1. Stressed Out?
2. Thermit Welding
3. Sulfur - It Stinks!
Is weld stress stressing you out? Not an uncommon situation. But before you reach out for a bottle of your favorite heartburn medication, take a deep breath, sit back, and think about the welding process. Welding is an inherently stress inducing process given the rapid rise and fall in weld temperature, often in a matter of milli-seconds. In addition to cooling rate, factors such as clamping methods, pre-weld residual stress, and material issues can add to an already stressful situation.
For starters, you can try and reduce cooling rate of the weld by providing a gradual reduction in energy. In pulsed welding, this can take the form of a separate cool down pulse or a taper down of the energy profile at the end of the welding pulse. In continuous energy welds, such as with most arc welds and CW laser welds, you can try to change the fixture design such that the fixtures do not cool the weld too rapidly. Changes could include fixture material or cutting down on cooling water. If the parts are coming in with significant residual stress from prior machining/forming/stamping operations, you can try and anneal/stress-relieve the parts to reduce residual stress. Keep in mind that any heat treatment can warp the parts and they may not fit together as well as they did before annealing; poor fitup can cause a whole new set of problems. Excessively rigid restraints during welding can also lead to high residual stress. Given a choice, one of the two parts being welded should be located firmly while the other one should have some liberty to adjust itself during the welding process. One way to do that is to tack weld the parts together and then securely hold one part while the other realigns during welding.
Rapid cooling rate can also produce undesirable microstructures in the weld zone as well as the HAZ (see previous newsletter at this link for a discussion on HAZ) which can further complicate the situation. This is especially true with high carbon content steels which can form harder phases at faster cooling rates. Pre-heating the parts, in additon to reducing the cooling rate, also allows hydrogen to escape from the weld, thus reducing the potential for hydrogen embrittlement. Slower cooling rate also reduces residual stresses. Keep in mind that rapid cooling rate is not always so bad. In instances where minor impurity elements can segregate along grain boundaries, a rapid cooling rate can actually prevent segregation and could help avoid cracking.
Thermit welding is a process used commonly to weld two segments of rails in the field. The word thermit comes from the fact that an exothermic reaction is used to generate heat in order to melt the steel. The process is very different than other welding process in that it acts more like a casting process. The charge, a mixture of aluminum powder and iron oxide, is heated to a high enough temperature to start the exothermic reaction where aluminum reduces iron oxide to form aluminum oxide and molten steel. Aluminum oxide forms a slag that floats to the surface. The molten metal is then allowed to flow down under action of gravity to fill the gap between the two rails being welded. The molten metal is contained around the rails with molds. Temperature of molten steel is high enough that it is able to include some of the parent metal into the fusion zone thus forming a strong bond. After cooling down, any excess material is removed by cutting and grinding.
Thermit welding is ideally suited for joining rails because of many reasons. First is its ability to melt a large quantity of metal out in the field without the need for complicated equipment or accurate temperature control. Secondly, since temperature of the molten steel produced during the thermit process is very high, almost 2500C, the molten steel is able to form a fusion layer all along the mating faces of the rails, something that would be almost impossible with any other welding process. Finally, given that it acts more like a casting process to fill the gap between two rails, thermit welding is not very sensitive to alignment or gap clearance. Thermit welding, a combination of casting and welding, is a great example of human ingenuity in coming up with a solution to a unique problem of rail welding.
Sulfur - It Stinks!
Sulfur is the most common nuisance element in welding, especially in welding of steels. Sulfur can combine with iron to form low melting (1200 C) iron sulfide (FeS); additionally, FeS can forms an even lower melting (988 C) eutectic composition with iron. Presence of iron sulfide can cause hot cracking (click here to see earlier newsletter on cracks in welds). Manganese is commonly added to carbon steels with the sole purpose of reacting with sulfur to form more stable manganese sulfide. Sulfur can also affect cracking during laser welding of stainless steels such as 316L which have just the right ratio of Cr:Ni to start solidification as primary austenite. Once again, low melting and low solubility iron sulfide is relegated to the grain boundaries and causes cracks. Typical solution is to specify ultra-low levels of sulfur or switch to other SS grades such as 304L that solidifies as primary ferrite and avoids cracking.
Free-machining steel grades are often made with higher amounts of sulfur, of the order of 0.1 to 0.3%, and can produce porosity and cracking in welds. Even though these steels can be welded under some circumstances, one should avoid welding free-machining steels as much as possible.
There is a bright side to sulfur when it comes to increasing weld penetration depth in TIG (GTAW) welding of steels. Sulfur concentration less than 0.008% produces a wide and shallow weld, while greater than 0.008% sulfur produces deeper and narrower welds. The difference is attributed to changes in surface tension of the melt pool and the corresponding changes in direction of molten metal flow.