport process from the heating chamber to immersion in oil was thoroughly and systematically analyzed and included: the sequence of load transport from the heating chamber into the quenching chamber; the sequence of internal door opening and closing; vacuum level during load transportation for quenching; the moment of backfill (with nitrogen) and gas dispersion during the backfill process in the quenching chamber; pressure above the oil; load immersion rate; oil agitator activation and rotational speed and quench-oil temperature.
As a result of the optimization of these factors, the intensity and quantity of the white layer was reduced and its presence limited to isolated areas in the microstructure, but it was not completely eliminated. The next step was to change the quench oil knowing that different oils have different vapor pressures (and the ability to produce varying amounts of oil vapors). Oil from a different supplier was used. Further testing did not solve the problem. The white layer still appeared occasionally but perhaps to a slightly lesser degree.
Unable to find a definitive solution to the problem during load transfer and in the quench chamber itself, the focus then turned to the heating chamber, particularly to that portion of the cycle where the temperature is lowered to final hardening temperature (the drop temperature stage in carburizing) and load stabilization just before load transfer to the quench.
It was discovered that, in some recipe settings, both the furnace and quench chambers were being pumped simultaneously by the same pumping system. In this arrangement both chambers are connected, and, theoretically, migration of oil vapors from the oil chamber to the heating chamber is possible. The test was repeated with a revised approach excluding simultaneous pumping of the two chambers. When this was done, the white layer disappeared completely. The tests were repeated multiple times with the same positive result, revealing that oil migration into the heating chamber was the root cause of the observed white-layer phenomenon.
The root cause of surface and near-surface white-layer formation is uncontrolled carburizing due to quench-oil vapor. This may occur in different stages of the process: during soak periods in the hot zone before transfer to the quench; during cooling of a hot load over the quench oil; and during load immersion into the oil. Which of these stages has the most influence on white-layer formation remains an open question.
In vacuum furnaces equipped with oil-quench tanks, the operational and process parameters should be set up and configured in such a way as to minimize the possibility of contact of quench-oil vapors with the hot load and to reduce the possibility of quench-oil vapor transfer to the heating chamber.
- Vanin, V.S., “White Layers in Vacuum-Quenched Steel,” UDC 621.785.51, Nikolaevsk Shipbuilding Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, p. 51, May, 1983
- Vanin, V. S., “The Influence of Pressure and of Carburizing Material on Carburization In Liquid Media,” Tr. Nikovaevsk, Korablestroit, Inst., 21 (1960)
- Vanin, V. S., G. I. Ermakova, and T. V. Shemeneva, “Prevention of Steel Decarburization by the Method of Nonisothermal Cyanidation,” Metalloved. Term. Obrab. Met., No. 11, 32 (1976)
- VSQ Vacuum Sealed Quench Brochure, CI Hayes