Intensive quenching could replace some oil-quenching processes as it becomes better understood and with the use of appropriately designed systems.
|Fig 1 Relationship of quench cracking of a 60SC7 steel (silicon alloy steel) cylinder quenched in water with cooling rate/quench severity|
Every metallurgist has been trained that increasing cooling rates, especially in the martensitic transformation region, leads to increasing potential for cracking as shown in Fig. 1. However, since the 1920s, there have been various, often little-known, industrial heat-treating processes that have been designated as intense, intensive, rapid, drastic, severe, or extreme quenching or shell-hardening methods[2-8]. The essence of these methods is to harden less hardenable steels using very fast cooling rates to impart high compressive stresses and improved fatigue properties to the quenched component. Therefore, in view of the classical training received by metallurgists, why are these processes not accompanied by quench cracking? Other questions put forth are: What is intensive quenching and would you recognize it if you saw it?
|Fig 2 Effect of cooling rate on the probability of cracking|
In 1964, Kobasko published the first of an extensive series of papers in which he used the term "intensive quenching." The experimental data included in this work provided numerical evidence showing that although it is true that increasing cooling rates result in increasing propensity for cracking (as historically recognized), there does exist a critical cooling rate above which cracking propensity decreases as shown in Fig. 2. Computer simulations were later used to validate and develop design criteria for optimal conditions for conducting intensive-quenching processes[10, 11]. Subsequently, various industrial intensive-quenching processes were developed and patented by Kobasko[12, 13]. A brief overview of a selected number of historical intensive-quenching applications is provided.
In 1935, it was recognized that the depth of hardening could be controlled by spray pressure of cold water impinging on rail steel and by the speed of the steel moving through the spray zone (U.S. Patent 1,828,325). No spray pressures and quantitative measures quench severity were provided. However, this appears to be one of the earliest published references suggesting the development of intensive quenching.
In 1986, Kern reported that the first known example of intense quenching was in the production of a AISI 1035 carbon-steel rear axle for the Ford Model T using a hot solution of 5% aqueous caustic soda. However, Kern also reported that the Ladish Co. used an intense quenching process using vigorously agitated quench oil to produce Rolls Royce P-51 engine crankshafts.
There are two issues to consider when comparing this application with the Model T application. The first is that oil is recognized to exhibit substantially less quench severity than cold water in a high-pressure spray. The second is that there is no quantitative criterion either of quench severity or to the precise meaning of "vigorous" agitation. Therefore, it is not clear what the process definition of the term "intense quenching" is as referred to by Kern.
In a series of patents published between 1967 to 1971, somewhat more quantification was provided for the production of various machinery parts[5-7]. Pressurized tanks (accumulators) were used to deliver very high volumes (approximately 3,000 gpm, or 11,355 liter/min) of quenchant to selected surface areas of the part being press quenched. Such agitation was designated as "extreme" and "drastic." The purpose stated for performing such high-volume delivery processes was to achieve maximum hardness uniformity. A critical cooling rate was reported in U.S. Patent 3,515,601, but the rate (value) was not given.
The necessity of hardening plain-carbon and boron-containing carbon steels using a cold-water, rapid-quenching process to achieve the optimal as-quenched properties (high tensile strength and good weldability) was discussed by Morio. Properties obtained using cold-water, rapid-quenching are superior to those obtained by means of oil hardening. However, it was recognized that rapid quenching leads to cracking and increased distortion, although some control is afforded through the heat-treating process and component shape design.
Another method of minimizing cracking and distortion reported by Morio was to quench the part using sufficiently high agitation rates to eliminate film boiling on the surface, which would provide a more uniform quench. The "critical cooling rate" for this process (drastic quenching) was the cooling rate that is necessary to eliminate film boiling. The agitation rate/quench severity correlation used to calculate the critical cooling rate was taken from the traditional Grossmann Quench Severity data shown in Table 1. According to Morio, the practical limit for water quenching was 1.5 to 2.0, unless high-pressure sprays were used. This approach is complicated by the absence of a definition of mild, moderate, good, etc. for agitation rate. Furthermore, it is impossible to determine by observing "surface roll," etc., because agitation in quench tanks is notoriously nonuniform.
