Spring 2013 (contents):
1. Temper Tantrums
2. MIG Welding
3. Rustless Steel?
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Temper Tantrums
On cooling rapidly, a steel weld can form sufficient amount of martensite to cause trouble. Martensite is a hard and brittle phase that is prone to cracking; it gets worse in the presence of hydrogen where you end up with hydrogen embrittlement. Martensite can form in the HAZ or in the weldment. Martensite can be tempered, a.k.a. softened to increase toughness (resistance to crack growth) and reduce hardness (resistance to deformation). Tempering process involves heating to an intermediate temperature and holding it there for some time to allow the martensite to transform into softer phases. Often times, it is not convenient to temper a welded structure due to its size or other parts of the weldment that may not be able to withstand heat treatment. Localized tempering of the weld is quite difficult as well except in case of resistance welding where a tempering cycle can be programmed right into the welding schedule.
Instead of trying to temper a brittle weld, it is usually better to take steps to avoid formation or detrimental levels of martensite in the first place. One option is to choose the right chemistry of parent metal that is low in carbon (<0.15%). If filler metal is to be added, its chemistry should also be selected carefully such that martensite formation is minimized or avoided. Of course, base metal and filler wire steels that are low in carbon or carbon-equivalent are also not very strong. If stronger steels with higher amounts of alloying elements are to be welded, then the other trick is to preheat the parts such that the weld then cools down much more slowly and spends a lot of time above the martensite transformation start temperature thus reducing or avoiding martensite formation. Sometimes, preheat is followed by a postheat after the weld; especially helpful for steels that easily hardenable. Holding a postheat also allows hydrogen to diffuse out of the weld. Trapped hydrogen in a steel matrix can cause hydrogen embrittlement - a phenomenon where hydrogen accumulates at a crack tip and forces the crack to grow. Whatever the situation, it is best not to allow the weld to throw a temper tantrum.
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MIG Welding
MIG is an acronym for "Metal Inert Gas" which is similar to TIG welding ( Fall 2012 Weld Nugget) except that the filler wire itself acts like an electrode. Used with a bare metal electrode, it utilizes a shielding gas to protect the weld pool. Over the years, applications have broadened to include reactive gases such carbon dioxide hence the new all-encompassing term GMAW - Gas Metal Arc Welding. GMAW can be thought of as the next generation of stick welding ( Summer 2012 Weld Nugget) where the electrode can be fed continuously and at much higher feed rates. GMAW is also much more amenable to automation and is often the process of choice for robotic welding. Welds require minimal cleaning due to absence of slag since it uses a bare wire. As with any process, there are a few limitations. Because shielding gas is used, welding has to be done in an environment free of drafts that could disperse the shielding gas thus limiting outdoor applications. The welding gun is also larger and makes it difficult to reach locations which would be welded with SMAW.
In automated MIG welding, the power supply attempts to hold a constant voltage across the arc thus setting up a fixed distance between wire tip and work piece. If there is any change in position of the electrode gun (with respect to the work piece) and the arc becomes longer/shorter, the current compensates to keep the same voltage/arc length. After some time, the machine equilibrates to a new stable position which might have different level of current flow and hence a different level of weld penetration. In some cases, a constant-current power supply is used but requires more operator skill as self-regulation is limited.
Since the filler wire feeds in across the arc, the metal transfer is in the form of drops that go gently across (short circuiting transfer) to the other extreme of being fired like bullets from a gun (spray transfer). Either case, the bead is not as clean as TIG welding where the filler wire is fed from the side into the molten metal puddle. Systems are now available where two wires are simultaneously fed into the weld pool thus more than doubling the deposition rate and travel speed. At the other extreme, a new technology called cold metal transfer (CMT) is an extension of short circuiting mode that allows a more controlled method of droplet detachment allowing welding of thinner gauge metals and even allows dissimilar metal welding. Given the wide variety of choices for metal deposition and ease of automation, it is no wonder that MIG welding is the most commonly used arc welding process in industry.
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Rustless Steel?
Back in the days when the industrial revolution was in full swing, there was this little annoying problem with rust and wear on steels. Problem was affecting makers of gun barrels and kitchen knives alike. In 1912, while trying to improve wear characteristics, Harry Brearley of Sheffield, England, cooked up a mixture of iron and chromium in an attempt to make steel with better wear resistance. In trying to etch it to see the microstructure he realized that the new composition was difficult etch and would not rust even though exposed to the elements for a long time. He came up with the name Rustless Steel, which sounds quite apt to an engineer. But when he tried to sell his "rustless" steel to Ernest Stuart, manager at a nearby cutlery manufacturer, the name did not fly. Mr. Stuart liked the knives that would not stain, but did not like the name "rustless", instead preferring to call the new steel "stainless steel." Shakespeare had said, "What is in the name, a rose by any other name would smell as sweet." But rustless steel? Really? Stainless steel sounds so much better.
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