1. Heat Balance - A Tightrope Walk
2. Underwater Welding - Going Deep
3. To Kill a Steel
Heat Balance - A Tightrope Walk
Practically all welding processes depend on adding heat to the weld joint to form a bond. Heat is added to soften or melt the required constituents to a planned temperature in order to facilitate formation of the bond. Processes vary from low temperature soldering to fusion welding. In all cases, it is of utmost importance to provide the right amount of heat, at the right time, and at the right place. Alas, controlling all the rights is one tough job and produces nightmares for process control. Because doing everything right means nothing more AND nothing less. Too little heat will often produce a weak weld while too much heat causes problems such as runoff of solders/braze, softening/warping of base materials, and excessive defects such as spatter and porosity in fusion welds. One of the tricky aspects of heat balance is the concept of net heat input into the weld zone. Not only does one have to worry about heat input to the weld (which can be easily programmed at the weld process controller), one must also control heat lost from the weld. Heat is lost mostly by conduction through the parts to be welded as well as fixtures used to hold the parts; there is minimal heat loss by convection and radiation. Timing also has to be just right - duration as well as time profile of energy delivery; combination of the two will produce the expected peak temperature at the weld junction. While lower than expected temperature will produce a weak weld, higher temperatures will invariably produce more defects. Alignment is also important to ensure both parts being welded get to the desired temperature.
Good heat balance, and consequently good weld quality, comes at a price. Process equipment which can accurately control amount and timing of heat delivery is expensive. Supplying heat to the right place entails quality fixturing to hold the parts and special tooling, sometimes equipped with sensors, for precision alignment. Achieving heat balance also requires higher component quality including better fitup, surface characteristics, cleanliness, and base material consistency. Users have to walk the fine line between cost and quality to optimize return on investment; a tightrope walk indeed.
Underwater Welding - Going Deep
Underwater welding is often used to repair damage to structures that are underwater and in use. Examples include offshore oil platforms, undersea pipelines, harbor facilities, and ships. Two of the biggest challenges (apart from becoming shark food or electrocuting yourself) are the rapid rate of cooling experienced by these welds and the presence of copious amounts of hydrogen. A typical arc weld takes about 10 seconds to cool to below 500C - a corresponding underwater weld takes only 1 second. Rapid rate of cooling produces significant amount of martensite in the heat affected zone ( Summer 2010 Newsletter) increasing the likelihood of hydrogen cracking. Since presence of hydrogen (from water) is unavoidable, methods to manage the problem include the use of consumables that can dissolve a lot of hydrogen, use special flux coatings to introduce alternate gases, use weld parameters to minimize hydrogen pickup, and design weld for minimum stress. One of the unique ways to reduce weld stress is the application of a temper bead which is a deposition of a weld bead over the main weld bead. The function of the temper bead is to reduces stress in the main weld bead, increases toughness of the HAZ, and reduces hardness in the weld area. The temper bead laid over the main weld bead is composed of austenitic steel which can be a reservoir for hydrogen and is able to extract some of the hydrogen from the HAZ.
Welds in water are made with stick welding a.k.a. GMAW. Not all welds are made directly in the water. If shape of the structure is suitable, a hyperbaric chamber is constructed over the weld zone to create a dry environment which makes the process easier but much more expensive. GMAW welds are made down to 600 feet while a hyperbaric chamber allows TIG and MIG welding down to 900 feet!!, now that's real deep.
To Kill a Steel
Steels can be "killed" by depriving them of oxygen, not unlike asphyxiation. However, the difference is that killing steels is a desirable outcome. Steels are killed by adding a small quantity of elements with greater affinity for oxygen in comparison to iron. Silicon is the most commonly used deoxidizer. It reacts with oxygen to form silica which floats to the surface as slag and is removed. It is important to eliminate silica particles since their presence as inclusions in steel are detrimental to metal working and mechanical properties. Stronger deoxidation can be achieved by addition of aluminum, titanium, or calcium. Aluminum is a strong deoxidizer with the additional benefit of producing fine grain microstructure. It can also remove nitrogen, an impurity that can reduce toughness of ferritic steels. Titanium and zirconium are even better at removing nitrogen. With the removal of oxygen and nitrogen, the killing process is complete and new steel is born.