I came into this thread late but I thought I would share a couple of tidbits. In the mid-80s, when we (Davidson Cycles) were building around 700 frames per year, we wanted some idea of what was going to happen to them in a crash, so borrowed a destructive testing device (I forget who had it. Bicycling Mag, maybe?) that simulated a front end crash. Made a bunch of front ends to test in it, with all different materials and methods but otherwise the same, and found, almost without exception, that the joint failed (came apart) when done with silver, and the tube failed behind the intact joint when brazed with brass. What does that tell you? Well, given a hard enough impact you will destroy a steel bike no matter, but the brass did make a stronger joint. Yes you can braze at a lower temperature with silver, but if you are burning the snot out of it getting it to move around then you will do a lot more harm. I think an experienced builder using brass will build a much superior bike to a rookie with silver.
It is not only the absolute temperature that is reached that degrades the steel, it is the length of time at that temperature. The longer it stays hot, the farther away from the joint the heat affected zone travels. It is that margin of the heat affected zone that is where a frame will usually fail in a crash. Keeping it closer to the lug, in the butted section, makes it more likely to withstand a crash. So brazing it quickly is the answer to making the strongest frame. We developed a way to braze them very quickly. We don't do a lot of lugged steel frames any more so I don't mine sharing the tricks. We made little rings of brass that were the shape of the inside of the joint, and assembled the frame with them in there. The joint is heated quickly and evenly with a large, broad flame until red hot. You can see the brass ring in there as a shadow. Once it is gone you know the brass has melted, we switch to a small torch and work the heat around until brass comes out all the edges evenly, and voila, you are done. Almost no cleanup involved afterwards. A few of our brazers would use the big torch in one hand and the small torch in the other. We had fork crowns made with a shelf that the steerer butted against, and stamped out washers of brass that fit right in there and did the same thing for the crown to steerer braze, and had rings inside the blade sockets too. Two guys could braze up 30 steerer, crown, blade assemblies an hour that way. Again with almost no cleanup. And it leaves a little fillet of brass on the inside of the joint too to further strengthen it. You would never get penetration that good by brazing from the outside only, and not nearly as fast.
Another tidbit, and all framebuilders know this, but 853 Reynolds is always brazed with brass. It doesn't get hot enough for the air-hardening to happen with silver.
cheers
Bob Freeman
Elliott Bay Bicycles
2116 Western Ave
Seattle, WA 98121
206-441-8144
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