Understanding Intensive Quenching
Intensive quenching, known as IntensiQuench(sm), is an alternative way to harden steel parts. IntensiQuench processes can be defined as cooling (usually using pure water or low-concentration water/salt solutions) at a rate several times faster than the rate of conventional quenching. Intensive quenching requires very high cooling rates for parts within the martensitic phase. Very fast and very uniform part cooling reduces the probability of part cracking and distortion, while improving the surface hardness and durability of steel parts.
The rapid cooling rate also provides greater hardened depth, which, in turn, improves mechanical properties. It creates high residual compressive stresses on the part surface, allows the use of lower alloyed steels or using a smaller (lighter) yet stronger part and makes the quenching process more cost effective. In addition, the process is clean and environmentally friendly.
IntensiQuench: What is it?
Animation "freeze frames" outline the critical points to the intensive quenching processes (Fig. 1). When carried out in this order and according to an IQ Technologies proprietary computer software model, intensive water quenching produces high hardness (with high compressive surface stresses) and low distortion at the same time.
Figure 1a shows the cross-sectional view of a ring before heating. Figure 1b shows inside the same ring heated to the austenitizing temperature (parts can be through-heated using any traditional means). Because the part is red hot uniformly throughout, the ring has zero compressive or tensile surface stresses on the accompanying graph (to the right of the illustration). The initiation of the intensive water quench is shown in Fig. 1c. At this moment, the surface is being cooled so fast and uniformly that a "shell" of cool austenite is formed on the surface layer of the part. As the austenite cools, before reaching martensite start, it shrinks and puts the surface under tensile stress (as shown on the graph to the right of the illustration). Figure 1d shows the point where the surface has reached maximum compression to an optimum depth over the entire surface of the part. At this point, the part surface is at the temperature of the quench water, and the core is still hot austenite. Also at this moment, the intensive water quench is interrupted, usually by removing the part to the air, where the part continues to cool uniformly by conduction through the wall of the shell. At this time, usually less than a minute into the intensive quench, the part surface shell is hard enough and deep enough to withstand the eventual swelling of the core as it reaches martensite phase (or a mixed phase of hardened structures, depending on the alloy.) Once formed, this uniform surface shell (under very high compressive stresses) now functions as a "die" on the outer surface of the part to "hold" the part, contain the swelling core and reduce distortion and eliminate part quench cracking.
The IQT computer model predicts the window of time for "interruption" and also tells the heat treater the minimum flow rates necessary to obtain the uniform shell. There is no maximum flow rate. One cannot quench too fast because once above the "intensive zone," the part cannot physically conduct the heat to the surface any faster. The IQT software model has been successfully tested in thousands of actual parts and validated by comparisons to other software models.
Figure 1e shows the core continuing to cool and to change from austenite to martensite (or a mixed structure). Austenite-phase steel will grow approximately 4% in volume as it converts to martensite. Normally, this would spell disaster for a heat treater who is water quenching, because the shell cannot contain the swelling (martensitic) core. However, because the intensive quenching was interrupted when shell was at maximum compressive stress to an optimum depth, it can and does contain the swelling core and actually produces a "packed martensite"' under the shell. Packed martensite, with its high dislocation densities, actually looks as though it was physically shot peened. The compressive stresses obtained by IntensiQuench are higher in amount and to a deeper level than those obtained with traditional case hardening.
In addition to the shear strength of the shell, the core is still hot and austenite is in a plastic stage when the intensive quench is interrupted, and the core shrinks (thermally) as the austenite cools. At interruption of the intensive quench, the austenitic core continues to deform in its plastic state under the cold shell until the core cools to the martensite start temperature (by uniform conduction) and begins to swell under the fully strengthened austenitic shell. This cooling of the austenitic core phenomenon is termed "pre-phase transformation shrinkage."
Figure 1f shows the core cooling to martensite (or a mixed structure) and swelling under the shell, which offsets or reduces some of the shell's compressive surface stress. Also, depending on the IntensiQuench recipe, the core heat may be used to temper the martensite in the shell for further toughening of the shell. However, at the end of the intensive quench cycle, the part emerges as fully hardened and yet with very low distortion.
Intensive quenching in action
When properly applied, IntensiQuench imparts very high as-quenched hardness, high residual compressive stresses and, at the same time, low distortion on the part. Figure 2 shows an intensive water-quenching unit in the down position with a part over the loading platform. Figure 3 shows the system in the quench position with the part visible in the clear tube. Because the water is moving fast enough to prevent any film boiling or nucleate boiling (for "direct convection cooling(sm)," the plastic pipe in the quench chamber does not get hot during quenching. The intensive water quench is "interrupted" (flow stopped and parts removed to air cool) when the compressive surface stresses on the part are at their maximum value (and also at their optimum depth). At the time of interruption, the shell is the temperature of the quenchant and the core is still very hot. The high compressive surface stresses reduces distortion because the compression functions like a die, or "shell," to hold the core of the part while it cools evenly by conduction through the cold surface layer.
The IQT proprietary computer model shows when the optimum time is to interrupt the quench for the particular part geometry and alloy type. The computer model also shows what flow rate is necessary to ensure that the part is uniformly, intensively quenched over its entire surface.
Parts having complex geometry or parts quenched in batches (in a 6,000-gal production unit) require two stages of intensive quench, called IQ-2. After the first few seconds of intensive quenching, the shell is allowed to "self-temper" using the heat from the core and then the intensive quench is reapplied to stop the self tempering and finish cooling the core. Again, the IQT computer model determines the times and the minimum flow rates for each step of the quenching process.
The first intensive quench tank (for the USA) supplied by IQ Technologies consists of a "U" tube that has a prop in one side and the parts are quenched in the water plume in the other end of the "U" (Fig 4). The quench rate is many times faster than that of oil, polymer and traditional water quenches. The interruption time is still key. Although the original quench tank is slower than the one pictured in Fig. 2 and 3, the intensive part of the quench is completed in a matter of seconds for most parts.