| Braze Joint Cracking Under Heavy Machining Question: Can you tell us why our braze joint is cracking during the machining operation? The brazement is made up of a series of Inconel* 600 rings and electrical iron rings alternately stacked and brazed together. Fixturing is accomplished by a step in the iron rings, which locates the alternative rings. Nicrobraz LM (BNi-2) filler metal paste is applied as a fillet at each joint. The part is then brazed at 2050° F (1120° C), and the I.D. is bored and O.D. contoured. The parts must be leak-free when tested with a helium leak detector. Answer:
With a given set of base metals, the major variables are (in order of significance): quantity of filler metal (or clearance), time at brazing temperature and brazing temperature (which is almost as important as time at temp.). Two noteworthy papers referenced below show the relationship of these variables. 1. Dr. Richard Johnson, "The Use of TETIG Diagrams in High-Temperature Brazing" (FL: Welding Journal, October 1981), p. 185S. 2. Dr. Eric Lugscheider and K.D. Partz, "High Temperature Brazing of Stainless Steels with Nickel Base Filler Metals BNi-2, BNi-5 and BNi-7" (FL: Welding Journal, June 1983), p. 160S. Both papers provide insight into how the secondary center phase reacts with clearance, time and temperature. The Lugscheider and Partz paper shows two sturdy bars placed side by side with one end at zero clearance and the other end with a 0.004 gap. The bars were tack welded (GTAW) at each end. The specimen was brazed at a specified time and temperature with a given filler metal. Then it was cross-sectioned, polished and etched with Marbles reagent, and the V-section observed with a metallograph. At the zero clearance end, a single phase exists all the way across the joint, and the phase remains essentially un-etched. As the clearance increases along the V-joint, a point is reached where a secondary phase appears in the center of the braze joint. This clearance is called the MBC point, or maximum brazing clearance to obtain a single-phase structure. The single phase structure is stronger, ductile and has a high re-melt temperature, which is more desirable on high stress parts. Both papers contain specimens that can be used to obtain data on the clearance, time and temperature required to obtain single-phase diffusion of the brazing filler metal in the joint. Boron-containing filler metals, such as Nicrobraz LM, have very interesting properties. At brazing temperature, boron, which accounts for the low melting properties of the filler metal, can be diffused out of the brazed joint and into the adjoining base metal. This diffusion accounts for the drop in filler metal hardness to as low as Rb 70 when brazing steel or iron-chromium base metals, (AISI 410) and the increase in the brazing joint re-melt temperature to above 2500° F (1370° C). Smaller elements can diffuse readily at higher brazing temperatures. The atomic diameters (angstrom or A) of some of these smaller elements are shown here.
The larger elements shown below diffuse very slowly compared to the above elements.
Thus, to obtain a joint strength that will withstand the high pressure loadings of heavy machining, it is necessary to have a fully diffused brazed joint. There should be no secondary phase in the center of the joint. Note: Subsequent parts were run using the minimum clearance, a brazing temperature of 2150° F (1175° C) and Nicrobraz LC (BNi-1a) filler metal. The butt joint was bent around a 5/8" diameter bar with the braze at a right angle to the direction of the bend. No fractures were observed in the braze joint (Inconel 600 to electrical iron), thus indicating the fully diffusion brazed joint was strong and ductile. A photomicrograph also confirmed that there was no secondary phase in this particular braze combination. Originally appeared in the Welding Journal, February 1991. Bob Peaslee authors a monthly Q & A column on brazing in the Welding Journal. * Trademark of Inco Alloys International. |
If diffusion brazing is not adequately accomplished, the braze joint may crack under high-stress loading. When brazing for short periods of time at temperature or with large joint clearances, particularly at lower brazing temperatures, the boron in the filler metal will not completely diffuse. Incomplete diffusion leaves a soft layer on each side of the braze joint with a hard center layer of low ductility, which can fracture when highly stressed.