Our resident brazing expert takes time to answer a question submitted by a reader.
QUESTION: We manufacture plate-type heat exchangers and EGR coolers in India. We currently use a continuous conveyor-belt induction heating inert atmospheric furnace and use mild-steel fixtures for brazing. After 35 passes, the fixtures must be scrapped. Can you suggest an alternative fixture material that has better life and is more cost-effective?
ANSWER: Thank you for your question about suggesting an alternate fixturing material that will have better life than the mild-steel fixtures you have been using. Please be aware that all mild steels will go through a phase change when they are heated up to brazing temperatures. All metals have orderly alignment of atoms in their structure, and many metals maintain such alignment of atoms when they are heated to high temperatures. However, mild steels do not.
Like all metals, the mild steel expands as it is heated, but only up to a point. When it is heated above approximately 910°C (1670°F), its atomic alignment (arrangement of atoms) changes, and the atoms rearrange themselves into a different pattern, which actually occupies less space than the room-temperature atomic alignment. Thus, the steel actually begins to shrink in size above 910°C. Technically speaking, the mild steel goes from a body-centered cubic (BCC) room-temperature alignment to a face-centered cubic (FCC) alignment above 910°C, and then back again when it goes back to room temperature. Thus, the mild steel expands at first before shrinking when it goes through its “phase transformation” temperature, which causes the steel to shrink when it is heated to brazing temperature. The mild steel then re-expands when it cools back down to room temp. When such cycles are repeated time and again, this expand-shrink-expand-shrink phenomenon typically results in significant distortion of the mild-steel fixtures.
Therefore, I do not recommend that you use mild steel for brazing fixtures. This same trouble can occur with alloy steels, low-carbon steels and even with 17-4PH to some extent. Instead, you might select an inexpensive stainless steel material since it does not go through such phase changes, and it can pay for itself by lasting far longer than the mild-steel fixtures that you have to replace again and again.