Handbook-Expansion & Contraction

8 Residual Stress We said that the weld metal always tends to shrink. If the sections being joined are locked into a structure so securely that they cannot move during the welding period, or during the cooling-down period, what happens? The weld metal stretches while it is cooling. Steel, at elevated temperatures, is quite ductile, so it can stretch quite a lot. (Cast iron, which is not very ductile, would probably crack.) Most of that stretch will represent permanent deformation. But some of that stretch will take place within the elastic limit. It is stretch that would disappear if the structural forces restraining the welded joint should suddenly be released. In place of that theoretical stretch, we have a ”residual” or ”locked-up” stress. That sounds alarming. Don’t let it worry you too much. Almost all welded structures contain residual stresses. Unless a residual stress in one section tends to reinforce the residual stress in another section, it’s not likely to cause trouble. When it seems necessary to get rid of residual stresses of considerable magnitude, they can be largely eliminated by post-heating the entire assembly, or the weld zone, and allowing it to cool slowly. In modern welding practice, residual stresses are often minimized by using ”multi-pass” instead of ”single-pass” welds in heavy sections. Each pass eliminates or reduces the residual stress created by previous passes. Coefficient of Thermal Expansion All metals expand when heated, but not to the same degree. The measure of expansion is termed the coefficient of thermal expansion. Most steels have a coefficient of about 0.000011 per degree Celsius. In metric terms, that means that an unrestrained steel bar, one meter long, will increase in length 11 millionths of a meter, or 11 thousandths of a millimeter, for each 1 degree C rise in temperature. A temperature rise of 10000C will cause a one-meter bar to increase 11 mm in length. In ”English” measure, you can calculate that a one-foot steel bar will increase in length about Ys inch when heated 1000⁰C.