Just how much atmosphere needs to be leaked into the furnace, i.e., how much of a “partial-pressure” is needed?
Many brazing shops indicate that they build up partial-pressures to about 200-microns or more for their partial-pressure brazing. Why did they select such a number, and what does that actually mean?
Well, to understand that, we’ll need to look again at another chart that I’ve referred to in past articles, namely, the Vapor Pressure Curves for Metals shown in Figure 2.
As you can see from the chart, the vertical axes are labeled differently on the left side and the right side. The left-side shows pressure in millimeters of Mercury (Hg), whereas the right side shows pressure in microns. What is the significance of such pressure measurements, where did they originate, and how do they relate to the chart shown in Fig. 1? To answer this, we need to take a brief look at the primary historical event in discovering the meaning and value of barometric pressure measurement.
As illustrated in Fig. 3, Evangelista Torricelli, back in the year 1643, inverted a tube of Mercury (Hg) into a dish, and the atmospheric pressure on the surface of the Hg that drained out into the dish, in conjunction with the Torricellian vacuum created in the closed space at the top of the column, kept the Hg suspended in the tube at a height of 760 mm (29.92 inches). This 760 mm of Hg came to represent “1 standard atmosphere of pressure” (when measured at sea-level and about 25°C).
Later on, scientists decided to give recognition to Torricelli’s work by naming each one of those 760 millimeter increments a “Torr” in his honor. Therefore:
1 Torr = I mm Hg
As you can then see, one standard atmosphere of pressure can also be called a pressure of 760-Torr.
Okay, let’s now go back to the chart shown in Fig. 2. The vertical axis on the right side of the chart is measured in “microns” of pressure. 1-micron (also called 1-millitorr) is 1/1000th of a Torr. Temperature on the chart is shown
along the horizontal axes.
Notice that each curve in Fig. 2 represents a specific metal. The curve for Chromium (Cr) runs approximately down the center of the chart from upper-right down to lower-left. When operating on the left side of the curve for any given
metal, that metal will not be volatilized. However, when operating on the right side of any given curve, that metal can volatilize (out-gas). It is always desirable to operate on the LEFT side of any given curve.
Let’s take Chromium (Cr) for example. You can see on the chart in Fig. 2 that the line for Cr crosses the dotted horizontal pressure line near the top of the chart (which represents one full atmosphere of pressure) at about 4350°F/2400°C. But, as a vacuum is pulled in a furnace, and the pressure drops, the Cr line begins to move down to the left. Thus, at 10-3 Torr pressure on the left vertical axis (equals 1 micron, or 1 millitorr on the right vertical axis) the Cr line crosses the 2100°F/1150°C temperature line. At 10-4 Torr, the Cr line is at only about 1900°F/1040°C. Remember, if your temp/vac-level combination puts you on the right side of a given metal’s curve, that metal will begin to volatilize (outgas).