Mei has taken the approach reported by Morio even further by stating that an agitation rate sufficient to provide a Grossmann Quench Severity value of > 6.0 is required to provide an intensive-quenching process. However, in addition to the limitations of using Grossmann H-values, the approach reported by Mei is based on trial-and-error experimentation.
What is intensive quenching?
The historical references discussed above indicate that intensive quenching is conducted using very high (extreme or violent) agitation rates with a Grossmann H-value of >6.0. Agitation is a focus of these papers and, unfortunately, alloys and cross-section sizes are not addressed quantitatively, with the exception of references 14 and 15. While this information is helpful to envision an intensive-quenching process, it is insufficient to properly design a quenching process to achieve optimal results in a particular application.
The simplest, correct working definition of intensive quenching might be those conditions that lead to maximum surface compressive stresses. However, to properly design a system, it is important to consider together those conditions that affect the formation of maximum surface compressive stresses including:
- Part shape
- Cross-section size
- Quenching cooling rate
- Final machining/grinding
Kobasko's definition of intensive quenching likely agrees with the above, depending on the materials, cross-section sizes and actual cooling rates achieved at the hot metal interface during quenching. Many of the processes described above either might not result in the parts being intensively quenched or they might not have possessed maximum surface compressive stresses.
Intensive-quenching system design
Recently, a process simulation procedure has been developed that enables the part designer to obtain higher performance from a given material while providing lower part distortion at the same time. It is the "other side" of Fig. 2; that is; a sufficiently high cooling rate to achieve desired physical properties and low distortion.
Intensive water quenching systems (IntensiQuenchSM) designed by IQ Technologies use very fast cooling, and high-performance mechanical properties are no longer as dependent on steel hardenability. The design process integrates material selection, part design and heat-treating process selection. (A discussion of intensive quenching and process design is available in a paper entitled "Theory of Intensive Quenching" which is available at www.intensivequench.com.)
In addition to using computer simulation, it is necessary to validate the proposed process conditions using appropriate trials based on accumulated knowledge and the simulated results. The computer model provides the parameters to ensure a robust part-processing methodology based on:
- Material properties (stress-strain conditions)
- Alloying element effects
- Mapping of part geometry
- Required minimum cooling rate for proper formation of the surface martensite shell
- Method used to make the shell uniform
- Parameters for the quenching equipment to form the shell repeatedly and reproducibly
- Minimization of distortion
- Elimination of internal and external cracks
- Proper window for interruption of the intensive phase of the quench (when compressive surface stresses are at their maximum value and at their optimum depth)
When all of these parameters are synchronized, IntensiQuench provides a unique microstructure of packaged, or packed, martensite having very high dislocation densities, a super-strengthened layer of compressive stress and low part distortion-all using lower alloy steel.
Another driver for adopting intensive water quenching is the elimination of oil quenching and the various associated hazards. Because IntensiQuench uses water and not oil, it offers the manufacturer the flexibility to place the heat-treating operation in line with the machining and grinding operations on the production floor within a manufacturing/machining cell. It is no longer necessary to batch carburize parts using long cycles and oil quench the parts to provide surface compressive stresses and a properly toughened core. Plug quenching can be eliminated because IntensiQuench builds the "die" on the outside of the part during the intensive part of the quench cycle.
|Parts that have been processed using intensive water quenching|
With the exception of Kobasko's ongoing work, there has been no other comprehensive and quantitative work on this process. Considering Kobasko's definition of intensive quenching as "Those processes that yield maximum surface compressive stresses? many of the earlier processes either may not have been intensive, or they may not have yielded maximum surface compressive stresses. Currently, as a computer simulation process, Intensi-Quench has been developed to aid in the design of intensive-quench systems that yield optimum results. It is likely that as intensive quenching becomes better understood and with the use of appropriately design systems, many oil-quenching processes, such as carburizing, could be replaced with intensive water quenching processes.